Analysis of the Scavenging Process of a Two-Stroke Free-Piston Engine Based on the Selection of Scavenging Ports Or Valves

energies

Article Analysis of the Scavenging Process of a Two-Stroke Free-Piston Engine Based on the Selection of Scavenging Ports or Valves

Boru Jia 1,2,* ID , Yaodong Wang 1, Andrew Smallbone 1 and Anthony Paul Roskilly 1

1 Sir Joseph Swan Centre for Energy Research, Newcastle University, Newcastle upon Tyne NE1 7RU, UK; [email protected] (Y.W.); [email protected] (A.S.); [email protected] (A.P.R.) 2 School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China * Correspondence: [email protected] or [email protected]; Tel.: +44-07547839154

Received: 1 December 2017; Accepted: 24 January 2018; Published: 2 February 2018

Abstract: The free-piston engine generator (FPEG) is a linear energy conversion device with the objective of utilisation within a hybrid-electric automotive vehicle power system. In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine (CE) of the same size, and the difference in the valve timing is compared for both port scavenging type and valve scavenging type, with the exhaust valve closing timing is selected as the parameter. A zero-dimensional simulation model is developed in Ricardo WAVE software (2016.1), with the piston dynamics obtained from the simulation model in Matlab/SIMULINK (R2017a). For the CE and FEPG using scavenging ports, in order to improve its power output to the same level as that of a CE, the inlet gas pressure is suggested to be improved to above 1.2 bar, approximately 0.2 bar higher than that used for a CE. If a CE cylinder with exhaust valves is adopted or referred to during the development of an FPEG prototype, the exhaust valve is suggested to be closed earlier to improve its power output, and a higher intake pressure is also suggested if its output power is expected to be the same or higher than that of a CE.

Keywords: free-piston engine; scavenging system; numerical modelling; optimisation

1. Introduction The free-piston engine (FPE) is a novel power device system, and the main difference from a conventional crankshaft engine is that the piston motion is linear and free to move between its dead centers [1,2]. For the FPEs, the elimination of the crankshaft system and the reduction of mechanical components signiﬁcantly reduces the complexity of the engine [3]. This gives a number of advantages: the heat transfer losses and NOx generation are supposed to be reduced due to faster expansion stroke; the frictional losses should be reduced due to the simpliﬁed mechanism and the elimination of the piston side force in conventional reciprocating engines; potentially lower maintenance cost due to a compact design; and multi-fuel/combustion mode possibility due to a possibility of variable compression ratio [4]. The FPE system is known to be coupled with different loads, which include air compressors, hydraulic pumps, and electric generators [1,5–7]. In this research, the FPE connected with a linear electric generator (free-piston engine generator, FPEG) is selected for future application in hybrid-electric automotive vehicles. Combustion takes place alternatively in each chamber of the engine makes the translator reciprocate and the linear electrical generator converts parts of the mover’s kinetic energy into electricity, which will be stored and/or used by an external load [8]. The effective efficiency is estimated

Energies 2018, 11, 324; doi:10.3390/en11020324 www.mdpi.com/journal/energies Energies 2018, 11, 324 2 of 14 to be up to 46% (including friction losses) at a power output level of 23 kW, which shows promising results in terms of engine performance and emissions [9]. Different FPEG prototypes have been reported in the recent years. Successful single-cylinder single-piston FPEG systems have been reported [4,10–12]. Researchers at West Virginia University described the development of a spark ignited dual-cylinder dual-piston engine generator as well as a compression ignition prototype [13,14]. Meanwhile, dual-cylinder dual-piston configurations have also been developed by researchers from a European Commission-funded Free-Piston Energy Converter (FPEC) project, Czech Technical University, Sandia National Laboratory, Beijing Institute of Technology, Shanghai Jiaotong University, and Newcastle University [15–21]. Most of the reported FPEG prototypes employ a two-stroke thermodynamic process, and the gas exchange process is performed with intake/exhaust ports or valves [22]. The cylinders adopted in an FPEG prototype at this stage are mainly from commercial products, and the others are designed and manufactured with a commercial engine taken as a reference [21,23,24]. For a conventional two-stroke engine, the crank angle is used as feedback to decide the valve timing. On the other hand, for the FEPG, the valve timing cannot be decided according to the crank angle as that of the reciprocating engine due to the elimination of the crankshaft system. However, very little research has been undertaken to examine its scavenging process based on the selection of scavenging ports or valves. Researchers at Beijing Institute of Technology investigated the scavenging process of an FPEG prototype using computational fluid dynamics [25]. A time-based numerical simulation program was developed in Matlab to describe the movement of the piston, and a parametric analysis was undertaken based on the simulation model. The influence of the engine effective stroke length, valve overlapping timing, system operating frequency, and intake pressure, etc. on the engine scavenging performance was discussed. From the simulation results, it was suggested that in order to obtain high scavenging and trapping efficiencies, a combination of high effective stroke length to bore ratio and long valve overlapping timing with low intake pressure should be employed [25]. The scavenging process of a two-stroke FPEG prototype was investigated by researchers from Sandia National Laboratories, aiming to improve its thermal efficiency and exhaust emissions [26]. The scavenging system was configured using computational fluid dynamics, 0/1 dimensional modelling, and single step parametric variations. Different design options were analysed and compared, including the application of loop, hybrid-loop, and uniflow scavenging methods, different charge delivery options, etc. Meanwhile, the intake/exhaust port arrangement, valve lift and valve timing setting, and charging pressure selection were adjusted and analysed. The computational results indicated that a stratified scavenging scheme employing a uniflow geometry supplied by a stable, low temperature/pressure charge should best optimise the engine efficiency and emissions characteristics [26]. In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine of the same size, and the difference in the valve timing is compared for both the port scavenging type and valve scavenging type, with the exhaust valve closing timing being selected as the parameter. A zero-dimensional simulation model is developed in Ricardo WAVE software, with the piston dynamics obtained from the simulation model in Matlab/SIMULINK. Engine performance with different intake pressures and various exhaust valve closing timings are predicted and analysed, aiming to provide guidance for the design and optimisation of the FPEG scavenging process with either scavenging ports or valves.

2. Fundamental Analysis

2.1. FPEG Conﬁguration FPEGs can be divided into three categories according to piston/cylinder configuration: single-piston with single combustion cylinder, dual-piston with two combustion cylinders, and opposed-piston with a shared combustion cylinder [1]. The basic operation principles are the same for each concept; differences between the concepts include the number of combustion chambers and compression stroke realisation [27]. EnergiesEnergies 20182018,, 1111,, 324x FOR PEER REVIEW 33 ofof 1414

compression stroke realisation [27]. The FPEG prototype in this research adopts the most popular Thedual-piston FPEG prototype type, the in this schematic research adoptsconfiguration the most popularof which dual-piston is shown type, in Figure the schematic 1. Figure configuration 1a is an ofillustration which is shownfor the inFPEG Figure with1. Figureintake/exhaust1a is an illustrationvalves, and for Figure the FPEG 1b is witha demonstration intake/exhaust of the valves, FPEG andwith Figure scavenging1b is a demonstrationports. of the FPEG with scavenging ports.

(a) Free-piston engine generator (FPEG) with valves.

(b) FPEG with scavenging ports.

Figure 1. FPEG schematic configuration [3]. Figure 1. FPEG schematic conﬁguration [3]. This FPEG comprises two internal combustion engines and a linear electric machine. The linear electricThis machine FPEG comprises is located two between internal the combustion two cylind enginesers, and andthe atwo linear pistons electric are machine. connected The with linear the electricmover. machineThe spark-ignited is located betweenFPEG employed the two cylinders,here uses anda two-stroke the two pistonsthermodynamic are connected cycle, with i.e., thethe mover.power Thestroke spark-ignited is controlled FPEG to take employed place alternat here usesely a two-strokein each cylinder, thermodynamic and to drive cycle, the i.e., compression the power strokestroke isof controlled the other tocylinder. take place The alternately pistons with in eachthe mover cylinder, oscillate, and to and drive the the linear compression electric generator stroke of theconverts other cylinder.part of the The mechanical pistons with energy the moverinto elec oscillate,tricity, andwhich the is linear stored electric by an generatorexternal load. converts part of the mechanical energy into electricity, which is stored by an external load. 2.2. Comparison of FPEG with a Conventional Engine (CE) 2.2. Comparison of FPEG with a Conventional Engine (CE) As the crankshaft mechanism is eliminated for an FPEG system, the piston movement is only affectedAs the by crankshaftthe forces mechanismacting on the is eliminatedpistons, as forshown an FPEG in Figure system, 2a. the The piston main movement forces are isthe only in-cylinder affected bygas the force forces from acting both on cylinders, the pistons, the as resistance shown in force Figure from2a. The the mainlinear forces electric are machine, the in-cylinder the mechanical gas force fromfrictional both cylinders,force, and thethe resistanceinertia of forcethe moving from the ma linearss. The electric corresponding machine, the dynamic mechanical equation frictional is derived force, andas: the inertia of the moving mass. The corresponding dynamic equation is derived as: → → → → d2x F++F++F++F==m (1)(1) l r e f dt2

where (unit: N) and (N) are the gas forces from the left and right cylinders respectively; (N) where Fl (unit: N) and Fr (N) are the gas forces from the left and right cylinders respectively; Fe (N) isis thethe forceforce outputoutput fromfrom thethe linearlinear electricelectric machine—amachine—a parameterparameter whichwhich isis variedvaried dependingdepending ifif thethe machine is operated in motoring or generation modes; (N) is the mechanical friction force; (kg) machine is operated in motoring or generation modes; Ff (N) is the mechanical friction force; m (kg) is theis the moving moving mass mass of the of pistonthe piston assembly assembly with with the mover the mover of the of electric the electric machine, machine, and x (m)and is the(m) piston is the distancepiston distance from the from cylinder the cylinder head. The head. calculation The calc ofulation the forces of the mentioned forces mentioned above and above model and validation model resultsvalidation can beresults found can elsewhere be found [ 3elsewhere,22,28]. [3,22,28].

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For a conventional crank shaft engine, a schematic ﬁgure is shown in Figure2b. The distance between the crank axis and the piston pin axis s is given by [29]:

1/2 s = r cos θ + l2 − r2 sin2 θ (2) Energies 2018, 11, x FOR PEER REVIEW 4 of 14 where l is the connecting rod length (m); r is the crank radius (m); and θ is the crank angle (degree). For a conventional crank shaft engine, a schematic figure is shown in Figure 2b. The distance As a result,between the the piston crank axis dynamics and the piston of an pin FPEG axis will is given be different by [29]: from that of a conventional two-stroke crankshaft engine with the same size. = cos + − sin ⁄ (2) The cylinder volume V at any crank position θ is: where is the connecting rod length (m); is the crank radius (m); and is the crank angle (degree). 2 As a result, the piston dynamics of an FPEGπB will be different from that of a conventional two- V = Vc + (l + r − s) (3) stroke crankshaft engine with the same size. 4 The cylinder volume at any crank position is: where V is the clearance volume (m3), and can be calculated from the swept volume V and the set c d =+ +− (3) compression ratio CR: 4

3 V = V /(CR − 1) (4) where is the clearance volume (m ), andc cand be calculated from the swept volume and the set compression ratio : The piston distance from the cylinder head of the crankshaft engine is then calculated by [29]: =⁄ − 1 (4) 2 The piston distance from the cylinder headV of theπ crankshaftB engine is then calculated by [29]: x = V/A = d + (l + r − s)/A (5) − CR 1 4 =⁄ = + +− (5) − 1 4

(a) FPEG system

(b) CE system

Figure 2. Schematic figures of an FPEG and a conventional engine (CE). Figure 2. Schematic ﬁgures of an FPEG and a conventional engine (CE). The engine of the FPEG is the same size as that of the CE. The specifications are summarised in Table 1. The input parameters for the FPEG is based on a prototype developed at Newcastle University, The engineand a more of the detailed FPEG development is the same process size can as thatbe found of the elsewhere CE. The [21]. speciﬁcations The piston diameter are summarised is the in Table1. Thesame input for parametersboth engines. For for the the FPEG, FPEG the is piston based stroke on a is prototype variable, and developed it is tuned atto Newcastlebe the same as University, and a morethat detailed of the CE development by changing the processinput fuel canand the be electric found load. elsewhere The operation [21]. speed The pistonof the CE diameter is set is the same for bothto be engines.the same asFor that theof the FPEG, FPEG, which the piston is 35.7 Hz stroke for the is FPEG, variable, and 2142 and rpm itis for tuned the CE. to be the same as that of the CE by changing the input fuel and the electric load. The operation speed of the CE is set to be the same as that of the FPEG, which is 35.7 Hz for the FPEG, and 2142 rpm for the CE.

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Energies 2018, 11, x FOR PEER REVIEW 5 of 14 Table 1. Speciﬁcations of FPEG and CE. Table 1. Specifications of FPEG and CE. Parameters (Unit) FPEG CE Parameters (Unit) FPEG CE PistonPiston diameter diameter (mm) (mm) 50.0 50.0 50.0 50.0 StrokeStroke (mm) (mm) 33.0 33.0 33.0 33.0 Compression ratio (-) 10.5 10.5 Compression ratio (-) 10.5 10.5 Connecting rod length (mm) - 49.5 Connecting rod length (mm) - 49.5 Crank radius (mm) - 16.5 Crank radius (mm) - 16.5

The simulatedThe simulated piston piston displacement displacement from from the the cylindercylinder head for for the the FPEG FPEG is isdemonstrated demonstrated in in FigureFigure3, with 3, thatwith ofthat the of CEthe shownCE shown in thein the same same figure figure as as a a comparison. comparison. For For a abetter better understanding understanding of theof difference the difference in piston in piston dynamics, dynamics, the the changing changing trendtrend of the the piston piston velocity velocity with with the thepiston piston displacementdisplacement is shown is shown in Figure in Figure4 with 4 with four four specific specific points points (points (points 1, 2, 3, 3, 4) 4) marked marked in inthe the figure. figure.

Figure 3. Comparison of FPE and CE piston displacement. Figure 3. Comparison of FPE and CE piston displacement. It canIt be can found be found that that the the piston piston of of the the FPEG FPEG moves moves fasterfaster after after combustion combustion takes takes place place (point (point 1 in 1 in FigureFigure4) as it4) is as not it is restricted not restricted by theby the crankshaft crankshaft system. system. For For thethe FPEG, the the expansion expansion process process is the is the corresponding compression process of the other cylinder, and the piston is driven by the combustion corresponding compression process of the other cylinder, and the piston is driven by the combustion gas force to overcome the compression force. As a result, the piston velocity of the FPEG becomes gas force to overcome the compression force. As a result, the piston velocity of the FPEG becomes slower soon after the top dead center (TDC) compared with that of the CE (point 2 in Figure 3). slowerThe soon peak after velocity the of top the dead CE is centerapproximately (TDC) compared4 m/s, which with is higher that than of the that CE achieved (point in 2 the in FigureFPEG 3). The peak(around velocity 3.4 m/s). of the CE is approximately 4 m/s, which is higher than that achieved in the FPEG (around 3.4When m/s). the piston is moving towards the TDC from its bottom dead center (BDC), the piston Whenvelocity the is pistonhigher in is the moving FPEG at towards the beginning the TDC of the from stroke its (point bottom 3 in dead Figure center 4), while (BDC), gets theslower piston velocityafterward is higher (point in the4 in FPEG Figure at4). the The beginning piston velocity of the at point stroke 3 in (point Figure 3 in4 is Figure higher4 because), while the gets piston slower afterwardis driven (point by 4the in high-pressure Figure4). The gas piston after velocity combusti aton. point As the 3 in FPEG Figure configuration4 is higher employed because the in this piston is drivenresearch by the is a high-pressure dual-piston type, gas the after compression combustion. process As (from the FPEG BDC conﬁgurationto TDC) is the employedcorresponding in this expansion process for the cylinder of the other side. Without the crankshaft mechanism, the piston research is a dual-piston type, the compression process (from BDC to TDC) is the corresponding velocity will be lower when the piston is approaching its TDC during the compression process. expansion process for the cylinder of the other side. Without the crankshaft mechanism, the piston velocity will be lower when the piston is approaching its TDC during the compression process.

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Figure 4. Comparison of FPEG and CE piston velocity. Figure 4. Comparison of FPEG and CE piston velocity. 2.3. Valve Opening Timing 2.3. Valve Opening Timing The cylinders adopted in an FPEG prototype at this stage are mainly from commercial products, Theand cylindersthe others adopted are designed in an and FPEG manufactured prototype atwi thisth a stage commercial are mainly engine from taken commercial as a reference. products, and theThe othersscavenging are designedprocess is andperformed manufactured by ports withand valves. a commercial If a scavenging engine port taken is asused, a reference.it is covered/uncovered by the piston movement. If valves are employed, then the scavenging process is The scavenging process is performed by ports and valves. If a scavenging port is used, it is controlled by opening/closing the valve. For a conventional two-stroke engine, the crank angle is covered/uncovered by the piston movement. If valves are employed, then the scavenging process is used as feedback to decide the valve timing. In contrast, for the FEPG, the valve timing cannot be controlleddecided by according opening/closing to the crank the angle valve. as For that a of conventional the reciprocating two-stroke engine engine,due to the the elimination crank angle of the is used as feedbackcrankshaft to decide system. the In valveorder to timing. make further In contrast, comparison for the of FEPG, the FPEG the valvewith CE, timing an equivalent cannot be crank decided accordingangle to (ECA) the crank is adopted angle for as the that FPEG of the to reciprocatingdescribe one operation engine due cycle to [3]: the elimination of the crankshaft system. In order to make further comparison of the FPEG with CE, an equivalent crank angle (ECA) is = ∙ 360° (6) adopted for the FPEG to describe one operation cycle [3]: where is the operation time (s) and is the operation duration for one cycle. t ◦ The piston displacement with the crankECA angle= for·360 the CE, or the ECA for the FPEG, is compared (6) T in Figures 5 and 6. If the FPEG uses the same engine as the CE, then difference in the valve timing wherewillt is be the compared operation for timeboth port (s) and scavengingT is the type operation and valve duration scavenging for onetype. cycle. The exhaust valve closing The(EVC) piston timing displacement is a crucial parameter with the crank that can angle affe forct the the scavenging CE, or the ECAefficiency for the and FPEG, the compression is compared in Figuresstroke5 and length,6. If the etc. FPEG Meanwhile, uses the the same difference engine in as piston the CE, displacement then difference is more in the significant valve timing when willthe be piston is moving from its BDC to TDC for the CE and FPEG. As a result, the EVC timing is selected compared for both port scavenging type and valve scavenging type. The exhaust valve closing (EVC) as the parameter to compare the valve timing of the CE and the FPEG. timing is a crucial parameter that can affect the scavenging efficiency and the compression stroke For the CE and FPEG engines using scavenging ports, the ports will be opened/closed when the length,piston etc. Meanwhile,reaches a certain the position. difference The in valve piston timing displacement can be adjusted is more during significant the design when process, the while piston is movingit cannot from be its varied BDC during to TDC the for operation. the CE If and a refe FPEG.rence AsEVC a is result, set to be the 32 EVCmm from timing the cylinder is selected head, as the parameterthen the to EVC compare for the the FPEG valve is approximately timing of the 10° CE earlie andr the than FPEG. that of the CE. As a result, the scavenging Forefficiency the CE and and trapping FPEG enginesefficiency using will scavengingbe affected if ports, an early the EVC ports or will short be scavenging opened/closed process when the pistonis adopted. reaches a certain position. The valve timing can be adjusted during the design process, while it cannot be varied during the operation. If a reference EVC is set to be 32 mm from the cylinder head, then the EVC for the FPEG is approximately 10◦ earlier than that of the CE. As a result, the scavenging efficiency and trapping efficiency will be affected if an early EVC or short scavenging process is adopted.

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Figure 5. Valve timing of CE and FPEG with port scavenging type. FigureFigure 5. Valve 5. Valve timing timing of of CE CE and and FPEG FPEG withwith port scavenging scavenging type. type. If a valveIf a valve scavenging scavenging type type engine engine isis adopted fo forr both both the the CE CEand andthe FPEG, the FPEG, the valve the timing valve can timing can bebe adjusted adjustedIf a valve duringduring scavenging the the operation operation type engine process process is adoptedwith with the thefocrrank both crank angle. the angle. CE If theand Ifexhaust the the FPEG, exhaust valve the is valve set to timing is close set to50°can close afterbe adjusted BDC, as during illustrated the operation in Figure process 6, the compresswith the crionank stroke angle. of If the the FPEG exhaust will valve be reduced is set to close(around 50° 50◦ after BDC, as illustrated in Figure6, the compression stroke of the FPEG will be reduced (around 4after mm BDC, shorter as thanillustrated that of in the Figure CE). 6,As the a resultcompress, theion peak stroke cylinder of the pressure FPEG will and be the reduced corresponding (around 4 mm shorter than that of the CE). As a result, the peak cylinder pressure and the corresponding engine-indicated4 mm shorter than work that is ofexpected the CE). to As be alower, result wh, theich peak will thencylinder reduce pressure the engine and thethermal corresponding efficiency engine-indicatedandengine-indicated the whole work system work is expectedefficiency. is expected to to be be lower, lower, which which willwill then reduce reduce the the engine engine thermal thermal efficiency efﬁciency and theand whole the whole system system efﬁciency. efficiency.

Figure 6. Valve timing of CE and FPEG with valve scavenging type. Figure 6. Valve timing of CE and FPEG with valve scavenging type. 2.4. Simulation MethodologyFigure 6. Valve timing of CE and FPEG with valve scavenging type. 2.4. Simulation Methodology In order to investigate the scavenging performance of the FPEG and further optimise its valve 2.4. Simulation Methodology timing,In ordera zero-dimensional to investigate thesimulation scavenging model performa is developednce of the in FPEGRicardo and WAVE further software, optimise withits valve the Inpistontiming, order dynamics toa zero-dimensional investigate obtained the from scavenging simulation the simulation performance model mo is deldeveloped ofof thethe FPEGFEPG in Ricardo and the further WAVECE in optimise Matlab/SIMULINK. software, its valvewith the timing, a zero-dimensionalThepiston simulation dynamics simulation process obtained is modelfromillustrated the is simulation developed in Figure mo in7. delThe Ricardo of simulation the WAVEFEPG andmodel software, the inCE WAVE within Matlab/SIMULINK. the is pistona two-stroke dynamics obtainedsingle-cylinderThe fromsimulation the simulation engine,process and is modelillustrated the piston of thein motion Figure FEPG profile7. and The the simulationis CEimported in Matlab/SIMULINK. model from inthe WAVE simulation is a The two-strokeresults simulation in Matlab/SIMULINK.single-cylinder engine, The and piston the motionpiston motionprofile ofprofile the FPEGis imported is described from thewith simulation crank angle results using in process is illustrated in Figure7. The simulation model in WAVE is a two-stroke single-cylinder engine, EquationMatlab/SIMULINK. (6). The piston motion profile of the FPEG is described with crank angle using and theEquation piston motion(6). profile is imported from the simulation results in Matlab/SIMULINK. The piston motion profile of the FPEG is described with crank angle using Equation (6).

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FigureFigure 7. 7.Simulation Simulation process.

The other engine input parameters for the model are summarised in Table 2. The parameters in The other engine input parameters for the model are summarised in Table2. The parameters in Table 2 are kept unchanged during the simulation for both the FPEG and the CE. The output from Table2 are kept unchanged during the simulation for both the FPEG and the CE. The output from the the simulation model in WAVE would be the engine performance, e.g., cylinder pressure, power simulation model in WAVE would be the engine performance, e.g., cylinder pressure, power output, output, scavenging efficiency, etc. The ignition timings of the FPEG and the CE are set to be the same, scavenging efﬁciency, etc. The ignition timings of the FPEG and the CE are set to be the same, which is ◦which is 15° before top dead centre (bTDC). The simulation in this research aims to provide the 15 followingbefore top information dead centre for (bTDC).the future The design simulation and optimisation in this research of the FPEG aims scavenging to provide process: the following information for the future design and optimisation of the FPEG scavenging process: (a) If an engine with scavenging ports is adopted for the FPEG prototype, information on the (a) Ifperformance an engine with of the scavenging scavenging ports process, is adoptedthe difference for the compared FPEG prototype,to that of the information CE with the same on the performancecylinder, and of the methods scavenging to improve process, the the sy differencestem scavenging compared process to that areof collected. the CE with the same (b)cylinder, If an engine and thewith methods valves isto adopted improve forthe the systemFPEG prototype, scavenging information process are on collected.how the valve timing (b) If anwill engine affect its with scavenging valves is process, adopted the for difference the FPEG compared prototype, to that information of the CE onwith how the thesame valve cylinder, timing willand affect the itsoptimised scavenging valve process, timing theare differencecollected. compared to that of the CE with the same cylinder, and the optimised valve timing are collected. Table 2. Simulation engine input parameters.

TableParameters 2. Simulation (Unit) engine input parameters. Value Engine speed (rpm) 2143 NumberParameters of valves (Unit) (-) Value 2 Intake valveEngine diameter speed (rpm) (mm) 2143 18.0 Exhaust Numbervalve diameter of valves (mm) (-) 2 18.0 IntakeValve valve lift (mm) diameter (mm) 18.0 4.2 ExhaustFuel valve type diameter (-) (mm) 18.0 Octane Fuel lower heatingValve lift valve (mm) (J/kg) 4.2 4.4 × 107 Fuel type (-) Octane Air fuel ratio (-) 14.7 Fuel lower heating valve (J/kg) 4.4 × 107 Intake airAir pressure fuel ratio (bar) (-) 14.7 1.0–2.0 IntakeIntake air temperature air pressure (K) (bar) 1.0–2.0 300.0 IntakeIgnition air timing temperature (°) (K) 300.0 15 bTDC Ignition timing (◦) 15 bTDC 3. Results and Discussion 3. Results and Discussion 3.1. FPEG with Scavenging Ports 3.1. FPEG with Scavenging Ports For the CE and FEPG using scavenging ports, and the EVC timing is set to be 32 mm from the cylinderFor the head, CE and as illustrated FEPG using in Figure scavenging 5, and the ports, inlet and gas the pressure EVC timingis 1.2 bar. is setThe to cylinder be 32 mm pressure from is the cylindershown head, in Figure as illustrated 8, and the in cylinder Figure5 ,pressures and the inlet with gas the pressure volume/clearance is 1.2 bar. volume The cylinder for both pressure engine is showntypes in are Figure compared8, and thein Figure cylinder 9. The pressures other input with theparameters volume/clearance are the same volume for the for CE both and engineFPEG. As types a areresult, compared the peak in Figure cylinder9. The pressure other of input the CE parameters is approximately are the same41 bar, for which the CE is 7 and bar FPEG. higher As than a result,that theof peak the cylinderFPEG (34 pressurebar), even of though the CE the is approximately same compression 41 bar, stroke which length is 7 is bar adopted higher for than both that of ofthe the engines. The peak value is obtained after the piston reaches its TDC. The pressure of the FPEG at the FPEG (34 bar), even though the same compression stroke length is adopted for both of the engines. end of the compression process is also lower than that of the CE. As the free-piston is not limited by The peak value is obtained after the piston reaches its TDC. The pressure of the FPEG at the end of the mechanical system, its velocity after TDC is higher than that of CE (as shown in Figure 4), and the compression process is also lower than that of the CE. As the free-piston is not limited by the mechanical system, its velocity after TDC is higher than that of CE (as shown in Figure4), and the

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the corresponding cylinder pressure is lower due to a faster expansion process. When the exhaust correspondingthe corresponding cylinder cylinder pressure pressure is lower is duelower to due a faster to a faster expansion expansion process. process. When When the the exhaust exhaust port port opens, the cylinder pressure is nearly the same for both engines. opens,port the cylinderopens, the pressure cylinder ispressure nearly is the nearly same the for same both for engines. both engines.

Figure 8. Cylinder pressure for the CE and FPEG with scavenging ports. FigureFigure 8. Cylinder 8. Cylinder pressure pressure for for the the CE CE and and FPEG FPEG withwith scavenging ports. ports. The scavenging efficiency for the FPEG is 94.1%, which is slightly lower than that of the CE The scavengingThe scavenging efﬁciency efficiency for for the the FPEG FPEG is is 94.1%, 94.1%, which which isis slightly lower lower than than that that of ofthe the CE CE (94.8%). For the engine output power, the power of the FPEG is supposed to be much lower than that (94.8%).(94.8%). For the For engine the engine output output power, power, the the powerpower of of the the FPEG FPEG is supposed is supposed to be to much be muchlower than lower that than of the CE. As can be seen from Figure 9, the area enclosed by the pressure-volume of the FPEG is less that ofof the the CE. CE. As As cancan be seen seen from from Figure Figure 9, 9the, the area area enclosed enclosed by the by pressure-volume the pressure-volume of the FPEG of the is less FPEG than that of the CE. From the calculated results of the WAVE software, the brake power of the is less thanthan that of the CE. From From the the calculated calculated results results of of the the WAVE WAVE software, software, the the brake brake power power of the of the single-cylinder CE engine is 2.1 kW, and that of a single-cylinder FPEG is 1.7 kW, with the same single-cylinder CE engine is 2.1 kW, and that of a single-cylinder FPEG is 1.7 kW, with the same single-cylinderengine size CE and engine operation is 2.1 speed. kW, and that of a single-cylinder FPEG is 1.7 kW, with the same engine size andengine operation size and speed. operation speed.

Figure 9. p-V diagram of FPEG with scavenging ports compared with CE. Figure 9. p-V diagram of FPEG with scavenging ports compared with CE. Figure 9. p-V diagram of FPEG with scavenging ports compared with CE. For the engine with scavenging ports, the opening/closing timing of the ports is controlled by For the engine with scavenging ports, the opening/closing timing of the ports is controlled by the piston movement. It can be adjusted during the design process, while it cannot be varied during Forthe the piston engine movement. with scavenging It can be adjusted ports, the during opening/closing the design process, timing while of the it cannot ports isbe controlled varied during by the engine operation. If the FPEG adopts a commercial cylinder with scavenging ports, the opening/closing engine operation. If the FPEG adopts a commercial cylinder with scavenging ports, the opening/closing pistonpositions movement. of the It canports be are adjusted fixed. As during a result, the the design engine process, scavenging while efficiency it cannot and be power varied output during of the engine positions of the ports are fixed. As a result, the engine scavenging efficiency and power output of the operation.FEPG If will the FPEGbe affected, adopts and a commercialthe performance cylinder is not with as good scavenging as that in ports, a CE. the Generally, opening/closing for a CE and positions an FEPG will be affected, and the performance is not as good as that in a CE. Generally, for a CE and an of the portsFPEG arewith fixed. the design As a result, parameters the engine shown scavenging in Tables efficiency1 and 2, a and compressor power output for inlet of thegas FEPGpressure will be FPEG with the design parameters shown in Tables 1 and 2, a compressor for inlet gas pressure affected, and the performance is not as good as that in a CE. Generally, for a CE and an FPEG with the design parameters shown in Tables1 and2, a compressor for inlet gas pressure increase is suggested to boost the pressure to above 1.2 for a better scavenging performance. The system scavenging efficiency with different inlet gas pressures for both the CE and FPEG is compared in Figure 10. Energies 2018, 11, x FOR PEER REVIEW 10 of 14

increaseEnergies is suggested 2018, 11, x FOR to PEER boost REVIEW the pressure to above 1.2 for a better scavenging performance.10 of 14 The systemincrease scavenging is suggested efficiency to boostwith differentthe pressure inlet to ga absove pressures 1.2 for afor better both scavenging the CE and performance. FPEG is compared The in Figuresystem 10. scavenging efficiency with different inlet gas pressures for both the CE and FPEG is compared Energies 2018, 11, 324 10 of 14 in Figure 10.

Figure 10. Scavenging efficiency with different intake pressures. Figure 10. Scavenging efficiency efficiency with different intake pressures.pressures. The engine-indicated power and peak pressure of both the CE and FPEG with different inlet gas Thepressures engine-indicated are shown in powerpower Figure and11. The peak engine pressure efficiency ofof both is supposed the CE to and show FPEG a similar with trend different compared inlet gas pressurespressuresto the areare engine-indicated shownshown inin FigureFigure power 1111.. The output engine due efficiencyeffici to theency setting is supposed of the fuel to injection. show a similar It is found trend that compared if the FPEG shares the same cylinder size and scavenging ports position with the CE, then the peak cylinder toto thethe engine-indicatedengine-indicated power output due to thethe settingsetting ofof thethe fuelfuel injection.injection. It is foundfound thatthat ifif thethe gas pressure and output brake power of the CE is higher than that of the FPEG, with an inlet gas FPEG shares thethe same cylinder size and scavenging portsports position with the CE, then the peak cylinder pressure range of 1.1–2.0 bar. However, by increasing the inlet gas pressure, both the peak cylinder gas pressure and output brake power of the CE is higher than that of the FPEG, with an inlet gas gas pressuregas pressure and outputand the engine brake output power brake of the power CE isincrease higher in than a nearly that linear of the relationship. FPEG, with The anpotential inlet gas pressure range of 1.1–2.0 bar. However, by increasing the inlet gas pressure, both the peak cylinder pressurereasons range for of this 1.1–2.0 couldbar. be that However, more air byand increasing fuel is drawn the toinlet the cylinder gas pressure, with higher both inlet the gas peak pressure cylinder gas pressure(the fuel andand injector the engine is a proportional output brake type, power which increase means the inin afuel nearly mass linear is proportional relationship. to the The inlet potential air reasonsreasonsmass), forfor thisthis and couldcould thus bethebe thatscavenging more airair efficiency and fuel is is improved. drawn to Asthe a cylinder result, for with an FPEG higher prototype inlet gas of pressure this (the(the fuelfuelkind, injectorinjector in order isis aato proportionalproportional improve its power type,type, output whichwhich to meansme theans same the level fuel massas that is of proportional a CE with the to same the inletsize, air mass), the andand inlet thusthus gas the the pressure scavenging scavenging is suggested efficiency efficiency to be is improved.improvis improved.ed by As approximately As a result, a result, for an0.2for FPEG baran comparedFPEG prototype prototype to that of this used of kind, this inkind, order infor toorder a CE. improve Meanwhile,to improve its power the its intake power output pressure output to the of sameto the the FPEG level same is as suggested level that ofasa tothat CE be withboostedof a CE the to with same above the size, 1.4 same bar the for inletsize, a better power output. gasthe pressureinlet gas pressure is suggested is suggested to be improved to be improv by approximatelyed by approximately 0.2 bar compared 0.2 bar compared to that used to that for aused CE. Meanwhile,for a CE. Meanwhile, the intake the pressure intake pressure of the FPEG of the is suggestedFPEG is suggested to be boosted to be toboosted above to 1.4 above bar for 1.4 a bar better for powera better output. power output.

Figure 11. Engine-indicated power and peak pressure with different intake pressures.

Figure 11. Engine-indicated power and peak pressurepressure with different intakeintake pressures.pressures.

Energies 2018, 11, 324 11 of 14

Energies 2018, 11, x FOR PEER REVIEW 11 of 14 3.2. FPEG with Valves 3.2. FPEG with Valves If a cylinder with exhaust valves is adopted to during the development of an FPEG prototype, If a cylinder with exhaust valves is adopted to during the development of an FPEG prototype, the exhaust valve will be set to close at 50◦ after BDC (or 230◦ from TDC), as illustrated in Figure6. the exhaust valve will be set to close at 50° after BDC (or 230° from TDC), as illustrated in Figure 6. The cylinder pressure vs the volume/clearance volume proﬁles for both engines are compared in The cylinder pressure vs the volume/clearance volume profiles for both engines are compared in FigureFigure 12. The 12. inputThe input gas gas pressure pressure is setis set to to 1.2 1.2 bar bar for for bothboth engines, and and all all of of the the input input parameters parameters exceptexcept for the for exhaust the exhaust valve valve closing closing timing timing are are identical. identical. It is found found that that the the peak peak cylinder cylinder pressure, pressure, the areathe enclosed area enclosed by the by p-Vthe p-V diagram, diagram, and and the the corresponding corresponding engine-indicated engine-indicated work work of the of theFPEG FPEG are are lowerlower than thatthan ofthat the of CE,the CE, as the as the compression compression stroke stroke of of thethe FPEG is is reduced. reduced. As As a result, a result, if an if FPEG an FPEG prototypeprototype uses theuses same the same cylinder cylinder as the as CEthe withoutCE withou adjustingt adjusting the the exhaust exhaust valve valve closing closing angle, angle, the the peak cylinderpeak pressure cylinder will pressure be reduced will be byreduced approximately by approximately 10 bar and10 bar the and engine the engine output output brake brake power power will be affectedwill by be 0.4 affected kW. by 0.4 kW.

Figure 12. p-V diagram of FPEG and CE with the same valve timing. Figure 12. p-V diagram of FPEG and CE with the same valve timing. The engine-indicated power and peak pressure with different EVC timings are compared and Theshown engine-indicated in Figure 13. As power the fuel and injection peak pressureis fixed for with each different operating EVC cycle, timings the engine are efficiency compared is and shownsupposed in Figure to 13show. As a thesimilar fuel trend injection compared is ﬁxed to that for of each the output operating brake cycle, power. the It engineis found efﬁciency that for is both the CE and FPEG, the peak cylinder pressure and output brake power decrease when a late supposed to show a similar trend compared to that of the output brake power. It is found that for both exhaust valve closing timing is employed. For the CE, the maximum brake power is achieved when the CE and FPEG, the peak cylinder pressure and output brake power decrease when a late exhaust the exhaust valve closes at 220°; while for the FPEG it is reached at 225°. With the same exhaust valve valve closing timingtiming, isthe employed. peak cylinder For the pressure CE, the maximumand power brake achieved power by is achievedthe FPEG when are thealways exhaust ◦ ◦ valveapproximately closes at 220 ;0.4 while kW lower for the than FPEG that itof isthe reached CE, due at to 225 a shorter. With compression the same exhauststroke. As valve a result, closing timing,for the an peakFPEG cylinder prototype pressure with valves, and the power exhaust achieved valve is by suggested the FPEG to are be closed always earlier approximately to improve 0.4its kW lowerpower than thatoutput of the(but CE,no earlier due to than a shorter 225°). However, compression a higher stroke. intake As pressure a result, is for also an suggested FPEG prototype if its with valves,output power the exhaust is expected valve to isbe suggestedthe same or toeven be higher closed than earlier that toof improvea CE. Meanwhile, its power the outputEVC timing (but no earlieris than not considered 225◦). However, useful to a contro higherl power intake output pressure for the is also FPEG suggested as the improvement if its output is very power low, is while expected to be theboosting same the or evenintake higher pressure than is overwhelmi that of a CE.ngly Meanwhile, better than the changing EVC timing the EVC is timing. not considered useful to control power output for the FPEG as the improvement is very low, while boosting the intake pressure is overwhelmingly better than changing the EVC timing.

Energies 2018, 11, 324 12 of 14 Energies 2018, 11, x FOR PEER REVIEW 12 of 14

Figure 13. Engine-indicated power and peak pressure with different exhaust valve closing (EVC) timings. Figure 13. Engine-indicated power and peak pressure with different exhaust valve closing (EVC) timings. 4. Conclusions 4. Conclusions In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventionalIn this research, engine theof the piston same size, dynamic and the characteristics difference in the ofvalve an timing FPEG is iscompared compared for both with port that of a conventionalscavenging enginetype and of valve the samescavenging size, andtype, the with difference the exhaust in thevalve valve closing timing timing is comparedselected as the for both port scavengingparameter. It typeshould and be valvenoted scavengingthat the current type, state- withof-study the exhaust is still in valve early closing stages, and timing a roadmap selected in as the parameter.research It ought should to be be mentioned. noted that Main the currentconclusions state-of-study and suggestions is still from in this early research stages, are and listed a roadmapbelow: in research(1) ought Compared to be with mentioned. a CE, the Mainpiston conclusions of the FPEG andmove suggestionss faster after fromcombustion this research takes place, are listed as it is below: not restricted by the crankshaft system, while becomes slower soon after this. The piston velocity (1) Comparedof an FPEG with will a CE, be thelower piston when of the the piston FPEG is moves approaching faster afterits TDC combustion during the takes compression place, as it is not restrictedprocess, and by the the peak crankshaft velocity system,achieved while is also becomes lower than slower that of soon a CE. after this. The piston velocity (2)of an For FPEG the CE will and be lowerFEPG using when scavenging the piston isports approaching with the EVC its TDC timing during set to the be compression32 mm from the process, andcylinder the peak head, velocity the peak achieved cylinder is alsopressure, lower engine than thatpower of output, a CE. and scavenging efficiency of (2) Forthe the FEPG CE and are FEPGfound to using be lower scavenging than that portsof a CE with with the the EVCsame timingsize and set operation to be 32 conditions. mm from the cylinderFor an head, FPEG the prototype peak cylinder of this kind, pressure, in order engine to improve power its output,power output and scavenging to the same efficiencylevel as of that of a CE with the same size, the inlet gas pressure is suggested to be improved to above 1.4 bar the FEPG are found to be lower than that of a CE with the same size and operation conditions. for a better power output, which is approximately 0.2 bar higher than that used for a CE. For an FPEG prototype of this kind, in order to improve its power output to the same level as that (3) If a CE cylinder with exhaust valves is adopted or referred to during the development of an of aFPEG CE with prototype, the same and size, the exhaust the inlet valve gas is pressure set to close is suggested at 50° after toBDC, be improvedthe engine-indicated to above work 1.4 bar for a betterof the power FPEG output,are found which to be lower is approximately than that of the 0.2 CE bar as higher the compression than that stroke used forof the a CE. FPEG is (3) If a CEreduced. cylinder For withan FPEG exhaust prototype valves with is adoptedvalves, the or ex referredhaust valve to during is suggested the development to be closed earlier of an FPEG prototype,to improve and its the power exhaust output valve (but isset no toearlier close than at 50 225°),◦ after and BDC, a higher the engine-indicated intake pressure workis also of the FPEGsuggested are found if its to output be lower power than is expected that of the to CEbe th ase same the compression or higher than stroke that of of a theCE. FPEGMeanwhile, is reduced. Forthe an FPEGEVC timing prototype is not considered with valves, useful the to exhaust control valve power is output suggested for the to FPEG be closed as the earlierimprovement to improve is very low, while boosting the intake pressure is overwhelmingly better than changing the its power output (but no earlier than 225◦), and a higher intake pressure is also suggested if its EVC timing. output power is expected to be the same or higher than that of a CE. Meanwhile, the EVC timing Acknowledgments:is not considered This useful work was tocontrol funded using power the EPSRC output (Enginee for thering FPEG and Physical as the Sciences improvement Research Council) is very low, Impactwhile Acceleration boosting the Account intake (E pressureP/K503885/1) is overwhelminglyand Thermal Energy better Challenge than Network changing (EP/P005667/1). the EVC timing. Data supporting this publication is openly available under an ‘Open Data Commons Open Database License’. Additional metadata are available at: http://dx.doi.org/10.17634/123306-4. Please contact Newcastle Research Acknowledgments:Data Service at [email protected] workk wasfor access funded instructions. using the EPSRC (Engineering and Physical Sciences Research Council) Impact Acceleration Account (EP/K503885/1) and Thermal Energy Challenge Network (EP/P005667/1). DataAuthor supporting Contributions: this publication Yaodong is openlyWang and available Boru Jia under proposed an ‘Open the topic Data and Commons methodology; Open DatabaseBoru Jia and License’. AdditionalAndrew metadata Smallbone are developed available at:the http://dx.doi.org/10.17634/123306-4 simulation model and analyzed the data;. Please Boru contact Jia wrote Newcastle the paper; Research DataAnthony Service at Paul [email protected] Roskilly contributed for access anal instructions.ysis tools and revised the paper. Author Contributions: Yaodong Wang and Boru Jia proposed the topic and methodology; Boru Jia and Andrew Smallbone developed the simulation model and analyzed the data; Boru Jia wrote the paper; Anthony Paul Roskilly contributed analysis tools and revised the paper. Conflicts of Interest: The authors declare no conflict of interest. Energies 2018, 11, 324 13 of 14

References

1. Mikalsen, R.; Roskilly, A.P. A review of free-piston engine history and applications. Appl. Therm. Eng. 2007, 27, 2339–2352. [CrossRef] 2. Zhao, Z.; Zhang, F.; Huang, Y.; Zhao, C. Determination of TDC in a hydraulic free-piston engine by a novel approach. Appl. Therm. Eng. 2014, 70, 524–530. [CrossRef] 3. Jia, B.; Smallbone, A.; Zuo, Z.; Feng, H.; Roskilly, A.P. Design and simulation of a two-or four-stroke free-piston engine generator for range extender applications. Energy Convers. Manag. 2016, 111, 289–298. [CrossRef] 4. Goto, S.; Moriya, K.; Kosaka, H.; Akita, T.; Hotta, Y.; Umeno, T.; Nakakita, K. Development of Free Piston Engine Linear Generator System Part 2—Investigation of Control System for Generator; SAE Technical Paper; SAE International: Detroit, MI, USA, 2014. 5. Tikkanen, S.; Vilenius, M. On the dynamic characteristics of the hydraulic free piston engine. In Proceedings of the ICMA’98: International Conference on Machine Automation, Tampere, Finland, 15–18 September 1998. 6. Achten, P.A.; van den Oever, J.P.; Potma, J.; Vael, G.E. Horsepower with Brains: The Design of the Chiron Free Piston Engine; SAE Technical Paper; SAE International: Milwaukee, WI, USA, 2000. 7. Van Blarigan, P.; Paradiso, N.; Goldsborough, S. Homogeneous Charge Compression Ignition with a Free Piston: A New Approach to Ideal Otto Cycle Performance; SAE Technical Paper; SAE International: San Francisco, CA, USA, 1998. 8. Jia, B.; Mikalsen, R.; Smallbone, A.; Zuo, Z.; Feng, H.; Roskilly, A.P. Piston motion control of a free-piston engine generator: A new approach using cascade control. Appl. Energy 2016, 179, 1166–1175. [CrossRef] 9. Max, E. FPEC, Free piston energy converter. In Proceedings of the 21st Electric Vehicle Symposium & Exhibition, EVS, Monaco, France, 2–6 April 2005. 10. Haag, J.; Kock, F.; Chiodi, M.; Mack, O.; Bargende, M.; Naumann, C.; Slavinskaya, N.; Heron, A.; Riedel, U.; Ferrari, C. Development Approach for the Investigation of Homogeneous Charge Compression Ignition in a Free-Piston Engine. Available online: https://doi.org/10.4271/2013-24-0047 (accessed on 25 January 2018). 11. Kosaka, H.; Akita, T.; Moriya, K.; Goto, S.; Hotta, Y.; Umeno, T.; Nakakita, K. Development of Free Piston Engine Linear Generator System Part 1-Investigation of Fundamental Characteristics. Available online: https://doi.org/10.4271/2014-01-1203 (accessed on 25 January 2018). 12. Kock, F.; Haag, J.; Friedrich, H.E. The Free Piston Linear Generator-Development of an Innovative, Compact, Highly Efﬁcient Range-Extender Module. Available online: https://doi.org/10.4271/2013-01-1727 (accessed on 25 January 2018). 13. Nandkumar, S. Two-Stroke Linear Engine; West Virginia University: Morgantown, WV, USA, 1998. 14. Houdyschell, D. A Diesel Two-Stroke Linear Engine; West Virginia University: Morgantown, WV, USA, 2000. 15. Hansson, J. Analysis and Control of a Hybrid Vehicle Powered by Free-Piston Energy Converter. Ph.D. Thesis, Kungliga Tekniska Hogskolan, Stockholm, Sweden, 2006. 16. Johnson, T.A.; Leick, M.T. Experimental Evaluation of the Free Piston Engine—Linear Alternator (FPLA); Sandia National Laboratories: Albuquerque, NM, USA, 2015. 17. Vysoký, O. Linear Combustion Engine as Main Energy Unit for Hybrid Vehicles; Proceedings of Transtec Prague; Czech Technical University: Prague, Czech Republic, 2007; pp. 236–244. 18. Feng, H.; Guo, C.; Jia, B.; Zuo, Z.; Guo, Y.; Roskilly, T. Research on the intermediate process of a free-piston linear generator from cold start-up to stable operation: Numerical model and experimental results. Energy Convers. Manag. 2016, 122, 153–164. [CrossRef] 19. Miao, Y.; Zuo, Z.; Feng, H.; Guo, C.; Song, Y.; Jia, B.; Guo, Y. Research on the Combustion Characteristics of a Free-Piston Gasoline Engine Linear Generator during the Stable Generating Process. Energies 2016, 9, 655. [CrossRef] 20. Li, Q.; Xiao, J.; Huang, Z. Simulation of a two-stroke free-piston engine for electrical power generation. Energy Fuels 2008, 22, 3443–3449. [CrossRef] 21. Hanipah, M.R. Development of a Spark Ignition Free-Piston Engine Generator. Ph.D. Thesis, Newcastle University, Newcastle upon Tyne, UK, 2015. 22. Jia, B.; Zuo, Z.; Tian, G.; Feng, H.; Roskilly, A.P. Development and validation of a free-piston engine generator numerical model. Energy Convers. Manag. 2015, 91, 333–341. [CrossRef] Energies 2018, 11, 324 14 of 14

23. Mao, J.; Zuo, Z.; Feng, H. Parameters coupling designation of diesel free-piston linear alternator. Appl. Energy 2011, 88, 4577–4589. [CrossRef] 24. Mao, J.; Feng, H.; Zuo, Z. Dimensionless Parametric Analysis of Spark Ignited Free-Piston Linear Alternator. Available online: https://www.intechopen.com/books/thermodynamics-interaction-studies-solids-liquids- and-gases/dimensionless-parametric-analysis-of-spark-ignited-free-piston-linear-alternator (accessed on 25 January 2018). 25. Mao, J.; Zuo, Z.; Li, W.; Feng, H. Multi-dimensional scavenging analysis of a free-piston linear alternator based on numerical simulation. Appl. Energy 2011, 88, 1140–1152. [CrossRef] 26. Goldsborough, S.S.; van Blarigan, P. Optimizing the Scavenging System for a Two-Stroke Cycle, Free Piston Engine for High Efficiency and Low Emissions: A Computational Approach. Available online: https://doi.org/ 10.4271/2003-01-0001 (accessed on 25 January 2018). 27. Aichlmayr, H.T. Design Considerations, Modeling, and Analysis of Micro-Homogeneous Charge Compression Ignition Combustion Free-Piston Engines. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, USA, 2002. 28. Jia, B.; Smallbone, A.; Feng, H.; Tian, G.; Zuo, Z.; Roskilly, A.P. A fast response free-piston engine generator numerical model for control applications. Appl. Energy 2016, 162, 321–329. [CrossRef] 29. Heywood, J.B. Internal Combustion Engine Fundamentals; Mcgraw-hill New York: New York, NY, USA, 1988; Volume 930.

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