Research On Operating Stability And Control Strategy Of Single-cylinder Hydraulic Free Piston Engine

The hydraulic free piston engine is a compact power machine, it integrates internalcombustion engine (ICE) and hydraulic pump into a whole unit. The ICE piston is directlyconnected to the hydraulic piston and eliminate mechanisms in between, as crankshaft andpiston drive mechanism of the hydraulic pump. The chemical energy of fuels transformsinto hydraulic energy directly with less energy loss and efficiency can be improved.Consequently, in the last three or four decades, with electronic control and hydraulictechnology progress, hydraulic free piston engine gets fast development.According to the piston configuration, hydraulic free piston engine is divided intothree categories: single piston, dual piston and opposed piston. The single-cylinderhydraulic free piston engine (SHFPE) is a basic configuration of the single piston one, andis also major study object for researchers. The main advantages of SHFPE are simplestructure, few parts, small size and light weight, low energy loss due to less friction andheat transfer, variable compression ration allows multi-fuel applications and goodcontrollability. However without restriction of any mechanism, the piston movement iseasily influenced by the force on itself, and the SHFPE operating stability is also influencedby the force. As a consequence, it needs to study on the related characteristics of theSHFPE to explore the influence factors and control method of the operating stability.In this dissertation, the SHFPE operating mode is divided to start condition, normaloperating condition and misfire condition. Control strategies under whole workingconditions are developed. The piston BDC position has an impact on the SHFPEperformance, based on reduced piston rebound distance, a fuel injection control strategy isproposed to improve control accuracy of the BDC position. Work cycle time of the SHFPEwould be affected by shock and vibration due to piston rapid movement. There are theretypes of buffer are designed, and the buffer effect is analyzed. Consider to the practicalapplication in mobile machines, a constant pressure power drive system is developed to eliminate interrelationship between load and SHFPE. The system’s performance issimulated. The main contents of this dissertation are summarized as follows:In chapter1, an overview of the characteristics, construction, history and researchstatus of hydraulic free piston engine is presented. Key technologies of the SHFPE areintroduced. Based on introducing the application of SHFPE, the research significance andbackground are presented, and main contents of this dissertation are summarized.In chapter2, the SHFPE construction and working principle are illustrated, includedcharacteristics of the frequency control and power output. The mathematics simulationmodel of the SHFPE is formulated, and characteristics and efficiency of the SHFPE areanalyzed. In addition, the SHFPE performance changed with operating parameters ispresented.In chapter3, the dissertation puts forward the control strategy of the whole operationcondition of the SHFPE, including starting condition, normal operation condition andmisfire condition, and this strategy meets the basic requirements for operating the SHFPEnormally. Moreover, the dissertation calculates the distance between the main port and theBDC, and analyzes the kinetic characteristic of piston affect by different distance and theability requirements of the frequency control valve. Furthermore, the dissertation studiesthe influence rules about the frequency control valve and control pulse width for theoperating characteristic of SHFPE, and confirms the frequency control valve openingcontrol strategy based on the displacement feedback.In chapter4, this dissertation analyzes the influence rules of the SHFPE performanceaffected by the position of the BDC, and determines the control objective of the BDC. Inaddition, the dissertation establishes the non-linear vibration equation of the SHFPE,proposes the closed-loop control strategy of fuel-injection which can make sure theprecision of the BDC. Besides, this chapter studies the influence factor of distance of thepiston rebound distance when it is at the BDC, which could provide the evidence forreducing the rebound of the piston. In this chapter, the dissertation analyzes the cushioningeffect of different method for the shock and vibration which are made by the piston.In chapter5，a constant pressure power drive system is developed, it is composed of SHFPE and variable hydraulic motor. With this system, the SHFPE and load are separated.The oil output pressure from the SHFPE pump chamber can be constant under varyingloads, so SHFPE can work steadily. The motor’s speed response characteristics is simulatedand analyzed.In chapter6, the research contents and conclusions are summarized, and the furtherresearch work is proposed.