Atkinson cycle is an ideal cycle for Otto engine exhausting to a gas turbine. In this cycle, the isentropic expansion 3-4 of an Otto cycle (1-2-3-4) is further allowed to the lowest cycle pressure so as to increase the work output. The cycle is shown below in p-V and T-s diagrams.
Where, Compression ratio = rk = v1/ v2,
the expansion ratio = re= v4/ v3