Research Of Four-stroke Free Piston Engine Compressed Natural Gas

The Extended-range Electric Vehicles (EREV) and the hybrid electric vehicle (HEV) have become the hot area worthy of research and development. The free piston engine based on the Atkinson cycle is highly suitable for application in EV and HEV owing to several potential advantages for the free piston engine, including the simple construction, multi-fuel operation, high efficiency etc. With the rapid development of automobile industry and the gradual depletion of petroleum resources, natural gas (NG) is considered to be the most available alternative fuel due to its abundant sources and clean combustion. The free piston NG engine can take into both the traditional petroleum energy and the clean energy into account, therefore it is being taken more seriously and great attentions by engine researchers. Thus in order to bring the advantage of the free piston into full play, and improve the dynamic and economic performances, the thesis utilizes one method which combines experimental research and numerical simulation, and the thermodynamic process of the free piston engine is analyzed as well in this paper. Furthermore, the main factors impacting on the free piston engine are discussed. The main work of the paper includes:In order to analyze the merits of the energy conversion of the four-stroke free piston as well as the main influence factors, the ideal thermodynamic cycle models of the four-stroke free piston engine are developed based on the working principle and simplified process of the four-stroke free piston engine. The analysis is thermal efficiency of the free piston engine and the Otto engine is compared according to the first law and the second law of the thermodynamics. The results show that both the first efficiency and exergy efficiency of the four stroke free piston engine are higher than the Otto cycle engine, the expansion stoke of the free piston engine can be lengthened without changing the compression stroke so the efficiency of the free piston engine is improved. There is an optimal expansion ratio for the potential power decreased at the later expansion process. The most optimal expansion ratio and the expansion limit are defined respectively. Howerew the expansion limit is smaller than the optimal expansion ratio. So the maximum thermal efficiency can’t be achieved. And only if in the case of that the temperature and pressure of the compression end can ignite the mixture, the compression outside the cylinder is better than the compression in-cylinder.Based on the prototype platform of gasoline free piston engine (FPE), the natural gas supply system has been successfully designed and the ignition system is also improved. In the paper several experimental studies were carried out and four stroke free piston CNG engine prototype were further developed based on the previous design. The feasibility of FPE with multi-fuels and the reliability of simulation model are verified and the combustion characteristic of gaseous fuel in the four-stroke FPE is also explored by experiments.Based on the dynamics principle of the FPE, the dynamic model of the free piston is built. With the help of the general fluid computing platform, the module of the free piston movement can be solved step-by-step is developed. Moreover the multi-dimensional transient numerical simulation model of the FPE working process is also established. The effects of the piston movement on the cylinder flow field are also explored. Results show the predicted curves of the cylinder pressure in good agreement with the experimental curves under the given movement of the free piston, and furthermore illustrated that it is reasonable and accurate to make such whole process numerical simulation.On the base of multi-dimensional transient numerical simulation of the FPE in-cylinder working process, the effects of the main parameters on the FPE performance are analyzed such as the valve timing, the piston top shape, spark timing, the cylinder independent compressed, compression ratio and expansion ratio etc. Under low operating frequency, the valve timing and valve lift have little effect on the cycle air intake amount, and quantity of the cycle air intake increases with the intake stroke length linearly. After the piston top shape changes from flat shape to convex shape, in the meantime also under the same frequency of the piston, the compression ratio is increased and the exchange efficiency is improved. Free piston is very sensitive to pressure, so the increase of compression ratio, the time advance of ignition timing, or any other factors which can accelerate the combustion reaction all can affect the movement of the piston. The compression ratio can be adjusted by the changing of the electromagnetic, and the knock can be restrained effectively through the variable compression ratio by the numerical simulation method.