Effects of hydrogen addition on operating characteristics in a glow plug assisted compression ignition free piston linear engine

^*A miniature free piston linear engine (FPLE) with glow plug assisted compression ignition was designed and manufactured. Effects of hydrogen addition with volume concentration of 0%, 2%, 3% and 5% to methane/air mixtures on the operating characteristics of the FPLE were studies under different external loads. It is found that the FPLE can be started successfully in one cycle manually and then steady operation of was achieved. The results show that the performance of FPLE is improved as the hydrogen addition due to the high flame speed of hydrogen. Compared with 3% and 0% hydrogen addition, the peak pressure and the peak indicated power are increased by 23.4% and 22.8% respectively for hydrogen addition. However, the engine performance cannot be further improved when the volume rate of hydrogen continues to increase due to the fact that more hydrogen addition can cause advanced combustion to occur in the cylinders, which deteriorates the combustion and engine performance. The high speed color images show the flame is lighter and less yellow compared with that of no hydrogen addition, indicating that the hydrogen addition to the fuel have significant impacts on suppressing the soot formation.