Hydraulic Free Piston Engine Research Articles

Simulation on stirling reverse osmosis desalination power system

In this article, a new power system based on the Stirling engine and hydraulic free piston engine is proposed to provide power for reverse osmosis (RO) desalination. First, the design of the Stirling free piston RO desalination power system was carried out, and the size parameters of the main structures such as the regenerator, heater, cooler, power piston, and valve distribution piston were obtained. A physical model was established to simulate the process of the Stirling engine driving the RO desalination power system based on finite element CFD. The flow field distribution cloud map and output power of the Stirling engine were analyzed. The results indicate that the gas motion of the Stirling engine is stable, and the entire power system can operate normally.

Optimal power distribution control in modular power architecture using hydraulic free piston engines

Vehicle modularization has become an emerging trend in the automotive industry, leading to research on modular configuration, composition, and related control strategies. In this paper, we propose a modular power system with a hydraulic free piston engine (HFPE) as the power unit and develop a power distribution control strategy to enhance the overall efficiency of the system. Firstly, we determine the configuration scheme of the modular power system and establish a simulation model of the HFPE using MATLAB/Simulink. We conduct principle verification of the simulation model. Secondly, based on the simulation model of HFPE, we research the power unit control strategy using the machine learning regression prediction algorithm, enabling dynamic working condition switching of the power unit. Next, we propose a power distribution optimization algorithm which is named as the Rule Based Double Iterative Optimization Algorithm (RBDI) and compare it with several mature optimization algorithms under the framework of model predictive control, considering related constraints. Finally, we validate the performance of the proposed power distribution control strategy using a hardware-in-loop system. The results demonstrate that the output power of the modular power system can be effectively ensured. Compared with the average distribution algorithm (AVE), the genetic algorithm (GA), and the ameliorated particle swarm optimization algorithm (APSO), the overall working efficiency of the modular power system using the proposed control strategy is increased by 6.57%, 6.13%, and 5.59%, respectively, under the three test driving cycles.

A novel hydraulic load-sensing control solution to working functions of mobile machine

The hydraulic load-sensing system is a more efficient system solution than constant pressure system in mobile machine. However, the overall system efficiency for multiple actuators is still low since this centralized hydraulic control system only adapts to the highest load pressure. Therefore, in this article, a distributed hydraulic control system where each function is powered by a hydraulic free piston engine is proposed. Compared with centralized solution, it is more efficient since the power of hydraulic free piston engine is tailored to each function. By integrating small inertia linear engine and pump piston in one unit, the hydraulic free piston engine is an efficient and fast response hydraulic power source. Since the hydraulic free piston engine is essentially a digital hydraulic power source, a digital load-sensing control is proposed in this study. The system architecture, modeling, controller design and performance of the digital hydraulic load-sensing control system are presented. A test bench is developed to verify the system feasibility and performance.

Research on Working Process and Stability of Hydraulic Free Piston Engine

This paper studies the working principle of the hydraulic free piston engine, and details the related contents of the working process of the hydraulic free piston engine. The related contents include the analysis of working state, the analysis of fuel injection characteristics, the analysis of the air exchange system, the analysis of the operating parameters, the working process in the cylinder, etc. The stability of the hydraulic free piston engine is studied by establishing a nonlinear model. Based on this, the stability control strategy is developed, which aims to accumulate reliable experimental data and lay a good application foundation for developing the dynamic control of hydraulic free piston engine in working process.

Research on the Operating Characteristics of Hydraulic Free-Piston Engines: A Systematic Review and Meta-Analysis

The hydraulic free-piston engine (HFPE) is a kind of hybrid-powered machine which combines the reciprocating piston-type internal combustion engine and the plunger pump as a whole. In recent years, the HFPE has been investigated by a number of research groups worldwide due to its potential advantages of high efficiency, energy savings, reduced emissions and multi-fuel operation. Therefore, our study aimed to assess the operating characteristics, core questions and research progress of HFPEs via a systematic review and meta-analysis. We included operational control, starting characteristics, misfire characteristics, in-cylinder working processes and operating stability. We conducted the literature search using electronic databases. The research on HFPEs has mainly concentrated on four kinds of free-piston engine, according to piston arrangement form: single piston, dual pistons, opposed pistons and four-cylinder complex configuration. HFPE research in China is mainly conducted in Zhejiang University, Tianjin University, Jilin University and the Beijing Institute of Technology. In addition, in China, research has mainly focused on the in-cylinder combustion process while a piston is free by considering in-cylinder combustion machinery and piston dynamics. Regarding future research, it is very important that we solve the instabilities brought about by chance fluctuations in the combustion process, which will involve the hydraulic system’s efficiency, the cyclical variation, the method of predicting instability and the recovery after instability.

Evaluation of Trajectory Based Combustion Control for Electrical Free Piston Engine

<div class="section abstract"><div class="htmlview paragraph">Previously, the authors have proposed a novel strategy called trajectory based combustion control for the free piston engine (FPE) where the shape of the piston trajectory between top and bottom dead centers is used as a control input to modulate the chemical kinetics of the fuel-air mixture inside the combustion chamber. It has been shown that in case of a hydraulic free piston engine (HFPE), using active motion control, the piston inside the combustion chamber can be forced to track any desired trajectory, despite the absence of a crankshaft, providing reliable starting and stable operation. This allows the use of optimized piston trajectory for every operating point which minimizes fuel consumption and emissions. In this work, this concept is extended to an electrical free piston engine (EFPE) as a modular power source. A dynamic model of a linear electrical free piston engine unit has been developed which consists of a single phase linear generator driven by a single cylinder engine. The linear generator unit not only provides the required electromagnetic force to ensure precise trajectory tracking for the piston in the combustion chamber, but also efficiently extracts the combustion energy to charge the battery. The concept has been experimentally validated in a hardware-in-loop setup. The combustion data corresponding to a predetermined piston trajectory is obtained from a controlled trajectory rapid compression and expansion machine (CT-RCEM) and the dynamic model is used to evaluate the electrical output corresponding to the combustion data.</div></div>

Study on HCCI combustion improvement by using dual assisted compression ignition (DACI) on a hydraulic free piston engine fueled with methanol fuel

A dual assisted compression ignition combustion mode is researched on a hydraulic free piston engine. The dual assisted compression ignition combustion mode is realized by combining a glow plug with a spark plug in the ignition system to assist the homogeneous charge compression ignition combustion. The effects of control parameters and operating parameters on dual assisted compression ignition combustion mode are revealed by experiments. It is found that as the compression ratio increases, the combustion characteristics of dual assisted compression ignition change from spark ignition to two-stage heat release. The dual assisted compression ignition combustion has more controllability on the combustion phase and the heat release rate than glow plug assisted compression ignition due to the direct and stable spark ignition. When the temperature of the glow plug is lower, the heat release phase is dominated by the spark plug. With the temperature further increases, the heat release process is mainly controlled by the glow plug. The dual assisted compression ignition combustion has a wider compression ratio range than spark assisted compression ignition to guarantee the high indicated thermal efficiency. The suitable methanol/air equivalence ratio range for dual assisted compression ignition combustion mode is between 0.6 and 0.8.

Independent Pressure and Flow Rate Control Enabled by Hydraulic Free Piston Engine

Fluid power consumes a significant portion of energy annually with relatively low efficiency, especially for mobile applications. This is mainly due to the current architecture where an internal combustion engine (ICE) driven hydraulic pump is used to supply fluid to various driving and working circuits. This architecture as well as the response time limitation of the ICE and the hydraulic pump cause significant fluid throttling loss and low-efficiency engine operation at part load conditions. As a novel alternative, the hydraulic free piston engine (HFPE) eliminates the mechanical crankshaft and integrates the ICE and the hydraulic pump into one unit. Due to its ultimate freedom on the piston motion and reduced inertia, the HFPE has the potential to produce variable fluid power in an efficient manner with fast response time. In this paper, such potential is investigated. The working principle of a prototype HFPE is described and a comprehensive HFPE model is developed. A novel control method, namely the independent pressure and flow rate control (IPFC), is proposed to regulate the HFPE's output flow rate at any given load pressure and realize throttleless fluid control. The effectiveness of the method is demonstrated through simulations, which show the capability of the control method in providing different output flow rates at variable load pressures within tens of milliseconds. Additionally, a detailed analysis of the HFPE efficiency, in terms of thermal efficiency of the engine and mechanical efficiency of the pump, has been conducted under different working conditions. The results show that with the IPFC, the HFPE is an efficient and flexible mobile fluid power source.

Energy and exergy analysis of hydraulic free-piston engines

A hydraulic free-piston engine is an unconventional reciprocating piston internal combustion engine in which the piston assembly motion is determined by in-cylinder gas pressure and load force. Fuel combustion energy is directly converted into hydraulic energy. These affect the work process of cylinder and efficiency of energy conversion. In order to study the energy utilization efficiency and to explore the recovery potential of waste heat energy of hydraulic free-piston engine, in this paper, the energy distribution and waste heat energy characteristics of hydraulic free-piston engine have been studied by combining energy and exergy analysis. The thermal efficiency was analyzed by the first law of thermodynamics, and exergy balance was analyzed by the second law. The effect of the characteristic parameters on the thermal and exergy efficiency was studied through the simulation analysis comparing the energy utilization of hydraulic free-piston engine and conventional engines. The results show that control of the injection timing parameter is effective for optimizing efficiency because the cycle characteristic parameters can be controlled by changing the injection timing. The experimental results show that the thermal efficiency is 40.8% and the exergy efficiency is 46.3%. The simulation result show that the thermal efficiency of hydraulic free-piston engine is 38.0% and the conventional diesel engine is 33.0%.

Design and optimisation of cushioning structure of hydraulic free‐piston engine

The velocity of the free-piston assembly (FPA) of the hydraulic free-piston engine (HFPE) is very high, which is up to 10 m/s. In order to avoid the collision of the FPA to the cylinder head when the FPA moves towards the dead centres, the cushioning device must be involved. A cushioning structure which combines the sidestep-shape cushion and flute-shape cushion is proposed according to the working principle of the HFPE. The mathematical model of the cushioning is set up, and key parameters are obtained. AMESim is employed to analyse the cushioning process and the influence of the key parameters on the cushioning process. The optimised parameters are obtained according to the simulation. The test rig is established to verify the effectiveness of the cushioning structure according to the optimised parameters. The experiment results show that the proposed cushioning structure is reliable. The maximum pressure during the cushioning process is 19 MPa, and cushioning time is 12 ms. The cushioning structure has little influence on the starting process of HFPE.

Study on adaptive behavior and mechanism of compression ratio (or piston motion profile) for combustion parameters in hydraulic free piston engine

The Hydraulic Free Piston Engine (HFPE) piston motion profile is controlled by the composite force of the in-cylinder gas and hydraulic fluid. Based on the analysis of HFPE piston motion by dynamic calculations, it is found that the piston motion profile and compression ratio (CR) are self-adaptive for the variation of combustion parameters. As the combustion timing is advanced, the piston commutation time is advanced, and the CR is decreased. Further, the existence of this self-adaptive features are demonstrated by a prototype test of HFPE, and it also proves that HFPE has self-adaptive features for the variation of mixture concentration. The self-adaptive features can make HFPE effectively avoid the phenomenon of knocking or post-combustion, make it stable of the maximum in-cylinder gas pressure and maximum value of heat release rate, reduce the loss of indicated work, and keep the output power relatively stable.

Stability, Control, and Constraint Enforcement of Piston Motion in a Hydraulic Free-Piston Engine

A free-piston engine (FPE) requires stabilizing feedback control to manage piston motion and ensure safe operation. A high-fidelity model, which simulates the FPE behavior by combining dynamic, thermodynamic, and hydraulic submodels, is exploited as an engine surrogate for control development. A control-oriented model implicitly describes the FPE piston turnaround position using energy balance principles for an idealized Otto cycle. Local eigenvalues of the control-oriented model suggest that the FPE system is open-loop unstable. The control system relies on dynamic inversion and state feedback to stabilize the plant and track a target piston turnaround position. A robust reference governor is introduced to manage load changes, such that the piston position does not violate constraints, even in the presence of parameter uncertainty. In high-fidelity simulations, the reference governor constrains piston turnaround position to ±0.5 mm of the target during a load change.

Tribological characteristics of piston rings in a single-piston hydraulic free-piston engine

PurposeA free-piston engine (FPE) is an unconventional engine that abandons the crank system. This paper aims to focus on a numerical simulation for the lubricating characteristics of piston rings in a single-piston hydraulic free-piston engine (HFPE).Design/methodology/approachA time-based numerical simulation program was built using Matlab to define the piston motion of the new engine. And a lubrication mode of piston rings was built which is based on the gas flow equation, hydrodynamic lubrication equation and the asperity contact equation. The piston motion and the lubrication model are coupled, and then the finite difference method is used to obtain the piston rings lubrication performances of the FPE. Meanwhile, the lubrication characteristics of the new engine were compared with those of a corresponding conventional crankshaft-driven engine.FindingsThe study results indicate that compared with the traditional engine, the expansion stroke of the HFPE is longer, and the compression stroke is shorter. Lubrication oil film of the new engine is thicker than the traditional engine during the initial stage of compression stroke and the final stage of the power stroke. The average friction force and power of the hydraulic free piston engine are slightly lower than those of the traditional engine, but the peak friction power of the FPE is significantly greater than that of the traditional engine. With an increase in load, the friction loss power and friction loss efficiency decrease, and with a decrease in equivalence ratio, the friction power loss reduces, but the friction loss efficiency decreases first and then increases.Research limitations/implicationsIn this paper, only qualitative analysis was performed on the tribological difference between conventional crankshaft engine and HFPE, instead of a quantitative one.Practical implicationsThis paper contributes to the tribological design method of HFPE.Social implicationsNo social implications are available now, as the HFPE is under the development phase. However, the authors are positive that their work will be commercialized in the near future.Originality/valueThe main originality of the paper can be introduced as follows: the lubrication and friction characteristics of the new engine (HFPE) were investigated and revealed, which have not been studied before; the effect of the HFPE’s special piston motion on the tribological characteristics was considered in the lubrication simulation. The results show that compared with the traditional crankshaft engine, the new engine shows a different lubrication performance because of its free piston motion.

Cold starting characteristics analysis of hydraulic free piston engine

The cold start characteristic of hydraulic free piston diesel engine may affect its stable operation. Therefore the specific cold start characteristics, such as BDC or TDC positions, pressure in-cylinder, heat release rate, should be investigated in detail. These parameters fluctuate in some regularity in the cod start process. With the development of the free piston engine prototype and the establishment of test bench, the results are obtained. For the dynamic results, the fluctuation range of TDC and BDC positions is 8 mm and decreases with time. The thermodynamic results show that the combustion process is not stable and the pressure in-cylinder fluctuates largely in the cold start process. In addition, the combustion is rapid and knock happens inevitably. In order to investigate the reasons, a CFD model is established for temperature analysis in-cylinder and heat transfer conditions. It is found that higher start wall temperature will lead to more uniform temperature distribution. The delay period may decreases and heat release will move forward. This reason is analyzed by thermodynamic derivation based on the first law of thermodynamics. Finally, the improvement suggestions of cold start strategy are proposed.

Experimental study of hydraulic electronic unit injector in a hydraulic free piston engine

The fuel injection system in two stroke engine is very important, therefore the hydraulic electronic unit injector system is developed and the injection characteristics of hydraulic electronic unit injector are investigated. Firstly the HFPE and the hydraulic electronic unit injector working principle are analyzed, and then PID control strategy is built by engine demand. In order to validate the feasibility of hydraulic electronic unit injector, the prototype test bench is established. The specific measurement principle is presented. Further the injection characteristics, such as the effect of injection pressure on injection delay and the effect of engine frequency on injection delay, are analyzed. In order to optimize the engine stability performance, the BDC control based on fuel injection control is investigated. The load control based on fuel injection is also discussed and the BDC feedforward control with the load variation is investigated. Experiment results of stead engine operation shows that the hydraulic electronic unit injector system based on PID control can be satisfied with the engine operation demand. In addition, cycle fuel injection quantity is tested online and off-line engine operation. It is obvious that the fuel injection quantity is affected by the hydraulic pressure. The fuel injection quantity variation can be improved with decreasing the fluctuation of drive pressure or adopting more suitable oil common rail instead of connected with exhaust valve hydraulic drive oil-way. The energy flow in HFPE is analyzed and the hydraulic output energy can be optimized by selecting suitable hydraulic valves parameters. The fuel injection quantity should be designed by the energy balance and the stable operation requirement in spite of the higher thermal efficiency.

Thermodynamic and energy saving benefits of hydraulic free-piston engines

The hydraulic free-piston engine integrates the internal combustion engine with a hydraulic pump. The piston of an HFPE is not connected to the crankshaft and the piston movement is determined by the forces that act upon it. These features optimize combustion and make higher power density and efficiency increase. In this paper, a detailed thermodynamic and energy saving analysis is performed to demonstrate the fundamental efficiency advantage of an HFPE. The thermodynamic results show that the combustion process can be optimized to an ideal engine cycle. The experimental results show that the HFPE combustion process is a nearly constant-volume process; the efficiency is approximately 50%; the piston displacement and velocity curves for a cycle are the same at any frequency, even at a 1.25 Hz. The maximum velocities are of the same value at high or low frequencies. Similarly, pump output flow is not influenced by frequency. The independent cyclic characteristics of HFPE determine that it should work in higher frequencies when the vehicle runs in Japanese 10–15 road conditions. It indicates that a higher working frequency will lead to the starting frequency of HFPE, and a lower frequency will decrease the pressurized pressure of the hydraulic accumulator.

Stability research of the compression process of two-cylinder four-stroke single-piston hydraulic free piston engine

An invariable compression ratio in certain conditions must be achieved to ensure a steady and efficient performance of the single-piston hydraulic free piston engine. The compression ratio is determined by the characteristics of the compression stroke. The mathematical models of the key components during the compression process are established. The kinematic characteristics of the free piston assembly are analyzed under two conditions. One condition is that free piston assembly is driven by the compression accumulator only and the other one is that free piston assembly is driven by the compression accumulator and the pump station. Pressures in compression accumulator and compression chamber are analyzed and compared in both conditions. According to the experimental results, pressures in the compression chamber and compression accumulator are not constant in both conditions during the compression process. The compression stroke and compression time vary with the changing of the pressure in the compression accumulator, which adds the complexity and changeability to the compression ratio control. An improved configuration is put forward to solve the pressure variation. The results show that the improved configuration can make the pressure invariable at the inlet of the compression chamber and the compression process keeps almost the same regardless of the pressure changing in the compression accumulator. With this new structure, there is about 2.5% energy loss, which is acceptable considering the stable compression stroke.

Research on Hydraulic Free Piston Engine with High Scavenging Efficiency

The poor scavenging process of the hydraulic free piston engine which uses two-stroke engine as its driver was presented. A two-cylinder, four-stroke diesel engine was proposed to drive the single-piston hydraulic free piston engine to improve the scavenging process. The intake and release valves mechanism and fuel injection system were redesigned to adapt the performance of the single-piston hydraulic free piston engine. Feasibility and reliability of this new structure are verified through simulation.

Stability analysis of hydraulic free piston engine

Based on the piston force analysis of the hydraulic free piston diesel engine, a nonlinear vibration model is established. Then the solution of the nonlinear vibration model is given by the generalized harmonic KBM method. The reasons that affect the constant amplitude of the nonlinear vibration model are analyzed according to the expression of the constant amplitude. The constant amplitude is limited by many factors and the system will be unstable if the correlated variables are out of range. The influential factors, such as the fuel injection position and quality, are investigated. In addition, the fluctuation of pump chamber caused by the variation of response characteristics of check value also influences the constant amplitude. All these reasons may lead to the unstability of the oscillatory system. Because the combustion condition is influenced by the air intake and fuel injection, the selection of unstable region is decided by the scavenging and combustion. Therefore the scavenging influenced by the port opening is researched and the results are validated by experiment. In order to get the optimal design of HFPE, the stability range of the piston constant amplitude and judging in equation are proposed, and the parameters in the stable region are obtained. All these efforts establish the theoretical foundation for control strategy of stability improvement.

Study on Energy Balance of Three-Cylinder HCPE

The energy balance of Hydraulic Confined Piston Engine (HCPE) is the key to understanding its working mechanism, improving its efficiency. The working principle of HCPE was presented, and the energy balance for the rated condition and application characteristics for the full conditions were also studied. Taking the low thermal value of fuel into account, the percentage of energy loss related to combustion is 54.99%, the percentage related to friction is 5.59%, the hydraulic loss is 1.92% and the other losses are 1.10%; the efficient work is 36.40%. The efficient work and the loss related to combustion are similar to those of a hydraulic free piston engine and medial speed diesel engine.