Performance Analysis Of A Free Piston Linear Alternator

In the past decades, by realizing the crisis of global warming and shortage of fossil fuels, people have devoted to exploring new energy conversion devices and environmental friendly fuels. The free-piston linear alternator (FPLA) is regarded as one of the promising devices to solve the crisis. It consists of two parts. One is the free piston linear engine, which contains dual opposed pistons, connected by a connecting rod and two relevant cylinders. The other is the linear alternator with a permanent magnet mounted on the connecting rod as the translator. Combustion occurs alternately in each cylinder, forcing the connecting rod to move back and forth. The magnetic flux linked with circular wound coil changes periodically to produce the current.Compared to the conventional rotary engine, the free piston engine has only one moving part and suffers less friction loss due to no piston side loads. This may also reduce the noise and extend the lifetime of the machine. The linear alternator is capable of directly utilizing the linear piston force without additional mechanical components. Therefore, a higher mechanical efficiency may be produced in free piston engine. Furthermore, the piston motion is not constrained so the compression ratio is variable, making the high compression ratio and multi-fuel type achieved easily.This thesis is based on the full design and test process of a free piston engine. A simulation model is developed in Matlab/Simulink, containing the thermodynamic system, the electrical system, and also the dynamic system. Based on the initial simulation results, some key features of this system are obtained. Besides, a preliminary prototype is designed. The whole thesis follows a standard design process that is from fundamental analysis to design and then the final experiment. It makes the first simple idea come true.The main contents of this thesis are as follows:1. The flat-type permanent magnet linear alternator is proposed for the free piston linear alternators instead of a tubular one. Using the finite element method (FEM), these two kinds of linear alternators are compared. The result shows that the flat-type permanent magnet linear alternator has higher efficiency and larger output specific power than the tubular one, so the flat-type permanent magnet linear alternator is more suitable for free piston engine. The thrust force of the linear alternator is also modified through the finite model. Then the accurate alternator force is introduced to the whole simulation model of the free piston linear alternator successfully. It is crucial to the final numerical simulation results.2. The modeling and simulation of a novel development of the free piston linear alternator are presented. It consists of two parts, thermodynamic model and dynamic model, and they are combined and solved by Matlab/Simulink respectively. The effects of various intake pressures, moving masses, ignition positions and resistance loads are also investigated. Its basic features are learned from the simulation. Some of the design specifications are further studied from the model.3. The first generation of free piston linear engine is developed. Its fuel system as well as its intake and exhaust system is designed separately. A unique starting device is also designed and tested on the prototype engine.4.The control system for the free piston engine is developed. It contains the hardware and the software parts. The hall sensor and the linear sensor are used together to judge the top dead center of the piston. The control system deals with the ignition and the injection of the engine. All the timing as well as the fuel quantity can be flexibly changed through software.5.Experiments are carried out to test the designed prototype. The influence of ignition time, moving mass as well as the equivalence ratios, etc. is studied. The real free piston motion characteristic are obtained and compared with the former simulation results. The reasons for misfire in this kind of engine are also studied.6.The free piston engine has no mechanical constrain on the piston, so the compression ratio can be higher. This may be suitable for applying the HCCI combustion. Therefore, this new performance of free piston linear alternator is further studied. The effects of various equivalence ratios are studied. The simulation results show that the FPLA has a buildup nature to change the top dead center according to loads and other parameters. The high compression ratio can be achieved easily by decreasing the equivalence ratio, which is an effective way to increase the indicated thermal efficiency. This provides a direction for the future modification.