A gasifying device is fed oil from the return spill rail of a diesel engine, or from a reservoir of oil and gasifies the lighter fractions of the oil using an electrical heater, the gasified products being fed to the air intake manifold of the engine. In one embodiment unvaporized oil is fed from the gasifying device to the oil supply line to the engine, thereby avoiding a build-up of heavy oil components in the device. Several embodiments are disclosed. The device improves the handling performance of a vehicle fitted with the engine, the exhaust emissions, and the fuel economy. The device may also be fitted to a petrol engine.