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We have developed an effective secondary air-injection system that reduces harmful substances such as HC and CO. The secondary air in this system is heated to 300°C and injected into the exhaust pipe. Though the temperature of the secondary air is relatively low, it can activate a three way catalyst more rapidly than conventional secondary air injection systems. Thus, in our system (a “Heated-Air-Injection System”) is expected to be very effective in reducing harmful substances in the cold transient phase of the US Federal Test Procedure. For designing the system and analyzing its performance, we developed a simulation model including the design parameters of the system, such as flow rate of heated air, heater power, and so on. Besides these design parameters, the model takes into account of heat transfer from exhaust gas to exhaust pipe, gas-conversion reactions in a three way catalyst, and heat transfer efficiency of the electric heater.