Disclosed is a modular engine assembly incorporating a primary engine, an auxiliary engine and a device for storing kinetic energy. The kinetic energy storage device is drivingly coupled to the primary engine. In response to demand for additional power, the auxiliary engine is selectively engaged with the energy storage device by a clutch. The auxiliary engine is thereby turned over and brought up to speed. Once the auxiliary engine is brought up to speed, fuel is supplied whereby the auxiliary engine starts or fires only after it is turning over at a high RPM. Once started, the auxiliary engine drives the transmission through the clutch and primary engine thus increasing available power upon demand.