Abstract: The auxiliary power unit includes an auxiliary liquid-cooled diesel internal combustion engine for driving power producing equipment on board the vehicle. A first conduit connects the main engine liquid coolant outlet in fluid communication with the auxiliary engine liquid inlet. A second conduit connects the main vehicle engine liquid inlet in fluid communication with the auxiliary engine outlet. A pump recirculates the liquid coolant through the conduits so that the auxiliary diesel engine heats the coolant and thus the main engine, when it is not in use. The engine is controlled remotely from the vehicle cab.