This paper reviews the properties of the Atkinson cycle engine in relation to environmental pollution. The combination of low compression ratio and high expansion ratio makes it possible to maintain efficient working with lower temperatures and pressures than are possible with the Otto cycle. As a result there is much less Nitrogen dioxide produced during combustion. The use of sleeve valves in conjunction with an approximation to straight line motion reduces both mechanical loss and noise. A design is presented which incorporates only one rotating member and includes an alternator providing 3 phase current suitable for light weight Hybrid drive motor cars and having an output of 4kW(e). The basic design is also suitable for Combined heat and power.