Patent ID: 8495910

Claim:
An air-fuel ratio estimating/detecting device, comprising: an intake air volume estimating unit configured to estimate intake air volume introduced into a cylinder of an engine; a fuel injection amount estimating unit configured to estimate an amount of fuel injected for each cycle based upon a driving time of a fuel injection valve; an oxygen concentration detecting element having an output transition region where detection output according to concentration of oxygen remaining in a combustion gas is generated and the detection output changes in a stepwise fashion in accordance with the concentration of the remaining oxygen corresponding to a theoretical air-fuel ratio; and a proportional constant determining unit configured to determine a proportional constant of an air-fuel ratio and the theoretical air-fuel ratio by using the intake air volume estimated by the intake air volume estimating unit when an output value of the oxygen concentration detecting element is in the output transition region and the amount of fuel injected estimated by the amount of fuel injected estimating unit, wherein when the output value of the oxygen concentration detecting element is not in the output transition region, the air-fuel ratio is estimated from the proportional constant determined by the proportional constant determining unit, the intake air volume, and the amount of fuel injected.