Title: Direct injection variable valve timing engine control system and method
Abstract: A method for controlling mode transitions, such as from stratified to homogeneous mode, in a direct injection engine adjusts an intake manifold outlet control device, such as a cam timing, to rapidly control cylinder fresh charge despite manifold dynamics. In addition, a coordinated change between an intake manifold inlet control device, for example a throttle, and the outlet control device is used to achieve the rapid cylinder fresh charge control. In this way, engine torque disturbances during the mode transition are eliminated, even when cylinder air/fuel ratio is changed from one cylinder event to the next.
Patent Number: 6,945,227 Issued on 09/20/2005 to Russell, et al.
| Inventors:
|
Russell; John David (Farmington Hills, MI);
Surnilla; Gopichandra (Westland, MI);
Cooper; Stephen Lee (Dearborn, MI)
|
| Assignee:
|
Ford Global Technologies, LLC (Dearborn, MI)
|
| Appl. No.:
|
279359 |
| Filed:
|
October 24, 2002 |
| Current U.S. Class: |
123/399; 123/90.15 |
| Intern'l Class: |
F02D 007/00 |
| Field of Search: |
123/9015,901.6,901.7,901.8,681,682,687,399
701/107
|
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Primary Examiner: Gimie; Mahmoud
Attorney, Agent or Firm: Lippa; Allan J., Allerman Hall McCoy Russell & Tuttle LLP
Parent Case Text
RELATED PATENT APPLICATIONS
This is a divisional of patent application No. 09/420,451 filed Oct. 18, 1999
now U.S. Pat. No. 6,470,869 and is a division of application Ser. No. 09/888,032
filed Jun. 22, 2001, now U.S. Pat. No. 6,467,442.
Claims
1. An article of manufacture, comprising:
a computer storage medium having a computer program encoded therein for controlling
an engine having an intake manifold, an inlet control device for controlling flow
entering the manifold, and an outlet control device for controlling flow from the
intake manifold into a cylinder, said computer storage medium comprising:
code for enabling direct injection of fuel into said cylinder to change said
cylinder air/fuel ratio from a first cylinder air/fuel ratio to a second cylinder
air/fuel ratio; and
code for calculating a change in an operating position of said outlet control
device based on an engine operating parameter, in response to said fuel injection,
said engine operating parameter comprises a first manifold pressure before said
cylinder air/fuel ratio change; and
code for enabling adjustment in said operating position of said outlet control
device in response to said calculated change in said operating position so that
a manifold pressure after said air/fuel ratio change approaches said first manifold
pressure.
Description
FIELD OF THE INVENTION
The field of the invention relates to mode transitions in a direct injection
spark ignited engine.
BACKGROUND OF THE INVENTION
In direct injection spark ignition engines, there are two modes of operation
that
are typically used. The first mode is termed stratified mode where fuel is injected
during the compression stroke of the engine. In the stratified mode of operation,
the air/fuel ratio is operated lean of stoichiometry. In the second mode of operation,
termed homogeneous operation, fuel is injected during the intake stroke of the engine.
During homogeneous operation, the air/fuel can operate either lean or rich
of stoichiometry. However, in some circumstances, the operable stratified operation
range of lean air/fuel ratios does not coincide with any operable homogeneous,
lean air/fuel ratio. Therefore, when switching between these two modes of operation,
air/fuel ratio from one cylinder event to the next cylinder event changes in a
discontinuous way. Because of this discontinuous change in air/fuel ratio, engine
torque is uncompensated, and has an abrupt change.
One method for eliminating abrupt changes in engine cylinder air/fuel ratio is
to adjust ignition timing so that abrupt changes in engine torque will be avoided.
Another solution is to adjust throttle position to reduce or increase fresh charge
flow entering the intake manifold and therefore compensate for changes in engine
torque during discontinuous cylinder air/fuel ratio changes.
The inventors herein have recognized disadvantages with the above approaches.
Regarding ignition timing adjustments to avoid abrupt changes in engine torque,
this method is only applicable when the magnitude of the torque change is small.
In other words, the range of authority of ignition timing is limited by engine
misfire and emission constraints. Therefore, the approach is not generally applicable.
Regarding throttle position adjustments to prevent abrupt changes in engine
torque, controlling flow entering the manifold cannot rapidly control cylinder
charge due to manifold volume. In other words, air entering the cylinder is governed
by manifold dynamics and therefore there is a torque disturbance when using the
throttle to compensate for discontinuous cylinder air/fuel ratio changes. For example,
if the throttle is instantly closed and no air enters the manifold through the
throttle, cylinder air charge, does not instantly decrease to zero. The engine
must pump down the air stored in the manifold, which takes a certain number of
revolutions. Therefore, the cylinder air charge gradually decreases toward zero.
Such a situation is always present when trying to change cylinder charge using
a control device such as a throttle.
SUMMARY OF THE INVENTION
An object of the present invention is to allow air/fuel mode transitions in direct
injection engines between respective air/fuel regions which do not overlap while
preventing abrupt changes in engine torque.
The above object is achieved and disadvantages of prior approaches overcome by
a method for controlling an engine during a cylinder air/fuel ratio change from
a first cylinder air/fuel ratio to a second cylinder air/fuel ratio, the engine
having an intake manifold and an outlet control device for controlling flow from
the intake manifold into the cylinder. The method comprises the steps of indicating
the cylinder air/fuel ratio change, and in response to said indication, changing
the outlet control device.
By using an outlet control device that controls flow exiting the manifold (entering
the cylinder), it is possible to rapidly change cylinder charge despite response
delays of airflow inducted through the intake manifold. In other words, a rapid
change in cylinder charge can be achieved, thereby allowing a rapid change in cylinder
air/fuel ratio while preventing disturbances in engine torque.
An advantage of the above aspect of the invention is that unwanted torque changes
can be eliminated when abruptly changing cylinder air/fuel ratio.
In another aspect of the present invention, the above object is achieved and
disadvantages
of prior approaches overcome by a method for controlling an engine during a cylinder
air/fuel ratio change from a first cylinder air/fuel ratio to a second cylinder
air/fuel ratio, the engine having an intake manifold, an inlet control device for
controlling flow entering the manifold, and an outlet control device for controlling
flow exiting the intake manifold. The method comprises the steps of indicating
the cylinder air/fuel ratio change, and in response to said indication, changing
the outlet control device and the inlet control device.
By changing both the inlet and outlet control devices, it is possible to rapidly
change the cylinder air charge despite response delays of airflow inducted through
the intake manifold. Since the cylinder air charge can be rapidly changed, the
cylinder air/fuel ratio change can be compensated and abrupt changes in engine
torque can be avoided. In other words, the present invention controls manifold
inlet and outlet flows in a coordinated way to allow a rapid change in cylinder
air charge regardless of manifold volume. This rapid cylinder air charge change
allows the air/fuel ratio to rapidly change while preventing abrupt changes in
engine torque, even during abrupt changes in cylinder air/fuel ratio.
An advantage of the above aspect of the invention is that unwanted torque changes
can be eliminated when abruptly changing cylinder air/fuel ratio.
Another advantage of the above aspect of the invention is that by using both
an outlet and an inlet control device, a more controlled rapid change in cylinder
charge is possible.
BRIEF DESCRIPTION OF THE DRAWINGS
The object and advantages of the invention claimed herein will be more readily
understood by reading an example of an embodiment in which the invention is used
to advantage with reference to the following drawings wherein:
FIG. 1 is a block diagram of an embodiment in which the invention is used to advantage;
FIGS. 2,3,6, and 7 are high level flowcharts which
perform a portion of operation of the embodiment shown in FIG. 1;
FIG. 4 is a graph depicting results using prior art approaches; and
FIG. 5 is a graph depicting results using the present invention.
DETAILED DESCRIPTION AND BEST MODE
Direct injection spark ignited internal combustion engine
10, comprising
a plurality of combustion chambers, is controlled by electronic engine controller
12. Combustion chamber
30 of engine
10 is shown in FIG. 1
including combustion chamber walls
32 with piston
36 positioned therein
and connected to crankshaft
40. In this particular example piston
30
includes a recess or bowl (not shown) to help in forming stratified charges of
air and fuel. Combustion chamber, or cylinder,
30 is shown communicating
with intake manifold
44 and exhaust manifold
48 via respective intake
valves
52a and
52b (not shown), and exhaust valves
54a and
54b (not shown). Fuel injector
66 is
shown directly coupled to combustion chamber
30 for delivering liquid fuel
directly therein in proportion to the pulse width of signal fpw received from controller
12 via conventional electronic driver
68. Fuel is delivered to fuel
injector
66 by a conventional high pressure fuel system (not shown) including
a fuel tank, fuel pumps, and a fuel rail.
Intake manifold
44 is shown communicating with throttle body
58
via throttle plate
62. In this particular example, throttle plate
62
is coupled to electric motor
94 so that the position of throttle plate
62
is controlled by controller
12 via electric motor
94. This configuration
is commonly referred to as electronic throttle control (ETC) which is also utilized
during idle speed control. In an alternative embodiment (not shown), which is well
known to those skilled in the art, a bypass air passageway is arranged in parallel
with throttle plate
62 to control inducted airflow during idle speed control
via a throttle control valve positioned within the air passageway.
Exhaust gas oxygen sensor
76 is shown coupled to exhaust manifold
48 upstream of catalytic converter
70. In this particular example,
sensor
76 provides signal EGO to controller
12 which converts signal
EGO into two-state signal EGOS. A high voltage state of signal EGOS indicates exhaust
gases are rich of stoiehiometry and a low voltage state of signal EGOS indicates
exhaust gases are lean of stoichiemetry. Signal EGOS is used to advantage during
feedback air/fuel control in a conventional manner to maintain average air/fuel
at stoichiometry during the steichiometric homogeneous mode of operation.
Conventional distributorless ignition system
88 provides ignition
spark to combustion chamber
30 via spark plug
92 in response to spark
advance signal SA from controller
12.
Controller
12 causes combustion chamber
30 to operate in
either a homogeneous air/fuel mode or a stratified air/fuel mode by controlling
injection timing. In the stratified mode, controller
12 activates fuel injector
66 during the engine compression stroke se that fuel is sprayed directly
into the bowl of piston
36. Stratified air/fuel layers are thereby formed.
The strata closest to the spark plug contains a stoichiometric mixture or a mixture
slightly rich of stoichiometry, and subsequent strata contain progressively leaner
mixtures. During the homogeneous mode, controller
12 activates fuel injector
66 during the intake stroke so that a substantially homogeneous air/fuel
mixture is formed when ignition power is supplied to spark plug
92 by ignition
system
88. Controller
12 controls the amount of fuel delivered by
fuel injector
66 so that the homogeneous air/fuel mixture in chamber
30
can be selected to be at stoichiometry, a value rich of stoichiometry, or a value
lean of stoichiometry. The stratified air/fuel mixture will always be at a value
lean of stoichiometry, the exact air/fuel being a function of the amount of fuel
delivered to combustion chamber
30. An additional split mode of operation
wherein additional fuel is injected during the exhaust stroke while operating in
the stratified mode is also possible.
Nitrogen oxide (NOx) absorbent or trap
72 is shown positioned downstream
of catalytic converter
70. NOx trap
72 absorbs NOx when engine
10
is operating lean of snoichiometry. The absorbed NOx is subsequently reacted with
HC and catalyzed during a NOx purge cycle when controller
12 causes engine
10 to operate in either a rich homogeneous mode or a stoichiometric homogeneous mode.
Controller
12 is shown in FIG. 1 as a conventional microcomputer
including: microprocessor unit
102, input/output ports
104, an electronic
storage medium for executable programs and calibration values shown as read only
memory chip
106 in this particular example, random access memory
108,
keep alive memory
110, and a conventional data bus. Controller
12
is shown receiving various signals from sensors coupled to engine
10, in
addition to those signals previously discussed, including: measurement of inducted
mass air flow (MAP) from mass air flow sensor
100 coupled to throttle body
58; engine coolant temperature (ECT) from temperature sensor
112
coupled to cooling sleeve
114; a profile ignition pickup signal (PIP) from
Hall effect sensor
118 coupled to crankshaft
40; and throttle position
TP from throttle position sensor
120; and absolute Manifold
9 Pressure
Signal MAP from sensor
122. Engine speed signal RPM is generated by controller
12 from signal PIP in a conventional manner and manifold pressure signal
MAP provides an indication of engine load. In a preferred aspect of the present
invention, sensor
118, which is also used as an engine speed sensor, produces
a predetermined number of equally spaced pulses every revolution of the crankshaft.
In this particular example, temperature Tcat of catalytic converter
70
and temperature Ttrp of NOx trap
72 are inferred from engine operation as
disclosed in U.S. Pat. No. 5,414,994 the specification of which is incorporated
herein by reference. In an alternate embodiment, temperature Tcat is provided by
temperature sensor
124 and temperature Ttrp is provided by temperature sensor
126.
Continuing with FIG. 1, camshaft
130 of engine
10 is shown
communicating with rocker arms
132 and
134 for actuating intake valves
52a,
52b and exhaust valve
54a,
54b.
Camshaft
130 is directly coupled to housing
136. Housing
136
forms a toothed wheel having a plurality of teeth
138. Housing
136
is hydraulically coupled to an inner shaft (not shown), which is in turn directly
linked to camshaft
130 via a timing chain (not shown). Therefore, housing
136 and camshaft
130 rotate at a speed substantially equivalent to
the inner camshaft. The inner camshaft rotates at a constant speed ratio to crankshaft
40. However, by manipulation of the hydraulic coupling as will be described
later herein, the relative position of camshaft
130 to crankshaft
40
can be varied by hydraulic pressures in advance chamber
142 and retard chamber
144. By allowing high pressure hydraulic fluid to enter advance chamber
142, the relative relationship between camshaft
130 and crankshaft
40 is advanced. Thus, intake valves
52a,
52b and
exhaust valves
54a,
54b open and close at a time earlier
than normal relative to crankshaft
40. Similarly, by allowing high pressure
hydraulic fluid to enter retard chamber
144, the relative relationship between
camshaft
130 and crankshaft
40 is retarded. Thus, intake valves
52a,
52b and exhaust valves
54a,
54b open
and close at a time later than normal relative to crankshaft
40.
Teeth
138, being coupled to housing
136 and camshaft
130,
allow for measurement of relative cam position via cam timing sensor
150
providing signal VCT to controller
12. Teeth 1, 2, 3, and 4 are preferably
used for measurement of cam timing and are equally spaced (for example, in a V-8
dual bank engine, spaced 90 degrees apart from one another), while tooth
5
is preferably used for cylinder identification. In addition, Controller
12
sends control signals (LACT,RACT) to conventional solenoid valves (not shown) to
control the flow of hydraulic fluid either into advance chamber
142, retard
chamber
144, or neither.
Relative cam timing is measured using the method described in U.S. Pat.
No. 5,548,995, which is incorporated herein by reference. In general terms, the
time, or rotation angle between the rising edge of the PIP signal and receiving
a signal from one of the plurality of teeth
138 on housing
136 gives
a measure of the relative cam timing. For the particular example of a V-8 engine,
with two cylinder banks and a five toothed wheel, a measure of cam timing for a
particular bank is received four times per revolution, with the extra signal used
for cylinder identification.
Referring now to FIG. 2, a routine is described for performing mode transitions
from either stratified mode to homogeneous mode or from homogeneous mode to stratified
mode. First, in step
210, a determination is made as to whether a mode transition
is required. When the answer to step
210 is YES, a determination is made
as to whether there is an overlapping air/fuel region based on the current engine
operating conditions. The determination is made using one of the following two
equations, depending upon whether the mode is being changed from stratified to
homogeneous or from homogeneous to stratified.
When transitioning from stratified to homogeneous, the following condition is used:
where the equation determines if the minimum indicated engine torque (T
i)
over available ignition timings (spark) for homogenous operation at the maximum
lean homogenous air/fuel ratio (a/f
maxhomogeneous) is greater
than the maximum indicated engine torque over available ignition timings for stratified
operation at the minimum lean stratified air/fuel ratio (a/f
maxhomogenous)
at the current operationg conditions defined by, for example, engine speed (RPM),
fresh air flow, exhaust gas recirculation amount, and any other variables known
to those skilled in the art to affect engine indicated torque. In other words,
if this condition is true, then the routine continues to step
216.
When transitioning from homogeneous to stratified, the following condition is used:
where the equation determines if the maximum indicated engine torque over available
ignition timings for stratified operation at the minimum lean stratified air/fuel
ratio (a/f
maxhomogeneous) is less than the minimum indicated
engine torque (T
i) over available ignition timings (spark) for homogenous
operation at the maximum lean homogenous air/fuel ratio (a/f
maxhomogeneous)
at the current operationg conditions defined by, for example, engine speed (RPM),
fresh air flow, exhaust gas recirculation amount, and any other variables known
to those skilled in the art to affect engine indicated torque. In other words,
if this condition is true, then the routine continues to step
216.
As described above herein, these equations determine whether the mode can be
changed
by simply changing the injection timing, changing the injection timing and the
ignition timing, or, according to the present invention using a combined strategy
where the electronic throttle and variable cam timing actuators are synchronized.
Continuing with FIG. 2, when the answer to step
212 is YES, the
routine continues to step
214 where the operating mode is changed by changing
the injection timing or by changing the injection timing and ignition timing. When
the answer to step
212 is NO, the routine continues to step
216 where
the operating mode is changed by coordinated control of variable cam timing and
throttle position, described later herein with particular reference to FIG.
3.
Referring now to FIG. 3, a routine for changing engine operating modes
by coordinated control of variable cam timing and throttle position is described
where abrupt changes in engine torque are avoided during the transition. In step
3, the current manifold pressure before the mode transition is determined
using the following equation if mass charge is known:
where {circumflex over (P)}
mt is the manifold pressure
before the mode transition, m
c is total mass charge and the parameters
a,b are determine based on engine operating conditions, including current cam timing
(VCT), engine speed, and manifold temperature. Also, the current indicated engine
torque (Te) is estimated using current engine operating conditions. Otherwise,
the current manifold pressure before the mode transition is determined by reading
the manifold pressure sensor. Alternatively, various methods known to those skilled
in the art for determining manifold pressure can be used.
Continuing with FIG. 3, in step
312, the new required cylinder
fresh charge after the mode transition is determined so that equal engine torque
is produced both before and after the mode transition. The new cylinder fresh charge
m
cairnew value is determined according
to the operating conditions after the mode using the limiting air/fuel ratio for
the mode to which the engine is transitioning such that the engine torque determined
in step
310 is produced. The value is determined based on characteristic
engine maps represented by the function g:
Other engine operating parameters such as engine speed, exhaust gas recirculation,
or any other parameter affecting engine torque can be included.
Alternatively, any method known to those skilled in the art for determining
the required fresh charge to produce a given amount of engine torque at a certain
air/fuel ratio and manifold pressure can be used.
Continuing with FIG. 3, in step
314, the new variable cam timing
angle is determined so that manifold pressure will be equal to the manifold pressure
determined in step
310 and the actual mass charge will be equal to the mass
charge determined in step
312 using the following equation. Here, the cam
timing value which makes this equation hold represent the new desired cam timing
value, VCT
new:
Next, in step
316, the new throttle position is determined that will
provide the new fresh charge value determined in step
312 at the manifold
pressure transition value, {circumflex over (P)}
mt and current
operating conditions. Any equation known to those skilled in the art to describe
compressible flow through a throttle can be used to find the necessary throttle
position based on the transition manifold pressure in step
314 and the new
fresh charge determined in step
312.
According to the present invention, using the method described above herein,
with particular reference to FIG. 3, the engine operating mode can be changed or
the engine air/fuel ratio can be instantaneously jumped while avoiding abrupt changes
in engine torque. By keeping manifold pressure relatively constant and simultaneously
changing the throttle position and the variable cam timing position according to
the equations above, cylinder charge can be rapidly changed to match the change
in air/fuel ratio, thereby preventing abrupt changes in engine torque. Also, the
present invention can be applied to any situation where the air/fuel ratio is abruptly
changed and it is desired to prevent engine torque abrupt changes.
Further, the invention can be applied to rapidly control engine torque using
airflow. In other words, engine torque control can be rapidly achieved despite
manifold volume and manifold dynamics. For example, improved idle speed control
can be achieved by using cam timing and electronic throttle together to rapidly
control engine torque.
Referring now to FIG. 4, a group of plots showing operation according to
prior art methods is described. In the top graph, throttle position is shown versus
time. In the second graph, fuel injection amount is shown versus time. In the third
graph, engine torque versus time is shown. Finally, in the fourth and bottom graph,
cylinder air charge is shown versus time. At the time indicated by the vertical
dashed line, a mode transition is executed where the engine transitions from operating
in a stratified mode to operating in a homogeneous mode. In this situation, overlapping
air/fuel ratio is not allowed so that equal torque can be produced, even using
variations in ignition timing. Therefore, prior art methods using airflow as a
method to control torque are used. As shown in the top two graphs, the throttle
position is instantaneously lowered to account for the otherwise increased torque
caused by the instantaneous change in fuel injection amount to prevent degraded
engine combustion. As shown in the third graph, engine torque is disturbed during
the transition and does not return to the desired level until sometime after the
transition, which is governed by the manifold dynamics, as shown by the fourth
graph in which cylinder air charge converges to the new value.
Referring now to FIG. 5, a mode transition from the stratified mode to
the homogeneous mode is shown according to the present invention. The first graph
shows throttle position versus time. The second graph shows fuel injection amount
versus time. The third graph shows engine torque versus time. The fourth graph
shows cylinder air charge versus time. The fifth and final graph shows variable
cam timing position versus time, where the vertical axis shows increasing cam retard.
At the time instant shown by the vertical dashed line, a mode transition occurs
from stratified mode to homogeneous mode. According to the present invention, both
the throttle position and the variable cam timing are changed in a coordinated
way, such that the air charge, as shown in the fourth graph, steps down to a lower
level. At the same time, the fuel injection amount is increased to avoid operating
the engine in regions that would produce poor combustion. As shown in the third
graph, abrupt changes in engine torque are avoided during the transition. This
is due to the coordinated changed between throttle position and cam timing, where
the amount of change of cam timing and throttle position is determined according
to the present invention.
Referring now to FIG. 6, a routine is described where the method according
to the present invention is improved upon using feedback from available sensors.
In particular, when a mass airflow signal is available, it can be used in conjunction
with the present invention to provide additional control and compensation for any
calculation errors. First, in step
610, a determination is made as to whether
a mode transition has occurred. When the answer to step
610 is YES, the
routine continues to step
612. In step
612, an error is calculated
between the new desired cylinder air charge multiplied by engine speed and the
number of cylinders and the current reading of the mass airflow sensor. Next, in
step
614, this error is used to adjust throttle position from the throttle
position calculated in step
316. Controller
12 then controls actual
throttle position to this adjusted throttle position. In this way, any calculation
errors used in determining the throttle position change that corresponds to the
variable cam timing position change to give equal engine torque at a mode transition
can be compensated. In an alternative embodiment, the cam timing can be adjusted
based on the error signal rather than the throttle position. In another alternative
embodiment, both the cam timing and the throttle position can be adjusted based
the error signal.
Referring now to FIG. 7, the routine is described where a manifold pressure
sensor is used to compensate for any imperfect calculations. First, in step
710,
a determination is made as to whether a mode transition has occurred. If the answer
to step
710 is YES, the routine continues to step
712 where a manifold
pressure error is calculated between the manifold pressure determined in step
310
and the current manifold pressure. Next, in step
712, the throttle position
is adjusted based on the manifold pressure error determined in step
712.
Controller
12 then controls actual throttle position to this adjusted throttle
position. In this way, abrupt changes in engine torque can be avoided during a
mode transition despite variations not accounted for in the equations described
in the present invention.
While the invention has been shown and described in its preferred embodiments,
it will be clear to those skilled in the arts to which it pertains that many changes
and modifications may be made thereto without departing from the scope of the invention.
For example, any device, herein termed an outlet control device, that affects flow
exiting intake manifold
44 and entering cylinder
30 can be used in
place of the variable cam timing unit. For example, a swirl control valve, a charge
motion control valve, an intake manifold runner control valve, an electronically
controlled intake valve can be used according to the present invention to rapidly
change cylinder fresh charge in order to control engine torque. Further, any device
that affects flow entering intake manifold
44, herein termed an intake control
device can be used in place of the throttle. For example, an EGR valve, a purge
control valve, an intake air bypass valve can be used in conjunction with the outlet
control device so rapidly change cylinder fresh charge in order to control engine torque.
While the invention has been shown and described in its preferred embodiments,
it will be clear to those skilled in the arts to which it pertains that many changes
and modifications may be made thereto without departing from the scope of the invention.
*
