1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an engine, such as an automobile engine, wherein vaporized fuel is purged from the fuel tank.
2. Description of Related Art
Typically, air-fuel control systems for engines of this type are designed and adapted to purge vaporized fuel only when a feedback control of air-fuel ratio is taking place. With such an air-fuel control system, fuel vaporized in the fuel tank is collected in a canister and is purged to an air intake system only when the feedback control of air-fuel ratio is being exercised so that fluctuations in air-fuel ratio caused by the purging of vaporized fuel are canceled or absorbed by means of this feedback control. In other words, the feedback control of air-fuel ratio provides a decrease in the amount of fuel in accordance with the amount of purged vaporized fuel. Such an engine air-fuel control system is known from, for instance, Japanese Unexamined Patent Publication No. 60-33316.
On the other hand, learning control of the air-fuel ratio, which is well known to those skilled in the art, is generally conducted in order to prevent deviations in air-fuel ratio due to deviations resulting from temperature characteristics and deterioration due to aging of a fuel system including a fuel injector and an intake system including an air flow sensor.
When the purging of vaporized fuel is initiated by, for instance, suddenly opening a purge valve to its full opening so as to purge a desired amount of fuel, the feedback control cannot follow up, possibly creating deviations in air-fuel ratio owing to a time lag in control. Therefore, the purge valve is gradually opened so as to cause a gradual increase in the purging amount of vaporized fuel until reaching the desired amount. If the learning control of air-fuel ratio is experienced in engines equipped with such a vaporized fuel supply system, upon a resumption of purging following a temporary interruption of purging after the execution of learning control under a full purging condition, a learned value is obtained during the full purging of evaporated fuel. In this instance, the learned value obtained during the full purging of evaporated fuel is reflected in the feedback control, even if a gradually increasing purging is conducted at the initiation of purging and, hence, a full purging is not achieved. Consequently, because a decrease correction of the supply of fuel during the air-fuel ratio control is made in excess, the air-fuel ratio is controlled to become more lean than the desired or target air-fuel ratio, so that deviations are generated in air-fuel ratio.
A high concentration of vaporized fuel is collected at the top of the canister and is supplied first at the initiation of purging of vaporized fuel, and, thereafter, a stabilized concentration of vaporized fuel is supplied. Therefore, if learning is performed even during this interval, deviations in air-fuel ratio are possibly generated due to the temporary albeit rapid fluctuations in the concentration of vaporized fuel.