Numerical Investigation on the Indicated Mean Effective Pressure and Integral Heat Release Rate Variations under Different Key Operating Parameters of a Spark-Ignited Free Piston Engine Generator

Free-piston engine generator is a new type of hybrid power device and is regarded as the next-generation energy conversion device which can replace the traditional internal combustion engine. This paper focused on the combustion stability and combines experimental results to study the key factors affecting the stability of the free-piston engine, such as ignition time, intake pressure, equivalence ratio, and operating frequency. The simulation results showed that as the ignition advance angle increased, the indicated mean effective pressure increased significantly and the coefficient of variation of the indicated mean effective pressure was effectively reduced from 15.55% to 1.02% as the advance in ignition time from -15 to -30°ECA. When the intake pressure was increased to 1.2 bar, the average value of indicated mean effective pressure reached about 5.15 bar. When the equivalence ratio was in the range of 1.0-1.4, the coefficient of variation of the indicated mean effective pressure can be kept below 10%. The indicated mean effective pressure decreased monotonically from 4.79 to 3.62 bar, and the coefficient of variation increased by five times as the engine speed increased as the engine speed increased from 1,000 to 2,500 RPM.