Cyclic processes and their effeciency, Carnot cycle Information card

Heat engines transform input heat into work in accordance with the laws of thermodynamics. For instance, as we learned in the previous Concept, increasing the temperature of a gas at constant volume will increase its pressure. This pressure can be transformed into a force that moves a piston.

The mechanics of a basic heat engine involve the following steps:

•         Heat is supplied to the engine from some source at a higher temperature (T_h).

•         Some of this heat is transferred into mechanical energy through work done (W).

•         The rest of the input heat is transferred to some source at a lower temperature (T_c) until the system is in its original state.

The efficiency of an engine is define as the ratio of net work performed to input heat; this is the fraction of heat energy converted to mechanical energy by the engine:

What can be the maximum efficiency for a heat engine?

An ideal engine, the most efficient theoretically possible, is called a Carnot Engine.

Its efficiency is given by the following formula, where the temperatures are, respectively, the temperature of the exhaust environment and the temperature of the heat input, in Kelvins.