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What Is Engine Efficiency?

A man filling up his gas tank. A standard gasoline engine is only about 30% efficient, at best.
Diesel engines are more efficient than gasoline ones, in part, because they use high compression to ignite fuel.
The inherent inefficiency of steam-driven piston engines led to the demise of steam locomotives.
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  • Written By: Lori Kilchermann
  • Edited By: Jacob Harkins
  • Last Modified Date: 15 April 2014
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Engine efficiency refers to an engine's ability to transform the available energy from its fuel into useful work power. The modern gasoline combustion engine operates at an average of roughly 20 to 30 percent engine efficiency. The remaining 70 to 80 percent of the gasoline's heat energy is expelled from the engine as either exhaust heat, mechanical sound energy or friction loss. At idle, the engine efficiency is zero since the engine is not moving the vehicle and is only operating accessories, such as the water pump and generator.

Diesel engines are a bit more efficient. The diesel engine uses high compression to ignite its fuel. This higher compression compensates for the engines heat robbing parasitic loss and results in roughly 40 percent engine efficiency from idle to nearly 2,000 revolutions per minute. This engine efficiency is only observed by direct injection diesel engines.

An engine's compression ratio will affect its ability to be efficient. This is due, in part, from the engine's ability to convert the heat from the ignition process into work producing energy. The typical gasoline automobile engine operates at no more than 10:1 compression ration. Conversely, the typical diesel engine may operate with a compression ration as high as 25:1. The higher the compression ratio, the better the overall engine efficiency.

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The amount of oxygen that an engine is able to engulf directly affects its ability to operate more efficiently. This is the reasoning for introducing nitrous oxide into a gasoline engine's fuel system. The nitrous oxide adds oxygen molecules into the fuel, allowing more fuel to be burned in the combustion chamber. This burning of the added fuel allows the engine to operate more efficiently.

Fuel type also directly affects an engine's efficiency rating. Gasoline with a higher octane rating will allow the engine to operate with a higher compression ratio. This in turn creates greater engine efficiency. Fuels such as nitromethane produce oxygen, thereby creating much more power by allowing more fuel to be burned within the engine.

Some engines are even less efficient. The piston-steam engine, for example, operates at roughly 8 percent engine efficiency. This was a primary factor in the demise of the steam-powered locomotive. Steam turbines on the other hand, operate at efficiency levels equal to or exceeding those of the diesel engine. This is why the steam turbine is used for electrical generating plants. The gas turbine engine is the most efficient of all engines when operated at full power. They are used to produce electricity during high-use periods and are shut down after the added need has been met.

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