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Emission regulations continue to be strengthened, and it is important to decrease cold start hydrocarbon concentrations in order to meet them, now and in the future. The HC concentration in engine exhaust gas can be reduced by optimizing the air-fuel ratio. However, a conventional air-fuel ratio feedback control does not operate for the first ten seconds after the engine has started because the air-fuel ratio sensor has not yet been activated. In this paper, we report on a study to optimize the air-fuel ratio using a crank angle sensor until the air-fuel ratio sensor has been activated. A difference in fuel properties was used as a typical disturbance factor. The control was applied to both a direct-injection engine (DI) and a port-injection engine (MPI). It was evaluated for two fuel types: one which evaporates easily and one which does not. The experimental results show the air-fuel ratio is optimized for both types of fuel.