U.S. patent application number 14/539547 was filed with the patent office on 2016-05-12 for fresh air flow and exhaust gas recirculation control system and method.
The applicant listed for this patent is DEERE AND COMPANY. Invention is credited to John L. Lahti, Michael J. Maney, Michael J. Pipho, Dustin W. Ridenour.
Application Number | 20160131057 14/539547 |
Document ID | / |
Family ID | 54360999 |
Filed Date | 2016-05-12 |
United States Patent
Application |
20160131057 |
Kind Code |
A1 |
Lahti; John L. ; et
al. |
May 12, 2016 |
FRESH AIR FLOW AND EXHAUST GAS RECIRCULATION CONTROL SYSTEM AND
METHOD
Abstract
A fresh air and exhaust gas control method and system for an
engine air system with an air throttle and exhaust gas
recirculation (EGR) valve. The method includes monitoring engine
parameters; generating engine state estimates using an engine
observer model; generating measured engine states based on the
monitored engine parameters; computing observer error based on the
differences between the measured and modeled engine states;
generating model correction factors; and generating commands for
adjusting the air throttle and EGR valve. An inverse engine
observer model can generate the desired air throttle and EGR valve
positions. The method can include generating feedback actuator
commands in generating the desired air throttle and EGR valve
positions. The correction factors can include fresh air, EGR and/or
turbine mass flow correction factors.
Inventors: |
Lahti; John L.; (Cedar
Falls, IA) ; Pipho; Michael J.; (Independence,
IA) ; Ridenour; Dustin W.; (Waterloo, IA) ;
Maney; Michael J.; (Waterloo, IA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DEERE AND COMPANY |
Moline |
IL |
US |
|
|
Family ID: |
54360999 |
Appl. No.: |
14/539547 |
Filed: |
November 12, 2014 |
Current U.S.
Class: |
701/103 |
Current CPC
Class: |
Y02T 10/47 20130101;
F02D 41/0077 20130101; F02D 2041/1416 20130101; F02M 26/47
20160201; F02D 2200/0402 20130101; F02D 41/0002 20130101; F02D
41/0072 20130101; F02M 26/22 20160201; F02D 2041/1434 20130101;
Y02T 10/40 20130101; F02D 2041/141 20130101; F02D 2041/1433
20130101; F02M 26/53 20160201; F02M 26/17 20160201; F02M 26/14
20160201; F02D 9/08 20130101 |
International
Class: |
F02D 41/00 20060101
F02D041/00; F02D 9/08 20060101 F02D009/08; F02M 25/07 20060101
F02M025/07 |
Claims
1. A fresh air and exhaust gas control method for an engine having
an air system with an air throttle and an exhaust gas recirculation
(EGR) valve; the control method comprising: monitoring parameters
of the engine using a plurality of sensors; modeling the air system
and generating engine state estimates using an engine observer
model; generating measured engine states based on the monitored
engine parameters from the plurality of sensors; computing an
observer error based on the differences between the measured engine
states and the engine state estimates; generating model corrections
based on the observer error, the model corrections being input to
the engine observer model; generating a desired air throttle
position and a desired EGR valve position based on setpoint
commands, the monitored engine parameters, the model corrections
and the engine state estimates; adjusting the air throttle based on
the desired air throttle position; and adjusting the EGR valve
based on the desired EGR valve position.
2. The fresh air and exhaust gas control method of claim 1; wherein
the generating a desired air throttle position and a desired EGR
valve position step is performed using an inverse engine model, the
inverse engine model comprising an inverse of the engine observer
model.
3. The fresh air and exhaust gas control method of claim 1; further
comprising: generating feedback actuator commands based on the
setpoint commands and the engine state estimates; and using the
feedback actuator commands in generating the desired air throttle
position and the desired EGR valve position.
4. The fresh air and exhaust gas control method of claim 1; wherein
the aft system further comprises a charge air cooler and an engine
intake manifold coupled to the air throttle, intake air to the
engine flowing through the charge air cooler, the air throttle and
the engine intake manifold; and the method further comprising:
estimating a pressure difference across the charge air cooler;
estimating a temperature difference across the charge air cooler;
and estimating an air throttle mass flow as part of the engine
state estimates based on the pressure and temperature differences
across the charge air cooler.
5. The fresh air and exhaust gas control method of claim 4; wherein
estimating a pressure difference across the charge air cooler
comprises: estimating a compressor outlet pressure; estimating an
intake manifold pressure; and estimating an air intake pressure at
a location between the charge air cooler and the air throttle by
starting at the previous value of the air intake pressure and
searching a percentage of the range between the compressor outlet
pressure and the intake manifold pressure for the air intake
pressure that makes the charge air cooler mass flow and the air
throttle mass flow the same or closest to the same.
6. The fresh air and exhaust gas control method of claim 1; wherein
the air system further comprises an EGR cooler coupled to the EGR
valve, recirculated exhaust gasses from the engine flowing through
the EGR cooler, the EGR valve and the engine intake manifold; and
the method further comprising; estimating a pressure difference
across the EGR cooler; estimating a temperature difference across
the EGR cooler; estimating an EGR valve mass flow as part of the
engine state estimates based on the pressure and temperature
differences across the EGR cooler.
7. The fresh air and exhaust gas control method of claim 6; wherein
estimating a pressure difference across the EGR cooler comprises:
estimating an exhaust manifold pressure; estimating an intake
manifold pressure; and estimating an EGR internal pressure at a
location between the EGR cooler and the EGR valve by starting at
the previous value of the EGR internal pressure and searching a
percentage of the range between the exhaust manifold pressure and
the intake manifold pressure for the EGR internal pressure that
makes the EGR cooler mass flow and the EGR valve mass flow the same
or closest to the same.
8. The fresh air and exhaust gas control method of claim 4; wherein
the air system further comprises an EGR cooler coupled to the EGR
valve, recirculated exhaust gasses from the engine flowing through
the EGR cooler, the EGR valve and the engine intake manifold; and
the method further comprising; estimating a pressure difference
across the EGR cooler; estimating a temperature difference across
the EGR cooler; estimating an EGR valve mass flow as part of the
engine state estimates based on the pressure and temperature
differences across the EGR cooler.
9. The fresh air and exhaust gas control method of claim 8; wherein
estimating a pressure difference across the charge air cooler
comprises: estimating a compressor outlet pressure; estimating an
intake manifold pressure; estimating an air intake pressure at a
location between the charge air cooler and the air throttle by
starting at the previous value of the air intake pressure and
searching a percentage of the range between the compressor outlet
pressure and the intake manifold pressure for the air intake
pressure that makes the charge air cooler mass flow and the air
throttle mass flow the same or closest to the same; and wherein
estimating a pressure difference across the EGR cooler comprises:
estimating an exhaust manifold pressure; estimating an EGR internal
pressure at a location between the EGR cooler and the EGR valve by
starting at the previous value of the EGR internal pressure and
searching a percentage of the range between the exhaust manifold
pressure and the intake manifold pressure for the EGR internal
pressure that makes the EGR cooler mass flow and the EGR valve mass
flow the same or closest to the same.
10. The fresh air and exhaust gas control method of claim 1;
wherein the air system further comprises a charge air cooler and an
engine intake manifold coupled to the air throttle, intake air to
the engine flowing through the charge air cooler, the air throttle
and the engine intake manifold; and wherein the generating model
corrections step comprises: generating a fresh air mass flow
correction term based on a pressure error at the engine intake
manifold.
11. The fresh air and exhaust gas control method of claim 1;
wherein the air system further comprises an EGR cooler and an
engine intake manifold coupled to the EGR valve, recirculated
exhaust gasses from the engine flowing through the EGR cooler, the
EGR valve and the engine intake manifold; and wherein the
generating model corrections step comprises: generating an EGR mass
flow correction term based on a difference between the modelled and
measured mass flows through the EGR cooler.
12. The fresh air and exhaust gas control method of claim 1;
further comprising: calculating a desired intake manifold diluent
mass fraction based on a desired diluent-to-air ratio setpoint, an
engine speed and a number of cylinders in the engine; calculating a
desired EGR mass flow based on the desired intake manifold diluent
mass fraction; and calculating the desired EGR valve position based
on the desired EGR mass flow; calculating a desired intake manifold
pressure based on the desired diluent-to-air ratio setpoint and the
number of cylinders in the engine; calculating a desired air
throttle mass flow based on the desired intake manifold pressure;
and calculating the desired air throttle position based on the
desired air throttle mass flow.
13. The fresh air and exhaust gas control method of claim 1;
further comprising: calculating a desired intake manifold diluent
mass fraction based on a desired diluent-to-air ratio setpoint, an
engine speed and a number of cylinders in the engine; calculating
an intake manifold EGR mass fraction and a closed loop EGR mass
flow based on the desired intake manifold diluent mass fraction;
calculating a maximum air throttle mass flow and a maximum EGR
valve mass flow; calculating a desired intake manifold pressure
based on the desired diluent-to-air ratio setpoint and the number
of cylinders in the engine; calculating a maximum intake manifold
mass flow based on the intake manifold EGR mass fraction, the
closed loop EGR mass flow, the maximum air throttle mass flow and
the maximum EGR valve mass flow; calculating a desired intake
manifold mass flow in based on the maximum intake manifold mass
flow and the desired intake manifold pressure; calculating a
desired air throttle mass flow and a desired EGR valve mass flow
based on the intake manifold EGR mass fraction, the closed loop EGR
mass flow and the desired intake manifold mass flow in; calculating
the desired EGR valve position based on the desired EGR valve mass
flow; and calculating the desired air throttle position based on
the desired air throttle mass flow.
14. A fresh air and exhaust gas control system for an engine having
an air system with an air throttle and an exhaust gas recirculation
(EGR) valve; the control system comprising: a plurality of sensors
for monitoring engine parameters of the engine; an engine observer
for modeling the air system and generating engine state estimates;
a comparator for generating measured engine states based on the
engine parameters from the plurality of sensors, and computing an
observer error based on the differences between the measured engine
states and the engine state estimates; an observer controller for
generating model corrections based on the observer error, the model
corrections being input to the engine observer; and an inverse
engine model for generating actuator commands based on setpoint
commands, the engine parameters from the plurality of sensors, the
model corrections and the engine state estimates; wherein the
positions of the air throttle and the EGR valve are adjusted based
on the actuator commands.
15. The fresh air and exhaust gas control system of claim 14,
wherein the engine observer, the comparator, the observer
controller, and the inverse engine model are run on an electronic
control unit of the engine.
16. The fresh air and exhaust gas control system of claim 14,
further comprising a feedback controller for generating feedback
actuator commands based on the setpoint commands and the engine
state estimates, the positions of the air throttle and the EGR
valve being adjusted based on the actuator commands and the
feedback actuator commands.
17. The fresh air and exhaust gas control system of claim 14,
wherein the air system further comprises a charge air cooler and an
engine intake manifold coupled to the air throttle, intake air to
the engine flowing through the charge air cooler, the air throttle
and the engine intake manifold; and the engine state estimates
generated by the engine observer include an air throttle mass flow
estimate, the air throttle mass flow estimate being based on
pressure and temperature differences across the charge air
cooler.
18. The fresh air and exhaust gas control system of claim 17;
wherein the air system further comprises an EGR cooler coupled to
the EGR valve, recirculated exhaust gasses from the engine flowing
through the EGR cooler, the EGR valve and the engine intake
manifold; and the engine state estimates generated by the engine
observer include an EGR valve mass flow estimate, the EGR valve
mass flow estimate being based on pressure and temperature
differences across the EGR cooler.
19. The fresh air and exhaust gas control system of claim 14;
wherein the air system further comprises a charge air cooler and an
engine intake manifold coupled to the air throttle, intake air to
the engine flowing through the charge air cooler, the air throttle
and the engine intake manifold; and the observer controller
generates a fresh air mass flow correction term based on a
difference between the modelled and measured intake manifold
pressures.
20. The fresh air and exhaust gas control system of claim 14;
wherein the air system further comprises an EGR cooler and an
engine intake manifold coupled to the EGR valve, recirculated
exhaust gasses from the engine flowing through the EGR cooler, the
EGR valve and the engine intake manifold; and the observer
controller generates an EGR mass flow correction term based on a
difference between the modelled and measured EGR cooler mass flows.
Description
FIELD OF THE DISCLOSURE
[0001] The present disclosure relates to engine control systems and
methods and more particularly to an engine air system model that
can run in the engine control unit (ECU) in real time.
BACKGROUND
[0002] Internal combustion engines use recirculated exhaust gas to
reduce emissions of nitrogen oxides (NOx). Achieving the optimum
combination of air and burnt gasses within the engine cylinder
prior to the start of combustion on each engine cycle may not be
possible due to the dynamics of the air system. To address this
problem, control strategies are used to attempt to regulate the
intake manifold pressure and the percentage of exhaust gas
recirculation (EGR). Traditional engine control strategies use open
loop tables and/or proportional-integral-derivative (PID)
controllers. These systems tend to have errors under transient
conditions because the open loop tables are calibrated for steady
state conditions and the closed loop PID controls depend on sensor
feedback, which inherently has lag. A physics-based feed forward
calculation can be used instead of the open loop tables and PID
controls. The feed forward calculation can provide better response
under transient conditions, prioritize control objectives, account
for system constraints, and be easier to calibrate.
[0003] It would be desirable to have engine control systems and
methods that regulate the intake manifold pressure and percent EGR
using physics-based feed forward calculations.
SUMMARY
[0004] A fresh air and exhaust gas control method is disclosed for
an engine having an air system with an aft throttle and an exhaust
gas recirculation (EGR) valve. The control method includes
monitoring parameters of the engine using a plurality of sensors;
modeling the air system and generating engine state estimates using
an engine observer model; generating measured engine states based
on the monitored engine parameters from the plurality of sensors;
computing an observer error based on the differences between the
measured engine states and the engine state estimates; generating
model corrections; generating actuator commands and adjusting the
air throttle and the EGR valve positions based on the actuator
commands. The model corrections are based on the observer error,
and the model corrections are input to the engine observer model.
Generating actuator commands includes generating a desired air
throttle position and a desired EGR valve position based on
setpoint commands, the monitored engine parameters, the model
corrections and the engine state estimates. The air throttle is
adjusted based on the desired air throttle position, and the EGR
valve is adjusted based on the desired EGR valve position.
[0005] Generating a desired air throttle position and a desired EGR
valve position can be performed using an inverse engine model that
is an inverse of the engine observer model. The method can also
include generating feedback actuator commands based on the setpoint
commands and the engine state estimates; and using the feedback
actuator commands in generating the desired air throttle and EGR
valve positions.
[0006] The air system can also include a charge air cooler and an
engine intake manifold coupled to the air throttle, such that
intake air to the engine flows through the charge air cooler, the
air throttle and the engine intake manifold. In this embodiment,
the method can also include estimating a pressure difference across
the charge air cooler; estimating a temperature difference across
the charge air cooler; and estimating an air throttle mass flow as
part of the engine state estimates based on the pressure and
temperature differences across the charge air cooler. Estimating a
pressure difference across the charge air cooler can include
estimating the charge air cooler inlet pressure; estimating the
intake manifold pressure; and estimating the charge air cooler
outlet pressure by starting at the previous value of the charge air
cooler outlet pressure and searching a percentage of the range
between the charge air cooler inlet pressure and the intake
manifold pressure for the charge air cooler outlet pressure that
makes the charge air cooler mass flow and the air throttle mass
flow the same or closest to the same.
[0007] The air system can also include an EGR cooler and an engine
intake manifold coupled to the EGR valve, recirculated exhaust
gasses from the engine flowing through the EGR cooler, the EGR
valve and the engine intake manifold. In this embodiment, the
method can also include estimating a pressure difference across the
EGR cooler; estimating a temperature difference across the EGR
cooler; and estimating an EGR valve mass flow as part of the engine
state estimates based on the pressure and temperature differences
across the EGR cooler. Estimating a pressure difference across the
EGR cooler can include estimating the EGR cooler inlet pressure;
estimating the intake manifold pressure; and estimating the EGR
cooler outlet pressure by starting at the previous value of the EGR
cooler outlet pressure and searching a percentage of the range
between the EGR cooler inlet pressure and the intake manifold
pressure for the EGR cooler outlet pressure that makes the EGR
cooler mass flow and the EGR valve mass flow the same or closest to
the same.
[0008] The generating model corrections step can include generating
a fresh air mass flow correction term based on a pressure error at
the engine intake manifold, and/or generating an EGR mass flow
correction term based on the difference between the modelled and
measured mass flows through the EGR cooler.
[0009] The method can also include calculating a desired intake
manifold diluent mass fraction based on a desired diluent-to-air
ratio setpoint, an engine speed and a number of cylinders in the
engine; calculating a desired EGR mass flow based on the desired
intake manifold diluent mass fraction; and calculating the desired
EGR valve position based on the desired EGR mass flow. The method
can also include calculating a desired intake manifold pressure
based on the desired diluent-to-air ratio setpoint and the number
of cylinders in the engine; calculating a desired air throttle mass
flow based on the desired intake manifold pressure; and calculating
the desired air throttle position based on the desired air throttle
mass flow.
[0010] The method can also include calculating a desired intake
manifold diluent mass fraction based on a desired diluent-to-air
ratio setpoint, an engine speed and a number of cylinders in the
engine; calculating an intake manifold EGR mass fraction and a
closed loop EGR mass flow based on the desired intake manifold
diluent mass fraction; calculating a maximum air throttle mass flow
and a maximum EGR valve mass flow; calculating a desired intake
manifold pressure based on the desired diluent-to-air ratio
setpoint and the number of cylinders in the engine; calculating a
maximum intake manifold mass flow based on the intake manifold EGR
mass fraction, the closed loop EGR mass flow, the maximum air
throttle mass flow and the maximum EGR valve mass flow; calculating
a desired intake manifold mass flow in based on the maximum intake
manifold mass flow and the desired intake manifold pressure;
calculating a desired air throttle mass flow and a desired EGR
valve mass flow based on the intake manifold EGR mass fraction, the
closed loop EGR mass flow and the desired intake manifold mass flow
in; calculating the desired EGR valve position based on the desired
EGR valve mass flow; and calculating the desired air throttle
position based on the desired air throttle mass flow.
[0011] A fresh air and exhaust gas control system is disclosed for
an engine having an air system with an air throttle and an exhaust
gas recirculation (EGR) valve. The control system includes a
plurality of sensors, an engine observer, a comparator, an observer
controller, and an inverse engine model. The plurality of sensors
monitor engine parameters of the engine. The engine observer models
the air system and generates engine state estimates. The comparator
generates measured engine states based on the engine parameters
from the plurality of sensors, and computes an observer error based
on the differences between the measured engine states and the
engine state estimates. The observer controller generates model
corrections based on the observer error, and the model corrections
are input to the engine observer. The inverse engine model
generates actuator commands based on setpoint commands, the engine
parameters from the plurality of sensors, the model corrections and
the engine state estimates. The positions of the air throttle and
the EGR valve are adjusted based on the actuator commands.
[0012] The engine observer, the comparator, the observer
controller, and the inverse engine model can run on an electronic
control unit of the engine. The inverse engine model can include an
inverse of the engine observer.
[0013] The fresh air and exhaust gas control system can also
include a feedback controller that generates feedback actuator
commands based on the setpoint commands and the engine state
estimates; and the positions of the aft throttle and the EGR valve
would be adjusted based on the actuator commands and the feedback
actuator commands.
[0014] The air system can include a charge air cooler and an engine
intake manifold coupled to the air throttle, such that intake air
to the engine flows through the charge air cooler, the air throttle
and the engine intake manifold. In this case, the engine state
estimates generated by the engine observer can include an air
throttle mass flow estimate based on pressure and temperature
differences across the charge air cooler.
[0015] The air system can include an EGR cooler and an engine
intake manifold coupled to the EGR valve, such that recirculated
exhaust gasses from the engine flow through the EGR cooler, the EGR
valve and the engine intake manifold. In this case, the engine
state estimates generated by the engine observer can include an EGR
valve mass flow estimate based on pressure and temperature
differences across the EGR cooler.
[0016] The observer controller can generate a fresh air mass flow
correction term based on the difference between the modelled and
measured intake manifold pressures. The observer controller can
generate an EGR mass flow correction term based on the difference
between the modelled and measured EGR cooler mass flows.
[0017] The above and other features will become apparent from the
following description and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The detailed description of the drawing refers to the
accompanying figures in which:
[0019] FIG. 1 illustrates an exemplary observer based control
system including a physical process and a process model;
[0020] FIG. 2 illustrates some of the components, state variables,
mass flows, and sensors used in an exemplary air system model;
[0021] FIG. 3 illustrates a high level view of an exemplary control
method;
[0022] FIG. 4 illustrates an exemplary air throttle and EGR valve
control system;
[0023] FIG. 5 illustrates an exemplary method for calculating
desired EGR valve and air throttle positions;
[0024] FIG. 6 illustrates another exemplary method for calculating
desired EGR valve and air throttle positions;
[0025] FIG. 7 illustrates an exemplary turbocharger control
model;
[0026] FIG. 8 illustrates an exemplary method for calculating
turbocharger vane positions; and
[0027] FIG. 9 illustrates an exemplary two dimensional binary
search for finding turbine interstage pressure and desired vane
position for a given turbine mass flow.
DETAILED DESCRIPTION
[0028] Internal combustion engines use recirculated exhaust gas to
reduce emissions of nitrogen oxides (NOx). Achieving the optimum
combination of air and burnt gasses within the engine cylinder
prior to the start of combustion on each engine cycle may not be
possible due to the dynamics of the air system. A physics-based
control strategy can be used that prioritizes the control
objectives of the air system and regulates the flow of fresh air
and exhaust gas recirculation (EGR) under transient conditions. A
state observer can be used to model the air system states, measured
states can be compared to estimated states, and corrections can be
made to the observer model to minimize errors. The model
information and model corrections can be used in feed forward
calculations to determine the desired air throttle position and the
desired EGR valve position. Feed forward control achieved using
this method can provide fast and accurate control within the
constraints of the system.
[0029] A state observer is a math model of a process that is being
controlled. The model can be run in an electronic control unit
(ECU) of an engine. FIG. 1 illustrates an exemplary observer based
control system 100 that includes a physical process 102 and a
process model 104. The process model 104 calculates state
estimates, for example pressures, temperatures, speeds, and
positions. Measured states from the physical process 102 can be
compared with the state estimates from the process model 104, and
corrections can be made based on the differences.
[0030] The exemplary observer based control system 100 includes the
physical process 102 that is being modeled by the process model
104, as well as an observer controller 106, a feedback controller
108 and a feed forward controller 110. The feed forward controller
110 is an inverse of the process model 104. The process model 104
is not a perfect model of the physical process 102 which
experiences disturbances 112 that are not accounted for by the
process model 104. Sensors monitoring the physical process 102
provide a measured state 114, and the process model 104 outputs a
measured state estimate 116. The difference between the measured
state 114 and the measured state estimate 116 provides an observer
error 118 that is input to the observer controller 106. The
observer controller 106 processes the observer error 118 and
generates a model correction 120 that is input to the process model
104 and to the feed forward controller 110. Setpoint commands 122
indicating desired values for process parameters are input to the
control system 100. The setpoint commands 122 are input to the feed
forward controller 110. The difference between the setpoint
commands 122 and a controlled state estimate 124 generated by the
process model 104 produces a control error 126 that is input to the
feedback controller 108. The sum of the outputs of the feedback
controller 108 and the feed forward controller 110 produces
actuator commands 128 that are input to the physical process 102
and the process model 104.
[0031] There are several advantages to using a state observer
within a control system. The observer can provide estimates of
states that are difficult, expensive, or impossible to measure
directly. Since the process model 104 provides state estimates,
fewer sensors may be required. The process model 104 with
corrections from the observer controller 106 can be used in the
feed forward calculation 110 for the actuator commands 128. This
inverse process model 110 has desired states or setpoints 122 as
the inputs, and the corresponding actuator commands 128 are the
outputs. Feed forward control of this type provides fast response
and can reduce the feedback control complexity. This method can
make it easier to implement system constraints because the
constraints may be treated as limits within the feed forward and
feedback control eliminating the need for separate controllers
modifying the actuator commands. Using an observer in the control
system may also improve operation in non-standard conditions
because the model can predict the effects of changes and adjust the
controls as needed.
[0032] Since the observer model runs in the ECU, it needs to be
efficient at calculating state estimates. Finite element models and
one-dimensional wave dynamic models are typically too complex to
run within the ECU at real time. Models capable or running within
the ECU or in off line simulations with sufficient fidelity for use
with a control system are typically mean value lumped parameter
models. A mean value model calculates the mass flow through the
engine as being continuous without the pulsating effects of a real
engine. If needed, individual cylinder masses can be calculated for
each cylinder event from the mean flow.
[0033] Flow into the engine cylinders can be predicted using the
speed density method which uses engine speed and engine
displacement to determine an ideal volumetric flow rate. A
correction factor called the volumetric efficiency can then be
applied to provide an estimate of the volumetric flow rate.
Finally, the volumetric flow rate can be multiplied by the intake
manifold density to determine the mass flow into the engine
cylinders. This is sometimes called the speed density mass
flow.
[0034] Engine manifolds and pipes can be modeled using the lumped
parameter (zero dimensional) method. Using this method, the entire
volume of the manifold is assumed to have uniform pressure,
temperature, and mixture composition; all mass flows out of the
manifold are assumed to have the same pressure, temperature, and
mixture composition as the contents of the manifold; and the pipes
are assumed to have no wave dynamics or transport delays.
[0035] FIG. 2 illustrates some of the components, state variables,
mass flows, and sensors used in an exemplary air system model. Air
enters the system at a compressor inlet 200 into a low pressure
compressor 202. Sensors can be located at the compressor inlet 200
to monitor various parameters, for example compressor inlet
temperature, pressure and humidity. The low pressure compressor 202
compresses the air and sends it to a high pressure compressor 204
where it is further compressed. The compressed air from the high
pressure compressor 204 is input to a charge air cooler (CAC) 206.
Various parameters, for example, pressure, temperature and diluent
mass fraction in the CAC 206 can be monitored. The output from the
CAC 206 passes through an air throttle 208 and into a mixer 210.
Sensors can be positioned between the CAC 206 and the air throttle
208 to monitor the CAC outlet temperature and pressure as well as
other parameters. A position sensor can be used to monitor the
position of the air throttle 208.
[0036] The mixer 210 mixes fresh air from the CAC 206 and
recirculated exhaust gasses from an exhaust gas recirculation (EGR)
cooler 212. The mixed gasses from the mixer 210 are fed to an
engine intake manifold 216, and the output of the engine intake
manifold 216 is divided between cylinders 242 of an engine 240. The
illustration of FIG. 2 shows an engine 240 with six cylinders 242.
Various parameters, for example, pressure, temperature and diluent
mass fraction at the intake manifold 216 can be monitored.
[0037] The exhaust gasses of the cylinders 242 of the engine 240
are fed to an engine exhaust manifold 218. The exhaust gasses from
the exhaust manifold 218 are either recirculated through the EGR
cooler 212 or expelled from the system. Various parameters, for
example, pressure, temperature and diluent mass fraction at the
exhaust manifold 218 can be monitored.
[0038] The output from the EGR cooler 212 passes through an EGR
valve 214 and into the mixer 210. Various parameters, for example,
pressure, temperature and diluent mass fraction in the EGR cooler
212 can be monitored. Sensors can be positioned between the EGR
cooler 212 and the EGR valve 214 to monitor the EGR cooler outlet
temperature and pressure as well as other parameters. A position
sensor can be used to monitor the position of the EGR valve 214. An
EGR flow Venturi can be located between the EGR valve 214 and the
mixer 210.
[0039] The exhaust gasses expelled from the exhaust manifold 218
pass through a high pressure turbine 224 and a low pressure turbine
222. The high pressure turbine 224 can be a variable geometry
turbocharger (VGT) with a vane position sensor to monitor the vane
positions. Various other parameters, for example, pressure,
temperature, turbocharger speed can be monitored. An exhaust back
pressure at the output of the low pressure turbine 222 can also be
monitored.
[0040] The system shown in FIG. 2 can be modeled by state equations
that are based on the conservation of mass, energy, and momentum.
Some relationships can be approximated using first order filters.
Each state equation describes a rate of change. The state value at
any given time can be determined by integration of the state
equation. Within the ECU, each state can be given an initial
condition and the state value at each time step of the controller
can be updated using numeric integration.
[0041] The rate of change of mass in the charge air cooler (CAC)
206 can be modeled by the difference between the rate of change of
mass coming in from the compressors 202, 204 and the rate of change
of mass going out to the air throttle 208 as:
m cac t = m . cmp - m . at ( 1 ) ##EQU00001##
[0042] The rate of change of mass in the intake manifold 216 can be
modeled by the sum of the rate of change of mass coming in from the
air throttle 208 and the EGR valve 214 minus the rate of change of
mass going into the engine 240 as:
m im t = m . at + m . egr - m . eng_in ( 2 ) ##EQU00002##
[0043] The rate of change of diluent mass in the intake manifold
216 can be modeled as the sum of the rate of change in mass coming
from the air throttle 208 times the mass fraction of diluent in the
charge air cooler 206 (can assume same as the mass fraction of
water in ambient air due to humidity) plus the rate of change of
mass coming in from the EGR valve 214 times the mass fraction of
diluent in the exhaust manifold 218, minus the rate of change of
mass going into the engine 240 times the mass fraction of diluent
in the intake manifold 216 as:
m d _ im t = m . at .chi. d _ cac + m . egr .chi. d _ em - m . eng
_ in .chi. d _ im ( 3 ) ##EQU00003##
The term diluent is used to describe everything other than dry air
(or fuel) that is included in a mixture.
[0044] The rate of change of mass in the exhaust manifold 218 can
be modeled by the rate of change of mass coming out of the engine
240 minus the rate of change of mass going into the EGR cooler 212
and the rate of change of mass going into the turbines 222, 224
as:
m em t = m . eng _ out - m . egr - m . trb ( 4 ) ##EQU00004##
[0045] The rate of change of diluent mass in the exhaust manifold
218 can be modeled by the difference between the rate of change of
mass coming in from the engine 240 times the mass fraction of
diluent coming in from the engine 240 and the sum of the rates of
change of mass going into the EGR cooler 212 and the turbines 222,
224 times the mass fraction of diluent in the exhaust manifold 218
as:
m d _ em t = m . eng _ out .chi. d _ eng _ out - ( m . egr + m .
trb ) .chi. d _ em ( 5 ) ##EQU00005##
[0046] The rate of change of temperature at the output of the CAC
206 can be modeled by the difference between the steady state
temperature at the output of the CAC 206 and the measured
temperature at the output of the CAC 206 divided by a time constant
for the CAC 206 as:
T caco t = ( T caco _ ss + T caco ) .tau. cac ( 6 )
##EQU00006##
[0047] The rate of change of temperature at the output of the EGR
cooler 212 can be modeled by the difference between the steady
state temperature at the output of the EGR cooler 212 and the
measured temperature at the output of the EGR cooler 212 divided by
a time constant for the EGR cooler 212 as:
T egrco t = ( T egrco _ ss - T egrco ) .tau. egrc ( 7 )
##EQU00007##
The steady state cooler outlet temperatures T.sub.caco.sub._.sub.ss
and T.sub.egrco.sub._.sub.ss can be calculated using a heat
exchanger effectiveness model. The effectiveness can be calibrated
using a table with mass flow as the input. An effectiveness of one
means the cooler outlet temperature is equal to the temperature of
the cooling fluid, and an effectiveness of zero means there is no
change in temperature between the cooler inlet and outlet.
[0048] The rate of temperature change of the intake manifold 216
can be calculated using the mass flows in and out, the temperature
in and out, and the change in mass within the intake manifold 216.
The rate of change of temperature of the intake manifold 216 can be
modeled by the rate of change of mass at the air throttle 208 times
C.sub.p times the temperature at the output of the CAC 206 plus the
rate of change of mass at the EGR valve 214 times C.sub.p times the
temperature at the output of the EGR cooler 212 minus the rate of
change of mass going into the engine 240 times C.sub.p times the
temperature of the intake manifold 216 minus the rate of change of
mass in the intake manifold 216 times C.sub.v times the temperature
of the intake manifold 216, all divided by the product of the mass
at the intake manifold 216 times C.sub.v as:
T im t = m . at C p T caco + m . egr C p T egrco - m . eng _ in C p
T im - m im t C v T im m im C v ( 8 ) ##EQU00008##
where C.sub.p is the specific heat at constant volume, and C.sub.v
is the specific heat at constant pressure. The specific heat can be
measured experimentally at constant volume or constant pressure.
C.sub.p is greater than C.sub.v because as the mixture is heated at
constant pressure it expands doing work on the container or the
fluid around it. C.sub.p can be used to calculate energy flow into
or out of the control volume. C.sub.v can be used to calculate the
change in energy within the control volume due to changes in
temperate and mass.
[0049] The rate of change of temperature at the exhaust manifold
218 can be modeled by the difference between the temperature at the
output of the engine 240 and the temperature of the exhaust
manifold 218 divided by a time constant for the exhaust manifold
218 as:
T em t = ( T eng _ out - T em ) .tau. em ( 9 ) ##EQU00009##
[0050] The rate of change in low pressure turbocharger speed can be
modeled by the sum of the torques on the low pressure turbocharger
shaft and blades divided by the inertia of the low pressure
turbocharger shaft and blades (Newton's second law for rotation).
The sum of the torques can be modeled by the difference between low
pressure turbine power and low pressure compressor power divided by
the low pressure turbocharger speed. The turbine power (or
compressor power) can be calculated from the change in enthalpy of
the gas as it goes through the turbine (or compressor). The
enthalpy change is equal to the mass flow rate times the specific
heat at constant pressure (C.sub.p) times the delta temperature
across the turbine (or compressor). When turbocharger speed is
expressed in units of revolutions per minute (rpm), it must be
converted to radians per second (rad/s) using a factor of .pi./30.
The inverse of this factor is squared in equation (10) to convert
N.sub.lpt in the denominator and also to express the solution
dN.sub.lpt/dt in units of revolutions per minute per second
(rpm/s).
N lpt t = 1 J lpt ( W . lp_trb - W . lp_cmp N lpt ) ( 30 / .pi. rpm
rad / s ) 2 ( 10 ) ##EQU00010##
[0051] The rate of change in high pressure turbocharger speed can
be modeled by the sum of the torques on the high pressure
turbocharger shaft and blades divided by the inertia of the high
pressure turbocharger shaft and blades (Newton's second law for
rotation). The sum of the torques can be modeled by the difference
between high pressure turbine power and high pressure compressor
power divided by the high pressure turbocharger speed. The turbine
power (or compressor power) can be calculated from the change in
enthalpy of the gas as it goes through the turbine (or compressor).
The enthalpy change is equal to the mass flow rate times the
specific heat at constant pressure (C.sub.p) times the delta
temperature across the turbine (or compressor). When turbocharger
speed is expressed in units or revolutions per minute (rpm) it must
be converted to radians per second (rad/s) using a factor of
.pi./30. The inverse of this factor is squared in equation (11) to
convert N.sub.hpt in the denominator and also to express the
solution dN.sub.hrt/dt in units of revolutions per minute per
second (rpm/s).
N hpt t = 1 J hpt ( W . h p_trb - W . hp _cmp N hpt ) ( 30 / .pi.
rpm rad / s ) 2 ( 11 ) ##EQU00011##
[0052] The air throttle and EGR valve can be modeled using the
compressible gas flow equation for an orifice as:
m . at = C d _ at ( u at ) A at P caco R T caco .PSI. ( P im P caco
) , and ( 12 ) m . egr = C d _ egr ( u egr ) A egr P egrco R T
egrco .PSI. ( P im P egrco ) ( 13 ) ##EQU00012##
[0053] where C.sub.d.sub._.sub.at and C.sub.d.sub._.sub.egr are the
discharge coefficients for the air throttle 208 and the EGR valve
214, respectively, which can be calibrated using a table with
actuator position (u.sub.at and u.sub.egr, respectively) as the
input. The fresh air flow correction factor CF.sub.air (calculated
below in equation (26)) and the EGR correction factor CF.sub.egr
(calculated below in equation (32)) can be used to multiply the
result of the air throttle and EGR valve mass flow equations (12)
and (13), respectively, as:
m . at = CF air C d _ a t ( u at ) A at P caco R T caco .PSI. ( P
im P caco ) , and ( 12 A ) m . egr = CF egr C d _ egr ( u egr ) A
egr P egrco R T egrco .PSI. ( P im P egrco ) ( 13 A )
##EQU00013##
The areas A can be calculated using the actuator bore diameters.
The term R is the gas constant. The compressible gas flow factor
.PSI. is a function of pressure ratio and can be calibrated using a
table with pressure ratio as the input. The table values can be
calculated off-line using the following equations:
.PSI. ( P out P in ) = ( P out P in ) 1 k 2 k k + 1 ( 1 - ( P out P
in ) k - 1 k ) for ( P out P in ) .gtoreq. ( 2 k + 1 ) k k - 1 ( 14
) .PSI. ( P out P in ) = k ( 2 k + 1 ) k + 1 k - 1 for ( P out P in
) < ( 2 k + 1 ) k k - 1 ( 15 ) ##EQU00014##
where the term k represents the ratio of specific heats
C.sub.p/C.sub.v.
[0054] The mass flow calculation for the charge aft cooler 206 and
the EGR cooler 212 can be based on the Darcy-Weisbach equation:
h f = f L d V 2 2 g ( 16 ) ##EQU00015##
[0055] This relationship was developed for a pipe, but the
structure of the coolers 206, 212 are similar. The term h.sub.f is
the pipe head loss, f is the Darcy friction factor, L is the pipe
length, d is the diameter, V is velocity, and g is the
gravitational constant.
[0056] Pressure drop is related to the head loss by:
.DELTA.P=.rho.gh.sub.f (17)
Substituting (16) into (17) results in:
.DELTA. P = f L d .rho. V 2 2 ( 18 ) ##EQU00016##
Velocity and mass flow are related by:
V = m . .rho. A ( 19 ) ##EQU00017##
Substituting (19) into (18) results in:
.DELTA. P = f L d 1 2 A 2 m . 2 .rho. ( 20 ) ##EQU00018##
[0057] Equation (20) shows a relationship between mass flow squared
over density and the pressure drop across the cooler. The remaining
terms are constant. An empirical model of this form can be fit to
experimental data. The experimental data may also show a
relationship to temperature change across the cooler. This finding
is reasonable because the density is changing as the gasses flow
through the cooler and not constant as indicated by equation (20).
These relationships can be calibrated using a regression model or
tables of the following form, where delta pressure and delta
temperature are the inputs:
m . cac 2 .rho. cac _ avg = CACMassFlowSquaredOverDensityTable (
.DELTA. P cac , .DELTA. T cac ) ( 21 ) m . egrc 2 .rho. egrc _ avg
= EGRCMassFlowSquaredOverDensityTable ( .DELTA. P egrc , .DELTA. T
egrc ) ( 22 ) ##EQU00019##
[0058] The inlet and outlet temperatures of each cooler 206, 212
can be modeled and/or the temperatures can be measured. Half of the
cooler can be assumed to be at the inlet temperature and half of
the cooler can be assumed to be at the outlet temperature. The
average cooler density can be calculated using the average
temperature and average pressure. Assuming a linear drop in
pressure and temperate across the cooler, the average temperature
and average pressure can be calculated by averaging inlet and
outlet values. The mass flow can be calculated by multiplying the
average density by the table output and then taking the square
root. The fresh air flow correction factor CF.sub.air (calculated
below in equation (26)) and the EGR correction factor CF.sub.egr
(calculated below in equation (32)) can be included in these mass
flow calculations for the coolers as:
m . cac = CF air .rho. cac_avg CACFlowSqOverDensityTbl ( .DELTA. P
cac , .DELTA. T cac ) ( 21 A ) m . egrc = CF egr .rho. egrc_avg
EGRCFlowSqOverDensityTbl ( .DELTA. P egrc , .DELTA. T egrc ) ( 22 A
) ##EQU00020##
[0059] The mass flow through the charge air cooler 206 and the air
throttle 208 can be assumed to be the same. The outlet pressure of
the charge air cooler 206 can be estimated using a binary search
algorithm that searches for a charge air cooler outlet pressure
that results in the mass flow of the charge air cooler 206 and the
mass flow of the air throttle 208 being the same. The outlet
pressure of the charge air cooler 206 should be between the inlet
pressure of the charge air cooler 206 and the pressure of the
intake manifold 216. This entire range does not have to be searched
at every ECU time step. To reduce processor load, the previous
solution can be used as a starting point and only a small range
searched around that value. Even though the solution may not be
within the search range under extreme transient conditions, the
solution should converge within a few ECU time steps.
[0060] The calculation can begin by evaluating mass flow through
the charge air cooler 206 and the air throttle 208 with the
previous outlet pressure of the charge air cooler 206. If the mass
flow of the air throttle 208 is lower than the mass flow of the
charge air cooler 206, then the outlet pressure of the charge air
cooler 206 can be adjusted up by a percentage of the maximum search
range (charge air cooler inlet pressure-intake manifold pressure).
Otherwise, the outlet pressure of the charge air cooler 206 can be
adjusted down by a percentage of the maximum search range. This
process can be repeated with percentage adjustments of, for
example, 4, 2, 1, 0.5 and 0.25 percent of the maximum search range
(charge air cooler inlet pressure-intake manifold pressure).
[0061] The same type of iterative solution technique can be used
for the mass flow through the EGR cooler 212 and the EGR valve
214.
[0062] The compressors 202, 204 and turbines 222, 224 can be
modeled using mass flow tables and efficiency tables (turbocharger
maps). The pressure ratio across the compressors (or turbines) and
the turbocharger speed can be used to determine the mass flow and
efficiency. These calculations can include corrections for
temperature and pressure when not operating at the standard
conditions for the turbocharger maps. The efficiency can be used to
calculate the outlet temperature and the turbocharger shaft power.
The turbocharger speed can be calculated using the compressor
power, turbine power, and inertia as shown by equations (10) and
(11).
[0063] A variable geometry turbocharger (VGT) 224 has adjustable
vanes on the turbine that change the turbine performance
characteristics. Moving the vanes in the closing direction causes
the pressure of the exhaust manifold 218 to increase. Up to some
point this also causes the turbine power to increase, which causes
the turbocharger to speed up and provide more compressor flow. If
the turbine vanes are closed too much the turbine flow can be
"choked" and the power may drop.
[0064] To model the turbine operation at various vane positions,
multiple turbine tables can be used that characterize the turbine
performance at different vane positions. For operation at vane
positions between the tables, interpolation can be used to
determine the mass flow and efficiency.
[0065] Two stage turbochargers have two compressors in series and
two turbines in series, as shown in FIG. 2. The interstage volume
between the compressors 202, 204 and between the turbines 222, 224
is small, making it difficult to dynamically model the pressure. A
small change in flow causes a large change in pressure. At the time
steps used in the ECU, the modeled interstage pressure can become
numerically unstable. A flow change over one time step can cause a
change in pressure that in turn causes another change in flow and
pressure causing the modelled pressures and flows to oscillate.
This can be overcome by using a very small time step but that is
not typically practical for calculations within an ECU.
[0066] To eliminate the numeric instability, the flow through each
compressor 202, 204 can be assumed to be the same. Likewise, the
flow through each turbine 222, 224 can be assumed to be the same.
The interstage pressure can then be solved iteratively using a
binary search algorithm that adjusts the interstage pressure until
the flow through each compressor (or turbine) is the same. This is
similar to the search algorithm described above for the charge air
cooler 206 and the air throttle 208.
[0067] The observer controller can make corrections to the air
system model so that the estimated states match the measured
states. The modeled states may have error due to modeling errors,
engine variation, changing engine characteristics over time, air
leaks and other sources. Three examples of model corrections are:
fresh air mass flow correction term, EGR mass flow correction term,
and turbine mass flow correction term. These correction terms can
be calculated within the observer controller based on: intake
manifold pressure error, EGR mass flow error, and exhaust manifold
pressure error, respectively. The model corrections can be
multiplicative correction factors, additive or offset correction
factors or other types of correction terms. Exemplary correction
term calculations are shown below. The correction terms can be
applied to the observer model as well as to the models used in
actuator control calculations.
[0068] A fresh air flow correction factor CF.sub.air can be applied
as a correction to the compressor mass flow estimates, charge air
cooler mass flow estimates, and air throttle mass flow estimates.
The pressure error at the intake manifold 216 is the difference
between the modelled and measured pressures:
P.sub.im.sub._.sub.obs.sub._.sub.error=P.sub.im.sub._.sub.sensor-P.sub.i-
m (23)
A proportional air flow correction factor CF.sub.air.sub._.sub.prop
can be calculated as a constant times the pressure error at the
intake manifold 216:
CF.sub.air.sub._.sub.prop=K.sub.p.sub._.sub.im.sub._.sub.obsP.sub.im.sub-
._.sub.obs.sub._.sub.error (24)
An incremental air flow intake correction factor
CF.sub.air.sub._.sub.int can also be calculated based on the
pressure error at the intake manifold 216 as:
CF.sub.air.sub._.sub.int=K.sub.i.sub._.sub.im.sub._.sub.obsP.sub.im.sub.-
_.sub.obs.sub._.sub.error+CF.sub.air.sub._.sub.int.sub._.sub.previous
(25)
A fresh air flow correction factor CF.sub.air can be estimated by
the sum of these air flow correction factors:
CF.sub.air=CF.sub.air.sub._.sub.prop+CF.sub.air.sub._.sub.int
(26)
[0069] A turbine mass flow correction factor CF.sub.trb can be
applied as a correction to the turbine mass flow estimates. The
pressure error at the exhaust manifold 218 is the difference
between the modelled and measured pressures:
P.sub.em.sub._.sub.obs.sub._.sub.error=P.sub.em.sub._.sub.sensor-P.sub.e-
m (27)
A proportional turbine correction factor CF.sub.trb.sub._.sub.prop
can be calculated as a constant times the pressure error at the
exhaust manifold 218:
CF.sub.trb.sub._.sub.prop=K.sub.p.sub._.sub.em.sub._.sub.obsP.sub.em.sub-
._.sub.obs.sub._.sub.error (28)
An incremental turbine intake correction factor
CF.sub.trb.sub._.sub.int can also be calculated based on the
pressure error at the exhaust manifold 218 as:
CF.sub.trb.sub._.sub.int=K.sub.i.sub._.sub.em.sub._.sub.obsP.sub.em.sub.-
_.sub.obs.sub._.sub.error+CF.sub.trb.sub._.sub.int.sub._.sub.previous
(29)
A turbine intake correction factor CF.sub.trb can be estimated by
the sum of these turbine correction factors:
CF.sub.trb=CF.sub.trb.sub._.sub.prop+CF.sub.trb.sub._.sub.int
(30)
[0070] An EGR mass flow correction factor CF.sub.egr can be applied
as a correction to the EGR cooler mass flow estimates, and the EGR
valve mass flow estimates. A mass flow sensor can be located
between the EGR valve 214 and the mixer 210, and the mass flow
error at the EGR cooler 212 can be estimated as the difference
between the modelled and measured mass flows:
{dot over (m)}.sub.egr.sub._.sub.obs.sub._.sub.error={dot over
(m)}.sub.egr.sub._.sub.sensor{dot over (m)}.sub.egr (31)
An EGR Flow Venturi can be located between the EGR valve 214 and
the mixer 210 to measure the EGR mass flow. The pressure drop from
the inlet to the throat of the Venturi (delta pressure) is related
to the mass flow rate. Some engines can use an EGR flow orifice,
and the measured pressure ratio across the orifice can be related
to the flow rate (e.g., using equation 13). An incremental EGR
correction factor CF.sub.egr can be calculated based on this mass
flow error as:
CF.sub.egr=K.sub.i.sub._.sub.egr.sub._.sub.obs{dot over
(m)}.sub.egr.sub._.sub.obs.sub._.sub.error+CF.sub.egr.sub._.sub.previous
(32)
[0071] Proportional plus integral (PI) controllers can be used for
the intake manifold pressure and exhaust manifold pressure control.
An integral controller can be used for the EGR mass flow error
control. The EGR mass flow correction factor can be applied
directly to the EGR mass flow estimate to make integral control
sufficient.
[0072] Fuel reacts with oxygen during combustion. When fuel reacts
with a quantity of air that has just enough oxygen to combine with
all the fuel molecules to form combustion products of carbon
dioxide and water, that ratio of fuel to air is called the
stoichiometric ratio. Diesel engines typically run leaner than the
stoichiometric ratio meaning they have excess air in the cylinder.
This excess air passes through the exhaust valves into the exhaust
with the burnt gasses.
[0073] Engines do not typically run on only dry air. The air that
enters the engine may have some water vapor. In addition, modern
engines commonly use exhaust gas recirculation (EGR) to reduce
combustion temperatures for controlling nitrogen oxides (NOx)
emissions. Recirculated exhaust gas may contain combustion products
as well as air.
[0074] The term "diluent" is used to describe everything other than
dry air (or fuel) that is included in a mixture. The mass fraction
of oxygen in dry air is 0.232. If the oxygen mass fraction in a
mixture is known, the air mass fraction can be determined by
dividing by 0.232. If there is no fuel, the diluent mass fraction
can be calculated by taking one minus the air mass fraction.
[0075] Within an air system state observer, the diluent mass
fraction can be calculated for the charge air cooler, intake
manifold, and exhaust manifold. The diluent mass fraction can be
chosen as a variable for convenience because it increases with
increasing EGR for the intake and exhaust manifolds. For the charge
air cooler, the diluent mass fraction increases with increasing
ambient relative humidity. The same modeling results can be
obtained using dry air mass fraction or oxygen mass fraction as
state variables.
[0076] Another way to describe the diluent concentration is as a
ratio of diluent to air (D/A). D/A can be used as a variable for
the in-cylinder concentration to be consistent with fuel-to-air
ratio (F/A), which is commonly used for engine control.
[0077] Two important parameters in air system control are the
in-cylinder diluent-to-air ratio (D/A) and fuel-to-air ratio (F/A).
D/A is controlled to regulate NOx. F/A is controlled to prevent
rich operation causing smoke, and to prevent lean operation causing
high cylinder pressure. These ratios can be calculated from the
desired fuel per cylinder, the air mass in-cylinder when the intake
valve closes, and the diluent mass in-cylinder when the intake
valve closes. The in-cylinder air mass has two components: air that
is present in the residual mass from the previous engine cycle, and
air that entered through the intake valve on the current engine
cycle:
m.sub.a.sub._.sub.cyl=m.sub.a.sub._.sub.cyl.sub._.sub.in+m.sub.a.sub._.s-
ub.res (33)
Likewise the in-cylinder diluent mass has two components: diluent
that is present in the residual mass from the previous engine
cycle, and diluent that entered through the intake valve on the
current engine cycle.
m.sub.d.sub._.sub.cyl=m.sub.d.sub._.sub.cyl.sub._.sub.in+m.sub.d.sub._.s-
ub.res (34)
The in-cylinder fuel-to-air ratio (F/A) is then:
F / A = m f_des m a_cyl ( 35 ) ##EQU00021##
and the in-cylinder diluent-to-air ratio (D/A) is:
D / A = m d_cyl m a_cyl ( 36 ) ##EQU00022##
[0078] A model can be used to predict the residual air mass and
residual diluent mass. The speed density mass flow model and the
intake manifold diluent mass fraction can be used to estimate the
air mass and diluent mass that enters through the valves.
[0079] FIG. 3 illustrates a high level view of an exemplary control
system 300. In this exemplary system, information is used to
convert the desired in-cylinder states to manifold states, the
desired flow rates to achieve those states are then calculated, and
then the flow models and model corrections are used to calculate
the actuator positions. The control system 300 includes an engine
302, an engine observer model 304, an observer controller 306, a
proportional feedback controller 308, a feed forward or inverse
engine model 310 and a flow-to-position module 312.
[0080] The physical engine 302 experiences disturbances 340 that
are not modeled by the observer model 304. Sensors monitoring the
engine 302 provide a measured state 314. The engine observer model
304 processes various inputs (including, for example, speeds,
masses, pressures, temperatures, model corrections, etc.) and
generates a measured state estimate 316 to be compared with the
measured state 314, as well as controlled state estimates 324 that
are provided to the inverse engine model 310 and the
flow-to-position module 312. The difference between the measured
state 314 and the measured state estimate 316 provides an observer
error 318 that is input to the observer controller 306.
[0081] The observer controller 306 generates model corrections 320
that are provided to the engine observer model 304, the inverse
engine model 310 and the flow-to-position module 312. The model
corrections generated by the observer controller 306 can include an
integral term that drives the steady state error to zero. This can
help keep operation of the engine observer model 304 close to that
of the real engine 302. Another option would be to only use
proportional control in the observer controller 306 and have a
proportional plus integral (PI) controller for the main feedback
control 308.
[0082] The inverse engine model or feed forward controller 310
takes the controlled state estimates 324 generated by the engine
observer model 304, the model corrections 320 generated by the
observer controller 306, desired state inputs 322 and various
system parameters 324 and calculates desired engine state commands
330 and feed forward mass flow terms 332 to achieve the desired D/A
and F/A values included in the desired state inputs 322.
[0083] The difference between the controlled state estimates 324
generated by the engine observer model 304 and the desired engine
state commands 330 generated by the inverse engine model 310 is
input to the feedback controller 308 which calculates proportional
flow commands 334. The proportional feedback controller 308 can be
used for transient control. To change a manifold pressure more or
less mass has to be stored in the manifold. This change in mass
storage can be achieved using the proportional controller 308.
Using the proportional controller 308 can provide smoother
operation than using a derivative term in the feed forward
calculation. The proportional feedback controller 308 also can
remove error that may not be covered by the model correction
factors because of the response time or limits that may be applied
to the correction factors.
[0084] The sum of the feed forward mass flows 332 generated by the
inverse engine model 310 and the proportional flow commands 334
generated by the proportional feedback controller 308 provides flow
commands 336 that are input to the flow-to-position module 312. The
flow-to-position module 312 also receives inputs of the model
corrections 320 generated by the observer controller 306 and the
controlled state estimates 324 generated by the engine observer
model 304. The flow-to-position module 312 takes these inputs and
an exhaust back pressure and calculates actuator position commands
328 that are input to the engine 302 and the engine observer model
304.
[0085] FIG. 4 illustrates an exemplary air throttle and EGR valve
control system 400 which includes a charge air cooler (CAC) 406, an
air throttle 408, an intake manifold 416, an EGR valve 414, an EGR
cooler 412 and a cylinder 442. Only one cylinder 442 is shown for
clarity but the engine would include multiple cylinders 442 and the
output from the intake manifold 416 would be distributed across the
multiple cylinders 442.
[0086] The air throttle 408 and EGR valve 414 provide fast response
for controlling the diluent mass fraction and pressure of the
intake manifold 416. The turbocharge vanes (see VGT 224 of FIG. 2)
can also be adjusted to control the EGR flow and fresh air flow but
this control is slower. Control of the turbocharger is typically
for providing sufficient exhaust manifold pressure and charge air
cooler pressure so that the air throttle 408 and EGR valve 414 can
be adjusted to achieve the desired setpoints.
[0087] The system can be designed to give the diluent-to-air-ratio
(D/A) control higher priority than the fuel-to-air-ratio (F/A)
control. D/A is controlled by regulating the diluent mass fraction
of the intake manifold 416. F/A is controlled by regulating the
pressure of the intake manifold 416. In general, D/A control is
faster than F/A control.
[0088] The desired diluent concentration in the intake manifold 416
can be calculated using the current in cylinder air mass instead of
a desired value calculated from the F/A setpoint because under
transient conditions it may take a long time to achieve the F/A
setpoint. FIG. 5 shows a block diagram for the control.
[0089] The desired diluent mass fraction of the intake manifold 416
can be calculated at block 510 using the following exemplary
method. The EGR control calculation can calculate the mass of
diluent that is desired in-cylinder using the estimated current air
mass in-cylinder and the desired D/A setpoint:
m.sub.d.sub._.sub.cyl.sub._.sub.des*m.sub.a.sub._.sub.cyl*D/A.sub.des
(37)
The mass of diluent to enter the cylinder can then be calculated by
subtracting the residual diluent mass from the desired in-cylinder
diluent mass:
m.sub.d.sub._.sub.cyl.sub._.sub.in.sub._.sub.des=m.sub.d.sub._.sub.cyl.s-
ub._.sub.des-m.sub.d.sub._.sub.res (38)
The desired diluent mass flow into the engine can then be
calculated using engine speed and the number of cylinders:
m . d_eng _in _des = m d_cyl _in _des n cyl N e ( rpm ) ( min 60
sec ) ( cycle 2 rev ) ( 39 ) ##EQU00023##
The desired intake manifold diluent mass fraction can then be
calculated by dividing the desired engine-in diluent mass flow by
the speed density mass flow:
.chi. d_im _des = m . d_eng _in _des m . eng_in ( 40 )
##EQU00024##
[0090] The desired mass flow through the EGR valve 414 can be
calculated at block 512 using the following exemplary method. The
following equation can be used to relate the desired diluent mass
fraction of the intake manifold 416 to the feed forward intake
manifold EGR mass fraction:
.chi..sub.d.sub._.sub.im.sub._.sub.des=.chi..sub.d.sub._.sub.em.chi..sub-
.egr.sub._.sub.im.sub._.sub.ff+.chi..sub.d.sub._.sub.cac(1-.chi..sub.egr.s-
ub._.sub.im.sub._.sub.ff) (41)
The mass fraction of diluent in the charge air cooler 406 can be
used in this calculation to account for water vapor that may enter
with the fresh air. Water vapor is considered a diluent. This
equation can be re-arranged to solve for the feed forward intake
manifold EGR mass fraction as:
.chi. egr_im _ff = .chi. d_im _des - .chi. d_cac .chi. d_em - .chi.
d_cac ( 42 ) ##EQU00025##
The feed forward EGR mass flow can be calculated by multiplying the
feed forward intake manifold EGR mass fraction by the engine in
mass flow estimate:
{dot over
(m)}.sub.egr.sub._.sub.ff=.chi..sub.egr.sub._.sub.im.sub._.sub.ff{dot
over (m)}.sub.eng.sub._.sub.in (43)
The intake manifold diluent mass fraction error is the difference
between the desired intake manifold diluent mass fraction and the
intake manifold diluent mass fraction estimate:
.chi..sub.d.sub._.sub.im.sub._.sub.error=.chi..sub.d.sub._.sub.im.sub._.-
sub.des-.chi..sub.d.sub._.sub.im (44)
[0091] The error can be multiplied by the proportional gain to
determine the closed loop term by:
{dot over
(m)}.sub.egr.sub._.sub.cl=K.sub.p.sub._.sub.egr.lamda..sub.d.sub._.sub.im-
.sub._.sub.error (45)
K.sub.p.sub._.sub.egr can be calibrated using a table with exhaust
manifold diluent mass fraction as the input. This allows higher
gains to be used when the exhaust manifold diluent mass fraction is
low. The desired mass flow through the EGR valve 414 is the sum of
the feed forward term and the closed loop term:
{dot over (m)}.sub.egr.sub._.sub.des={dot over
(m)}.sub.egr.sub._.sub.ff+{dot over (m)}.sub.egr.sub._.sub.cl
(46)
The calculation of the desired position of the EGR valve 414 at
block 514 will be described below along with the calculation of the
desired position of the air throttle 408 at block 524.
[0092] The air throttle feed forward command can be calculated
using the engine in mass flow estimate and the EGR valve mass flow
estimate as:
{dot over (m)}.sub.at.sub._.sub.ff={dot over
(m)}.sub.eng.sub._.sub.in-{dot over (m)}.sub.egr (47)
This air throttle feed forward term can be used to achieve a flow
that will keep the pressure of the intake manifold 416 constant.
The closed loop term that will be calculated next is used to change
the pressure of the intake manifold 416.
[0093] The desired air mass in-cylinder can be calculated using the
desired fuel command and the desired F/A lean setpoint as:
m a_cyl _lean _des = m f_des F / A lean_des ( 48 ) ##EQU00026##
A lean setpoint can be used here because the air throttle command
is only used to close the air throttle 408 when the engine
operation becomes too lean. The turbocharger can use a F/A rich
setpoint to increase the engine boost when the engine operation
becomes too rich. Having this control deadband between the rich and
lean setpoints can allow the air throttle and turbocharger vanes to
remain more open more of the time than they would be if controlling
to one F/A setpoint.
[0094] The desired pressure of the intake manifold 416 can be
calculated at block 520 using the following exemplary method. The
desired diluent mass in-cylinder can be calculated using the
desired in-cylinder lean air mass and the desired D/A setpoint
as:
m.sub.d.sub._.sub.cyl.sub._.sub.lean.sub._.sub.des=m.sub.a.sub._.sub.cyl-
.sub._.sub.lean.sub._.sub.desD/A.sub.des (49)
The mass of air to enter the cylinder can then be calculated by
subtracting the residual air mass from the desired in-cylinder air
mass:
m.sub.a.sub._.sub.cyl.sub._.sub.in.sub._.sub.lean.sub._.sub.des=m.sub.a.-
sub._.sub.cyl.sub._.sub.lean.sub._.sub.des-m.sub.a.sub._.sub.res
(50)
The mass of diluent to enter the cylinder can be calculated by
subtracting the residual diluent mass from the desired in-cylinder
diluent mass:
m.sub.d.sub._.sub.cyl.sub._.sub.in.sub._.sub.lean.sub._.sub.des=m.sub.d.-
sub._.sub.cyl.sub._.sub.lean.sub._.sub.des-m.sub.d.sub._.sub.res
(51)
The total mass to enter the cylinder can be calculated as the sum
of the air mass to enter the cylinder and the diluent mass to enter
the cylinder:
m.sub.cyl.sub._.sub.in.sub._.sub.lean.sub._.sub.des=m.sub.a.sub._.sub.cy-
l.sub._.sub.in.sub._.sub.lean.sub._.sub.des+m.sub.d.sub._.sub.cyl.sub._.su-
b.in.sub._.sub.lean.sub._.sub.des (52)
The pressure needed to achieve this mass can be found using the
ideal gas law and the volumetric efficiency by:
P im_lean _des = n cyl m cyl_in _lean _des R T im V d VE ( 53 )
##EQU00027##
This equation is related to the speed-density calculation but does
not account for speed because mass per cylinder is used instead of
a mass flow. The desired intake manifold lean pressure may be
limited to a certain range for component protection and engine
performance. Low pressures are sometimes avoided to prevent high
oil consumption and poor combustion quality.
[0095] The desired mass flow through the air throttle 408 can be
calculated at block 522 using the following exemplary method. The
intake manifold pressure lean error is the difference between the
desired intake manifold lean pressure and the intake manifold
pressure estimate:
P.sub.im.sub._.sub.lean.sub._.sub.error=P.sub.im.sub._.sub.lean.sub._.su-
b.des-P.sub.im (54)
The error can be multiplied by the proportional gain to determine
the closed loop term:
{dot over
(m)}.sub.at.sub._.sub.cl=K.sub.p.sub._.sub.imP.sub.im.sub._.sub.lean.sub.-
_.sub.error (55)
The desired air throttle mass flow is the sum of the feed forward
term from equation (47) and the closed loop term from equation
(55):
{dot over (m)}.sub.at.sub._.sub.des={dot over
(m)}.sub.at.sub._.sub.ff+{dot over (m)}.sub.at.sub._.sub.cl
(56)
[0096] The control described above and shown in FIG. 5 uses the EGR
valve 414 to control the intake manifold diluent mass fraction and
the air throttle 408 to control the intake manifold pressure. Under
transient conditions when there is insufficient engine delta
pressure to provide the required EGR mass flow the resulting intake
manifold diluent mass fraction may be too low. An alternative
control strategy can be used to resolve this issue by using the air
throttle 408 to control the fresh air flow entering the intake
manifold 416 when needed to achieve the desired intake manifold
diluent mass fraction. This is different in that it calculates the
maximum mass flow for the air throttle 408 and EGR valve 414, then
determines the maximum combined flow that is possible while
achieving the desired intake manifold diluent mass fraction. With
this method, the intake manifold pressure is controlled by
calculating a combined mass flow term that is within the maximum
limit. The individual mass flows for the air throttle 408 and the
EGR valve 414 are then calculated as shown in FIG. 6. Many of the
calculations are the same as described above, and the differences
are described below.
[0097] The desired intake manifold diluent mass fraction can be
calculated at block 602 using the method disclosed with equation
(40). The feed forward intake manifold EGR mass fraction can be
calculated at block 604 using the method disclosed with equation
(42). The closed loop EGR mass flow can be calculated at block 604
using the method disclosed with equation (45). The desired intake
manifold pressure can be calculated at block 608 using the method
disclosed with equation (53).
[0098] The maximum air throttle and EGR valve mass flows can be
calculated at block 606 using the maximum air throttle and EGR
valve positions in equation (12) and (13), and equations (21) and
(22). An iterative process can be used to find the mass flow since
the corresponding cooler outlet pressure must be found that
provides the same flow through the actuator and cooler. A similar
iterative process can be used to the one described above for the
charge air and EGR cooler mass flows for the observer, except that
the maximum actuator position could be used instead of the current
actuator position. The maximum actuator position does not have to
correspond to a wide open valve position. The maximum actuator
position may be calibrated to a value less than wide open under
some operating conditions.
[0099] The maximum mass flow rate at the intake manifold 416 can be
based on the EGR valve mass flow limit calculated using the maximum
EGR valve position in equation (13), equation (22), the closed loop
EGR mass flow term from equation (45), and the feed forward intake
manifold EGR mass fraction from equation (42) as:
m . im_in _max _egr _ltd = m . egr_max - m . egr_cl .chi. egr_im
_ff ( 57 ) ##EQU00028##
The closed loop EGR mass flow is subtracted from the maximum EGR
valve mass flow to help ensure there is room to add it later as
shown in equation (63).
[0100] The maximum intake manifold mass flow based on the air
throttle mass flow limit can be calculated using the maximum air
throttle position in equation (12), equation (21), the closed loop
EGR mass flow term from equation (45), and the feed forward intake
manifold EGR mass fraction from equation (42) as:
m . im_in _max _at _ltd = m . at_max + m . egr_cl ( 1 - .chi.
egr_im _ff ) ( 58 ) ##EQU00029##
The closed loop EGR mass flow is added to the maximum air throttle
mass flow because later it will be subtracted as shown in equation
(64).
[0101] The maximum intake manifold mass flow can be calculated at
block 610 as the minimum of the two values calculated by equations
(57) and (58):
{dot over (m)}.sub.im.sub._.sub.in.sub._.sub.max=min({dot over
(m)}.sub.im.sub._.sub.in.sub._.sub.max.sub._.sub.egr.sub._.sub.ltd,{dot
over
(m)}.sub.im.sub._.sub.in.sub._.sub.max.sub._.sub.at.sub._.sub.ltd)
(59)
[0102] The feed forward intake manifold mass flow term can be
equated to the engine in mass flow estimate:
{dot over (m)}.sub.im.sub._.sub.in.sub._.sub.ff={dot over
(m)}.sub.eng.sub._.sub.in (60)
The closed loop intake manifold mass flow term can be estimated as
a proportional gain times the intake manifold pressure lean error
that was calculated in equation (54):
{dot over
(m)}.sub.im.sub._.sub.in.sub._.sub.cl=K.sub.p.sub._.sub.imP.sub.im.sub._.-
sub.lean.sub._.sub.error (61)
The desired intake manifold mass flow in calculated at block 612
can be estimated as the sum of the feed forward term and the closed
loop term:
{dot over (m)}.sub.im.sub._.sub.in.sub._.sub.des={dot over
(m)}.sub.im.sub._.sub.in.sub._.sub.ff+{dot over
(m)}.sub.im.sub._.sub.in.sub._.sub.cl (62)
[0103] The desired EGR valve mass flow and air throttle mass flow
can be calculated at block 614 from the desired intake manifold
mass flow as:
{dot over (m)}.sub.egr.sub._.sub.des={dot over
(m)}.sub.im.sub._.sub.in.sub._.sub.des.chi..sub.egr.sub._.sub.im.sub._.su-
b.ff+{dot over (m)}.sub.egr.sub._.sub.cl (63)
{dot over (m)}.sub.at.sub._.sub.des={dot over
(m)}.sub.im.sub._.sub.in.sub._.sub.des(1-.lamda..sub.egr.sub._.sub.im.sub-
._.sub.ff)-{dot over (m)}.sub.egr.sub._.sub.cl (64)
The feed forward intake manifold EGR mass fraction and the closed
loop EGR mass flow terms do not affect the total mass flow entering
the manifold, only the split between the EGR valve and the air
throttle.
[0104] The desired position for the air throttle 408 and the
desired position for the EGR valve 414 can be calculated from the
desired flow rate for each actuator. The cooler inlet pressure and
intake manifold pressure can be used in each case.
[0105] For the air throttle 408, the pressure drop across the CAC
406 can be evaluated using equation (21). This can be done by
searching the CACMassFlowSquaredOverDensityTable described above to
find the delta pressure that gives the desired mass flow.
Alternatively, a separate table can be used with delta pressure as
the output and mass flow squared over density as one of the inputs.
The outlet pressure of the CAC 406 can then be calculated by
subtracting this pressure drop from the CAC inlet pressure.
[0106] The pressure ratio across the air throttle 408 can be
calculated by dividing the pressure of the intake manifold 416 by
the outlet pressure of the CAC 406. Equation (12) can then be used
to solve for the discharge coefficient C.sub.d.sub._.sub.at. The
discharge coefficient C.sub.d.sub._.sub.at is related to actuator
position u.sub.at. A table defining this relationship was described
for equation (12) of the observer model. The table for equation
(12) could have two columns (u.sub.at and C.sub.d.sub._.sub.at),
where u.sub.at is the input and C.sub.d.sub._.sub.at is the output.
For the feed forward calculation a similar table can be used, but
in this case C.sub.d.sub._.sub.at is the input and u.sub.at is the
output.
[0107] For the EGR valve 414, the pressure drop across the EGR
cooler 412 can be evaluated using equation (22). This can be done
by searching the EGRCMassFlowSquaredOverDensityTable described
above to find the delta pressure that gives the desired mass flow.
Alternatively, a separate table can be used with delta pressure as
the output and mass flow squared over density as one of the inputs.
The outlet pressure of the EGR cooler 412 can then be calculated by
subtracting this pressure drop from the EGR cooler inlet
pressure.
[0108] The pressure ratio across the EGR valve 414 can be
calculated by dividing the pressure of the intake manifold 416 by
the outlet pressure of the EGR cooler 412. Equation (13) can then
be used to solve for the discharge coefficient
C.sub.d.sub._.sub.egr. The discharge coefficient
C.sub.d.sub._.sub.egr is related to actuator position u.sub.egr. A
table defining this relationship was described for equation (13) of
the observer model. The table for equation (13) could have two
columns (u.sub.egr and C.sub.d.sub._.sub.egr), where u.sub.egr is
the input and C.sub.d.sub._.sub.egr is the output. For the feed
forward calculation a similar table can be used, but in this case
C.sub.d.sub._.sub.egr is the input and u.sub.egr is the output.
[0109] Within these calculations the desired mass flows can be
modified by dividing by the corresponding mass flow correction
factors that were calculated for the air system observer. This
allows the correct position to be calculated for the desired mass
flow.
[0110] The turbocharger control can provide sufficient exhaust
pressure to achieve a desired EGR mass flow and provide sufficient
intake manifold pressure to achieve a desired fuel-to-air ratio.
The vanes on the turbocharger can be adjusted to regulate the mass
flow through the turbine. Moving the vanes in the closing direction
tends to reduce the mass flow, increase the exhaust manifold
pressure, and increase the turbine power. A flow based strategy can
be used with the turbocharger control. The desired turbine mass
flow can be calculated and then the turbocharger maps can be used
to calculate the vane position needed to achieve the desired mass
flow.
[0111] FIG. 7 illustrates an exemplary turbocharger control
schematic 700 which includes a cylinder 742, an exhaust manifold
718, an EGR cooler 712, a high pressure variable geometry
turbocharger 724, a low pressure turbine 722 and an exhaust outlet
730. Only one cylinder 742 is shown for clarity but the engine
would include multiple cylinders 742 and the output from the
multiple cylinders 742 would be input to the exhaust manifold 718.
The vanes of the variable geometry turbocharger 724 can be adjusted
to control the EGR flow and fresh air flow.
[0112] FIG. 8 illustrates an exemplary turbocharger vane position
control model. At block 810, the desired EGR rich mass flow and
desired intake manifold pressure rich setpoint are calculated. At
block 814, the minimum exhaust manifold pressure to achieve the
desired EGR rich mass flow is calculated. An integral controller
can be used to regulate the intake manifold pressure at block 818
by adjusting the exhaust manifold pressure. At block 822, the
desired exhaust manifold pressure is limited to the minimum
pressure calculated for EGR mass flow. At block 826, the turbine
mass flow to achieve that pressure is calculated. Finally, at block
830, the vane position is calculated.
[0113] The feed forward term for the turbine mass follow can be
calculated by subtracting the EGR mass flow from the engine out
mass flow.
{dot over (m)}.sub.trb.sub._.sub.ff={dot over
(m)}.sub.eng.sub._.sub.out-{dot over (m)}.sub.egr (65)
This turbine flow will tend to keep the exhaust manifold pressure
constant. The closed loop term is used to make changes in
pressure.
[0114] The desired EGR mass flow at block 810 can be calculated by
first calculating the desired exhaust manifold pressure. The
desired air mass and diluent mass in-cylinder can be calculated
as:
m a_cyl _rich _des = m f_des F / A rich_des ( 66 ) m d_cyl _rich
_des = m a_cyl _rich _des D / A des ( 67 ) ##EQU00030##
The desired mass of air, diluent, and total mass entering the
cylinder can be calculated using the desired in-cylinder values and
the residual values
m.sub.a.sub._.sub.cyl.sub._.sub.in.sub._.sub.rich.sub._.sub.des=m.sub.a.-
sub._.sub.cyl.sub._.sub.rich.sub._.sub.des-m.sub.a.sub._.sub.res
(68)
m.sub.d.sub._.sub.cyl.sub._.sub.in.sub._.sub.rich.sub._.sub.des=m.sub.d.-
sub._.sub.cyl.sub._.sub.rich.sub._.sub.des-m.sub.d.sub._.sub.res
(69)
m.sub.cyl.sub._.sub.in.sub._.sub.rich.sub._.sub.des=m.sub.a.sub._.sub.cy-
l.sub._.sub.in.sub._.sub.rich.sub._.sub.des+m.sub.d.sub._.sub.cyl.sub._.su-
b.in.sub._.sub.rich.sub._.sub.des (70)
The desired intake manifold rich diluent mass fraction can be
calculated by dividing the desired cylinder in rich diluent mass by
the desired cylinder in rich mass.
.chi. d_im _rich _des = m d_cyl _in _rich _des m cyl_in _rich _des
( 71 ) ##EQU00031##
The desired intake manifold rich diluent mass fraction can be
related to the desired intake manifold rich EGR mass fraction
by:
.chi..sub.d.sub._.sub.im.sub._.sub.rich.sub._.sub.des=.chi..sub.d.sub._.-
sub.em.chi..sub.egr.sub._.sub.im.sub._.sub.rich.sub._.sub.des+.chi..sub.d.-
sub._.sub.cac(1-.chi..sub.egr.sub._.sub.im.sub._.sub.rich.sub._.sub.des)
(72)
The mass fraction of diluent in the charge air cooler is used in
this calculation to account for water vapor that may enter with the
fresh air. Water vapor is considered a diluent. Equation (72) can
be rearranged to solve for the desired intake manifold rich EGR
mass fraction as:
.chi. egr_im _rich _des = .chi. d_im _rich _des - .chi. d_cac .chi.
d_em - .chi. d_cac ( 73 ) ##EQU00032##
The desired engine in rich mass flow is calculated using the
desired cylinder in rich mass, number of cylinders, and engine
speed as:
m . eng_in _rich _des = m cyl_in _rich _des n cyl N e ( rpm ) ( min
60 sec ) ( cycle 2 rev ) ( 74 ) ##EQU00033##
The desired rich EGR mass flow can then be calculated by
multiplying the desired intake manifold rich EGR mass fraction by
the desired engine in rich mass flow as:
{dot over
(m)}.sub.egr.sub._.sub.rich.sub._.sub.des=.chi..sub.egr.sub._.sub.im.sub.-
_.sub.rich.sub._.sub.des{dot over
(m)}.sub.eng.sub._.sub.in.sub._.sub.rich.sub._.sub.des (75)
[0115] At block 814, the minimum exhaust manifold pressure is found
that provides the desired EGR mass flow. When the desired EGR valve
position u.sub.egr.sub._.sub.des is less than the maximum
u.sub.egr.sub._.sub.max, then there is sufficient exhaust manifold
pressure for EGR flow control with the EGR valve. Under this
condition, the minimum exhaust manifold pressure limit can be
calculated using the desired EGR rich mass flow and desired intake
manifold pressure rich setpoints. The exhaust pressure calculation
can take the desired rich intake manifold pressure and add the
calculated pressure drop across the EGR cooler and EGR valve (with
the EGR valve at its maximum position).
[0116] When the EGR valve is at the maximum position
u.sub.egr.sub._.sub.max, then the EGR valve is not controlling the
flow and the flow can be controlled by regulating the exhaust
pressure. Under this condition, the desired EGR valve mass flow
from the EGR valve control calculation can be used with the current
intake manifold pressure estimate to calculate the required exhaust
manifold pressure.
[0117] An exemplary calculation for the minimum exhaust manifold
pressure under the two conditions described above is:
If u.sub.egr.sub._.sub.des<u.sub.egr.sub._.sub.max
P.sub.em.sub._.sub.min.sub._.sub.for.sub._.sub.egr=ExhManifPressMinForEG-
R({dot over
(m)}.sub.egr.sub._.sub.rich.sub._.sub.des,P.sub.im.sub._.sub.rich.sub._.s-
ub.des)
Else
P.sub.em.sub._.sub.min.sub._.sub.for.sub._.sub.egr=ExhManifPressMinForEG-
R({dot over (m)}.sub.egr.sub._.sub.des,P.sub.im) (76)
[0118] The desired intake manifold pressure can be determined at
block 810 by computing the pressure needed to achieve the desired
engine in mass flow which can be found using the ideal gas law and
the volumetric efficiency as:
P im_rich _des = n cyl m cyl_in _rich _des R T im V d VE ( 77 )
##EQU00034##
This equation is related to the speed-density calculation but does
not account for speed because the mass per cylinder is used instead
of a mass flow. The desired intake manifold rich pressure may be
limited to a certain range for component protection and engine
performance. Low pressures are sometimes avoided to prevent high
oil consumption and poor combustion quality.
[0119] At block 818, the error in intake manifold rich pressure can
be calculated as the difference between the desired intake manifold
rich pressure and the estimated intake manifold pressure by:
P.sub.im.sub._.sub.rich.sub._.sub.error=P.sub.im.sub._.sub.rich.sub._.su-
b.des-P.sub.im (78)
This pressure error can be multiplied by the integral gain and
added to the previous desired exhaust manifold pressure to
determine the unlimited desired exhaust manifold pressure before
the pressure limits are applied as:
P.sub.em.sub._.sub.des.sub._.sub.unlimited=K.sub.i.sub._.sub.imP.sub.im.-
sub._.sub.rich.sub._.sub.error+P.sub.em.sub._.sub.des.sub._.sub.previous
(79)
[0120] At block 822, the unlimited desired exhaust manifold
pressure may be limited by minimum pressure limits to the minimum
exhaust manifold pressure for EGR from equation (76), and may also
have a calibrate-able minimum limit for engine performance:
P.sub.em.sub._.sub.des.sub._.sub.min.sub._.sub.limited=maximum(P.sub.em.-
sub._.sub.des.sub._.sub.unlimited,P.sub.em.sub._.sub.min.sub._.sub.for.sub-
._.sub.egr,P.sub.em.sub._.sub.min.sub._.sub.for.sub._.sub.perf)
(79A)
Keeping the exhaust manifold pressure higher than necessary at low
load can improve the transient response of the engine because the
turbocharger speed is higher before the load is applied. This
improvement in transient response performance comes with a
reduction in fuel economy because the higher exhaust manifold
pressure creates more pumping work for the engine.
[0121] The desired exhaust manifold pressure may be reduced when it
is determined that controlling to a lower exhaust manifold pressure
produces more turbine power. As the exhaust manifold pressure
increases, the turbine vanes move in the closing direction to
achieve the desired turbine mass flow. As the turbine vanes move in
the closing direction the angle at which the gasses enter the
turbine changes. For a given set of operating conditions and
turbocharger speed there will be an optimum turbine vane position
that maximizes turbine power. Closing the vanes more than what is
optimum for maximum turbine power increase the exhaust manifold
pressure, increases engine pumping work, reduces fuel economy, and
reduces turbine power. When it is determined that operating at a
lower exhaust manifold pressure produces more turbine power as
described below, the desired exhaust manifold pressure can be
calculated by taking the previous desired exhaust manifold pressure
and subtracting a small calibrate-able value. Since this reduction
in pressure occurs at each time step in the software the desired
exhaust manifold pressure is reduced over time in the form of a
ramp.
If Maximum_Turbine_Power_Exceeded
P.sub.em.sub._.sub.des.sub._.sub.trb.sub._.sub.pwr.sub._.sub.limited=P.s-
ub.em.sub._.sub.des.sub._.sub.previous-.DELTA.P.sub.em.sub._.sub.ramp.sub.-
_.sub.trb.sub._.sub.pwr.sub._.sub.exceeded
Else
P.sub.em.sub._.sub.des.sub._.sub.trb.sub._.sub.pwr.sub._.sub.limited=P.s-
ub.em.sub._.sub.des.sub._.sub.min.sub._.sub.limited (79B)
[0122] The desired exhaust manifold pressure may also be
constrained by a maximum engine delta pressure limits for engine
protection. Engine delta pressure is the difference between the
exhaust manifold pressure and intake manifold pressure. If the
engine delta pressure becomes too large, the exhaust pressure can
push the exhaust valve open during the intake stroke of the engine
which is called valve float. A maximum exhaust manifold pressure
limit for engine delta pressure protection can be calculated
as:
P.sub.em.sub._.sub.max.sub._.sub.for.sub._.sub.eng.sub._.sub.dp=P.sub.im-
+.DELTA.P.sub.eng.sub._.sub.max (79C)
The desired exhaust manifold pressure may also be constrained by a
maximum pressure limit for component protection (to protect
turbocharge seals, exhaust manifold gaskets, etc.). This pressure
limit can be calculated using the ambient (barometric) pressure and
the gage pressure limit for the exhaust manifold components as:
P.sub.em.sub._.sub.max.sub._.sub.for.sub._.sub.comp=P.sub.amb+P.sub.em.s-
ub._.sub.max.sub._.sub.comp.sub._.sub.gage (79C)
The desired exhaust manifold pressure can be calculated by taking
the minimum of the turbine power limited desired exhaust manifold
pressure, the maximum pressure for engine delta pressure
protection, and the pressure limit for component protection:
P.sub.em.sub._.sub.des=minimum(P.sub.em.sub._.sub.des.sub._.sub.trb.sub.-
_.sub.pwr.sub._.sub.limited,P.sub.em.sub._.sub.max.sub._.sub.for.sub._.sub-
.eng.sub._.sub.dp,P.sub.em.sub._.sub.max.sub._.sub.for.sub._.sub.comp)
(79D)
In addition to the magnitude limits described here, rate limits can
also be applied to prevent sudden changes to the desired exhaust
manifold pressure that would cause large transient changes to the
desired turbine mass flow and large transient changes to the
desired turbine vane position.
[0123] The closed loop turbine mass flow can be calculated by first
calculating the exhaust manifold pressure error as:
P.sub.em.sub._.sub.error=P.sub.em.sub._.sub.des-P.sub.em (80)
where P.sub.em is the exhaust manifold pressure estimate from the
observer. The estimate can be used instead of the measured value
since the measured value may have lag due to the sensor response
time and any filtering that may be used to reduce signal noise, and
the estimated value may continue to be used if a sensor fails.
Alternatively, pressure sensors can respond fast compared to the
air system response so using the measured pressure instead of the
estimate can be done with similar results. The exhaust manifold
pressure error can be multiplied by the proportional gain to
determine the closed loop term by:
{dot over
(m)}.sub.trb.sub._.sub.cl=-K.sub.p.sub._.sub.emP.sub.em.sub._.sub.error
(81)
The negative sign is used because turbine flow has to go down to
increase exhaust manifold pressure. This allows a positive gain to
be used in calculating the closed loop turbine mass flow. The
desired turbine mass flow is the sum of the feed forward term and
the closed loop term:
{dot over (m)}.sub.trb.sub._.sub.des={dot over
(m)}.sub.trb.sub._.sub.ff+{dot over (m)}.sub.trb.sub._.sub.cl
(82)
[0124] The control method described above and illustrated in FIG. 8
is intended for use when the desired lean F/A setpoint is less than
the desired rich F/A setpoint. This results in the air throttle
attempting to maintain a higher intake manifold pressure than the
turbine vane position control. This separation can create a control
band in which both the air throttle and turbine vanes move in the
opening direction. Under steady state conditions the air throttle
typically moves to the maximum position, which minimizes the
pumping work across the air throttle.
[0125] When the desired lean F/A setpoint is greater than or equal;
to the desired rich F/A setpoint, a different control method can be
used to prevent the air throttle and turbine vane controls from
conflicting. Without the intake manifold pressure setpoint
separation, the air throttle control and turbine vane control may
oscillate or the air throttle may close completely. This problem
can be overcome by using the turbine vanes to control the charge
air cooler pressure instead of using the turbine vanes to control
the intake manifold pressure.
[0126] In this situation, the desired air throttle rich mass flow
and the desired charge air cooler pressure can be calculated first.
The desired air throttle rich mass flow can be calculated using the
desired intake manifold rich EGR mass fraction and the desired
engine in rich mass flow as:
m.sub.at.sub._.sub.rich.sub._.sub.des=(1-.chi..sub.egr.sub._.sub.im.sub.-
_.sub.rich.sub._.sub.des){dot over
(m)}.sub.eng.sub._.sub.in.sub._.sub.rich.sub._.sub.des (83)
[0127] The desired charge air cooler rich pressure can be estimated
by the desired intake manifold rich pressure and the pressure drop
across the air throttle and charge air cooler. This calculation can
be shown as a function by:
P.sub.cac.sub._.sub.rich.sub._.sub.des=DesCACPress(u.sub.at.sub._.sub.ma-
x,{dot over
(m)}.sub.at.sub._.sub.rich.sub._.sub.des,P.sub.im.sub._.sub.rich.sub._.su-
b.des,T.sub.caci,T.sub.caco) (84)
This function has two parts. First, equation (12) can be used to
find the desired charge air cooler outlet pressure that provides
the desired mass flow through the air throttle. Then equation (21)
can be used to find the charge air cooler inlet pressure that is
sufficient to overcome the pressure drop across the charge air
cooler. The maximum air throttle position u.sub.at.sub._.sub.max
can be used since it minimizes the pumping work across the air
throttle. These equations can be solved by an iterative solution
method to determine the desired charge air cooler rich
pressure.
[0128] The charge air cooler pressure rich error is the difference
between the desired charge air cooler rich pressure and the
estimated charge air cooler pressure:
P.sub.cac.sub._.sub.rich.sub._.sub.error=P.sub.cac.sub._.sub.rich.sub._.-
sub.des-P.sub.cac (85)
This error can be multiplied by the integral gain and added to the
previous desired exhaust manifold pressure to determine the desired
exhaust manifold pressure as:
P.sub.em.sub._.sub.des=K.sub.i.sub._.sub.cacP.sub.cac.sub._.sub.rich.sub-
._.sub.error+P.sub.em.sub._.sub.des.sub._.sub.previous (86)
The desired exhaust manifold pressure can then be limited to the
minimum value calculated by equation (76), and it may also be
limited to a certain range for component protection and engine
performance. The desired exhaust manifold pressure can then be used
to calculate the desired turbine mass flow as shown by equations
(80) through (82).
[0129] The desired turbine vane position u.sub.vgt.sub._.sub.des
can be calculated using the desired turbine mass flow {dot over
(m)}.sub.trb.sub._.sub.des and the state information for the
observer model. The state information can include, for example,
exhaust manifold pressure, exhaust manifold temperature, exhaust
back pressure, high pressure turbocharger speed, and low pressure
turbocharger speed.
[0130] The interstage pressure between the high and low pressure
turbines 724, 722 can also be determined in finding the vane
position u.sub.vgt.sub._.sub.des that provides the desired turbine
mass flow {dot over (m)}.sub.trb.sub._.sub.des. The turbine
interstage pressure and desired vane position can be found for a
given turbine mass flow using a two dimensional binary search, an
example of which is shown in FIG. 9.
[0131] At block 910, the desired turbine mass flow {dot over
(m)}.sub.trb.sub._.sub.des from equation (82) is input. At block
912, the exhaust manifold pressure and temperature, the exhaust
back pressure, and the speeds of the low and high pressure turbines
724, 722 are input. At block 914, the current desired vane position
is set to the desired vane position from the calculation that was
performed at the previous time step, the turbine delta pressure is
set to the difference between the exhaust manifold pressure and the
exhaust back pressure, and a search index and vane adjustment value
are initialized. Block 916 is an outer loop check that allows
i.sub.max iterations of the outer loop. Block 920 is an inner loop
that performs j.sub.max iterations adjusting the turbine interstage
pressure to find a resulting turbine interstage pressure for the
current desired vane position that provides the same or closest to
the same mass flow through each of the turbines 724, 722. This step
is similar to the search described in [0052] for the observer model
but in this case the model is being used to evaluate a possible
vane position rather than the final desired vane position. At block
930, the outer loop takes the mass flows through each of the
turbines 724, 722, and calculates an average turbine mass flow by
averaging the high and low pressure turbine mass flows. At block
930, the outer loop also fine tunes the turbine position adjustment
step. At block 934, the average turbine mass flow is compared with
the desired turbine mass flow. If the average turbine mass flow is
too high, then at block 936 the current desired vane position is
adjusted in the closing direction (increasing
u.sub.vgt.sub._.sub.des). Otherwise, at block 938 the current
desired vane position is adjusted in the opening direction
(decreasing u.sub.vgt.sub._.sub.des). Control returns to block 916,
and the process is repeated as part of the binary search. After all
the iterations of the outer loop are complete, at block 950 the
final desired vane position u.sub.vgt.sub._.sub.des that provides
the desired turbine mass flow {dot over (m)}.sub.trb.sub._.sub.des
is output. This process can be repeated with percentage adjustments
to the vane position of, for example, 4, 2, 1, 0.5 and 0.25
percent.
[0132] The modeled mass flow through the high and low pressure
turbines 724, 722 in the search for the desired vane position can
each be corrected using the turbine mass flow correction factor
shown in equation (30). This allows the turbine vane position to be
determined with modeled flow correction so that the desired turbine
mass flow is achieved.
[0133] The turbine interstage temperature can be assumed to be
equal to the high pressure turbine outlet temperature, which can be
calculated using the turbine efficiency, pressure ratio across the
high pressure turbine, and the exhaust manifold temperature. The
efficiency can be determined using tables with inputs of corrected
turbocharger speed and pressure ratio. The turbocharger speed can
be adjusted using a correction factor to compensate for operating
with exhaust manifold temperatures that are different than the
reference temperature used when creating the efficiency table.
Efficiency tables can be generated for different vane positions and
interpolation can be used when the vane position is between two of
the provided tables. The turbine interstage pressure and
temperature can be used to calculate the low pressure turbine mass
flow.
[0134] For a given turbine mass flow rate and turbocharger speed
there will be an optimum turbine vane position that maximizes
turbine power. This vane position will provide the best combination
of flow velocity and flow angle for the exhaust gas as it enters
the turbine blades. Opening the vanes more than the maximum turbine
power point causes lower velocity, less optimum flow angle, and
less turbine power but it also reduces the exhaust manifold
pressure which tends to reduce engine pumping work. For normal
control when the maximum turbine power is not needed the vanes are
typically positioned more open than the maximum turbine power point
because this reduces the engine pumping work.
[0135] Closing the vanes more than the maximum turbine power point
causes higher velocity but the flow angle is less optimum, there is
less turbine power, and the exhausts manifold pressure is higher.
This region of operation is typically avoided. One exception is
when there is a need to raise the exhaust temperature for
after-treatment system control. In this case, the increase in
pumping work is beneficial because it puts additional load on the
engine. The extra load on the engine requires more fuel to be
consumed, raising the exhaust temperature.
[0136] For normal control it is desirable to have the turbine vanes
between the maximum turbine power position and wide open. When
calculating the desired vane position for a desired turbine mass
flow, exhaust manifold pressure, and turbocharger speed as
described above, a check can also be performed to see if the
maximum turbine power has been exceeded. When this happens it means
the desired exhaust manifold pressure is too high.
[0137] This maximum turbine power check can be performed after the
vane position calculation described above to see if controlling to
a slightly lower exhaust manifold pressure (5 kPa lower for
example) would provide more turbine power. If the turbine power is
higher with reduced exhaust manifold pressure that means the
exhaust manifold pressure for maximum turbine power has been
exceeded. When the maximum turbine power is exceeded, the desired
exhaust manifold pressure can be reduced at a calibrate-able rate
until the maximum turbine power is no longer exceeded, as shown by
equation (79B).
[0138] While the disclosure has been illustrated and described in
detail in the drawings and foregoing description, such illustration
and description is to be considered as exemplary and not
restrictive in character, it being understood that illustrative
embodiment(s) have been shown and described and that all changes
and modifications that come within the spirit of the disclosure are
desired to be protected. It will be noted that alternative
embodiments of the present disclosure may not include all of the
features described yet still benefit from at least some of the
advantages of such features. Those of ordinary skill in the art may
readily devise their own implementations that incorporate one or
more of the features of the present disclosure and fall within the
spirit and scope of the present invention as defined by the
appended claims.
* * * * *