Performance Analysis And Optimization Of Internal Combustion-Linear Generator Integrated Power System

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 generator is regarded as one of the promising devices to solve the crisis. It coupled with free-piston engine and linear generator, the chemical energy of the fuel can be converted into electricity power. Compared to the conventional rotary engine, the free-piston linear generator has only one moving part and without additional mechanical components. There are many performance advantages in it, such as compact structure, good fuel adaptability, high conversion efficiency, etc. A novel four-stroke free-piston linear generator was proposed and its performance was also analysed and optimized. The main works and fruits of the thesis are as follows:(1) The structure of a novel internal combustion linear generator integrated power system was described and the four-stroke cycle used by the system was also presented. Interdisciplinary model of the system was created. The dynamic and circuit simulation model were made based on the Matlab/Simulink and SimPowerSystems, and the simulation was done based on the given parameters of a prototype. The dynamic variations of displacement, velocity, incylinder gas pressure, intake and exhaust flow rate, electric power were given out. The influence of parameters on the dynamic characteristics of the system was analyzed. The piston motion characteristic of the system was also revealed. The feasibility and effectiveness of the system was validated and it provided theoretical basis for further performance optimization.(2) Based on the analysis of the thermodynamic cycle, a novel working cycle was proposed to achieve higher power density. This new cycle was characterized by independent compression and controllable intake thermodynamic parameters. The working principle of the new cycle was expounded and some new performance evaluation indexes were also defined. The influence of some parameters such as intake pressure, intake temperature, expansion stroke on the performance of the system was analyzed. The circular energy flowing process was also studied from the point of view of energy conservation. A ideal thermodynamic model was established by finite time thermodynamic method. The internal and external irreversibility feature of the system was investigated through this method. Compression ratio, intake temperature and other factors have a great effect on the thermal efficiency. Compared to traditional Otto cycle, the theoretical analysis shows that the new cycle has great improvement on the thermal efficiency.(3) The piston motion of this system was optimized in the thesis. When the system output work was set as the optimiztion objective and some relevent constraint conditions were also set, a piston motion optimization model was built. Based on the separate phase characteristics of the piston motion, gauss pseudospectral method was used to discrete optimal control problems and sequential quadratic programming algorithm was used to solve nonlinear programming problems. Numerical calculation results show that the piston motion law has great improvement on the system performance by controlling the incylinder thermodynamic processes such as combustion, heat transfer and ignition.(4) The control system of the prototype was built based on DSP and the experimental bench was also built. Before the experimental study, some preparations were made such as connected the inlet pipeline, debugged the ignition circuit, checked the gas tightness. Experimental study was carried out to investigate the performance of this prototype and stable running was realized. By comparing the experimental data of different intake parameters, the results proved that the proposed cycle has a high energy convention efficiency. Otherwise, by comparing the experimental data of different piston motion laws, the results proved that the system performance can be improved through optimizing the piston motion laws.(5) In order to apply the muli-cylinder internal combustion linear generator for extended range electric vehicle, a hybrid energy storage system was proposed. This hybrid energy storage system combined supercapacitors and batteries. Based on the study of the system structure design and parameter matching, the mathematical model was built and the topology circuit of the system was simulated. This system achieved stable working under suitable control strategy and a high energy storage efficiency, which verified the validity of the theoretical analysis.