Lean air/fuel control system for an internal combustion engine

An air/fuel control system for an engine (10) provides an air/fuel indicating signal linearly related to average engine air/fuel operation from a two-state exhaust gas oxygen sensor (76). Fuel delivered to the engine is modulated with a periodic or modulation signal (244). The modulation signal is offset in either a fuel increasing or a fuel decreasing direction when the air/fuel indicating signal is respectively saturated at either a lean or a rich value (394-420).

BACKGROUND OF THE INVENTION 
The field of the invention relates to engine air/fuel control systems 
including systems which rapidly warm the catalytic converter. 
U.S. Pat. No. 5,211,011 discloses a system in which the fuel delivered to 
an engine is alternated between rich and lean values while ignition timing 
is retarded to more rapidly heat the catalytic converter. 
The inventors herein have recognized numerous problems with the above 
approach. For example, fuel modulation, under certain operating 
conditions, may cause an excessively lean shift in engine air/fuel ratio 
resulting in rough engine operation. Still another problem recognized by 
the inventors is that the air/fuel ratio may drift lean and cause 
misfires, or may drift rich causing excessive emissions. 
SUMMARY OF THE INVENTION 
An object of the invention claimed herein is to provide rapid warm-up of 
the catalytic converter using fuel modulation without incurring 
excessively lean or rich operation 
The problems of prior approaches are overcome, and the objects and 
advantages of the claimed invention achieved, by providing both a control 
method and a control system for an engine having an exhaust gas oxygen 
sensor and a catalytic converter positioned in the engine exhaust. In one 
particular aspect of the invention, the method comprises the steps of: 
modulating fuel delivered to the engine by a modulation signal; generating 
an indicating signal from the exhaust gas oxygen sensor output; and 
offsetting the modulation signal by a first predetermined offset in a fuel 
increasing direction when the indicating signal reaches a first 
preselected value and offsetting the modulation signal by a second 
predetermined offset in a fuel decreasing direction when the air/fuel 
indicating signal reaches a second preselected value. 
An advantage of the above aspect of the invention is that rapid warm-up of 
the catalytic converter is provided without incurring excessively lean or 
excessively rich engine air/fuel operation. 
In another aspect of the invention, the system comprises: an exhaust gas 
oxygen sensor with a two state output having first and second states 
respectively corresponding to exhaust gases being rich or lean of 
stoichiometry; a fuel controller delivering fuel to the engine in response 
to a desired fuel signal from an engine controller; the engine controller 
modulating the desired fuel signal with a modulation signal and averaging 
an output of the exhaust gas oxygen sensor to provide an air/fuel 
indicating signal having an amplitude related to engine air/fuel 
operation; and the engine controller offsetting the modulation signal by a 
first predetermined offset in a fuel increasing direction when the 
air/fuel indicating signal reaches a first preselected limit and 
offsetting the modulation signal by a second predetermined offset in a 
fuel decreasing direction when the air/fuel indicating signal reaches a 
second preselected limit. Preferably, the controller generates the 
modulation frequency with a first predetermined frequency until a first 
value corresponding to the preselected exhaust gas temperature is reached 
and thereafter generates the modulation signal at a second predetermined 
frequency which is less than the first frequency. 
An advantage of the above aspect of the invention is that rapid warm-up of 
the catalytic converter is provided without incurring excessively lean or 
excessively rich engine air/fuel operation. 
Another advantage of the aspect of the above invention is that drift in 
air/fuel operation is avoided thereby avoiding excessively lean or 
excessively rich air/fuel operation.

DESCRIPTION OF AN EXAMPLE OF OPERATION 
Internal combustion engine 10 comprising a plurality of cylinders, one 
cylinder of which is shown in FIG. 1, is controlled by electronic engine 
controller 12. Engine 10 includes combustion chamber 30 and cylinder walls 
32 with piston 36 positioned therein and connected to crankshaft 40. 
Combustion chamber 30 is shown communicating with intake manifold 44 and 
exhaust manifold 48 via respective intake valve 52 and exhaust valve 54. 
Intake manifold 44 is shown communicating with throttle body 58 via 
throttle plate 62. Intake manifold 44 is also shown having fuel injector 
66 coupled thereto for delivering liquid fuel 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 fuel system (not shown) including a fuel tank, fuel pump, and 
fuel rail. 
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 a desired air/fuel ratio and a low voltage state 
of signal EGOS indicates exhaust gases are lean of the desired air/fuel 
ratio. Typically, the desired air/fuel ratio is selected at stoichiometry 
which falls within the peak efficiency window of catalytic converter 70. 
Idle bypass passageway 94 is shown coupled to throttle body 58 in parallel 
with throttle plate 66 to provide air to intake manifold 44 via solenoid 
valve 96 independently of the position of throttle plate 62. Controller 12 
provides pulse width modulated signal ISCDTY to solenoid valve 96 so that 
airflow is inducted into intake manifold 44 at a rate proportional to the 
duty cycle of signal ISCDTY for controlling engine idle speed. 
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 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, 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: measurements of inducted mass air flow (MAF) from 
mass air flow sensor 100 which is coupled to throttle body 58 upstream of 
air bypass passageway 94 to provide a total measurement of airflow 
inducted into intake manifold 44 via both throttle body 58 and air bypass 
passageway 94; 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. 
A description of various air/fuel operations performed by controller 12 is 
now provided with initial reference to the flow charts shown in FIGS. 
2A-2B. During step 200, the fuel command (shown as desired fuel quantity 
Fd) is calculated by dividing the product of desired air/fuel ratio AFD 
times feedback variable FV into the product of inducted mass flow 
measurement MAF times correction value K. In this particular example, 
desired air/fuel ratio AFD is the stoichiometric value of the fuel blend 
used which is 14.3 pounds of air per pound of fuel for a low emissions 
fuel blend. Feedback variable FV and correction value K are each generated 
by the feedback routines, responsive to EGO sensor 76, which are described 
later herein with particular reference to respective FIGS. 2B and 5. 
Continuing with FIG. 2A, feedback variable FV is initially set to a fixed 
value for open loop air/fuel operation (step 202). Stated another way, 
desired fuel quantity Fd provides an open loop fuel command which is 
related to signal MAF and is not adjusted by feedback. In this particular 
example, feedback variable FV is set to unity which would correspond to 
operation at desired air/fuel ratio AFD under ideal operating conditions 
without any engine component aging. It is well known, however, that this 
open loop operation may not result in engine air/fuel exactly at 
stoichiometry. Correction by correction value K, however, will be provided 
as described below. 
When engine coolant temperature ECT is less than predetermined temperature 
T1 (step 206), engine temperature is too low to enter the subroutine for 
converter warm-up. The subroutine described with reference to steps 
208-210 is then entered to minimize the time required to start and 
reliably warm-up engine 10. In step 208, ignition timing is first set 
using the cold start table stored in ROM 10. Various sub steps are then 
performed during step 210. Open loop air/fuel operation proceeds by adding 
a rich offset to desired fuel quantity Fd. In this particular example, 
feedback variable FV is set to a fixed value less than unity. Correction 
value K is then extrapolated from two tables stored in ROM 10 which store 
correction K for cold engine operation and hot engine operation, 
respectively. In this example, the extrapolation occurs as a function of 
engine coolant temperature ECT. 
In the event engine coolant temperature ECT is greater than temperature T1 
(step 206), it is compared to temperature T4 (step 214) which is 
associated with hot engine operation and normal air/fuel ratio control. If 
engine coolant temperature CT is less than temperature T4, an inference of 
the temperature of catalytic converter 70 (ICAT) is compared to 
temperature T2 (step 216). 
When inferred temperature ICAT is less than temperature T2, ignition timing 
and engine idle speed are set per the warm-up schedules (step 218) 
provided for rapid catalyst warm-up. That is, ignition timing is retarded 
from its nominal value and idle speed elevated. Desired engine air/fuel 
ratio AFd is set to a lean value (AFLEAN) which is lean of stoichiometry 
by a preselected amount as shown in step 222. In this particular example, 
stoichiometry is 14.3 pounds of air per pound of fuel and AFLEAN is 14.6 
pounds of air per pound of fuel. During step 224, reference signal REF is 
see equal to lean value REFLEAN which corresponds to desired lean air/fuel 
ratio AFLEAN. 
On the other hand, if inferred temperature ICAT is greater than temperature 
T2, normal ignition timing and idle speed tables are utilized (step 228). 
Desired air/fuel ratio APd is then set equal to the air/fuel ratio 
corresponding to stoichiometry (AFSTOIC) as shown in step 232. During step 
234, reference signal REF is set equal to a value corresponding to the 
stoichiometric air/fuel ratio (REFSTOIC). 
Desired fuel quantity Fd is generated during step 240 which corresponds to 
the amount of liquid fuel to be delivered to engine 10. More specifically, 
desired fuel quantity signal Pd is generated by dividing the product of 
desired air/fuel ratio AFd and feedback variable FV into measurement of 
inducted mass air flow MAF times a correction value (not shown). Feedback 
variable FV is modulated during step 244 by modulation signal MODSIG which 
is read from the subroutine described later herein with particular 
reference to FIG. 6. 
A rolling average of signal EGO is generated during step 248. Error signal 
ERROR is generated during step 252 by subtracting reference signal REF 
from the rolling average of signal EGO (252). The feedback variable FV is 
then generated by applying a proportional plus integral (PI) controller to 
signal ERROR as shown in step 256. More specifically, signal ERROR is 
multiplied by proportional gain value P and the product added to the 
integral of signal ERROR. 
The operation and advantageous effects of steps 222-256 will be better 
understood by reviewing an example of operation with particular reference 
to the waveforms shown in FIGS. 3A-3C. Before discussing FIGS. 3A-3C, the 
description of updating the cold K and hot K tables is completed with 
continuing reference to FIG. 2B. 
When engine coolant temperature ECT is greater than temperature T3 (step 
282), but less than temperature T4 (step 284), each correction value K is 
interpolated from the cold K and hot K tables stored in ROM 10 for each 
engine speed load range (step 288). In the event engine coolant 
temperature ECT is greater than temperature T4 (step 284), each correction 
value K is selected from the hot K tables of ROM 10 (step 290). 
It is noted that correction values K for the hot K table are generated by 
adaptive learning as described later herein with particular reference to 
FIG. 5. By generating two sets of correction values (K) for cold and hot 
engine operation, and either extrapolating (step 210) or interpolating 
(step 288) between the tables, more accurate air/fuel operation is 
obtained. Once again, engine air/fuel operation is provided at either 
stoichiometry or preselected air/fuel ratios lean of stoichiometry by a 
preselected amount far more accurately than heretofore possible. 
Referring now to FIGS. 3A-3C and FIG. 4, graphical representations are 
shown which correspond to process steps 222-256 which were previously 
described with particular reference to FIGS. 2A-2B. In this particular 
example which depicts steady state lean air/fuel operation, reference 
signal REF is set to lean value REFLEAN to provide an average air/fuel 
ratio lean of stoichiometry while feedback variable FV is being modulated 
with a triangular wave (FIG. 3C). Such modulation occurs until an 
indication is provided that catalytic converter 70 has reached a desired 
temperature. 
In this particular example, the effect of such modulation and selection of 
lean reference value REFLEAN provides the exhaust air/fuel ratio shown in 
FIG. 3A. The average value of this air/fuel ratio is shown as the dashed 
line labeled AFLEAN which is lean of the stoichiometric air/fuel ratio 
labeled AFSTOIC. Corresponding signal EGO from sensor 76 is shown in FIG. 
3B wherein a high voltage state is indicative of air/fuel operation rich 
of stoichiometry and a low voltage state is indicative of air/fuel 
operation lean of stoichiometry. The rolling average of signal EGO is the 
air/fuel indicating signal (FIG. 4). In this example showing steady state 
operation, the rolling average of signal EGO is forced to the same value 
as lean reference value REFLEAN. 
Referring to FIG. 4, a hypothetical graphical representation of the rolling 
average of signal EGO, which is the lean air/fuel indicating signal, in 
relation to the average engine air/fuel ratio is shown. It is seen that an 
advantage of the invention claimed herein is that a linear air/fuel 
indicating signal is provided from a two-state exhaust gas oxygen sensor. 
In this particular example, the air/fuel indicating signal is used to 
operate engine 10 at an average value lean of stoichiometry using accurate 
feedback control. 
The adaptive learning subroutine for learning correction value K during 
both cold engine and hot engine operation is now described with reference 
to the flowchart shown in FIG. 5. Operation for entering closed loop 
air/fuel control is first determined in step 300 as soon as EGO Sensor 76 
reaches its operating temperature and engine coolant temperature ECT is 
not less than T1 in step 206 in FIGS. 2A-2B. Engine speed and load are 
then read during step 304 and the correction values generated below stored 
in tables for each speed load range. 
When engine coolant temperature ECT is less than temperature T4 (step 306) 
and also less than T3, the cold K tables are updated as now described. If 
feedback variable FV is greater than its nominal value (unity in this 
example) plus the lean offset introduced as previously described with 
reference to FIGS. 2A-2B (step 310), then the cold K table speed/load cell 
is decremented by .increment.K (step 312). On the other hand, if feedback 
variable FV is less than unity plus the lean offset (step 310), the 
corresponding speed/load cell in the cold K table incremented by 
.increment.K (step 314). 
Operation proceeds in a similar manner to adaptively learn correction value 
K during hot engine operation when engine coolant temperature ECT is 
greater than temperature T4 (step 306). More specifically, when feedback 
variable FV is greater than unity (step 320), the speed/load cell of the 
hot K table is decremented by .increment.K (step 322). Similarly, when 
feedback variable FV is less than unity (step 320), the speed/load cell of 
the hot K table is incremented by .increment.K (step 324). 
The subroutine described above with respect to FIG. 5 provides an adaptive 
learning of the difference or error between actual engine air/fuel 
operation and the desired air/fuel ratio. It is also operable when the 
desired air/fuel ratio is offset from stoichiometry by a preselected 
offset. 
Concurrently referring to the flowchart shown in FIG. 6 and the 
corresponding signals shown in FIGS. 7A-7C, the subroutine for generating 
modulation signal MODSIG is now described. Periodic or modulation signal 
MODSIG is used to modulate fuel delivered to the engine as previously 
described with particular reference to FIGS. 2A-2B. 
The temperature of exhaust gas oxygen sensor 76 (EGOT) is first estimated 
in step 394. In this particular example, the estimate is provided by 
measuring the peak-to-peak excursion of signal EGO. It is well-known that 
the peak-to-peak excursion in the output of an exhaust gas oxygen sensor 
is related to temperature. 
When estimated temperature EGOT is less than a temperature associated with 
warm and therefore stable operation of sensor 76 (temperature Te) as shown 
in step 396, modulation signal MODSIG is generated as a square wave at 
frequency F1 (step 398). In this particular example, frequency F1 is on 
the order of 4 hertz which is considerably greater than the modulation 
frequency desired for normal operation. Using square wave modulation at 
such a frequency was found to provide well-mixed exhaust gas having 
relatively high concentrations of both CO and O.sub.2. Rapid warm-up of 
catalytic converter 70 is thereby provided when combined with the ignition 
timing retard strategy and fuel enleanment previously described herein 
with reference to FIGS. 2A-2B. 
When estimated temperature EGOT is greater than preselected temperature Te 
(step 396), modulation signal MODSIG is generated as a triangular wave 
with frequency F2 (step 400). In this particular example, frequency F2 is 
in the range of 1-2 hertz which has been found to be suitable for accurate 
engine air/fuel control. 
After delay time T1 (step 402), the AIR/FUEL INDICATING SIGNAL is read 
during step 404. If the AIR/FUEL INDICATING SIGNAL is saturated rich (step 
406), modulation signal MODSIG is shifted or offset a preselected lean 
offset (step 420), after delay time T2 (step 418). This lean offset of 
modulation signal MODSIG is shown graphically in FIG. 7C. 
When the AIR/FUEL INDICATING SIGNAL is not detected as being saturated rich 
during step 406, it is read again during step 410 after delay time T3 
(step 408). If the AIR/FUEL INDICATING SIGNAL is then detected as being 
saturated lean (step 412), modulation signal MODSIG is offset by a 
preselected rich offset (step 416) after delay time T4 (step 414). This 
rich offset is shown graphically in FIG. 7B at time T4. 
On the other hand, if the AIR/FUEL INDICATING SIGNAL is not saturated lean 
(step 412), no offset is provided to modulation signal MODSIG (step 424). 
Modulation signal MODSIG without such an offset is shown in FIG. 7A. 
Although one example of an embodiment which practices the invention has 
been described herein, there are numerous other examples which could also 
be described. For example, analog devices, or discreet IC's may be used to 
advantage rather than a microcomputer. Further, different feedback 
controllers other than proportional plus integral may be used to 
advantage. The invention is therefore to be defined only in accordance 
with the following claims.