Steam Engine

Carnot Efficiency

Graphical description of the Carnot Cycle and Carnot Efficiency. This is a PV graph showing the 4 stages operating between T_h and T_c = T_l.[17]

The maximum heat that can be converted to mechanical work is fully described by the Carnot efficiency; the ratio of the output work to the input heat of a system.  As a percentage of input heat the output work (ηth) can be expressed by the equation below, where T_c = T_l is the temperature of the cold sink and T_h is the temperature of the heat source (in Kelvin).

There are many sources of irreversibility which contribute to loss of efficiency.[14]  The main contributing sources of irreversibility are: heat flowing directly from the source to the sink; the full temperature gradient may not be available due to thermal resistance across the path of heat flow; some of the work generated is converted back to heat by friction; the expansion may do no work and only increase the entropy (as in the case of expansion into a vacuum, no work is done).