Fuel economy can be improved by reducing engine displacement, thanks to the resulting smaller friction losses and pumping losses. However, smaller engines frequently operate at high-engine speed and high-load, when pressure on the accelerator increases during acceleration and at high speed.To protect exhaust system components from thermal stress, exhaust gas temperature is reduced by fuel enrichment.To improve fuel economy, it is important to increase the frequency of stoichiometric operation at high-engine speed and high-load.Usually, the start timing of fuel enrichment is based upon temperature requirements to protect the catalyst. In the high-engine speed and high-load zone, the threshold temperature of catalyst protection is attained after some time because of the heat mass. Therefore, stoichiometric operation can be maintained until the catalyst temperature reaches the threshold temperature.The existing system is operated stoichiometrically by estimating the time when the catalyst temperature will reach threshold temperature. Currently, catalyst temperature is not measured directly and the estimation of catalyst temperature is not accurate, therefore the engine can only be operated stoichiometrically-allowing an adequate margin-until the catalyst temperature reaches threshold temperature.The system reported in this paper was developed to achieve better fuel enrichment than existing systems by the installation of a high-accuracy exhaust gas temperature sensor to the inlet of the manifold catalyst to accurately estimate the catalyst temperature based on exhaust temperature.