The LiquidPiston engine is a pistonless rotary engine that operates on the High Efficiency Hybrid Cycle. This cycle consists of compressing air (with no fuel) to a very high ratio, as is typical in the Diesel cycle. The air is then isolated in a constant volume chamber. Fuel is injected and allowed to combust fully under constant volume conditions, which is how Otto cycle combustion is modeled. Finally, the combustion products are expanded to atmospheric pressure, utilizing the Atkinson cycle.
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Liquidpiston Wikipedia(Text) CC BY-SA