Effect of ignition timing on the combustion process of a port injection free piston linear generator: A system level multi-physics coupling method

Free piston linear generators (FPLGs) have the advantages of high power density, high compactness and flexible compression ratio, rendering FPLGs as potential power sources in pure electric range extension, hybrid power and various outdoor power fields. In the present study, the influence of ignition timing on the performance of a dual-piston dual-cylinder free-piston engine generator was experimentally investigated. A FPLG multi-physics coupled simulation-experimental bench was specially developed, and the effect of ignition timing on the engine performance was investigated through the three-dimensional numerical simulation method. The simulation results showed that with the increase of spark advance, both the peak in-cylinder pressure and the heat release rate raised, and the maximum peak pressure could reach over 30 bar under given scenarios. With the increase of ignition timing, the start of combustion was significantly advanced, and the maximum advance in combustion initiation was about 19°ECA over the investigated engine speeds. When the ignition timing ranges were −35°ECA ∼ -20°ECA, the combustion durations were roughly varied between 20°ECA ∼ 30°ECA. At the specified engine speed, when the ignition timing was earlier than − 20°ECA, the indicated power and indicated thermal efficiency were not sensitive to ignition timing. As the engine speed and spark advance increased, the NOx mass fraction in the combustion process were raised. The heat transfer loss decreased with the retarding of the ignition timing, and it increased as the engine speed decreased.