Numerical comparison of HCCI combustion mode between a free piston linear engine and traditional crankshaft engine

The free piston linear engine (FPLE) eliminates the crankshaft components and supports variable compression ratio operation. This characteristic makes it as one of the most appropriate choices to adopt HCCI combustion mode. This work aims to reveal the preliminary characteristics of HCCI combustion in FPLE and compare it with traditional crankshaft engines (TCE). The HCCI combustion model of TCE and FPLE was established by using a chemical kinetic reaction mechanism, and it then was numerically calculated to obtain the combustion and emission performance. The results indicate that the typical two-stage reaction characteristic of HCCI combustion also occur in the FPLE. Although the final cumulative heat release of the two types of engines is approximately the same, the low-temperature reaction stage of FPLE comes later, which also leads to the lag of its high-temperature reaction stage. In addition, the TCE features with earlier heat release rate, higher peak pressure and temperature, and longer duration of high temperature, which are beneficial to promote the reaction of soot, thereby slightly reducing soot emissions, but also increasing NO emissions.