Otto cycle

The Otto cycle is an idealized thermodynamic cycle for spark-ignition engines. It models compression and expansion as adiabatic and heat addition and rejection as constant-volume processes.

Isentropic process: A thermodynamic process with constant entropy, reversible and adiabatic in the ideal case. The cycle idealizes compression and expansion as isentropic processes.
First law of thermodynamics: The principle that energy is conserved and changes form rather than being created or destroyed. It accounts for the heat and work exchanged during each cycle process.
Spark-ignition engine: An internal-combustion engine in which an electrical spark ignites the fuel–air mixture. The Otto cycle is the ideal thermodynamic model associated with this engine type.
Nikolaus Otto: A German engineer who developed a successful four-stroke internal-combustion engine in the nineteenth century. The cycle is named after Otto, whose engine established the practical four-stroke design.
Constant-volume process: A thermodynamic process in which the system's volume remains fixed. The ideal cycle adds and rejects heat during constant-volume processes.
Internal energy: The microscopic kinetic and potential energy contained within a thermodynamic system. For an ideal gas, temperature changes track the internal-energy changes across the cycle.
Air-standard assumptions: Simplifying assumptions that model an engine cycle with air as a fixed-mass working fluid. They make the Otto cycle calculable without modeling combustion and gas exchange in detail.
Étienne Lenoir: A Belgian-born engineer who built an early commercially successful internal-combustion engine. His engine preceded Otto's and provides context for the development of gas engines.
Compression ratio: The ratio of a cylinder's maximum volume to its minimum volume. This ratio is the main geometric parameter governing ideal Otto-cycle efficiency.
Ideal gas: A gas model whose particles have negligible volume and interact only through elastic collisions. The standard air-cycle analysis treats the working fluid as an ideal gas.

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