Numerical analysis of two-stroke free piston engine operating on HCCI combustion

An opposed-piston hydraulic free piston engine operating with homogenous charge compression ignition (HCCI) combustion, has been proposed by State Key Laboratory of Engines as a means of significantly improving the IC engine’s cycle thermal efficiency and lowering exhaust emissions. Single and multi-zone Chemkin model with detailed chemical kinetics, and unique piston dynamics extracted from one dimensional gas dynamic model, have been used to analyze the combustion characteristics and engine performance. Intake heating, variable compression ratio and internal EGR are utilized to control the combustion phasing and duration in the cycle simulations, revealing the critical factors and possible limits of performance improvement relative to conventional crank engines. Furthermore, real engine effects such as heat transfer with air swirl, residual mass fraction, thermal stratification, and heat loss fraction between zones are considered in the sequential CFD/multi-zone method to approach the realistic engine performance at an acceptable knock level.

► Single and multi-zone Chemkin model and sequential CFD/multi-zone method are used. ► Real engine effects such as heat transfer with air swirl are considered in the simulation. ► The simulation results can approach the realistic engine performance at an acceptable knock level. ► The critical factors and possible limits of performance improvement for free piston engines are revealed.