Isentropic efficiency is a measure of the energy loss in a system. Since an isentropic process is an idealized process that occurs without entropy change, isentropic efficiency is often used to compare real-world losses to an idealized process.
The real-world efficiency of a device is always less than the isentropic efficiency. Examples of devices for which isentropic efficiencies are frequently calculated include turbines, compressors, nozzles and pumps.
Isentropic turbine efficiency is the ratio of actual work to the maximum theoretical work per mass flow. For compressors, the isentropic efficiency is the ratio of minimum theoretical work going into the compressor to the actual work per mass flow.
Typical isentropic turbine and compressor efficiencies range from 70 to 90 percent, depending on design and size. Nozzle isentropic efficiency is typically over 90 percent and can reach over 95 percent.
Many devices involve a number of individual processes, each of which are often analyzed in terms of isentropic efficiency. For example, a number of different isentropic efficiencies make up the efficiency of the cycle of a combustion engine, such as an internal combustion piston engine or a gas turbine engine. These individual efficiencies include the compression and expansion processes, as well as the inlet and exhaust flow through the ports or nozzles. By increasing the isentropic efficiency of each of these processes, designers seek to improve the overall efficiency of the engine.