<P> An engine - generator or portable generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of equipment . This combination is also called an engine - generator set or a gen - set . In many contexts, the engine is taken for granted and the combined unit is simply called a generator . </P> <P> In addition to the engine and generator, engine - generators generally include a fuel supply, a constant engine speed regulator (governor) and a generator voltage regulator, cooling and exhaust systems, and lubrication system . Units larger than about 1 kW rating often have a battery and electric starter motor; very large units may start with compressed air either to an air driven starter motor or introduced directly to the engine cylinders to initiate engine rotation . Standby power generating units often include an automatic starting system and a transfer switch to disconnect the load from the utility power source when there is a power failure and connect it to the generator . </P> <P> Engine - generators are available in a wide range of power ratings . These include small, hand - portable units that can supply several hundred watts of power, hand - cart mounted units, as pictured below, that can supply several thousand watts and stationary or trailer - mounted units that can supply over a million watts . Regardless of the size, generators may run on gasoline, diesel, natural gas, propane, bio-diesel, water, sewage gas or hydrogen . Most of the smaller units are built to use gasoline (petrol) as a fuel, and the larger ones have various fuel types, including diesel, natural gas and propane (liquid or gas). Some engines may also operate on diesel and gas simultaneously (bi-fuel operation). </P> <P> Many engine - generators use a reciprocating engine, with fuels mentioned above . This can be a steam engine, such as most coal - powered fossil - fuel power plants use . Some engine - generators use a turbine as the engine, such as the industrial gas turbines used in peaking power plants and the microturbines used in some hybrid electric buses . </P>

When is a gasoline engine used as the prime mover for a generator
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