The Otto cycle idealizes a spark-ignition reciprocating engine using a fixed mass of ideal gas.

It consists of four internally reversible processes:

1. isentropic compression;
2. constant-volume heat addition;
3. isentropic expansion;
4. constant-volume heat rejection.

With constant specific heats, its thermal efficiency is

$$\eta_{Otto}=1-\frac{1}{r^{k-1}},$$

where

$$r=\frac{v_1}{v_2}$$

is the compression ratio and $k=c_p/c_v$.

For $r=10$ and $k=1.4$,

$$\eta_{Otto}=1-10^{-0.4}\approx0.602.$$

The result explains why a larger compression ratio improves the ideal cycle. Real engines depart from the model through finite-rate combustion, heat losses, friction, gas exchange, variable specific heats and material or combustion constraints.

The Otto cycle is therefore a thermodynamic benchmark for engine architecture, not a literal description of every process inside a real cylinder.