Ideal thermodynamic cycle analysis of free piston engine based on expansion ratio

One of the major alternatives of the Otto engine has been examined to determine its potential for increased thermal efficiency in the spark ignited combustion engine is to lengthen the expansion process relative to the compression process. In order to compare the thermal efficiency of the free piston engine and the Otto engine, the ideal thermodynamic cycle models of the four-stroke free piston engine developed based on the supercharged theory of Atkinson cycle and the conventional four-stroke engine based on Otto cycle in the paper. For the ideal thermodynamic cycle, the compression and power processes are adiabatic and reversible, and the heat transfer may be ignored. Sensitivity analysis of the cycle efficiency of a free piston engine versus the Otto cycle engine was also performed. The results show that, other things being held constant, the efficiency can be improved by at least 10% under an appropriately high expansion ratio.