The Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines. The cycle consists of four processes, which are:
- Isentropic compression: The gas is compressed adiabatically and reversibly from state 1 to state 2.
- Heat addition at constant volume: The gas is heated at constant volume from state 2 to state 3.
- Isentropic expansion: The gas is expanded adiabatically and reversibly from state 3 to state 4.
- Rejection of heat at constant volume: The gas is cooled at constant volume from state 4 to state 1.
The Otto cycle is constructed from a pair of quasi-parallel and isentropic processes at the top and bottom of the loop, and a pair of parallel isochoric processes at the left and right sides of the loop. In the case of a four-stroke Otto cycle, there are two additional processes: one for the exhaust of waste heat and combustion products at constant pressure (isobaric), and one for the intake of cool oxygen-rich air also at constant pressure.
The efficiency of an ideal Otto cycle is shown as a function of the compression ratio in Figure 3.11. As the compression ratio increases, the efficiency increases, but so does the risk of the mixture igniting without a spark (at the wrong location in the cycle) . The Otto cycle provides the energy for most transportation and was essential for the modern world.
