Overview
In general, energy conversion efficiency is the ratio between the useful output of a device and the input, in energy terms. For thermal efficiency, the input, to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, or heat, or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is
From the first law of thermodynamics, the energy output cannot exceed the input, so
When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines' efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency, and a large coal-fueled electrical generating plant peaks at about 46%. The largest diesel engine in the world peaks at 51.7%. In a combined cycle plant, thermal efficiencies are approaching 60%. Such a real-world value may be used as a figure of merit for the device.
For engines where a fuel is burned there are two types of thermal efficiency: indicated thermal efficiency and brake thermal efficiency. This efficiency is only appropriate when comparing similar types or similar devices.
For other systems the specifics of the calculations of efficiency vary but the non dimensional input is still the same. Efficiency = Output energy / input energy
Read more about this topic: Thermal Efficiency