Abstract: Abstract: 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. The modified Otto engine is called a free piston engine. Free piston engines received more attention among researchers in the recent past as a promising technology for a hybrid electric vehicle, and the potential advantages of the free piston engine are including a variable compression ratio and expansion ratio, possible multi-fuel operation, small friction and so on. 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. The length of the compression stroke and that of the expansion stroke are equal in the Otto cycle engine, while it is a very different situation in the free piston engine. For the piston is being connected by the linear electric generator not the crankshaft, thus the Atkinson cycle can be easily realized by changing the level of current. In other words, the expansion stoke of the free piston engine can be lengthened with the compression stroke unchanged. Furthermore, the 462 engine is taken as the computing object of the Otto cycle and Atkinson cycle, the calculation using gasoline as fuel on the both cycles. Under the condition of a stoichiometric air fuel ratio and the effective compression ratio being kept to a precondition, that is the compression ratio is 7,8,9, the expansion temperature, pressure, and the thermal efficiency are calculated respectively. The simulated curves show that the expansion temperature decreases with increasing expansion ratio while the compression process is unaltered. The expansion temperature of a free piston engine can be decreased 500k more than the Otto cycle engine through high expansion with the same speed and compression ratio, so the automobile gas emission decreases along with the expansion temperature reduction. 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. Compared to a conventional Otto cycle engine, the free piston engine has a greater work output and a higher thermal efficiency than the Otto cycle engine at the same operating condition. In addition, the concept of expansion limit is also given, and the expansion ratio is optimized through numerical simulation. For an ideal thermodynamic cycle, all the processes are considered reversible, and heat losses do not occur, either with an Otto cycle or with an Atkinson cycle. However, in the cycle of a real engine, the situation is different, for the combustion process is not adiabatic, and there are heat losses. However, our present experiment shows that the power generation efficiency of a four-stroke engine can reach more than 30%, which is much higher than the efficiency of a two-stroke free piston engine reported abroad.