Flow characteristic analysis of intake and end of compression for free piston engine

Abstract: Compared with the conventional four-stroke engine, the four-stroke free piston engine has benefits in terms of high efficiency, weight reduction, and variable compression ratio and expansion ratio. At the same time, both the freedom and convenience control of the movement of the four-stroke free piston are improved. Without the restriction of the crankshaft, the movement of the piston that is determined by the interaction of a number of forces acting on the mover, makes it possible to further improve the performance through design optimization and technology innovation. Based on the dynamics principle of the free piston engine, the dynamic model of the free piston is built. The dynamics of the free piston assembly obeys Newton's second law. On the basis of the movement of the free piston, the geometrical model and the simulation model of the compressed natural gas (CNG) free piston engine are established. With the help of the general fluid computing platform, the module of the free piston movement that can be solved step by step is developed. Moreover, the multi-dimensional transient numerical simulation model of the free piston engine's working process is also established. The effects of the piston movement on the cylinder flow field are also explored. The results show that the fast moving of the piston can get high in-cylinder unit mass kinetic energy and turbulent kinetic energy. When the time of the intake stroke is only 45 percent of the sum time of the intake and compress stroke, the maximum of the in-cylinder unit mass kinetic energy and the turbulent kinetic energy can be 208.8 J/kg and 25.11 m2/(s2·kg) respectively. But the fast moving piston in intake stroke has little impact on the in-cylinder unit mass kinetic energy and turbulent kinetic energy for the intake end. When the time of the intake stroke is only 45 percent of the sum time of the intake and compress stroke, the in-cylinder unit mass turbulent kinetic energy is only 3 percent higher than that of the intake stroke whose time is 55 percent of the sum time of the intake and compress stroke. Under the conditions of the same compression time, the mid-range acceleration is superior to the acceleration of late course during the compression stroke. This is due to that this movement of the mid-range acceleration during the compression stroke can also enhance the turbulent kinetic energy of the compression end. So the mid-range acceleration is adopted in the compression stroke. The fast moving piston in compression stroke can increase the turbulent kinetic energy of the compression end. When the compression time is equal to 45% of the sum of the intake and the compression time, the turbulent kinetic energy of the compress end can be 4.883 m2/(s2·kg), and it is 40% higher than that of the compression time equal to 55% of the sum of the intake and the compression time. So the piston movement with slow intake and fast compression is adopted, which can increase the gas movement intensity of the compression end and accelerate the flame propagation.