Research On Linear Generator In Free Piston Internal Combustion Generator System

Problems such as environmental pollution and energy shortage make people realize the importance of energy saving and emission reduction.Free piston internal combustion generator,which is a new type of power device,can be used in hybrid electric vehicles.It can help to relieve the pressure of energy and environment.The free piston internal combustion generator is composed of a free piston engine and a linear generator and can realize the conversion of mechanical energy and electric energy.As an important part of this power device,the performance of the linear generator can affect the stable operation of the whole system and the efficiency of the energy conversion.So it is of great significance to research the linear generator in this device.Firstly,this paper summarizes the factors which should be considered when design the linear generator suitable for free piston internal combustion power generation.Based on the analysis of those factors,this paper decides to choose permanent magnetic axial flux linear generator whose structure is long secondary and short primary.On this basis,this paper analyses the properties of generator under different magnetization methods.The results show that the amplitude of unloaded back EMF of the mixed magnetization is increased by 14.66% than that of the radial magnetization,but this causes that 23.76% more of the permanent magnet material is used.And the detent force amplitude of mixed magnetization is increased by 34.32% than that of the radial magnetization.The amplitude of the unloaded back EMF of the axial magnetization is 75.90% lower than that of the radial magnetization,and its distortion rate of the waveform is as high as 11.01%.The radial magnetization method can balance the amplitude of the no-load back EMF,the waveform distortion,the amplitude of detent force and the amount of permanent magnets.So this paper decides to choose radial magnetization method.In addition,this paper compares the unloaded and loaded performance of the cylindrical and flat type linear generator.The results indicate that the amplitude of unloaded back EMF of the cylindrical linear generator is 21.36% larger than that of the flat type,and the distortion rate of the waveform is 60.96% lower.The maximum output power,the maximum power density and the efficiency under the maximum output power of the cylindrical linear generator is higher than that of the flat type.Besides,the mass of the moving part and total mass of cylindrical linear generator are smaller than that of the flat type.Therefore,the unloaded and loaded performance of the cylindrical linear generator is better.So the cylindrical linear generator is more suitable for the free piston internal combustion generator system.Furthermore,this paper studies the unloaded and loaded characteristics of the generator with surface mounted permanent magnet and buried permanent magnet.The results show that the amplitude of the unloaded back EMF of the buried permanent magnet linear generator is 8.41% lower than that of the surface mounted.The output power and power density of the surface mounted permanent magnet linear generator are larger than the buried one under the same load.The maximum output power of the surface mounted permanent magnet linear generator is 19.05% higher than that of buried one,and the maximum power density is 23.34% higher.Therefore,this paper chooses surface mounted permanent magnet linear generator.At last,the detent force of the generator is studied.The influence of the slot width,the auxiliary slot,the height of edge tooth,the pole arc coefficient,the air gap width and the primary iron core length on the detent force are analyzed.In addition,the pole arc coefficient,the air gap width and the convex primary length are optimized based on the orthogonal experimental method.The result shows that when the polar arc coefficient is 0.85,the width of the air gap is 1mm,and the convex primary length is 6.25 mm,the amplitude of the detent force can be reduced and the amplitude of the EMF can also be taken into account.After optimization,the amplitude of the detent force of the generator is reduced by 76.83%,and the amplitude of the back EMF is increased by 2.72%.