Experimental Research On The Optimization Of Combustion Process Of SI Hydraulic Free Piston Engine

Spark ignition engines have the advantages of direct control of combustion phase,zero soot emissions in combustion process,and convenient disposal of harmful emissions.However,it is difficult to improve the compression ratio and increase cylinder diameter due to the detonation phenomenon.So low thermal efficiency of spark ignition engines is always a difficult problem to solve in the internal combustion engine field for a long time.In this paper,unique piston movement law of hydraulic free piston engine,the flexible controllable compression ratio and multi-spark plugs ignition technology were combined to improve the thermal efficiency of SI engine and to find the way to optimize the combustion process of ignition hydraulic free piston engine.In this paper,the effect of the structure and control parameters such as spark plug number,ignition phase,piston movement law and compression ratio on thermal efficiency and detonation tendency were studied by means of engine Indicator diagram,heat release rate analysis and simulation in a single hydraulic free piston engine.The results show that:Using multi-spark plug ignition can shorten the duration of combustion,increase the maximum pressure in the cylinder,and improve the thermal efficiency of the single cycle.The ignition efficiency of the four spark plugs and seven spark plugs are improved by 4.2 % and 4.9 %,compared with those of the single spark plugs.However,multi-spark plug ignition will increase the knocking tendency which was revealed from the reduction of the limit compression ratio:1,4,7 limit compression ratio of one spark plug ignition,four spark plugs ignition and seven spark plugs is 13,12.5,11.7 respectively.By changing the motion quality of the piston,it can realize different piston movement laws.reducing the motion quality of the piston can improve the speed of piston movement and reduce the residence time of the piston in the top dead center.The knocking tendency is obviously reduced: in this research,the compression ratio is increased to 16.5 without knock;but with the piston speed increases,indicating the thermal efficiency is reduced;compression ratio 15.8 is the highest thermal efficiency,which means the compression ratio exceeds 15.8 but thermal efficiency will reduce although no knocking phenomenon.Based on the above results,it is concluded that the combustion process of the ignition free piston engine is optimized by using multi spark plug ignition,increasing the piston velocity and using the appropriate compression ratio(15.8).The indicated thermal efficiency is increase by 20 % without knocking phenomenon compared to the original program after optimization.The results of this research have certain reference value in the aspects of suppression knocking phenomenon in SI combustion process,realization way of high compression ratio SI mode,and development of clean and high efficiency hydraulic free piston engines.