The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and, more particularly, to an air-fuel ratio control apparatus for an internal combustion engine having a purge system, which can acquire the correct learned value by increasing the opportunities to learn an air-fuel ratio feedback coefficient.
In order to improve the fuel mileage and prevent air pollution, a fuel vapor purge system is being employed in recent vehicles. The purge system temporarily adsorbs fuel vaporized in the fuel tank of a vehicle by means of a canister and then feeds (purges) the adsorbed fuel vapor as part of the fuel delivered to the intake pipe at the proper timing. In an internal combustion engine that also employs air-fuel ratio control, however, the fuel vapor that is supplied via the purge system becomes an external disturbance to the air-fuel ratio control. In this respect, there is a demand for a purge method which has less influence on the air-fuel ratio control.
There is conventional air-fuel ratio control designed in consideration of a time-dependent change in the characteristics of an air flow meter or a fuel injection valve in an internal combustion engine. This air-fuel ratio control learns a base air-fuel ratio feedback coefficient which reflects the influence of a time-dependent change in the characteristics of the air flow meter or the fuel injection valve. It is therefore very important that when purging is carried out during learning of the base air-fuel ratio feedback coefficient, the purged fuel vapor should not affect the learned value.
An air-fuel ratio control apparatus, as a solution to the above problem, is disclosed in Japanese Unexamined Patent Publication No. 62-206262. This air-fuel ratio control apparatus is provided with a map having a plurality of drive sections set in accordance with the running state of an internal combustion engine. Base air-fuel ratio feedback coefficients are registered in the individual drive sections. When the running state of the internal combustion engine lies in a drive section in which an associated base air-fuel ratio feedback coefficient has not yet been registered, purging of fuel vapor is stopped.
The purge system must to carry out purging for as long a period as possible. Since the drive section frequently changes according to the running state, however, purging is frequently switched on and off when there are many drive sections in which associated base air-fuel ratio feedback coefficients have not yet been registered. The frequent purge-OFF action is contrary to against the demand to purge for a long period. Further, the frequent ON/OFF switching of purging results in inaccurate learning of the base air-fuel ratio feedback coefficient. When a lot of fuel vapor is accumulated in the canister, the ON/OFF switching of purging significantly affects the air-fuel ratio so that the air-fuel ratio control apparatus may not implement accurate control.
Japanese Unexamined Patent Publication No. 7-293362 and Japanese Unexamined Patent Publication No. 6-10736 disclose, as a solution to the above problem, air-fuel ratio control apparatuses that learn the base air-fuel ratio feedback coefficient based on the concentration of fuel vapor to be purged. Those control apparatuses measure the concentration of the fuel vapor to be purged and learn the base air-fuel ratio feedback coefficient. When that concentration is small, the base air-fuel ratio feedback coefficient is learned on the assumption that the fuel vapor to be purged does will not have much influence on the air-fuel ratio.
The control apparatus of Japanese Unexamined Patent Publication No. 7-293362 inhibits learning of the base air-fuel ratio feedback coefficient once that coefficient is learned. If the base air-fuel ratio feedback coefficient is learned inaccurately somehow, therefore, the learned value cannot be change to a correct value. In addition, since the base air-fuel ratio feedback coefficient is also used to learn the purge concentration, the purge concentration is also wrongly learned.
With the purge concentration set to the wrong value, therefore, when the running state enters a drive section having no registered associated base air-fuel ratio feedback coefficient, the control apparatus also inaccurately learns the base air-fuel ratio feedback coefficient in that section. Further, when the running state enters a drive section for which the base air-fuel ratio feedback coefficient has been learned correctly but where the wrong purge concentration has been learned, the air-fuel ratio of the internal combustion engine cannot be controlled precisely. This may bring about problems with emission and drivability.
The control apparatus described in the latter Japanese Unexamined Patent Publication No. 6-10736 frequently learns the base air-fuel ratio feedback coefficient when fuel vapor to be purged is lean. If the base air-fuel ratio feedback coefficient has been learned inaccurately, this coefficient seems to be set to the correct value in the next learning. This control apparatus however determines that fuel vapor to be purged is lean when the learned value of the purge concentration is small. The learned value of the purge concentration that is the criterion for the decision, like the base air-fuel ratio feedback coefficient, is acquired based on the amount of deviation of the air-fuel ratio feedback coefficient. The learned value of the purge concentration is complementary to the base air-fuel ratio feedback coefficient and is obtained in accordance with the air-fuel ratio feedback coefficient. That is, the learned value of the purge concentration indirectly indicates the concentration of fuel vapor to be purged and is likely to include a relatively large error with respect to the fuel concentration in the actual fuel vapor to be purged. If the learned value of the base air-fuel ratio feedback coefficient for a given drive section absorbs a deviation of the air-fuel ratio feedback coefficient based on the purged fuel vapor, for instance, the learned value of the purge concentration may indicate that the purged fuel vapor is lean. When the running state enters another drive section with the inadequate learned value of the purge concentration, the base air-fuel ratio feedback coefficient in that section is learned inadequately.
Japanese Unexamined Patent Publication No. 63-129159 discloses another control apparatus that halts purging every predetermined period and learns the base air-fuel ratio feedback coefficient. Because this control apparatus frequently misses opportunities to purge, however, it cannot overcome the aforementioned problems.