U.S. patent application number 17/530179 was filed with the patent office on 2022-03-10 for engine torque smoothing.
The applicant listed for this patent is Tula Technology, Inc.. Invention is credited to Steven E. CARLSON, Kian EISAZADEH-FAR, Mohammad R. PIRJABERI, Louis J. SERRANO.
Application Number | 20220074361 17/530179 |
Document ID | / |
Family ID | |
Filed Date | 2022-03-10 |
United States Patent
Application |
20220074361 |
Kind Code |
A1 |
PIRJABERI; Mohammad R. ; et
al. |
March 10, 2022 |
ENGINE TORQUE SMOOTHING
Abstract
Methods, devices, estimators, controllers and algorithms are
described for estimating the torque profile of an engine and/or for
controlling torque applied to a powertrain by one or more devices
other than the engine itself to manage the net torque applied by
the engine and other device(s) in manners that reduce undesirable
NVH. The described approaches are particularly well suitable for
use in hybrid vehicles in which the engine is operated in a skip
fire or other dynamic firing level modulation manner--however they
may be used in a variety of other circumstances as well. In some
embodiments, the hybrid vehicle includes a motor/generator that
applies the smoothing torque.
Inventors: |
PIRJABERI; Mohammad R.; (San
Jose, CA) ; EISAZADEH-FAR; Kian; (Berkeley, CA)
; CARLSON; Steven E.; (Oakland, CA) ; SERRANO;
Louis J.; (Los Gatos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tula Technology, Inc. |
San Jose |
CA |
US |
|
|
Appl. No.: |
17/530179 |
Filed: |
November 18, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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17002309 |
Aug 25, 2020 |
11208964 |
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17530179 |
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16278075 |
Feb 16, 2019 |
10787979 |
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17002309 |
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16038622 |
Jul 18, 2018 |
10436133 |
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16278075 |
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15679462 |
Aug 17, 2017 |
10060368 |
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16038622 |
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15340291 |
Nov 1, 2016 |
10221786 |
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15679462 |
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14992779 |
Jan 11, 2016 |
9512794 |
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15340291 |
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62379357 |
Aug 25, 2016 |
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62137539 |
Mar 24, 2015 |
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62102206 |
Jan 12, 2015 |
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International
Class: |
F02D 41/00 20060101
F02D041/00; F02D 41/30 20060101 F02D041/30; F02D 41/14 20060101
F02D041/14 |
Claims
1. A control system configured to control the transition of an
engine between different displacements in a hybrid vehicle having
an internal combustion engine and an additional power source/sink,
the control system comprising: an engine control unit configured to
direct operation of the engine, including directing the transition
of the engine from a first operational displacement to a second
operational displacement that is different than the first
operational displacement; a torque profile estimator configured to
determine an engine torque profile associated with the transition
from the first operational displacement to the second operational
displacement; and an additional power source/sink controller
configured to determine a smoothing torque based at least in part
of the determined engine torque profile and to direct the
additional power source/sink to apply the smoothing torque during
the transition from the first operational firing fraction.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 17/002,309 filed Aug. 25, 2020, which is a Continuation of U.S.
application Ser. No. 16/278,075 filed Feb. 16, 2019 (now U.S. Pat.
No. 10,787,979 issued Sep. 29, 2020), which is a Continuation of
U.S. application Ser. No. 16/038,622, filed on Jul. 18, 2018 (now
U.S. Pat. No. 10,436,133, issued Oct. 8, 2019), which is a
Continuation of U.S. application Ser. No. 15/679,462, filed on Aug.
17, 2017 (now U.S. Pat. No. 10,060,368, issued on Aug. 28, 2018),
which claims priority of U.S. Provisional Patent Application No.
62/379,357, filed Aug. 25, 2016. U.S. application Ser. No.
15/679,462 is also a Continuation-in-Part of U.S. application Ser.
No. 15/340,291, filed on Nov. 1, 2016 (now U.S. Pat. No.
10,221,786, issued Mar. 5, 2019), which is a Continuation of U.S.
application Ser. No. 14/992,779, filed on Jan. 11, 2016 (now U.S.
Pat. No. 9,512,794, issued on Dec. 6, 2016). U.S. application Ser.
No. 14/992,779 claims priority of U.S. Provisional Patent
Application Nos. 62/102,206, filed on Jan. 12, 2015, and
62/137,539, filed on Mar. 24, 2015. Each of these referenced
priority applications are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to hybrid vehicles
powered by internal combustion engines operating under skip-fire
control having another source of power in addition to the internal
combustion engine. The torque profile of the skip-fire controlled
engine is estimated and the additional source of power is used to
smooth the torque profile.
BACKGROUND
[0003] Fuel efficiency of internal combustion engines can be
substantially improved by varying the displacement of the engine.
This allows for the full torque to be available when required, yet
can significantly reduce pumping losses and improve thermal
efficiency by using a smaller displacement when full torque is not
required. The most common method today of implementing a variable
displacement engine is to deactivate a group of cylinders
substantially simultaneously. In this approach the intake and
exhaust valves associated with the deactivated cylinders are kept
closed and no fuel is injected when it is desired to skip a
combustion event. For example, an 8 cylinder variable displacement
engine may deactivate half of the cylinders (i.e. 4 cylinders) so
that it is operating using only the remaining 4 cylinders.
Commercially available variable displacement engines available
today typically support only two or at most three
displacements.
[0004] Another engine control approach that varies the effective
displacement of an engine is referred to as "skip-fire" engine
control. In general, skip-fire engine control contemplates
selectively skipping the firing of certain cylinders during
selected firing opportunities. Thus, a particular cylinder may be
fired during one engine cycle and then may be skipped during the
next engine cycle and then selectively skipped or fired during the
next. In this manner, even finer control of the effective engine
displacement is possible. For example, firing every third cylinder
in a 4 cylinder engine would provide an effective displacement of
1/3.sup.rd of the full engine displacement, which is a fractional
displacement that is not obtainable by simply deactivating a set of
cylinders.
[0005] U.S. Pat. No. 8,131,445 (which is incorporated herein by
reference) teaches a skip-fire operational approach, which allows
any fraction of the cylinders to be fired on average using
individual cylinder deactivation. In other skip-fire approaches a
particular firing sequence or firing density may be selected from a
set of available firing sequences or fractions. In a skip-fire
operational mode the amount of torque delivered generally depends
heavily on the firing density, or fraction of combustion events
that are not skipped. Dynamic skip fire (DSF) control refers to
skip-fire operation where the fire/skip decisions are made in a
dynamic manner, for example, at every firing opportunity, every
engine cycle, or at some other interval.
[0006] In some applications referred to as multi-level skip fire,
individual working cycles that are fired may be purposely operated
at different cylinder outputs levels--that is, using purposefully
different air charge and corresponding fueling levels. By way of
example, U.S. Pat. No. 9,399,964 (which is incorporated herein by
reference) describes some such approaches. The individual cylinder
control concepts used in dynamic skip fire can also be applied to
dynamic multi-charge level engine operation in which all cylinders
are fired, but individual working cycles are purposely operated at
different cylinder output levels. Dynamic skip fire and dynamic
multi-charge level engine operation may collectively be considered
different types of dynamic firing level modulation engine operation
in which the output of each working cycle (e.g., skip/fire,
high/low, skip/high/low, etc.) is dynamically determined during
operation of the engine, typically on an individual cylinder
working cycle by working cycle (firing opportunity by firing
opportunity) basis. It should be appreciated that dynamic firing
level engine operation is different than conventional variable
displacement in which when the engine enters a reduced displacement
operational state, a defined set of cylinders are operated in
generally the same manner until the engine transitions to a
different operational state.
[0007] The combustion process and the firing of cylinders using
skip fire or other firing level modulation techniques can introduce
unwanted noise, vibration and harshness (NVH). For example, the
engine can transfer vibration to the body of the vehicle, where it
may be perceived by vehicle occupants. Sounds may also be
transmitted through the chassis into the vehicle cabin. Under
certain operating conditions, the firing of cylinders generates
undesirable acoustic effects through the exhaust system and
tailpipe. Vehicle occupants may thus experience undesirable NVH
from structurally transmitted vibrations or air transmitted
sounds.
[0008] A challenge with skip fire engine control is obtaining
acceptable NVH performance. While prior approaches work well, there
are continuing efforts to develop new and improved approaches for
managing NVH during firing level modulation operation of an
engine.
SUMMARY
[0009] A variety of methods, devices, estimators, controllers and
algorithms are described for estimating the torque profile of an
engine and/or for controlling torque applied to a powertrain by one
or more devices other than the engine itself to manage the net
torque applied by the engine and other device(s) in manners that
reduce undesirable NVH. The described approaches are particularly
well suitable for use in hybrid vehicles in which the engine is
operated in a skip fire or other dynamic firing level modulation
manner--however they may be used in a variety of other
circumstances as well. In some embodiments, the hybrid vehicle
includes a motor/generator that applies the smoothing torque.
[0010] In some embodiments, periods are identified in which an
instantaneous torque or an instantaneous acceleration produced by
the engine is expected to exceed a designated threshold. A
counteracting torque is then applied to the powertrain in a
controlled manner by an energy source or sink during the identified
periods such that the expected net powertrain torque does not
exceed the designated threshold. In some embodiments, the
designated threshold may vary as a function of engine speed and/or
transmission gear. In some embodiments, the counteracting
(smoothing) torque is applied in short impulses timed to counteract
torque spikes generated during skip fire or dynamic firing level
modulation operation of the engine.
[0011] In some hybrid vehicle embodiments, when an estimated engine
torque profile is determined to provide acceptable NVH, the hybrid
vehicle is operated solely on the output of the internal combustion
engine. However, when the estimated engine torque profile is
determined to provide unacceptable NVH, the both the internal
combustion engine and an auxiliary power source/sink are utilized,
with the auxiliary power source/sink being arranged to provide a
smoothing torque to reduce NVH to an acceptable level.
[0012] In some embodiments, the overall engine torque profile and
the determination of the counteracting smoothing torque is updated
each firing opportunity such that need for and magnitude of the
counteracting smoothing torque is updated for each firing
opportunity.
[0013] In some skip fire or other dynamic firing level modulation
embodiments, the torque profile estimations are used in the
selection of the (effective) operational firing fraction. In such
embodiments, the fuel efficiency of various candidate firing
fractions may be compared after considering the fuel efficiency
implications of any smoothing torques that may be required when
operating at the respective firing fractions to meet desired
drivability criteria.
[0014] In some embodiments, the torque profile for the engine may
be determined by summing the contribution of each of the working
chambers (e.g. cylinders). In some embodiments, the torque profile
for a particular cylinder may be accomplished by selecting or
determining a normalized torque profile for the cylinder's
operational state and then scaling the normalized torque profile
based on current engine operating parameters. During skip fire
engine operation, the normalized torque profile utilized will vary
based on the skip/fire firing decision for that particular
cylinder. In some embodiments, the normalized torque profile will
be based at least in part on intake manifold pressure. In some
embodiments, the normalized torque profile may be scaled based on
one or more current operating parameters such as engine speed,
sparking timing, valve timing/lift, engine firing history, cylinder
firing history, etc.
[0015] In some embodiments, the engine torque profile is filtered
to identify selected harmonic components of the torque profile. A
counteracting smoothing torque to apply to the powertrain may then
be based on the filtered results. In some such embodiments, the
filtered results may be amplified based on one or more current
engine parameters. The filtered signal may be delayed to align with
the torque predicted to be produced by the engine. The amplified
filtered signal may be inverted and used in the control of an
electric motor/generator to source/sink torque based on the
inverted torque signal.
[0016] In some embodiments the smoothing torque may be applied as
one or more oscillating (e.g. sinusoidal) signal, whereas in others
the smoothing torque may be applied as impulses intended to offset
portions of expected torque spikes.
[0017] In various embodiments, the smoothing torque can effectively
be applied by devices that draw energy from the powertrain by
increasing or decreasing their respective loads appropriately.
Similarly the torque applied by devices that add torque to the
powertrain can be increased or decreased to effectively provide the
desired smoothing torque. When devices such as a motor/generator
that can both add and subtract torque are used, either of these
approaches may be used or the devices may be varied between torque
contributing and torque drawing states to provide the desired
smoothing torque.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention and the advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings in which:
[0019] FIG. 1 is a diagrammatic illustration of a representative
hybrid powertrain according to an embodiment of the present
invention.
[0020] FIG. 2 is a diagrammatic illustration of a representative
control architecture for a hybrid powertrain according to an
embodiment of the present invention.
[0021] FIGS. 3A and 3B show a cylinder torque profile versus crank
angle for multiple firings at different MAP values.
[0022] FIGS. 4A and 4B show a normalized torque profile versus
crank angle for a combustion stroke at different MAP values
according to an embodiment of the present invention.
[0023] FIGS. 5A and 5B show a normalized torque profile versus
crank angle for a compression stroke at different MAP values
according to an embodiment of the present invention.
[0024] FIG. 6 shows an exemplary table showing values for the
normalized torque profile for different values of MAP according to
an embodiment of the present invention.
[0025] FIG. 7 shows an exemplary table showing the torque scaling
factor for different values of MAP and engine speed according to an
embodiment of the present invention.
[0026] FIG. 8 shows an exemplary torque profile versus crank angle
at an average engine speed of 1500 rpm and firing fraction of 3/4
for a 4 cylinder engine according to an embodiment of the present
invention.
[0027] FIG. 9 shows the torque profile of FIG. 8 converted into the
time domain according to an embodiment of the present
invention.
[0028] FIG. 10 shows the amount of the torque being added to the
powertrain (positive value) and removed from the powertrain
(negative value) by the second power source/sink of the hybrid
engine according to an embodiment of the present invention.
[0029] FIG. 11 shows a comparison of total power train torque
between internal combustion engine only operation and operation of
the engine in conjunction with a second power source according to
an embodiment of the present invention.
[0030] FIG. 12 is an exemplary schematic flow diagram of a method
to select the most fuel efficient firing sequence according to an
embodiment of the present invention.
[0031] FIG. 13 is an exemplary schematic flow diagram of a harmonic
cancellation method according to an embodiment of the present
invention.
[0032] FIG. 14 shows a timeline illustrating the timing of a
smoothing torque determination for a particular working cycle
relative to the associated working cycle according to an embodiment
of the present invention.
[0033] FIG. 15 shows exemplary filter characteristics according to
an embodiment of the present invention.
[0034] FIG. 16 shows a representative engine torque profile and
resultant filtered signal appropriate for driving an additional
power source/sink according to an embodiment of the present
invention.
[0035] FIG. 17 shows suppression of the first and second order
frequencies according to an embodiment of the present
invention.
[0036] FIGS. 18 A-D show an example of cross fading during a firing
fraction transition according to an embodiment of the present
invention.
[0037] In the drawings, like reference numerals are sometimes used
to designate like structural elements. It should also be
appreciated that the depictions in the figures are diagrammatic and
not to scale.
DETAILED DESCRIPTION
[0038] The present invention describes methods and systems for
reducing NVH and improving fuel efficiency in a hybrid engine using
a skip fire or firing level modulation controlled internal
combustion engine as one source of power. An auxiliary power
source/sink is capable to adding and/or removing torque from the
powertrain in a controlled manner that helps reduce engine
generated NVH.
[0039] Skip fire operation most commonly includes cylinder
deactivation whereby intake and exhaust valves are kept closed
during the nominal gas exchange phases of a 4-stroke engine cycle.
Performing cylinder deactivation requires the engine controller to
control power driver outputs that actuate the cylinder deactivation
elements. For cam operated valves, cylinder deactivation may be
realized by actuating solenoids that operate hydraulic oil control
valves, which allow the valve lifters to either remain rigid (a
fired cylinder) or collapse (a skipped cylinder). Such a system may
be referred to as a "lost-motion" deactivation system. Cylinder
deactivation can be achieved using other mechanisms for cam
operated valves. Alternatively electromechanical actuators may be
used to control the intake and/or the exhaust valves. Independent
of the cylinder deactivation method there is a time lag between
making a fire/no-fire decision and intake valve opening of a firing
cylinder.
[0040] The varying and sometimes irregular firing pattern in a skip
fire controlled internal combustion engine can lead to unacceptable
NVH with some firing patterns. One approach to dealing with such
problems is to not use particular firing fractions or firing
sequences that are known to produce unacceptable NVH levels.
Instead, other firing fractions or firing sequences are used and
the cylinder output is adjusted accordingly (e.g., by adjusting the
manifold absolute pressure, spark advance, etc.) so that the
desired engine output is delivered. These allowed firing fractions
are chosen based on their desirable NVH properties, i.e. the NVH
produced while operating at these firing fractions is acceptable.
Various approaches of this kind are described in co-assigned U.S.
patent application Ser. Nos. 13/654,244 and 14/638,908, which are
incorporated herein in their entirety for all purposes. Co-assigned
U.S. patent application Ser. No. 14/992,779, which is incorporated
herein in its entirety for all purposes, describes some systems and
methods for integrating an additional power source/sink with a
dynamic skip fired controlled engine. Forcing a skip fire engine to
operate at only a limited number of firing fractions reduces the
fuel efficiency gains that can be realized with skip fire control,
since torque control must use other actuators such as spark timing,
MAP, and cam. Use of these other actuators to control torque output
is generally less fuel efficient than control based exclusively on
the firing fraction.
[0041] This application describes various control methods in which
a second power source/sink, in addition to the internal combustion
engine, is operated in a manner that generate a smoothing torque
that is applied to a vehicle powertrain. The smoothing torque is
any torque that is applied to help cancel out or reduce a variation
in torque generated by the internal combustion engine. The
smoothing torque can be generated by any suitable energy
storage/capture/release device. One example would be an electric
motor/generator with a battery and/or capacitor to store and
release energy. Alternatively any system or device that stores and
captures/releases energy mechanically, pneumatically or
hydraulically may be used. For example, a flywheel with a variable
mechanical coupling, or a high pressure fluid reservoir with valves
controlling fluid flow to and from a turbine or similar device may
be used to capture/release energy from a powertrain. The smoothing
torque is applied in a manner such that noise and vibration
generated by the skip fire firing sequence is at least partially
reduced or canceled out.
[0042] FIG. 1 schematically illustrates an exemplary hybrid
electric vehicle powertrain and associated components that can be
used to in conjunction with the present invention. These figures
shows a parallel hybrid electric powertrain configuration however,
it should be appreciated that the same concepts can be applied to
other hybrid powertrains including series hybrid electric
configurations, power-split electric configurations and hydraulic
hybrid configurations, although the largest improvements in fuel
efficiency are expected for the parallel and series electric hybrid
configurations.
[0043] FIG. 1 show a skip fire controlled engine 10 applying torque
to a powertrain drive shaft which is connected to a transmission
12, which in turn drives selected wheels 20 of a vehicle. A
motor/generator 14 is also coupled to the powertrain and is capable
of either simultaneously generating electrical power (thereby
effectively subtracting torque from the drive shaft) or
supplementing the engine torque, depending on whether the engine is
producing surplus torque or deficit torque relative to a desired
powertrain torque output. When the engine produces surplus torque,
the surplus torque causes the motor/generator 14 to generate
electricity which gets stored in the energy storage device 24,
which may be a battery and/or a capacitor, after conditioning by
the power electronics 26. The power electronics 26 may include
circuitry to convert the output voltage on the energy storage
device 24 to a voltage suitable for delivering/receiving power from
the motor/generator 14. When the engine produces deficit torque the
engine torque is supplemented with torque produced by the
motor/generator 14 using energy previously stored in the energy
storage device 24. Use of a capacitor as energy storage device 24
may lead to a larger improvement of the overall fuel economy of the
vehicle, since it largely avoids the energy losses associated with
charging and discharging conventional batteries, which is
particularly advantageous when relatively frequent storage and
retrieval cycles are contemplated as in the current invention.
[0044] FIG. 2 shows a hybrid vehicle control system suitable for
controlling the hybrid vehicle powertrain shown in FIG. 1 according
to a particular embodiment. The vehicle control system 100 includes
an engine control unit (ECU) 130, an internal combustion engine
150, a powertrain 142, and an additional power source/sink 140. The
additional power source/sink may include power electronics, a
motor/generator, and an energy storage device. The ECU 130 receives
an input signal 114 representative of the desired engine output.
The input signal 114 may be treated as a request for a desired
engine output or torque. The signal 114 may be received or derived
from an accelerator pedal position sensor (APP) 163 or other
suitable sources, such as a cruise controller, a torque calculator,
etc. An optional preprocessor 105 may modify the accelerator pedal
signal prior to delivery to the engine controller 130. However, it
should be appreciated that in other implementations, the
accelerator pedal position sensor may communicate directly with the
engine controller 130.
[0045] The ECU 130 may include a firing sequence generator 202, a
torque model module 204, a power train parameter module 206, a
firing control unit 210, and an NVH reduction module 208. These
units and modules communicate with each other and work
cooperatively to control the vehicle. The firing sequence generator
202 determines the sequence of skips and fires of the cylinders of
engine 150. The firing sequence may be generated based on a firing
fraction and an output of a delta-sigma converter or may be
generated in any appropriate manner such as described in U.S. Pat.
Nos. 8,099,224, 9,086,020, and 9,200,587, which are incorporated
herein by reference in their entirety. In operation the firing
sequence generator may investigate the fuel efficiency associated
with various firing sequences and chose the firing sequence that
offers optimal fuel economy while meeting the torque request. In
some cases the powertrain torque may be supplemented or reduced by
the power source/sink 140. The output of the firing sequence
generator is a drive pulse signal 113 that may consist of a bit
stream, in which each 0 indicates a skip and each 1 indicates a
fire for an associated cylinder firing opportunity thereby defining
a firing sequence. The firing decision associated with any firing
opportunity is generated in advance of the firing opportunity to
provide adequate time for the firing control unit 210 to correctly
configure the engine 150, for example, deactivate a cylinder intake
valve on a skipped firing opportunity. The torque model module 204
determines an estimated torque based on the firing sequence and
power train parameters determined by the powertrain parameter
module 206. These power train parameters may include, but are not
limited to, intake manifold absolute pressure (MAP), cam phase
angle, spark timing, exhaust gas recirculation level, and engine
speed. The power train parameter module 206 may direct the firing
control unit 210 to set selected power train parameters
appropriately to ensure that the actual powertrain output
substantially equals the requested output. The firing control unit
210 may also actuate the cylinder firings. The NVH reduction module
208 may use the output of the torque model module 204 to determine
an NVH associated with any particular firing sequence and set of
power train parameters. In certain cases the NVH reduction module
208 may direct additional power source/sink 140 to add or subtract
torque from the powertrain 142. It should be appreciated that the
various modules depicted in FIG. 2 may be combined or configured in
a different manner without impacting the overall functionality of
the vehicle control system 100.
Torque Profile
[0046] In order to determine whether it is necessary to supply a
smoothing torque, and what that smoothing torque should be, it is
advantageous to estimate the overall torque profile of the internal
combustion engine. This estimate must be done in an accurate,
computationally efficient manner so that the engine torque profile
can be predicted in real time. The predicted torque profile may
then be used to determine what, if any, smoothing torque is
required.
[0047] In various approaches, the above smoothing torque may be
applied selectively. That is, many firing fractions and firing
sequences deliver an engine torque profile with acceptable levels
of NVH, and thus the smoothing torque need not be applied in those
circumstances. In other circumstances, a firing fraction or firing
sequence may generate undesirable levels of NVH. In these cases a
smoothing torque may be applied to reduce NVH to an acceptable
level. In other cases a different firing fraction or firing
sequence may be used that has acceptable NVH characteristics. A
smoothing torque may optionally be used with this firing fraction
or sequence. In various embodiments, the smoothing torque system is
arranged to analyze the energy costs of the available options and
select the most fuel efficient approach that also brings NVH to
acceptable levels.
[0048] A single cylinder, normalized torque profile can be used to
model the overall torque profile of a skip fire controlled internal
combustion engine. Normalized profiles for fired and skipped
cylinders may be recorded in a look up table. Tables can be
generated for various levels of intake manifold absolute pressure
(MAP), such as MAP increments of 10 kPa. Intermediate values may be
determined by interpolation from these tables. An estimated torque
profile for each cylinder can then be determined based on scaling
and shifting the normalized torque by factors such as spark and cam
phase angle, which controls the opening and closing times of the
intake and/or exhaust valves. Different normalized profiles can be
used for fired and skipped cylinders. When different firing levels
are used, the different firing levels can be modeled differently by
beginning with different normalized profiles for each different
firing level and/or by scaling and shifting differently based on
different spark and cam settings used. The estimated torque profile
for all engine cylinders can be summed with the appropriate phasing
to obtain the overall engine torque profile. The method described
herein can be used to determine the engine torque with a resolution
of 0.5.degree. of crank angle, although as described below, courser
resolution can often be used to reduce computational time without
significantly impacting model accuracy.
[0049] FIGS. 3A and 3B shows a torque profile associated with two
different MAP values for an engine operating over a range of
speeds. FIG. 3A is for an average MAP of 70 kPa and FIG. 3B is for
an average MAP of 40 kPa. In both cases the vertical scale is
torque and the horizontal scale is engine crank angle. Both graphs
are for a fired cylinder. The figures show the torque profile in
increments of 0.5.degree. of crank angle. The various individual
cycle profiles shown represent a range of engine speeds and cam
angles. Spark timing has been adjusted for optimum fuel efficiency
in all cases.
[0050] FIGS. 3A and 3B depict the cylinder torque profile for a
4-stroke engine. Such an engine completes an engine cycle in
720.degree. of crank rotation. An engine cycle can be divided into
four phases or strokes, intake, compression, combustion (power),
and exhaust. Each stroke extends over 180.degree. of crank angle
rotation. The stroke transitions correspond to successive top dead
center (TDC) and bottom dead center (BDC) piston positions. The
torque here is zero, since the lever arm on the crankshaft is zero
at TDC and BDC.
[0051] Inspection of FIGS. 3A and 3B shows that the maximum torque
generated in the combustion stroke is significantly higher at 70
kPa compared to 30 kPa, since more air and fuel are inducted into
the cylinder at higher MAP valves. Also, the pumping losses,
denoted by the negative torque regions in the intake stroke are
larger at the smaller MAP value. Skip fire engine operation tends
to operate at higher MAP values to minimize these pumping losses
and thereby improve fuel economy.
[0052] The torque profiles at each MAP and cam angle may be
normalized. FIGS. 4A and 4B show such a normalized torque profile
for the combustion stroke of an engine cycle with a 30.degree. cam
angle and for an average MAP of 70 kPa and 40 kPa, respectively. In
these figures the vertical axis is normalized torque and the
horizontal axis is crank angle. An important, unexpected
observation is that by normalizing the torque profiles to the
highest instantaneous torque, all the normalized torque profiles
associated with each firing are substantially identical for all
engine speeds. FIGS. 5A and 5B show such a normalized torque
profile for the compression stroke of an engine cycle for a
30.degree. cam angle and an average MAP of 70 kPa and 40 kPa. In
this figure the vertical axis is normalized torque and the
horizontal axis is crank angle. Again all the individual torque
profiles have substantially identical normalized torque profiles.
Similar normalized profiles can be generated for the intake and
exhaust strokes of a fired cylinder.
[0053] Likewise, similar profiles can be generated for a skipped
cylinder. A skipped cylinder has no power producing combustion or
high temperature exhaust gases. As such, the "intake" and
"combustion" stroke may have generally similar profiles when low
pressure gas springs are used, as are the "compression" and
"exhaust" strokes. The nature of the torque profile during a
skipped firing opportunity will vary depending on valve motion
during the skipped opportunity. A skipped cylinder may be
deactivated, where either one or both the intake and exhaust valves
stay closed during an engine cycle, so that no air is pumped
through the cylinder. If both valves are closed during the cycle,
the hot exhaust gases may be trapped in the cylinder or the hot
exhaust gases may be released prior to closing the valves. These
situations may be referred to as forming a "low pressure" spring
(venting exhaust gases prior to cylinder deactivation) or a "high
pressure" spring (trapping exhaust gases by deactivating the
exhaust valve prior to the exhaust of a prior firing). These cases
will have different torque profiles that can be modeled. In some
cases, a skipped cylinder may not deactivate the valves and may
pump air through the cylinder. Again this case can be modeled as
well. To aid in understanding the current invention the following
graphs and description will assume that a skipped cylinder is
operating in a "low pressure" spring mode, but this is not a
requirement.
[0054] FIG. 6 shows table 400, which illustrates profiles similar
to those shown in FIGS. 4A, 4B, 5A and 5B in a table format. In
table 400 the rows correspond to crank angle and the columns
correspond to different MAP values. The columns are normalized in
the tables so that the profiles associated with each MAP value
cover the same area, although different types of normalizations may
be used. A separate table may be constructed for each engine
stroke, i.e. intake, compression, combustion (power), and exhaust
of an activated cylinder. Likewise a separate table may be
constructed for the two different crankshaft rotations of a skipped
engine cycle, i.e. the intake/compression rotation and the
compression/exhaust rotation. Separate tables for each stroke or
crankshaft rotation are useful, since depending on the engine
operating conditions the scaling factor may be different between
the different strokes in any given engine cycle.
[0055] Since the normalized torque profile associated with any
given firing or skip is known, an estimated torque profile
associated with each firing opportunity can be determined by
scaling the normalized torque profile by the appropriate scaling
factors. FIG. 7 shows a portion of an exemplary table 500 for
scaling a normalized torque profile. The table entries are
proportional to the total torque produced in a stroke for a given
MAP (rows in the table) and average engine speed (columns in the
table). The average engine speed in vehicle applications is known
on a real time basis based on vehicle sensors that monitor the
engine speed. The table shown in FIG. 5 is for a cam phase angle of
30.degree.. Other similar tables may be constructed for other cam
phase angles. In engines using dual cams, different tables may use
different combinations of intake and exhaust valve timing.
[0056] The impact of spark timing on the torque profile may be
handled in different ways. One method would be to construct tables
similar to that of tables 400 and 500 for different values of spark
timing. It is likely that only a table for the combustion stroke
would be necessary, since spark timing will typically have
relatively little impact on the other engine strokes. An
alternative method of handling spark timing would be to generate a
spark timing multiplier, which can be multiplied to the values in
table 500 to adjust for the spark timing. In some embodiments, the
impact of varying cam phase angle may be incorporated in the torque
model by use of a simple multiplier, rather than constructing
alternative tables 400 and 500 for different cam phase angles.
[0057] An alternative method of including spark timing is to
represent actual torque profiles for various spark timings, i.e.
construct a set of tables 400 similar to those shown in FIG. 6, for
sets of cam and spark timing. Then a simple multiplication step
between the normalized torque profile of table 400 (FIG. 6) and
scaling factor of table 500 (FIG. 7) would be all that is required
to generate the actual torque profile.
[0058] Multiplication of the normalized torque profile of table 400
by the appropriate scaling factor of table 500 provides a real time
estimate of the torque profile in degrees of crank angle for any
given cylinder. Once the estimated torque profile associated with
each cylinder has been determined, it is a simple matter to simply
sum the individual cylinder torque profiles. The cylinder profiles
will be offset in crank angle and thus time. For a 4 cylinder, 4
stroke engine the cylinder firings will be offset by 180.degree. of
crank angle. The sum of successive firings and skips associated
with all the cylinders is the engine torque profile. FIG. 8 shows
an example of such an engine torque profile for a four cylinder,
4-stroke engine operating at an average engine speed of 1500 rpm at
a firing fraction of 3/4. The vertical axis is total net torque
from all cylinders and the horizontal axis is crank angle. In this
example the firing pattern repeats every 720.degree.. There are
three engine torque spikes 813 every 720.degree., which are
associated with the three cylinders that fire per engine cycle.
Each of the torque spikes is relatively short in duration. The
skipped firing opportunity shows a torque dip. In this example,
cylinder load is approximately 65% of its maximum value. Often
operating at about 65% of the maximum cylinder load corresponds
minimizing the brake specific fuel consumption (BSFC). Over an
engine cycle the maximum instantaneous delivered torque is more
than 175 N*m, which may yield unacceptable NVH performance. Without
the addition of a smoothing torque, a less fuel efficient firing
fraction may have to be selected to provide the requested
torque.
Scaling Multipliers Based on Firing History
[0059] In some embodiments, one or more additional multipliers
based on the firing history may be used to further scale the
normalized torque profile model to more accurately the delivered
torque. These multipliers can be based on the firing history of the
particular cylinder and/or the firing history of the immediately
preceding engine firing opportunities (the firing sequence). During
skip fire operation of an engine, the amount of torque provided by
any particular firing will vary as a function of both (a) the
firing history of the particular cylinder; and (b) the firing
history of the immediately preceding engine firing opportunities.
Generally, when other things are equal, a particular cylinder that
is fired after it was skipped in its previous working cycle will
generate more torque then when that same cylinder is fired after it
was fired in its previous working cycle. This is due in part due to
differences between the valve actuation schemes between a fired
working cycle that follows a skipped working cycle vs. a fired
working cycle that follows another fired working cycles. More
particularly, when a fired working cycle follows another fired
working cycle, the exhaust valve opening from the previous working
cycle will typically overlap with the intake valve opening in the
following working cycle. This causes a different amount of air to
be introduced to the cylinder as compared to a circumstance in
which the exhaust valve opening does not overlap with the intake
valve opening as typically occurs when a fired working cycle
follows a skipped working cycle in the same cylinder. Another
factor that affects the air charge is the cooling of the cylinder
which allows more air (and correspondingly, fuel) to be introduced
to the cylinder fired. When the cylinder was skipped in its two
previous firing opportunities, even more cooling can occur and the
air charge (and thus the cylinder torque output) may further
increase accordingly. With all other parameters being equal, the
torque output for different firing opportunities of the same
cylinder can vary by more than 10% based on that particular
cylinder's firing history. Typically, the skip/fire status of the
cylinder's immediately preceding working cycle has the most
significant impact on the torque output of a particular cylinder
during a particular working cycle--however, the effects can be seen
based on the skip/fire status of several previous working
cycles.
[0060] Similarly, the overall engine cylinder firing history can
also impact the output of any particular cylinder firing.
Generally, when the previous cylinder in the cylinder firing order
was skipped, it does not have an associated intake event. When no
intake event occurs, the pressure within the intake manifold will
increase somewhat--which causes more air to be introduced when an
intake event occurs for the following cylinder in the cylinder
firing order. The effects of intake events associated with several
preceding cylinders (i.e., the engine firing history) affects the
air charge somewhat like the individual cylinder firing history.
Again, the torque output for different firing opportunities in an
engine cycle can vary by more than 10% based on the then current
engine firing history.
[0061] The effects of either or both the cylinder firing history
and the engine firing history can be accounted for by using
appropriate firing history based multipliers taken from firing
history tables or other appropriate constructs.
[0062] By way of example, the following two tables illustrate one
particular table implementation that accounts for the effects of
the engine firing sequence. The first table illustrates multipliers
that are based on the number of firings that will have occurred
since the last skip. In this example, if the present fired cylinder
is the first firing in the engine firing sequence following a skip,
a torque multiplier of 1.05 is used. If the present fired cylinder
is the second consecutive firing in the engine firing sequence
following a skip, a torque multiplier of 1.01 is used. If the
present fired cylinder is the third consecutive firing in the
engine firing sequence following a skip, a torque multiplier of
0.98 is used. If the present fired cylinder is the fourth
consecutive firing in the engine firing sequence following a skip
or higher, a torque multiplier of 0.96 is used. It should be
appreciated that this table is particularly useful when using
firing fractions of greater than 1/2 where there is an expectation
that the firing sequence generated may include multiple firings in
a row.
TABLE-US-00001 Number of Firings After skip Multiplier 1 1.05 2
1.01 3 .98 4 .96
[0063] A second table can be used to account for the effects of
multiple sequential skips in the firing order immediately before
the present fired cylinder. In this table the number of consecutive
skips that occurred before the present firing is used as the index.
In this example, if the present fired cylinder follows a single
skip in the engine firing sequence, a multiplier of 0.98 is used.
If the present fired cylinder follows two consecutive skips in the
engine firing sequence, a multiplier of 0.99 is used. If the
present fired cylinder follows three consecutive skips in the
engine firing sequence, a multiplier of 1.03 is used. If the
present fired cylinder follows four or more consecutive skips in
the engine firing sequence, a multiplier of 1.04 is used. It should
be appreciated that this table is particularly useful when using
firing fractions of less than 1/2 where there is an expectation
that the firing sequence generated may include multiple skips in a
row.
TABLE-US-00002 Number of Skips Before Firing Multiplier 1 0.98 2
0.99 3 1.03 4 1.04
[0064] The specific multipliers used in the aforementioned engine
firing history tables will vary based on a number of engine related
factors such as the intake manifold dynamics, the nature of the
engine, and the characteristics of the normalized torque
profile.
[0065] Separate tables may be used to determine the appropriate
multiplier to account for the firing history of the cylinder
itself. One such table illustrated below that is suitable for use
when the fired cylinder was skipped in its previous working cycle
utilizes the intake manifold pressure (MAP) and the CAM advance as
its indices. In this example, when the manifold pressure is 50 kPa,
and the cam advance is 0 degrees, a multiplier of 1.0 is used. If
the cam advance is 10 degrees, a multiplier of 1.02 is used. If the
cam advance is 30 degrees, a multiplier of 1.07 is used. If the cam
advance is 60 degrees, a multiplier of 1.10 is used. Suitable
values are provided for other manifold pressures as well. When the
current intake manifold pressure and/or the current cam advance is
between index values in the table, interpolation can be used to
obtain more accurate multipliers.
TABLE-US-00003 CAM Advance (degrees) MAP (kPa) 0 10 30 60 30 1 1.02
1.07 1.10 50 1 1.02 1.07 1.10 70 1 1.02 1.05 1.09 90 1 1.01 1.03
1.04 110 1 1.01 1.02 1.04 50
[0066] Again, the specific multipliers used will vary based on a
variety of engine related characteristics.
Transformations to the Time Domain
[0067] In some implementations, it may be desirable to transform
information available in the crank angle domain to the time domain.
A rough method of transforming a crank angle domain to a time
domain is to simply use the average engine speed. We have:
.DELTA.t.sub.avg=.DELTA.(crankangle)/(average engine speed) (Eq.
1)
For example if the average engine speed is 1500 rpm, then
0.5.degree. of crank angle equals approximately 0.056 msec, and the
crank angle domain can be readily transformed into a time
domain.
[0068] Alternatively a more precise method of transforming crank
angle into time may be used. Most vehicles monitor engine speed in
real time using an engine speed sensor. The sensor typically
measures the time between passage of successive marks on a flywheel
rotating with the engine past a fixed sensor to determine the
engine speed. The mark spacing is typically 6.degree. of crank
angle. Variations in the torque supplied to the powertrain will
cause variations in the engine speed, which can be measured with
the engine speed sensor. For example, the torque spike associated
with a cylinder firing will cause the engine/vehicle to speed up
and a torque dip associated with a skipped firing opportunity will
cause the engine/vehicle to slow down.
[0069] An engine controller can compare recent variations in engine
torque, determined by the previously described torque model, with
recently measured variations in engine speed and establish a
correlation between the two. The controller may then extrapolate
this relationship for the future estimated torque profile to help
transform a crank angle domain into a time domain. It should be
appreciated that the transformation of a crank angle domain into a
time domain is not limited to the previously described methods, but
any suitable method may be used.
[0070] FIG. 9 shows transformation of the torque profile of FIG. 8
into a time domain rather than a crank angle domain. In this figure
the vertical axis is applied torque and the horizontal axis is
time. The variation in the engine speed with applied torque was
included in the transformation to the time base. The total elapsed
time in the figure, 240 msec, corresponds to the same three engine
cycles depicted in FIG. 8. Aside from transforming the horizontal
axis from a crank angle domain to a time domain, FIG. 9 also
depicts a courser resolution model. In this case, the torque
profile was modeled in 6.degree. crank increments rather than the
previously described 0.5.degree. increments. The result is a more
stair step like torque profile. In practice we have found that
6.degree. modeling yields sufficient resolution for engine control
and diagnostic purposes. In some cases, even coarser resolution,
such as 12.degree., 30.degree., or even 60.degree. resolution may
be sufficient. An advantage of using courser resolution is a
reduction in memory and computational demands on the engine control
unit. Note that the overall shape of the torque profile is very
similar whether a crank angle domain (FIG. 8) or time domain (FIG.
9) is used, with only slight changes resulting from the
transformation.
Application of Torque Profile
[0071] Knowledge of the torque profile may be advantageously used
in a number of ways. In particular knowledge of the torque profile
associated with upcoming firing opportunities may be used to
control a smoothing torque applied in parallel to the powertrain to
cancel or partially cancel variations in the overall powertrain
torque. This smoothing torque may be positive (adding torque to the
powertrain) or negative (subtracting torque from the powertrain) or
both. The smoothing torque may be supplied by a motor/generator or
some other means as previously described.
[0072] An engine controller may determine torque profiles for
various firing fractions and firing sequences that deliver the
requested torque. Some of these profiles may require application of
a smoothing torque to provide acceptable NVH characteristics. The
engine controller may then select from this set of firing fractions
or firing sequence that fraction or sequence, which provides the
requested torque with a minimum of fuel consumption. Generally the
selected firing fraction or sequence will provide the required
torque with each cylinder operating at or near its optimum
efficiency.
[0073] A set of torque limit calibration tables may be constructed
for different engine speeds and transmission gears. These tables
compile the maximum allowed instantaneous torque for different
operating conditions. If any point on the torque profile, like that
shown in FIG. 9 exceeds the torque limit value in the calibration
table, then that firing fraction or firing sequence is not allowed,
unless a smoothing torque is applied to the vehicle's powertrain.
For example, if the calibration torque limit 917 corresponding to
an engine speed of 1500 rpm and the vehicle being in third gear is
110 N*m, then the torque profile depicted in FIG. 9 would not be
allowed, since the maximum instantaneous significantly exceeds this
value.
[0074] In addition to, or in place of, a torque limit calibration
table other measures of NVH may be compiled. For example, angular
jerk, the time derivative of torque, may be determined for
different torque profiles. If angular jerk exceeds a certain value
within a defined frequency range, the firing sequence may be not
allowed or a smoothing torque may be added to reduce angular jerk.
In still other embodiments, the limits may be expressed in terms of
a weighted RMS vibration threshold. That is, a weighted RMS average
of the instantaneous torque variations may be determined and that
value may be compared to a maximum permissible weighted RMS
vibration threshold.
[0075] FIG. 10 shows a smoothing torque that may be applied to the
vehicle's powertrain by an additional power source/sink to reduce
the maximum instantaneous torque to the calibration limit. In this
figure the vertical axis is applied torque and the horizontal axis
is time. A positive applied torque represents torque added to the
powertrain and a negative torque represents torque removed from the
powertrain. Inspection of FIG. 10 shows there are periods when
there is no applied torque, periods with a negative applied torque,
and periods of positive applied torque. The three successive
periods of negative torque 1013 in an engine cycle overlap with the
portions three torque spikes 813 corresponding to the cylinder
firings of the internal combustion engine that exceed the torque
limit. The one positive period of applied torque overlaps the
torque trough associated with the skipped cylinder. The profile of
the smoothing torque may be chosen to substantially match the shape
of the torque profiles associated with a firing cylinder. This
results in a more repetitive torque profile, which may be perceived
as having lower NVH.
[0076] It should be appreciated that the portions of the engine
torque spikes 813 that are offset by the negative torque impulses
in the smoothing torque are quite short in duration, with each
impulse corresponding to less than 180 degrees of crankshaft
rotation, and typically less than 90 degrees of crankshaft
rotation.
[0077] The amounts of positive and negative power supplied by the
additional power source/sink can be controlled so that they are
equal, less losses associated with the energy
capture/storage/release system. Control in this manner will result
in the amount of stored energy remaining relatively fixed about
some appropriate level. If more stored energy is desired the amount
of power drawn from the powertrain may be increased and if less
stored energy is desired, the amount of power delivered to the
powertrain may be increased. In some embodiments, the energy
extracted from the powertrain is returned (minus losses) within the
cyclic pattern, which in some cases is within the same engine
cycle. More specifically, the extracted energy is preferably
returned within a period equal to the degrees of crank angle
associated with each firing opportunity (sometimes referred to
herein as the firing opportunity period) times the denominator of
the firing fraction. In an 8 cylinder engine, each firing
opportunity is associated with 90 degrees of crankshaft rotation
(the firing opportunity period); in a 6 cylinder engine, each
firing opportunity is associated with 120 degrees of crankshaft
rotation; and in a 4 cylinder engine, each firing opportunity is
associated with 180 degrees of crankshaft rotation. Thus, for
example, when a firing fraction having a denominator of 5 is used
(e.g., 1/5, , 3/5, 4/5) in an eight cylinder engine, the energy is
preferably returned within 450 degrees of crankshaft rotation
(90*5)--whereas a 4 cylinder engine operating at the same firing
fraction would return its energy within 900 degrees of crankshaft
rotation (180*5). Of course, the actual period in which the energy
will be returned will vary as a function of both the number of
cylinders available and the operational firing fraction.
[0078] FIG. 11 shows a comparison of the powertrain torque profile
between a skip fire controlled engine without a smoothing torque
and a skip fire controlled engine practicing the current invention.
The dashed line depicts the torque profile of the internal
combustion engine alone, without any compensation. This curve is
identical to that shown in FIG. 9. The solid line depicts the
torque profile of the combination of the engine with a
motor/generator that can both add and remove torque from the
powertrain. It is obtained by adding the smoothing torque of FIG.
10 to the internal combustion engine torque profile. Inspection of
FIG. 11 shows that the instantaneous torque profile always remains
below 110 N*m, which was the limit in this example. It should be
appreciated that the torque limit varies with engine speed and
transmission gear ratio and may also depend on other variables,
such as the tip-in or tip-out rate of the accelerator pedal.
[0079] In some embodiments the predicted torque profile may be
determined for a number of future firing opportunities assuming
different firing fractions or firing sequences. The prediction may
extend at least several firings into the future relative to the
current firing opportunity. Preferably the prediction extends far
enough into the future so that the engine controller can
activate/deactivate the engine valves as appropriate for a
fire/skip. This lead time may correspond to 3 to 9 future firing
opportunities depending on the engine speed and valve actuation
mechanism. In some circumstances both longer and shorter prediction
periods may be used. In some embodiments the predicted torque
profile may extend over the period between making a firing decision
and implementing that firing decision.
[0080] The engine controller may determine the NVH and fuel
consumption associated with several of the firing fractions or
firing sequences that deliver the requested torque. For some firing
fraction or firing sequences a smoothing torque may be required to
provide acceptable NVH. The controller may then choose to operate
the engine on the firing fraction or firing sequence, and
optionally smoothing torque, which provides acceptable NVH while
minimizing fuel consumption. In making the decision of the
appropriate firing fraction or firing sequence, the engine
controller may also consider other variables such as the storage
level in the energy storage device associated with the auxiliary
power source/sink that provides the smoothing torque as well as the
conversion efficiency to and from the energy storage device. The
engine controller may use additional knowledge such as whether the
energy in the energy storage device is obtained from the internal
combustion engine or some other power source, such as the electric
power grid in a plug-in hybrid. Use of this invention will allow
operation on previously disallowed firing fractions improving fuel
efficiency.
[0081] FIG. 12 schematically illustrates a method 1200 of
determining the most fuel efficient firing sequence according to an
embodiment of the current invention. In this method one or more
candidate firing sequences may be generated at step 1210 by the
firing sequence generator 202 (FIG. 2) based on the torque request.
The candidate firing sequences may be generated by any known
method, such as those described in U.S. Pat. Nos. 8,099,224,
9,086,020, 9,200,587 and 9,200,575 and U.S. patent application Ser.
Nos. 14/638,908 and 14/704,630, which are incorporated herein by
reference in their entirety. These sequences are input into a
torque model 1220. Also input into the torque model are various
engine parameters, such as spark timing, cam phase angle, engine
speed, MAP, etc. The torque model 1220 determines the torque
profile for these candidate firing sequences at step 1230. An
assessment may then be made at step 1240 whether a smoothing torque
is necessary for the candidate firing sequence to provide an
acceptable NVH level. The vehicle transmission gear setting may be
used in making this assessment. If a smoothing torque is not
required, the flow diagram can proceed to step 1260. If a smoothing
torque is required an assessment is made at step 1250 whether there
is adequate stored energy to supply the smoothing torque. If
insufficient stored energy is available, that candidate firing
sequence cannot be used. If sufficient energy is available, then
the method proceeds to step 1260, where the fuel efficiency of the
evaluated firing sequences are compared and the firing sequence
providing optimum fuel efficiency is selected as the operational
firing sequence. The method then proceeds to step 1270 where the
engine is operate on the selected operational firing sequence. The
method 1200 may be repeated for each firing opportunity to
determine an optimal firing sequence.
[0082] Generating a smoothing torque to compensate for internal
combustion engine torque variations is an application of the
previously described torque model. During engine operation
variables input to the model may include cam angle (controlling
valve timing), MAP, engine speed, spark timing, crank angle, firing
sequence, and firing fraction are known. The torque model can
generate the instantaneous engine torque profile. Knowing the
instantaneous torque at a particular crank angle, an engine
controller may control the smoothing torque needed to be removed
from the powertrain, for example, by a generator, or added into the
powertrain, for example, by an electric motor. The electric
motor/generator may be integrated into a single unit in
communication with an electrical energy storage device, such as a
battery or capacitor.
[0083] In the description of FIGS. 9-11 above, the torque profile
and the smoothing torque are shown in the time domain. It should be
appreciated that in other embodiments the smoothing torque can be
determined and applied in the crank angle domain rather than
converting to the time domain. This can be advantageous in some
applications because the crank angle is always available to the
engine controller. In such embodiments, the drawing of torque may
be directed to starts at "x" degrees and ends at "y" degrees, or
the addition of torque may start at "m" degree and ends at "n"
degrees. As suggested above, the values of "x, y, m, n" might be
arranged as a table and determined according to current RPM.
Transient Conditions
[0084] The preceding description has generally been directed at
selecting the optimum combination of engine firing fraction,
cylinder load, and smoothing torque during operation under
nominally steady-state conditions. While this is important, a
skip-fire controlled vehicle will often be switching between
allowed firing fractions to deliver the required torque. A historic
problem with skip fire, as well as variable displacement, engines
has been unacceptable NVH generated during transitions between the
number of firing cylinders i.e. changes in the firing fraction.
[0085] A smoothing torque may be applied during any transition,
such as the transition associated with changing firing fraction
levels. As described in co-pending U.S. patent application Ser.
Nos. 13/654,248, 14/857,371 and U.S. provisional patent application
62/296,451, which are incorporated herein by reference in their
entirety, transitions between firing fraction levels may be the
source of unacceptable NVH. Use a smoothing torque during those
transitions may shorten the required transition time and reduce the
use of fuel wasting spark retard during the transition.
[0086] One method of handling transient conditions may be referred
to as harmonic cancellation. In this method a theoretically
predicted engine torque profile is sent through a specially
designed FIR (Finite Impulse Response) band-pass filter in the
crank angle domain to extract the DSF frequency components that
causes excessive vibration in real time. The engine torque profile
may be determined using the previously described methods. The
filtered signal can be used to create a smoothing torque via an
electric motor/generator to reduce the overall powertrain torque
variation. The filtering may be accomplished using a set of FIR
filters that may be run in parallel, each extracting a particular
frequency band in the crank angle domain. An advantage of the
harmonic cancellation methods is that the same filter algorithm can
be used for quantifying DSF caused vibration in both steady state
and transient conditions.
[0087] Harmonic cancellation provides a real time target torque
signal in a numerically efficient way that can be used in hybrid
vehicle application for vibration reduction. It may be particularly
applicable to micro-hybrids where the starter motor serves as the
motor/generator and energy storage capacity is limited. This type
of system can handle the relatively small and short duration torque
requirements associated with firing fraction transitions, which
typically last less than two seconds.
[0088] To apply harmonic cancellation, a torque profile may be
determined using the previously described methods or any other
suitable method. For example, once a "Fire" or "Skip" decision is
made on a cylinder by an ECU, a torque waveform is created based on
the engine parameters (such as engine speed, MAP, cam angle, etc.)
in the crank domain. The total torque waveform may be assembled by
combining torque waveforms of all cylinders. The total engine
torque signal may then be directed through a set of FIR filters to
extract the vibration energy (harmonics) caused by DSF operation.
Since lower frequencies tend to have a greater NVH impact, the
filter set may consist of a bandpass filter on the first and second
DSF orders in the crank angle domain. Filtering in the crank angle
domain means that the "frequencies" on the first and second DSF
orders may be fixed with respect to engine speed, so the filter
parameters may not require adjustment with engine speed. The FIR
filters can have a linear phase shift, so that the delays of all
filters are similar. This minimizes distortion in the filtered
signal. The filtered values of the engine torque profile may be
used to help generate a counter or smoothing torque in the crank
angle domain. Phase in and out functions, sometimes referred to as
a cross fading, may be used when switching between filters for
smoothing transitions. Alternatively, filtered signal may to
directed through a secondary filter to minimize discontinuities
during the transient.
[0089] FIG. 13 shows an embodiment of the harmonic cancellation
method. Inputs to the method include various engine parameters,
such as MAP, cam phase angle, engine speed, and spark timing. A
further input to the model is the firing fraction or firing
sequence, which defines the pattern of upcoming skips and fires.
These values are input into an engine torque model as previously
described. The engine speed and firing information may be input
into a filter coefficient determination module. The module
determines the filter coefficients for the various DSF orders of
interest, for example, the first and second order. In some cases
previously used filter coefficients may be used in an upcoming
calculation. The future torque profile and filter coefficients are
input into a filter bank. The filter bank may be a single FIR
filter or may consist of an array of FIR filters, one for each
frequency band of interest. An advantage of using multiple FIR
filters is that it allows application of different phase
compensation to be applied to offset different phase shifts in
generating a physical torque to the powertrain. The filter bank is
configured to calculate an appropriate smoothing torque to cancel
low order torque oscillations in the crank angle domain. The filter
coefficients used in the calculation may be sent to the filter
coefficient determination module for use in a subsequent
calculation.
[0090] The output of the filter bank is directed to a crank angle
to time domain conversion module. This module may use the engine
speed and calculated future torque profile to transform the input
crank domain signal to an output time domain signal. The conversion
may be simply based on average engine speed or may optionally
include calculated speed variations based on the calculated torque
profile. Output of the time domain conversion module may be
directed to the power electronics unit 26 (see FIG. 1) of the
motor/generator. The power electronics unit 26 controls the
motor/generator, which adds or subtracts torque from the powertrain
as specified by the time domain conversion module signal. The
resultant powertrain torque has been smoothed to remove torque
fluctuations that would cause undesirable NVH.
[0091] FIG. 14 illustrates some of the timing constraints required
to successfully practice the methods described in FIG. 13. FIG. 14
shows a time line illustrating decision points, implementation
windows, and engine positions associated with some embodiments of
implementing the methods described in FIG. 13. At point D a
decision is made whether to skip or fire a given cylinder. As
described in co-pending U.S. patent application Ser. No.
14/812,370, which is incorporated herein by reference in its
entirety, that decision is generally made 3 to 9 firing
opportunities in advance of the implementation of that decision.
Generally it is desirable to minimize the lag between making and
implementation of the firing decision to improve engine
responsiveness; however, delays of this magnitude are sufficient
for responsive vehicle control. The start of a working cycle
corresponding to the firing opportunity associated with the point D
decision is denoted as point S on the time line of FIG. 14.
[0092] Once the decision to skip or fire is made, the cylinder
torque profile for that firing opportunity can be determined. In
FIG. 14 the time to calculate that torque profile is illustrated as
window A. The filter bank has a known delay, which is represented
as window B in FIG. 14. This represents the time required for the
engine torque signal to be processed by the filter bank of FIG. 13.
Window C in FIG. 14 represents the time required for conversion of
the filtered signal output by the motor/generator to torque on the
powertrain. As long as the endpoint of window C precedes point S,
the start of the firing opportunity, the approach described in
conjuction with FIG. 13 may be successfully implemented. Window D
in FIG. 14 represents the extra, unallocated time available to
complete the process if that were to become necessary.
[0093] FIG. 15 shows representative filter responses for a variety
of firing fraction denominators for a 4 cylinder, 4-stroke engine.
Columns in FIG. 15 correspond to various firing fractions, n/2,
n/3, n/4, and n/5 where n is an integer greater than zero and less
than the denominator and the numerator and denominator have no
common factors. The first row in FIG. 15 represents the filter
characteristics associated with the first order vibration of the
engine. The horizontal axis on these graphs is a normalized
frequency, expressed in terms of engine order. Here an engine order
of one corresponds to one cylinder firing per engine revolution.
The second row corresponds to the second order engine vibration
frequency. The third row corresponds to the composite frequency
response of the two filters. Inspection of FIG. 15 shows that for
n/2 the first order frequency is at an engine order of 1, i.e. at a
firing fraction of 1/2 in 4 cylinder, 4-stroke engine there is one
firing per engine revolution. For the case of n/3 the first order
vibration is at an engine order of 2/3, for n/4 the first order
vibration is at an engine order of 1/2, and for n/5 the first order
vibration is at engine order of . The second order frequencies are
at twice the frequency of the first order. The sum of the two
frequency responses are the broader peaked curves shown in the
bottom row. In FIG. 15 the shape of the filter coefficients has
been adjusted to so as to provide a substantially constant, linear
phase shift for all filters. While the peak gain is generally near
a harmonic frequency, peak gain need not correspond exactly with
these frequencies. Rather the gain at the harmonic frequency can be
set at a defined value, 1 in the examples shown in FIG. 15 and the
filter characteristics adjusted to provide for a linear phase
response.
[0094] FIG. 16 shows an exemplary resultant filtered signal for a
specific engine operating condition. In this case the engine is
operating with cam phase angle of 40.degree., a speed of 1500 rpm,
a MAP of 50 kPa, and a firing fraction of 2/3. The resultant engine
torque profile under these conditions is shown by curve 1510 in
FIG. 16. As expected curve 1510 shows two torque spikes, associated
with firing cylinders, followed by a torque dip associated with a
skipped cylinder. The red curve 1520 and purple curve 1530 show the
filtered signal for crank angle resolutions of 1.degree. and
30.degree., respectively. The curves are substantially identical,
with at most a 6% difference in the filtered signal value. The
relative insensitivity of the filtered signal to the filter
resolution indicates that accurate results can be obtained even
using course resolution. Use of course resolution dramatically
decreases the computation time required to make the calculations,
for example, determining the filtered signal at a resolution of
1.degree. takes approximately 130 times longer than determining the
resolution at 30.degree.. This allows the calculations to be made
on a real time basis in an ECU or some other vehicle control module
with only modest processing power and speed.
[0095] FIG. 17 shows the resultant suppression of the first and
second order vibrations in the powertrain. In this figure the
horizontal axis is engine order, effectively normalized frequency,
and the vertical axis is the amplitude of the powertrain vibration
at that frequency. The grey curve shows the response without the
addition of any smoothing torque. Inspection of the figure shows
significant vibration at an engine order of 0.5 and 1. The green
curve shows the resultant powertrain vibrations with the addition
of the smoothing torque shown generated by the filtered signal of
FIG. 16. As evident in the figure, the first and second order
oscillations have been almost completely eliminated.
[0096] Transient conditions may be handled using a cross fading
technique as shown in FIGS. 18A-D. FIG. 18A is the filtered output
of one filter bank, denoted as filter A, and FIG. 18B is the
filtered output of a second filter bank, denoted as filter B. The
output of the two filter banks is summed according to a switching
function illustrated in FIG. 18C. FIG. 18D is a sum of the filtered
outputs of filter A and filter B weighted by the switching function
shown in FIG. 18C. While the switching function is shown as linear
in FIG. 18C, this is not a requirement. Use of cross fading allows
the filtered signal to transition seamless during a firing fraction
transition.
[0097] Some of the advantages of the harmonic cancellation method
are that it automatically handles transient conditions. The
filtering can be independent of engine speed and cylinder load. It
is energy efficient since it only damps certain frequency
components, which is especially important in micro-hybrid
application. Phase-in and phase-out methods allow smoothly
switching the filters. Furthermore, the method has a low
computation overhead for determining the filtering and gain
settings and is numerically efficient, both in terms of computation
and memory usage.
Exit from DCCO (Decel Cylinder Cut-Off)
[0098] One particular transient condition that can occur in a skip
fire controlled engine is DCCO (decel cylinder cut-off). Operation
of a dynamic skip fire controlled engine during DCCO has been
described in co-pending U.S. patent application Ser. No.
15/009,533, which is incorporated herein by reference in its
entirety. Use of DCCO improves fuel economy, since the cylinders
are not being fueled during deceleration when no torque is being
requested (e.g. when the accelerator pedal is not depressed). Use
of DCCO further improves fuel economy relative to the more commonly
used (DFCO) (decel fuel cut-off) because the cylinders during DCCO
have been deactivated so that they do not pump air. The pumped air
compromises the oxidation/reduction balance required in a 3-way
catalytic converter, so its use may be limited and/or extra fuel
may be required to restore the catalyst balance.
[0099] One problem with DCCO is that the intake manifold fills with
air during a DCCO event. When torque is again requested the high
MAP may result in high cylinder loads causing a torque surge
leading to unacceptable NVH. Solutions to this problem include
reducing engine efficiency by retarding spark timing and/or
skipping some cylinders without deactivating the valves to help
pump down the intake manifold. Both these solutions have
limitations. Retarding spark reduces fuel economy. Pumping air
through the engine oxidizes the catalytic converter, which may
require additional fuel to restore the oxidation/reduction balance,
again reducing fuel economy.
[0100] During an exit from a DCCO event MAP will generally drop
from atmospheric or near atmospheric pressure to a value
appropriate for delivering the requested torque for example 70 or
80 kPa. The previous described torque model may be used to
determine the engine torque when exiting a DCCO event. In this
case, MAP will be changing over successive engine cycle. MAP
changes can be modeled using methods described in co-pending U.S.
patent application Ser. No. 13/794,157, 62/353,218, and 62/362,177,
which are incorporated herein by reference in their entirety. Other
methods of MAP estimation may be used. As the MAP drops the output
per fired cylinder will generally decrease in a roughly
proportional manner.
[0101] The torque surge may be cancelled or reduce by use of a
smoothing torque. The smoothing torque may be chosen so that the
powertrain torque gradually increases from zero, the value during
DCCO, towards the requested torque level. Unlike some of the
previously described cases the smoothing torque in this case will
not necessarily display a regular cyclic behavior and the smoothing
torque will generally be removing torque from the powertrain during
the transient period associated with exiting a DCCO event. The
energy associated with the removed torque may be stored in an
energy storage device, such as a capacitor or battery, and used to
help power the vehicle at a future time. Application of a smoothing
torque from the additional power source/sink during an exit from a
DCCO event improves fuel efficiency and does not impact the
catalytic converter oxidation/reduction balance.
[0102] More generally this same type of control strategy may be
used whenever there is a firing fraction transition from a low
firing fraction to a higher firing fraction. These transitions have
a tendency to create an engine torque surge, which can be mitigated
by absorbing some or all of the excess torque in an energy storage
device. Similarly, transitions from a high firing fraction to a low
firing fraction may cause a torque dip in engine output. This dip
may be partially or completely filled in using energy from the
energy storage device.
Controlling Accessories to Help Manage Torque
[0103] In most of the examples given above, the smoothing torque is
applied by a bidirectional energy source/sink such as an electric
motor/generator that is capable of both adding torque to the
powertrain and drawing torque from the powertrain, with the excess
energy being stored in a storage device such as a capacitor or
battery. Although electric hybrid vehicles are particularly well
suited for applying the smoothing torque, similar effects can be
obtained in some circumstances in non-hybrid vehicles through the
active control of certain accessories. For example most non-hybrid
automotive engines include an alternator. When generating
electricity, the alternator puts a load on the engine. During
normal driving, the alternator is often configured to generate
electricity to charge the battery. The output of an alternator can
be controlled by controlling the field winding current of the
alternator. Thus, in some embodiments, the output of the alternator
can be varied to load and unload the powertrain in a manner that
effectively applies a smoothing torque to the powertrain.
[0104] When more power is needed from the engine, the alternator
field current can be reduced or removed--which will cause the
output of the alternator to drop thereby reducing the load on the
powertrain which makes more torque available to the drivetrain.
When less power is needed from the engine, alternator can be
commanded to produce more power which provides a higher load on the
engine. Thus, the alternator field current can be modulated in a
manner that varies its load on the powertrain to offset vibration
inducing torque surges. Pulse-width modulated signals are typically
used to drive the alternator field current and can readily be
controlled to produce higher (or lower) alternator output voltages
to charge the battery and momentarily increase (or decrease) the
drag load applied to the powertrain by the alternator. When the
battery charge is already high and more battery charging is not
desirable, devices such as the rear window heater or the front
windshield heater can be turned on to absorb the electrical load.
The use of the alternator in this manner is particularly effective
at handling torque surges in applications such as transitioning out
of DCCO operation back to skip fire operation of an engine.
[0105] Another accessory that can sometimes be used in a similar
manner is an air conditioner in operating circumstances where the
air conditioner is operating. Specifically, since, the precise
output of an air conditioning unit is generally not critical, its
output can be modulated to provide some of the described torque
smoothing functions.
Other Embodiments
[0106] The embodiments described above have primarily been
described in the context of smoothing the torque in conjunction
with skip fire operation of an engine. However, it should be
appreciated that the described techniques are equally applicable in
embodiment that utilize multi-charge level or other types of firing
level modulation engine operation. Furthermore, many of the
described techniques can be used to improve operation during
traditional variable displacement operation of an engine--including
during both transitions between different displacements and during
steady state operation at a particular displacement.
[0107] Another application of the torque model described above
would be engine calibration. Engine calibration is much easier
using this method. A calibrated table based on engine speed and
firing fraction or firing sequence for each gear would indicate
what operating conditions provide acceptable NVH. If the engine
torque excursions exceeded the allowed torque, i.e. the output of
the vibration calibration table, a smoothing torque may be added to
bring the overall torque profile within acceptable levels.
[0108] The invention has been described in conjunction with
specific embodiments, it will be understood that it is not intended
to limit the invention to the described embodiments. On the
contrary, it is intended to cover alternatives, modifications, and
equivalents as may be included within the spirit and scope of the
invention as defined by the appended claims. The present invention
may be practiced without some or all of these specific details. In
addition, well known features may not have been described in detail
to avoid unnecessarily obscuring the invention. For example, there
are many forms of hybrid engines, parallel hybrids, series hybrids,
micro hybrids, mild hybrid, full hybrids depending on the relative
size of the two power sources, the storage capacity of the
auxiliary energy source, and the mechanisms used to store the
auxiliary energy. The invention described herein is applicable to
all these types of hybrid vehicles.
[0109] In accordance with the present invention, the components,
process steps, and/or data structures may be implemented using
various types of operating systems, programming languages,
computing platforms, computer programs, and/or computing devices.
In addition, those of ordinary skill in the art will recognize that
devices such as hardwired devices, field programmable gate arrays
(FPGAs), application specific integrated circuits (ASICs), or the
like, may also be used without departing from the scope and spirit
of the inventive concepts disclosed herein. The present invention
may also be tangibly embodied as a set of computer instructions
stored on a computer readable medium, such as a memory device.
* * * * *