**Mathematical Definition**

The efficiency of engine is defined as ratio of the useful **work done** to the heat provided.

where, is the heat absorbed and is the work done.

Please note that the term **work done** relates to the power delivered **at the clutch** or **at the driveshaft**.

This means the friction and other losses are subtracted from the work done by thermodynamic expansion. Thus an engine not delivering any work to the outside environment has zero efficiency.

Read more about this topic: Engine Efficiency

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