Free-piston Engine Research Articles

Numerical analysis of ammonia HCCI combustion in a free piston engine through trajectory-based combustion control

Ammonia is a promising carbon–neutral fuel, able to power heavy-duty vehicles, non-road machineries and ocean ships with zero CO2 emission. However, practical challenges still exist when applying ammonia in traditional internal combustion engines (ICEs) due to its high ignition energy, slow flame propagation and significant NOx emissions. Free piston engine (FPE) is a flexible alternative to ICE whose piston can move freely without the constraints of crankshaft. Therefore, its piston trajectory can be leveraged as a new control means to facilitate ammonia combustion. In this paper, a numerical analysis investigating ammonia HCCI combustion in FPE was presented. A physical-based model was developed at first, including a unique trajectory synthesizing mechanism and a detailed reaction mechanism of ammonia oxidation. Various asymmetric piston trajectories were then employed into the model and the simulated HCCI combustion were analyzed through six characteristics, namely indicated thermal efficiency, location of peak pressure, exhaust gas temperature, combustion duration, ignition delay, and NOx emissions. This investigation not only determines the best compression ratio (CR = 25) for ammonia ignition, but also shape the optimal piston trajectory (symmetric trajectory with Ω = 0.5) for ammonia HCCI combustion to enhance thermal efficiency and reduce NOx emission simultaneously.

Numerical Investigation on NOx Emission of a Hydrogen-Fuelled Dual-Cylinder Free-Piston Engine

The free-piston engine is a type of none-crank engine that could be operated under variable compression ratio, and this provides it flexible fuel applicability and low engine emission potential. In this work, several 1-D engine models, including conventional gasoline engines, free-piston gasoline engines and free-piston hydrogen engines, have been established. Both engine performance and emission performance under engine speeds between 5–11 Hz and with different equivalent ratios have been simulated and compared. Results indicated that the free-piston engine has remarkable potential for NOx reduction, and the largest reduction is 57.37% at 6 Hz compared with a conventional gasoline engine. However, the figure of NOx from the hydrogen free-piston engine is slightly higher than that of the gasoline free-piston engine, and the difference increases with the increase of engine speed. In addition, several factors and their relationships related to hydrogen combustion in the free-piston engine have been investigated, and results show that the equivalent ratio φ=0.88 is a vital point that affects NOx production, and the ignition advance timing could also affect combustion duration, the highest in-cylinder temperature and NOx production to a large extent.

Stability evaluation of a free-piston Stirling engine using linear dimensionless models and stability curves

A free-piston Stirling engine (FPSE) is a dynamic type of Stirling engine that uses springs to drive the piston and displacer. FPSEs differ from traditional Stirling engines in that the phase, stroke, and operating frequency of the piston and displacer cannot be directly determined by its driving mechanism. Therefore, determining the operation criterion of FPSEs is critical. This study investigated the operation criterion of FPSEs by using a theoretical model. The displacement of the piston and displacer was predicted using equations of motion. The pressure forces exerted on the piston and displacer from the working fluid were established using Schmidt’s assumption. The proposed model was simplified by introducing 13 dimensionless design parameters. The equations of motion were linearized, and their eigenvalues were obtained and used to identify the operation criterion and operating frequency. The stability curve and operating frequency were plotted in the temperature ratio–pressure force ratio domain. The operation region of FPSEs was then determined and evaluated under various amounts of spring stiffness and various damping coefficients. A prototype FPSE was developed and tested in parallel to validate the model. The results revealed a suitable charged pressure region for starting the FPSE. The FPSE could operate under a minimum heating temperature when the charged pressure was at an optimum value. Both the pressure force and spring force exerted on the displacer were in the same order. The operating frequency was close to the natural frequency of the displacer, with a difference of ± 8 %. The experimental results indicate that the proposed FPSE can start at a heating temperature of 125 ℃ with a charged pressure of 3 bar. By using the stability curve, the stability of FPSEs could be analyzed systematically. The proposed method can be used to design other FPSEs.

Effect of Disturbances on the Operation Process of a Methane-Fueled Free-Piston Engine Generator

Effect of Disturbances on the Operation Process of a Methane-Fueled Free-Piston Engine Generator

Development of a coupling model and parametric analysis of an opposed free-piston engine linear generator

The opposed free-piston engine linear generator shows unique dynamic characteristics due to the elimination of the complex crankshaft and connecting rod mechanism. This paper investigated the behaviors of the design parameters and operating conditions on engine performance based on the simulation and experimental results. Considering the unique dynamic characteristics and the coupling of free-piston engine and linear generator, a numerical model was established and validated with prediction errors of 2 %. According to the experimental results, an optimal ignition position of 27 mm was obtained, and the both of indicated power and indicated work achieved their maximum value of 1.85 kW and 87.37 J improved by 17.1 % and 20.9 %, respectively. Further parametric analysis about the design parameters was carried out based on the coupling model. Simulation results showed that the brake thermal efficiency could be improved significantly from 31.95 % to 36.06 % considering the coupling effects of the piston assembly mass and the stroke length. Additionally, higher indicated power could be obtained with a shorter stroke length and lower piston assembly mass.

Combustion stability control of a single-piston free piston engine generator

Free piston engine generator is an effective alternative to conventional range ex-tender, which is able to facilitate the transportation sector decarbonization attributed to its high thermal efficiency and ultimate fuel flexibility. However, due to the lack of crankshaft mechanism, stable and robust operation of free piston engine generator is difficult to achieve since various disturbances may exist during its operation. In this paper, a combustion stability control based on active disturbance rejection control algorithm was proposed for a single piston free piston engine generator. To develop this control, a thermodynamic model of the free piston engine generator system was calibrated and validated through experimental data. Afterward, a 2nd order active disturbance rejection control based speed control was developed by leveraging the developed free piston engine generator model. The proposed active disturbance rejection control was employed into multiple abnormal combustion scenarios through a simulation, including various fuel heat released, ignition positions and burn durations, aimed to verify the control ability of rejecting disturbances and enhancing the robustness of the free piston engine generator operation.

Operation Range of Intermittent Velocity Control for Improving Top-Dead-Center Accuracy in Dual-Sided Free-Piston Engine Linear Generator

Operation Range of Intermittent Velocity Control for Improving Top-Dead-Center Accuracy in Dual-Sided Free-Piston Engine Linear Generator

Design and modeling of a free-piston engine generator

Design and modeling of a free-piston engine generator

Effect of ignition timing on the combustion process of a port injection free piston linear generator: A system level multi-physics coupling method

Free piston linear generators (FPLGs) have the advantages of high power density, high compactness and flexible compression ratio, rendering FPLGs as potential power sources in pure electric range extension, hybrid power and various outdoor power fields. In the present study, the influence of ignition timing on the performance of a dual-piston dual-cylinder free-piston engine generator was experimentally investigated. A FPLG multi-physics coupled simulation-experimental bench was specially developed, and the effect of ignition timing on the engine performance was investigated through the three-dimensional numerical simulation method. The simulation results showed that with the increase of spark advance, both the peak in-cylinder pressure and the heat release rate raised, and the maximum peak pressure could reach over 30 bar under given scenarios. With the increase of ignition timing, the start of combustion was significantly advanced, and the maximum advance in combustion initiation was about 19°ECA over the investigated engine speeds. When the ignition timing ranges were −35°ECA ∼ -20°ECA, the combustion durations were roughly varied between 20°ECA ∼ 30°ECA. At the specified engine speed, when the ignition timing was earlier than − 20°ECA, the indicated power and indicated thermal efficiency were not sensitive to ignition timing. As the engine speed and spark advance increased,the NOx mass fraction in the combustion process were raised. The heat transfer loss decreased with the retarding of the ignition timing, and it increased as the engine speed decreased.

Modeling results of a new high performance free liquid piston engine

Free piston engines, Ericsson hot air engines and liquid piston machines are concepts which, each independently, are considered highly relevant. These technologies have been associated to design a Free Liquid Piston Ericsson Engine (FLPEE). Both ends of a U-tube partly filled with water are equipped with valves. Air is the working fluid. One leg of the U-tube acts to compress the air, while the other arm allows the expansion of the compressed air previously heated by the hot source of the engine. This free liquid piston system is coupled with a micro-expander. A dynamic model of this engine is developed in the Matlab/Simulink computing environment, where simulations are performed in order to obtain suitable operation conditions and predict the performance of this machine. Cycle averaged simulations results provide an understanding of how operational parameters impact system performance. Intracycle simulation results based on a suitable operational data set highlight the instantaneous evolution of pressures, mass flow rates and forces acting on the liquid piston. For the chosen data set, the theoretical net mechanical power produced by the system is about 1.17 kW with a theoretical efficiency of 34% for a maximum pressure of 2.5 bar.

Development and performance measurements of a 2.5 kW-class free-piston stirling converter with detailed design and fabrication processes

This study presents the development and performance measurement of a 2.5 kW class free-piston Stirling engine with detailed design and fabrication processes, considering flexure spring stiffness effects on the engine dynamics and performance. The development of the test engine includes tube type heat exchangers and planar flexure springs. In particular, the flexure springs are designed via finite element analyses and verified through stiffness measurements and fatigue life tests. For the performance measurements, an engine test rig is constructed with an LNG combustion heat supply. The thermal efficiency is evaluated in terms of combustion gas components based on the reaction formula of methane. As a result, the electric output and efficiency of the test engine were measured to be 2.46 kW and 19.46%, respectively. Furthermore, the effects of displacer spring stiffness are experimented with a stack design of flexure springs by varying the number of springs. As a result, increases in displacer stiffness increased engine operating frequency and decreased both the piston amplitudes, while power piston velocity remained relatively constant as well as the engine power output. The experimental results are analyzed through a simple linear dynamic model, which showed reasonable agreements.

Advances on a free-piston Stirling engine-based micro-combined heat and power system

As a promising supplement to traditional central electric generation technologies, a distributed combined heat and power (CHP) system is conducive to renewable energy deployment and mitigation of carbon emissions. Among the candidate technologies, the free-piston Stirling engine (FPSE)-based CHP technology is competitive in micro- or small-scale applications. In this work, the performance of a FPSE-based micro-CHP system is improved through a combination of numerical optimization and experimental validation. First, by using a third-order model and thermoacoustic analysis, the impact of regenerator length, displacer moving mass, and force factor of the alternator on the free-piston Stirling generator (FPSG) inside acoustic field, supplied thermal power and exergy loss is studied numerically. The numerical results indicate a rational increase of the force factor, the regenerator length as well as the displacer moving mass is positive for improving the CHP performance. Based on the numerical optimization, the FPSG prototype is updated accordingly, and adequate measurements of the internal acoustic field of the FPSG have been deployed. Then, a series of experiments are conducted to evaluate the CHP performance of the refurbished FPSG prototype. The experimental results show that the highest CHP efficiency reaches 95.7%, the output electric power is 4.03 kW, and the corresponding thermal-to-electric efficiency is 32.2% when the average temperature of supply and return water is as high as 75 °C. Compared with the work before upgrade, the output electric power and CHP efficiency are improved significantly by 39% and 9.4%, respectively. An insight into the impact of the temperature difference between supply and return water indicates that it has a slight impact on electric power generation, especially under higher charge pressure. In contrast, thermal production exhibits an apparent negative correlation with the temperature difference.

Study on the coupling between engine and alternator in a free-piston Stirling generator

The free-piston Stirling generator (FPSG) is composed of a free-piston Stirling engine (FPSE) and a linear alternator. The coupling relationship between the two parts directly affects the FPSG’s performance. The published literature about simulation models mainly focuses on investigating the FPSE and the alternator separately, which could hardly show the coupling between them. From the perspective of acoustic impedance matching, this paper established a complete simulation model of the FPSG based on SAGE software. The coupling relationship between the engine and the alternator and the influence of some vital parameters on FPSG’s performance were systematically investigated. The results showed that the linear alternator parameters directly influence the acoustic impedance at the surface of the power piston facing the FPSE, thereby affecting the FPSG’s performance. The working pressure affects the linear alternator by changing the working frequency. The acoustic-to-electric efficiency is only determined by the linear alternator parameters, external load and working frequency. To verify the simulation model, a FPSG prototype was established. Its characteristics showed good agreement with the simulation. With the heating temperature of 610 ℃, it could output 1040 W electric power with a thermal-to-electric efficiency of 36.2 %.

Effect of Nonlinear Spring Characteristics on the Efficiency of Free-Piston Engine Generator

Piston motion is an important factor in improving the energy conversion efficiency of free-piston engine generators (FPEGs), and an air cylinder is an important component affecting piston motion. In this study, the effect of the air cylinder specifications on the piston drive frequency and energy conversion efficiency is clarified. By considering that the repulsion force of an air cylinder is nonlinear, the main factors that change the piston drive frequency were investigated by simulation. In addition, a piston drive frequency diagram was drawn based on the top surface area of the air cylinder and the compression ratio to discuss the effect of the air cylinder specifications on the piston drive frequency. The results indicate that the air cylinder specifications affect the piston drive frequency and generation force of a linear machine. Moreover, the structure of the air cylinder and the constraints on the maximum generation force of the linear machine narrow the possible operating range in the piston drive frequency. The air cylinder specifications based on the piston drive frequency diagram improved the FPEG energy conversion efficiency by 0.5%.

Recent progress on performance and control of linear engine generator

Linear engines, or widely called free piston engines, have their advantages compared to conventional engines with crankshafts, for example, lower friction loss, compact structure, fuel adaptability, flexible compression ratio, etc. Among the linear engine conceptual designs and prototypes, Linear Engine Generator (LEG) incorporating a linear engine (internal or external combustion) and a linear alternator provides an alternative and attractive solution for hybrid electrical powertrains in transport applications. Due to the advantages of linear engines, hydrogen, synthetic hydrogen carrier fuels and biofuels are easier to adopt on LEG, making it an appealing technology for decarbonisation. This paper explicitly reviews LEG research and development on state-of-art solutions and recent progress in linear engine improvement, linear alternator design optimisation, and integrated LEG system control. Key performance indicators of the linear engine generator, like power output, thermal and electrical efficiency, etc., are summarised to present a general picture of up-to-date LEG development. Recommendations for further LEG development are provided at the end of the paper regarding the review outcomes.

Sensitivity and effect of key operational parameters on performance of a dual-cylinder free-piston engine generator

Sensitivity and effect of key operational parameters on performance of a dual-cylinder free-piston engine generator

Experimental analysis on the operation process of opposed-piston free piston engine generator

As a novel energy conversion device, opposed-piston free piston engine generator (FPEG) has high energy conversion efficiency, high power density, low heat transfer loss and excellent NVH performance. The opposed-piston FPEG test platform is built, and the continuous and stable operation of the prototype is realized. The influence laws of key parameters such as fuel injection timing, ignition timing, fuel injection quantity, scavenging pressure and rebound cylinder initial pressure on the performance of the system are systematically analyzed, the suitable value for the opposed-piston FPEG is determined, and the stable operation mechanism and synchronization error of the system are analyzed. It is found that there is a matching relationship between injection timing and ignition timing. The injection position should be set at 37 mm-39 mm, the ignition position should be 25–27 mm, and the injection and ignition interval should be controlled at about 12 mm. The peak in-cylinder pressure increases with the increase of fuel injection quantity, and the fuller the P-V diagram is. When the scavenging pressure is 1.5 bar and the ignition position is 25 mm-27 mm, the basic indexes have extreme points. When the ignition position is 27 mm, the peak in-cylinder pressure is the largest and the engine indicated thermal efficiency is the highest. When the initial pressure of rebound cylinder is 1.7 bar, the indicated thermal efficiency is the highest. Under the trajectory tracking control strategy, the system realizes continuous and stable operation and can deal with large cylinder pressure fluctuation. The synchronization error changes periodically and is relatively large in the middle of the expansion stroke and near the outer dead center.

Design of a Slot-Spaced Permanent Magnet Linear Alternator Based on Numerical Analysis

Linear alternators work seamlessly with Free-Piston Stirling Engines (FPSE) in the energy conversion process. This research concentrates on the design and development of a tubular slot-spaced Permanent Magnet Linear Alternator (PMLA) to be coupled with the FPSE. In an attempt to increase the power density of the machine over conventional PMLAs, a slot space is added to reduce the total mass of the stator and a parametrical study is carried out for the same. Numerically analyzing the fundamental parameters such as change of stator and magnet materials, and operating conditions frequency and stroke length. The effects of slotting are studied to understand the skinning effects on the tooth and the performance variation of the machine. A modified model is obtained to surpass 100 W from the numerical analysis of the parametric variations. Putting the machine to extreme limitations, the study conducted upon variations of parameters obtained a stable maximum power density of 186 W/kg and produced a power of 921 W for the designed PMLA. The study outlines the variation seen in the performance of the machine in such diverse conditions they go through during their life cycle.

Simulation Research of the n-Butanol Proportion Influence on HCCI Combustion of Free-Piston Diesel Engine Generators

For researching the influence of n-butanol proportion in diesel fuel on homogeneous charge compression ignition (HCCI) combustion of the free-piston diesel engine generator (FPDEG), a three-dimensional (3D) moving mesh computational fluid dynamics (CFD) simulation model of a FPDEG prototype was developed. A detailed chemical reaction mechanism of diesel fuel was selected as the HCCI combustion mechanism and coupled in the established HCCI combustion simulation model of the FPDEG prototype. The validity of the established HCCI combustion simulation model is proved by comparing the simulation and experimental pressure curves under the condition of pure diesel fuel. The simulation results of different n-butanol proportions in diesel fuel showed that as the n-butanol proportion increased from 0 to 60%, the maximum heat release rate decreased to 59.6 J/deg, the calculated indicated thermal efficiency augmented to 4.6%, the calculated indicated mean effective pressure increased to 0.057 MPa, and the final NOx and CO content decreased to 0.239 and 0.57 g/kg fuel, respectively, but the final soot content increased to 0.000562 g/kg fuel. Therefore, the n-butanol proportion of diesel fuel played a vital role in combustion and emission progress of the FPDEG.

Pneumatic Damper Multicriteria Optimization in the Free-Piston Stirling Engine Displacer

The free-piston Stirling engine is a complex self-oscillating system, which parameters should be strictly coordinated with each other to ensure the unit operability. One of such coordination tasks is to ensure the balance of energy input to and output from the engine displacer. In this case, energy output depends on the dissipative forces, and energy output is provided by the displacer rod and depends on its diameter. The need to select rod diameter complicates and slows down the engine design. In addition, the system possible instability could lead to the engine racing. To solve this problem, it is proposed to install a pneumatic damper on the displacer rod ensuring the dissipative forces growth with an increase in the displacer amplitude above its nominal value. Thermodynamic model was created to evaluate the damper characteristics and select its parameters. MATLAB software package was used as the development environment. The system of differential equations was integrated using the Runge --- Kutta family method. The damper multicriteria optimization problem was formalized, and two objective functions with five independent parameters were compiled. Optimization was performed using the genetic algorithm, and the Pareto front was built. To evaluate results of the work performed, the damper mathematical model was integrated into the Stirling engine mathematical model, and the working process was calculated with selected damper parameters. It is demonstrated that the developed pneumatic damper prevents an increase in the displacer oscillation amplitude ensuring system stability and reducing time for the unit design