Experimental Study Of Achieving Rapid Combustion With Multi-spark Ignition

The free piston engine is paid more and more attention for its great potential in high thermal efficiency and reduced emissions as the energy crisis and environmental problems become more and more serious.But the hydraulic free piston engine has a problem of a short residence time in the top dead center,it needs rapid combustion technology in order to get high rate of heat release.In order to find out a rapid and controllable combustion mode which is suitable for free piston engine,characteristics of multi point ignition are studied using a rapid compression machine that has a similar principle of operation with a free piston engine in this paper.The ECU which controls the multi-points ignition was developed and the data acquisition as well as analysis system were optimized based on the rapid compression and expansion machine experimental platform of multi points ignition of methanol.Using the experimental platform,the influence of the number of spark plugs on indicated work,the maximum combustion pressure,the maximum pressure rise rate,the heat release rate,the knock characteristics and cycle fluctuation were studied.The conclusions are as follow: the compression ratio of RECM is affected by the comprehensive effects of hydraulic pressure,ignition and gas phase components in the cylinder.The compression ratio of RECM increases with the increased hydraulic pressure,the delay of ignition and the decreased mixture of specific heat ratio.When conditions which conclude the pressure of hydraulic pressure and mixed gas concentration are the same,with the increase of the ignition phase,that is to say the ignition time is advanced,indicated work increases at first and then decreases.There is an optimum ignition phase.The optimum ignition phase can be delayed accordingly as the number of spark plugs increases.The heat release rate,the maximum combustion pressure,the pressure rise rate and the indicated work of the multi points ignition are significantly higher than those of the single point ignition,and the increase rate of the ignition is not linear with the number of spark plugs,the increase rate of the burning and the improvement of the indicator are more obvious in the thin mixed gas using multi spark plugs.Multi-point ignition is more inclined to cause knock when higher hydraulic pressure.Multi spark plug ignition makes better repeatability,which may lead a lower the cycle fluctuation of the engine.In brief,the fast controllable combustion can be realized by using a flexible change of the number of multiple spark plugs,increasing the pressure of the hydraulic pressure,reducing the concentration of mixture appropriately,and selecting the optimum ignition phase.In this paper,the characteristics of multi-point ignition combustion of free piston engine were studied by using the fast compression and expansion machine.The idea of realizing the fast and controllable combustion which is instructive to the high efficiency operation and engineering application of hydraulic free piston engine was proposed.