Abstract: A fuel injection system for an internal combustion engine wherein an oxygen sensor for detecting a limit electric current is employed for providing a signal indicating an oxygen partial pressure in the gas introduced into the engine. The sensor detects an amount of air newly introduced into the engine. In addition to a map for calculating a basic fuel injection amount from the oxygen partial pressure, a second map is provided for calculating the basic fuel amount from a parameter such as a degree of opening of the throttle valve. A degree of activation of the oxygen sensor is determined by detecting the impedance of the sensor. When it is determined that the oxygen sensor is not activated, the second map is employed in place of the first map to calculate the basic fuel amount based on the degree of opening of the throttle valve.

Abstract: An engine control system having an oxygen density sensor arranged in an intake system at a position downstream of a throttle valve for detecting the amount of air newly introduced into the engine. The sensor is a limit electric current detection type capable of detecting a continuously changing density of oxygen. An amount of fuel to be injected is calculated in accordance with the output level of the sensor by using mapped data of the output level of the sensor signal. The sensitivity of the sensor is detected and multiplied by the output value of the sensor for obtaining a precise oxygen partial pressure. Instead of calculating the basic fuel amount from the output level of the oxygen density sensor, a normal intake pressure sensor with an intake pressure-engine speed map can be employed. In the latter case, the oxygen partial pressure from the oxygen density sensor is used for correcting the calculated basic fuel amount. The sensitivity of the sensor is also corrected.