U.S. patent application number 11/138045 was filed with the patent office on 2006-12-28 for control of autoignition timing in a hcci engine.
Invention is credited to Jialin Yang.
Application Number | 20060288966 11/138045 |
Document ID | / |
Family ID | 32654166 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060288966 |
Kind Code |
A1 |
Yang; Jialin |
December 28, 2006 |
Control of autoignition timing in a HCCI engine
Abstract
Method and system embody a valve timing strategy to control the
autoignition timing of a four stroke internal combustion engine
(10) operated in an HCCI mode at different engine operating
conditions such as different engine speed and torque. A particular
valve timing strategy varies lift timing of the intake valve (20)
relative to the exhaust valve (28), or vice versa, and relative to
top dead center in response to a change in engine torque, for
example, to vary amount of trapped residual burned gas in the
combustion chamber (12) flowing to an intake or exhaust port
(16,18) and back to the combustion chamber during which the
residual gas is cooled. Control of the flow of residual gas between
the combustion chamber and intake or exhaust port and thus its
temperature by the valve timing strategy, in turn, is used to
control the temperature of the fresh air/residual gas/fuel mixture
in the combustion chamber (12) and thus autoignition timing in
response to a change in engine torque.
Inventors: |
Yang; Jialin; (Canton,
MI) |
Correspondence
Address: |
ALLEMAN HALL MCCOY RUSSELL & TUTTLE, LLP
806 S.W. BROADWAY, SUITE 600
PORTLAND
OR
97205
US
|
Family ID: |
32654166 |
Appl. No.: |
11/138045 |
Filed: |
August 29, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10248349 |
Jan 13, 2003 |
7093568 |
|
|
11138045 |
Aug 29, 2005 |
|
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Current U.S.
Class: |
123/27R ;
123/90.11 |
Current CPC
Class: |
F02D 13/0265 20130101;
Y02T 10/128 20130101; F02D 13/0207 20130101; F02D 41/3035 20130101;
Y02T 10/12 20130101; Y02T 10/18 20130101; F02D 13/0273 20130101;
F02D 2041/001 20130101; F02D 13/0215 20130101; F02B 1/12
20130101 |
Class at
Publication: |
123/027.00R ;
123/090.11 |
International
Class: |
F02B 1/12 20060101
F02B001/12; F01L 9/04 20060101 F01L009/04 |
Claims
1-19. (canceled)
20. A method for operating a 4-stroke, auto-ignited, internal
combustion engine having a cylinder with an intake valve and an
exhaust valve coupled thereto, comprising: determining an operator
demanded engine torque; adjusting an opening time of the intake
valve; and adjusting an air-fuel ratio supplied to the cylinder
wherein the intake valve opening time and said air-fuel ratio are
adjusted in response to said operator demanded engine torque.
21. The method of claim 20 wherein an exhaust valve closing time is
substantially fixed before top dead center and said intake valve
opening time is varied after top dead center over successive engine
cycles to provide successive intake events that change autoignition
timing.
22. The method of claim 20 wherein said air-fuel ratio is adjusted
over successive engine cycles to provide successive intake events
that change autoignition timing.
23. The method of claim 20 wherein said intake valve opening time
and said air-fuel ratio are adjusted to substantially provide said
operator demanded engine torque.
24. The method of claim 20, further comprising: adjusting an
exhaust valve closing time such that the intake valve and exhaust
valve are open simultaneously wherein said exhaust valve closing
time adjustment causes said air-fuel ratio adjustment.
25. A compression-ignited internal combustion engine, comprising: a
cylinder; an intake valve coupled to said cylinder; an exhaust
valve coupled to said cylinder; an accelerator pedal depression
sensor; and an electronic control unit electronically coupled to
said intake valve, said exhaust valve, and said pedal sensor, said
electronic control unit determining an operator demanded engine
torque based on a signal from said pedal sensor, said electronic
control unit adjusting both an opening time of said intake valve
and a closing time of said exhaust valve to cause the engine to
provide substantially said operator demanded engine torque.
26. The engine of claim 25 wherein said exhaust valve dosing time
adjustment affects an air-fuel ratio in said cylinder.
27. The engine of claim 26 wherein said intake valve closing time
adjustment affects an amount of backflow through said intake valve
from said cylinder to an engine intake thereby affecting
temperature of the gases in said cylinder and said autoignition
timing.
28. The engine of claim 25 wherein said adjustments are made over
successive engine cycles.
29. The engine of claim 25 wherein said electronic control unit
commands said intake valve to open an additional time said
additional time occurring prior to top dead center and said
adjusted valve opening occurring after top dead center.
30. The engine of claim 25 wherein said electronic control unit
commands said exhaust valve to open an additional time said
additional time occurring after top dead center and said exhaust
valve closing occurring prior to top dead center.
31. A computer readable storage medium having stored data
representing instructions executable by a computer for controlling
an internal combustion engine having a cylinder with an intake
valve and an exhaust valve coupled thereto comprising: instructions
to determine an operator demanded engine torque; instructions to
adjust an opening time of the intake valve; and instructions to
adjust a closing time of the exhaust valve wherein said intake
valve opening time and said exhaust closing time are adjusted in
response to said operator demanded engine torque.
32. The media of claim 31, further comprising: instructions to
determine a desired autoignition time to provide said operator
demanded engine torque wherein said opening time of the intake
valve affects said autoignition time.
33. The media of claim 31, further comprising: instructions to
cause said adjustments in intake valve opening time and exhaust
valve closing time to be made over successive engine cycles.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to methods and systems for
controlling autoignition timing of an internal combustion engine
operated in a homogeneous-charge compression-ignition mode.
[0003] 2. Background Information
[0004] A conventional gasoline-fueled internal combustion engine
employs spark ignition where the fuel and air am premixed and a
spark initiates a flame that propagates through the fuel/air
mixture in the combustion chamber. The other common type of
internal combustion engine employs compression ignition where the
fuel and air are purposely kept separate until shortly before top
dead center in the engine when the temperature of the air in the
combustion chamber is high due to the compression. The fuel then is
quickly injected into the combustion chamber as a very fine mist,
which partially mixes with the air and autoignites in the
combustion chamber. The timing of the fuel injection timing thus
controls the autoignition timing. Diesel engines are illustrative
of this type of compression ignition engine.
[0005] Homogeneous-charge compression-ignition (HCCI) internal
combustion engines are known and offer the potential to reduce fuel
consumption and NO.sub.x emissions. An HCCI engine employs a
premixed fuel/air charge to the combustion chamber as in a spark
ignition engine, while the charge is ignited by compression
ignition as in a diesel engine when the temperature of the air-fuel
charge reaches an autoignition temperature in the combustion
chamber. HCCI engines typically are provided with a conventional
spark plug for each cylinder and relatively low compression ratios,
typically close to those of spark ignition (SI) engines, to permit
switching of operation of the engine from the HCCI mode at lower
engine torques to the S1 mode at higher engine torques without
engine knocking.
[0006] Control of autoignition timing in an HCCI engine is more
difficult than in a diesel engine, which controls fuel injection
timing to control autoignition timing. In an HCCI engine, the
composition and temperature of the fuel-gas mixture in the
combustion chamber must be controlled to control autoignition
timing.
[0007] It has been proposed to control HCCI autoignition timing
using what has been called a negative valve overlap strategy that
provides internal exhaust gas recirculation in the combustion
chamber. Negative valve overlap control strategy involves trapping
hot residual burned gas in the cylinder to subsequently mix with
fresh air inducted into the combustion chamber. The trapped burned
gas raises the temperature of the air-burned gas mixture to promote
autoignition. Autoignition timing (delay) is represented by the
equation: t=A exp(E/RT), where t is the time it takes for the
mixture in the combustion chamber to autoignite, often called the
ignition delay, A is an empirical constant, E is an activation
energy and is a function of the composition of the mixture, such as
type of fuel, fuel/air mixture amount of residuals, etc., and R is
the universal gas constant. Because the equation expresses an
exponential relationship, it is evident that temperature of the
mixture plays a key role in determining if and importantly when
autoignition will occur.
[0008] Pursuant to negative valve overlap control strategy, the
exhaust valve doses before top dead center (TDC) and the intake
valve opens after TDC such that both valves are closed at TDC of
the exhaust stroke. Such strategy controls trapping of hot residual
burned gas in the combustion chamber to, in turn control the
autoignition timing. FIG. 5 shows a plurality of intake and exhaust
valve lift curves versus crank angle for an HCCI engine for
purposes of illustrating the negative valve overlap strategy where
different negative valve overlaps are shown for use at different
engine torques. In particular, for different engine torques,
different pairs of intake and exhaust valve lift curves (e.g.,
curves 1I, 1E; 2I, 2E; 3I, 3E: and so on) are employed in
coordination with one another to provide the desired negative
overlap for a particular engine torque. That is, intake and exhaust
valve lift curves 1I, 1E would be used in coordination for a
particular engine torque, different intake and exhaust valve lift
curves 2I. 2E would be used in coordination for a different
particular engine torque, and so on. The negative valve overlap
control strategy is described by Willard et al. in "The knocking
syndrome--its cure and its potential". SAE 982483, 1998.
[0009] When engine speed or torque changes, the autoignition timing
of the HCCI engine tends to change. For example, at higher torque
autoignition timing tends to advance, resulting in the increase in
hear transfer losses, NO.sub.x emissions, and combustion noise.
Therefore, the engine control system should adjust to move the
autoignition timing back to the optimum crank angle. At lower
engine torque, autoignition timing tends to be retarded resulting
in an increase of CO emissions and lower combustion efficiency. The
engine control system should adjust to move the autoignition timing
back to the optimum crank angle.
[0010] Moreover, it is desirable to operate the engine with a
stoichiometric air-fuel mixture and with a conventional three-way
catalyst for after-treatment of exhaust gases. Control of the mass
of trapped hot residual burned gas in the cylinder can provide
control of autoignition timing during HCCI engine operation. There
is a need to also control air-fuel ratio to provide a
stoichiometric mixture for engine operation over a wide range of
climate and weather conditions without altering the autoignition
timing.
[0011] However, use of negative valve overlap as a single control
variable in HCCI engine control strategy to control both the
autoignition timing and the air-fuel ratio at different operating
conditions is problematic in that use of a single negative valve
overlap variable in the control strategy offers insufficient
degrees of freedom to control the air-fuel ratio, in-cylinder gas
temperature, and residual fraction of burned gas in the in-cylinder
gas in a manner to provide favorable values for all of these
parameters at different operating conditions.
SUMMARY OF INVENTION
[0012] The present invention provides a method and system embodying
a particular valve timing strategy to control the autoignition
timing of a four stroke internal combustion engine operated in the
HCCI mode at different engine operating conditions such as at
different operator (driver) demanded engine torques. A-particular
valve timing strategy varies lift timing of the intake valve
relative to the exhaust valve, or vice versa, and relative to top
dead center in response to a change in operator demanded engine
torque, for example, to vary amount of trapped residual burned gas
in the combustion chamber flowing to an intake or exhaust port and
back to the combustion chamber by which the residual gas loses
thermal energy and is cooled. Such control of the flow of residual
burned gas between the combustion chamber and intake or exhaust
port and thus its temperature by the valve timing strategy is used
to control the temperature of the fresh air/residual burned gas
mixture in the combustion chamber into which fuel is mixed and thus
the autoignition timing to suit a given engine torque demand.
[0013] In an illustrative embodiment of the invention, the exhaust
valve timing is substantially fixed before TDC over successive
engine cycles to control the air-fuel ratio in the combustion
chamber. The opening time of the intake valve is varied relative to
TDC (e.g., advanced toward TDC) over successive intake cycles in a
manner that changes the temperature of the fresh air/residual
burned gas mixture in the combustion chamber into which the fuel is
mixed and thus the autoignition timing. The exhaust valve timing
and/or the fuel injection pulse width can be adjusted slightly to
compensate for the effect of the temperature change of the mixture
on the mass of the inducted fresh air in the combustion chamber.
Further, for each intake event, an initial intake valve opening
event preferably is provided immediately after the exhaust valve
closes and before TDC followed by a main intake valve event
occurring after TDC in a manner to reduce or minimize engine
pumping losses.
[0014] In another illustrative embodiment of the invention, the
intake valve lift timing is substantially fixed after TDC over
successive engine cycles to control the air-fuel ratio in the
combustion chamber. The closing time of the exhaust valve is vaned
relative to TDC (e.g., retarded toward TDC) over successive exhaust
cycles in a manner that changes the temperature of the fresh
air/residual burned gas mixture in the combustion chamber into
which fuel is mixed and thus the autoignition timing. The intake
valve timing and/or the fuel injection pulse width can be adjusted
as needed in order to compensate for the effect of the temperature
change of the mixture on the mass of the inducted fresh air in the
combustion chamber. For each exhaust event, a first main exhaust
valve opening event preferably is provided before TDC followed by a
subsequent secondary exhaust valve event occurring after TDC
immediately before opening of the intake valve in a manner to
reduce or minimize engine pumping losses.
[0015] The above advantages of the present invention will become
more readily apparent from the following description taken with the
following drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a schematic view of an internal combustion engine
and an electronic engine control unit for practicing an embodiment
of the invention.
[0017] FIG. 2 is diagram illustrating intake and exhaust valve
lift-curves versus crank angle (where TDC is bottom dead center and
TDC is top dead center) at a given engine speed and torque for an
embodiment pursuant to the invention.
[0018] FIG. 3 is diagram illustrating intake and exhaust valve lift
curves versus crank angle at a given engine speed and torque for
another embodiment pursuant to the invention having double intake
valve events.
[0019] FIG. 4 is diagram illustrating intake and exhaust valve lift
curves versus crank angle at a given engine speed and torque for
another embodiment pursuant to the invention having double exhaust
valve events.
[0020] FIG. 5 is a diagram illustrating conventional coordinated
intake and exhaust valve lift curves versus crank angle (where BDC
is bottom dead center and TDC is top dead center) of an HCCI engine
at different engine torques to provide different negative valve
overlaps wherein intake and exhaust lift curves 1I, 1E are employed
at a given torque: curves 2I, 2E are employed at a different
torque: and so on.
DESCRIPTION OF THE INVENTION
[0021] Referring to FIG. 1, a four cycle internal combustion engine
10 is illustrated as comprising a combustion chamber 12 formed by a
conventional cylinder head 13, cylinder 14, and piston 15. The
combustion chamber 12 is expanded and contracted by the piston 15
reciprocating in the engine cylinder 14. An intake port 16 and
exhaust port 18 of the engine 10 communicate with the combustion
chamber 12 in conventional manner. An intake valve 20 is provided
in the intake port 16. An intake passage 22 of the engine
communicates with the intake port 16. Air is aspirated from the
intake passage 22 through the intake port 16 into the combustion
chamber 12 when the intake valve 20 is open due to the piston
descending in the cylinder. A throttle 23 is provided in intake
passage 22 for adjusting the intake air flow rate of the engine in
a spark ignition (S) mode. In HCCI mode, the throttle 23 is
preferably fully open as shown in FIG. 1. A conventional fuel
injector 24 and spark plug 26 are provided on the cylinder head so
as to communicate with the combustion chamber 12. Fuel injected
into the combustion chamber 12 by fuel injector 24 is mixed with
fresh air aspirated from the intake port 16 and some fraction of
residual burned gas in the SI mode of engine operation. In the HCCI
mode, fuel injected into the combustion chamber 12 is mixed with a
fresh air-residual burned gas mixture having a much higher fraction
of residual burned gas for subsequent compression in combustion
chamber 12 by the piston 15. Alternately, the fuel injector 24 can
be mounted in the intake port in the same manner as a
port-fuel-injection engine.
[0022] An exhaust valve 28 is provided in the exhaust port 18.
Burned gas is discharged from the exhaust port 18 through an
exhaust passage 30 when the exhaust valve 28 is open during the
exhaust stroke.
[0023] Variable valve timing mechanisms 32, 34 are provided on the
engine to change the open/close timing of the intake valve 20 and
exhaust valve 28, respectively. The variable valve timing
mechanisms 32, 34 each can comprise a plural cam-type mechanism, a
solenoid-actuated mechanism, and other valve timing mechanisms
known in the art for adjusting the open/close timing of intake and
exhaust valves of internal combustion engines. U.S. Pat. No.
6,295,964 describes a particular variable valve timing mechanism
for an internal combustion engine.
[0024] Although only one combustion chamber 12 and cylinder 14 with
piston 15 therein are shown in FIG. 1, those skilled in the art
will appreciate that the engine 10 typically will include other
similar combustion chambers/cylinders/pistons and associated intake
valves, exhaust valves, fuel injectors, and spark plugs as shown in
FIG. 1. Further, more than one intake valve 20 and more than one
exhaust valve 28 can be provided for each combustion chamber 12, in
addition, although the fuel injector 24 is illustrated as injecting
fuel directly into cylinder 15, the invention alternately can be
practiced using fuel injection into the intake port 16.
[0025] An electronic control unit (ECU) 40 is provided to control
the fuel injection amount and injection timing, the spark timing of
the spark plug 26, the opening of throttle 23, the open/close
timing of the intake valve 20 and exhaust valve 28 by variable
valve timing mechanisms 32, 34. The ECU 40 comprises a
microcomputer including a central processing unit, read-only
memory, a random access memory, and a keep-alive memory, which
retains information when the engine ignition key is turned-off for
use when the engine is restarted, and an input/output interface.
The ECU 40 can be embodied by an electronically programmable
microprocessor, a microcontroller, an application-specific
integrated circuit, or a like device to provide a predetermined
engine control logic.
[0026] The ECU 40 receives a plurality of signals from the engine
10 via the input/output interface. Such signals can include, but
are not limited to signals from an air flow meter 42 which detects
intake air flow rate in the intake passage 22, a crank angle sensor
44 which detects crank angle of the engine 10, an accelerator pedal
depression sensor 45 which detects the amount of depression of the
accelerator pedal, and a starter switch 46 which detects star-up of
the engine 10.
[0027] The ECU 40 processes these signals received from the engine
sensors and generates corresponding signals, such as a fuel
injector pulse waveform signal that is transmitted to each fuel
injector 24 of each cylinder 15 on a signal line to control the
amount and timing of fuel delivered by each fuel injector 24 to
combustion chamber 12. ECU 40 provides corresponding signals to
control the spark timing of each spark plug 26, the opening of
throttle 23, and the open/dose timing of each intake valve 20 and
exhaust valve 28 by each variable valve timing mechanisms 32,
34.
[0028] Referring to FIG. 1, the ECU 40 includes a combustion
pattern selecting section SO implemented by a software program or
programs for selecting a particular combustion mode; namely, a
spark ignition mode 52 or a HCCI (compression autoignition) mode
54, depending on engine operating conditions. For example, ECU 40
can select a combustion mode based on an engine speed signal from
crank angle sensor 44 and on an accelerator pedal position
(indicative of a operator demand for engine torque) signal from
accelerator pedal depression sensor 45. ECU 40 typically selects
the compression autoignition engine operating mode 54 in a
predetermined engine operating region characterized by relatively
low engine speed and low to medium engine torque, and selects the
spark ignition mode in a very low engine torque region and in a
region of high engine speed and/or high engine torque. When the
compression autoignition mode 54 is selected. ECU 40 can deactivate
the spark plug 26 or alternatively continue sparking of the spark
plug 26.
[0029] The present invention provides a method and system using a
particular valve lift timing strategy to control the autoignition
timing and the air-fuel ratio during engine operation in the HCCI
mode 54. A particular valve timing strategy pursuant to the present
invention controls lift timing of one of the intake valve relative
to the exhaust valve, or vice versa, and relative to top dead
center to control autoignition timing at a given fixed engine speed
and operator demanded engine torque. The air-fuel ratio also is
controlled at the given fixed engine speed and torque. As is known,
the piston 15 generates maximum compression of gases in combustion
chamber 12 at TDC, the top of its stroke. Before TDC, the piston 15
moves toward combustion chamber 12, and, after TDC, the piston 15
is moving away from the combustion chamber 12.
[0030] FIG. 2 shows an illustrative embodiment of the present
invention where the air-fuel ratio is controlled by controlling the
mass of trapped residual burned gas in the combustion chamber 12
that mixes with inducted fresh air at the time before the
compression stroke of engine 10 when the engine is operated at a
fixed geometric compression ratio (e.g., in the range of 10:1 to
15:1). In FIG. 2, the exhaust valve lift (represented by curve EV)
from its opening time EVO to its dosing time EVC is plotted versus
crank angle of the engine 10. As shown, the exhaust valve opening
and closing times under fixed operating conditions of engine speed
and torque are substantially fixed or constant relative to TDC for
each exhaust stroke. With fixed exhaust valve opening time and
closing time, the amount of residual burned gas that does not flow
into the exhaust port 18 is, in turn, fixed regardless of the
intake valve timing. Thus, at a fixed intake (in-cylinder)
pressure, the mass of fresh intake air inducted into the combustion
chamber 12 is substantially fixed such that the air-fuel ratio can
be controlled. At a given engine speed, the exhaust valve timing is
used to control air-fuel ratio in combustion chamber 12, which in
turn provides the operator (driver) demanded engine torque. As used
above, substantially fixed fresh air mass means that there is at
most a minor change in the mass of fresh air inducted into the
combustion chamber 12 as a result of the temperature change of the
burned gas with which the air is mixed in the combustion chamber 12
as described below. This minor change in fresh air mass can be
accommodated as also described below.
[0031] FIG. 2 illustrates varying (e.g., advancing) intake valve
opening (IVO) of the intake valve 20 after the exhaust valve 28
closes as indicated by valve lift curves 1, 2, 3, 4, 5, 6 over
successive intake events. Such varying (e.g., advancement) of
intake valve opening time gradually changes (e.g., reduces) the
temperature of the fresh air-residual burned gas mixture into which
fuel is mixed in the combustion chamber 12 and thus the
autoignition timing before compression. The autoignition timing can
be changed in response to changes in operator demanded engine
torque using such valve timing. intake valve lift curves IV,
numbered 1 through 6, illustrate intake valve lifts from IVO to
intake valve closing IVC time of this embodiment of the invention.
Curve EV together with curve 0 represent a negative valve overlap
condition where none of the trapped residual burned gas flows out
of the combustion chamber 12 such that the air/residual burned gas
mixture will have the highest mixture temperature at a time before
the compression.
[0032] In effect, varying (e.g., advancing) of the time of opening
of the intake valve 20 as indicated by valve lift curves 1, 2, 3,
4, 3, 6 over successive intake events gradually increases the
intake time period so as to permit more and more trapped residual
burned gas to be pushed out or from the combustion chamber 12 into
the intake port 16 after the exhaust valve 28 doses and then to
flow back to the combustion chamber when the intake valve opens and
the piston descends. That is, a greater and greater portion of the
original hot trapped residual burned gas is caused to flow (by
higher cylinder pressure generated by compression in the exhaust
stroke after the exhaust valve doses) into the intake port 16 as
permitted by advanced opening of intake valve 20 and then dawn by
the intake stroke from the intake port 16 back into the combustion
chamber 12. Transmission of the residual burned gas between the
combustion chamber and the intake port in this manner reduces
thermal energy of the residual burned gas by heat transfer to
adjacent intake port walls without reducing the mass of the
residual-burned gas in the combustion chamber 12. Such transmission
is effective to control the mass ratio of original hot trapped
residual burned gas to the cooler recycled burned gas so as to
gradually decrease (or increase) the temperature of the fresh
air/residual burned gas mixture into which fuel is mixed in the
combustion chamber before compression. Autoignition timing thereby
can be controllably changed by gradually changing the intake valve
opening time over successive engine cycles (one engine cycle equals
four strokes or two revolutions) relative to exhaust valve timing
in response to changes in operator demanded engine torque.
Typically, autoignition timing is controlled to occur near TDC such
as, for example, the time of 50% completion of combustion occurs
within a range of 5 to 10 degrees after TDC.
[0033] When the temperature of the residual burned gas in the
combustion chamber 12 is changed, the mass of the fresh intake air
inducted into the combustion chamber and mixed with the residual
burned gas mixture will also be accordingly changed by a minor
amount despite the intake (incylinder) pressure and geometric
compression ratio of the engine remaining unchanged. The invention
envisions ECU 40 slightly adjusting the exhaust valve closing time
and/or the fuel injection pulse width during the period that the
intake valve opening timing is being changed as may be needed in
order to compensate for this effect of temperature change of the
residual burned gas mixture on the mass of the fresh air inducted
into the combustion chamber 12. For example, ECU 40 can move the
exhaust valve closing time closer to TDC during the period when the
intake valve opening timing is changed to increase the amount of
hot trapped residual burned gas exhausted from the combustion
chamber 12 and thereby increase the mass of inducted fresh air.
[0034] According to this embodiment of the invention, at any fixed
engine speed, the air-fuel ratio in combustion chamber 12 can be
controlled to the stoichiometric proportion by ECU 40 determining
engine torque and controlling the exhaust valve opening time and
closing time as described above in response to the determined
engine torque. The autoignition timing is adjusted by ECU 40 by
gradually changing the intake valve opening time as illustrated,
for example, in FIG. 2 by curves 1 through 6 over successive intake
events.
[0035] FIG. 3 illustrates another similar valve timing strategy
that minimizes or eliminates engine pumping losses while
controlling autoignition timing and air-fuel ratio.
[0036] The valve timing strategy of FIG. 3 is similar to that of
FIG. 2 with, however, the inclusion of an additional initial intake
event IVZ before TDC Similar to FIG. 2, at a fixed engine speed,
the air-fuel ratio in combustion chamber 12 can be controlled to
the stoichiometric proportion by ECU 40 determining engine torque
and controlling the exhaust valve timing as described above in
response to the demanded engine torque. Control of autoignition is
achieved by advancing the intake valve opening time IVO as
illustrated by curves 1, 2, 3 in FIG. 3 relative to TDC. To avoid
engine pumping losses, the additional intake event IVZ is provided
immediately after the exhaust valve 28 doses in the exhaust stroke
as shown in FIG. 3 to allow some residual burned gas to be pushed
into the intake port 16 due to continued upward movement of the
piston 14 in the exhaust stroke. The intake valve closing time IVC
of the intake event IV2 occurring before TDC is varied depending on
the amount of advancement of the intake opening time of main intake
event IV occurring after TDC That is, curve 1' of the additional
intake event would be employed when curve 1 represents the main
intake event occurring after TDC curve 2' of the additional intake
event would be employed when curve 2 represents the main intake
event occurring after TDC, and so on. As is apparent from FIG. 3,
the crank angle from the end of the additional. Initial intake
event IVZ (curve 1', 2, or 37 to TDC and the crank angle from TDC
to the beginning of the subsequent main intake event (curve 1, 2,
or 3) should be essentially equal to minimize engine pumping
losses.
[0037] In the embodiments of FIGS. 2 and 3, the fuel injection
timing is controlled by ECU 40 to occur typically after TDC since
after TDC, the gases flow into the combustion chamber due to the
downward movement of the piston. If an engine uses in-cylinder
(direct fuel injection, the fuel injection timing as controlled by
ECU 40 can play a role in control of the mixture temperature, hence
the autoignition timing, due to the charge cooling effect of fuel
evaporation. In general, later fuel injection results in lower
mixture temperature before compression. That is, the charge before
fuel injection (i.e., without charge cooling by fuel evaporation)
is hotter, increasing heat transfer from the hot burned gas to the
port walls. The fuel injection timing is constrained by the
requirement of fuel-air mixing. Fuel droplets need time to vaporize
and mix with the air.
[0038] FIG. 4 illustrates another embodiment of the invention where
the intake valve opening time IVO is controlled in a manner to
control the air-fuel ratio in combustion chamber 12 and the closing
time EVC of the exhaust valve 28 is varied relative to TDC (e.g.,
retarded toward TDC) over successive exhaust cycles in a manner
that changes the temperature of the air/residual burned gas mixture
into which fuel is mixed in the combustion chamber 12 and thus the
autoignition timing.
[0039] For example, FIG. 4 illustrates an embodiment of the present
invention where the intake air mass is controlled by the intake
valve opening time and dosing time so long as in-cylinder pressure
at the time of intake valve opening is fixed. As shown the intake
valve opening and closing times IVO, IVC under fixed operating
conditions of engine speed and torque are substantially fixed or
constant relative to TDC for each intake stroke. At a fixed engine
speed, the air-fuel ratio in combustion chamber 12 can be
controlled to the stoichiometric proportion by ECU 40 determining
engine torque and controlling the intake valve opening time in
response to the determined engine torque.
[0040] The exhaust valve lift timing is used to control the
temperature of the fresh air-residual burned gas mixture in the
combustion chamber 12 and thus the autoignition temperature before
compression. When the exhaust valve dosing times are retarded over
successive exhaust strokes relative to TDC as represented by curves
1, 2, 3 of the initial exhaust event EV of FIG. 4, more and more
hot trapped residual burned gas can flow out of the combustion
chamber 12 into the exhaust port 18 and then flow back into the
combustion chamber during the subsequent second exhaust event EV2
occurring after TDC represented by curves 1', 2', 3' to reduce
thermal energy by heat transfer and thereby control the temperature
of the burned gas mixture in the cylinder, The mass of the residual
burned gas that mixes with fresh air inducted into combustion
chamber 12 remains essentially unchanged despite the changes of
exhaust valve closing timing. The second exhaust event EVZ ends at
the time when the intake valve 20 opens so as to control the
in-cylinder pressure at the time of intake valve opening. This
enables control of the intake air mass by the timing of the intake
valve opening as described above for air-fuel ratio control
purposes.
[0041] In the embodiment of FIG. 4, the fuel injection timing is
controlled by ECU 40 typically to occur after TDC since after TDC,
the gases flow into the combustion chamber due to the downward
movement of the piston. Therefore, the injected fuel after TDC will
not flow out of the combustion chamber to the exhaust port despite
the exhaust port being open. The injection timing can be adjusted
by ECU 40 to affect the mixture temperature as described above for
incylinder (direct) fuel injection.
[0042] When the temperature of the residual burned gas in the
combustion chamber 12 is changed, the mass of the fresh intake air
inducted into the combustion chamber and mixed with the burned gas
mixture will also be accordingly changed by a minor amount despite
the intake (in-cylinder) pressure and effective compression ratio
of the engine remaining unchanged. The invention envisions ECU 40
slightly adjusting the intake valve opening time and for the fuel
injection pulse width during the period when the exhaust valve
dosing timing is changed as may be needed in order to compensate
for this effect of temperature change of the burned gas mixture on
the mass of the fresh air inducted into the combustion chamber 12.
For example. ECU 40 can move the intake valve opening time doser to
TDC during the period of changing of the exhaust valve closing
timing to increase the mass of fresh air inducted into the
combustion chamber 12.
[0043] To avoid engine pumping losses, the additional exhaust event
EV2 is provided immediately after the exhaust valve 28 doses in the
exhaust stroke and after TDC as shown in FIG. 4 to allow some
residual burned gases to be drawn from the exhaust part 18 by
piston motion. The exhaust valve opening time EVO of the second
exhaust event IV2 occurring after TDC is varied depending on the
amount of advancement of the exhaust valve dosing time EVC of main
intake event EV occurring before TDC. That is, curve 1' of the
additional exhaust event would be used when curve 1 represents the
main intake event occurring after TDC, curve 2' of the additional
intake event would be used when curve 2 represents the main intake
event occurring after TDC, and so on. As is apparent from FIG. 4,
the crank angle from the end of the initial main exhaust event F
(curves 1, 2, 3) to TDC and the crank angle from TDC to the
beginning of the subsequent exhaust event EV2 (curves 1', 2', 3')
should be essentially equal to minimize engine pumping losses.
[0044] According to this embodiment of the invention, at any fixed
engine speed the air-fuel ratio in combustion chamber 12 can be
controlled to the stoichiometric proportion by ECU 40 determining
engine torque and controlling the intake valve open/close timing as
described above in response to the determined engine torque. The
autoignition timing is adjusted by ECU 40 by changing the exhaust
valve closing timing as illustrated, for example, in FIG. 4 by
curves 1 through 3 over successive exhaust events.
[0045] Although the invention has been described above with respect
to FIG. 1 for controlling the intake valve 20 and the exhaust valve
28, those skilled in the art will appreciate that more than one
intake valve (e.g., two intake valves) and more than one exhaust
valve (e.g.. two exhaust valves) can be controlled in a manner to
achieve advantages of the invention, for example, for an engine
with more than two valves per cylinder, the open/close timing of
the intake valves or the-exhaust valves of a cylinder can be
controlled either in unison or differently. For example, FIG. 3
shows two intake events per cycle. For an engine with four valves
per cylinder, the two intake valves can open and close differently
such that the initial intake event IV2 is realized by one intake
valve and the main intake event IV is realized by the other intake
valve. Likewise, the two exhaust valves can be controlled to open
and close differently when there are two exhaust events as shown in
FIG. 4 such that the main exhaust EV is realized by one exhaust
valve and the subsequent exhaust event EVZ is realized by the other
exhaust valve.
[0046] While the invention has been described in terms of specific
embodiments thereof, it is not intended to be limited thereto but
rather only as set forth in the appended claims.
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