•Friction mechanisms were reviewed and compared between a FPE and CSE.•Sub-models were developed to describe friction force from friction mechanisms.•FPE doesn’t show advantage on piston ring ...friction force over the CSE.•Total friction loss of the FPE is nearly half of the CSE.
Friction work in free-piston engines is expected to be lower than in crankshaft engines due to the elimination of the crank mechanism. In this paper, friction mechanisms were reviewed and compared between a free-piston and crankshaft engine of similar size. The main friction mechanisms were identified to be the piston assembly including piston rings and piston skirt, valve train system, the crank and bearing system for the CSE, and the linear electric generator for the FPE. The frictional loss of each friction mechanism was estimated and discussed. A Stribeck diagram was used to simulate the piston ring friction during hydrodynamic lubrication, mixed lubrication, and boundary condition. It is found that the FPE doesn’t show advantage on piston ring friction force over the CSE, and the frictional loss from the piston ring is even higher. While the elimination of the crankshaft system reduces the frictional loss of the FPE, and the total friction loss of the FPE is nearly half of the CSE.
•Detailed friction models of each component in FPEG and TCE are presented.•Dynamic and friction characteristics of FPEG and TCE are compared and analyzed.•Effects of lubricating oil viscosity and ...temperature on friction loss are studied.•Total mean friction power in FPEG is obviously lower than that in TCE.
Compared with traditional crankshaft engines (TCEs), free-piston engine generators (FPEGs) present many potential advantages due to the abandonment of crankshaft mechanism. The friction loss in FPEGs is considered to be lower than that in TCE. However, few studies have calculated and analyzed the friction differences between them in any great detail. This paper presents an equivalent design of two-stroke TCE based on an existing FPEG prototype size, and then determines the main friction components of their prototypes. The comprehensive simulation models of FPEG and TCE are established in MATLAB/Simulink. Following this, the friction models and lubrication characteristics of each part in FPEG and TCE are described and compared based on the fundamental lubrication theory. The simulation results show good agreement with the experimental data for both the cold start and combustion power generation. And the overall error between them is less than 5%, indicating that the simulation model is reliable and can predict the prototype performance. Moreover, the effects of lubricating oil grade, temperature and operating frequency on the friction performance of FPEG are also investigated. Results show that the piston assembly friction power is less powerful in FPEG (118.3 W) than that in TCE (142.1 W), while the friction power of piston rings is slightly more powerful in FPEG than TCE. The total mean friction power is significantly lower in FPEG than that in TCE, with the proportion of total mean friction power to the indicated power being 11% in TCE and 6.7% in FPEG. This distribution of friction power indicates that when concerning friction loss, FPEG has an advantage over TCE. The piston ring friction power increases with an increase in lubricating oil temperature and operating frequency, and decreases with an increase in lubricating oil viscosity. The proportion of mean friction power to the indicated power shows a decreasing trend as the viscosity and operating frequency increase, and an increasing trend as the lubricating oil temperature increases.
•The piston trajectory that maximizes the thermodynamic efficiency is found.•The expansion and the discharge processes are shortened.•Thermodynamic losses due to leakage and heat transfer are ...reduced.•The volumetric efficiency is increased with respect to a baseline compressor.•The optimized compressor is less affected by the pressure ratio.
Energetic efficiency is one of the main drivers in the design of hermetic reciprocating compressors. Although many studies have been conducted on this subject, there is still room for further improvement, mainly by mitigating thermodynamic losses. This study applies a simulation model based on an unsteady lumped formulation of the mass and energy conservation equations to predict the piston trajectory that provides the highest thermodynamic efficiency for a reciprocating compressor, disregarding the constraints associated with the driving mechanism. The optimum piston trajectory (OPT) is described by four constant-speed stages of the piston and brings about shorter times for the expansion, suction and discharge processes, but a longer time for the compression process with respect to a baseline crank-rod compressor. The OPT resulted in the improvement of the thermodynamic efficiency from 88.3% to 92.1%, and the volumetric efficiency from 70.9% to 72.0%, mainly by reductions of the heat transfer and leakage losses. Furthermore, the performance of the optimized compressor was evaluated for other conditions of the operating envelope. It was found that the optimized compressor is more efficient than the baseline for all conditions assessed and is less affected by the pressure ratio.
•Mathematical model of an axial piston pump is written in a state-space form.•The Extended Kalman filter is adapted to estimate pressure inside piston chambers.•Pressure estimates are used to ...identify the leaky piston successfully.
Volumetric losses are essential to ensure proper lubrication of moving components in an axial piston pump (APP). However, amplification of these volumetric losses can be observed when one or more pistons of the APP degrade due to friction and contaminated fluid. This amplification of volumetric losses due to a worn piston is often called a piston leak. The latter disturbs the output pressure signal and considerably reduces the efficiency of the pump. It also generates significant vibrations that can lead to the resonance of the pump structure. In this context, it is necessary to implement a diagnosis tool to identify the worn piston among the others. This will allow effective maintenance interventions by changing only the worn piston instead of all pistons.
This paper presents a new approach based on the physical model of the pump to identify the worn piston from the healthy ones. It begins by modelling the dynamic comportment of the pump in a nonlinear state model. Then, the extended Kalman filter (EKF) is adapted to estimate pressure in piston chambers. This estimation gives the possibility to observe the pressure into each piston chamber and then allows the identification of the worn piston. The proposed approach is validated on an APP test rig. The obtained results prove the efficiency of the proposed approach in identifying the worn piston.
•A novel dual piston linear compressor prototype is presented.•The experimental results verify the established numerical model.•Piston operation is a direct control variable, which means the ...volumetric efficiency can be regulated.•The output characteristics, especially linear motor efficiency, depend heavily on the operating frequency and stroke.
A novel dual-piston linear compressor consisting of two piston-cylinder assemblies and a linear electric motor was constructed. To explore the performance of the dual-piston linear compressor, a numerical model that contains a thermodynamic sub-model, dynamic sub-model, three-phase linear motor sub-module, and control system sub-model was built. Several piston trajectories and different operating conditions were assessed to investigate the performance of the dual piston linear compressor. Results from both the simulation and experiments indicate that the system with the triangle curve trajectory exhibits the best performance. The output characteristics (mass flow rate, motor efficiency, mechanical efficiency, and overall efficiency) depend heavily on the operating frequency and the stroke of the piston. For the dual-piston linear compressor system that was designed, the overall efficiency can reach up to 57.59% when the system works at a frequency of 12 Hz and a stroke of 26 mm.
The paper is focused on understanding the flow losses and the resulting flow/pressure dynamics in a piston pump. Initially, equations to evaluate leakages in all piston pump gaps will be presented ...and tested against numerical models, later the equations will be linked to determine the general pressure/flow pump dynamic characteristics. The model will also provide the temporal pressure in each piston/cylinder chamber and the temporal leakage in all pump clearances. A test rig able to measure the dynamic pressure inside a piston chamber was build and employed to evaluate pressure ripple dynamics as a function of turning speed, outlet pressure and swash plate angle. The comparison between experimental and simulated results is very good, giving confidence to the model presented. The advantage of using the analytical approach is that explicit equations allow a more direct understanding of the effect of dimension changes and operating conditions on pump dynamics. Fluid used hydraulic oil ISO 32.
The piston/cylinder pair is the critical lubricating interface of axial piston pumps. It suffers from excessive wear, especially under high output pressure. The performance degradation of the ...piston/cylinder pair is significant to be clarified. In this paper, a wear prediction method of the piston/cylinder pair is established by coupling the load-bearing and lubrication parameters calculation model and the wear calculation model. The models are validated through experiments. The experimental and simulated results show that the wear of two ends of the cylinder bore is severe in the specific ranges of circumferential angle. The time-varying wear process of the piston/cylinder pair can be obtained by using this method; therefore, the maintenance time can be predicted.
•A wear prediction method which can obtain the time-varying wear process of the piston/cylinder pair is established.•The interaction between load-bearing and lubrication parameters and surface wear is taken into account.•The simulation and experimental results show that the wear of two ends of the cylinder bore is relatively serious.•The wear process can be divided into two wear stages according to the wear rate and the wear depth.
•Theoretical groundwork presented in developing UAV hydrogen-fueled power plants.•Lithium ion battery modeling reproduces UAV specified endurance level.•Internal combustion engine demonstrates ...superior performance at a cost of complexity.•Free piston engine simulations highlight direct thermal energy to electricity conversion.•Fuel cells can be readily integrated without convoluted energy conversion system.
The relatively low energy density of lithium ion (Li-ion) batteries that power small Unmanned Aerial Vehicles (UAVs) limits their operating range. The use of hydrogen can provide a significant range improvement given its magnitude increase in mass and volume specific energy over Li-ion batteries. As a result, this effort investigates the potential of hydrogen-fueled power plants for small UAVs. Here, five powertrain options are simulated, namely Li-ion battery (LiNiCoAlO2 and Li-air), internal combustion engine (ICE) with integrated generator, parallel hybrid ICE, free piston engine (FPE) with integrated linear generator, and proton exchange membrane fuel cell (PEMFC). The three major outcomes of this study include: (1) Though the performance characteristics of an ICE are superior to a FPE, the ICE has a relatively high manufacturing cost due to a more complex architecture, whereas a FPE can directly convert thermal energy to electricity; (2) PEMFCs can be readily integrated into an UAV since they provide electric power directly without a convoluted energy conversion system, reducing the overall weight of an UAV; and (3) Theoretically, the use of a Li-air battery pack would result in a longest flight time and simplest configuration; however, the necessary chemistry is still years away from practical implementation.
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•Volumetric efficiency ranged from 88.3% to 93.6% due to large intake port area.•Crank angle corresponding to peak in-cylinder pressure was around 10° CA after TDC.•Combustion ...duration of this engine fuelled with hydrogen was 22–30 °CA.•Low engine speed led to higher NO formation and higher peak in-cylinder temperature.•Power density and NO emissions factor were 69.2 kW·L−1 and 10.60 g·(kW·h)−1.
Popularisations of hybrid vehicles and range extender electric vehicles promote the development of high power density and small scale internal combustion engines. Opposed rotary piston (ORP) engines characterised by compact designs, few moving parts and high power density are an ideal power source for the above mentioned vehicles. Due to the short cyclic period of the ORP engine, hydrogen fuel was applied to decrease the combustion duration. This paper investigated the in-cylinder combustion and emissions characteristics of the hydrogen fuelled ORP engine using 3D numerical simulation method at various engine speeds and full load conditions. In-cylinder pressure evolutions, heat release rates, nitrogen monoxide (NO) formations, and power density were analysed to evaluate the engine performance. The results indicated that volumetric efficiency of this ORP engine was higher than 88.3% for all the given scenarios, being benefited from large area of intake ports. Peak in-cylinder pressure decreased significantly with engine speeds, which was mainly resulted from low fuel mass burn fraction before top dead centre (TDC) for high engine speed conditions. As long as the combustion chambers passed TDC, combustion flame propagated from the bowls into the gaps between end faces of adjacent pistons rapidly. In the exhaust stroke, free discharge process of this ORP engine lasted longer duration than reciprocating engines, which would lead to more energy losses. NO was mainly formed after TDC, with the accumulated NO mass being in the range of 0.07 mg–0.5 mg per cycle per cylinder in the engine speed range of 1000–5000 r/min. Maximum power density and NO emissions factor of this engine fuelled with hydrogen was approximately 69.2 kW·L−1 and 10.60 g·(kW·h)−1, respectively. Indicated thermal efficiency dropped from 36.2% to 26.5% when the engine speed increased from 1000 to 5000 r/min.
Taken the deep-sea gravity-piston corer as the prototype, a model of the corer penetrating was built based on energy conservation and by analysing the coring process. The effects of influencing ...parameters of the deep-sea gravity-piston corer on the penetration process and coring length were analysed. In addition, a comparison between the calculated values and the real examples was performed. The results show that corer weight, sediment properties and penetration velocity affect the coring performance. The penetration depth significantly increases with an increase in corer weight, and the corer weight plays an important role in the process of the corer penetrating. Moreover, the penetration depth decreases with the friction coefficient of increasing sediment, while it gradually increases with an increase in penetration velocity. It is also revealed that the penetration depth decreases approximately linearly as the core barrel diameter. Consequently, these will provide the basic data for the design and optimisation of deep-water gravity-piston corer.
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