Single Piston Hydraulic Free-piston Engine Research Articles

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.

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.

Asymmetric vibration characteristics of two-cylinder four-stroke single-piston hydraulic free piston engine

The structure and working principle of a two-cylinder four-stroke single-piston hydraulic free piston engine (HFPE) were introduced. The basic vibration equation of free piston assembly (FPA) was established based upon the energy conversion between the injected fuel and the friction together with the load. Both the theoretical and numerical results show that the vibration system of FPA is a nonlinear conservative autonomous system in one cycle. The FPA vibration is symmetric with constant amplitude when FPA is only driven by the compression pressure in the compression accumulator and that in the combustion chamber. When considering the friction and load, FPA could still achieve a stable vibration after a few cycles’ adjustment whether the input energy is equal to the consumed energy or not. The vibration characteristics are different when FPA vibrates in the compression stroke and the expansion stroke, which is the unique feature of the single-piston HFPE.

Simulation Research on the Basic Performance of a Single Piston Hydraulic Free-Piston Engine

Hydraulic free-piston engine is a compact machine which is composed of a conventional engine and a hydraulic pump. Compared to the conventional technology, the free piston engine has many potential advantages. In this paper, a simulation model for a single piston hydraulic free-piston engine (SPHFPE) is described, and the differences in motion characteristics between the free-piston engine and the conventional engine are presented. And varying piston mass and starting pressures have an impact on the motion characteristics of the free-piston engine.

Fuel combustion under asymmetric piston motion: Tested results

This paper presents the measured characteristics of a single piston hydraulic free-piston engine prototype. The detailed results about the piston motion and the fuel combustion are presented. The results show that there is a strong interaction between the piston motion and the fuel combustion in the free-piston engine. The gas force plays a leading role in the innate asymmetric piston motion around top dead centre. The small moving piston inertia makes the asymmetric characteristics more visible. The combustion energy conversion of the single piston hydraulic free-piston engine is fast. Most of the fuel injected burns in 1.0 ms in the pre-mixed combustion phase. Because of the asymmetric characteristics, the mixing-controlled combustion phase in the single piston hydraulic free-piston engine is much shorter than the one in the conventional diesel engine. The short combustion duration decreases the rated power of the free-piston engine. The asymmetric piston motion is unfavourable for the rated power increase of the free-piston engine. The smaller rated power has a negative effect on the free-piston engine application development.

Influence of Pressure Build-Up Time of Compression Chamber on Improving the Operation Frequency of a Single-Piston Hydraulic Free-Piston Engine

A single-piston hydraulic free-piston engine with a two-cylinder four-stroke diesel engine as its driver is introduced. It takes the free-piston assembly a certain time to move after the pressure in the compression chamber starts to increase. The time difference between the pressure increasing and the piston starting to move is defined as the pressure build-up time. The characteristics of the pressure build-up time and its influence on the performance of the free-piston engine are introduced and analyzed. Based on the basic law of dynamics of the free-piston assembly, the parameters which influence the pressure build-up time are analyzed. And then improvement and optimization are proposed to shorten the pressure build-up time.

On–off motion of a hydraulic free-piston engine

The current paper presents the on–off characteristics of a compression ignition single-piston hydraulic free-piston engine. A pilot control circuit and a control method for on–off control of the engine are introduced. Detailed piston motion under on–off control of the engine is investigated. The results indicate that the piston on–off motion of 1 cycle can be divided into a pilot phase and a wave phase. The piston motion in the pilot phase is determined by the flow capacity of the frequency control valve. The piston acceleration during the accelerated compression process peaks in the pilot phase. The wave phase consists of a rebound stage, a decaying stage and a creep stage. The rebound stage determines the initial conditions of the other two stages and affects the piston frequency control. The control method proposed is immune from the bottom-dead-centre cycle variation. A high piston frequency results in a better engine fuel economy.

Opening and closing of a novel high-speed switching valve

The current paper presents a high-speed switching valve for the piston motion control of a single-piston hydraulic free-piston engine. A performance prediction model is developed and a test method for the valve spool movement without using displacement sensors is provided. The agreement between experiments and simulations indicates that the model can predict the valve performance. The valve response is rapid enough for the piston motion control of the engine. The flow capacity of the valve is not reduced by the fast response requirement. The time between the step of the control signal and the response of the pilot valve is important for reducing the valve response lag. A smaller gap is helpful in improving the valve response.

Compression Stroke Characteristics of Single Piston Hydraulic Free-piston Engine

To achieve a high efficiency and reliable compression process and satisfy the requirement for compression ignition of a single piston hydraulic free-piston engine,characteristics of the compression stroke are analyzed.The compression process is divided into different categories based on the system configuration.The main characteristics of the categories are presented.The prime control parameters in the compression process realized by hydraulic power are confirmed.A compression model describing the gas cylinder and hydraulic chamber states under different operation conditions is given.The characteristics of the compression stroke under different operation conditions are investigated by combining with simulation and test results.The results indicate that the requirement for compression ignition can be satisfied easily in the low pressure compression process.The pressure stabilizing effect of the compression accumulator should be eliminated and the effect of throttling action of the frequency control valve should be limited in the high pressure compression process in non-normal working state.