A hybrid vehicle, which includes an engine and a motor as a driving power source, can achieve zero emission traveling by traveling only using the motor with the engine being stopped. Meanwhile, generally, at least during high-speed traveling or acceleration in which driving power becomes large, the hybrid vehicle travels with the engine being operated. Hence, in order to remove emission from the exhaust gas of the engine, an exhaust emission purifier such as a three-way catalyst is provided. In the description below, such exhaust emission purifiers are also collectively and simply referred to as “catalyst”.
Japanese Patent Laying-Open No. 2007-237794 (PTL 1) describes a technique for decreasing temperature of a catalyst when the temperature of the catalyst becomes excessively high. Specifically, it is described that when the level of state of charge (SOC) in the battery is high, the motor is controlled to increase motor output so as to rotate the engine at a predetermined rotational speed or more using the motor output, and that when the level of the SOC is low, the air-fuel ratio of the engine is enriched and engine output is increased to get out of a low-load region.
Further, Japanese Patent Laying-Open No. 2004-204707 (PTL 2) describes engine control for avoiding occurrence of knocking in a hybrid vehicle. Specifically, when operating state of the engine is transitioned from an initial state to a final state in response to a request for high-load output, the engine is temporarily controlled to operate in a transient state, which is an intermediate state between the initial state and the final state. While operating in the transient state, the motor generator operates to compensate insufficiency of the engine output.