The paper discusses the concept, specification and overall performance of a 10 kW electric power range extender suitable for electric plug-in and series hybrid vehicles, based on a single cylinder, high speed, four stroke internal combustion engine, tested and developed at Istituto Motori CNR of Italy. This unit has been conceived from the beginning as a compact on board recharging system for the mentioned kind of means, and especially for city cars and small commercial vehicles. The paper starts by defining some characteristics, advantages and drawbacks of an electric city car, followed by the criteria adopted to characterize the nominal power of the range extender. Then, the ratio which leaded to the adoption of a single cylinder internal combustion engine is discussed, followed by an explanation of the main design characteristics of the whole unit. The latter has been re-designed and constructed by means of mass produced elements only, in order to reduce cost coming from new design and manufacture. The use of liquid cooling for the engine and the electric generator permitted to make the whole unit very compact and completely packaged, allowing an easy application on the vehicle. After field test, a complete description of its running behavior has been reported; the unit has been tested with conventional unleaded gasoline, but it's ready to be converted to ethanol and methanol gasoline blends or LPG if required.