The Otto cycle with polytropic compression and expansion is a thermodynamic cycle that describes the operation of an ideal spark-ignition engine. It consists of four processes: isentropic compression, constant-volume heat addition, isentropic expansion, and constant-volume heat rejection.
In the first process, the air-fuel mixture is compressed adiabatically, which increases its temperature and pressure. Then, the mixture is ignited, causing a rapid increase in pressure and volume due to the combustion. This is followed by an isentropic expansion, which converts the thermal energy into mechanical work. Finally, the exhaust gases are expelled in the fourth process, completing the cycle.
Isentropic compression/expansion refers to a process that occurs without any heat transfer or change in entropy. This means that the change in temperature is only due to the work done on or by the gas. On the other hand, polytropic compression/expansion takes into account the heat transfer and changes in entropy, resulting in a non-constant temperature change.
The main advantage of the Otto cycle with polytropic compression and expansion is its high efficiency. The addition of the isentropic compression and expansion processes allows for a more complete combustion and better utilization of the thermal energy. Additionally, the constant-volume heat addition and rejection processes result in a smoother and more efficient power delivery.
The efficiency of the Otto cycle with polytropic compression and expansion can be increased by increasing the compression ratio, which leads to a higher peak temperature during combustion. Using higher octane fuel can also improve the efficiency by reducing the chances of pre-ignition and allowing for a higher compression ratio. Additionally, optimizing the timing of the combustion and exhaust processes can further improve the efficiency of the cycle.