The operation characteristics of a liquefied petroleum gas (LPG) spark-ignition free piston engine

A prototype of dual-piston type spark-ignition free piston engine coupled with a linear alternator was built for electric power generation. Liquefied petroleum gas consisting of 98% propane (and 2% butane) was used as the fuel. A gas fuel was selected for better mixture preparation with air due to limited operation range of the engine. The effects of air flow rate and the alternator load on the operation characteristics of the free piston engine were studied. The final electric power output was determined by the alternator load (or the electric conductance value inside the electric circuit). The piston dynamics was varied by the alternator load due to the changes in the induced current and the voltage. High induced current deteriorated the piston kinetic energy due to high magnetic resistive force applied on the piston. Therefore, an optimum electric power output value existed right before when the increased alternator load (increased current) was cancelled out by the deteriorated piston velocity (decreased voltage). The engine power output change with respect to the alternator load was also mainly due to the changes in the piston dynamics. The cyclic variation of the piston work was higher at lower alternator load condition due to higher piston kinetic energy. The variation of the operation frequency and the stroke were coupled together. They decreased simultaneously with respect to the increase of the alternator load. The instantaneous piston velocity at the spark timing was derived to be a key factor for the control and engine performance.