An engine control method and related engine control method are disclosed for an engine with an auxiliary device of a vehicle wherein auxiliary device control means controls a drive torque of the auxiliary device, engine control means executes an engine torque control to vary the output torque of the engine, and failure detecting means detects a failure giving an adversely affect to engine torque control executed with the engine control means. The auxiliary device control means alters a drive control mode of the auxiliary device in response to the failure detected with the failure detecting means. In preferred embodiment, the auxiliary device is driven in a collaborative control mode in the presence of an extremely mild failure, a fixed voltage control mode in the presence of a mild failure and a gradual change control mode in the presence of a fatal failure.