In an engine control system operated by closed control loop, an indispensable component is an exhaust gas sensor which detects a stoichiometric air-fuel ratio. This exhaust gas sensor is mounted at the exhaust side of the engine and, therefore, a time delay occurs in controlling the intake mixture gas. Further, when the engine temperature is low, such as during engine warm-up, the exhaust gas sensor fails to operate or operates insufficiently and, therefore, control by this sensor is impossible. The only method conceivable for accurate engine control to attain a predetermined air-fuel ratio under such a condition is by controlling the fuel injection amount at an improved detection accuracy of the air flowmeter.
In view of the above-mentioned fact that closed loop control of the engine by the exhaust gas sensor is impossible under such a special engine operating condition, the prior art control systems employ a method in which an appropriate air-fuel ratio is read out of a table stored in a read-only memory (ROM) in accordance with the intake air flow rate and engine number of revolutions (engine speed), and the air-fuel ratio thus read out is used for engine control. In such a conventional method for engine control, however, an inaccurate air-fuel ratio is likely to be read out of the memory due to variations in the characteristics of the air flowmeter or changes in the detection sensitivity thereof with time. A method for overcoming this problem is disclosed by U.S. Pat. No. 4,130,095 and U.S. Pat. Application Ser. No. 950,572 dated Oct. 12, 1978, now Pat. No. 4,201,161 in which a correction factor for correcting the air-fuel ratio stored in the table is tabulated and stored in a random access memory (RAM) and this correction factor is appropriately rewritten during the process of closed loop control based on the exhaust gas sensor.
However, a RAM, in spite of capability of writing and reading the data, or the erasure thereof, has the disadvantage that the data stored therein disappears at the time of power off or failure. This type of engine control data is used recurrently for driving the engine and therefore must be kept unerased. For this reason, a method is conceivable in which an EAROM is used instead of a RAM to prevent the required data from being erased even when power is cut off. By the way, EAROM is an abbreviation of an electrically alterable ROM which makes up a non-volatile memory.
Although this EAROM is available for repeated write and read operations and has an excellent feature that the storage therein is not erased at the time of power cut-off, its disadvantage is that the reliability thereof is reduced after repeated operations.