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What is Engine Efficiency?

By Lori Kilchermann
Updated Feb 16, 2024
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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.

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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Discussion Comments

By anon962743 — On Jul 25, 2014

How do I find the overall efficiency of a power train?

By Melonlity — On Jul 07, 2014

@Logicfest -- true, but all of those innovations have made the hobbyist mechanic almost a thing of the past. There was a time when you opened a car hood and saw an engine and its supports, accessories such as generators and the ground. Looking into a modern engine bay is completely different. Finding the engine itself is a challenge at times.

My wife needed a new battery in her SUV not long ago and even changing that was a chore due to a weird kind of exoskeleton that was on top of the battery (I'm still not sure what that complicated bit of machinery is for). If changing a battery is hard, just imaging how difficult it is to do something more complex like setting timing.

Increases in engine efficiency are great, but they have come at the cost of making engines almost impossible for anyone but trained mechanics to figure out and repair.

By Logicfest — On Jul 06, 2014

Ever wonder how modern auto manufacturers are able to squeeze over 300 horsepower out of a six cylinder engine and achieve 30 miles per gallon on the interstate? They are increasing the overall efficiency of the engine.

Innovations like fuel injection, better air intake systems, higher compression ratios and other factors have all led to improved efficiency over the years. We still have a long way to go, but auto manufacturers seem to be on the right track, at least.

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