Ottociklus

Ottociklus, known in English as the Otto cycle, is an idealized thermodynamic cycle used to model the operation of spark-ignition internal combustion engines. It describes four distinct processes carried out by the working fluid in a cylinder: rapid compression, heat addition at nearly constant volume, expansion that performs work, and heat rejection at nearly constant volume to return to the initial state. The cycle is commonly used as a theoretical basis for analyzing gasoline engines.

In the ideal Otto cycle, the sequence is: 1-2 is an adiabatic (isentropic for an ideal gas) compression, 2-3 is a constant-volume heat addition due to combustion, 3-4 is an adiabatic expansion, and 4-1 is a constant-volume heat rejection. The working fluid is typically treated as an ideal gas, and the gas constant and gamma (Cp/Cv) govern the relationship between temperatures and volumes during the processes.

Thermal efficiency of the ideal Otto cycle depends on the compression ratio r = V1/V2 and the specific heat ratio gamma. The standard expression is η = 1 − 1/r^(gamma−1). Higher compression ratios can improve efficiency but are limited by factors such as knocking, detonation, material strength, and lubrication in real engines. While the Otto cycle provides a useful framework for understanding spark-ignition engines, real engines deviate due to non-ideal heat transfer, friction, and varying combustion characteristics.

See also: internal combustion engine, thermodynamic cycles, four-stroke engine.