Abstract: An air-fuel ratio control device includes an air-fuel ratio sensor configured such that an output current value thereof varies linearly in accordance with an oxygen concentration, and air-fuel ratio feedback control means capable of executing air-fuel ratio feedback control for feedback-controlling a fuel injection amount on the basis of a detection value from the air-fuel ratio sensor so that exhaust gas of an internal combustion engine reaches a predetermined air-fuel ratio. The air-fuel ratio control device further includes prohibiting means for prohibiting the feedback control when the air-fuel ratio reaches or exceeds a predetermined rich air-fuel ratio. The air-fuel ratio control device permits the feedback control for a predetermined period after the air-fuel ratio reaches or exceeds the predetermined rich air-fuel ratio.

Abstract: An air-fuel ratio control device includes an air-fuel ratio sensor configured such that an output current value thereof varies linearly in accordance with an oxygen concentration, and air-fuel ratio feedback control means capable of executing air-fuel ratio feedback control for feedback-controlling a fuel injection amount on the basis of a detection value from the air-fuel ratio sensor so that exhaust gas of an internal combustion engine reaches a predetermined air-fuel ratio. The air-fuel ratio control device further includes prohibiting means for prohibiting the feedback control when the air-fuel ratio reaches or exceeds a predetermined rich air-fuel ratio. The air-fuel ratio control device permits the feedback control for a predetermined period after the air-fuel ratio reaches or exceeds the predetermined rich air-fuel ratio.

Abstract: Provided is a device for performing degradation diagnosis of a downstream-side exhaust gas sensor. When a diagnosis condition is met, an intake-air quantity is successively accumulated to obtain an intake-air-quantity accumulation value, an intake-air-quantity average value is obtained as an average of the intake-air quantity from the start of diagnosis, and, on the basis of this intake-air-quantity average value, an intake-air-quantity accumulation threshold value is set. When the intake-air-quantity accumulation value reaches the intake-air-quantity accumulation threshold value, the diagnosis is terminated, and an output reversal number of the downstream-side exhaust gas sensor during the diagnosis period is compared with a threshold value to judge whether the sensor is normal or degraded.

Abstract: The objective of the present invention is to prevent an increase in the amount of oxygen stored in a catalyst due to a fuel cut by performing a rich spike, even when the fuel is again supplied after a fuel cut in some of the cylinders. When a prescribed fuel cut condition has been satisfied (t1, t6), first, the fuel to some of the cylinders is cut, and after a prescribed period of time (A1) has elapsed, the supply of fuel to all of the cylinders is cut (A2). When the fuel to only some of the cylinders has been cut and a prescribed fuel cut recovery condition has been satisfied (t3), fuel is again supplied to the aforementioned cylinders, and during a prescribed period (C1) after the supply of the fuel has been restarted a rich spike (D1) is executed, whereby the amount of fuel being supplied is increased and the exhaust air-fuel ratio is controlled so as to be richer than the stoichiometric air-fuel ratio.

Abstract: The objective of the present invention is to prevent an increase in the amount of oxygen stored in a catalyst due to a fuel cut by performing a rich spike, even when the fuel is again supplied after a fuel cut in some of the cylinders. When a prescribed fuel cut condition has been satisfied (t1, t6), first, the fuel to some of the cylinders is cut, and after a prescribed period of time (A1) has elapsed, the supply of fuel to all of the cylinders is cut (A2). When the fuel to only some of the cylinders has been cut and a prescribed fuel cut recovery condition has been satisfied (t3), fuel is again supplied to the aforementioned cylinders, and during a prescribed period (C1) after the supply of the fuel has been restarted a rich spike (D1) is executed, whereby the amount of fuel being supplied is increased and the exhaust air-fuel ratio is controlled so as to be richer than the stoichiometric air-fuel ratio.

Abstract: Provided is a device for performing degradation diagnosis of a downstream-side exhaust gas sensor. When a diagnosis condition is met, an intake-air quantity is successively accumulated to obtain an intake-air-quantity accumulation value, an intake-air-quantity average value is obtained as an average of the intake-air quantity from the start of diagnosis, and, on the basis of this intake-air-quantity average value, an intake-air-quantity accumulation threshold value is set. When the intake-air-quantity accumulation value reaches the intake-air-quantity accumulation threshold value, the diagnosis is terminated, and an output reversal number of the downstream-side exhaust gas sensor during the diagnosis period is compared with a threshold value to judge whether the sensor is normal or degraded.

Abstract: A leak check apparatus performs a leak check of a fuel vapor processing apparatus based on a change in a negative pressure in a purge line during leak down. The pressure change during leak down is corrected by measuring an amount of vapor generated after the purge line is opened to the atmosphere. A first pressure change rate of the purge line is measured after a predetermined negative pressure is applied to the purge line, and the purge line is sealed at the negative pressure. A second pressure change rate of the purge line is measured after the first pressure change rate of the purge line is measured, the cut valve is opened to expose the purge line to the atmosphere, and the purge valve and the cut valve are closed to seal the purge line at the atmospheric pressure. The second pressure change rate measurement does not begin until the degree of pressure increase in the purge line exceeds a predetermined reference value.

Abstract: A lag time is set in a period from the time when a leak down is finished to the time when measurement of the amount of fuel vapor is started for the purpose of returning a deformed fuel tank to its original shape. A fuel vapor amount estimating means is provided that estimates the amount of fuel vapor produced in the fuel tank during the time of the leak down. This means is so set as to make the lag time shorter as the amount of fuel vapor increases. With this, undesired precision in measuring the amount of fuel vapor produced under a condition wherein the amount of fuel vapor is large can be avoided.

Abstract: A leak check apparatus performs a leak check of a fuel vapor processing apparatus based on a change in a negative pressure in a purge line during leak down. The pressure change during leak down is corrected by measuring an amount of vapor generated after the purge line is opened to the atmosphere. A first pressure change rate of the purge line is measured after a predetermined negative pressure is applied to the purge line, and the purge line is sealed at the negative pressure. A second pressure change rate of the purge line is measured after the first pressure change rate of the purge line is measured, the cut valve is opened to expose the purge line to the atmosphere, and the purge valve and the cut valve are closed to seal the purge line at the atmospheric pressure. The second pressure change rate measurement does not begin until the degree of pressure increase in the purge line exceeds a predetermined reference value.

Abstract: A lag time is set in a period from the time when a leak down is finished to the time when measurement of the amount of fuel vapor is started for the purpose of returning a deformed fuel tank to its original shape. A fuel vapor amount estimating means is provided that estimates the amount of fuel vapor produced in the fuel tank during the time of the leak down. This means is so set as to make the lag time shorter as the amount of fuel vapor increases. With this, undesired precision in measuring the amount of fuel vapor produced under a condition wherein the amount of fuel vapor is large can be avoided.