Free-piston Engine Research Articles

Advanced combustion free-piston engines: A comprehensive review

A reciprocating engine without a crank-slider mechanism is called a free-piston engine. If the piston is directly connected to a linear alternator, it is called a free-piston linear alternator. Free-piston engines and free-piston linear alternators have the potential to offer solutions for future hybrid electric vehicles and stationary power generation, by enabling direct conversion of mechanical energy to electricity. They benefit from reduced friction losses compared to conventional engines and can have variable compression ratio, which enables combustion control and optimization. Their widespread application has been limited by the necessity for high-speed control strategies. However, their operating characteristics can provide high efficiency, especially when used with low temperature combustion strategies. Low temperature combustion combines the high thermal efficiency of diesel engines, with the low soot emissions of spark-ignition engines, and low NO x emissions because of low burned gas temperatures. This article provides a comprehensive review of free-piston engine technology, with a focus on advanced combustion processes and their potential for use in future powertrain systems.

Low pressure direct injection strategies effect on a small SI natural gas two-stroke engine’s energy distribution and emissions

This paper examines methods to simultaneously improve efficiency and reduce emissions from a 34 cc air-cooled two-stroke engine configured to operate on natural gas, using tuned intake plus exhaust resonators designed from Helmholtz resonance theory to promote effective scavenging. The engine was developed for novel application in a small, free piston engine for decentralized power generation. Operation occurred at wide-open-throttle at a speed of 5400 RPM for both port injection (PI) and low-pressure direct injection (LPDI). In both cases, the electronic ignition timing was adjusted for maximum brake torque, while the fuel was adjusted such that both rich and lean combustion were examined. The LPDI engine was then operated over a variety of engine speeds. An energy balance was completed for both PI and LPDI operation, which quantified all energy pathways, as engine operating regimes changed. Results showed that exhaust chemical energy (ECE) for LPDI was reduced significantly when compared to PI, mostly due to less fuel slip. The fuel slip rate ranged from 35 to 40% for PI operation while it was in the range of 6–20% for LPDI operation. However, mixture stratification with LPDI operation increased carbon monoxide emissions. The start-of-injection (SOI) was also examined, targeting a SOI to provide the highest trapping efficiency and most stable combustion. For LPDI operation, trapping efficiency, which is function of engine speed, showed significant effects on overall efficiency and fuel slip. Air volumetric efficiency increased due to the absence of gaseous fuel within the intake manifold. It was shown that, relative to PI, the overall engine indicated efficiency increased by 90% to a maximum indicated efficiency of 30% for LPDI operation with a SOI of 180 CAD BTDC.

Wall Heat Transfer Analysis of Internal Combustion Engine with Free Piston Linear Motion

The manuscript should contain an abstract. The abstract should be self-contained and citation-free and should not exceed 200 words. The abstract should state the purpose, approach, results and conclusions of the work. The author should assume that the reader has some knowledge of the subject but has not read the paper. Thus, the abstract should be intelligible and complete in it-self (no numerical references); it should not cite figures, tables, or sections of the paper. The abstract should be written using third person instead of first person. Intensive researches are being carried out on the main power generator for free piston linear generator(FPLG) by both the academic and industrial research group due to its potential as a high fuel efficiency and low emission engine. The linear generator, which is a coil and a translator positioned to move linearly back and forth relative to each other. The study investigates the heat transfer data of internal combustion engine with free piston linear motion profile and compared with the conventional reciprocating engine for one cycle motion only. Engine simulation software GT-Power is employed which utilize the 1-D thermodynamic modeling. All parameters for both free-piston engine are set-up to be the same except for the piston motion profile and the injection timing. Both conventional and free piston engine models are built, simulation settings are set up, and simulations are launched in GT-ISE. Once simulation is done, results are viewed in GT-POST, the data collected was analysed and compared to investigate the dictinct effect of piston motion to heat transfer profile and data. The overall trend shows that free piston engine have a lower heat transfer rate throughout majority of the cycle. This finding agrees that due to less time of piston near top dead centre area, heat transfer losses to the wall per cycle are reduced. The heat transfer profile of the free piston also shown distinct feature compared to conventional with rapid increase and decrease of heat transfer rate, followed by a secondary peak of gradual decline of the profile.

METHODS OF STRUCTURAL AND PARAMETRIC SYNTHESIS OF COMBINED GENERATOR OF RECIPROCATING POWER PLANTS FOR ROBOTIC SYSTEMS ON THE BASIS OF A FREE PISTON ENGINE

Power supply system of advanced robotic systems requires the development of electromechanical energy converters with high energy and minimum weight and overall dimensions. In this regard, a free piston engine with an electric generator is considered as a promising plant.Interest in the study of power plants based on free piston engines is caused by several advantages compared to conventional internal combustion engines with a crank mechanism: relative simplicity of the design, 40 % fewer elements, which reduces the overall capacity, specific gravity and metal content of the free piston engine in 2.5–3 times. In addition, the fuel consumption is 30 % lower. Also an important design advantage of power plants based on free piston engines is a relatively simple modular construction. Reciprocating electric generators with transverse increment of the magnetic flux are the most commonly used ones in currently developed power plants based on free piston engines of foreign countries (USA, Russia, Germany, China, UK, Japan, Sweden, Israel, etc.) as the electrical AC machines. The main disadvantage of this type of generators are the absence of coordination of electrical and mechanical subsystems of the power plant at the extreme points of the operating cycle, which limits the efficiency of the free piston engine and reduces the reliability of the power plant.To solve this problem it is proposed to use Electromechanical energy Converter with transverse and longitudinal increment of the magnetic flux (combo generator). However, currently there is no scientifically valid method of synthesis of this type of generator. To address this problem we have developed the methodology of structural and parametric synthesis of combined generator of the reciprocating type for a plant on the basis of free piston engine, which is based on the use of the specific gravity of the combined generator as the objective function. It allows synthesizing electric machine of the reciprocating type with the specified efficiency and minimum specific mass.

Operation Range of Generation Braking Force to Achieve High Efficiency Considering Combustion in a Free-Piston Engine Linear Generator System

Operation Range of Generation Braking Force to Achieve High Efficiency Considering Combustion in a Free-Piston Engine Linear Generator System

Design of a Linear Synchronous Generator and Examination of the Driving Range for a Free-Piston Engine Linear Generator System

Design of a Linear Synchronous Generator and Examination of the Driving Range for a Free-Piston Engine Linear Generator System

Free-Piston Engine (FPE) Technology with Different Applications

The free-piston engine (FPE) is a linear engine in which the requirement for a crankshaft system is eliminated and the piston assembly has a free and linear motion[1]. First proposed around 1930, FPEs were in use in the period 1930-1960 as air compressors and gas generators and provided some advantages over presenttime conventional combustion engines and gas turbine systems[2]. They are known to have a greater thermal efficiency (40-50%) than an equivalent and more conventional reciprocating engine (30-40%)[3]. A driving force behind the interest in free-piston engine generators is the automotive industry’s increasing interest in hybrid-electric vehicle technology. Much work has been undertaken by number of research groups worldwide, including the authors’ group, to explore the operation characteristics of FPEs[4,5].After initial investigations and development of freepiston related products during the early to mid-20th century, recent advances in control and real time actuation systems have enabled the technology to become a viable alternative to reciprocating technologies, and as such, research is now being carried out by number of groups worldwide [3,6-9]. Modern applications of the FPE concept have been proposed for the generation of electric and hydraulic power, typically in hybrid electric vehicles[10-15]. Known FPE applications include electric generators, hydraulic pumps and air compressors[2], which are summarised in the Table 1 below.

Modeling and experimental investigation of a free-piston Stirling engine-based micro-combined heat and power system

In recent years, combined heat and power (CHP) systems have attracted increasing attention worldwide. Owing to their advantages of high overall thermal efficiency, fuel flexibility, low noise and vibration, and low emissions, Stirling engines, especially dynamic Stirling engines (i.e., free-piston Stirling engines, FPSEs) are promising candidates for micro-CHP systems. In this paper, recent progress in Stirling engine-based micro-CHP systems and FPSE modeling and analysis is first briefly reviewed, and then a hybrid calculation model based on thermoacoustic theory is proposed and developed to simulate the entire micro-CHP system. Finally, the construction and testing of a pilot setup is described in detail. The obtained experimental results clearly validate the numerical model and scheme, with the primary deviation within approximately 10%. CHP performance tests revealed a maximum CHP efficiency of 87.5% and an output electrical power of 2.9 kW, corresponding to a 28% thermal-to-electric efficiency, when the delivering temperature was above 60 °C. Furthermore, acoustic impedance analysis indicated that the CHP efficiency remains high over a large temperature lift, which was also confirmed experimentally.

Constrained control of free piston engine generator based on implicit reference governor

The free piston engine generator (FPEG) is a novel power plant concept for series hybrid electric vehicles (HEV) that requires reliable control to regulate piston motion and guarantee safe operation during load transitions. This paper focuses on the control and constraint enforcement in a FPEG using a reference governor.A discrete, implicit, control oriented model describing the piston motion in a two-stroke two-cylinder FPEG at the turnaround point is derived based on energy balance and a feedback controller is designed to track the desired turnaround position by regulating fuel. An implicit reference governor is developed to guarantee safe piston motion by managing the load transitions. Thereference governor utilizes Newtons method applied to an implicitnonlinear model for response predictionand a bisection search algorithm to enforce the constraints for all the future time instants byadjusting the reference command. Additionally, the error in applying one iteration of Newtons method in predictingthe response of the implicit nonlinear system is estimated and accounted for in constraint tightening to guarantee that constraints are robustly enforced. The simulation results show that the feedback control scheme incorporating the developed implicit reference governor can effectively enforce the prescribed constraints during load transition.

Thermal analysis and experimental verification of a staggered‐teeth transverse‐flux permanent‐magnet linear machine

To overcome the inherent demerit of low power factor existing in transverse-flux permanent-magnet (TFPM) machines, a tubular staggered-teeth TFPM linear machine is presented here. Linear alternator integrated with free-piston Stirling engines could offer a great potential in a wide variety of applications ranging from solar energy generation to space power supply. The thermal behaviour of the machine is studied using a three-terminal lumped-parameter thermal network (LPTN) to solve the problem of temperature overestimation of the traditional LPTN. The determination of thermal resistances and thermal parameters are introduced in detail. The temperatures of various components of the machine under different load conditions are calculated by both the three-terminal LPTN model and the numerical thermal model. Sensitivity analysis is carried out to study the influence of critical thermal parameters on the temperature rise of the machine. On this basis, the effectiveness of the forced air cooling is investigated. A prototype is fabricated and temperature experiment indicates that there is a good agreement between measurement and calculation.

Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine

The free-piston engine (FPE) is a new crankless engine, which operates with variable compression ratio, flexible fuel applicability and low pollution potential. A numerical model which couples with dynamic, combustion and gas exchange was established and verified by experiment to simulate the effects of different hydrogen addition on the combustion and emission of a diesel FPE. Results indicate that a small amount of hydrogen addition has a little effect on the combustion process of the FPE. However, when the ratio of hydrogen addition (RH2) is more than 0.1, the RH2 gives a positive effect on the peak in-cylinder gas pressure, temperature, and nitric oxide emission of the FPE, while soot emission decreases with the increase of hydrogen addition. Moreover, the larger RH2 induces a longer ignition delay, shorter rapid combustion period, weaker post-combustion effect, greater heat release rate, and earlier peak heat release rate for the FPE. Nevertheless, the released heat in rapid combustion period is significantly enhanced by the increase of RH2.

Research on the engine combustion characteristics of a free-piston diesel engine linear generator

The free-piston diesel engine linear generator (FPDLG) shows a great deal of merits due to its unique structure and operation characteristics. For this reason, most researchers mainly investigated on the FPDLG characteristics. However, there has not been any report on the engine operation characteristics. So this paper focuses on the engine operation characteristics of the FPDLG during the generating process. According to the experimental results, it is found that the output power of the designed prototype can reach 5.16 kW with the thermal efficiency of up to 38.5%. Experimental test results are obtained and processed both in time and crank-angle coordinates. Compared to the conventional diesel engines, the ignition delay of the FPDLG is shorter and the premix combustion quality is better. By analyzing the operating cyclical variations of the prototype, the coefficient of variation (COV) of the compression ratio is found to be higher than 27%. While the peak in-cylinder gas pressure and pressure rising ratio cyclical variation can maintain a stable operation (similar to the conventional diesel engines), higher COVs of the compression ratio can always complicates the engine combustion. Therefore proper control strategies are necessary for the FPDLG.

A study and comparison of frictional losses in free-piston engine and crankshaft engines

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.

Effect of fuel injection rate shapes on mixture formation and combustion characteristic in a free-piston diesel engine generator

The free-piston engine generator is a new power machinery, which removes the crank-shaft mechanism. This article aims to study the impact of different injection rate shapes (left triangle, rectangl...

A Control-Oriented Model for Trajectory-Based HCCI Combustion Control

Previously, the authors have proposed the concept of piston trajectory-based homogeneous charge compression ignition (HCCI) combustion control enabled by a free piston engine (FPE) and shown its benefits on both engine thermal efficiency and emissions by implementing various piston trajectories. In order to realize the HCCI trajectory-based combustion control in practical applications, a control-oriented model with sufficient chemical kinetics information has to be developed. In this paper, such a model is proposed and its performance, in terms of computational speed and model fidelity, is compared to three existing models: a simplified model using a one-step global reaction, a reduced-order model using Jones–Lindstedt mechanism, and a complex physics-based model including detailed chemical reaction mechanisms. A unique phase separation method is proposed to significantly reduce the computational time and guarantee the prediction accuracy simultaneously. In addition, the paper also shows that the high fidelity of the proposed model is sustained at multiple working conditions, including different air-fuel ratios (AFR), various compression ratios (CR), and distinct piston motion patterns between the two end positions. Finally, an example is presented showing how the control-oriented model enables real-time optimization of the HCCI combustion phasing by varying the trajectories. The simulation results show that the combustion phasing can be adjusted quickly as desired, which further demonstrates the effectiveness of the piston trajectory-based combustion control.

Impact of Exhaust Gas Recirculation on Performance and Emissions of Free-Piston Electrical Generator Fueled by DME

AbstractThis study presents a novel and interesting investigation regarding the use of exhaust gas recirculation (EGR) in free-piston engines by exploring EGR impact on both engine performance and ...

Higher order modeling of a free-piston Stirling engine: analysis and experiment

This paper focuses on higher order modeling and design of the free-piston Stirling engine (FPSE) based on Ant Colony Optimization (ACO). First, the governing thermodynamics and dynamical equations of the engine have been derived. Then, the design parameters of the engine are selected taking into account the finite heat transfer coefficient (resulting in a fifth-order model) and pressure drop (resulting in a sixth-order model) in the dynamical system and the corresponding differential equations are derived in detail. In the following, the mentioned methods and their performance in modeling the FPSE dynamics are investigated. The simulated results show that the effect of the pressure drop on the places of the closed-loop poles of the system is not significant, while the heat transfer coefficient has a considerable effect on the engine dynamics. Accordingly, a fifth-order model along with ACO algorithm is proposed to justify the FPSE behavior. To validate the presented modeling scheme, the prototype engine SUTECH-SR-1 was experimented. It is found that the values of parameters obtained from the proposed design method are close to those of the experiment. Besides, the presented higher order model predicts the engine behavior with an acceptable accuracy through which the validity of the design technique is affirmed.

Experiment on the ignition performances of a free-piston diesel engine alternator

The free-piston engine alternator (FPEA) is an unconventional engine. The initialization of the new engine differs from the conventional engine due to the feature that it eliminates the crankshaft and flywheel system. This study performed an experiment to analyze the ignition characteristics of a compression ignited diesel FPEA. Meanwhile, the effects of injection position and starting force on the ignition were also investigated. The experimental result shows that the larger starting force does not absolutely bring about higher ignition efficiency for the initialization of FPEA, although it brings in a greater possibility for the engine to gain a higher compression ratio from its resonant reciprocation. The large, but inappropriate starting force may lead to a serious incomplete combustion for the ignition, and the level of combustion completion of ignition is influenced by the injection triggering advance position (ITAP). The experiment suggests that properly enlarging the ITAP can enhance the combustion efficiency of the ignition and is beneficial to enlarge the compression ratio of opposite cylinder, but an excessive ITAP may advance the injection and ignition cycle and further induce a misfire for the FPEA.

Comparative assessment of the effectiveness of a free-piston Stirling engine-based micro-cogeneration unit and a heat pump

In this article, a comparison of the thermodynamic and economic effectiveness of two heating systems dedicated to residential applications is presented: a natural gas-fueled micro-cogeneration unit based on a free-piston Stirling engine that generates additional electric energy, and a heat pump system. The measurements of the heat pump system as well as those of the energy (electricity and heat) demand profiles in the analyzed heating season were conducted in a single-family house. The measurements of the μCHP unit were made using a laboratory stand prepared for simulating a variable heat demand. The efficiency of electric energy generation in the μCHP unit was in the range of 7.2%–12.7%, and the overall efficiency was in the range of 88.6%–92.4%. The economic evaluation of the μCHP unit revealed a slight loss compared to the operation of the heat pump throughout the heating season. Sensitivity analysis of the unit price of natural gas and electric energy was performed, and revealed that a reduction in the gas price or an increase in the purchase price of electricity at 2% would have a beneficial financial effect when operating a μCHP system as compared to a heat pump.

Effect of fuel injection characteristics on the performance of a free-piston diesel engine linear generator: CFD simulation and experimental results

The free-piston diesel engine linear generator (FPDLG), as a novel energy conversion device, is investigated by many researcher groups. The injection characteristics are important for the FPDLG operation performance. Therefore, the effects of fuel injection characteristics on the FPDLG are researched by CDF simulation and experimental results in this paper. According to the results, it is found that the trends of the compression ratio and the peak in-cylinder pressure vs the injection timing are the identical with convex function curves. And the IMEP of the FPDLG also show the same trend. Although the heat-release rises in the rapid combustion period, the IMEP vs the injection timing various law present convex function curve on account of reduced heat-release in the normal combustion period. Four injection rate-profiles, i.e. rectangle, wedge, trapezium and triangle are simulated and compared. It is observed that the combustion process with the trapezium rate-profile curve is smoother. The peak in-cylinder pressure and the thermal efficiency can be kept high value while the center of heat-release curve near the TDC. Therefore the trapezium rate-profile is suggested is appropriate and can keep high efficiency of the FPDLG in order to get high engine efficiency.