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Fig. 4.3 - The execution of the Otto cycle in a piston–cylinder device (a), together
with P-v diagram (b), and T-s diagram (c)

Now the piston moves upward one more time, purging the exhaust gases through the
exhaust valve (the exhaust stroke), and down a second time, drawing in fresh air–fuel
mixture through the intake valve (the intake stroke). Notice that the pressure in the
cylinder is slightly above the atmospheric value during the exhaust stroke and slightly
below during the intake stroke.
In two-stroke engines, all four functions described above are executed in just two
strokes: the power stroke and the compression stroke.
The thermodynamic analysis of the actual four-stroke or two-stroke cycles described is
not a simple task. However, the analysis can be simplified significantly if the air-
standard assumptions are utilized. The resulting cycle, which closely resembles the
actual operating conditions, is the ideal Otto cycle. It consists of four internally
reversible processes:
1-2 Isentropic compression
2-3 Constant-volume heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection
The execution of the Otto cycle in a piston–cylinder device together with P-v diagram
and T-s diagram is illustrated in Fig. 4.3.

The thermal efficiency of the Otto cycle is
1
  1 (4.3)
th 1 - k

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