A reciprocating engine is a machine that uses the kinetic energy of repetitively moving pistons to create pressure that is then converted to rotating motion in a crankshaft. The crankshaft can then be connected to the shaft of a generator. In a reciprocating engine, pistons are moved by heating a gas through ignition of a fuel air mixture or through contact with a hot heat exchanger. In the case of ignition, the engine is called an internal combustion engine. The internal combustion engine is what drives most automobiles and can be fueled by a variety of liquid fuels including diesel, gasoline, heavy fuel oils, compressed natural gas, liquefied petroleum gas, and biofuels. The most common generator driven by a reciprocating engine is the diesel genset often used for back-up generation, or primary generation in areas remote from the electrical grid. But larger natural gas reciprocating engine/gensets are now commonly used as utility-scale peaking units.