Development of a spark ignition free-piston engine generator

Abstract

A dual-piston type two-stroke spark-ignition free-piston engine generator prototype has been developed. A comprehensive review on recent published researches and patent documents from academia and industrial organisations on free-piston engine generator, especially on the applications for series hybrid electric vehicles, was conducted. Relevant parameters affecting the operating performance and a number of challenges had been identified as the common denominator for this technology. Modelling and simulations using one-dimensional tools were conducted in parallel with the development activities. Three main simulation models for the crankshaft engines were developed, validated and optimised before converted into the free-piston engine model. This was done by using imposed-piston motion sub-model. The two-stroke free-piston engine model had undergone parametric study for valve timing optimisation. This model was validated by using motoring experimental results using the developed free-piston engine generator prototype. From the experimental results, the free-piston engine generator motoring performance was able to meet the targeted cyclic speed and compression pressure for starting. However, the free-piston engine generator operating speed was limited to 5Hz and below due to valve delay inherent in the pneumatic actuators. The motoring results were used to validate the free-piston engine model which showed a good agreement at various starting speeds. Finally, performance and parametric investigations were conducted using the final validated and refined free-piston engine model. From the simulation, it was found that the free-piston engine had similar response to air-fuel ratio and ignition position variations compare to crankshaft engine with the free-piston engine performance was slightly reduced. Further, the reduced frictional losses contributed little to its performance gain. However, the high influence of piston motion around TDC on the engine performance, observed in free-piston engine, could be manipulated to increase its performance significantly.