Various Processes of Ideal Otto Cycle
The ideal Otto cycle comprises of two isentropic (constant entropy) and two constant volume processes. The Otto cycle is an open cycle or non-cyclic process since the fresh air-fuel is mixture is inducted inside the engine during each cycle and the burnt mixture is released to the atmosphere. To understand these processes let us consider piston and cylinder engine air-fuel mixture as the working fluid. Refer the P-V diagram given at the top.
1) Air-fuel intake process 1-2: During this process the inlet valve of the engine is open, the piston moves towards the bottom position inducting air-fuel mixture at constant pressure.
2) Isentropic compression process 2-3: During this process the inlet and exhaust valves of the engine remain close and the air-fuel mixture, which has been inducted inside the cylinder, is compressed to the minimum volume.
3) Combustion of air-fuel mixture at constant volume 3-4: Thereafter, the air-fuel mixture inducted inside the cylinder is combusted by the spark at constant volume; hence these engines are called spark ignition engines. This leads increase in temperature and pressure inside the cylinder.
4) Isentropic expansion process 4-5: Due to extremely high pressure, the piston is pushed again towards the bottommost position of the cylinder. It is during this process that the actual work is produced from the engine.
5) Constant volume process 5-6: During this process the exhaust valve opens and all the exhaust gases are ready to be released to the atmosphere. The pressure inside the cylinder falls drastically.
6) Exhaust process 6-1: During this process the exhaust valve is open and the piston moves upwards and removes all the exhaust gases inside the cylinder at constant pressure.
Thereafter the exhaust valve closes, the piston starts moving in downward direction, the inlet valve opens and fresh air-fuel mixture is inducted. The whole cycle is completed in four strokes of engine, hence it is called four-stroke engine.