namely TH, and 〈TC〉 the lowest, namely TC. For other less efficient thermodynamic cycles, 〈TH〉 will be lower than TH, and 〈TC〉 will be higher than TC. This can help illustrate, for example, why a reheater or a regenerator can improve the thermal efficiency of steam power plants and why the thermal efficiency of combined-cycle power plants (which incorporate gas turbines operating at even higher temperatures) exceeds that of conventional steam plants. The first prototype of the diesel engine was based on the Carnot cycle. Carnot heat engine as an impractical macroscopic construct A Carnot heat engine is a heat engine performing a Carnot cycle, and its realization on a macroscopic scale is impractical. For example, for the isothermal expansion part of the Carnot cycle, the following conditions must be satisfied simultaneously at every step in the expansion: The hot reservoir temperature TH is infinitesimally higher than the system gas temperature T so heat flow (energy transfer) from the hot reservoir to the gas is made without increasing T (via infinitesimal work on the surroundings by the gas as another energy transfer); if TH is significantly higher than T, then T may be not uniform through the gas so the system would deviate from thermal equilibrium as well as not being a reversible process (i.e. not a Carnot cycle) or T might increase noticeably so it would not be an isothermal process. The force externally applied on the piston (opposite to the internal force on the piston by the gas) needs to be infinitesimally reduced somehow. Without this external assistance, it would not be possible to follow a gas PV (Pressure-Volume) curve downward at a constant T since following this curve means that the gas-to-piston force decreases (P decreases) as the volume expands (the piston moves outward). If this assistance is so strong that the volume expansion is significant, the system may deviate from thermal equilibrium as well as not being a reversible process (i.e. not a Carnot cycle). These (and other) "infinitesimal" requirements make the Carnot cycle take an infinite amount of time. Other practical requirements that make the Carnot cycle hard to realize (e.g., fine control of the gas, thermal contact with the surroundings including high and low temperature reservoirs), so the Carnot engine should be thought as the theoretical limit of macroscopic scale heat engines rather than a practical device that could ever be built. See also