Thermal efficiency measures energy output divided by input, with a maximum of 100%. Gasoline engines use heat released from combustion as input and mechanical work as output. Refrigerators use mechanical work as input and remove heat as output. Real-world efficiency is lower due to friction and other factors.
Thermal efficiency is a measure of the energy output divided by the energy input into a system. Must be between 0% and 100%. A level of 100% would mean that all energy entering a system exits, albeit in a different form. Both heat engines and refrigerators have associated thermal efficiencies, although they are trying to achieve opposite goals. Real-world thermal efficiencies generally drop significantly below 100% due to a variety of reasons.
In a gasoline engine, the energy input is stored in the chemical bonds of a hydrocarbon fuel. A hydrocarbon molecule consists entirely of hydrogen and carbon. When these molecules combine with oxygen, they can chemically react and form carbon monoxide and water; essentially, the hydrocarbon molecule is split apart and combined with oxygen atoms. The part of this reaction that is useful to an engine, however, is the heat that is released. The heat released by the combustion of gasoline is the relevant input energy in thermal efficiency.
The energy output in calculating the efficiency of an engine is not heat, but mechanical work. In physics, work is the amount of energy transferred by a force acting over a distance. Pushing a box across the carpet a certain distance requires a finite amount of work; this is equal to the product of the distance traveled and the average force exerted. Similarly, a petrol engine works when it moves the wheels of a car.
In the case of a refrigerator or an air conditioner, the heat-work ratio is reversed. The desired result in this situation is to remove heat from a system and vent it to the outside environment. The available input, therefore, is mechanical work, often provided by an electrically powered compressor. Calculating efficiency, however, still requires dividing the energy output by the energy input. The difference to a petrol engine, of course, is that the output is heat and the input is work.
A typical automotive engine has a thermal efficiency of less than 35%. This number seems low for two important reasons. First, there is a theoretical upper limit to the thermal efficiency of any heat engine which has to do with system temperature versus ambient temperature. The greater the temperature difference, the higher the maximum efficiency that an ideal, frictionless motor can achieve. This is called Carnot efficiency.
The second reason why car engines have apparently low efficiency is that the engines cannot be made to work ideally. Friction between moving parts constantly tends to slow down the engine. Some of the heat escapes from the combustion chamber and becomes useless to the engine. Fuel does not always burn at its highest temperature, reducing the amount of heat released. For these reasons, thermal efficiency in real-world devices tends to be well below 100%.
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