Air-fuel ratio control system for an engine

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Drawing sheet — no readable text.

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Drawing sheet — no readable text.

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START

CAL CULATE

CAL CULATE

CAL CULATE

CACULATE

O5 ACTUAL AIR-FUEL

RATIOAFa

106 DESIRED

EAOYES

AFd CHANGED

sEd

STOP TIME

ZERO

YES 3.Bd Derive stop tie

CALCULATE CALCULATE

CAL CULATE

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Drawing sheet — no readable text.

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ing sensing means for sensing engine operating condi

AIR-FUEL RATIO CONTROL SYSTEM FOR AN tions and for producing operating condition signals, a ENGINE sensor provided for sensing oxygen concentration of

BACKGROUND OF THE INVENTION

exhaust gas of the engine and for producing a feedback 5 signal dependent on the concentration, first means re

The present invention relates to an air-fuel ratio con sponsive to the operating condition signals for produc trol system for an engine of a motor vehicle, and more ing a desired air-fuel ratio signal, second means for particularly for a lean mixture engine. comparing the feedback signal with the desired air-fuel The lean mixture engine operates on lean mixture at ratio signal and for producing an error signal, control light and middle load and on stoichiometry mixture at 10 means responsive to the error signal for deciding heavy load. A feedback air-fuel ratio control is pro amount of fuel supplied to the engine, third means for vided for supplying the air-fuel mixture at large air-fuel detecting variation of the desired air-fuel ratio and for ratio (lean mixture) or stoichiometric air-fuel ratio in producing a variation signal, a look-up table provided accordance with engine operating conditions. for storing a plurality of time data dependent on engine The feedback system is provided with a lean mixture operating condition, the time data being decided to sensor for sensing the oxygen concentration of the ex correspond to a control delay time of the control sys haust gas of the engine, the output voltage of which is tem, fourth means responsive to the variation signal for proportional to the oxygen concentration. In a fuel fixing the error signal for a period derived from the injection system for the lean mixture engine, a plurality look-up table.

of desired air-fuel ratios are stored in a look-up table in 20 In an aspect of the invention, the sensor is a lean accordance with engine operating conditions. A feed mixture sensor for sensing oxygen concentration in back signal from the lean mixture sensor is compared burnt exhaust gas of lean mixture, and the delay time is with a desired air-fuel ratio AFd to produce an error the sum of a transport time delay and first-order lag. signal. A feedback coefficient KFB for fuel injection is calculated based on the error signal. On the other hand, 25 be apparentlyobjects

The other and features of this invention will understood from the following descrip an air-fuel ratio coefficient KAF and a miscellaneous tion with reference to the accompanying drawings. coefficient COEF including a plurality of coefficients based on various operating conditions such as coolant BRIEF DESCRIPTION OF DRAWINGS temperature, intake air temperature and others are ob tained. Fuel injection time TI of injected fuel is calcu- 30 system of1 the

FIG. is a schematic diagram showing a control present invention;

lated as follows.

FIG. 2 is a graph showing an output characteristic of a lean mixture sensor;

FIG. 3 is a block diagram showing the control system where 35 of the present invention;

K is a correcting coefficient, FIG. 4 is a flowchart showing the operation of the Q is intake air flow rate, system; and

N is engine speed. FIGS. 5 and 6 are graphs showing variations of air By injecting fuel during the calculated time, air-fuel fuel ratios in the system of the present invention and in ratio is controlled to the desired air-fuel ratio. 40 a conventional system.

When the desired air-fuel ratio AFd varies from one DETAILED DESCRIPTION OF THE value to another value dependent on changing of engine PREFERRED EMBODIMENT operating condition during the looking-up the table, the control of air-fuel delays. As a result, the control system Referring to FIG. 1, an air flow meter 2 for produc oscillates, so that actual air fuel ratio AFa oscillates as 45 ing an air flow signal Q, throttle position sensor 3 and shown in FIG. 6. fuel injector 4 are mounted on an intake pipe 1 of an SUMMARY OF THE INVENTION engine E. In an exhaust pipe 1a, a lean mixture sensor 5 and a catalytic converter 6 are provided. Mounted on

The object of the present invention is to provide a the engine E are a coolant temperature sensor 7 and a system which may control the air-fuel ratio without 50 crankangle sensor 8 which produces an engine speed oscillating the system. signal N. Output signals of those sensors are applied to In accordance with the present invention, when the a control unit 10. As shown in FIG. 2, the output volt desired air-fuel ratio varies from one value to another age of the lean mixture sensor 5 is proportional to the value dependent on changing of engine operating con air-fuel ratio of lean mixture.

dition during the looking-up the table, the feedback 55 FIG. 3 shows the control unit 10. The control unit 10 control operation is stopped for a time which is decided has a desired air-fuel ratio table 11 from which a desired by a delay time dependent on engine operating condi air-fuel ratio AFd is derived. The desired air-fuel ratio tions. The delay time is the sum of a transport time AFd is applied to an air-fuel ratio coefficient KAF and delay and a first-order lag of the system. The transport miscellaneous coefficient COEF calculator 12, to an time delay is a period from the time when the desired 60 adder 15, and to a feedback stopping timer look-up table air-fuel ratio varies to the time when the change of 17. The adder 15 produces an error signal dependent on oxygen concentration in the exhaust gas in accordance the difference between the desired air-fuel ratio AFd with the change of the desired ratio is sensed by the lean and the actual air fuel ratio AFa calculated from the mixture sensor. When the desired air-fuel ratio does not feedback signal from the lean mixture sensor 5. The vary during the time, the feedback control operation is 65 error signal is applied to a feedback coefficient calcula restarted. tor 16 which produces a feedback coefficient KFB. On According to the present invention, there is provided the other hand, the timer table 17 stores a plurality of an air-fuel ratio control system for an engine, compris stop periods dependent on engine operating conditions

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such as engine speed N and intake air flow rate Q. As While the presently referred embodiment of the pres described hereinbefore, the stopping period is the sum ent invention has been shown and described, it is to be of the transport time delay and the first-order lag. The understood that this disclosure is for the purpose of timer table 17 produces a stop period signal in response illustration and that various changes and modifications to the variation of the desired air-fuel ratio, for tem may be made without departing from the spirit and poraril stopping the feedback operation at the changing scope of the invention as set forth in the appended claim.

of the desired air-fuel ratio caused by changing of en What is claimed is:

gine operating conditions. 1. An air-fuel ratio control system for an engine, The coefficients KdF, COEF and KFB are multiplied O comprising sensing means for sensing engine operating at a multiplier 13 and the product is applied to a fuel conditions and for producing operating condition sig injection time calculator 14 where the above described nals;

calculation TI is made to produce a fuel injection signal. a sensor sensing oxygen concentration of exhaust gas The fuel injection signal is applied to an engine E to of the engine and producing a feedback signal de inject fuel during the time TI. 15 pendent on the concentration; FIG. 4 shows the operation of the system. From a first means responsive to the operating condition step 101 to a step 105 shows the above described opera signals for producing a desired air-fuel ratio signal; tions for obtaining the various data. When the desired second means for comparing the feedback signal with air-fuel ratio AFd does not change and the output timer the desired air-fuel ratio signal and for producing table 17 is zero (steps 106, 107), the comparison of AFd an error signal;

and AFa is made at a step 108. In accordance with the control means responsive to the error signal for de result of the comparison, the feedback coefficient KFB is ciding amount of fuel supplied to the engine; corrected at steps 109 to 112, and fuel injection time TI third means for detecting variation of the desired is calculated at a step 116. When the desired air-fuel 25 a look-upair-fuel ratio and for producing a variation signal; ratio changes, the stopping period is derived from the table storing a plurality of time period data table 17 at a step 113 and the feedback coefficient KFB dependent on an engine operating condition, the is fixed to the value before the changing of the desired time period being decided to correspond to a con air-fuel ratio. Thus, the feedback operation is stopped trol delay time of the control system; during the set period and the fuel injection period is 30 fourth means responsive to the variation signal for fixing the error signal for a time period derived calculated based on the fixed value of KFB. If the varia from the look-up table.

tion of the desired air-fuel ratio has ceased during the 2. The system according to claim 1 wherein the sen set time, the feedback operation is restarted. sor is a lean mixture sensor for sensing oxygen concen Thus, in accordance with the present invention, since tration in burnt exhaust gas of lean mixture. feedback operation is the stopping period correspond 35 3. The system according to claim 1 wherein the con ing to a control delay time which causes the oscillation trol means is means for calculating fuel injection time. of the system, the air-fuel ratio AFa is converged to the 4. The system according to claim 1 wherein the delay desired air-fuel ratio AFd since the feedback coefficient time is the sum of a transport time delay and first-order KFB is fixed to the value without oscillating as shown in lag.

FIG.S. 40