An auxiliary power unit for a motor vehicle includes an auxiliary engine with an output shaft directly connected to a generator. The generator has a field winding and three output windings. A first output winding produces 120 volts AC that is applied to standard electrical outlets in the vehicle. A second output winding produces a low AC voltage that is converted to 12 volts DC and applied to the battery of the truck, thereby charging the battery and powering the vehicle's electrical system when the primary engine of the vehicle is off. An auxiliary winding in which another AC voltage is produced that is used by a controller to excite the field winding. A novel enclosure for the auxiliary power unit also is described.