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Description  |
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FIELD OF THE INVENTION
This invention relates to improvements in air-fuel ratio control in
internal combustion engines.
BACKGROUND OF THE INVENTION
In conventional automotive engine control, the extent that certain
well-known engine and emission system performance goals can be achieved is
largely determined by the capacity to control the engine air-fuel ratio.
In general, many conventional vehicle powertrain controllers PCMs attempt
to maintain the engine air-fuel ratio at the well known stoichiometric
ratio (.lambda.=1). This ratio is generally found to yield satisfactory
engine performance.
Engine control systems that are capable of controlling fuel, air, and
recirculated exhaust gas EGR, attempt to maintain the air-fuel ratio at
stoichiometry by coordinating control of the quantity of fuel, air, and
EGR admitted into the engine, based on predetermined relationships between
those control parameters calibrated for the specific engine application,
and based on the present engine operating condition.
Such control may not account for manufacturing variations or for
disturbances to the control system, for example the inevitable system
performance changes due to aging. As such, it is common in the art of
engine control to sense the performance of the air-fuel ratio control
itself, for example using an oxygen sensor located in the exhaust path of
the engine to observe, in a conventional manner, the actual engine
air-fuel ratio. The observed (sensed) air-fuel ratio may then be fed back
to the engine controller, which may trim (adjust) one of the three control
parameters in order to compensate for the variations or disturbances.
In many such systems, fuel is a high resolution control parameter, making
it an attractive candidate when precise air-fuel ratio control is desired.
However, such systems may be "fuel-lead" systems in that the driver
directly sets a fuel command which is directly related to engine torque,
and only indirectly sets the air and EGR commands. As such, fuel command
adjustments tend to be more perceptible to the driver in these systems.
Such perceptibility is generally considered to be a disadvantage, as it
disturbs the torque command--particularly in transient maneuvers.
Alternatively, these fuel-lead systems may trim the quantity of air
admitted into the engine in engine air-fuel ratio control. Because air,
unlike fuel, is not directly controlled by the driver in these systems,
air trim is less perceptible to the driver. However, air trim does not
provide the resolution available with fuel trim, and air trim can only be
used in certain engine operating regions.
Further, these fuel-lead systems may trim the quantity of EGR admitted into
the engine for air-fuel ratio control. Like air trim, EGR trim is less
likely to be perceived by the driver in many of these systems. Further,
when the quantity of EGR and the ratio of fuel to air in the engine rise
or fall together, such as when EGR is trimmed for air-fuel ratio control,
the desired air-fuel ratio correction may be achieved while limiting the
creation of oxides of nitrogen (an undesirable combustion product) in the
engine. Still further, the engine spark command is less sensitive to EGR
trim than to fuel or air trim. However, EGR is not available or desirable
in certain engine operating regions, such as in high engine load regions,
or at idle. Further, EGR control does not have the resolution available
with fuel control.
In the above-described systems, there are advantages and disadvantages
associated with trimming fuel, air or EGR in order to control air-fuel
ratio. What is needed is a system control strategy that controls engine
air-fuel ratio using all three control parameters in a manner that retains
the benefits of each and minimizes their weaknesses.
SUMMARY OF THE INVENTION
The present invention comprises a comprehensive air-fuel ratio control
method for an engine controller that is not limited to control of a single
engine parameter, but selects and adjusts the parameter best suited to the
present engine operating condition.
In general, the method senses the engine operating condition and
determines, based on that condition, which control parameter is best
suited for air-fuel ratio control in terms of the benefits and detriments
it provides at that operating condition.
For example, at low load operating conditions, air has insufficient
resolution for air-fuel ratio control. Additionally, EGR is typically not
active at such operating conditions, due to concerns over control
stability. Accordingly, fuel is the parameter that is trimmed in the
closed loop air-fuel ratio control, as it has sufficient resolution for
the delicate control in the low load operating region and, as the driver
is typically not engaging the accelerator pedal in the low load region,
concern that the driver will perceive the fuel trim is minimized.
Engine control systems often abandon stoichiometry at extremely high engine
loads, allowing the engine to operate with a slightly "rich" air-fuel
mixture. Prior to that "extremely high" engine load range, there is a high
load range in which it is desirable to maintain a stoichiometric mixture.
In this range, air is at or near its maximum flow capacity, providing
little usefulness as a control parameter. Controlling around EGR is
possible in such a region, but too much EGR can erode engine torque yield,
which is generally considered a disadvantage in higher engine load
operating ranges. As such, fuel is trimmed in such regions for
stoichiometric control.
Finally, in the operating region between the described low and high load
regions, EGR may be trimmed in air-fuel ratio control. Fuel is avoided to
minimize driver perception of the control. Air trim also may be used for
air-fuel ratio control in the region, but EGR trim is preferred thereover
in that it is likely to be even less perceptible than air. However, if EGR
trim becomes saturated (runs out of authority), the system may further
refine the air-fuel ratio by holding the EGR trim steady, and by trimming
either air or fuel.
This method combines the advantages of the three potential control
variables to provide air-fuel ratio control over the applicable engine
operating range that does not substantially compromise power when it is
desired, does not compromise control precision, and minimizes driver
perception.
DESCRIPTION OF THE ILLUSTRATIONS
FIG. 1 is a general diagram of an engine control system; and
FIGS. 2, 3a, 3b, 4, 5a, 5b and 5c are computer flow diagrams illustrating
the operation of the system of FIG. 1 in accord with the principles of
this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, an internal combustion engine 10 is provided fuel by
some conventional fuel delivery system, such as by fuel injectors 12
mounted in proximity to each of the cylinders of the engine. The injectors
may be controlled in a conventional manner, for example by an injection
pulse width generated in the powertrain controller PCM 16, the pulse width
being indicative of the amount of time an injector should be injecting
fuel into the engine 10.
In the preferred embodiment, the quantity of fuel to be administered into
the engine is directly related to the driver's request, such as by the
driver's positioning of the vehicle accelerator pedal 24. Such systems,
often called fuel-lead systems, make the fuel command directly related to
the driver's request, and make air and EGR commands only indirectly
related to the driver's request.
Air is drawn into the engine through a bore, wherein a throttle valve 14 is
located to control the amount of air allowed into the engine according to
a command generated by the PCM 16. In this embodiment, the valve consists
of a flat blade 14 which rotates with respect to the air inlet according
to the angular position of a rotary actuator 18 connected to the blade by
a shaft in a conventional manner. The angular position of the actuator is
controlled by the PCM 16.
The angular position of the blade 14 is monitored by an angular position
sensor 20, such as a conventional rotary potentiometer, mounted in
proximity to the blade, for example on the shaft. The position sensor 20
monitors the blade angle and transmits the measurement to the PCM 16. The
inventors contemplate that other air control means may be used in the
system incorporating this invention, such as engine valve control which
meters the air ingested into the engine via PCM controlled valve lift
actuators.
An apparatus 24 by which the vehicle operator may apprise the PCM 16 of a
desired engine operating point is located in the vehicle passenger
compartment. This apparatus may simply be a conventional accelerator pedal
24, with a pedal position sensor associated therewith so as to monitor the
pedal position and transmit the measurement to the PCM 16.
The fuel injected into the engine 10 by means of the fuel injectors 12 is
combined with air admitted into the engine by the throttle valve 14, and
is distributed to the engine cylinders, where it is ignited in a
conventional manner, the product of the ignition being expelled as exhaust
gas through an exhaust conduit 26.
A closed conduit 28 is attached between the engine exhaust conduit 26 and
the intake manifold for transporting a quantity of the exhaust gas back to
be recirculated into the engine 10 with the engine intake air, according
to the conventional process of exhaust gas recirculation EGR.
A valve 30 is situated in the conduit 28 to meter the quantity of exhaust
gas recirculated. The valve 20 can be any conventional valve capable of
controlling the flow of a gaseous substance, such as a conventional
butterfly valve. The valve is controlled by a rotary actuator 32 which may
be connected to the valve 30 by a common drive shaft. The actuator 32 is
controlled according to a command generated in the PCM 16. The position of
the valve is monitored by a conventional valve position sensor 36, such as
a rotary potentiometer, located in proximity to the EGR valve. The
measured EGR position is transmitted to the PCM 16.
The ignited air-fuel mixture creates a force which ultimately rotates the
crankshaft of the engine in the usual manner. A conventional rotational
speed sensor (not shown) is located in proximity to the crankshaft of the
engine 10 to sense the rotational speed of the crankshaft. The sensed
value is transmitted to the PCM 16 as a measurement of engine speed RPM.
A conventional oxygen sensor 34 is located in proximity to the engine
exhaust conduit in such a manner that a substantial portion of the engine
exhaust gas flows by the sensor. The sensor generally indicates the oxygen
content in the exhaust gas, the indication being transmitted to the PCM
16. The PCM periodically reads and categorizes the indication as
describing a "rich" engine operating condition, where the ratio of air to
fuel is below the stoichiometric ratio, or a "lean" operating condition
where the air to fuel ratio is above the stoichiometric ratio.
In predetermined engine operating regions, the PCM will attempt to maintain
the air to fuel ratio close to the stoichiometric ratio. Necessarily, such
operation will cause frequent switching of the oxygen sensor indication
from a "lean" indication to a "rich" indication. The PCM, in a
conventional manner, categorizes the sensor indication as either rich or
lean. Once categorized, the air-fuel ratio status is stored in memory for
use in accord with the principals of this invention.
The PCM 16 takes the form of a standard digital computer, such as a
Motorola MC68HC11 single chip microcomputer. The principles of this
invention are implemented in the form of an operating program stored in
the computer's memory.
Automotive engine air-fuel ratio control methods attempt to drive the
actual air-fuel ratio to a desired value when operating in predetermined
operating ranges. Fuel-lead engine control systems, which are systems that
directly control fuel as a function of an operator command such as the
operator's positioning of the vehicle accelerator pedal, and that control
air, EGR and spark corresponding to the fuel command, will adjust (trim)
only one of the fuel, air, and EGR commands to correct for deviations from
the desired air-fuel ratio. Often, such adjustments are stored in a memory
location corresponding to a "cell" pertaining to the operating conditions
at which the adjustment was deemed necessary.
The engine operating region in which closed loop air-fuel ratio control is
to be carried out may be divided into a predetermined number of such
cells. The values in the memory locations corresponding to these cells are
initialized (or re-initialized after memory has been cleared) to values
corresponding to "no correction". Later, when operating in a certain cell,
any air-fuel ratio adjustments made will be recorded in some conventional
fashion in the cell. The active cell value will then be used in the
ultimate calculation of the desired quantity of the parameter to be
admitted into the engine.
It has been shown that certain engine operating regions are better suited
to say, fuel trim, while others are be better suited to either air or EGR
trim. The present invention takes this information into account in that it
does not limit the parameter to be trimmed to any one of the three
mentioned parameters, but trims the parameter that provides the most
desirable operation depending on the present engine operating point.
In this embodiment, each operating cell may be defined by engine operating
conditions, such as by engine speed or engine load or both. A series of
operating cells over the engine operating range is provided for in system
memory in the form of a memory lookup table. In this embodiment, one
lookup table is provided in non-volatile memory to store block learn
values for each of the three engine control parameters of air, fuel and
EGR. Individual entries in the above-described tables contain closed loop
control information for the cell associated therewith.
In accord with the principles of this invention, while closed-loop air-fuel
ratio control is active, only one of the three of air, fuel, or EGR will
be active for air-fuel ratio control. The active parameter has a closed
loop correction factor CLCF which is adjusted in a rapid manner in
response to the sensed actual air-fuel ratio and a block learn table, one
value within the table being adjusted more slowly than the CLCF, in
response to the CLCF. In general, the CLCF value and the value in the
block learn table as adjusted are used to drive the actual engine air-fuel
ratio toward the stoichiometric air-fuel ratio.
For example, the CLCF value associated with the active parameter is trimmed
or adjusted rapidly in accord with a short time constant in response to
the rich or lean output of the oxygen sensor 34 and in direction to
restore the air-fuel ratio to the stoichiometric ratio. The rapid response
is primarily used to provide quick compensation for variations in the
air-fuel ratio from stoichiometry. The CLCF value is a multiplier which,
when increased in magnitude increases the air-fuel ratio, and when
decreased in magnitude decreases the air-fuel ratio. The fuel, air, and
EGR commands are multiplied by their CLCF multipliers before the commands
are issued to the respective actuators. In this embodiment, a CLCF value
of 128 represents no correction to the command, or a unity gain
multiplier, with the value being increased or decreased therefrom as
necessary. The three CLCF values are stored in volatile memory.
The value associated with the active cell in the block learn table that is
operating is trimmed or adjusted more slowly than the CLCF values, but
still is adjusted to drive the actual air-fuel ratio toward stoichiometry.
There is one block learn value available for each cell in the tables
associated with each of fuel, air and EGR. The individual block learn
values, instead of being trimmed directly in response to the actual
air-fuel ratio as sensed by the oxygen sensor 34, are rather adjusted
according to the state of the corresponding CLCF value. However, the
effect of trimming the block learn values is the same as the effect of
trimming the CLCF values. For example, increasing the block learn value
will increase the air-fuel ratio, and decreasing the block learn value
will decrease the air-fuel ratio. Further, like the CLCF values, a block
learn value of 128 represents unity gain, and the value is increased or
decreased therefrom as necessary.
The active block learn value is adjusted in the same direction as the
corresponding CLCF value, so as to ultimately drive that CLCF value back
toward the unity gain value. For example, if the CLCF value corresponding
to the one of fuel, air, and EGR that is active is increased beyond its
unity value, the corresponding block learn value is, in time, increased.
The effect of the block learn increase is an increased air-fuel ratio
which, when sensed by the oxygen sensor 34, will force the corresponding
CLCF value back toward the unity gain value.
The adjustment of the block learn value is slow in accord with a long time
constant. As such, unnecessary excursions are avoided, such as from CLCF
response to transient disturbances in the system, such as sensor noise.
Unlike the CLCF values, the block learn values are stored in non-volatile
memory to provide a long-term correction to the evolving needs of the
system. They are intended to provide a more permanent, more careful
adaptation to system changes than the CLCF values. By combining the CLCF
values with the block learn values, the system has the capacity to quickly
respond to changing system needs (via the CLCF values), while
"remembering" any adjustments needed to operate at the desired air-fuel
ratio in the longer term (via the block learn values).
Referring to FIG. 2, when power is first applied to the system, such as
when a conventional vehicle ignition switch is turned to its "on"
position, the PCM initiates the engine control program at step 50 and then
proceeds to step 52 where the PCM provides for system initialization. For
example, at this step data constants are transferred from read only memory
locations to random access memory locations and counters, flags and
pointers are initialized. Additionally at this step, a general counting
variable is initialized to zero.
The routine then proceeds to step 54 where the above described three
closed-loop correction factors CLCFs are initialized. The CLCF values are
stored in volatile memory, such that they must be initialized at step 54
whenever memory keep-alive power is dropped. In the preferred embodiment,
these values are initialized to 128, which corresponds to a unity gain
factor in the closed loop control, as discussed.
The routine then proceeds to step 55, to determine whether there has been a
loss of non-volatile memory since the last operation of the PCM, for
example by a battery disconnect or by some system power failure. If such a
memory loss has occurred, the routine proceeds to step 57, to initialize
non-volatile memory to appropriate initial values. Most importantly in the
context of the present invention, block learn values stored in the three
block learn tables are initialized to 128 (a unity gain value in this
embodiment) at this point.
Next, or if there has been no loss of non-volatile memory, the routine
moves to step 56 where interrupts used in vehicle control, including
engine control in accord with this invention, are enabled. The interrupt
pertaining to execution of the routine incorporating the principles of
this invention, called the real-time interrupt, is enabled at this step to
occur approximately every 6.25 milliseconds. The PCM then proceeds to a
background loop at step 60 which is continuously repeated. This loop may
include system diagnostic and maintenance routines. In this embodiment,
the PCM interrupts the background loop upon occurrence of the real-time
interrupt to execute a general real-time interrupt service routine
incorporating the principles of this invention.
This general routine is illustrated in FIG. 3, and is entered at step 70.
The PCM proceeds to step 72 to execute general engine control functions
well known in the art of engine control, especially those functions
necessary for execution of the routine incorporating the principles of
this invention. For instance, at this step a desired idle air command is
generated, and the engine air-fuel ratio AFRAT is calculated.
The routine then proceeds to step 73 to determine engine load EL, and
engine speed RPM. These two values are commonly used in engine control as
indicative of the engine operating point. EL may be calculated as a
function of the operator requested engine operating point, such as from
the position of an accelerator pedal in a vehicle, and RPM may be
determined in a conventional manner, such as from a sensor located in
proximity to an engine output shaft that rotates at a speed proportional
to the engine speed.
The routine then advances to step 74, to determine the active cell to be
used in the current iteration of this routine for the purpose of trimming
one of air, fuel, or EGR in accord with this invention. As discussed, the
active cell is related to the engine operating point, which may be
ascertained from the above-determined engine load and speed.
The routine then moves to step 75 to ascertain whether the engine operating
point has changed to the extent that the present active cell as determined
at step 74 differs from the cell that was active in the previous iteration
of this routine. If so, the routine proceeds to step 76, where the three
CLCF values may be reset to their unity gain values, as any air-fuel ratio
correction information that may have been contained in the CLCF values
from the previous iteration may not be in accord with the needs of the
engine in its current operating state. Accordingly, that potentially
obsolete information may, at the option of the system designer, be
discarded, and new corrections may take place which are more likely to be
in accord with the needs of the engine in its current operating state.
Next, or if the active cell did not change, the routine moves to step 77,
to determine a fuel command in a conventional manner, according to the
following equation
Fuel Command=(EL*BL.sub.F /128* CL.sub.F /128)/(AFRAT)
where EL is engine load, BL.sub.F is the block learn value associated with
the active cell in the fuel block learn lookup table, CL.sub.F is the fuel
closed loop correction factor, and AFRAT is the calculated actual engine
air-fuel ratio. As discussed, the value 128 represents unity gain for both
the block learn value and the closed loop correction factor. This is
illustrated in the above fuel equation, where both of these values are
divided by 128.
After computing the fuel command, the routine proceeds to steps 78 and 80,
to compute the desired EGR command. First, at step 78, a base EGR command
is calculated. The routine for computing the base EGR command may be any
conventional EGR computation routine which determines a desirable quantity
of exhaust gas to be recirculated into the engine intake to be combined
with the engine intake air. After computing the base EGR command, the
overall EGR command is calculated at step 80 in a conventional manner.
In the preferred embodiment, the overall EGR command is calculated
according to the following equation
EGR Command=BL.sub.E /128*base EGR command*CL.sub.E /128
where BL.sub.E is the block learn value associated with the active cell in
the EGR block learn lookup table, and CL.sub.E is the EGR closed loop
correction factor. Both BL.sub.E and CL.sub.E are divided by 128 for
scaling purposes, as in the case of the already discussed fuel command
calculation. The calculation at step 80 merely trims the conventional base
EGR command according to the previously calculated stored block learn and
CLCF values, so as to drive the actual air-fuel ratio toward the desired
air-fuel ratio.
Next, at steps 82 and 84, the desired quantity of air to be admitted into
the engine is calculated. First, at step 82, a base air command is
determined. This term is used as a general purpose calibration factor that
may be determined in a conventional engine calibration process, as a
desired amount of air to be admitted into the engine at the current engine
operating point. In this embodiment, the base air command is determined as
a function of the engine operating range as indicated by engine speed and
engine load, and further is based on the ratio of the total air charge
(including both the quantity of EGR admitted to the engine and the
quantity of "fresh" air admitted to the engine) to the fresh air charge.
After determining the base air command, the routine advances to step 84, to
determine the air command according to the following equation
Air Command=base air command/((BL.sub.A /128)*(CL.sub.A /128))
where BL.sub.A is the block learn value associated with the active cell in
the air block learn lookup table, CL.sub.A is the air closed loop
correction factor, and BL.sub.A and CL.sub.A are divided by 128 for
scaling purposes, as discussed above for the fuel and EGR command
calculations.
After computing the air command, the routine proceeds to step 88, to select
an air-fuel ratio trim mode in accord with the principles of this
invention. The mode select is carried out by the routine illustrated in
FIG. 4, and will be discussed shortly.
The routine then proceeds to step 90, to perform any necessary adjustments
of the closed loop correction factor CLCF for the active one of fuel, air,
and EGR parameters, as selected in the routine illustrated in FIG. 4. The
CLCF adjustment step is well known in the art of engine air-fuel ratio
control, as discussed. In this embodiment, an adjustment may be made once
per real-time interrupt, so as to provide quick response to the evolving
needs of the system.
After adjusting the CLCF value, the routine proceeds to steps 92 through
98, to make any necessary adjustments to the block learn value associated
with the active cell in the block learn lookup table corresponding to the
mode selected in the routine illustrated in FIG. 4. The block learn
adjustment, unlike the CLCF adjustment, is not carried out each time the
real-time interrupt service routine is executed, but rather is carried out
after a predetermined number of real-time interrupts occur. For instance
in the preferred embodiment, it is desired that the block learn values be
updated approximately every 200 milliseconds, which requires about 32
iterations of the real-time interrupt between successive iterations of the
block learn adjustment routine.
Accordingly, at step 92, a general counting value i, which is reset upon
system power-up at step 52 of the routine illustrated in FIG. 2, and is
incremented upon every execution of the real-time interrupt service
routine, is compared to a predetermined value n. If i equals n, the
routine proceeds to step 96 to reset i so as to set up the next delay
period. The routine then proceeds to step 98 to carry out the block learn
routine, which is illustrated in FIG. 6, and will be described shortly.
Returning to step 92, in the preferred embodiment, the value of n is set to
32 so as to provide approximately a 200 millisecond delay period between
successive executions of the block learn calculation routine.
After executing the block learn calculation routine at step 98, the routine
moves to step 99, to issue, in any conventional manner the above
determined commands to their respective actuators so as to administer the
desired amount of fuel, EGR and air to the engine. Next, the routine
proceeds to step 100, to re-arm the real-time interrupt in preparation for
the next iteration of this routine. The routine then moves to step 102,
where it returns to the background loop illustrated as step 58 in FIG. 2.
Returning to step 92, if i does not equal n, meaning that 200 milliseconds
have not yet passed since the previous iteration of the block learn
calculation routine, the routine proceeds to step 94, to increment the
counter as an indication that the routine is one step closer to another
execution of the block learn calculation routine. The routine then
proceeds to step 100 to re-arm the real-time interrupt, and then returns
to the background loop of FIG. 2, via step 102, as discussed.
The mode select routine, called from step 88, is illustrated in FIG. 4, and
is entered at step 130. This routine generally selects the operating
parameter to be adjusted so as to drive the air-fuel ratio toward a
desired air-fuel ratio. As previously discussed, the engine air-fuel ratio
may be "fine-tuned" by trimming or adjusting the quantity of fuel, air or
EGR admitted into the engine, assuming the engine has the capacity to
control each of these quantities.
Practical constraints exist in conventional engine control that make it
advantageous to trim some engine parameters in certain engine operating
ranges, and other parameters in other ranges. Further, certain parameters
simply cannot be used in some engine operating ranges. For instance, when
the engine of many conventional systems is operating at idle, air cannot
be trimmed, as the air valve is near closed, and has poor resolution.
Similarly, in high power modes of operation, air typically cannot be
trimmed because the air valve is likely to have run out of authority.
It is well known in the art that EGR control has an undesirable effect on
idle stability, and as such should not be used at idle. Further, EGR trim
is generally avoided in high power modes of operation, as it can attenuate
engine power. Still further, EGR trim has limited authority, such that
even extreme (minimum or maximum) amounts of EGR trim may not provide
sufficient air-fuel ratio compensation.
Fuel trim is generally only used in engine operating ranges where air and
EGR trim cannot or should not be used, as fuel trim is typically more
perceptible to the driver in any operating range, which is considered to
be a disadvantage in engine control.
The routine of FIG. 4, in accord with the principles of this invention,
accounts for the above described advantages and disadvantages associated
with trimming the three control parameters in engine control by selecting
the parameter considered to provide the most benefit at the most recent
sensed engine operating conditions. The closed loop correction factor
associated with the selected parameter and the block learn value for the
cell corresponding to the present engine operating conditions for that
selected parameter will then be trimmed as necessary for air-fuel ratio
control at steps 90 and 98 of the routine of FIG. 3, as discussed.
Additionally, if EGR trim is determined not to provide sufficient
compensation at its extreme values, supplemental compensation may be
provided by trimming fuel or air. In this embodiment, air is selected in
the routine of FIG. 4 as the parameter by which the additional
compensation is provided when EGR trim is found to be inadequate for
complete compensation.
Specifically, the routine starts at step 130 of the routine illustrated in
FIG. 4, and proceeds to step 132 to determine if the engine is in an idle
state. The idle state may be diagnosed using the most recent measurements
of engine speed and engine load in a conventional manner. If the engine is
in an idle state, the above discussed practical considerations make it
most beneficial to use fuel trim to control air-fuel ratio. As such, the
routine proceeds to step 134 to enable fuel mode, and to disable the other
two modes, which may have been ac | | |
