Research On The Movement Mechanism And Characteristics Of Four-stroke Single-piston Hydraulic Free Piston Engine

Hydraulic Free Piston Engine (HFPE) is an emerging power source for hydraulic systems, which has a much simpler structure, smaller number of components and larger power to weight ratio compared to the traditional hydraulic pump driven by the crankshaft engine. Due to the linear movement of free piston assembly (FPA), HFPE reduces the FPA wear and the maintenance costs with longer lifetime cycle. Without the restriction of any mechanism, the dead centers of FPA are free. Therefore, HFPE has a variable compression ratio which allows multi-fuel applications. Since hydraulic system has good adaptability to recover energy, it is possible that HFPE could fit into the urban buses and cranes to recover energy, such as the bus frequently starts/stops or the crane lifts/drops loads. HFPE is associated with the combination of the internal combustion engine, hydraulics, sensor, condition monitoring and control technologies. The development of HFPE could bring new technologies and promote the development of the related disciplines.A new structure design of the four-stroke HFPE has been proposed, which enables separate exhaust stroke and intake stroke to work more effectively, hence the fuel usage and particle emission can be reduced. The gas exchange efficiency with the four-stroke structure of HFPE can be improved by15%compared to the two-stroke structure according to simulation study. FPA is the only moving part of the four-stroke HFPE, its nonlinear vibration equation has been established with the force of the hot gas in the combustion chamber, fluid forces both in the pump chambers and compression chamber together with the friction forces. The energy conversion between the energy released from the injected fuel and the hydraulic energy has been analyzed during FPA vibration. The vibration characteristics of FPA are dissimilar between the compression and expansion strokes because the forces acting on FPA are different in the two strokes. As a result, the vibration time of the two strokes are different. The vibration time ratio of the expansion and compression strokes has been adjusted to0.65through regulating the operation parameters. The volume change ratio of the pump chambers in the expansion and compression has been set to0.65to make the average output flow rate equal and to reduce the flow pulsation accordingly. The average flow rate of the expansion stroke are almost equal to that of the compression stroke and the flow pulsation ratio is around1.7calculated in simulation. As a comparison to HFPE without control of the flow pulsation ratio, the ratio was around2.1as shown in the literature. The study in this thesis can be used as a reference for theoretical analysis and design consideration of HFPE.In chapter1, an overview of the features, key technologies and the research state of art on the free piston engine, especially about the single-piston HFPE (SHFPE), are presented. The objectives of thesis study are addressed. The main contents are outlined.In chapter2, the three basic structures of SHFPE are introduced. The structure with two pump chambers is chosen to be the study objective by comparing the compression energy, the pressure pulsation, fuel consumption, engine body vibration and efficiency of SHFPE.In chapter3, the basic structure and operation process of SHFPE are presented. The vibration equation of FPA is established. Vibration characteristics of FPA under different conditions are discussed. Then the influence of the different characteristics of FPA vibration on the performance of SHFPE is presented.In chapter4, the servo motor is used to substitute the crankshaft to drive the valve mechanism and the injection system. And the camshaft is redesigned to suit the operation of SHFPE. The cushion and the compression characteristics of the compression chamber are designed and calculated. The reduction of the flow pulsation and the stability of FPA movement are assured through the overall design of SHFPE.In chapter5, the performance of SHFPE is simulated based on the design parameters and theoretical analysis. The kinematics characteristics, pressure characteristics of hydraulic chambers and combustion chambers are analyzed. The energy distribution within HFPE in one working cycle is discussed. The simulation results can be used as a reference to improve the engine efficiency and to optimize the SHFPE design.In chapter6, the subsystems of the compression, pump and reseting after misfire have been tested and the results have shown that the desired performance of the subsystems can be achieved. The pressure build-up time of the compression chamber has significant influence on the frequency of SHFPE. In addition, the influence factors on the pressure build-up time are analyzed and the optimization methods are proposed.In chapter7, conclusions are summarized and future research outlooks are suggested.