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For an engine working with maximum efficiency, one, that is, which neither works against friction nor allows heat to slip across a temperature gradient fruitlessly, the equation can be extended:

_ Q , - Q 2 _ V i v Q, T,

where Q is the heat released from the engine to the sink and and T2 are the absolute temperatures of the source and sink respectively. (In fact, this equation defines the absolute thermodynamic temperature scale, but that is another matter.) This equation shows that high efficiency is obtained by ensuring a large temperature dif­ ference between source and sink. The efficiency is sometimes given as a percentage instead of as a decimal fraction.