U.S. patent application number 09/824409 was filed with the patent office on 2001-08-16 for direct injection engine system and method.
Invention is credited to Grizzle, Jessy W., Sun, Jing.
Application Number | 20010013330 09/824409 |
Document ID | / |
Family ID | 23663613 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013330 |
Kind Code |
A1 |
Grizzle, Jessy W. ; et
al. |
August 16, 2001 |
Direct injection engine system and method
Abstract
A method for controlling mode transitions, such as from
stratified to homogeneous mode, in a direct injection engine
adjusts a number of cylinders carrying out combustion to prevent
engine torque disturbances. Cylinder activation is used when
changes in cylinder air/fuel ratio cannot be compensated using
ignition timing adjustments. In addition, the number of cylinders
to activate is also determined and measures are taken if the number
of cylinders determined are not available to be activated.
Inventors: |
Grizzle, Jessy W.; (Ann
Arbor, MI) ; Sun, Jing; (Bloomfield, MI) |
Correspondence
Address: |
John D. Russell
Parklane Towers East - Suite 600
1 Parklane Boulevard
Dearborn
MI
48126
US
|
Family ID: |
23663613 |
Appl. No.: |
09/824409 |
Filed: |
April 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09824409 |
Apr 2, 2001 |
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09419754 |
Oct 18, 1999 |
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6244242 |
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Current U.S.
Class: |
123/295 ;
123/198F; 123/481 |
Current CPC
Class: |
F02D 41/307 20130101;
F02B 17/005 20130101; F02D 2041/389 20130101; F02D 41/0087
20130101; F02D 17/02 20130101; F02D 41/3029 20130101; F02B 2075/125
20130101; Y02T 10/12 20130101; Y02T 10/123 20130101 |
Class at
Publication: |
123/295 ;
123/481; 123/198.00F |
International
Class: |
F02B 017/00; F02D
041/04 |
Claims
We claim:
1. 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 method comprising the steps of:
determining a first number of cylinders to enable to perform
combustion so that the cylinder air/fuel ratio change can occur
without a disturbance in engine torque; and enabling said first
number of cylinders and changing the cylinder air/fuel from the
first cylinder air/fuel ratio to the second cylinder air/fuel ratio
when said number of cylinders are currently disabled.
2. The method recited in claim 1 further comprising the step of
disabling a second number of cylinders and changing the cylinder
air/fuel ratio from the first cylinder air/fuel ratio to a third
cylinder air/fuel ratio when said number of cylinders are not
currently disabled.
3. The method recited in claim 2 further comprising the step of
performing said step of enabling said first number of cylinders and
changing the cylinder air/fuel from the first cylinder air/fuel
ratio to the second cylinder air/fuel ratio after performing said
step of disabling said second number of cylinders and changing the
cylinder air/fuel ratio from the first cylinder air/fuel ratio to
said third cylinder air/fuel ratio.
4. The method recited in claim 3 wherein said third air/fuel ratio
is based on said first number of cylinders.
5. The method recited in 4 wherein said third air/fuel ratio is
further based on said second number of cylinder.
6. The method recited in claim 2 wherein said step of enabling said
first number of cylinders is performed simultaneously with changing
the cylinder air/fuel from the first cylinder air/fuel ratio to the
second cylinder air/fuel ratio.
7. The method recited in claim 2 wherein said step of disabling a
second number of cylinders is performed simultaneously with
changing the cylinder air/fuel ratio from the first cylinder
air/fuel ratio to said third cylinder air/fuel ratio otherwise
8. The method recited in claim 2 wherein said step of determining
said first number of cylinders to enable further comprises
determining said first number of cylinders to enable based on a
torque difference between a first torque produced by the first
cylinder air/fuel ratio and a second torque produced by the second
cylinder air/fuel ratio.
9. The method recited in claim 2 further comprising the step of
determining said first number of cylinders to enable based on a
torque difference between a first torque produced by a current
engine operating mode at the first cylinder air/fuel ratio and a
requested torque.
10. The method recited in claim 2 wherein said first number of
cylinder to enable is further based on a number of cylinders
currently carrying out combustion.
11. The method recited in claim 2 wherein changing said cylinder
air/fuel from said first cylinder air/fuel ratio to said second
air/fuel ratio represents a mode transition, wherein the engine is
a direct injection engine.
12. The method recited in claim 2 wherein said first cylinder
air/fuel ratio represents a homogeneous air/fuel ratio limit in
homogeneous operation.
13. The method recited in claim 2 wherein said second cylinder
air/fuel ratio represents a stratified air/fuel ratio limit in
stratified operation.
14. The method recited in claim 12 wherein said third cylinder
air/fuel ratio represents a homogenous air/fuel ratio greater than
said first cylinder air/fuel ratio.
15. A method for controlling an engine during a mode transition
from homogeneous operation to stratified operation, wherein a
cylinder air/fuel ratio changes from a first cylinder air/fuel
ratio to a second cylinder air/fuel ratio, the method comprising
the steps of: determining a first number of cylinders to enable to
perform combustion so that the cylinder air/fuel ratio change can
occur without a disturbance in engine torque; and enabling said
first number of cylinders and changing the operating mode from
homogeneous mode to stratified mode when said number of cylinders
are currently disabled, said enabling step further comprising
changing cylinder air/fuel from the first cylinder air/fuel ratio
to the second cylinder air/fuel ratio; disabling a second number of
cylinders and changing the cylinder air/fuel ratio from the first
cylinder air/fuel ratio to a third homogeneous cylinder air/fuel
ratio otherwise.
16. The method recited in claim 15 further comprising the step of
performing said enabling step after performing said disabling
step.
17. The method recited in claim 16 wherein said third air/fuel
ratio is based on said first number of cylinders and said second
number of cylinders.
18. The method recited in claim 17 wherein said enabling step is
performed simultaneously with changing the cylinder air/fuel from
the first cylinder air/fuel ratio to the second cylinder air/fuel
ratio.
19. The method recited in claim 18 wherein said step of determining
said first number of cylinders to enable further comprises
determining said first number of cylinders to enable based on a
torque difference between a first torque produced by the first
cylinder air/fuel ratio and a second torque produced by the second
cylinder air/fuel ratio.
20. The method recited in claim 18 further comprising the step of
determining said first number of cylinders to enable based on a
torque difference between a first torque produced by a current
engine operating mode at the first cylinder air/fuel ratio and a
requested torque.
21. An article of manufacture comprising: a computer storage medium
having a computer program encoded therein 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, said computer
storage medium comprising: code for determining a first number of
cylinders to enable to perform combustion so that the cylinder
air/fuel ratio change can occur without a disturbance in engine
torque; code for enabling said first number of cylinders and
changing the cylinder air/fuel from the first cylinder air/fuel
ratio to the second cylinder air/fuel ratio when said number of
cylinders are currently disabled; code for disabling a second
number of cylinders and changing the cylinder air/fuel ratio from
the first cylinder air/fuel ratio to a third cylinder air/fuel
ratio otherwise; and code for performing said enabling code after
performing said disabling code.
Description
FIELD OF THE INVENTION
[0001] This invention relates to mode transition control of a
direct injection spark ignition engine.
BACKGROUND OF THE INVENTION
[0002] In direct injection spark ignition engines, the engine
operates with stratified air/fuel operation in which the combustion
chamber contains stratified layers of different air/fuel mixtures.
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.
[0003] The engine may also operate in a homogeneous mode of
operation with a homogeneous mixture of air and fuel generated in
the combustion chamber by early injection of fuel into the
combustion chamber during the intake stroke. Homogeneous operation
may be either lean of stoichiometry, at stoichiometry, or rich of
stoichiometry.
[0004] Direct injection engines are also coupled to conventional
three-way catalytic converters to reduce CO, HC, and NOx. When
operating at air/fuel mixtures lean of stoichiometry, a NOx trap or
catalyst is typically coupled downstream of the three-way catalytic
converter to further reduce NOx.
[0005] The stratified mode of operation is typically utilized when
the engine is operating in light to medium loads. The homogeneous
mode of operation is typically used from medium to heavy load
operating conditions. In certain conditions, it is necessary to
transition from one engine mode of operation to the other. During
these mode transitions, it is desired to deliver the requested
engine output torque to provide good drive feel. However, in some
circumstances, the range of acceptable lean air/fuel ratios of
stratified operation do not overlap with the acceptable air/fuel
ratios of homogeneous operation. Therefore, during the mode
transition, a torque shock occurs because of the step change in
engine air/fuel ratio.
[0006] One method of preventing the engine torque disturbance
during mode transition is to change the injection mode one cylinder
at a time according to the required amount of fuel to be injected.
This reduces a large torque disturbance to several smaller torque
disturbances. Such a method is described in U.S. Pat. No.
5,170,759.
[0007] The inventors herein have recognized a disadvantage with the
above approach. Even though the large torque jump during mode
transition is avoided, there are still several smaller torque jumps
experienced. In other words, a single, large torque disturbance is
substituted with multiple smaller torque disturbances which are
still noticeable by the vehicle driver.
SUMMARY OF THE INVENTION
[0008] An object of the invention herein is to control an engine
during mode transitions to provide a smooth torque output.
[0009] 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 method comprising the
steps of determining a first number of cylinders to enable to
perform combustion so that the cylinder air/fuel ratio change can
occur without a disturbance in engine torque, enabling said first
number of cylinders and changing the cylinder air/fuel from the
first cylinder air/fuel ratio to the second cylinder air/fuel ratio
when said number of cylinders are currently disabled, and disabling
a second number of cylinders and changing the cylinder air/fuel
ratio from the first cylinder air/fuel ratio to a third cylinder
air/fuel ratio otherwise.
[0010] By using cylinder activation as an additional degree of
freedom, abrupt changes in average engine torque can be avoided
even during air/fuel ratio changes. Thus, during a mode transition
where engine air/fuel ratio is constrained to change due to, for
example, combustion limits, abrupt changes in average engine torque
can be avoided by changing a number of active cylinders. Also, a
check is made as to whether the required number of cylinders to
activate are currently deactivated.
[0011] An advantage of the above aspect of the invention is that
abrupt changes in engine output torque can be avoided during mode
transitions.
[0012] Another advantage of the above aspect of the invention is
that the range of stratified operation can be extended since the
range of available transitions is increased.
[0013] Yet another advantage of the above aspect of the invention
is that emissions can be reduced since the engine can be operated
farther from air/fuel ratio combustion limits.
[0014] In another aspect of the invention, the above object is
achieved by a method for controlling an engine during a mode
transition from homogeneous operation to stratified operation,
wherein a cylinder air/fuel ratio changes from a first cylinder
air/fuel ratio to a second cylinder air/fuel ratio. The method
comprising the steps of determining a first number of cylinders to
enable to perform combustion so that the cylinder air/fuel ratio
change can occur without a disturbance in engine torque, enabling
said first number of cylinders and changing the operating mode from
homogeneous mode to stratified mode when said number of cylinders
are currently disabled, said enabling step further comprising
changing cylinder air/fuel from the first cylinder air/fuel ratio
to the second cylinder air/fuel ratio, and disabling a second
number of cylinders and changing the cylinder air/fuel ratio from
the first cylinder air/fuel ratio to a third homogeneous cylinder
air/fuel ratio otherwise.
[0015] By disabling cylinders when it is determined that cylinder
activation is required yet there are insufficient cylinders to
activate, it is possible to perform mode transitions using cylinder
activation.
[0016] An advantage of the above aspect of the invention is that
abrupt changes in engine output torque can be avoided during mode
transitions.
[0017] Another advantage of the above aspect of the invention is
that the range of stratified operation can be extended since the
range of available transitions is increased.
[0018] Yet another advantage of the above aspect of the invention
is that emissions can be reduced since the engine can be operated
farther from air/fuel ratio combustion limits.
DESCRIPTION OF THE DRAWINGS
[0019] 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:
[0020] FIG. 1 is a block diagram of an embodiment in which the
invention is used to advantage; and
[0021] FIGS. 2-3 are high level flowcharts which perform a portion
of operation of the embodiment shown in FIG. 1.
DESCRIPTION OF AN EXAMPLE OF OPERATION
[0022] 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 36 includes a recess or bowl (not
shown) to help in forming stratified charges of air and fuel.
Combustion chamber 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.
[0023] 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.
[0024] 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
stoichiometric homogeneous mode of operation.
[0025] 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.
[0026] 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 so
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.
[0027] 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 stoichiometry. 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.
[0028] 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 (MAF) 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 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.
[0029] In this particular example, temperature Tcat of catalytic
converter 70 and temperature Ttrap 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 Ttrap is provided by temperature sensor
126.
[0030] Referring now to FIG. 2 a routine for performing mode
transitions from stratified to homogeneous, and from homogeneous to
stratified is described. First, a determination is made in step 210
if a mode transition is required. A mode transition is required
based on engine operating conditions. For example, as the required
engine torque is gradually decreased from a large value to a small
value, a transition from homogeneous mode of operation to
stratified mode may be required. Conversely, if the required engine
torque is gradually changed from a low value to a high value,
transition from stratified mode to homogeneous mode may be
required. Also, fuel economy requirements or emission device
conditions may dictate mode transitions. For example, when trap 72
is reaches a certain capacity of stored NOx, a transition may be
required from stratified mode to homogeneous mode so that a rich or
stoichiometric air/fuel ratio can be combusted, thereby allowing
purging of trap 72. Further, this transition is required to be
smooth so that driver comfort is not affected.
[0031] Continuing with FIG. 2, if the answer to step 210 is yes, a
determination is made in step 212 if the engine is currently
operating in the stratified mode. If the answer to step 212 is yes,
a new determination is made in step 214 if cylinder deactivation is
required. The determination in step 214 is made using engine
mapping data described by the following equation:
min.sub.sparkT.sup.i(spark,a/f.sub.max.sup.homogeneous)>max.sub.sparkT.-
sup.l(spark,a/f.sub.min.sup.stratified)
[0032] where the equation determines if the minimum indicated
engine torque (T.sup.1) over available ignition timings (spark) for
homogenous operation at the maximum lean homogenous air/fuel ratio
(a/f.sub.max.sup.homogeneous) 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.sub.max.sup.homogeneous) at the current operating conditions
define 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 a torque disturbance will
occur when transitioning from stratified to homogenous mode if only
ignition timing is used to compensate for the mode change. Also,
these torque calculations are scaled for the current number of
active cylinders (Ncyl). Thus, one of ordinary skill in the art
will recognize the general applicability of the present invention
to any number of cylinders including when some cylinders may be
deactivated.
[0033] When this condition holds, cylinder deactivation is used. If
cylinder deactivation is not used, a mode transition can be carried
out using other control variables such as spark timing in step 216.
Otherwise, in step 218, the number of cylinders to deactivate is
determined. The number of cylinders to deactivate is based on the
size of the torque gap and several other factors, as described
later herein with particular reference to FIG. 3. Next, in step
220, the determined number of cylinders are deactivated while at
the same time the operating mode of the remaining cylinders is
changed. In addition, during the mode change, the air/fuel ratio is
also abruptly changed. However, according to the present invention,
the average output torque of the engine avoids abrupt changes.
[0034] Continuing with FIG. 2, if the answer to step 212 is no,
then the engine is currently operating in the homogeneous mode and
a transition from the homogeneous mode to the stratified mode is
required. In step 230, a determination is made if cylinder
reactivation is required based on the following equation:
min.sub.sparkT.sup.l(spark,a/f.sub.max.sup.homogeneous)<max.sub.sparkT.-
sup.l(spark,a/f.sub.min.sup.stratifed)
[0035] If the answer to step 230 is no, the routine continues to
step 216 previously described herein. Otherwise, in step 232, the
number of cylinders to enable is determined as described later
herein with particular reference to FIG. 3. Next, in step 233, a
determination is made as to whether the required number of
cylinders to enable (P) are currently deactivated. In other words,
it is not possible to enable cylinders if there are currently no
cylinders deactivated. When the answer to step 233 is Y, then, in
step 234, the determined cylinders are enabled and the engine
operating mode is simultaneously changed from the homogeneous mode
to the stratified mode. In addition, during this transition, the
air/fuel ratio in the cylinders is jumped.
[0036] When the answer to step 233 is NO, the mode transition is
postponed until there are (P) cylinders disabled in step 236. Then,
in step 238, (P) cylinders are disabled and the remaining cylinders
continuing to carry out combustion are enriched. In other words,
the air/fuel ratio of the cylinder continuing to carry about
combustion is decreased by changing the fuel injection amount to
those cylinders at the same time fuel injection is stopped to the
(P) cylinders to be deactivated. The new air/fuel ratio for the
remaining cylinders is found as described by the following
equation: 1 ( air fuel ) new = ( N cvl - ( P - V ) N cvl ) ( air
fuel ) current
[0037] where V is the current number of cylinder deactivated. The
routine can then return to step 234 and enable the disabled
cylinders and change operating mode from homogeneous to
stratified.
[0038] Referring now to FIG. 3, a routine for determining the
number of cylinders to enable/disable is described. In step 312 a
determination is made if the engine is currently operating in the
stratified mode. If the answer to step 312 is yes, the number of
cylinders to deactivate (M) is determined in step 314 from the
following equations, where: 2 T = min spark T i ( spark , a / f max
homogeneous ) - T requested M = roundup [ T min spark T i ( spark ,
a / f max homogeneous ) N cyl ]
[0039] Again, these torque calculations are scaled for the current
number of active cylinders (Ncyl). Thus, one of ordinary skill in
the art will recognize the general applicability of the present
invention to any number of cylinders including when some cylinders
may already be deactivated.
[0040] If the answer to step 312 is no, the number of cylinders to
activate (P) is determined in step 316 from the following equation,
where: 3 T ' = T requested - max spark T ' ( spark , a / f min
stratified ) P = roundup [ T ' max spark T i ( spark , a / f min
stratified ) N cyl ]
[0041] Thus, according to the present invention, during the mode
transition, where the air/fuel ratio is discontinuously jumped due
to engine performance constraints, abrupt changes in engine torque
are avoided by either activating, or deactivating, cylinders
concurrently with the air/fuel ratio jump.
[0042] Also, the "round up" function is used since the number of
cylinders to activate or deactivate to compensate the engine torque
during cylinder air/fuel ratio changes may not be exactly equal to
an integer value. Therefore, the remaining torque difference can be
compensated using methods known to those skilled in the art, such
as ignition timing.
[0043] In an alternative embodiment, the torque difference can be
calculated according to the formula:
.DELTA.T=.DELTA.T'=.vertline.min.sub.sparkT.sup.i(spark,a/f.sub.max.sup.ho-
mogeneous)-max.sub.sprkT.sup.l(spark,a/f.sub.min.sup.stratified).vertline.-
.
[0044] In another an alternative embodiment, where cylinder
air/fuel ratio is abruptly changed for reasons other than mode
transitions, the method described above with particular reference
to FIG. 2-3 can be applied. In particular, increases or decreases
in engine torque can be used for selecting either enabling, or
disabling cylinders. For example, when an increase in engine torque
will be produced (current engine torque is less than the future
engine torque, equivalent to switching from stratified to
homogenous mode), cylinder deactivation can be used. Similarly,
when a decrease in engine torque will be produced (current engine
torque is greater than the future engine torque, equivalent to
switching from homogenous to stratified mode), cylinder activation
can be used. The corresponding current and future engine torques
and air/fuel ratios can be substituted into the above
equations.
[0045] 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.
Accordingly, it is intended that the scope of the invention be
limited only by the following claims.
* * * * *