An Otto cycle

An Otto cycle is an idealized thermodynamic cycle which
describes the functioning of a typical spark ignition
reciprocating piston engine, the thermodynamic cycle most
commonly found in automobile engines.
The processes are described by
Process 1-2 is an isentropic compression of the air as the
piston moves from bottom dead centre (BDC) to top dead
centre (TDC).
Process 2-3 is a constant-volume heat transfer to the air
from an external source while the piston is at top dead
centre. This process is intended to represent the ignition of
the fuel-air mixture and the subsequent rapid burning.
Process 3-4 is an isentropic expansion (power stroke).
Process 4-1 completes the cycle by a constant-volume
process in which heat is rejected from the air while the
piston is a bottom dead centre.y