The Flex-Oecos—A Novel Optically Accessible Test Rig for the Investigation of Advanced Combustion Processes Under Engine-Like Conditions

energies

Article The Flex-OeCoS—a Novel Optically Accessible Test Rig for the Investigation of Advanced Combustion Processes under Engine-Like Conditions

Bruno Schneider 1,* , Christian Schürch 1, Konstantinos Boulouchos 1, Stefan Herzig 2 , Marc Hangartner 2, David Humair 2, Silas Wüthrich 2, Christoph Gossweiler 2 and Kai Herrmann 2

1 Aerothermochemistry and Combustion Systems Laboratory LAV, Institute of Energy Technology, ETH Zürich, CH-8092 Zürich, Switzerland; [email protected] (C.S.); [email protected] (K.B.) 2 Institute of Thermal and Fluid Engineering (ITFE), School of Engineering (HST), University of Applied Sciences and Arts Northwestern Switzerland (FHNW), CH-5210 Windisch, Switzerland; [email protected] (S.H.); [email protected] (M.H.); [email protected] (D.H.); [email protected] (S.W.); [email protected] (C.G.); [email protected] (K.H.) * Correspondence: [email protected]

Received: 27 February 2020; Accepted: 7 April 2020; Published: 8 April 2020

Abstract: A new test rig has been designed, built and commissioned, and is now jointly pursued to facilitate experimental investigations into advanced combustion processes (i.e., dual fuel, multi-mode) under turbulent conditions at high, engine-like temperature and pressure levels. Based on a standard diesel engine block, it offers much improved optical access to the in-cylinder processes due to its separated and rotated arrangement of the compression volume and combustion chamber, respectively. A fully variable pneumatic valve train and the appropriate preconditioning of the intake air allows it to represent a wide range of engine-like in-cylinder conditions regarding pressures, temperatures and turbulence levels. The modular design of the test rig facilitates easy optimizations of the combustion chamber/cylinder head design regarding different experimental requirements. The name of the new test rig, Flex-OeCoS, denotes its Flexibility regarding Optical engine Combustion diagnostics and/or the development of corresponding Sensing devices and applications. Measurements regarding in-cylinder gas pressures, temperatures and the flow field under typical operating conditions are presented to complete the description and assessment of the new test rig.

Keywords: Research Facilities; Experimental Apparatus; Engine Combustion; Dual Fuel; High Pressure High Temperature; Optical Access; Fine Wire Thermocouples; Pressure Indication; PIV measurements

1. Introduction Over the last years, the emphasis in the development goal for new combustion engines in the field of heavy duty, power generation and marine applications has increasingly shifted from engine efficiency towards the reduction of harmful exhaust gas emissions including CO2. In this context, lean premixed or dual fuel combustion offers an inherent advantage over pure diesel type combustion since a large part of the liquid fuel can be replaced with gaseous, less carbon rich fuels. Further, due to the absence of liquid fuel and/or highly variable fuel–air ratios in the combustion zone, other harmful exhaust gas emissions are reduced as well. Thanks to new, advanced combustion concepts, the thermal efficiency of such engines can reach levels comparable to diesel engines [1]. Dual fuel combustion

Energies 2020, 13, 1794; doi:10.3390/en13071794 www.mdpi.com/journal/energies Energies 2020, 13, 1794 2 of 23 enginesEnergies 2020 also, 13 off, erx FOR a high PEER degree REVIEW of flexibility towards the use of different fuels, including bio-generated2 of 22 fuels (multi fuel) [2,3], and variations of the combustion process itself (multi-mode) [4]. Dual fuel combustion includes processes from both premixed and diffusion type combustion, Dual fuel combustion includes processes from both premixed and diffusion type combustion, the presence of a different fuel type in the premixed cylinder charge also changes the auto ignition the presence of a different fuel type in the premixed cylinder charge also changes the auto ignition process of the injected liquid fuel. A further development of the technology therefore necessitates a process of the injected liquid fuel. A further development of the technology therefore necessitates a deeper understanding of many different and sometimes interlocked fundamental in-cylinder deeper understanding of many different and sometimes interlocked fundamental in-cylinder processes: processes: liquid or gaseous fuel injection, spray breakup, evaporation and mixing, (auto)ignition liquid or gaseous fuel injection, spray breakup, evaporation and mixing, (auto)ignition and flame and flame propagation in the evaporated fuel spray and in the premixed cylinder charge. Apart from propagation in the evaporated fuel spray and in the premixed cylinder charge. Apart from even even more detailed computer simulations, experimental investigations using (laser-)optical more detailed computer simulations, experimental investigations using (laser-)optical measurement measurement techniques are still indispensable tools to understand those complex processes in techniques are still indispensable tools to understand those complex processes in detail. detail. However, on the experimental side in the portfolio of the involved research institutions (Figure1), However, on the experimental side in the portfolio of the involved research institutions (Figure a test rig that could support in-depth (optical) investigations for model validation under engine relevant 1), a test rig that could support in-depth (optical) investigations for model validation under engine conditions was missing so far (as indicated by the empty box). relevant conditions was missing so far (as indicated by the empty box).

Figure 1. Research portfolio of the involved institutions related to the field of combustion engines. Figure 1. Research portfolio of the involved institutions related to the field of combustion engines. The empty box indicates the missing part of the experimental side. The empty box indicates the missing part of the experimental side. Test rigs suitable for this type of investigations are quite challenging: Although excellent optical Test rigs suitable for this type of investigations are quite challenging: Although excellent optical access to the investigated phenomena is very important, it must also be able to withstand the high access to the investigated phenomena is very important, it must also be able to withstand the high pressure and temperature levels. Further, the in-cylinder flow field and turbulence level must be pressure and temperature levels. Further, the in-cylinder flow field and turbulence level must be representative of engine conditions. A high degree of variability in its configuration, and flexibility representative of engine conditions. A high degree of variability in its configuration, and flexibility regarding its operating conditions, are also desirable to allow for investigations into premixed, diffusion regarding its operating conditions, are also desirable to allow for investigations into premixed, or dual/multi fuel combustion processes at the required conditions without the need for major test diffusion or dual/multi fuel combustion processes at the required conditions without the need for rig modifications. major test rig modifications. Therefore, a new experimental facility suitable to fill this gap has been designed and built jointly Therefore, a new experimental facility suitable to fill this gap has been designed and built jointly in the context of a Swiss Competence Center for Energy and Mobility (CCEM-CH) project [5]. in the context of a Swiss Competence Center for Energy and Mobility (CCEM-CH) project [5]. To further clarify the positioning of the new test rig, Figure2 gives an overview of the facilities To further clarify the positioning of the new test rig, Figure 2 gives an overview of the facilities that are in use (or have been used) for experimental investigations in the context of combustion engines that are in use (or have been used) for experimental investigations in the context of combustion at the involved research institutes. Roughly, they can be divided into test rigs more suitable for engines at the involved research institutes. Roughly, they can be divided into test rigs more suitable fundamental investigations, where processes can be separately investigated (to some extent), and test for fundamental investigations, where processes can be separately investigated (to some extent), and rigs that are more representative of real, full metal multi cylinder combustion engines. test rigs that are more representative of real, full metal multi cylinder combustion engines.

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FigureFigure 2.2. TheThe newnew testtest rigrig (Flex-OeCoS)(Flex-OeCoS) inin thethe contextcontext ofof other,other, alreadyalready existingexisting testtest facilitiesfacilities atat thethe involvedinvolved research research institutes. institutes. True True RCMs RCMs (rapid (rapid compression compression machines) machines) that lockthat thelock piston the piston at top at dead top centerdead center position position are not are available. not available.

ConstantConstant volume, volume, high high pressurepressure and and high high temperaturetemperature cells cells such such asas thethe HTDZHTDZ (high(high temperaturetemperature andand highhigh pressurepressure cell)cell) [[6–9],6–9], oror thethe muchmuch largerlarger SCCSCC (spray(spray combustioncombustion chamber)chamber) [[10–13],10–13], areare valuablevaluable tools tools to to investigate investigate fundamental fundamental processes processes related related to to di ffdiffusionusion combustion combustion with with liquid liquid fuel fuel or high-pressureor high-pressure gas gas jets jets [14 [14].]. Experiments Experiments with with premixed premixed or or dual dual fuel fuel combustion combustion however however areare veryvery didifficultfficult due due to to long long residency residency time time of the of ignitable the igni fueltable air fuel mixture air atmixture high temperatures at high temperatures (in electrically (in heatedelectrically cells), heated or because cells), the or pre-combustion because the pre-comb requiredustion to reach required high gas to temperatures reach high consumesgas temperatures all fuel presentconsumes inthe all cellfuel beforepresent the in actualthe cell experiment before the (i.e.,actual liquid experiment fuel injection (i.e., liquid/ignition). fuel injection/ignition). OnOn thethe otherother sideside ofof thethe spectrumspectrum areare thethe (optically(opticallyaccessible) accessible) single singlecylinder cylinder engines. engines.Full Full metalmetal enginesengines (as(as inin [[15–17])15–17]) cancan oofferffer onlyonly limitedlimited (endoscopic)(endoscopic) accessaccess intointo thethe cylinder,cylinder, butbut imposeimpose nono limitslimits regardingregarding cylindercylinder pressurespressures andand temperatures temperatures and and operating operating times times (i.e., (i.e., thermal thermal load). load). WithWith improvingimproving opticaloptical access,access, limitationslimitations regardingregarding peakpeak pressures,pressures, temperaturestemperatures andand thermalthermal loadingloading (number(number of of operating operating cycles) cycles) play play an increasinglyan increasingly important important role [ 18role]. Since [18]. opticalSince optical engines engines are usually are intendedusually intended to operate to in operate conditions in conditions closely approaching closely approaching those of real, those full of metal real, engines,full metal they engines, need large they gasneed exchange large gas valves, exchange which valves, pose which severe pose limitations severe forlimitations the optical for access. the optical access. RCEMsRCEMs (rapid(rapid compressioncompression andand expansionexpansion machines)machines) cancan bebe positionedpositioned somewheresomewhere betweenbetween constantconstant volumevolume cells and optical optical single single cylinder cylinder engines. engines. They They provide provide a single a single compression compression and andexpansion expansion stroke stroke comparable comparable to real to engines real engines and ha andve therefore have therefore no difficulties no diffi toculties work in to premixed work in premixed[19–21] or [ 19dual–21 ]fuel or dualcombustion fuel combustion modes [22,23]. modes The [22 ,23filling]. The and filling emptying and emptying process of process the cylinder of the cylindervolume volumeis decoupled is decoupled from the from fast thecompression fast compression and expansion, and expansion, respectively; respectively; thus, no thus, large no intake large intakeand exhaust and exhaust valves valvesare required are required that limit that the limit optica thel access. optical Usually access. the Usually cylinder the cylinderhead is a headrelatively is a relativelysimple part simple that partcan be that modified can be modified quite easily quite towa easilyrds towardsparticular particular experimental experimental needs. Due needs. to the Due long to thefilling long process, filling process, however, however, almost almostno flow no and flow turbulence and turbulence is present is present in the in thecylinder cylinder at the at the start start of ofcompression. compression. Options Options to to create create a aflow flow field field and and turb turbulenceulence at at TDC TDC (top (top dead center) position exist,exist, butbut its its e effectsffects are are quite quite limited limited [24 [24,25].,25]. True True RCMs RCMs (rapid (rapid compression compression machines), machines), which which do do not not exist exist in thein involvedthe involved research research institutes, institutes, differ fromdiffer RCEMs from byRCEMs locking by the locking piston the at the piston end theat compressionthe end the stroke.compression The fast stroke. compression The fast stroke compression creates a high-temperaturestroke creates a high-temperature and high-pressure and environment high-pressure in a constantenvironment volume in resultinga constant from volume the locked resulting piston. from This the type locked of machine piston. This is mostly type usedof machine for fundamental is mostly studiesused for concerning fundamental ignition studies and concerning combustion ignition processes and ofcombustion various fuel processes types. of various fuel types.

2. Concept of the New Test Rig The new test rig is intended to fill the gap among: (1) constant volume cells (which are not suitable for premixed or dual fuel combustion); (2) rapid compression and expansion machines (which are limited in operating pressure and the achievable in-cylinder flow fields/turbulence levels);

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2. Concept of the New Test Rig The new test rig is intended to fill the gap among: (1) constant volume cells (which are not suitable for premixed or dual fuel combustion); (2) rapid compression and expansion machines (which are limited in operating pressure and the achievable in-cylinder flow fields/turbulence levels); and (3) optically accessible engines (which are limited concerning operating pressures and optical access) in the context of the involved research institutes. The major design goals are as follows:

Excellent optical access to the combustion chamber (“see through” configuration). • Variability: Easy exchangeable cylinder heads that are optimized for specific experimental • requirements (diffusion, premixed and dual fuel/multi fuel combustion). Flexibility: Able to vary operating parameters (i.e., cylinder charge properties at the time of the • experiment) over a wide range. Realize engine-like end of compression pressures/temperatures (>100 bar/ 1000 K) and peak • ≥ combustion pressures (>200 bar). Provide flow velocities and turbulence levels at the time of the experiment comparable to conditions • found in real engines. No continuous operation required (single cycle experiments/limited cycle sequences only). • The following are goals not targeted by the new design:

Processes and properties related to the late combustion phase (e.g., overall engine/combustion • eﬃciency, or exhaust gas emissions)

Initially, the development work was focused on a hydraulically operated RCEM: since such a machine could perform an intake stroke similar to a real engine, it could also create engine-like in-cylinder flows and turbulence levels. Although the evolving design and the corresponding calculations proved the feasibility of such a concept, the necessary hydraulic piston driving system would be quite large and costly. The largest and fastest hydraulic servo-valves available on the market would have been required to control the high volume, high-pressure oil flows quickly enough. Due to the expected costs and the substantial technical risk, the initial design was therefore abandoned in favor of a concept that does not need a large hydraulic apparatus. The new and finally realized concept is based on the simple and well-proven crankshaft/piston combination of a normal combustion engine. Since single-cycle experiments cannot be realized by quickly starting and stopping the crankshaft (which would require no less effort than the full hydraulic solution), the cylinder head is equipped with a fully variable valve train that can change the valve timing at any time. This way the actual experiment cycle can be separated from the preceding gas preparation cycle(s) and the following exhaust cycle(s). Each gas exchange valve is an independent unit which can be positioned anywhere on the cylinder head where it does not block the optical access. This renders the test rig particularly flexible regarding the use of different cylinder head layouts, as shown in the example configurations in Figure3 (the names for the configurations are derived from the “size class” of the observable spray length respectively cylinder/piston bowl dimensions). A change of the test rig configuration only requires the adaption of the intake and exhaust piping to the new valve locations. Energies 2020, 13, 1794 5 of 23 Energies 2020, 13, x FOR PEER REVIEW 5 of 22 Energies 2020, 13, x FOR PEER REVIEW 5 of 22

FigureFigure 3. SketchSketch of of the the new new test test rig rig and and possible possible optical optical cylinder head configurations conﬁgurations (named Figure 3. Sketch of the new test rig and possible optical cylinder head configurations (named accordingaccording to to the the observable fuel fuel spray/gas spray/gas jet leng lengthth “class”). The The “car” “car” type type cylinder cylinder head head has has been been according to the observable fuel spray/gas jet length “class”). The “car” type cylinder head has been realizedrealized initially. initially. realized initially.

TheThe designdesign selected selected for for the the initial initial realization realization is the “car”is the type “car” conﬁguration type configuration where the combustionwhere the The design selected for the initial realization is the “car” type configuration where the combustionchamber size chamber is comparable size is comparable to small car engineto small cylinders car engine and cylinders piston bowl and piston dimensions bowl ofdimensions larger diesel of combustion chamber size is comparable to small car engine cylinders and piston bowl dimensions of largertype car diesel engines. type car The engines. emphasis The in emphasis the design in ofthe this design head of was this primarily head was optical primarily access. optical Therefore, access. larger diesel type car engines. The emphasis in the design of this head was primarily optical access. Therefore,the combustion the combustion chamber (i.e., chamber the cylinder (i.e., the volume cylinder at volume TDC position) at TDCis position) rotated byis rotated 90◦ and by placed 90° and on Therefore, the combustion chamber (i.e., the cylinder volume at TDC position) is rotated by 90° and placedtop of theon top engine of the block. engine This block. way, This the circularway, the combustion circular combustion chamber ischamber almost is completely almost completely optically placed on top of the engine block. This way, the circular combustion chamber is almost completely opticallyaccessible accessible from four from sides. four The drawbacksides. The of drawback this approach of this is thatapproach there is is little that space there available is little forspace the optically accessible from four sides. The drawback of this approach is that there is little space availablegas exchange for the valves. gas exchange However, valves. thanks However, to the fully thanks variable to the valve fully train, variable it is valve possible train, to useit is multiplepossible available for the gas exchange valves. However, thanks to the fully variable valve train, it is possible toengine use multiple cycles to engine prepare cycles or scavenge to prepare the or in-cylinder scavenge gasthe mixture.in-cylinder gas mixture. to use multiple engine cycles to prepare or scavenge the in-cylinder gas mixture.

3.3. Design Design of of the the Flex-OeCoS Flex-OeCoS 3. Design of the Flex-OeCoS TheThe basis basis of of the new test rig shown in Figure 44 isis aa four-cylinderfour-cylinder engineengine blockblock withwith eight-litereight-liter The basis of the new test rig shown in Figure 4 is a four-cylinder engine block with eight-liter displacementdisplacement (D944, (D944, donated donated by by Liebherr Liebherr SA, SA,Bulle, Bulle, Switzerland). Switzerland). Only one Only cylinder one cylinder is used, isand used, the displacement (D944, donated by Liebherr SA, Bulle, Switzerland). Only one cylinder is used, and the otherand thecylinders other cylindersare open to are the open environment to the environment (all pistons (all are pistonsinstalled are to installedpreserve tomass preserve balancing). mass other cylinders are open to the environment (all pistons are installed to preserve mass balancing). Thebalancing). engine Theis driven engine by is an driven electric by anmotor; electric a large motor; flywheel a large ﬂywheelin the connecting in the connecting shaft reduces shaft reduces speed The engine is driven by an electric motor; a large flywheel in the connecting shaft reduces speed oscillations.speed oscillations. oscillations.

Figure 4. An overview of the new test rig: (left) the CAD design and (right) the real setup including Figure 4. An overview ofof thethe newnew test test rig: rig: (left) (left) the the CAD CAD design design and and (right) (right) the the real real setup setup including including all all cables, pipes, pressurized air supply tubes, etc. cables,all cables, pipes, pipes, pressurized pressurized air air supply supply tubes, tubes, etc. etc.

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Combustion chamber: To reach sufficiently high compression pressures and temperatures, the size of the combustion chamber has been fixed at Ø60 mm diameter and 20 mm depth. Connection with the engine cylinder (neck): The neck has the same cross section as the combustion chamber itself (60 mm 20 mm). Since this forms a “dead volume” that reduces the compression Energies 2020, 13, x FOR ×PEER REVIEW 6 of 22 ratio, the length of the neck is kept at a minimum. Two factors limit this length: Firstly, the optical path throughCombustion the circular chamber: and To reach the side sufficiently windows high shouldcompression not pressures be blocked and temperatures, by engine or the engine size head components.of the combustion Secondly, chamber the rim has around been fix theed cylinder at Ø60 mm head diameter neck and must 20 bemm sti depth.ff enough to ensure that the prescribed contactConnection pressures with the engine of the cylinder cylinder (neck): head The gasketneck has is the maintained same cross evensection under as the fullcombustion load. chamber itself (60 mm × 20 mm). Since this forms a “dead volume” that reduces the compression Gas exchange valves: There are four fully variable, pneumatically actuated valves (Ø16 mm). ratio, the length of the neck is kept at a minimum. Two factors limit this length: Firstly, the optical Airpath supply: throughThe the engine circular intake and the air side (pressure windows up sh toould 7 bar) not can be blocked be preheated by engine to temperaturesor engine head> 200 ◦C to reachcomponents. the desired Secondly, end of compressionthe rim around gasthe cylinder pressure head and neck temperature. must be stiff enough to ensure that the Fuelprescribed gas supply: contactFor pressures experiments of the cylinder in the head context gasket of is maintained premixed- even or dualunder/multi-fuel full load. combustion, the (usuallyGas gaseous) exchange valves: fuel canThere be are injected four fully into variable, the airpneumatically supply pipes actuated just valves before (Ø16 the mm). intake valves. The timing ofAir this supply: injection The engine is coupled intake air to (pressure the timing up to of 7 the bar) intake can bevalves; preheated metering to temperatures is done >200 by controlling °C to reach the desired end of compression gas pressure and temperature. the injectionFuel duration gas supply: and For the experiments fuel pressure. in the context of premixed- or dual/multi-fuel combustion, the Thermal(usually conditioning: gaseous) fuelAn can externalbe injected unit into is the used air tosupply control pipes the just water before temperature the intake valves. and flowThe rate in the enginetiming blocks of this cooling injection circuit. is coupled The to water the timing is also of the used intake to heatvalves; and metering cool the is done engine by controlling oil, respectively. Enginethe injection blockmotoring duration and and the drive fuel train:pressure.An electric motor/generator is used to control the engine speed. To reduceThermal speedconditioning: variations, An external a large unit flywheelis used to control (moment the water of inertia temperature 48 kgm and2) isflow mounted rate in in the the engine blocks cooling circuit. The water is also used to heat and cool the engine oil, respectively. shaft connecting the electric motor and the engine block. Engine block motoring and drive train: An electric motor/generator is used to control the engine Enginespeed. piston: To reduceThe speed piston variations, bowl of a large the piston flywheel in (moment the actually of inertia used 48 cylinderkgm2) is mounted is filled in in the to reduce the compressionshaft connecting volume. the electric For safety motor reasons,and the engine valve block. pockets have been machined into the piston; no collision betweenEngine piston: intake The valves piston andbowl theof the piston piston canin the occur actually (the used exhaust cylinder valves is filled are in to far reduce away the from the piston).compression volume. For safety reasons, valve pockets have been machined into the piston; no Controlcollision system: betweenAn intake FPGA valves (field-programmable and the piston can occur gate (the array)-based exhaust valves system are far controls away from gas the exchange piston). valve timing and other crankshaft position dependent signals (liquid and gaseous fuel injection, triggers Control system: An FPGA (field-programmable gate array)-based system controls gas exchange for experimentalvalve timing devices). and other Another,crankshaft PLCposition (programmable dependent signals logic (liquid controller)-based and gaseous fuel system, injection, is used to controltriggers temperatures, for experimental various devices). pressures Another, and PLC the electric(programmable motor. logic It also controller)-based monitors all system, safety is relevant signalsused and shutsto control down temperatures, the test rig various in case pressures of an error. and the electric motor. It also monitors all safety Compressionrelevant signals volume: and shutsFigure down5 shows the test the rig dimensionsin case of an error. of the in-cylinder and combustion chamber volume of theCompression currently volume: realized Figure cylinder 5 shows the head dimensions configuration. of the in-cylinder The total and end combustion of compression chamber volume volume of the currently3 realized cylinder head configuration. The total end of compression volume amountsamounts to 155,600 to 155,600 mm mm, which3, which results results in in a a compressioncompression ratio ratio of of13.8. 13.8.

FigureFigure 5. Dimensions 5. Dimensions of theof the internal internal volume of ofthe the opti opticalcal cylinder cylinder head (at head TDC (atpiston TDC position). piston For position). For thethe base base conﬁguration, configuration, thethe total internal internal volume volume amounts amounts to 155,600 to 155,600 mm3 (compression mm3 (compression ratio 13.8). ratio 13.8).

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Energies 2020, 13, x FOR PEER REVIEW 7 of 22 3.1. Gas Exchange Valves 3.1. Gas Exchange Valves The space available for the gas exchange valves is quite limited since the emphasis in the design was putThe onspace optimal available optical for access. the gas There exchange are two valves intake is valvesquite limited (Ø16 mm) since going the emphasis directly into in the the design engine wascylinder, put on and optimal two exhaust optical valves access. (Ø16 There mm asare well) two sitting intake on valves top of (Ø16 the combustion mm) going chamber, directly asinto shown the enginein Figure cylinder,6. and two exhaust valves (Ø16 mm as well) sitting on top of the combustion chamber, as shown in Figure 6.

FigureFigure 6. 6. TheThe pneumatically pneumatically operated operated intake intake and and exhaus exhaustt valves. valves. Multiple, Multiple, fast fast switching switching pneumatic pneumatic valvesvalves are used for each cylinder to open and close the gas exchange valves as quickly as possible.

PneumaticPneumatic cylinders are usedused toto openopen thethe valves; valves; adjustable adjustable pneumatic pneumatic springs springs close close the the valve valve at atthe the end end of theof gasthe exchange.gas exchange. To realize To realize faster andfaster better and controllable better controllable valve closing valve times, closing the pneumatictimes, the pneumaticpistons are pistons actively are pushed actively back pushed as well. back Throttle as well valves. Throttle and valves end position and end damping position in damping the pneumatic in the pneumaticcylinders ensures cylinders smooth ensures valve smooth closing valve and closing prevents an hardd prevents impacts hard of the impacts valves of on the the valves valve seats.on the valveThe seats. gas exchange valves are not intended to operate as real engine valves; they open and close as quicklyThe as gas possible exchange to maximizevalves are the not achievable intended to gas oper flowate rates as real (trapezoidal engine valves; valve they lift curves;open and opening close astime quickly about 5as ms; possible closing to time maximize about 8 the ms). achievable For this, each gas engine-valve flow rates (trapezoidal actuation cylinder valve islift equipped curves; openingwith multiple time about fast, high5 ms; flow closing rate time pneumatic about 8 valves ms). For (Bibus this, Matrix each engine-valve series 750). Eachactuation engine-valve cylinder is is equippedequipped with amultiple valve lift fast, sensor; high those flow signalsrate pneumatic are used valves in the control (Bibus systemMatrix toseries monitor 750). e Eachffective engine- valve valveopening is equipped and closing with times. a valve The valveslift sensor; can bethose opened signals at any are time used during in the enginecontrol cycles; system valve to monitor pockets effectivein the piston valve prevent opening collisions and closing between times. the The intake valves valves can be and opened the piston. at any time during engine cycles; valveThe pockets gas flowin the through piston prevent the engines collisions gasexchange between the valves intake is veryvalves fast and and the highly piston. turbulent. To preventThe highgas flow temperature through lossesthe engines of the preheatedgas exchange intake valves air-gas is very mixture fast passingand highly the turbulent. engine intake To preventvalves, theyhigh can temperature be electrically losses heated of the to preheated up to 200 ◦inC.take air-gas mixture passing the engine intake valves, they can be electrically heated to up to 200 °C. 3.2. Optical Access, Experimental Configurations 3.2 OpticalDepending Access, on Experimental the location Configurations of the injection (or ignition) source, two different basic configurations are used (see Figure7). In the first configuration (“C” for central position), one of the circular windows Depending on the location of the injection (or ignition) source, two different basic configurations is replaced with a metal holder to mount the injection and ignition source. A special holder with an are used (see Figure 7). In the first configuration (“C” for central position), one of the circular integrated high temperature mirror allows the use of the double pass Schlieren imaging technique in windows is replaced with a metal holder to mount the injection and ignition source. A special holder this configuration. The second basic configuration (“T” for top position), with injection and ignition with an integrated high temperature mirror allows the use of the double pass Schlieren imaging from the top, allows full, “see through” optical access to the combustion chamber through the large, technique in this configuration. The second basic configuration (“T” for top position), with injection andcircular ignition windows from and the thetop, smaller allows rectangular full, “see windowsthrough” onoptical the side. access to the combustion chamber through the large, circular windows and the smaller rectangular windows on the side.

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Figure 7. Figure 7: The two basicbasic configurationsconfigurations ofof thethe testtest rig:rig: (left) thethe setupsetup withwith aa centralcentral multi-holemulti-hole injector/prechamber/ignition source mounted in place of one of the circular windows (configuration injector/prechamber/ignition source mounted in place of one of the circular windows (configuration “C”); and (right) the configuration with the injection/prechamber/ignition source mounted on top of “C”); and (right) the configuration with the injection/prechamber/ignition source mounted on top of the cylinder head. This configuration provides “see through” optical access from two directions and the cylinder head. This configuration provides “see through” optical access from two directions and maximizes the observable fuel spray/gas-jet length. maximizes the observable fuel spray/gas-jet length. 3.3. Standard Instrumentation 3.3. Standard Instrumentation The standard equipment used for pressure measurements in all experimental campaigns consists of anThe M8 piezostandard electric equipment sensor (Kistler used for 6044 pressure anti-strain me type)asurements mounted in inall the experimental engine cylinder, campaigns an M5 piezoconsists electric of an sensorM8 piezo (Kistler electric 6052CU40) sensor (Kistler mounted 6044 inanti-strai the opticallyn type) accessiblemounted in combustion the engine chambercylinder, andan M5 two piezo M14 piezoelectric resistive sensor pressure (Kistler sensors6052CU40) (Kistler mounted 4049B) in mounted the optically before accessible the two intake combustion valves (seechamber also Section and two 4.1 M14). piezo resistive pressure sensors (Kistler 4049B) mounted before the two intake valvesThe (see intake also air–gassection mixture4.1). temperature is measured with standard thermocouples (K type, Ø 1 mm, externalThe intake junction) air–gas in mixture the air stream; temperature the intake is meas air massured flowwith is standard determined thermocouples with a Coriolis (K masstype, flowmeterØ 1 mm, external (type Endress junction) & in Hauser the air Promass stream; 80Fthe inta DN15).ke air To mass isolate flow the is dete massrmined flow meter with a from Coriolis the largemass pressureflowmeter fluctuations (type Endress in the & intakeHauser air Promass piping system,80F DN15). it is To installed isolate in the front mass of flow a large meter pressure from oscillation-dampingthe large pressure fluctuations vessel. Although in the intake this arrangement air piping system, greatly it reducesis installed the in mass front flow of a fluctuations large pressure in theoscillation-damping flow meter, it also ve requiresssel. Although a long time this (typically arrangement 100 cycles) greatly to reduces reach stable the mass conditions flow fluctuations necessary for in accuratethe flow measurements.meter, it also requires a long time (typically 100 cycles) to reach stable conditions necessary for accurate measurements. The crankshaft position is measured with a photo sensor and a slotted disk (1200 slits = 0.3◦ resolution)The crankshaft mounted onposition the engine’s is measured flywheel. with The a FPGA photo control sensor system and a usesslotted a PLL disk (phase (1200 locked slits = loop) 0.3° resolution) mounted on the engine’s flywheel. The FPGA control system uses a PLL (phase locked to generate a crankshaft position signal with 0.1◦ resolution from the optical angle encoder signals. Thisloop) crank to generate angle resolution a crankshaft is used position for all signal crank anglewith 0.1° related resolution control signalsfrom the and optical measurements. angle encoder signals. This crank angle resolution is used for all crank angle related control signals and 3.4.measurements. Technical Data of the Flex-OeCoS Table1 presents the technical data of the base engine block used for the Flex-OeCoS. Three of the 3.4 Technical Data of the Flex-OeCoS four Liebherr D944 engine cylinders are closed off, only one cylinder is in use. Table 1 presents the technical data of the base engine block used for the Flex-OeCoS. Three of the four Liebherr D944 engine cylinders are closed off, only one cylinder is in use.

Table 1. Engine block data.

Dimension, Operating Parameter Value Cylinder bore Ø130 mm Stroke length 150 mm Connecting rod length 237.1 mm

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Table 1. Engine block data.

Dimension, Operating Parameter Value Cylinder bore Ø130 mm Stroke length 150 mm Connecting rod length 237.1 mm Displacement per cylinder 1990 cm3 Gap height 1.25 mm 1 Max. engine speed 1800 rpm 2 Max. cylinder pressure 230 bar 3 Operating temperature of water and oil 80 ◦C 1 Measured at TDC from piston top to the bottom of the head. 2 Normal operating range: 300–1200 rpm. 3 Controlled with an external unit.

The key component of the new test rig is the easily exchangeable and adaptable cylinder head/combustion chamber. Initially, the “Car” type cylinder head has been realized; the corresponding technical data are summarized in Table2.

Table 2. Cylinder head data (current “Car” conﬁguration).

Dimension, Operating Parameter Value Chamber bore diameter Ø60 mm Chamber and neck depth 20 mm Neck width 60 mm Neck length 45 mm 1 Cylinder head heating temperature: 200 C 2 ≤ Total Internal volume: 155,600 mm3 Compression ratio: 13.8 1 Measured from the circular combustion chamber to the bottom of the head. 2 Electrical heating, which can be switched oﬀ during experiments.

The test rig is supplied with externally compressed air that is electrically heated in the two 3-m-long intake pipes leading to the two intake valves. The key parameters of the air supply system and the gas exchange valves are presented in Table3.

Table 3. Air supply and gas exchange valve data.

Component, Operating Parameter Value Engine valves 2 intake, 2 exhaust valves Ø16 mm × × Engine valve operation Pneumatic valve actors, fully variable timing Valve lift curve Trapezoid, fully open in 5 ms, closed in 8 ms ≈ ≈ Intake pressure range p 7 bar ≤ Intake air temperature range >200 ◦C Intake valve heating temperature 200 C ≤ ◦

3.5. Structure of Experiment Sequences As mentioned above, multiple intake cycles are required to prepare the desired gas mixture in the cylinder prior to the actual experiment cycle(s). Usually multiple exhaust cycles are required to completely empty/ﬂush the cylinder volume as well to reach reproducible conditions in all experiment cycles. As shown in Figure8, up to ﬁve consecutive sequence types can be used in experiments:

Each sequence type has its own, separate gas exchange valve timing and experimental • trigger setting. Each sequence type can be repeated several times if desired. • Energies 2020, 13, 1794 10 of 23

In some sequence types, the valves for adding gaseous fuels into the air intake can be activated • as well. More trigger outputs are available in the actual experiment sequence type to control measurement • devices and, if needed, fuel injection and/or ignition systems. Energies 2020, 13, x FOR PEER REVIEW 10 of 22

Figure 8. Basic structure of experiment cycles in the Flex-OeCoS. Figure 8: Basic structure of experiment cycles in the Flex-OeCoS. The complete cycle of the consecutive sequences can be repeated several times if desired. However, The complete cycle of the consecutive sequences can be repeated several times if desired. the following possibly limiting factors must be considered: However, the following possibly limiting factors must be considered: Cylinder- and combustion chamber wall temperatures: Due to the inflow of preheated intake air Cylinder- and combustion chamber wall temperatures: Due to the inflow of preheated intake air and and the gas temperature increase from compression and (in most experiments) combustion, the wall the gas temperature increase from compression and (in most experiments) combustion, the wall temperature in the combustion chamber and engine cylinder will rise over time (so far there is no temperature in the combustion chamber and engine cylinder will rise over time (so far there is no active cooling of the cylinder head). To slow down this temperature rise, the electrical cylinder head active cooling of the cylinder head). To slow down this temperature rise, the electrical cylinder head heating can be switched off during the experiment cycles. Further, the compression ratio in the gas heating can be switched off during the experiment cycles. Further, the compression ratio in the gas exchange sequences can be reduced with an early opening of the exhaust valves. exchange sequences can be reduced with an early opening of the exhaust valves. Stable pressure levels and air mass flow: To achieve stable, repeatable conditions and a constant air Stable pressure levels and air mass flow: To achieve stable, repeatable conditions and a constant air mass flow during the experiments cycle repetitions, it is necessary to optimize valve timing in the mass flow during the experiments cycle repetitions, it is necessary to optimize valve timing in the gas gas exchange sequences to reach the same air mass flow. This way a stable pressure distribution can exchange sequences to reach the same air mass flow. This way a stable pressure distribution can establish itself in the intake piping system. establish itself in the intake piping system. Minimum cylinder pressure: The pressure in the cylinder must never fall below ambient pressure Minimum cylinder pressure: The pressure in the cylinder must never fall below ambient pressure during the operation of the engine to prevent lube oil intrusion into the cylinder/combustion chamber. during the operation of the engine to prevent lube oil intrusion into the cylinder/combustion Initial in-situ analysis: After an experiment cycle has finished, the control system will immediately chamber. present a graphical overview of important experimental parameters such as mass flows, cylinder Initial in-situ analysis: After an experiment cycle has finished, the control system will immediately pressures, peak pressures (i.e., pressure and corresponding crank angle), wall temperatures and present a graphical overview of important experimental parameters such as mass flows, cylinder calculated approximate ignition delays of all recorded cycles. The operator thus immediately receives pressures, peak pressures (i.e., pressure and corresponding crank angle), wall temperatures and a feedback that allows him to check if the actual experimental conditions have been correct, and how calculated approximate ignition delays of all recorded cycles. The operator thus immediately receives the conditions changed in the course of the measurement. a feedback that allows him to check if the actual experimental conditions have been correct, and how 4.the Characterization conditions changed of the inFlex-OeCoS the course of Test the Rigmeasurement.

4. CharacterizationIt is important to of characterize the Flex-OeCoS a new Test test Rig rig after its commissioning: Firstly, it must be ensured that its desired performance regarding experimental conditions (pressures, temperatures, flow, and reproducibility)It is important to can characterize be achieved. a new Secondly, test rig af itter is neededits commissioning: to obtain the Firstly, test rigit must settings be ensured (valve timings,that its desired sequence performance repetitions, regarding wall/air heatingexperimental temperatures, conditions etc.) (pressures, required temperatures, to realize the flow, actually and desiredreproducibility) experimental can be conditions achieved. (pressures, Secondly, temperatures,it is needed to flow).obtain Thirdly,the test rig a database settings can(valve be createdtimings, thatsequence contains repetitions, all measured wall/air quantities heating for temperatures, different settings etc.) ofrequired the desired to realize experimental the actually conditions. desired Thisexperimental database canconditions be used (pressures, in future projectstemperatures, to validate flow). and Thirdly, tune models a database in accompanying can be created CRFD that simulationscontains all in measured the test rig. quantities for different settings of the desired experimental conditions. This database can be used in future projects to validate and tune models in accompanying CRFD simulations in the test rig. The following sections present the results of the assessment of the test rig regarding cylinder and combustion chamber pressures, measurements of the actual gas temperatures in the combustion chamber using fine wire thermocouples and, lastly, measurements of the flow field in the combustion chamber using 2D high speed PIV (particle image velocimetry). Unless noted otherwise, the standard settings, as shown in Table 4, are used for the measurements presented here.

Table 4. Standard measurement conditions. Combustion TDC defined at 0° CA (crank angle).

Energies 2020, 13, 1794 11 of 23

The following sections present the results of the assessment of the test rig regarding cylinder and combustion chamber pressures, measurements of the actual gas temperatures in the combustion chamber using fine wire thermocouples and, lastly, measurements of the flow field in the combustion chamber using 2D high speed PIV (particle image velocimetry). Unless noted otherwise, the standard settings, as shown in Table4, are used for the measurements presented here.

Table 4. Standard measurement conditions. Combustion TDC deﬁned at 0◦ CA (crank angle).

Operating Parameter Value Engine speed (motored) 600 rpm Peak compression pressure 100 bar 1 Intake valve timing Open –370◦ CA, close –180◦ CA Exhaust valve timing Open 180◦ CA, close –370◦ CA Cylinder head/intake valve temperature 150 ◦C 2 Intake manifold air temperature 150 ◦C 1 Intake pressure adjusted accordingly. 2 Target peak compression temperature 1000 K.

Starting from those standard settings, measurements with a variation of engine speeds, peak compression pressures and peak compression temperatures have been carried out. Due to small modiﬁcations in the ongoing test rig commissioning process, the actually used intake manifold air pressure and temperature varied slightly over time in these initial measurement campaigns.

4.1. Cylinder Pressure Measurements Measurements of the cylinder pressure form the basis of heat release calculations and other combustion diagnostics regarding variability, knock detection, etc. Therefore, it is important to achieve a maximum accuracy in all parts of the pressure measurement setup. This applies even more to the new test rig, since it will also be used for the further development of pressure sensing devices and related equipment. The in-cylinder volume of the Flex-OeCoS has a quite uncommon configuration: the optically accessible combustion chamber on top is attached to the actual engine cylinder as an oversized prechamber. This leads to a two-stage combustion under premixed/dual-fuel conditions since the fuel in the actual engine cylinder only starts to burn after the flame has passed through the “neck” that connects the two volumes. Pressure compensation between the two volumes is not instantaneous; therefore, both pressures must be measured individually. The cylinder head of the Flex-OeCoS has mounting holes for up to four M 5 piezoelectric pressure transducers in the optically accessible combustion chamber and two M 8 sensors in the engine cylinder. For experiments where the optical accessibility is less important than pressure signals, the radial and/or axial windows can be replaced with inserts that provide space for up to six additional pressure sensors per insert. In Figure9a, the inside volume of the Flex-OeCoS is shown with pressure sensors mounted in all standard locations in the cylinder head and in two sensor inserts mounted instead of windows (piston position 20 mm before TDC). A specially designed, thermal-oil cooled sensor adapter for use in the Flex-OeCoS test rig is shown in Figure9b. The heat fins and cooling path arrangement optimize heat transfer and heat distribution in the adapter, respectively, aiming for a uniform sensor temperature during experiments. Due to its complicated shape, it is manufactured with the selective laser melting technique. A subsequent heat treatment increases the mechanical strength of the insert and reduces the strain on the package. Thanks to the wide operating temperature range of the thermal oil that is used as a coolant, it is possible to realize both positive and negative heat transfers from the insert and pressure sensor to and from the gas in the combustion chamber, respectively. Energies 2020, 13, x FOR PEER REVIEW 11 of 22

Operating Parameter Value Engine speed (motored) 600 rpm Peak compression pressure 100 bar 1 Intake valve timing Open –370° CA, close –180° CA Exhaust valve timing Open 180° CA, close –370° CA Cylinder head/intake valve temperature 150 °C Intake manifold air temperature 150 °C 2 1 Intake pressure adjusted accordingly. 2 Target peak compression temperature 1000 K. Starting from those standard settings, measurements with a variation of engine speeds, peak compression pressures and peak compression temperatures have been carried out. Due to small modifications in the ongoing test rig commissioning process, the actually used intake manifold air pressure and temperature varied slightly over time in these initial measurement campaigns.

Figure 9. (a) The inside (gas) volume of the engine cylinder (piston 20 mm before TDC) and the combustion chamber with pressure sensors mounted in all standard locations plus two inserts. (b) Window insert for a single, thermally especially well stabilized pressure sensor.

Pegging of the Cylinder Pressure Piezo-electric pressure sensors, as are normally used to measure cylinder pressures, must be tied to a known absolute pressure to obtain the effective in-cylinder pressure. The most common pegging methods are either based on a thermodynamic calculation assuming a fixed polytropic exponent within a part of the engine cycle, or based on the assumption that the cylinder pressure is equal to the intake manifold pressure at a certain point during the engine cycle [26]. In the case of the Flex-OeCoS, the first method is difficult to apply due to the different wall-heat transfer regimes in the engine cylinder, the optically accessible combustion chamber and the connecting “neck” in-between. The second method cannot be applied easily as well because the gas exchange valves are very small compared to normal engine valves, thus the two pressures are never equal during a cycle. To develop the appropriate pegging method for the new test rig, an experimental campaign was carried out that used an additional high fidelity, absolute pressure sensor mounted in the optically accessible combustion chamber to measure the effective chamber pressure for different engine speeds and intake with respect to peak compression pressures. It was found that, within a certain crank angle range near BDC (bottom dead center), there is a consistent linear relation between intake manifold pressure and chamber pressure. Thus, in future measurement campaigns, tabulated coefficients can be used to peg the cylinder pressure to the absolute intake manifold pressure in the defined crank angle range. The set of coefficients depend on both engine speed and valve timings. Through comparisons with measurements using reference pressure sensors, it is possible to estimate the uncertainty of the measurements when using the standard instrumentation only. At a typical load point, as shown in Figure 10, the estimated maximum measurement error of the cylinder with respect to combustion chamber pressure is in the order of 30 mbar during the gas exchange phases, 50 mbar during the compression and expansion stroke (without combustion) and 100 mbar during the combustion phase. Energies 2020, 13, x FOR PEER REVIEW 12 of 22

Figure 9: (a) The inside (gas) volume of the engine cylinder (piston 20 mm before TDC) and the combustion chamber with pressure sensors mounted in all standard locations plus two inserts. (b) Window insert for a single, thermally especially well stabilized pressure sensor.

A specially designed, thermal-oil cooled sensor adapter for use in the Flex-OeCoS test rig is shown in Figure 9b. The heat fins and cooling path arrangement optimize heat transfer and heat distribution in the adapter, respectively, aiming for a uniform sensor temperature during experiments. Due to its complicated shape, it is manufactured with the selective laser melting technique. A subsequent heat treatment increases the mechanical strength of the insert and reduces the strain on the package. Thanks to the wide operating temperature range of the thermal oil that is used as a coolant, it is possible to realize both positive and negative heat transfers from the insert and pressure sensor to and from the gas in the combustion chamber, respectively.

4.1.1. Pegging of the Cylinder Pressure Piezo-electric pressure sensors, as are normally used to measure cylinder pressures, must be tied to a known absolute pressure to obtain the effective in-cylinder pressure. The most common pegging methods are either based on a thermodynamic calculation assuming a fixed polytropic exponent within a part of the engine cycle, or based on the assumption that the cylinder pressure is equal to the intake manifold pressure at a certain point during the engine cycle [26]. In the case of the Flex- OeCoS, the first method is difficult to apply due to the different wall-heat transfer regimes in the engine cylinder, the optically accessible combustion chamber and the connecting “neck” in-between. The second method cannot be applied easily as well because the gas exchange valves are very small compared to normal engine valves, thus the two pressures are never equal during a cycle. To develop the appropriate pegging method for the new test rig, an experimental campaign was carried out that used an additional high fidelity, absolute pressure sensor mounted in the optically accessible combustion chamber to measure the effective chamber pressure for different engine speeds and intake with respect to peak compression pressures. It was found that, within a certain crank angle range near BDC (bottom dead center), there is a consistent linear relation between intake manifold pressure and chamber pressure. Thus, in future measurement campaigns, tabulated coefficients can be used to peg the cylinder pressure to the absolute intake manifold pressure in the defined crank angle range. The set of coefficients depend on both engine speed and valve timings. Through comparisons with measurements using reference pressure sensors, it is possible to estimate the uncertainty of the measurements when using the standard instrumentation only. At a typical load point, as shown in Figure 10, the estimated maximum measurement error of the cylinder with respect to combustion chamber pressure is in the order of 30 mbar during the gas exchange Energiesphases,2020 50, 13mbar, 1794 during the compression and expansion stroke (without combustion) and 10013 mbar of 23 during the combustion phase.

Figure 10. Pressure traces and valve lift curves for an experiment with combustion (400 rpm, premixed

fuel methane λ = 2, pilot injection with dodecane, peak combustion pressure 140 bar). In the blue

colored area (around –190◦ CA), the cylinder pressure can be pegged to the absolute intake pressure with the help of tabulated coeﬃcients.

4.2. Gas Temperature Measurements Since direct measurements are difficult, the actual gas temperature in combustion engines is usually calculated with the help of heat release calculations. For well-known standard engine configurations, this works quite well, but, for the new test rig with its two large, separate combustion chambers, it is rather more difficult. Therefore, to gain better knowledge about the effective gas conditions inside the combustion chamber of the Flex-OeCoS, detailed temperature measurements have been carried out [27]. These measurements also form a good basis for the development of the heat release calculation code intended to be used for the new test rig. Tracking the rapid changes of the working gas temperature inside internal combustion engines requires robust sensors with very low response times. Only TCs (thermocouples) consisting of very thin sensor wires ( 25 µm) with exposed welding beads are fast enough to keep up with the rapid changes ≤ of the gas temperature inside the cylinder. Owing to the fragility of those sensors, all measurements were carried out without combustion. Due to the heat transfer processes between the gas and the thermocouple, the measurements have a certain response time (lag) relative to the actual gas temperature change. Especially in very unsteady environments as in the cylinder and combustion chamber of IC engines, this can lead to highly delayed and inaccurate sensor readings—as illustrated by the Ø0.5 mm TC temperature measurement shown in Figure 11. In principle, the response time of a thermocouple measurement can be described by a first-order differential equation: Tg T = τ ∂T∂t, where Tg is the actual gas temperature, T is the thermocouple − · temperature and τ is the thermocouple time constant. However, the time constant τ is not just a fixed property of the thermocouple but depends heavily on the gas state and gas flow around it as well. To overcome this, multiple techniques for thermocouple response time estimations have been proposed in the literature. All are based on the simultaneous measurement with two thermocouples that have different wire diameters but are positioned in close proximity, so they are exposed to the same gas state and flow condition. For the measurements presented here, the best results were found using a technique based on [28,29]. Figure 11 shows a comparison between the measured thermocouple temperatures (TC diameters 12.5 µm and 25 µm) and the reconstructed actual gas temperature (measurement position in the center of the combustion chamber). Energies 2020, 13, x FOR PEER REVIEW 13 of 22

Figure 10. Pressure traces and valve lift curves for an experiment with combustion (400 rpm, premixed fuel methane λ = 2, pilot injection with dodecane, peak combustion pressure 140 bar). In the blue colored area (around –190° CA), the cylinder pressure can be pegged to the absolute intake pressure with the help of tabulated coefficients.

4.2. Gas Temperature Measurements Since direct measurements are difficult, the actual gas temperature in combustion engines is usually calculated with the help of heat release calculations. For well-known standard engine configurations, this works quite well, but, for the new test rig with its two large, separate combustion chambers, it is rather more difficult. Therefore, to gain better knowledge about the effective gas conditions inside the combustion chamber of the Flex-OeCoS, detailed temperature measurements have been carried out [27]. These measurements also form a good basis for the development of the heat release calculation code intended to be used for the new test rig. Tracking the rapid changes of the working gas temperature inside internal combustion engines requires robust sensors with very low response times. Only TCs (thermocouples) consisting of very thin sensor wires (≤25 µm) with exposed welding beads are fast enough to keep up with the rapid changes of the gas temperature inside the cylinder. Owing to the fragility of those sensors, all measurements were carried out without combustion. Due to the heat transfer processes between the gas and the thermocouple, the measurements have a certain response time (lag) relative to the actual gas temperature change. Especially in very unsteady environments as in the cylinder and combustion chamber of IC engines, this can lead to Energieshighly 2020delayed, 13, 1794 and inaccurate sensor readings—as illustrated by the Ø0.5 mm TC temperature14 of 23 measurement shown in Figure 11.

Energies 2020, 13, x FOR PEER REVIEW 14 of 22 measured thermocouple temperatures (TC diameters 12.5 µm and 25 µm) and the reconstructed actual gas temperature (measurement position in the center of the combustion chamber).

The new test rig is highly flexible in multiple respects: gas conditions in the cylinder and combustionFigure 11.11: chamber Traces ofregarding the measured temperature, temperatures pressure, (thermocouples(thermocouples and flow characteristicsØ12.5 µmµm and Ø25can µµm)bem) varied and the over a widereconstructed range by adjusting actualactual gasintakegas temperature.temperature. manifold pressure,For For compar comparison, temperature,ison, the the temperature temperature (gaseous) that that fuel is content, measured cylinder with aa head heating,common valve Ø0.5Ø0.5 timing, mmmm sheathedsheathedand engine thermocouplethermocouple speed. This isis shownshown allows asas optimizing wellwell (blue(blue dotteddotted the experimental line).line). environment for investigations using various fuel types, fuel delivery to ignition systems, and combustion processes. The new test rig is highly flexible in multiple respects: gas conditions in the cylinder and FigureIn principle, 12 shows the response the measured time of (nota thermocoup reconstrucleted) measurement gas temperatures can be described (Ø12.5 µm by aTC) first-order in the combustion chamber regarding temperature, pressure, and flow characteristics can be varied over a combustiondifferential chamberequation: at 𝑇 two −𝑇=𝜏∙𝜕𝑇⁄𝜕𝑡 locations for a, variatiowhere nT ofg isthe the intake actual air preheatinggas temperature, temperature. T is Thethe wide range by adjusting intake manifold pressure, temperature, (gaseous) fuel content, cylinder head temperaturethermocouple at temperature the position nearand τthe is thetop thermocouple(Pos 1, 26 mm timefrom constant.the center) However, is about 40the K time lower constant compared τ is heating, valve timing, and engine speed. This allows optimizing the experimental environment for tonot the just center a fixed of the property combustion of the chamber thermocouple (Pos Z), bu bot thdepends located heavilyin the middle on the plane gas state(between and thegas largeflow investigations using various fuel types, fuel delivery to ignition systems, and combustion processes. circulararound itwindows). as well. To Since overcome the end-of-compressionthis, multiple techniques pressure for thermocouple was kept at response100 bar timein all estimations cases (by Figure 12 shows the measured (not reconstructed) gas temperatures (Ø12.5 µm TC) in the adaptinghave been the proposed intake air in pressure), the literature. this temperature All are based variation on the issimultaneous also a density measurement variation. Since with intake two combustion chamber at two locations for a variation of the intake air preheating temperature. pressurethermocouples and temperature that have different can be varied wire diametersindependentl but y,are constant positioned density in close cond proximity,itions at different so they gasare The temperature at the position near the top (Pos 1, 26 mm from the center) is about 40 K lower temperaturesexposed to the can same be realized gas state as and well. flow condition. For the measurements presented here, the best compared to the center of the combustion chamber (Pos Z), both located in the middle plane (between resultsThe were reconstructed found using temperature a technique in based the center on [2 8,29].of the Figure combustion 11 shows chamber a comparison allows definingbetween the the large circular windows). Since the end-of-compression pressure was kept at 100 bar in all cases (by temperature operation point in experiments. Still ongoing, however, is the analysis of a full series of adapting the intake air pressure), this temperature variation is also a density variation. Since intake measurements that are required to obtain a full spatial temperature distribution field in the pressure and temperature can be varied independently, constant density conditions at different gas combustion chamber. This will benefit both further experimental campaigns and the accompanying temperatures can be realized as well. CRFD simulations.

Figure 12.12: Measurement of the gas temperature (with a single Ø12.5 µmµm TC) at two locations inside the combustion chamber. The variation of the preheated intakeintake airair temperaturetemperature TTinin results in a density density variation, since the peak compression pressurepressure waswas keptkept atat 100100 barbar forfor allall cases.cases.

Another field of interest closely related to accurate in-cylinder gas temperature measurements is the delicate matter of wall heat fluxes, as well as the measurement thereof. By using near-wall measurements of the actual gas temperature, combined with fast and precise wall temperature measurements, it is possible to calculate the local heat flux to and from the wall with the “Zero Crossing Method” (German: “Nulldurchgangsmethode” [30]). Detailed studies are currently under way to measure the wall heat fluxes in the Flex-OeCoS test rig with this method and then, in combination with the PIV flow field measurements, to develop appropriate wall heat transfer models that can be used in heat release calculations and CFD simulations of the new test rig.

4.3. Flow Field Measurements using PIV Due to its rotated orientation in the new test rig, the flow field inside the combustion chamber is different from the in-cylinder flow fields in normal engines. Swirl in the engine cylinder would become tumble in the combustion chamber, and vice versa. However, large structures break up while

Energies 2020, 13, 1794 15 of 23

The reconstructed temperature in the center of the combustion chamber allows defining the temperature operation point in experiments. Still ongoing, however, is the analysis of a full series of measurements that are required to obtain a full spatial temperature distribution field in the combustion chamber. This will benefit both further experimental campaigns and the accompanying CRFD simulations. EnergiesAnother 2020, 13 field, x FOR of PEER interest REVIEW closely related to accurate in-cylinder gas temperature measurements15 of 22 is the delicate matter of wall heat fluxes, as well as the measurement thereof. By using near-wall measurementspassing through of the the “neck” actual that gas connects temperature, the two combined volumes with (except fast if and they precise are aligned wall with temperature the long measurements,side of the “neck”). it is possible Therefore, to calculate the flow thein the local comb heatustion flux to chamber and from is the quite wall different with the from “Zero and Crossing largely Method”independent (German: of the flow “Nulldurchgangsmethode” field in the actual engine [30 cylinder.]). Detailed studies are currently under way to measureSince the good wall knowledge heat fluxes about in the the Flex-OeCoS flow field in test the rig combustion with this methodchamber and is important, then, in combination a high speed withPIV themeasurement PIV flow field campaign measurements, was carried to develop to assess appropriate flow-field, wall flow-structures, heat transfer modelsflow-velocities that can and be usedturbulence in heat levels release for calculations a variation and of CFD engine simulations speeds ofand the intake new test and rig. therefore peak compression pressure levels [31]. All experiments have been carried out without combustion. An overview of the 4.3.experimental Flow Field Measurementssetup is shown using in Figure PIV 13. DueLaser to sheet: its rotated The laser orientation from a dual in the cavity, new testfrequency rig, the doubled flow field Nd:YAG inside thelaser combustion (Photonic Industries chamber isDM60 different 532 DH, from 6 themJ/pulse, in-cylinder 200 ns flow pulse fields width) in normalis transformed engines. into Swirl a light in thesheet engine (45 mm cylinder wide, would0.5 mm becomethickness) tumble by using in the lenses combustion and a slit chamber, aperture and just vice in versa. front However,of the cylinder large structureshead. The breaklight sheet up while (i.e., passingmeasurement through plane) the “neck” is aligned that along connects the center the two of volumesthe combustion (except chamber. if they are aligned with the long side ofSeeding: the “neck”). To reduce Therefore, abrasive the wear flow in in the the engine, combustion particles chamber made isof quite boron di nitridefferent have from been andlargely chosen independentinstead of the of more the flow commonly field in theused actual titan enginedioxide cylinder. particles. With a mean diameter of 1 µm, they are –5 ableSince to follow good the knowledge flow structures about very the well flow (relaxation field in the time combustion ≤ 5 µs, Stokes chamber number isimportant, ~ 10 (engine a high time speedscale τ PIVe = Stroke measurement length/Mean campaign piston was speed)). carried The to assessparticles flow-field, are seeded flow-structures, into the intake flow-velocities air with four andseparately turbulence activated levels blowpipe for a variations, as shown of engine in Figure speeds 13. and intake and therefore peak compression pressureCamera: levels A [ 3112-bit]. All monochrome experiments CMOS have been camera carried (Photron out without FASTCAM combustion. SA-X2) equipped An overview with of a the105 experimentalmm f/4 Micro-NIKKOR setup is shown objective in Figure was 13used. to record the seeding particles. The chosen field of view (seen on the right in Figure 13) results in a resolution of approximately 17 pixels per mm.

FigureFigure 13.13.Optical Optical setupsetup forfor thethe PIVPIV ﬂowflow ﬁeldfield measurementsmeasurements in in the the Flex-OeCoS. Flex-OeCoS.

LaserTiming: sheet: A Thedouble laser image from of a dualthe particles cavity, frequency was reco doubledrded at every Nd:YAG crank laser angle. (Photonic The crank Industries angle DM60dependent 532 DH, time 6 mJdelay/pulse, between 200 ns pulsethe pulses width) from is transformed the two laser intoa cavities light sheet was (45 optimized mm wide, towards 0.5 mm thickness)maintaining by an using average lenses particle and adisplacement slit aperture of just five in pixels front between of the cylinder the double head. images. The light sheet (i.e., measurementPost-processing: plane) The raw is aligned PIV images along have the center been ofprocess the combustioned with commercially chamber. available software (LaVision, DaVis 8.4). A particle intensity normalization filter was used to eliminate the local background noise before the calculation of the flow velocities from the double images with a multi- pass, cross-correlation algorithm. The interrogation window size was set to 24 × 24 pixels with 50% overlap. The resulting two-dimensional velocity field has a vector spacing of 0.71 mm. Starting from the standard point (600 rpm and 100 bar peak pressure), measurements with an engine speed variation (400 and 800 rpm) and a peak compression pressures variation (70 and 130 bar) have been made. The intake air temperature was kept constant at 200 °C for all cases; the intake air pressure was adjusted for each parameter set to reach the desired peak compression pressure.

Energies 2020, 13, 1794 16 of 23

Seeding: To reduce abrasive wear in the engine, particles made of boron nitride have been chosen instead of the more commonly used titan dioxide particles. With a mean diameter of 1 µm, they are able to follow the flow structures very well (relaxation time 5 µs, Stokes number ~ 10–5 (engine time ≤ scale τe = Stroke length/Mean piston speed)). The particles are seeded into the intake air with four separately activated blowpipes, as shown in Figure 13. Camera: A 12-bit monochrome CMOS camera (Photron FASTCAM SA-X2) equipped with a 105 mm f/4 Micro-NIKKOR objective was used to record the seeding particles. The chosen field of view (seen on the right in Figure 13) results in a resolution of approximately 17 pixels per mm. Timing: A double image of the particles was recorded at every crank angle. The crank angle dependent time delay between the pulses from the two laser cavities was optimized towards maintaining an average particle displacement of five pixels between the double images. Post-processing: The raw PIV images have been processed with commercially available software (LaVision, DaVis 8.4). A particle intensity normalization filter was used to eliminate the local background noise before the calculation of the flow velocities from the double images with a multi-pass, cross-correlation algorithm. The interrogation window size was set to 24 24 pixels with 50% overlap. × The resulting two-dimensional velocity field has a vector spacing of 0.71 mm. Starting from the standard point (600 rpm and 100 bar peak pressure), measurements with an engine speed variation (400 and 800 rpm) and a peak compression pressures variation (70 and 130 bar) have been made. The intake air temperature was kept constant at 200 ◦C for all cases; the intake air pressure was adjusted for each parameter set to reach the desired peak compression pressure. PIV data have been acquired at every crank angle over N = 200 individual cycles for each parameter set. The ensemble averaged mean velocity →u ens was then calculated for every crank angle θ over all N cycles: N 1 X (k) →u (θ) = →u (θ) (1) ens N k=1

0 The corresponding turbulence intensity →u ens at the crank angle θ is: v tu N 1 X (k) 2 →u 0 (θ) = ∆→u (θ) (2) ens N k=1

(k) (k) where ∆→u (θ) is the diﬀerence between the instantaneous velocity →u at the crank angle θ in the current cycle k and the ensemble averaged velocity →u ens at the same crank angle θ:

(k) (k) ∆→u (θ) = →u (θ) →u ens(θ) (3) − The development of the ensemble averaged flow field for selected crank angles during compression is shown in Figure 14. Initially, until approximately 30◦ CA before TDC, the flow enters the chamber from bottom left towards top right and creates a counter-clock rotating vortex on the left side. The flow in this compression phase is dominated by the tumble motion in the engine cylinder (due to the location of the two intake valves near the cylinder wall on one side). Since the tumble is aligned with the neck orientation (cf. Figures5 and6), it endures the passage into the combustion chamber. Energies 2020, 13, x FOR PEER REVIEW 16 of 22

PIV data have been acquired at every crank angle over N = 200 individual cycles for each parameter set. The ensemble averaged mean velocity 𝑢⃗ was then calculated for every crank angle 𝜃 over all N cycles: 1 𝑢⃗ (𝜃) = 𝑢⃗()(𝜃) (1) 𝑁 The corresponding turbulence intensity 𝑢⃗ at the crank angle 𝜃 is:

1 𝑢⃗ (𝜃) = ∆𝑢⃗()(𝜃) (2) 𝑁 where ∆𝑢⃗()(𝜃) is the difference between the instantaneous velocity 𝑢⃗() at the crank angle 𝜃 in the current cycle k and the ensemble averaged velocity 𝑢⃗ at the same crank angle 𝜃:

() () ∆𝑢⃗ (𝜃) =𝑢⃗ (𝜃) −𝑢⃗(𝜃) (3) The development of the ensemble averaged flow field for selected crank angles during compression is shown in Figure 14. Initially, until approximately 30° CA before TDC, the flow enters the chamber from bottom left towards top right and creates a counter-clock rotating vortex on the left side. The flow in this compression phase is dominated by the tumble motion in the engine cylinder (due to the location of the two intake valves near the cylinder wall on one side). Since the tumble is aligned with the neck orientation (cf. Figures 5 and 6), it endures the passage into the combustion chamber. Towards the end of the compression stroke, however, the flow entering the chamber alters its direction (now from bottom right to top left), and another (clockwise rotating) vortex forms itself at the right side. This is likely due to the slightly eccentric position of the combustion chamber/neck relative to the engine cylinder (cf. Figure 5): the squish-flow from the larger volume of the right side is stronger. The initially formed, counter-clock rotating vortex is pushed into the top left corner. EnergiesAt2020 TDC,, 13, 1794both vortices are still present (at an engine speed of 600 rpm), but the velocity17 of 23 magnitude in the top/central area of the chamber has fallen to 3 m/s or less.

Figure 14. Ensemble averaged mean velocities (color: magnitude; arrows: flow ﬂow direction) at selected crank angles in the compression stroke for the standard settings (600 rpm and peak compression pressure 100 bar).

ItTowards was found the endthat ofa variation the compression of the peak stroke, compre however,ssion pressure the flow enteringhas no significant the chamber influence alters on its thedirection flow field. (now A from change bottom of engine right to speed, top left), however, and another has a (clockwise strong effect rotating) as expected. vortex formsFigure itself 15 shows at the theright ensemble side. This averaged is likely flow due tofields the slightlyfor three eccentric engine positionspeeds (400/600/800 of the combustion rpm) at chamber 40° CA/neck before relative TDC (Figureto the engine 15a) and cylinder at TDC (cf. itself Figure (Figure5): the 15b). squish-flow At the fromlowest the engine larger speed volume (400 of therpm), right the side counter-clock is stronger. rotatingThe initially vortex formed, formed counter-clock early in the rotating compression vortex st isroke pushed dissipates into the before top left TDC; corner. the velocity field in the centralAt TDC, and both upper vortices part areof the still chamber present (at is analmost engine uniform speed ofwith 600 velocity rpm), but magnitudes the velocity in magnitude the order ofin the1.5 topm/s./central The two area cases of the with chamber higher has engine fallen tospeeds 3 m/s (600 or less. and 800 rpm) show comparable flow structuresIt was both found at 40° that CA a variation before TDC of the (Figure peak 15a) compression and at TDC pressure itself (Figure has no significant15b); they differ influence mainly on inthe the flow location field. of A changethe vortex of enginecenters speed,and the however, velocity hasmagnitudes. a strong effect as expected. Figure 15 shows the ensemble averaged flow fields for three engine speeds (400/600/800 rpm) at 40◦ CA before TDC (Figure 15a) and at TDC itself (Figure 15b). At the lowest engine speed (400 rpm), the counter-clock rotating vortex formed early in the compression stroke dissipates before TDC; the velocity field in the central and upper part of the chamber is almost uniform with velocity magnitudes in the order of 1.5 m/s. The two cases with higher engine speeds (600 and 800 rpm) show comparable flow structures both at 40◦ CA before TDC (Figure 15a) and at TDC itself (Figure 15b); they differ mainly in the location ofEnergies the vortex 2020, 13 centers, x FOR PEER and REVIEW the velocity magnitudes. 17 of 22

FigureFigure 15.15. Ensemble averaged averaged mean mean velocities velocities (color: (color: ma magnitude,gnitude, arrows: arrows: flow flow direction) direction) at 40° at 40CA◦ CAbefore before TDC TDC (a) (aand) and at atTDC TDC itself itself (b ()b )for for a a variation variation of of engine speedsspeeds (400(400/600/800/600/800 rpm).rpm). PeakPeak compressioncompression pressure pressure 100 100 bar bar in in all all cases. cases. This means that it is possible to select the desired flow structure in the area of interest by adjusting This means that it is possible to select the desired flow structure in the area of interest by engine speed and experiment timing (injection and ignition timing). Since the actual cylinder pressure adjusting engine speed and experiment timing (injection and ignition timing). Since the actual has little influence on the flow field, the desired pressure and temperature at the time of the experiment cylinder pressure has little influence on the flow field, the desired pressure and temperature at the can be adjusted separately by regulating the intake air pressure (boost pressure) and temperature time of the experiment can be adjusted separately by regulating the intake air pressure (boost (charge air preheating). pressure) and temperature (charge air preheating). An overview of the spatially averaged flow velocity and turbulence intensity around TDC for the An overview of the spatially averaged flow velocity and turbulence intensity around TDC for standard measurement settings is shown in Figure 16. All velocity values have been normalized with the standard measurement settings is shown in Figure 16. All velocity values have been normalized the mean piston speed cm (3 m/s at 600 rpm). On the left side (Figure 16a), the mean values for the two with the mean piston speed cm (3 m/s at 600 rpm). On the left side (Figure 16a), the mean values for turbulence intensity components are shown together with the gas pressure and temperature traces. the two turbulence intensity components are shown together with the gas pressure and temperature The arrows beside the trace labels indicate the flexibility of the Flex-OeCoS to vary the desired operating traces. The arrows beside the trace labels indicate the flexibility of the Flex-OeCoS to vary the desired conditions in terms of pressure, temperature and flow/turbulence conditions, as mentioned above. operating conditions in terms of pressure, temperature and flow/turbulence conditions, as mentioned above. On the right side (Figure 16b), the flow velocities ux and uy (top) and the turbulence intensities u’x and u’y (bottom) are shown. The colored background in the plots shows how often a certain flow velocity ux/uy or turbulence intensity u’x/u’y has been measured at each crank angle in the recorded experiment cycles (yellow = never; dark blue = very often).

Figure 16. Normalized, spatially averaged flow velocity magnitude and ensemble averaged turbulence intensity for the standard case in the late compression/early expansion stroke without combustion. (a) Overall quantities including combustion chamber pressure and temperature trace; The arrows beside the labels indicate the flexibility of the Flex-OeCoS to vary operating parameters individually. (b) (top) The normalized spatial mean velocities in x and y direction including spatial probability density functions; (bottom) the corresponding quantities for the turbulence intensities.

As shown in Figure 13, the horizontal x component is aligned perpendicular to the piston/cylinder axis, and the vertical y component is aligned parallel to the cylinder axis. During the

Energies 2020, 13, x FOR PEER REVIEW 17 of 22

Figure 15. Ensemble averaged mean velocities (color: magnitude, arrows: flow direction) at 40° CA before TDC (a) and at TDC itself (b) for a variation of engine speeds (400/600/800 rpm). Peak compression pressure 100 bar in all cases.

This means that it is possible to select the desired flow structure in the area of interest by adjusting engine speed and experiment timing (injection and ignition timing). Since the actual cylinder pressure has little influence on the flow field, the desired pressure and temperature at the time of the experiment can be adjusted separately by regulating the intake air pressure (boost pressure) and temperature (charge air preheating). An overview of the spatially averaged flow velocity and turbulence intensity around TDC for the standard measurement settings is shown in Figure 16. All velocity values have been normalized with the mean piston speed cm (3 m/s at 600 rpm). On the left side (Figure 16a), the mean values for the two turbulence intensity components are shown together with the gas pressure and temperature traces. The arrows beside the trace labels indicate the flexibility of the Flex-OeCoS to vary the desired operating conditions in terms of pressure, temperature and flow/turbulence conditions, as mentioned above. On the right side (Figure 16b), the flow velocities ux and uy (top) and the turbulence intensities u’x and u’y (bottom) are shown. The colored background in the plots shows how often a certain flow velocityEnergies 2020 ux,/13uy, or 1794 turbulence intensity u’x/u’y has been measured at each crank angle in the recorded18 of 23 experiment cycles (yellow = never; dark blue = very often).

Figure 16.16.Normalized, Normalized, spatially spatially averaged averaged ﬂow velocityflow velo magnitudecity magnitude and ensemble and ensemble averaged turbulenceaveraged turbulenceintensity for intensity the standard for the case standard in the latecase compression in the late compression/early/early expansion stroke expansion without stroke combustion. without combustion.(a) Overall quantities (a) Overall including quantities combustion including chambercombustion pressure chamber and pressure temperature and temperature trace; The arrows trace; Thebeside arrows the labels beside indicate the labels the indicate ﬂexibility the of flexibilit the Flex-OeCoSy of the toFlex-OeCoS vary operating to vary parameters operating individually. parameters individually.(b) (top) The ( normalizedb) (top) The spatial normalized mean spatial velocities mean in velocities x and y direction in x and includingy direction spatial including probability spatial probabilitydensity functions; density (bottom) functions; the (bottom) corresponding the correspon quantitiesding for quantities the turbulence for the intensities.turbulence intensities.

AsOn theshown right in side Figure (Figure 13, 16 b),the the horizontal flow velocities x componentux and uy (top)is aligned and the perpendicular turbulence intensities to the piston/cylinderu’x and u’y (bottom) axis, areand shown. the vertical The coloredy component background is aligned in the parallel plots showsto the cylinder how often axis. a certain During flow the u u u’ u’ velocity x/ y or turbulence intensity x/ y has been measured at each crank angle in the recorded experiment cycles (yellow = never; dark blue = very often). As shown in Figure 13, the horizontal x component is aligned perpendicular to the piston/cylinder axis, and the vertical y component is aligned parallel to the cylinder axis. During the compression stroke, the spatial probability density function of the horizontal velocity component ux shows quite a large variability of the mean values due to the variable entrance time of large flow structures into the combustion chamber. After TDC, no new sources for horizontal velocity components enter the chamber; the already present turbulent structures quickly break up. Therefore, the horizontal velocity ux drops towards zero as well. The direction of the vertical velocity component uy (aligned with the piston axis) represents the general movement of the gas charge in the combustion chamber due to the piston motion (note the positive values of uy before TDC and negative values afterwards). The two turbulence intensity components u’x and u’y shown below the velocities exhibit comparable characteristics. The turbulence level falls quickly after TDC due to the decaying turbulence already present in the chamber, and due to the loss of new turbulence sources brought in with the inflow and produced by the inflow into the combustion chamber. The turbulence intensity u’ as shown in Figure 16 encompasses all variable velocity components including cyclic variations and the influence of large flow structures. Often, however, it would be important to be able to separate the overall turbulent kinetic energy into the contributions brought in from the different flow scales. To achieve this, the Proper Orthogonal Decomposition Method (POD) was applied to the measurements. This processing technique decomposes the measured flow fields into a set of empirical eigenfields φm(θ) (θ = Crank angle), each of which is weighted with a factor (k) cm (θ) that represents a fraction of the overall total kinetic energy present:

(k) XM (k) →u (θ) = c (θ) φm(θ) (M = Number of measurements) (4) m=1 m · Energies 2020, 13, x FOR PEER REVIEW 18 of 22 compression stroke, the spatial probability density function of the horizontal velocity component ux shows quite a large variability of the mean values due to the variable entrance time of large flow structures into the combustion chamber. After TDC, no new sources for horizontal velocity components enter the chamber; the already present turbulent structures quickly break up. Therefore, the horizontal velocity ux drops towards zero as well. The direction of the vertical velocity component uy (aligned with the piston axis) represents the general movement of the gas charge in the combustion chamber due to the piston motion (note the positive values of uy before TDC and negative values afterwards). The two turbulence intensity components u’x and u’y shown below the velocities exhibit comparable characteristics. The turbulence level falls quickly after TDC due to the decaying turbulence already present in the chamber, and due to the loss of new turbulence sources brought in with the inflow and produced by the inflow into the combustion chamber. The turbulence intensity u’ as shown in Figure 16 encompasses all variable velocity components including cyclic variations and the influence of large flow structures. Often, however, it would be importantEnergies 2020 ,to13 be, 1794 able to separate the overall turbulent kinetic energy into the contributions brought19 of in 23 from the different flow scales. To achieve this, the Proper Orthogonal Decomposition Method (POD) was applied to the measurements. This processing technique decomposes the( kmeasured) flow fields The eigenfields φm are sorted with decreasing energy content (factor cm ), thus, by selecting into a set of empirical eigenfields 𝜙(𝜃) (θ = Crank angle), each of which is weighted with a factor the() appropriate cut-offs it is possible to separate the most energy rich dominant structures from the 𝑐 (𝜃) that represents a fraction of the overall total kinetic energy present: coherent structures and the smallest scale, least energy containing real turbulent structures [32]: () () 𝑢⃗ (𝜃) = ∑ 𝑐 (𝜃) ∙𝜙(𝜃) (M = Number of measurements) (4) (k) (k) (k) (k) →u (θ) = →u dominant(θ) + ∆→u coherent(θ) + ∆→u turbulent((θ) (5) The eigenfields ϕm are sorted with decreasing energy content (factor 𝑐 ), thus, by selecting the appropriate cut-offs it is possible to separate the most energy rich dominant structures from the The definition of the used cut-offs between the structures is based on [33,34]. After the coherent structures and the smallest scale, least energy containing real turbulent structures [32]: decomposition, any velocity vector field can be approximated by the superposition of the included () () () () eigenfields: To get the small-scale𝑢⃗ (𝜃) =𝑢⃗ turbulent(𝜃) velocity ⃗Δ𝑢 field, for(𝜃) example, ⃗Δ𝑢 the weighted(𝜃) superposition(5) of the dominant and coherent eigenfields are subtracted from the overall mean velocity field at this crank The definition of the used cut-offs between the structures is based on [33,34]. After the angle θ. decomposition, any velocity vector field can be approximated by the superposition of the included Figure 17 shows some exemplary velocity fields from single modes on the right side. On the left eigenfields: To get the small-scale turbulent velocity field, for example, the weighted superposition side, the development of the turbulent kinetic energy during the compression and early expansion of the dominant and coherent eigenfields are subtracted from the overall mean velocity field at this stroke is shown. Please note that the turbulent kinetic energy shown here is calculated from the two crank angle θ. measured dimensions; the contribution of the unknown third dimension u’z is assumed to be similar Figure 17 shows some exemplary velocity fields from single modes on the right side. On the left to the two measured components (u0z = 0.5 u0x + u0y ). side, the development of the turbulent kinetic· energy during the compression and early expansion As expected, the turbulent kinetic energy contained in the flow is much lower if the dominant or stroke is shown. Please note that the turbulent kinetic energy shown here is calculated from the two both dominant and coherent flow structures have been removed. The trends of the three turbulent measured dimensions; the contribution of the unknown third dimension u’z is assumed to be similar kinetic energy curves however are the same (constant cut-off values). to the two measured components (𝑢′ =0.5∙𝑢′ 𝑢′).

FigureFigure 17. 17. CrankCrank angle angle dependent dependent turbulent turbulent kinetic kinetic energy energy in the in combustion the combustion chamber chamber of theof Flex- the OeCoSFlex-OeCoS for the for standard the standard measurement measurement conditions conditions (600 rpm/100 (600 rpm bar)./100 bar).Shown Shown are the are overall the overall values values that that contain all ﬂow structures and the reduced values after the subtraction of the dominant structures

or the dominant and coherent ﬂow structures. The modes have been calculated with the Proper Orthogonal Decomposition Method (POD).

An overall assessment of the ﬂow ﬁeld measurement in the Flex-OeCoS shows that

Flow velocity and turbulence intensity magnitude scale with engine speed. • The pressure level has little influence on the flow structure and flow velocity or turbulence • intensity magnitude.

Due to the special arrangement of combustion chamber and engine cylinder, the turbulence intensity level in the Flex-OeCoS is relatively high compared to conditions found in “normal” combustion engines. At the end of the compression stroke for standard measurement conditions (600 rpm, cm = 3 m/s, 100 bar peak compression pressure), the turbulence intensities u’x and u’y are both in the order of 3.4 m/s (i.e., about 1.1–1.2 times the mean piston speed cm). This puts the Flex-OeCoS at Energies 2020, 13, x FOR PEER REVIEW 19 of 22

contain all flow structures and the reduced values after the subtraction of the dominant structures or the dominant and coherent flow structures. The modes have been calculated with the Proper Orthogonal Decomposition Method (POD).

As expected, the turbulent kinetic energy contained in the flow is much lower if the dominant or both dominant and coherent flow structures have been removed. The trends of the three turbulent kinetic energy curves however are the same (constant cut-off values). An overall assessment of the flow field measurement in the Flex-OeCoS shows that • Flow velocity and turbulence intensity magnitude scale with engine speed. • The pressure level has little influence on the flow structure and flow velocity or turbulence intensity magnitude. Due to the special arrangement of combustion chamber and engine cylinder, the turbulence intensity level in the Flex-OeCoS is relatively high compared to conditions found in “normal”

Energiescombustion2020, 13 ,engines. 1794 At the end of the compression stroke for standard measurement conditions20 of(600 23 rpm, cm = 3 m/s, 100 bar peak compression pressure), the turbulence intensities u’x and u’y are both in the order of 3.4 m/s (i.e., about 1.1–1.2 times the mean piston speed cm). This puts the Flex-OeCoS at thethe upper upper end end of of the the range range given given in in a a collection collection of of measurements measurements presented presented in in [35 [35]] (Figures (Figure 8–12, 8–12, page page 363).363).

5.5. OutlookOutlook TheThe optically optically accessible accessible combustion combustion chamber chamber of the Flex-OeCoSof the Flex-OeCoS test facility test allows facility the applicationallows the ofapplication various optical of various (high-speed) optical techniques (high-speed) in combination techniques within combination corresponding with time corresponding resolved pressure time andresolved temperature pressure measurements and temperature as well measurements as acquisition as ofwell further as acquisition boundary conditions.of further boundary Initially, theconditions. acquisition Initially, of the turbulentthe acquisition flow field of the (velocities, turbulen turbulentt flow field intensities (velocities, and turbulent kineticintensities energy) and hasturbulent been performed kinetic energy) by means has been of high-speed performed PIV. by means of high-speed PIV. Currently,Currently, simultaneous simultaneous high-speedhigh-speed SchlierenSchlieren//OH*OH* chemiluminescencechemiluminescence imagingimaging isis appliedapplied toto determinatedeterminate ignition ignition properties properties (delay (delay and location), and location), flame propagation flame propagation and combustion and characteristics. combustion Incharacteristics. particular, ignition In particular, delay and ignition combustion delay and stability combustion (cyclic variability) stability (cyclic in terms variability) of air–fuel in terms ratio as of wellair–fuel as start ratio and as durationwell as start of injection and duration is under of injection investigation. is under investigation. FigureFigure 1818 givesgives anan insight,insight, illustrating the the experimental experimental setup setup at at the the Flex-OeCoS Flex-OeCoS by by means means of ofsimultaneously simultaneously applied applied Schlieren/OH* Schlieren/ OH*chemiluminescence chemiluminescence (Figure (Figure 18a) along 18a) with along some with exemplary some exemplaryimages ofimages a recording of a recording sequence sequence (Figure (Figure 18b, 18Schlierenb, Schlieren images images in inblack black and and white, OH*OH* chemiluminescencechemiluminescence intensity intensity in in magenta). magenta).

FigureFigure 18. 18. OpticalOptical setupsetup atat thethe FlexFlex OeCoSOeCoS forfor dual-fuel dual-fuel investigations investigationsby by means means of of simultaneous simultaneous SchlierenSchlieren/OH*/OH* chemiluminescence chemiluminescence ( a(a);); and and an an exemplary exemplary high-speed high-speed recording recording sequence sequence of of overlaid overlaid SchlierenSchlieren and and (colorized) (colorized) OH* OH* chemiluminescence chemiluminescence (b ).(b Operating). Operating conditions: conditions: methane methane/air/air charge chargeλ = λ2, = engine2, engine speed speed 600 rpm,600 rpm, pressure pressure/temperature/temperature conditions cond ofitions 70 bar of/ 76070 bar/760 K at (hydraulic) K at (hydraulic) start of injection start of ofinjection –20◦ CA of (dodecane –20° CA pilot (dodecane fuel, single pilot hole fuel, nozzle single Ø90 holeµm, nozzle pinj = 1000Ø90bar) µm, with pinj an= energizing1000 bar) with time ofan 525energizingµs. time of 525 µs.

The two high-speed measurement images (36 kHz recording speed and 0.1◦ CA resolution) have been overlaid (and for OH* colorized) to identify liquid pilot spray evaporation (Schlieren), the (auto) ignition process (OH* as tracer of initial high temperature reactivity), combustion transition (auto-ignition to premixed ﬂame) and the subsequent propagation of the turbulent premixed ﬂame (Schlieren). A post-processing procedure determines start of injection, ignition delay, start of injection and start of combustion based on the optical measurements in combination with corresponding heat release analysis. Detailed results for a wide range of operating conditions will be published.

6. Conclusions Thus far, optically accessible experimental test rigs for fundamental investigations are often limited in terms of operation conditions related to real engines, in particular concerning peak pressure levels. To investigate in-cylinder phenomena of modern combustion processes and fuel types (e.g., gas/dual fuel) close to engine relevant conditions, the “optical engine” facility “Flex-OeCoS” was Energies 2020, 13, 1794 21 of 23 developed—the designation indicating its Flexibility regarding Optical engine Combustion diagnostics and/or development of corresponding Sensing devices. The experimental facility consists of an optical combustion chamber connected to one cylinder of a motored heavy-duty engine block. Intake air or air/(gaseous) fuel mixture (at initial conditions of up to 7 bar and >200 ◦C) is fed through two inlet valves of the working cylinder and afterwards compressed into the combustion chamber. The timing of the two inlet and exhaust valves can be adjusted independently of the crankshaft position, allowing full flexibility of engine operation. Moreover, the drive train offers complete engine speed flexibility, adjusting the process gas charge flow velocity and turbulence conditions. The combustion chamber head offers optical accessibility with sapphire windows located at each margin, providing excellent optical access. The rectangular side windows allow application of optical light-sheet techniques, whereas the round main windows offer complete view (“see through”) of the entire combustion chamber. Comprehensive instrumentation of the combustion chamber determines in-cylinder pressure, process gas temperature and other necessary boundary conditions. In addition, the setup offers enormous flexibility to adapt individually designed plug-in inserts as replacement of the windows. Therefore, it is possible to mount specific instrumentation such as fast thermocouples, pressure sensors for calibration or even heat flux sensors. Important operating parameters and limitations such as peak pressure, characteristics of the valve train and achievable gas temperatures have been examined. A comprehensive set of precise boundary and process conditions has been determined. The Flex-OeCoS test facility features adaptable operation at engine relevant conditions: compression and combustion pressure (up to 160 bar, respectively 230 bar), working gas temperature (700 to >1000 K), tunable flow/turbulence (turbulence intensity within 2–6 m/s depending on motor speed of typically 400–1000 rpm), adjustable gas/air charge composition and high procedure variance by pneumatic driven valve train. Moreover, the specific design is widely adaptable by different designs of optical combustion chambers enabling investigations of distinct combustion concepts. In terms of dual fuel combustion processes, this is related to both low-pressure premixed gas/air pilot or (prechamber) flame jet ignited and high-pressure direct gas injection pilot ignited. The variability of the Flex-OeCoS test facility and its flexible operation modes offer advantageous features for various combustion research investigations.

Author Contributions: Conceptualization, B.S., K.B., and C.G.; Funding acquisition, K.B. and C.G.; Investigation, C.S., S.H., M.H., D.H., and S.W.; Supervision, K.B., C.G., and K.H.; Writing—original draft, B.S., S.H., D.H., S.W., and K.H.; and Writing—review and editing, B.S. and K.H. All authors have read and agree to the published version of the manuscript. Funding: The development and realization of the new test rig was funded by the CCEM (Competence Center Energy and Mobility) Project “Flex-Fi-Dual” (901) and the SFOE (Swiss Federal Office of Energy) Projects “Dual Fuel Verbrennungssysteme” (SI/500970-01) and “next ICE” (SI/501020-01). Moreover, the authors gratefully thank FHNW School of Engineering as well as LAV ETH Zurich for complementary financial support. Acknowledgments: We would like to thank Liebherr SA, Bulle, Switzerland, for contributing the base engine block for the new test rig. Conflicts of Interest: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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