Theoretical And Experimental Studies On A Micro Power Generation System Based On A Three-arm Free-piston Swine Engine

At present,the power supplies for Micro-Electro-Mechanical-Systems(MEMS),such as the micro computers,micro robots and portable detection equipments,are mainly batteries.However,the batteries have disadvantages like low energy density,large mass or volume,short power supply time and containing toxic substances,so the developments of the MEMS are greatly limited.Therefore,it is necessary to develop the power MEMS to replace the batteries.The hydrocarbon fuel has advantages such as high energy density,fast fuel supplement,environment-friendly combustion products(water and carbon dioxide).Therefore,the micro power generation system based on the fuel combustion has become the most promising alternative to batteries.In this paper,a micro heat engine based on n-butane combustion has been developed to replace the batteries.Among various micro heat engine concepts,this paper selected the micro swing engine.The operating frequency of this engine concept is low,so no special requirements are needed for material strength,bearing and lubrication.And the abrasion between the piston and cylinder is small.Besides the swing engine can utilize the cylinder volume more effectively due to its fan-shaped combustion chamber design,thus it has a high power density.In addition,because of its sandwich structure,the swing engine is easy to be fabricated and assembled.Therefore,the swing engine is very promising for micro power generation.In this paper,the three-arm free-piston swing engine concept was proposed,and theoretical and experimental studies of the engine/generator system have been carried out.In chapter 2,based on the angular momentum,mass and energy conservation equations,governing equations of the zero-dimensional model for the three-arm swing engine/generator system were firstly derived.Besides the leakage,heat loss and friction loss models as well as the combustion model were established.Then the calculation process and convergence criteria of the system simulations were given.Operating characteristics and thermodynamic cycle of the swing engine were obtained via numerical simulations.The results show that the swing engine can start by itself without the aid of the starter,and the engine startup is very quick,and there is no "dead point" in the operating process.In addition,after the two-stroke engine reaches steady operations,the thermodynamic cycle can be approximated by the Otto cycle.In chapter 3,the influences of the engine load,intake parameters,combustion characteristics and engine size on the system performance have been studied.For a fixed mass load,there exists an optimal electric load torque coefficient to maximize the thermal efficiency or indicated power.The larger the mass load is,the higher the load capacity is,and the higher peaks the thermal efficiency or indicated power can achieve.For a small inlet port,the thermal efficiency increases with the increase of the inlet pressure.But the efficiency decreases with increasing the inlet pressure for a large inlet port.On the other hand,the indicated power gets larger with the increase of the inlet pressure or inlet port diameter.Regarding to the characteristic parameters of the combustion,there exists an optimal ignition advance coefficient to maximize the thermal efficiency or indicated power,for a given combustion duration time.Both the efficiency and power peaks decrease with increasing the combustion duration time.As to the engine size,it shows that the indicated power is proportional to the square of the engine size.And the power density and operating frequency is inversely proportional to the engine size.Neither the thermal efficiency nor the swing amplitude changes with the engine size.Moreover,the sensitivities of the system performance to parameters were also analyzed.The results show that the sensitivities of the thermal efficiency to various parameters in order of decreasing importance are as follows:the combustion duration time,the inlet port diameter>the inlet pressure>the electric load torque coefficient>the ignition advance coefficient>mass load coefficient.The sensitivities of the indicated power to these parameters in order of decreasing importance are found to be:the inlet pressure>inlet port diameter>>the combustion duration time>the mass load coefficient>the electric load torque coefficient>the ignition advance coefficient.Additionally,in chapter 3,the effects of the leakage,heat loss and friction loss on the system performance have been respectively studied.And the mechanisms of these losses have been revealed,which can help to find the loss control measures.This paper has also compared how great influences of the three losses on the system perforrmance.The results show that the importance of the three losses for the engine in decreasing order is:heat loss>leakage>>friction loss.The friction loss is far less important than the former two.With the reduction of the engine size,the effects of the leakage on the performance grow the fastest,followed by heat loss.Influences of the friction loss increase slowly and are very small.In chapter 4,employing a simplified swing engine model,the fundamental researches have been conducted with experiments.Firstly,by using the driving motor and crank rocker mechanism,experiments of the engine air tightness were carried out.Combined with the numerical simulations,the key parameters affecting leakage were revealed.It is found that the leakage loss can be reduced by either decreasing the gap size or increasing the operating frequency.Then,the compressed air was used to drive the swing engine instead of the fuel combustion,and the operating characteristics and energy conversion have been studied.The operating characteristics of the free-piston swing engine,such as fast engine startup/shutdown and load following,were verified firstly by experiments.And influences of the inlet pressure and resistance torque,charging duration time and charging advance coefficient on the compression ratio,operating frequency,exergy efficiency and indicated power were respectively obtained.The results show that the exergy efficiency of the engine becomes higher with a smaller inlet pressure,a shorter charging time,and a proper charging advance coefficient and resistance torque.In addition,the power generation experiments of the swing engine system have also been carried out.The results show that the peak value of the output voltage at the two ends of the LED lamp board is 5.8V,and the corresponding peak power is 6.3W.Additionally,the platform for the experiments with fuel combustion has been built,including the closed loop ignition control and multi parameter dynamic acquisition subsystem.Using n-butane as the fuel,the single ignition experiment was conducted to verify the engine structure design(i.e.free-piston,opposed cylinder arrangement)and thermodynamic process(Otto cycle).Thus the feasibility of the swing engine was validated.In addition,based on the open-loop control equal interval ignition system,the continuous engine operations were achieved and preliminary researches showed that the ignition delay and the scavenging efficiency are the two main parameters affecting the operating characteristics of the swing engine.Finally,based on the experimental results of the simplified prototype,the three-arm swing engine design was improved and optimized,and the fabrication of the three-arm prototype was completed.