U.S. patent application number 15/963948 was filed with the patent office on 2018-08-30 for fuel limiter for a uniflow-scavenged, two-stroke cycle, opposed-piston engine.
This patent application is currently assigned to ACHATES POWER, INC.. The applicant listed for this patent is ACHATES POWER, INC.. Invention is credited to NISHIT NAGAR, ARUNANDAN SHARMA.
Application Number | 20180245520 15/963948 |
Document ID | / |
Family ID | 57882182 |
Filed Date | 2018-08-30 |
United States Patent
Application |
20180245520 |
Kind Code |
A1 |
NAGAR; NISHIT ; et
al. |
August 30, 2018 |
FUEL LIMITER FOR A UNIFLOW-SCAVENGED, TWO-STROKE CYCLE,
OPPOSED-PISTON ENGINE
Abstract
Control of fuel flow in a uniflow-scavenged, two-stroke cycle,
opposed-piston engine includes limiting an amount of torque or fuel
in response to a torque demand, based upon a comparison and a
selection of fuel delivery options derived from a global airflow
parameter and/or a trapped airflow parameter.
Inventors: |
NAGAR; NISHIT; (San Diego,
CA) ; SHARMA; ARUNANDAN; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ACHATES POWER, INC. |
San Diego |
CA |
US |
|
|
Assignee: |
ACHATES POWER, INC.
San Diego
CA
|
Family ID: |
57882182 |
Appl. No.: |
15/963948 |
Filed: |
April 26, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15062868 |
Mar 7, 2016 |
9957901 |
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15963948 |
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62279301 |
Jan 15, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01B 7/14 20130101; F02D
41/182 20130101; F02B 2075/025 20130101; F02B 75/02 20130101; F02D
1/06 20130101; F02B 75/282 20130101; Y02T 10/12 20130101; F02D
2400/04 20130101; F02D 41/10 20130101; F02D 2250/26 20130101; F02D
2250/38 20130101; F02D 41/0007 20130101; F02D 41/1458 20130101;
F02B 25/08 20130101; F02M 63/0225 20130101; Y02T 10/144
20130101 |
International
Class: |
F02D 1/06 20060101
F02D001/06; F02B 75/02 20060101 F02B075/02; F02M 63/02 20060101
F02M063/02; F02B 25/08 20060101 F02B025/08 |
Claims
1. A method for controlling operation of a uniflow-scavenged,
two-stroke cycle, opposed-piston engine with a fuel source during
demands for increased torque, comprising: operating the engine in
one of a fuel mode and a torque mode; providing charge air to
cylinders of the engine; receiving a torque demand for a desired
torque level of the engine; determining a minimum value of an air
to fuel ratio (AFR) of the engine based on engine speed and the
desired torque level; determining a value of mass airflow through
the engine based on air flow conditions of the engine; determining
a maximum fuel quantity limit based on the minimum value of AFR and
the value of mass airflow through the engine; limiting a quantity
of fuel provided to the engine based the lesser of a maximum
allowable torque derived from the maximum fuel quantity limit and
the desired torque level; and, if the engine is operated in the
fuel mode, providing a limited fuel command indicative of the
limited quantity of fuel to the fuel source; otherwise, if the
engine is operated in the torque mode, providing a limited torque
command indicative of the limited quantity of fuel to the fuel
source.
2. The method of claim 1, wherein the value of mass airflow is
based on one of: a mass of charge air provided to a cylinder of the
engine during a cycle of engine operation; and, a mass of charge
air trapped in the cylinder by a last port of the cylinder to close
during a cycle of engine operation.
3. The method of claim 2, wherein the minimum value of AFR is a
smoke-limited AFR.
4. The method of claim 2, wherein the value of a mass of charge air
provided to a cylinder of the engine during a cycle of engine
operation is determined by measurement using one of a mass airflow
sensor and a NOx/0.sub.2 sensor.
5. The method of claim 2, wherein the value of a mass of charge air
trapped in a cylinder by the last port of the cylinder to close
during one cycle of engine operation is determined by measurement
using a virtual sensor.
6. The method of claim 1, wherein the AFR is one of: a global AFR
that is determined with reference to the total mass of charge air
delivered to a cylinder during one cycle of engine operation; and,
a trapped AFR that is determined with reference to the total mass
of charge air trapped in a cylinder by the last port of the
cylinder to close during one cycle of engine operation.
7. The method of claim 6, wherein the minimum value of AFR is a
smoke-limited AFR.
8. The method of claim 6, wherein the total mass of charge air
provided to a cylinder of the engine during a cycle of engine
operation is determined by measurement using one of a mass airflow
sensor and a NOx/0.sub.2 sensor.
9. The method of claim 6, wherein the total mass of charge air
trapped in a cylinder by the last port of the cylinder to close
during one cycle of engine operation is determined by measurement
using a virtual sensor.
10. A method for limiting emission of smoke by a uniflow-scavenged,
two-stroke cycle, opposed-piston engine with a fuel source during
demands for increased torque, comprising: receiving a torque demand
for a desired torque level of the engine; obtaining a minimum value
of a smoke-limited AFR (air to fuel ratio) of the engine from a
smoke map based on engine speed and the desired torque level;
determining a measured value of a mass airflow of the engine;
determining a maximum fuel quantity limit based on the minimum
value of the smoke-limited AFR and the measured value of mass
airflow of the engine; limiting a quantity of fuel provided to the
engine based on the lesser of a maximum allowable torque derived
from the maximum fuel quantity limit and the desired torque level;
and, providing a limited torque command indicative of the limited
quantity of fuel to the fuel source.
11. The method of claim 10, wherein the smoke-limited AFR is based
on one of: a mass of charge air provided to a cylinder of the
engine during a cycle of engine operation; and, a mass of charge
air trapped in the cylinder by a last port of the cylinder to close
during a cycle of engine operation.
12. The method of claim 10, wherein the measured value of mass
airflow is based on one of: a mass of charge air provided to a
cylinder of the engine during a cycle of engine operation; and, a
mass of charge air trapped in the cylinder by a last port of the
cylinder to close during a cycle of engine operation.
13. The method of claim 12, wherein the value of a mass of charge
air provided to a cylinder of the engine during a cycle of engine
operation is determined by measurement using one of a mass airflow
sensor and a NOx/0.sub.2 sensor.
14. The method of claim 12, wherein the value of a mass of charge
air trapped in a cylinder by the last port of the cylinder to close
during one cycle of engine operation is determined by measurement
using a virtual sensor.
15. The method of claim 10, wherein the AFR is one of: a global AFR
that is determined with reference to the total mass of charge air
delivered to a cylinder during one cycle of engine operation; and,
a trapped AFR that is determined with reference to the total mass
of charge air trapped in a cylinder by the last port of the
cylinder to close during one cycle of engine operation.
16. A uniflow-scavenged, two-stroke cycle, opposed-piston engine,
comprising: at least one cylinder, two pistons disposed
crown-to-crown in a bore of the at least one cylinder for
reciprocating movement in opposing directions, and an inlet port
and an exhaust port near respective ends of the at least one
cylinder; an air handling system for transporting charge air to the
inlet port and exhaust from the exhaust port; a fuel handling
system for providing fuel to the at least one cylinder to be mixed
with charge air therein and ignited by heat of charge air
compressed by the two pistons; and, a smoke limiter means for:
accepting a desired load signal from a sensor of the engine;
responsively to the desired load signal, determining a quantity of
fuel that can be provided to the at least one cylinder without
exceeding a smoke-limited air/fuel ratio that limits a production
of smoke by the opposed-piston engine in response to a rich mixture
of charge air and fuel; and, providing a command to the fuel
handling system indicative of the quantity of fuel.
17. The uniflow-scavenged, two-stroke cycle, opposed-piston engine
of claim 16, further comprising an exhaust gas recirculation (EGR)
loop.
18. The uniflow-scavenged, two-stroke cycle, opposed-piston engine
of claim 16, in which the fuel handling system comprises a common
rail fuel handling system for delivering fuel to the at least one
cylinder by direct injection into the at least one cylinder.
19. The uniflow-scavenged, two-stroke cycle, opposed-piston engine
of claim 18, in which the common rail fuel handling system
comprises multiple fuel injectors mounted for direct injection into
space in the at least one cylinder between top dead center
locations of end surfaces of the two pistons.
20. The uniflow-scavenged, two-stroke cycle, opposed-piston engine
of claim 18, in which the common rail fuel handling system includes
two fuel injectors, at least one rail/accumulator mechanism, and a
fuel pump.
Description
PRIORITY
[0001] This application claims priority to co-pending U.S. patent
application Ser. No. 15/062,868, filed in the United States Patent
and Trademark Office on 7 Mar. 2016, which claims priority to U.S.
Provisional Application Ser. No. 62/279,301, filed in the United
States Patent and Trademark Office on 15 Jan. 2016.
RELATED APPLICATIONS
[0002] This application contains subject matter related to the
subject matter of the following US applications, which are commonly
owned herewith: U.S. patent application Ser. No. 13/654,340, filed
Ser. No. 10/17/2012, published as US 2013/0104848 A1; U.S. patent
application Ser. No. 13/926,360 filed on Jun. 25, 2013, published
as US 2014/0373814 A1, and issued as U.S. Pat. No. 9,206,751 B2 on
Dec. 8, 2015; U.S. patent application Ser. No. 14/039,856, filed on
Sep. 27, 2013, published as US 2014/0026563 A1; and U.S. patent
application Ser. No. 14/378,252, filed on Aug. 12, 2014, published
as US 2015/0033736 A1.
FIELD
[0003] The field is control and operation of air handling systems
and fuel handling systems for two-stroke cycle, opposed-piston
engines. In particular, the field covers limitation of a fuel
quantity or a torque quantity in uniflow-scavenged, two-stroke
cycle, opposed-piston engines in response to demands for increased
torque.
BACKGROUND
[0004] A two-stroke cycle engine is an internal combustion engine
that completes a cycle of operation with a single complete rotation
of a crankshaft and two strokes of a piston connected to the
crankshaft. The strokes are typically denoted as compression and
power strokes. In a two-stroke cycle, opposed-piston ("OP2S")
engine two pistons are disposed crown-to-crown in the bore of a
cylinder for reciprocating movement in opposing directions. The
cylinder has longitudinally-spaced inlet and exhaust ports formed
in the cylinder sidewall near respective ends of the cylinder. Each
of the opposed pistons controls a respective one of the ports,
opening the port as it moves toward a bottom dead center (BDC)
location during a power stroke (also called an expansion stroke),
and closing the port as it moves from BDC toward a top dead center
(TDC) location during a compression stroke. One of the ports
provides passage for the products of combustion out of the bore,
the other serves to admit pressurized air into the bore; these are
respectively termed the "exhaust" and "intake" ports.
[0005] OP2S engines typically operate according to the
compression-ignition principle. During a compression stroke,
turbulent pressurized air ("charge air") enters the bore of a
cylinder through the intake port and is compressed between the end
surfaces of the two pistons as they move from BDC toward TDC. Fuel
injected between the approaching piston end surfaces mixes with the
turbulent air. Near TDC of the pistons, the air/fuel mixture is
ignited by the heat of the compressed air, and combustion
follows.
[0006] In a uniflow-scavenged OP2S engine, near the end of a power
stroke, charge air entering a cylinder through the intake port
displaces exhaust gas flowing out of the cylinder through the
exhaust port. Thus gas flows through the cylinder in one direction
("uniflow")--from intake port to exhaust port. A continuous
positive pressure differential must exist from the intake ports to
the exhaust ports of the engine in order to maintain the desired
unidirectional flow of gas in the cylinders. In many cases, this
need is met by an engine air handling system that includes a fast
acting, positive-displacement pump, upstream from the engine's
intake ports. This role is typically filled by a supercharger (or
blower) which is driven mechanically by an engine crankshaft,
either by direct connection, as with a belt, chain. The air
handling system may include a turbocharger, powered by exhaust
energy, and including a compressor that is placed ahead (upstream)
of the supercharger in order to increase the density of intake air
fed into the supercharger. Depending on design requirements, a
uniflow-scavenged, OP2S engine may include either or both of a
mechanically-driven supercharger and an exhaust-driven
turbocharger.
[0007] Operation of an OP2S engine subjects it to demands for
torque which may result from acceleration, deceleration,
transmission shifts, switching accessories (like air conditioning)
on or off, pulling a trailer, climbing a hill, and so on.
[0008] A demand for torque generates a requirement to increase the
quantity of fuel provided to the engine in order to raise the level
of energy released by combustion. This requires a concurrent
provision of additional air to burn the additional fuel. A
uniflow-scavenged, OP2S engine equipped with
electronically-controlled air and fuel handling systems may produce
smoke, or produce other hydrocarbon emissions, or experience
misfiring in response to a torque demand if the response time of
the air handling system lags that of the fuel handling system. For
example, in diesel applications of an OP2S engine, without timely
provision of additional air in response to a torque demand the
increased level of diesel fuel may be incompletely burned, which
can result in the transport of particulate matter comprising
unburned fuel droplets, incompletely burned fuel, and soot in the
exhaust stream, thereby creating smoke.
[0009] Therefore, it is desirable to limit the consequences of
incomplete combustion due to air handling lag in response to a
torque demand during operation of a uniflow-scavenged, OP2S engine.
Consequently, during a torque demand, a limiting factor for the
engine's operation may be defined by the delay in providing
sufficient air to burn the additional fuel provided in response to
the demand.
[0010] Provision of fuel and air in the engine are governed by an
engine control mechanization that senses various engine operating
parameters and regulates the flow of gasses (air and exhaust)
through the engine and the injection of fuel into the engine. It is
particularly desirable that the engine control mechanization be
able to limit the additional fuel quantity provided in response to
a torque demand while the air handling system increases the airflow
to a level sufficient to support full combustion at the requested
torque level.
SUMMARY
[0011] In a uniflow-scavenged, OP2S engine equipped with a fuel
handling system and an air handling system, a torque demand
initiates limitation of the fuel quantity provided in response to
sensing, measurement, or estimation of one or more airflow
parameter levels. A minimum allowable airflow parameter level is
provided as a function of the torque demand and candidate
quantities of fuel are determined based on the minimum allowable
airflow parameter level and a sensed airflow level. A limited fuel
quantity is determined based upon a comparison and a selection from
among the candidate quantities of fuel.
[0012] In some aspects, wherein fuel provision and airflow are
governed by an engine control mechanization, the selected limited
fuel quantity may be provided as one of a limited fuel command and
a limited torque command, depending upon a governing mode of engine
control.
[0013] In some other aspects, the one or more minimum allowable
airflow parameters comprise one of a global airflow parameter based
on total airflow through the engine and a corresponding trapped
airflow parameter based on charge air trapped in a cylinder by the
last port of the cylinder to close.
[0014] In yet other aspects, the one or more minimum allowable
airflow parameters comprise both of a global airflow parameter
based on total airflow through the engine and a corresponding
trapped airflow parameter based on charge air trapped in a cylinder
by the last port of the cylinder to close.
[0015] In particular, these aspects may include a global air/fuel
ratio and a trapped air/fuel ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic illustration of a uniflow-scavenged,
two-stroke cycle, opposed-piston (OP2S) engine of the prior
art.
[0017] FIG. 2 is a schematic diagram illustrating a fuel injection
system embodiment for an OP2S engine.
[0018] FIG. 3 is a schematic diagram illustrating an air handling
system embodiment for an OP2S engine.
[0019] FIG. 4 is a schematic diagram illustrating a control
mechanization embodiment for controlling provision of fuel and
airflow in an OP2S engine.
[0020] FIG. 5 is a schematic diagram illustrating a process
embodiment for limiting an increase in an amount of torque or fuel
provided to an OP2S engine in response to a torque demand.
[0021] FIG. 6 is a schematic diagram illustrating a process
embodiment for limiting an increase in an amount of torque or fuel
provided to an OP2S engine in response to a torque demand.
[0022] FIG. 7 is a data table representing an airflow map.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] FIG. 1 is a schematic representation of a uniflow-scavenged,
two-stroke cycle opposed-piston (0P2S) engine 8 of the compression
ignition type that includes at least one cylinder. Preferably, the
engine 8 has two or more cylinders. In any event, the cylinder 10
represents both single cylinder and multi-cylinder configurations
of the OP2S engine 8. The cylinder 10 includes a bore 12 and
longitudinally displaced intake and exhaust ports 14 and 16
machined or formed in the cylinder, near respective ends thereof.
An air handling system 15 of the engine 8 manages the transport of
charge air into, and exhaust out of, the engine. Each of the intake
and exhaust ports includes one or more circumferential arrays of
openings in which adjacent openings are separated by a solid
portion of the cylinder wall (also called a "bridge"). In some
descriptions, each opening is referred to as a "port"; however, the
construction of a circumferential array of such "ports" is no
different than the port constructions in FIG. 1. Fuel injectors 17
include nozzles that are secured in threaded holes that open
through the sidewall of the cylinder for direct side injection into
the cylinder. Two pistons 20, 22 are disposed in the bore 12 with
their end surfaces 20e, 22e in opposition to each other. For
convenience, the piston 20 is referred to as the "intake" piston
because it opens and closes the intake port 14. Similarly, the
piston 22 is referred to as the "exhaust" piston because it opens
and closes the exhaust port 16. Preferably, but not necessarily,
the intake piston 20 and all other intake pistons are coupled to a
crankshaft 30 disposed along one side of the engine 8; and, the
exhaust piston 22 and all other exhaust pistons are coupled to a
crankshaft 32 disposed along the opposite side of the engine 8.
[0024] Operation of the OP2S engine 8 is well understood. In
response to combustion the opposed pistons move away from
respective TDC locations where they are at their innermost
positions in the cylinder 10. While moving from TDC, the pistons
keep their associated ports closed until they approach respective
BDC locations where they are at their outermost positions in the
cylinder and the associated ports are open. The pistons may move in
phase so that the intake and exhaust ports 14, 16 open and close in
unison. Alternatively, one piston may lead the other in phase, in
which case the intake and exhaust ports have different opening and
closing times.
[0025] As charge air enters the cylinder 10 through the intake port
14, the shapes of the intake port openings cause the charge air to
rotate in a vortex 34 about the cylinder's longitudinal axis, which
spirals in the direction of the exhaust port 16. A swirl vortex 34
promotes air/fuel mixing, combustion, and suppression of
pollutants. Swirl velocity increases as the end surfaces 20e and
22e move together.
[0026] FIG. 2 shows an embodiment of a fuel handling system for an
OP2S engine. Preferably, the fuel handling system is embodied as a
common rail direct injection fuel handling system. The fuel
handling system 18 delivers fuel to each cylinder 10 by injection
into the cylinder. Preferably, each cylinder 10 is provided with
multiple fuel injectors mounted for direct injection into cylinder
space between the TDC locations of piston end surfaces. For
example, each cylinder 10 has two fuel injectors 17. Preferably,
fuel is fed to the fuel injectors 17 from a fuel source 40 that
includes at least one rail/accumulator mechanism 41 to which fuel
is pumped by a fuel pump 43. A fuel return manifold 44 collects
fuel from the fuel injectors 17 and the fuel source 40 for return
to a reservoir from which the fuel is pumped. Elements of the fuel
source 40 are operated by respective computer-controlled actuators
that respond to fuel commands issued by an engine control unit.
Although FIG. 2 shows the fuel injectors 17 of each cylinder
disposed at an angle of less than 180.degree., this is merely a
schematic representation and is not intended to be limiting with
respect to the locations of the injectors or the directions of the
sprays that they inject. In a preferred configuration, best seen in
FIG. 1, the injectors 17 are disposed for injecting fuel sprays in
diametrically opposing directions of the cylinder 8 along an
injection axis. Preferably, each fuel injector 17 is operated by a
respective computer-controlled actuator that responds to injector
commands issued by an engine control unit.
[0027] FIG. 3 shows an embodiment of an air handling system that
manages the transport of charge air provided to, and exhaust gas
produced by, a uniflow-scavenged OP2S engine. A representative air
handling system construction includes a charge air subsystem and an
exhaust subsystem. In the air handling system 15, a charge air
source receives fresh air and processes it into charge air. The
charge air subsystem receives the charge air and transports it to
the intake ports of the engine 8. The exhaust subsystem transports
exhaust products from exhaust ports of the engine for delivery to
other exhaust components.
[0028] The air handling system 15 includes a turbocharger system
comprising one or more turbochargers. For example, a turbocharger
50 includes a turbine 51 and a compressor 52 that rotate on a
common shaft 53. The turbine 51 is coupled to the exhaust subsystem
and the compressor 52 is coupled to the charge air subsystem. The
turbocharger 50 extracts energy from exhaust gas that exits the
exhaust ports and flows into an exhaust channel 55 directly from
engine exhaust ports 16, or from an exhaust manifold assembly 57
that collects exhaust gasses output through the exhaust ports.
Preferably, in a multi-cylinder OP2S engine, the exhaust manifold
57 comprises an exhaust plenum or chest that communicates with the
exhaust ports 16 of all cylinders 10, which are supported in a
cylinder block 75. The turbine 51 is rotated by exhaust gas passing
through it to an exhaust outlet 58. This rotates the compressor 52,
causing it to generate charge air by compressing fresh air.
[0029] Exhaust gasses from the exhaust ports of the cylinders 50
flow from the exhaust manifold assembly 57 into the inlet of the
turbine 51, and from the turbine's outlet into the exhaust outlet
channel 55. In some instances, one or more after-treatment devices
79 are provided in the exhaust outlet channel 55. While the air
handling system 15 may be constructed to reduce NOx emissions
produced by combustion by recirculating exhaust gas through the
ported cylinders of the engine, the details of an exhaust gas
recirculation loop are not necessary to an understanding of fuel
limitation according to this disclosure.
[0030] The charge air subsystem may provide inlet air to the
compressor 52 via an air filter 80. As the compressor 52 rotates it
compresses inlet air, and the compressed inlet air flows into the
inlet 71 of a supercharger 60. Air pumped by the supercharger 60
flows through the supercharger's outlet 72 into the intake manifold
62. Pressurized charge air is delivered from the intake manifold 62
to the intake ports 14 of the cylinders 10. Preferably, in a
multi-cylinder OP2S engine, the intake manifold 68 comprises an
intake plenum or chest that communicates with the intake ports 14
of all cylinders 10.
[0031] The charge air subsystem may further include at least one
cooler coupled to receive and cool charge air before delivery to
the intake ports of the engine 8. In these instances, charge air
output by the compressor 52 flows through a cooler 67, whence it is
pumped by the supercharger 60 to the intake ports. A second cooler
69 may be provided between the output of the supercharger 60 and
the intake manifold 68.
[0032] With further reference to FIG. 3, the air handling system 15
is equipped for control of gas flow at separate control points in
the charge air and exhaust subsystems. In the charge air subsystem,
charge air flow and boost pressure are controlled by operation of a
shunt path 80 coupling the output 72 of the supercharger to the
supercharger's input 71. The shunt path 80 includes a shunt valve
82 that governs the flow of charge air into, and thus the pressure
in, the intake manifold 68. More precisely, the shunt valve 82
shunts the charge air flow from the supercharger's outlet 72 (high
pressure) to its inlet 71 (lower pressure). A backpressure valve 90
in the exhaust channel 55 governs the flow of exhaust out of, and
thus the backpressure in, the exhaust subsystem. As per FIG. 3, the
backpressure valve 90 is positioned in the exhaust channel 55,
between the output 58 of the turbine 51 and the after-treatment
devices 79. A wastegate valve 92 diverts exhaust gasses around the
turbine, which enables control of the speed of the turbine.
Regulation of the turbine speed enables regulation of the
compressor speed which, in turn, permits control of charge air
boost pressure. These valves 90 and 92 are opened and closed by
respective computer-controlled actuators that respond to rotational
commands issued by an engine control unit.
[0033] In some instances, additional control of gas flow and
pressure is provided by way of a variable speed supercharger. In
these aspects, the supercharger 60 is mechanically coupled by a
drive mechanism 95 (Drive) to a crankshaft 30 or 32 of the engine
8, to be driven thereby. The drive mechanism 95 may comprise a
stepwise transmission device, or continuously variable transmission
device (CVD), in which cases charge air flow, and boost pressure,
may be varied by varying the speed of the supercharger 60 in
response to a speed control signal provided to the drive mechanism
95. In other instances, the supercharger may be a single-speed
device with a mechanism to disengage the drive, thus giving two
different drive states. The drive mechanism 95 is operated by a
computer-controlled actuator that responds to drive commands.
[0034] In some aspects, the turbine 51 may be a variable-geometry
turbine (VGT) device having an effective aspect ratio that may be
varied in response to changing speeds and loads of the engine.
Alteration of the aspect ratio enables control of the speed of the
turbine. Regulation of the turbine speed enables regulation of the
compressor speed which, in turn, permits control of charge air
boost pressure. Thus, in many cases, a turbocharger comprising a
VGT does not require a wastegate valve. A VGT device is operated by
a computer-controlled actuator that responds to turbine commands
issued by an engine control unit.
[0035] In some instances, the air handling system may include an
EGR loop to control undesirable emissions. The EGR loop may
comprise a high pressure configuration, as shown in FIG. 3.
Alternatively, it may comprise a low pressure configuration or a
medium pressure configuration.
[0036] In this disclosure, an engine control mechanization is a
computer-based system that governs the operations of various engine
systems, including the fuel handling system, the air handling
system, a cooling system, a lubrication system, and other engine
systems. The engine control mechanism includes one or more
electronic control units coupled to associated sensors, actuators,
and other machine devices throughout the engine. As per FIG. 4,
control of the fuel handling system of FIG. 2 and the air handling
system of FIG. 3 (and, possibly, other systems of a
uniflow-scavenged, OP2S engine) is implemented by a control
mechanization 93 that includes an ECU (engine control unit) 94
programmed to execute fuel handling algorithms and air handling
algorithms under various engine operating conditions. For a common
rail direct injection system, the ECU 94 controls injection of fuel
into the cylinders by issuing Fuel commands for provision of fuel
by the fuel source 40 to the injectors 17, and by issuing Injector
commands for operation of the injectors 17. For the air handling
system the ECU 94 controls the transport of gas (charge air and
exhaust) through the engine by issuing Backpressure, Wastegate and
Shunt commands to open and close the exhaust backpressure valve 90,
the wastegate valve 92, and the supercharger shunt valve 82,
respectively, and also by issuing Drive commands to actuate the
supercharger drive 93 and VGT commands to open/close the aspect
ratio of the turbine 51 (if configured as a VGT).
[0037] When the OP2S engine 8 runs, the ECU 94 determines the
current engine operating state based on engine load and engine
speed, and governs the amount, pattern, and timing of fuel injected
into each cylinder 10 by control of common rail fuel pressure and
injection duration, based on the current operating state. For this
purpose, the ECU 94 receives a Desired Load signal from an
accelerator sensor 96 (or a speed governor or a cruise control
system, or equivalent means) that detects accelerator position, an
engine speed sensor 97 that detects the rotational speed (RPM) of
the engine, and a fuel supply sensor 98 that detects a quantity of
fuel being provided to the engine. At the same time, the ECU 94
configures the air handling system 15 to provide the optimal AFR
for the current operational state. For this purpose, the ECU
receives signals from a mass air flow sensor 100 that detects the
mass flow of air into the inlet of the compressor 52, an intake
manifold pressure sensor 101 that detects charge air pressure in
the intake manifold 68, an exhaust manifold pressure sensor 102
that detects exhaust pressure in the exhaust manifold 57, a
supercharger intake pressure sensor 103 that detects charge air
pressure at the intake of the supercharger 60, a supercharger
outlet pressure sensor 104 that detects charge air pressure at the
outlet of the supercharger 60, and, possibly other sensors.
[0038] FUEL LIMITATION: During transient engine operations
occurring when a torque demand is being accepted, a limiting factor
for the engine response is defined by how quickly the air handling
system can transition the airflow through the engine while keeping
the emissions under control. The air handling system generally lags
behind the fuel handling system in meeting a desired operational
setpoint for the torque demand, and so it becomes desirable to
limit the quantity of fuel being injected while the
slower-responding air handling system catches up with the faster
fuel handling system. Further, it becomes useful to relate the
limitation in fuel quantity to an airflow parameter that indicates
the response of the air handling system so as to ensure a smooth
transition of air and fuel to desired setpoints. Such airflow
parameters include mass airflow, lambda, EGR flow, intake manifold
pressure, and others. One useful example of an airflow parameter
that indicates the response of the air handling system is air/fuel
ratio.
[0039] An air/fuel ratio (AFR) is a ratio of the mass of air to the
mass of fuel in a mixture of air and fuel consumed by a combustion
event in a cylinder. When the AFR is stoichiometric, the fuel and
air are fully consumed by combustion, that is to say all of the
fuel is burned and no air remains. When there is insufficient air
to fully combust the fuel (or too much fuel to be burned by the
mass of air available) the mixture is rich and AFR is lower than
stoichiometric. In the exemplary case of a uniflow-scavenged OP2S
engine operated in a diesel mode, a rich mixture can result in
particulate matter (including smoke or soot) being expelled in the
exhaust. Too much air (or too little fuel) produces a high AFR as
compared to the stoichiometric ratio; the mixture is lean, and the
excess air combines with nitrogen in the presence of the heat of
combustion to produce NOx elements in the exhaust.
[0040] When the air-handling system lags behind the fuel-handling
system in meeting the torque demand, one result is a reduction of
AFR for a short period of time during which the fuel and air
handling systems respond to (accept) the demand. Reduction of AFR
may produce undesirable effects in emissions (smoke, hydrocarbons)
or in engine operation (misfires). For example, in diesel
operation, the reduction of AFR can cause a significant spike in
soot emissions. Accordingly, it is desirable to implement a fuel
quantity limitation capability for a uniflow-scavenged OP2S engine
in order to achieve and maintain desired AFR values.
[0041] In a uniflow-scavenged OP2S engine, some of the mass of air
delivered to a cylinder during a cycle of engine operation
("delivered air") flows out of the exhaust port during scavenging
and thus is not available for combustion. Hence, a desirable
measure of AFR for controlling combustion uses the mass of charge
air retained ("trapped") in the cylinder when the last port of the
cylinder is closed. Depending on engine design, either the exhaust
port or the intake port may be the last to close; however, in most
instances, the intake port is the last to close. It is further the
case that, in addition to the trapped charge air, a measurable mass
of residual exhaust gas may sometimes be trapped in the cylinder by
closure of the exhaust port and/or by recirculation into the
cylinder with the charge air. In this case, the total mass of
trapped gas (charge air and exhaust) is referred to as "trapped
charge."
[0042] The uniflow-scavenging process of an OP2S engine is defined
by two attributes: scavenge ratio and scavenging efficiency.
Depending upon the scavenge ratio and scavenging efficiency, the
amount of trapped charge air and delivered air can be significantly
different. Both these attributes impact the trapped conditions in a
cylinder of an OP2S engine. Accordingly it is desirable in some
instances to utilize either or both of a global AFR and a trapped
AFR to initialize fuel quantity limitation in an OP2S engine.
[0043] Global AFR is determined with reference to the total mass of
charge air delivered to a cylinder during one cycle of engine
operation. Delivered charge air mass may be measured using a mass
airflow (MAF) sensor 100 (FIGS. 3 and 4) at the inlet to
compressor, or, possibly, a NOx/0.sub.2 sensor at the exhaust
outlet channel 55. Alternatively, delivered charge air mass may be
obtained from a digital model. On the other hand, trapped AFR is
determined with reference to the total mass of charge trapped
(retained) in a cylinder by the last port of the cylinder to close
during one cycle of engine operation. Trapped charge (trapped
charge air mass plus the mass of any trapped exhaust) can be
measured by a virtual sensor, or estimated in real time on the
basis of trapped conditions in the cylinder. See, for example, the
procedures for determining trapped lambda and trapped burn fraction
that are disclosed in commonly-owned U.S. Pat. No. 9,206,751; US
2014/0373815; and US 2014/0373816.
[0044] With reference to FIG. 5, the ECU 94 may execute a fuel
limiter algorithm 110 that references one or more airflow
parameters in order to limit fuel quantity during transition of
fuel and air handling systems in response to a torque demand. In
this specification a torque demand is embodied by some indication
of a desired engine load, such as a current accelerator position.
Further, depending on a preferred mode of fuel control, a current
accelerator position may be converted by known means to a desired
fuel quantity level or a desired torque level.
[0045] In performing control algorithms for a uniflow-scavenged,
OP2S engine, including the fuel limiter algorithm 110, the ECU 94
makes use of calibration tables which contain calibration data
representative of optimal engine performance that is obtained from
test engines operating in dynamometers. The calibration tables
(also called "maps") are used by the ECU 94 to model the engine in
terms of performance, emissions, and fuel economy. The ECU 94
determines the quantity of fuel required to achieve the desired
torque and the amount of charge air necessary to yield a desired
AFR based on calculations using computer models and calibration
data stored in maps. Using the calibration data, engine speed, mass
airflow, and the required engine load, and, possibly, other engine
operating parameters, the ECU 94 calculates the quantity of fuel to
be provided.
[0046] In performing the fuel limiter algorithm, the ECU 94
utilizes maps that covert engine operating parameter values to
airflow parameter values, maps that convert torque load demands to
desired fuel quantities and desired torque quantities, maps that
convert desired fuel quantities to desired torque quantities, maps
that convert desired torque quantities to desired fuel quantities,
and various other maps. Further, the ECU 94 is enabled to control
fuel provision according to various methods, including a torque
control method and a fuel control method. In this regard, a torque
control method of fuel provision relates engine speed to an
allowable fuel quantity in view of engine horsepower and torque
limits. A fuel control method relates engine speed to an allowable
fuel quantity in view of a desired acceleration.
[0047] Referring to FIG. 5, the ECU 94 executes the fuel limiter
algorithm 110 using an airflow parameter map 111, which may be
embodied as a look-up table accessed by an engine speed signal
(RPM) indicating a current engine speed and a desired engine load
signal (Desired Load), which may comprise, for example, an
accelerator position signal. The map 111 outputs a minimum
allowable value of the airflow parameter for the current engine
speed and accelerator position values. For example, the map 111
outputs a minimum value of AFR for the current values of RPM and
Desired Load. A maximum fuel quantity limit due to the minimum
allowable value of AFR is derived at 112 by division of the
measured airflow value by the minimum allowable airflow value. For
example, the divider 112 outputs a maximum fuel quantity limit due
to a minimum AFR value obtained from the map 111 and a value of
mass airflow obtained by a sensor (physical or virtual), or by
estimation or calculation. Presuming that the ECU 94 is governing
fuel provision according to a fuel control method, the maximum fuel
quantity limit is compared at 113 with a desired fuel quantity
derived from the Desired Load signal, and the minimum of the
maximum and desired fuel quantities is output from the comparator
113 as a Limited Fuel signal indicative of a limited quantity of
fuel to be injected. In most instances, the Limited Fuel signal is
output by the ECU 94 as a Limited Fuel command to the fuel source
40 (FIGS. 2 and 4).
[0048] Presuming that the ECU 94 is governing fuel provision
according to a torque control method, the maximum fuel quantity
limit produced at 112 is converted at 114 to a maximum allowable
torque at the current RPM, and the maximum allowable torque is
compared with a desired torque quantity derived from the Desired
Load signal at 115. The minimum of the maximum and desired torque
quantities is output from the comparator 115 as a Limited Torque
signal indicative of a limited quantity of fuel to be injected in
order not to exceed a torque limit. In most instances, the Limited
Torque signal is output by the ECU 94 as a Limited Torque Fuel
command to the fuel source 40 (FIGS. 2 and 4).
[0049] The fuel limiter algorithm 110 may be constructed and
programmed for either a fuel control method comprising the map 111,
the divider 112, and the comparator 113, or a torque control
method, comprising the map 111, the divider 112, the conversion
114, and the comparator 115. In some instances, as suggested by
FIG. 5, the ECU 94 may be constructed and programmed for both a
fuel control method and a torque control method, in which case, the
fuel limiter algorithm further comprises a control selector 116
programmed to output a Limited Fuel command if the ECU 94 is
implementing fuel control method, or a Limited Torque Fuel command
if the ECU 94 is implementing a torque control method.
[0050] Further, the fuel limiter algorithm 110 may be constructed
for fuel quantity limitation controlled by global AFR or for fuel
quantity limitation controlled by trapped AFR. In the first
instance, the map 111 would contain minimum allowable global AFR
values, which would be combined with delivered air mass values in
the divider 112. In the second instance, the map 111 would contain
minimum allowable trapped AFR values, which would be combined with
trapped charge mass values in the divider 112.
[0051] With reference to FIG. 6, the ECU 94 may execute a fuel
limiter algorithm 120 that references global and trapped airflow
parameters in order to limit fuel quantity during transition of
fuel and air handling systems in response to a torque demand. In
this specification a torque demand is embodied by some indication
of a desired engine load, such as a current accelerator position.
Further, depending on a preferred mode of fuel control, a current
accelerator position may be converted by known means to a desired
fuel quantity level and/or a desired torque level. The ECU 94
executes the fuel limiter algorithm 120 using a trapped airflow
parameter map 121 and a global airflow parameter map 123. The map
121 outputs a minimum allowable value of the trapped AFR parameter
for the current engine speed and desired load values. The map 123
outputs a minimum allowable value of the global AFR parameter for
the current engine speed and desired load values.
[0052] A maximum fuel quantity limit due to the minimum allowable
value of the trapped AFR is derived at 124 by division of the
measured trapped AFR value by the minimum allowable trapped AFR
value. For example, the divider 124 outputs a maximum fuel quantity
limit due to a minimum trapped AFR value obtained from the map 121
and a value of trapped mass airflow obtained by a sensor (physical
or virtual), or by estimation or calculation. A maximum fuel
quantity limit due to the minimum allowable value of the global AFR
is derived at 126 by division of the measured global AFR value by
the minimum allowable global AFR value. For example, the divider
126 outputs a maximum fuel quantity limit due to a minimum global
AFR value obtained from the map 123 and a value of global mass
airflow obtained by a sensor (physical or virtual), or by
estimation or calculation.
[0053] Presuming that the ECU 94 is governing fuel provision
according to a fuel control method, the maximum fuel quantity limit
due to trapped AFR, the maximum fuel quantity limit due to global
AFR, a desired fuel quantity derived from the Desired Load signal
are compared at 130, and the minimum of the three values is output
from the comparator 130 as a Limited Fuel signal indicative of a
limited quantity of fuel to be injected.
[0054] A maximum allowable torque quantity limit due to the minimum
allowable value of the trapped AFR is derived at 127 by conversion
of a maximum fuel quantity limit due to the minimum allowable value
of the trapped AFR output by the divider 124, based upon current
RPM. Similarly, a maximum allowable torque quantity limit due to
the minimum allowable value of the global AFR is derived at 128 by
conversion of a maximum fuel quantity limit due to the minimum
allowable value of the global AFR output by the divider 126, based
upon current RPM.
[0055] Presuming that the ECU 94 is governing fuel provision
according to a torque control method, the maximum torque quantity
limit due to trapped AFR, the maximum torque quantity limit due to
global AFR, a desired torque quantity derived from the Desired Load
signal are compared at 132, and the minimum of the three values is
output from the comparator 132 as a Limited Torque signal
indicative of a limited quantity of fuel to be injected.
[0056] In most instances, the Limited Fuel signal is output by the
ECU 94 as a Limited Fuel command to the fuel source 40 (FIGS. 2 and
4). Presuming that the ECU 94 is governing fuel provision according
to a torque control method, the maximum fuel quantity limit is
converted at 114 with a desired torque quantity derived from the
Desired Load signal, and the minimum of the maximum and desired
torque quantities is output from the comparator 115 as a Limited
Torque signal indicative of a limited quantity of fuel to be
injected in order not to exceed a torque limit. In most instances,
the Limited Torque signal is output by the ECU 94 as a Limited
Torque Fuel command to the fuel source 40 (FIGS. 2 and 4).
[0057] The fuel limiter algorithm 120 may be constructed and
programmed for either a fuel control method comprising the maps 121
and 123, the dividers 124 and 126, and the comparator 130, or a
torque control method, comprising the maps 121 and 123, the
dividers 124 and 126, the convertors 127 and 128, and the
comparator 132. In some instances, as suggested by FIG. 6, the ECU
94 may be constructed and programmed for both a fuel control method
and a torque control method, in which case, the fuel limiter
algorithm further comprises a control selector 140 programmed to
output a Limited Fuel command if the ECU 94 is implementing fuel
control method, or a Limited Torque Fuel command if the ECU 94 is
implementing a torque control method.
[0058] The maps 111, 121, and 123, and other maps used by the ECU
94 (such as fuel maps and torque maps), are multidimensional arrays
that output data from locations indexed by table inputs. A
representative AFR map is illustrated in FIG. 7. The AFR map is
indexed by inputs including engine speed (RPM) and a load command
indicating a desired load; such a load command may comprise, for
example an accelerator position For FIG. 5, maps of minimum
allowable global or trapped AFRs are generated during calibration
on the basis of engine speed and engine load. For FIG. 6,
look-up-tables of minimum allowable global and trapped AFRs are
generated during calibration on the basis of engine speed and
engine load. The ECU 94 then uses measured or estimated global
airflow and trapped airflow values in conjunction with the maps to
generate the maximum fuel quantity (or torque quantity) that should
be allowed to maintain minimum global or trapped AFR values.
[0059] In diesel-powered applications of a uniflow-scavenged, OP2S
engine, if global or trapped AFR goes below a threshold, incomplete
combustion can result thereby generating smoke. A smoke-limiter
process regulates fuel-flow rate into the engine in order to
prevent a global or trapped AFR value from going below a certain
threshold. A trade-off associated with calibrating a smoke-limiter
process aggressively is that although it reduces soot emissions, it
also causes slower torque response to driver's torque request.
[0060] Either of the fuel quantity limiter algorithms 110 and 120
may be appropriately programmed as smoke limiters. In this regard,
a smoke limiter is a routine based on calibration air/fuel ratios
that produce smoke from rich air/fuel mixtures. Thus, in response
to torque demands, either algorithm will determine a maximum
allowable quantity of diesel fuel that can be provided without
exceeding a smoke-limited AFR-a known air/fuel ratio that limits
production of smoke from rich conditions. This quantity is passed
in a Fuel command to the fuel source 40. As a smoke limiter, either
algorithm 110 and 120 will limit smoke produced by the OP2S engine
during response to a torque demand when the airflow requirements in
the engine are not immediately met due to transport delay, turbo
lag, or possibly, other factors. Thus, if global or trapped AFR
goes below a smoke threshold, it can result in incomplete
combustion thereby generating smoke. The smoke-limiter process
regulates fuel-flow rate into the engine in order to prevent global
or trapped AFR value from going below a smoke threshold.
[0061] The contents of priority application U.S. Provisional
Application Ser. No. 62/1,279,301 filed in the United States Patent
and Trademark Office on 15 Jan. 2016, are hereby incorporated by
reference.
[0062] Although control of fuel quantity in a uniflow-scavenged,
two-stroke cycle, opposed-piston engine in response to a torque
demand has been described with reference to presently preferred
embodiments, it should be understood that various modifications can
be made without departing from the scope of the following
claims.
* * * * *