Ottosüklis

Ottosüklis, in Estonian terminology often rendered as the Otto cycle, is a thermodynamic cycle used in internal combustion engines to convert the chemical energy of fuel into mechanical work. It is the basis for most spark-ignition (gasoline) engines and describes a four-stroke sequence consisting of intake, compression, combustion with heat addition at constant volume, and expansion, followed by exhaust.

In the Otto cycle, the processes are typically defined as: 1-2 an adiabatic (isentropic) compression of the air–fuel mixture, 2-3 heat addition at constant volume, 3-4 adiabatic expansion producing useful work, and 4-1 heat rejection at constant volume. The cycle is an idealization that helps analyze engine performance, recognizing that real engines experience heat transfer, friction, and non-ideal gas behavior.

Historically, the cycle was developed in the 1870s by Nikolaus Otto and collaborators, evolving from earlier gas compression experiments. It became the foundational model for gasoline-powered engines and has influenced the design and optimization of compression ratios, ignition timing, and fuel delivery systems. The term Ottosüklis reflects the adaptation of the Otto cycle concept in Estonian-language technical literature.

Thermal efficiency for the ideal Otto cycle (with an ideal gas of constant specific heats) is given by eta = 1 − 1/(r^(gamma−1)), where r is the compression ratio and gamma is the ratio of specific heats. In practice, real engines achieve lower efficiency due to heat losses, friction, and changes in specific heats during operation. The Otto cycle remains a reference point against which other cycles, such as the Diesel, Atkinson, and Miller cycles, are compared.

See also: Diesel cycle, Atkinson cycle, Miller cycle, spark-ignition engines.