1.  Field of the Invention
The present invention relates to a fuel supply control system for an engine. 
2.  Description of the Related Art
Japanese Unexamined Patent Publication No.  5-26118 discloses a fuel vapor emission control system for an engine, in which the system includes a canister connected to an intake passage downstream of a throttle valve arranged therein via a purge passage, for temporarily storing fuel vapor, and a duty-controlled purge control valve arranged in the purge passage for controlling an amount of a purge gas, that is, air containing fuel vapor.  An opening ratio of the purge control valve is controlled to make the amount of purge gas equal to a target amount determined in accordance with an engine operating condition.  In a typical engine having such a system, during the purging operation, the amount of fuel to be injected from a fuel injector to the engine is reduced in accordance with the amount of the purge gas, and more precisely, in accordance with an amount of fuel vapor contained in the purge gas, to thereby make the air-fuel ratio of the engine equal to a target air-fuel ratio.  In this engine, if the amount of the purge gas is obtained, the air-fuel ratio is easily maintained at the target air-fuel ratio.  Therefore, in the above-mentioned system, the purge control valve is controlled to make the amount of the purge gas equal to a target amount to thereby maintain the air-fuel ratio at the target air-fuel ratio. 
In the above-mentioned system, an opening ratio of the purge control valve is controlled by, for example, a duty ratio.  The duty ratio is defined as a ratio of a period during which the purge control valve is opened to a duty cycle time.  When the opening ratio of the valve is controlled by the duty ratio, if the duty cycle time is set as in a very short period, such as a few millisecond, the opening and closing operation of the valve must be carried out in the very short period.  Accordingly, a load on the valve becomes larger.  Further, in this condition, the response of the valve deteriorates and thereby a controllability of the amount of the purge gas also deteriorates.  To solve this problem, in a typical system, the duty cycle time is set in a relatively long period, such as 100 ms, to thereby decrease the load of the valve.  In this connection, the duty ratio for each duty cycle is calculated before the corresponding duty cycle starts, in accordance with the engine operating condition at that time. 
When the purging operation is being carried out, if the engine operating condition varies drastically, namely, if, for example, the engine is rapidly accelerated, the target amount of the purge gas also varies drastically.  However, as mentioned above, the duty ratio is calculated before the duty cycle starts.  Namely, during the duty cycle, the duty ratio is not changed, even if the target amount of the purge gas varies.  As a result, if the engine operating condition varies during the cycle, the actual amount of the purge gas deviates from the target amount thereof.  Accordingly, a problem occurs that, in an engine in which an amount of fuel to be fed to the engine is controlled in accordance with the amount of the purge gas, if the amount of the purge gas deviates from the target amount thereof, the air-fuel ratio is not be maintained to the target air-fuel ratio.  JPP '118 does not teach this problem at all.