In all engines with internal combustion, the gas involved is changed after each working cycle, i.e. exhaust gas is expelled and fresh mixture (fresh gas) is supplied. The unused combustion heat that escapes with the exhaust gas is included in the power loss.
Modern engines compress the gas supplied to the working chamber, then combustion is initiated under pressure. The gas heats up strongly and the pressure increases. The engine relaxes the hot gas (for example with a retreating piston), the pressure and temperature of the gas fall and the volume increases. In the process, it performs mechanical work. Depending on the construction and function of the engine, these processes are carried out in different ways. Fundamental to its function as an engine is that, because of the combustion of the fuel-air mixture, the expansion of the mixture occurs at higher pressure than compression. The maximum efficiency possible depends on the temperature levels at which the heat of combustion is supplied and removed, and depends on the compression ratio and the cycle. Large two-stroke diesel engines achieve efficiencies of just over 50%. Modern automotive gasoline engines achieve an effective efficiency of 40 % at the best operating point (approximately in the middle of the speed band and just below the full load curve). For automotive diesel engines, it is 43 %. The efficiency is lower at high speeds and drops sharply as the load decreases because the mechanical losses in the engine hardly change over the load. They amount to about 10 % of the full load power and depend almost exclusively on the speed. (see consumption map). This is of particular importance for automotive engines in road traffic, as they are mainly operated in the lower partial load range. The average efficiency of a motor vehicle engine is therefore much lower than the maximum values. Crastan, for example, gives an average efficiency of 20 % for a conventional vehicle with a petrol engine.
