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A gas engine means an engine running on a gas, such as coal gas, producer gas biogas, landfill gas, or natural gas. In the UK, the term is unambiguous.

In the US, due to the widespread use of "gas" as an abbreviation for gasoline, such an engine might also be called a gaseous fueled engine, spark ignited engine, or natural gas engine.

Generally the term gas engine refers to a heavy-duty, slow-revving industrial engine capable of running continuously at full output for periods approaching a high fraction of 8,760 hours per year, for many years, with indefinite lifetime, unlike, say, a gasoline automobile engine, which is lightweight, high-revving and typically runs for no more than 4,000 hours in its entire life. Typical power ranges from 10 kW (13 hp) to 4,000 kW (5,364 hp).

Current manufacturers

Manufacturers of gas engines include Rolls-Royce with the Bergen-Engines AS, Kawasaki Heavy Industries, MTU Friedrichshafen, GE Jenbacher, Caterpillar Inc., Perkins Engines, MWM, Cummins, Wrtsil, Dresser-Waukesha, Guascor, Deutz, MTU, MAN, Doosan, and Yanmar. Output ranges from about 10 kW (13 hp) micro CHP [combined heat and power] to 18 MW (24,000 hp). Generally speaking, the modern high-speed gas engine is very competitive with gas turbines up to about 50 MW (67,000 hp) depending on actual circumstances, and the best ones are much more fuel efficient than the gas turbines. Rolls-Royce with the Bergen Engines, Caterpillar and many other manufacturers base their products on a diesel engine block and crankshaft. GE Jenbacher are the sole company whose engines are designed and dedicated to gas alone.

Typical applications

Typical applications are baseload or high-hour generation schemes, including combined heat and power, landfill gas, mines gas, well-head gas and biogas (where the waste heat from the engine may be used to warm the digesters). For typical biogas engine installation parameters see. For parameters of a large gas engine CHP system, as fitted in a factory, see. Gas engines are rarely used for standby applications, which remain largely the province of diesel engines. One exception to this is the small (<150 kW) emergency generator often installed by farms, museums, small businesses, and residences. Connected to either natural gas from the public utility or propane from on-site storage tanks, these generators can be arranged for automatic starting upon power failure. The natural gas engines (LNG) are getting more into the marine market, as the lean-burn gas engine can meet the new emission requirements whitout any extra fuel treatment or exhaust cleaning systems.