Abstract: A vehicle location system detects the location where a vehicle is position. A control system controls a fuel injection system of an engine of the vehicle in such a manner that emission of the engine varies in accordance with the detected vehicle location.

Abstract: Feedback correction of an air-fuel ratio is performed, a feedback correction amount is learned, and applied as a learning value on the next occasion that control is performed. It is determined whether or not an engine is in a high load state, and when the high load state continues for a predetermined time, the feedback correction amount is fixed at a predetermined value. The learning value is fixed at its value based on the feedback correction amount before fixing. The predetermined time is set such that the learning value in the high load state converges within this time. By applying open loop control using this learning value, surging of the vehicle due to the air-fuel ratio feedback control in the high engine load state is prevented, while the air-fuel ratio in this state is controlled precisely to the stoichiometric air-fuel ratio.

Abstract: An oxygen concentration detecting apparatus precisely and easily performs diagnosis of a limit current type oxygen sensor. The limit current type oxygen sensor has an oxygen concentration detecting element for outputting limit current proportional to the oxygen concentration and a heater for heating the detecting element. A CPU of a microcomputer controls energization of the heater to activate the oxygen sensor. The CPU calculates element resistance based on the voltage applied to the oxygen sensor and the current detected in the sensor. In a sensor diagnosis routine, the CPU determines whether preconditions for the diagnosis have been met. If all the preconditions have been met, the CPU executes the diagnosis. That is, the CPU determines whether the element resistance is within a predetermined range. If it is below the range, the CPU determines that the sensor has high element temperature abnormality.

Abstract: A method is provided for controlling the combustion parameters of an internal combustion engine without a PCV solenoid during a boil-off condition. The methodology records the temperature of the engine when the vehicle is turned off. When the vehicle is restarted, the methodology determines the amount the engine has cooled since it was shut down. Based on the difference in engine temperature and the current engine temperature, the methodology determines the amount of boil-off corruption remaining. The methodology then determines a new boil-off corruption fuel multiplier based on the amount of boil-off corruption remaining. The new boil-off corruption fuel multiplier is used to modify fuel control until the boil-off condition ceases.

Abstract: The present invention overcomes these problems by providing an oxygen sensor located in the exhaust stream of an internal combustion engine, a load sensing device which generates a signal indicative of an internal combustion engine load, a central processing unit which calculates a running average of the oxygen content in the exhaust stream indicated by the oxygen sensor and a fuel injection device which supplies fuel to the combustion air in response to the central processing unit. The central processing unit uses the running average of the oxygen content in the exhaust stream, in lieu of the instantaneous oxygen content, to modify the fuel equation for determining the proper air fuel mix when transitioning from closed to open loop operation. The resulting air fuel mixture thus reflects the average oxygen value in the exhaust over a period of time. This average serves to cancel out the extreme surplus and deficiency of oxygen in the exhaust stream due to dithering.

Abstract: In an engine air-fuel ratio controller, a target frequency of an air-fuel ratio oscillation is set to a value different from a natural vibration frequency of a drive mechanism, and an air-fuel ratio feedback correction coefficient is set so that the air-fuel ratio oscillation frequency coincides with the target frequency, and the air-fuel ratio is rich for a longer time than the time for which it is lean. In this way, resonance of the air-fuel ratio oscillation frequency with the natural vibration frequency of the drive system is prevented, the air-fuel ratio is prevented from remaining lean in the high load region for a long period of time, and decline of driving performance is avoided.

Abstract: An air/fuel control system (8) and method for an engine (28) having two engine banks coupled to a single catalytic converter (50) uses first and second exhaust gas oxygen sensors (44, 55) coupled to respective first and second exhaust manifolds (56, 57) and various engine operating parameters. During a first set of engine operating conditions, air/fuel control system (8) maintains the exhaust air/fuel ratio oscillations of the two bank in phase with one another. During a second set of engine operating conditions, air/fuel control system (8) maintains the exhaust air/fuel ratio oscillations of the two bank 180 degrees out of phase with one another. When transitioning, emission impacts are minimized.

Abstract: The invention relates to a process for controlling the fuel-air ratio for a gas engine which is provided with one or more .lambda. (1) sensors and a three-way catalyst, as well as a control system for adjusting the fuel-air ratio by means of a control element on the basis of the signal of a first .lambda. (1) sensor, comprising stepwise adjusting the .lambda. (1) value at regulated intervals, at constant load of the engine, and measuring the corresponding activity of the catalyst, determining the corresponding measuring signals of the .lambda. (1) sensor, and determining the value of the measuring signal at which the activity of the catalyst considerably increases or decreases, followed by adjusting the value of the control signal of the .lambda. (1) sensor which is maintained as the desired control value on the basis of the latter measuring signal.

Abstract: An apparatus for controlling operation of an engine is disclosed. The apparatus includes a passage for supplying air to the engine and a fuel injection valve. The engine burns air-fuel mixture to produce torque. A valve adjusts the quantity of air supplied to the engine. An ECU actuates the valve based on the driving state of the engine. The ECU feedback controls the air-fuel ratio by altering the position of the valve to prevent torque shocks caused by changes in the air supply. The feedback control employs two correction factors, the first of which is representative of the air-fuel ratio of the mixture, and the second of which is representative of an average deviation of the first factor from a stoichiometric air-fuel ratio value. The air-fuel ratio of the mixture is feedback controlled as a function of both correction factors.

Abstract: The present invention provides a method for controlling combustion parameters of an internal combustion engine during wide open throttle or part throttle enrichment operation. Oxygen sensor rich and lean counts above and below a stoichiometric air/fuel ratio are counted during an injector pulse width period while a fuel concentration determination is being made at wide open throttle and part throttle enrichment. The ratio of lean counts to rich counts is compared to a calibratable value for incrementing the fuel concentration delivered to the internal combustion engine. The fuel concentration is further controlled if a boil-off condition is detected.

Abstract: A compensator is described for use on those manifold fuel injectors for gasoline engines which utilize a gas pressure cycler to vary the instantaneous rate of liquid fuel injection in proportion to the instantaneous rate of engine intake air flow into each engine cylinder during each intake stroke. The compensator adjusts liquid fuel injection pressure by bleeding adjustable air quantities out of the gas pressure cycler during pressure rise, and then subsequently returning these bleed masses into the gas pressure cycler during pressure decrease. A compensator of this invention is of relatively simple mechanical design, and uses a moderately complex electronic controller.

Abstract: Several embodiments of feedback control systems for either two or four-cycle engines that employ sensors such as oxygen sensors. The systems include an arrangement wherein the sensor condition is determined by comparing the response time with a normal response time. If the time of response is outside of the normal range, compensation may be made so as to permit continued feedback control. Embodiments are also describe that adjust the maximum permissible injection amount and also revert to an open control if the deterioration is more any predetermined amount.

Abstract: The present invention provides a method of controlling combustion parameters of an internal combustion engine without a PCV solenoid during a boil-off condition. The methodology detects a boil-off condition and implements one of two modes of boil-off compensation depending on the active status of an inferred alcohol content multiplier update system. The methodology also determines the nature of the air-flow through the internal combustion engine and determines the level of boil-off corruption present and the time when the corruption is complete. After the boil-off condition is determined to be complete, control of the combustion parameters are returned to a normal regime of a flexible fuel compensation system using the inferred alcohol content from the employed mode of boil-off compensation.

Abstract: A fuel supply control system for an internal combustion engine. A load condition sensor detects a load condition of the engine, and an ECU increases the amount of fuel to be supplied to the engine when a predetermined high load condition of the engine is detected by the load condition sensor, and sets a delay time period from detection of the predetermined high load condition of the engine by the load condition sensor to execution of increase of the amount of fuel, according to operating conditions of the engine. A time period elapsed from the detection of the predetermined high load condition is counted. The set delay time period is corrected based on a ratio of the set delay time period to the counted time period and the load condition of the engine detected by the load condition sensor before the detection of said predetermined high load condition.

Abstract: There is provided an air-fuel ratio control system for an internal combustion. An air-fuel ratio sensor arranged in the exhaust system detects an air-fuel ratio of exhaust gases emitted from the engine. An amount of fuel to be supplied to the engine is controlled in response to the output from the air-fuel ratio sensor by using a controller of a recurrence formula type, such that an air-fuel ratio of a mixture supplied to the engine is converged to a desired air-fuel ratio, to thereby effect feedback control of the air-fuel ratio of the mixture. The desired air-fuel ratio is determined based on operating conditions of the engine. The desired air-fuel ratio is smoothed when the engine is in a predetermined operating condition. The feedback control of the air-fuel ratio is carried out by the use of the smoothed desired air-fuel ratio.

Abstract: An engine control device for controlling an air-fuel ratio of an engine, wherein an air-fuel ratio is set at 24 when an output torque of the engine is not more than a first point. It is changed from the value of 24 toward a stoichiometric ratio in accordance with a magnitude of the output torque when the output torque of the engine is in a range of from the first point to a second point and it is set at the stoichiometric ratio (14.7) when the output torque of the engine is more than the second point.

Abstract: A fuel metering control system for an internal combustion engine including a feedback loop having an adaptive controller and an adaptation mechanism that estimates controller parameters .theta.. The adaptive controller corrects the quantity of fuel injection to bring a controlled variable obtained at least based on an output of said air/fuel ratio sensor, to a desired value. The adaptation mechanism is input with the controlled variable once per prescribed crank angle such as a TDC of a certain cylinder and estimates the controller parameters. Since, however, the input is limited to a specific cylinder's air/fuel ratio, the air/fuel ratio is averaged for all cylinders and used in the calculation. Similar averaging is made for the other parameters input to the mechanism or output from the controller.

Abstract: An engine feedback control system that includes an arrangement for setting lower tolerance limits on the rich and lean side in response to certain engine running characteristics so as to avoid unstable running as might occur under abnormal conditions when wider limits are set as with prior art type constructions. In addition, the increasing limit of fuel supply is set different from the decreasing limits so as to maintain the engine operation within the stable running area regardless of whether the conditions are normal or abnormal.

Abstract: On an exhaust pipe of an internal combustion engine, an A/F sensor is disposed at an upstream side of a three-way catalyst and a downstream side O.sub.2 sensor is disposed on a downstream side thereof. A CPU determines according to an exhaust gas temperature whether the operational state of the engine is in a high load. In an early stage of the engine operation of high load, CPU sets a "rich" side target air-fuel ratio within a range that enables the downstream side O.sub.2 sensor to make linear detection of air-fuel ratio and executes feedback control of air-fuel ratio by using the set target air-fuel ratio. Also, when the level of the load has increased, CPU sets a "rich" side target air-fuel ratio according to the exhaust gas temperature and executes feedback control of air-fuel ratio by using the set target air-fuel ratio. Further, when the "rich" width deviates from a range that enables the A/F sensor to make its detection, CPU performs open-loop control with respect to the increment in fuel.

Abstract: A control system senses air quantity entering an engine, and oxygen in the engine exhaust. A controller controls an amount of fuel injected into the engine according to the sensed oxygen level. As the car changes speed, air flow to the engine changes. The controller converts sensed air flow into an efficiency parameter, and compares it to a setpoint. When an air flow changes and the setpoint is reached, the feedback control operation is suspended, and the controller changes the I-gain in a series of gradually changing steps. The gradual change in I-gain accordingly changes the amount of fuel injected in a series of gradually changing steps.

Abstract: Feedback control systems for engines employing combustion condition sensors. The feedback control is varied in response to various other parameters such as back pressure, engine speed, engine temperature, engine load and initial start-up operation so as to provide more accurate control under varying and transient conditions.

Abstract: An air-fuel ratio control system for an engine for controlling an amount of a fuel mixture to be delivered into a combustion chamber so that an air-fuel ratio of the fuel mixture attains a target air-fuel ratio performs selecting one of a plurality of predetermined target air-fuel ratios according to engine operating conditions, controlling an amount of a fuel mixture so as to attain a selected target air-fuel ratio, and changing a speed at which the target air-fuel ratio is changed from one to another ratio higher with an increase in deviation of the other target air-fuel ratio from the one target air-fuel ratio.

Abstract: A system for controlling fuel metering for a multi-cylinder internal combustion engine, having a feedback loop which has an adaptive controller and an adaptation mechanism coupled to the adaptive controller for estimating controller parameters .theta.. The adaptive controller calculates a feedback correction coefficient using internal variables that include the controller parameters .theta., to correct a basic quantity of fuel injection obtained by retrieving mapped data by engine speed and engine load, to bring a detected air/fuel ratio to a desired air/fuel ratio. In the system, the internal variables of the adaptive controller are determined in response to detected engine operating conditions, when the engine operation has shifted from an open-loop control region to the feedback control region.

Abstract: A feedback control system for a multi-cylinder engine using a combustion condition sensor that senses the condition in only one cylinder. The total air flow to the engine is measured and the amount of fuel supplied to other than the sensed cylinders is varied in response to known variations in air flow for a given engine running conditions. The sense cylinder has direct feedback control with no correction.

Abstract: A fuel metering control system for an internal combustion engine including a feedback loop having an adaptive controller and an adaptation mechanism that estimates controller parameters .theta.. The adaptive controller calculates a feedback correction coefficient that corrects the quantity of fuel injection to bring a controlled variable obtained at least based on an output of an air/fuel ratio sensor, to a desired air/fuel ratio. The convergence speed of the controller parameters are determined by a gain matrix. The gain matrix is determined in response to at least one of the detected engine operating conditions.

Abstract: A feedback control system for a multi-cylinder internal combustion engine that employs a combustion condition sensor that senses the condition in only one combustion chamber at the end of the combustion cycle. The power of the engine can be reduced by disabling cylinders other than that from which the combustion products are sensed.

Abstract: A fuel supply control system for an engine comprises a canister for temporarily storing fuel vapor, a purge passage extending between the canister and a intake passage downstream of a throttle valve, and a purge control valve arrange in the purge passage for controlling the amount of a purge gas, that is, air containing fuel vapor, flowing through the purge passage. The purge control valve is controlled by a duty ratio, which is defined as a ratio of a period during which the purge control valve is opened to a duty cycle time. Namely, the purge control valve is driven to make an opening ratio of the purge control valve equal to the duty ratio. The duty cycle for each cycle is calculated before the corresponding cycle starts, in accordance with an engine operating condition at that time. During each cycle, a corrected duty cycle is calculated in accordance with the engine operating condition at that time.

Abstract: The invention is directed to a method of metering fuel to the engine during warm-up operation which precedes normal operation of the engine during which a first amount of fuel is metered to the engine. The engine is equipped with a lambda control and a device for detecting a variable suitable for distinguishing warm-up operation and normal operation of the engine. A predetermined corrective factor (FWL) is provided in dependence upon the variable. A second amount of fuel is metered to the engine during the warm-up operation which is increased by the corrective factor (FWL) so that the second amount is greater than the first amount. Lambda control is engaged during the warm-up operation and provides a desired lambda value (.lambda..sub.des). A quantity is formed indicative of the mean deviation of the actual lambda value (.lambda..sub.act) from the desired lambda value (.lambda..sub.des) when the lambda control is engaged.

Abstract: An fuel injection control system suitable for use with internal combustion vehicle engines fueled by compressed natural gas or the like. A liquid-fuel type closed-loop system, modified for gaseous fuel, establishes the duration of each injection command signal by the combination of a P.I.D. controller responsive to the level of exhaust oxygen, an adaptive table which stores previously determined fuel delivery rate values for particular engine speed and load conditions, an injector supply voltage table and an added table modifies the fuel delivery rate correction values which compensate for unwanted pressure variations which occur in the fuel supply due to injector sequencing effects, firing order, fuel flow effects, temperature effects, and fuel sensor errors.

Abstract: An Electronic Engine Controller (EEC) is calibratable to control the rate at which fuel is delivered to an intake of an internal combustion engine in order to achieve a bias in the air/fuel ratio combusted by the engine. The EEC operates under closed-loop control to alter the fuel delivery rate in response to an output of an oxygen sensor which detects the concentration in exhaust gas produced by the engine. The EEC achieves a predetermined bias, in the air/fuel ratio, which is in a rich or lean direction from stoichiometry, by maintaining the fuel delivery rate at a jump value upon the indication by the oxygen sensor of an air/fuel ratio in the same direction from stoichiometry as the desired bias. The fuel delivery rate is maintained at the jump value for a variable period of time, which preferably depends upon the engine speed and load. Upon expiration of the variable period of time, the fuel delivery rate is abruptly altered to a fuel delivery rate corresponding to a predetermined air/fuel ratio.

Abstract: An air/fuel control system (8) and method to maintain the air/fuel ratio of each engine bank (28) out of phase with one another. A feedback variable is generated (410-428) in response to a comparison of a difference in exhaust gas oxygen sensors (44, 55) from respective left and right engine banks (56, 57) to a reference (104-120). Separate feedback variables are generated for each engine bank by phase inverting the feedback variable between the banks and adding an adjustment signal (450-462). The adjustment signal is generated from a PI controller responsive to a comparison of the sum of the left and right exhaust gas oxygen sensors (44, 55) to a reference (440-448).

Abstract: An engine-control system is structured in such a way that a timing for closing the intake valve is retarded and the air-fuel ratio is so set as to become a rich air-fuel ratio richer than, for example, the stoichiometric air-fuel ratio, in a region where the load is low. On the other hand, in a region where the load is high, the timing for closing the intake valve is set earlier and the air-fuel ratio is so set as to become a lean air-fuel ratio leaner than, for example, the stoichiometric air-fuel ratio.

Abstract: A control system for an internal combustion engine according to one aspect of the invention calculates a total amount of evaporative fuel purged from a canister based on dynamic characteristics and concentration of purged gas. The system calculates an amount of evaporative fuel supplied to the combustion chamber of the engine based on the total amount of evaporative fuel and depending on dynamic characteristics of the purged gas. Then, the system determines a required amount of fuel to be injected by a fuel injection valve based on the amount of evaporative fuel supplied to the combustion chamber.

Abstract: An apparatus for diminishing the amount of NO.sub.x in the exhaust gases of an engine by a catalytic converter installed in its exhaust system. The apparatus includes an injector associated with the intake system of the engine, or the exhaust system upstream of the catalytic converter for injecting fuel into the intake, or exhaust system to ensure that the catalytic converter receives a sufficient supply of hydrocarbon. The amount of NO.sub.x in the exhaust gases leaving the engine is estimated from the number of revolutions thereof and the angle of a control lever, or the concentration of NO.sub.x in the exhaust gases leaving the catalytic converter is detected by a sensor. The amount of fuel injection into the intake, or exhaust system is controlled in accordance with the estimated amount of NO.sub.x, or the output of the sensor.

Abstract: An apparatus and method for controlling an air/fuel mixture ratio for an internal combustion engine which can start the air/fuel mixture ratio at a earlier timing than the conventional air/fuel mixture ratio controlling apparatus. For example, a control unit determines whether the engine is revolved and, thereafter, accumulates an intake air quantity. When the accumulated value exceeds the predetermined value SQ.sub.ST and the control unit determines that the activation of an oxygen sensor sufficiently occurs, the start of the air/fuel mixture ratio feedback control is determined. At this time, a fuel supply quantity T.sub.I is corrected so that the air/fuel mixture ratio which tends to be inclined to a lean side is shifted to the rich side.

Abstract: An engine air/fuel control system includes an apparatus and method for adaptively learning a reference voltage. Fuel delivered to the engine is trimmed by a feedback variable provided by integrating a two-state signal resulting from a comparison between the reference voltage and the exhaust gas oxygen sensor output. Each sample period of a microprocessor, a high voltage signal and low voltage signal are generated which track the outer envelope of the sensor signal. Calculation of a midpoint between high and low voltage signals provides the reference which instantaneously tracks the midpoint of the sensor signal.

Abstract: In an air-fuel ratio control system for an internal combustion engine, a proportional part for determining a feedback correction factor is corrected by a value proportional to an intake air flow corresponding value, whereas an integral part for determining the feedback correction factor is corrected by a value proportional to a square of the intake air flow corresponding value.

Abstract: An air-fuel ratio control system for an engine, to which evaporated fuel in a canister is supplied, is used for feedback controlling an air-fuel ratio based on a feedback control value calculated according to the quantity of evaporated fuel supplied to the engine. A calculation is made, in a first range of engine operating conditions, of a difference of a first control value between the first control value determined during supplying of the evaporated fuel to the engine and the first control value determined during not supplying of the evaporated fuel to the engine in the first range. A second control value for a second range of engine operating conditions is established different from the first range based on a feedback control value for controlling an air-fuel ratio of fuel in the first range, a ratio of the quantity of intake air into the engine between the first and second ranges, and a ratio of the supplying quantity of the evaporated fuel into the engine between the first and second ranges.

Abstract: An air/fuel mixture control system for an internal combustion engine uses a closed-loop controller which varies the air/fuel mixture in response to the oxygen level in the engine's exhaust emissions to achieve stoichiometry. The oxygen level sensor produces a binary signal indicating either a rich or a lean mixture. The controller responds by generating a fuel delivery rate control signal which has three components: an integral (ramp function) component, a proportional step function component which abruptly jumps the control signal to an intermediate level at the time of each oxygen level change, and a differential overshoot component which (1) injects a compensating volume of fuel into the engine intake at the onset of each lean signal, and (2) subtracts a compensating volume of fuel from the intake at the onset of each rich signal. The compensating fuel volume minimizes the effects of fuel puddling in the intake manifold to reduce the effective closed loop delay time for better control.

Abstract: An engine air/fuel feedback control system adjusts the engine air/fuel ratio in response to a modified output of an exhaust gas oxygen sensor. A sensor output is compared to a reference at its nominal midpoint to develop a two-state signal indicating operation rich or lean of stoichiometry. This two-state or step output is shifted towards the peak efficiency window of a catalytic converter by pumping current through a sensor electrode in response to an error signal derived from a downstream sensor. Shifts in the upstream sensor amplitude caused by current pumping are corrected by a correction factor which is adaptively learned during a test cycle.

Abstract: An internal combustion engine includes a lambda control device having a lambda sensor and a lambda control for regulating a fuel-air mixture to be supplied to an internal combustion engine to a set-point value, outside of full load operation, as a function of an output signal of the lambda sensor, and a mixture control device for setting the fuel-air mixture to a value being less than the set-point value, during full load operation of the internal combustion engine and with the lambda control switched off.

Abstract: An air-fuel ratio control system for an internal combustion engine calculates an amount of fuel to be supplied to the engine in response to the rotational speed of the engine and a parameter indicative of load on the engine, and an amount of fuel injected into an intake passage and adhering to the inner surface of the intake passage, based upon a direct supply ratio and a carry-off ratio. A feedback correction value is set to such a value that an air-fuel ratio in exhaust gases from the engine becomes equal to a desired value, and the amount of fuel calculated as above is corrected by the direct supply ratio, the carry-off ratio, and the feedback correction value set as above. A control speed at which the feedback correction value is corrected is set based upon at least one of the direct supply ratio and the carry-off ratio.

Abstract: In an air-fuel ratio control system for an internal combustion engine, an output from a first air-fuel ratio sensor arranged upstream of a three-way catalyst mounted across an exhaust passage of the engine is compared with a reference value during feedback control of the air-fuel ratio. The reference value is controlled based on an output from a second air-fuel ratio sensor arranged downstream of the three-way catalyst. The reference value is changed in dependence on the engine load detected.

Abstract: An exhaust gas cleaning device for controlling the air-to-fuel ratio of the air-fuel mixture supplied to an internal combustion engine (1) provided with a catalytic converter (14) disposed in the exhaust pipe (13). The air-to-fuel ratio is oscillated around the central level of the integral control or proportional plus integral feedback control signal based on the output of an air-to-fuel ratio sensor (12); the amplitude of oscillation and the proportional control amount are, or alternatively, the frequency of the oscillation is, varied in accordance with the operating condition of the engine.

Abstract: An air-fuel ratio control apparatus for an alcohol engine driven by fuel containing an alcohol component, where oxygen density of the exhaust gas is detected by an O.sub.2 sensor and an air-fuel ratio is controlled to approach a theoretical air-fuel ratio in response to the oxygen density detected by the O.sub.2 sensor, is disclosed. The apparatus comprises an engine load detecting means for detecting engine load, and a control unit for correcting the air-fuel ratio to be rich in response to engine load such that the degree of the correction becomes larger as the engine load becomes small.

Abstract: A method for controlling the air-fuel ratio in an internal combustion engine equipped to operate from a gaseous alternative fuel. The output signal of the engine's exhaust gas oxygen sensor is used to measure the variable engine transport delay. The variable is used to approximate a reciprocal of the delay. The reciprocal is used as a representation of the variable engine mass air-flow. A limit cycle operating in a transport-delay-oscillator mode is modified by extending the duration of the enrichment phase in proportion to the mass air-flow. The resulting exhaust gas concentrations are biased rich and thus shifted within an optimum operating window of the engine's gasoline catalytic converter. A significant reduction of unwanted exhaust emissions is observed.

Abstract: An air-fuel ratio control system for an internal combustion engine, which detects the actual air-fuel ratio of a mixture supplied to the engine, detects operating conditions of the engine, calculates a desired air-fuel ratio based on the detected operating conditions of the engine, and calculates an air-fuel ratio correction value applied for feedback-controlling the actual air-fuel ratio to the calculated desired air-fuel ratio. A change rate at which the air-fuel ratio correction value is to be changed, is set such that when a change has occurred in the operating mode of the engine, the change rate is set to and held at a smaller value than a value assumed when no change has occurred in the operating mode, over a predetermined time period from the time the change occurred.

Abstract: In an internal combustion engine where in a predetermined driving state, the air-fuel ratio is feedback-controlled based on a detection signal of an air-fuel ratio sensor and in other driving state, the air-fuel ratio is feed-forward-controlled, the presence or absence of a misfire is judged from the detection signal of the air-fuel sensor at the time of the feed forward control, and when the presence of a misfire is judged, a warning is given.