Engine air/fuel control with monitoring

A control system and method maintains engine air/fuel operation near stoichiometry in response to exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter. A converter efficiency test cycle is generated during air/fuel feedback control after engine operation occurs in a plurality of airflow ranges for a period determined by a count in transitions of a feedback variable derived from the upstream sensor. Upon each transition in the downstream sensor, the measurement of inducted airflow at which such transition occurred is accumulated. At the end of the test cycle, converter efficiency is determined by an average of such airflow accumulations.

FIELD OF THE INVENTION 
The invention relates to controlling engine air/fuel ratio while 
concurrently monitoring the efficiency of a catalytic converter coupled to 
the engine's exhaust. 
BACKGROUND OF THE INVENTION 
Air/fuel engine control systems responsive to exhaust gas oxygen sensors 
positioned both upstream and downstream of a catalytic converter are well 
known. Various attempts have been made to provide an indication of 
converter efficiency in response to outputs derived from the upstream and 
downstream sensors. In one approach a comparison of downstream to upstream 
sensor amplitudes over a predetermined time provides an indication of 
converter efficiency. In another known approach, a frequency ratio of 
downstream to upstream sensor outputs over a predetermined time was used 
to provide an indication of converter efficiency. 
The inventors herein have recognized numerous problems with these prior 
approaches. For example, variations in the manner in which a vehicle is 
driven during the predetermined time period in which converter efficiency 
was tested may result in variations of test results. 
SUMMARY OF THE INVENTION 
An object of the invention herein is to provide accurate monitoring of 
catalytic converter efficiency while concurrently maintaining engine 
air/fuel control. The above object is achieved, and problems of prior 
approaches overcome, by providing both a control system and method for 
controlling engine air/fuel ratio while concurrently monitoring efficiency 
of the converter. In one particular aspect of the invention, the method 
comprises the steps of: adjusting the engine air/fuel ratio in response to 
a feedback variable derived from an output of an upstream exhaust gas 
oxygen sensor positioned upstream of the converter; accumulating one of a 
plurality of airflow values upon each transition in output states of a 
downstream sensor positioned downstream of the converter, each of the 
airflow values being related to one of a plurality of inducted airflow 
ranges and the airflow value which is accumulated upon the downstream 
sensor output transition is related to the airflow range in which the 
downstream sensor output transition occurred; and averaging the 
accumulated airflow values over a test period and providing an indication 
of converter degradation when the average falls below a preselected 
average. Preferably, the test period is completed when the engine has 
operated within each of a plurality of inducted airflow ranges for at 
least a minimum duration in each of the airflow ranges. The minimum 
duration is preferably determined when a preselected number of upstream 
sensor output transitions has occurred for each of the airflow ranges. 
An advantage of the above aspect of the invention is that the operating 
conditions under which the converter is tested remain relatively stable 
from one test to another despite variations in the manner in which the 
vehicle is operated during the test period. Highly accurate and consistent 
test results are thereby obtained.

DESCRIPTION OF AN EMBODIMENT 
Controller 10 is shown in the block diagram of FIG. 1 as a conventional 
microcomputer including: microprocessor unit 12; input ports 14; output 
ports 16; read-only memory 18; random access memory 20; keep-alive memory 
22; and a conventional data bus. Controller 10 is shown receiving various 
signals from sensors coupled to engine 28 including: measurement of 
inducted mass airflow (MAF) from mass airflow sensor 32; engine coolant 
temperature (T) from temperature sensor 40; indication of engine speed 
(rpm) from tachometer 42; output signal FEGO derived by conventional 
filtering and threshold comparison of the output from conventional exhaust 
gas oxygen sensor 44 positioned upstream of catalytic converter 50; and 
signal REGO derived by conventional filtering and threshold comparison of 
the output from another conventional exhaust gas oxygen sensor (52) 
positioned downstream of catalytic converter 52. 
Intake manifold 58 of engine 28 is shown coupled to throttle body 60 having 
primary throttle plate 62 positioned therein. Throttle body 60 is also 
shown having fuel injector 76 coupled thereto for delivering liquid fuel 
in proportion to the pulse width of signal fpw from controller 10. Fuel is 
delivered to fuel injector 76 by a conventional fuel system including fuel 
tank 80, fuel pump 82, and fuel rail 84. 
Other engine components and systems such as an ignition system are not 
shown because they are well known to those skilled in the art. Although a 
central fuel injection system is shown, the invention claimed herein may 
be used to advantage with other types of systems such as sequential fuel 
injection or carbureted systems. Those skilled in the art will also 
recognize that the invention claimed herein is applicable to other engine 
control configurations such as "stereo" control systems wherein the fuel 
injectors for each bank are controlled by a separate exhaust gas oxygen 
sensor positioned in each of the exhaust manifolds in engines having a "V" 
configuration. 
Referring now to FIG. 2, a flowchart of a routine performed by controller 
10 to generate fuel trim signal FT is now described. In the particular 
example described herein, closed-loop air/fuel control is commenced (step 
104) when engine temperature is within a predetermined range, the engine 
has been operating for at least a preselected time, and throttle position 
is within a preselected range. When closed-loop control commences, signal 
REGO is read (step 108), multiplied by gain constant GI (step 126), and 
the resulting product added to products previously accumulated (GI * 
REGO.sub.i-1) in step 128. Stated another way, signal REGO is integrated 
each sample period (i) in steps determined by gain constant GI. 
During step 132, signal REGO is multiplied by proportional gain GP. The 
integral value from step 128 is added to the proportional value from step 
132 during addition step 134 to generate fuel trim signal FT. 
The routine executed by controller 10 to generate the desired quantity of 
liquid fuel delivered to engine 28 is now described with reference to FIG. 
3. During step 158, an open-loop fuel quantity is first determined by 
dividing measurement of inducted mass airflow (MAF) by desired air/fuel 
ratio AFd which is typically the stoichiometric value for gasoline 
combustion. This open-loop fuel charge is then adjusted, in this example 
divided, by feedback variable FV which is generated as now described with 
respect to steps 160-178. 
After determining that closed-loop control is desired (step 160), by 
monitoring engine operating conditions such as those previously described 
herein with reference to step 104 in FIG. 2, signal FEGO is read during 
step 162. Signal FEGO is then trimmed (in this example by addition) by 
trim signal FT which is transferred from the routine previously described 
with reference to FIG. 2 to generate trimmed signal TS. The product of 
integral gain value KI times trimmed signal TS (step 170) is generated and 
added to the previously accumulated products (step 172). That is, trimmed 
signal TS is integrated in steps determined by gain constant KI each 
sample period (i) during step 172. A product of proportional gain KP times 
trimmed signal TS (step 176) ms then added to the integration of KI * TS 
during step 178 to generate feedback variable FV. 
Alternatively, the process described above with particular reference to 
FIG. 3 may be performed by biasing signal FV, rather than trimming signal 
FEGO, with fuel trim signal FT. In one such alternative embodiment, two 
proportional gain constants (KP.sub.1 and KP.sub.2) are used to advantage. 
Proportional gain KP.sub.1 multiplies signal FEGO when it switches from a 
lean to a rich indicating state and proportional gain KP.sub.2 multiplies 
signal FEGO when it switches from a rich to a lean state. Proportional 
term KP.sub.1 is incremented when fuel trim signal FT indicates a lean 
bias is desired and proportional term KP.sub.1 is decreased (or KP.sub.2 
incremented) when a rich bias is desired by fuel trim signal FT. 
Referring now to FIGS. 4A-4B, signals FV and REGO are band pass filtered 
and then rectified during respective steps 302 and 304. A hypothetical 
signal FV is shown in FIG. 5A and in FIG. 5B after it is band pass 
filtered. Similarly, a hypothetical signal REGO is shown in FIG. 6A and 
after band pass filtering in FIG. 6B. 
Returning to FIGS. 4A-4B, initial engine conditions are checked during step 
310 before entering a test cycle or period which is now described. The 
inducted airflow range in which engine 28 is operating is determined 
during steps 320, 324, and 326. Engine 28 is operating within airflow 
range (1), when measurement of inducted airflow MAF is between minimum 
value MIN.sub.1 and maximum value MAX.sub.1 (step 320). However, if the 
count in transitions of feedback variable FV (CFV.sub.1) is greater than a 
maximum value (CFV.sub.1max), then catalytic converter 50 has been fully 
tested for inducted airflow range (1) and, accordingly, the test will 
proceed when engine 28 operates in the other airflow ranges (step 322). 
Assuming the test of converter 50 is proceeding for inducted airflow range 
(1), count signal CFV.sub.1 is incremented each transition between states 
of signal FV until it reaches its maximum count CFV.sub.1max (steps 332 
and 336). The same procedure is followed when engine 28 is operating 
within airflow range (n) as shown in steps 326, 362, 372, and 376. The 
converter test cycle or period is completed when engine 28 has operated in 
each of "n" airflow ranges during a preselected number of transitions in 
signal FV for each of the "n" airflow ranges. A similar result may also be 
achieved by counting transitions in a signal derived from EGO sensor 34 in 
place of transitions in signal FV. 
While engine 28 is operating in each of the airflow ranges and, transitions 
in downstream EGO sensor 52 (i.e., signal REGO) are monitored (step 338). 
A transition in signal REGO indicates exhaust emissions have exceed the 
capacity of converter 50 at the particular inducted airflow range and 
measurement of inducted airflow MAF is stored as signal MAFBi (see step 
340 assuming operation in airflow range (1)). Measurement of inducted 
airflow MAF is stored as signal MAFBi whenever signal REGO transitions 
during any of the "n" inducted ranges (e.g., steps 376-380 for airflow 
range "n"). 
Each transition of signal REGO, the total count in transitions of signal 
REGO which is designated as signal CREGO, is incremented in step 390. In 
addition, for each transition of signal REGO, the corresponding 
measurement of inducted airflow (signal MAFBi) is added to the previously 
accumulated inducted airflow measurements which occurred at prior 
transitions of signal REGO. A total or accumulated airflow measurement 
corresponding to converter breakthrough (shown as signal MAFBT in step 
394) is thereby generated. 
During step 398, average signal MAFBA is generated by dividing signal CREGO 
into signal MAFBT. Stated another way, a representation of average 
inducted airflow at catalytic breakthrough is generated by averaging a 
total measurement of inducted airflow at breakthrough (MAFBT) by the 
corresponding counts of signal REGO (CREGO) at which each measured 
breakthrough occurred. 
A determination of whether converter 50 has been tested over all inducted 
airflow ranges is made in step 402. More specifically, the test cycle is 
completed when the count in transitions of feedback variable FV for each 
of the inducted airflow ranges (CFV.sub.1 . . . CFV.sub.n) has reached its 
respective maximum value (CFV.sub.1max . . . CFV.sub.nmax). When the test 
cycle is completed, signal MAFBA is compared to reference value REF (step 
406), and when it is below reference value REF the converter flag is set 
in step 408. 
Those skilled in the art will recognize that actual measurements of 
inducted airflow (MAF and MAFBi) need not be used, but any value 
correlated with the airflow ranges may be used such as scaling factors 
linearly related to such measurements. It is also recognized that the test 
period or cycle is completed when converter 50 has been tested over a 
plurality of airflow ranges referred to as sub-test. And each of these 
sub-test last for a duration indicated by a predetermined count in 
transitions of feedback variable FV. In this manner, the converter is 
tested when an indication that steady-state rather than transient 
operation is provided. Accordingly, consistent test results should be 
achieved regardless of the manner in which the vehicle is driven during 
the test period. 
An example of operation is presented herein where both upstream sensor 44 
and downstream sensor 52 are two-state exhaust gas oxygen sensors. The 
invention claimed herein, however, may be used to advantage with other 
sensors such as proportional sensors. Other modifications will become 
apparent to those skilled in the art without departing from the spirit and 
scope of the invention. Accordingly, it is intended that the scope of the 
invention be defined only by the following claims.