The two commonly used cycles on which the I.C. engines operate are:
1Otto cycle. 2 Diesel cycle.
1 OTTO CYCLE: theoretical Otto cycle is shown in the cycle was originally introduced by bean de Rocha's but Otto was the first to succeed in working an engine on this cycle. The cycle consists of two adiabatic and two constant volumes lines. At cylinder is full of air. The combustion takes place at constant volume, from (2) to (3) after which the air expends adiabatically from (3) to (4).
Where r= compression ratio
Γ = ratio of specific heats.
The actual P-V diagram of the cycle differs slightly from the theoretical.
2 DIESEL CYCLE: Theoretical diesel cycle P-V diagram shows that the popular engine works on this cycle. This is similar to Otto cycle, except that the combustion here takes place at constant pressure. The air standard efficiency, η, of this cycle is given as:
Where ρ= volume ratio at cut off= volume at (3)/volume at (1)