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409 D L Reuss, T-W Kuo, G Silvas, V Natarajan, and V Sick410 Experimental metrics for identifying origins ofcombustion variability during spark-assistedcompression ignition D L Reuss*, T-W Kuol, G Silvas, V Natarajan, and V Sick Powertrain Systems Research Laboratory, General Motors R & D Center, Warren, Michigan, USA University of Michigan, Ann Arbor, Michigan, USA The manuscript was accepted after revision for publication on 2 June 2008.DOI: 10.1243/14680874JER01108 Abstract: Spark-assisted compression ignition, SACI, can be used to control the combustionphasing of compression-ignition gasoline engines. However, implementation of this techniquecan be confounded by cyclic variability. The purpose of this paper is to define experimentalmetrics that describe the SACI process and to demonstrate the use of these metrics foridentifying the source(s) of cyclic variability during the SACI process. This study focused on a light load condition (7 mg/cycle, 200 kPa i.m.e.p.), where spray-guideddirect fuel injection with spark ignition and an exhaust-rebreathing strategy was employed toachieve flame propagation, which led to compression ignition. This study employed acombination of measurements including pressure-based heat-release analysis, spark-dischargevoltage/current measurements, and cycle-resolved combustion imaging. Based on thesemeasurements, four distinct combustion periods were identified; namely, the spark discharge,the early kernel growth (EKG),flame propagation, and the compression ignition periods. Metricswere defined to characterize each period and used to identify the contribution of each period tothe cyclic variability of the main heat release. For the light load condition studied here, the EKGperiod had the largest effect on the crank angle (CA) position of 50 per cent mass burned, CA50.The spark-discharge event may affect CA50 indirectly through its influence on EKG. However,this could not be definitively assessed here since the camera was incapable of recording both thespark-discharge event and the flame images during cycles of the same tests. Keywords:homogeneous charge compression ignition (HCCI) engine, spark-assisted com-pression ignition, spark-assisted HCCI, cyclic variability INTRODUCTION Ignition and the burning rate of homogeneouscharge compression ignition (HCCI), engines arecontrolled by chemical reaction rates; therefore,phasing is dominated by the in-cylinder tempera-ture and composition. In practical gasoline com-pressionignition(CI) engines, the in-cylindermixture temperature varies from cylinder to cylin-der and during transients due to variations in ( *Corresponding author: G eneral Motors R&D, MS 480-106-252, 30500 Mound Road, Warren, MI 48090-9055, US A . email: david.l.reuss@gm.com ) ( H ere the combustion phasing is d e fined as the crank angle of 50 per cent mass fraction b urned, CA50. ) intake-charge ttemperature,residual-chargetem-perature and amount, and wall temperatures. Con-sequently, control methodologies are required [1, 2]. Spark-assisted compression ignition (SACI) com-bustion, is one method to achieve control of HCCI ingasoline engines. Where several studies have focusedon higher loads, especially transition from HCCI tospark ignition (SI) operation [3-9], this study focuseson phasing control at low load. As implemented withstratified-charge direct-injection in this study, SACIis achieved by injecting a portion of the fuel early inthe intake stroke to create a nearly homogeneousfuel, air, and residual mixture;;then aasecondinjection, late in the compression stroke, creates astratified charge that is spark ignited. Thus, SACIcombustionbeginsasflameepropagation ina stratified-charge spark-ignited direct-injection en-gine configuration that increases the unburned fuel-air charge to pressures and temperatures abovethose that would otherwise be achieved with thepiston compression alone. This causes CI combus-tion of the unburned gas to occur at an earlier CAthan would occur with piston compression alone,analogoustoJ6traditionalSI knock butt withacontrolled heat-release rate. SACI combustion suffers from high cycle-to-cyclevariability as demonstrated by Perrson et al. [7],which potentially has the same causes of variabilitypresent in both conventional SI and stratified-charge/spark-ignited direct-injected (SC/SIDI), en-gines. Cyclic variability of combustion phasing inhomogeneous SI engines has been traced to thespark discharge event itself; it is dominated by theflow-velocity variability, as long as the dilution level(air, residual, or EGR) is low enough to supportrobust combustion [10]. However, SC/SIDI cyclicvariability is also increased by fuel stratification andflow induced by the injection event. In principle,SACI also experiences variability during the CIcombustion; however, it will be shown here thatthe CI duration is small and actually decreases thevariability resulting from the ignition and flame-propagation phases of the combustion process. The purpose of this study is to establish measuredmetrics that characterize the SACI combustionprocess and identify the influence of each periodon CI-phasing variability; cycle-to-cycle variability ofcylinder mass and composition are not addressed here. A single light-load engine-operating conditioniss used for this demonstration (7mg/cycle fuel,200kPaai.m.e.p.). Four combustion periodsaredefined during SACI, as illustrated in Fig. 1, namely,the spark discharge, the EKG, flame propagation,and the CI periods. Thispaper describesthecharacterization and quantification of SACI combus-tion and its cyclic variability achieved using steady state emissions; pressure-based heat release; spark discharge voltage and current measure-ments: (d)imaging of the spark and combustion process. This manuscript describes the engine and com-bustion characterization in the order just enumer-ated. This is followed by a discussion identifyingthe causes of cyclic variability by correlating theignition and flame parameters against each otherand CA50. 2 BACKGROUND This background contains two sections. Section 2.1provides a description of the four periods of SACIalready defined based on the results of this study;this summary is provided prior to the results in orderto provide the reader with a perspective for thedetails that follow. Section 2.2 describes the cyclicvariability of SACI combustion, which is the motiva-tion for this study. Fig.1 Definition of the four SACI combustion periods using the imaged flame area and heat-release rate, HRR, for ensemble-averaged cycles conditionally sampled on CA50=2 ATDC 2.1 The four periods of SACI combustion This study focused on a light-load condition, wherespray-guided direct fuel injection was required toachieve flame propagation. The four combustionperiods are defined based on the spark current,measured flame areas, and heat release rate, HRR.These four periods are illustrated in Fig. 1 as thespark discharge, the early kernel growth (EKG), theflame propagation, and the CI combustion periods.The beginning of the spark discharge is the spark setpoint, CASpk. The end of the spark ignition event,EOSpk, is defined as time when the spark-dischargecurrent returns to zero and by the definition here isequal to the beginning of the EKG event. Conceptually, EKG is the period after the sparkdischarge where a small reaction kernel exists, butwith weak chemiluminescence, slow area growth,and too little heat release to be reliably measuredwith pressure-based heat-release analysis. This is alight load phenomenon where the flame is too weakto propagate strongly. Eventually heat release pro-ceeds at a more rapid rate, presumably because theearly kernel encounters a more favourable air-fuel ratio; encounters higher temperatures due to com-pression; or (c)CI combustion occurs before the kernel be-comes robust. The end of the early-kernel growth, EOEkg, isdefined as the crank angle where the second deri-vative of the flame area, dAilm/dCA, is maximum,and is defined here as being equal to the beginningof the flame propagation. This definition of the EKGperiod is conceptually similar to the model ofAbraham et al. [11], which describes an ignitionkernel in a conventional homogeneous SI engine thatfirst grows slowly on a laminar-flame timescale andthen transitions to rapid growth on a turbulent-flametimescale. The ‘flame'is defined here as the period where thechemiluminescence becomes more intense, areagrowth faster than during EKG, and the heat releaseismeasurable (Fig.1). ‘Flame’ here should beinterpreted as a convenient label rather than takenliterally, since the measured heat release may be acombination of CI and true flame* combustion, ( *Here a true’ f lame is considered t o h a ve th e properties a deflagration wave with a propagation rate t hrough a partially premixed s t ratified f uel-air m ixture t h at is co n trolled by aconvective-diffusive balance. ) which the measurements here cannot discriminate.This interpretation of the combustion is analogousto the initial slow heat release, ISHR, defined byPerrson et al. [7], which was presumed to be a flamealso.The end of the flame propagation is defined tobe the CA where the combustion transitions to CIcombustion, CATrans. CI heat release rates occurover chemical-reaction timescales and are fasterthan HRRs of turbulent flame propagation. Thus,CATrans is considered to be the time when theoverall HRR changes from the slower ‘flame’rate tothe faster CI rate, here measured as the point wherethe second derivative of the HRR, dHRR/dCA, ismaximum as in [7]. The EKG and flame period definitions are notbased on fundamental definitions of deflagrationwaves, but rather, are based on the observed ratechanges. These parameters become useful engineer-ing metrics that will be used here to define the seriesof event(s) and to identify which, if any, of the eventsdominate cyclic variability of the CI combustionphasing. 2.2CCyclic variability of SACI combustion The light-load engine-operating condition studiedhere was fuelled with 7 mg/cycle gasoline. For pureCI operation all 7 mg of the fuel was injected at -330ATDC.t The water temperature, intake temperatures,and injection strategies used for SACI (Table 1) andshown graphically in Fig. 2 were derived as follows.Firstly the engine was operated at the pure CIcondition with a hot wall (90℃ cooling water tothe head and cylinder, Twall) and the intaketemperature, Tintk, sufficiently hot (205C) to yielda location of peak pressure, LPP, equal to 9 ATDC.Next, the water temperature was reduced to 65℃,(retaining EOI=-330 and Tintk=205℃) to retardthe LPP to 12.5 ATDC. SACI was then used (retainingTintk=205°℃ and Twall=65°C) to regain 9 ATDCLPP, where 4 mg/cycle injected at EOI=-330, 3 mg/cycle injected at -45 ATDC, and spark ignitionadvanced to -45 ATDC (dotted line, Fig. 2). This wasthe minimum second-injection fuelling rate requiredto readvance the LPP, and thereby overcome theenthalpy deficit created by the cold coolant. The difference between the CI and SACI HRRs andtheir cyclic variability are shown in Fig. 3. Figure 3(a)shows the ensemble-averaged HRRs, correspondingto three extreme conditions indicated by the large ( ATDC indicates crank angle degrees after top dead centre of th e compression stroke. ) Table 1Engine hardware and operating parameters Borexstroke (mm) 86×94.6 Speed 1000r/min Compression ratio 11:1 Intake map 100.5kPa EVO1,10 mm 138 ATDC Exhaust map 101.5kPa EVC1 373 ATDC A/F 20:1 EVO2, 4 mm 378 ATDC cI SACI EVC2 521 ATDC Twall (C) 90 65 65 IVO,4mm 297 ATDC Tintake (C) 205 205 205 IVC 565 ATDC Texh port (C) 328 338 332 Piston bowl VVA3 TIvc (C) 264 265 267 Injector 8 hole, 60°, 15 cc/s Residual% 60 60 60 Fuel Indolene [22] Injection timing, EOI ATDC -330 -330 -330/-45 P fuel 11MPa Fuelling rate, mg/cycle 7 7 4/3 Spark plugs Traditional and double pin,9 mm long centre electrode Spark timing, ATDC n.a n.a -45 circles in Fig. 2. Figure 3(a) reveals that the peak,average HRR of the SACI combustion (black dotted) isphased the same as the original hot-coolant CI, whichwas expected based on LPP in Fig. 2. The condition-ally averaged HRRs of the SACI cycles in Fig.3(a)provide insight into the nature of the combustion.The curves in Fig. 3(a) are ensemble averages of theindividual-cycle HRR(CA) curves conditionally sam-pled on those cycles for which the CA at 10 per centmass-burned fractions, CA10, occurred at -10.6 and-2.6± 0.5 ATDC (red and blue dotted). Consider firstthe early burning cycles CA10=-10.6; these earlySACI cycles have nearly the same phasing as the hotwall (90°℃) CI operation but with more heat releasebetween -20 and-5 CA (albeit slow compared to theprimary heat release), which here is attributed toflame propagation. The late CA10 cycles are phasedlater than the pure CI at the same fuelling rate andcoolant temperature. These conditionally averagedHRR curves demonstrate that the ensemble averageof all SACI cycles is a poor representation of either the Fig. 2Combustion-pressure phasing used to establishthe SACI operating condition early or late SACI cycles, implying high cyclicvariability. The cyclic variability is shown succinctlyin Fig. 3(b), where both CA10 and CA50 of SACI havefive times the range compared to that of either hot orcold coolant CI. The phasing variations in Fig. 3(b)result in the i.m.e.p. variations shown in Fig.3(C),which directly affect the noise, vibration, and harsh-ness engine operation. Revealing the causes of thisvariability was the motivation for this study. Figure 4(a) shows i.m.e.p. versus the amount offuel burned during a given cycle; this demonstratesthat i.m.e.p. variability is directly related to theamount of fuel burned during a given cycle.Combining the results of Fig.4(a) with Fig. 3(c),Fig. 4(b) shows the relationship between massburned per cycle and CA50, and implies that earlycombustion phasing is required to burn more of thefuel or vice versa. The plot of CATrans and CA10 inFig. 4(c) demonstrates that the transition to CI atthis operating condition always occurs after 10 percent of the mass has burned, CA10, and that thephase variability of the main combustion (CI) isestablished prior to both CATrans and CA10. Thus,to determine the cause of the cyclic variability incombustion phasing, it is necessary to investigatevariability prior to CA10. Since accurate heat releasecalculations from the cylinder pressure are notor-iously difficult prior to CA10, the spark and imagingdiagnostics were employed as described below. 3EXPERIMENTAL HARDWARE ANDPROCEDURES The experiments herein required four recordingsystems and correlation of the data from each.Steady state engine-out emissions were recordedaveraged over the duration of each 300-cycle test. In-cylinder pressure was recorded once per CA for all300 cycles and used for individual-cycle heat releaseanalysis. Spark-discharge voltage and currents were 35 CI, Twall-90C, All Cycles CI, a 30 Twall=65C, All Cycles ··SACI, Twall-65C, All Cycles ·SACI,Twall-65C, ca10=-10.5 25 ·SACI,Twall-65C, ca10=-25 · SACI, Twall=65°C, covlMEP=10 50 · Cl, Twall=65°C, covlMEP= 7 CI, Twall=90°C, covIMEP= 1.6 0· -5 0 5 10 15 20 ca50 Fig. 3 Comparison of heat release parameters for thethree operating conditions indicated by thelarge circles in Fig. 2 - Cl, ·SACI, Twall=65°C,covlMEP=10 Twall=65°C, covlMEP= 7 a , R2=0.99 Burned Fuel (mg) b Fig.4 Correlation. oCf individual-cycle heat-releaseparameters Fig.5Optical engine and piston configuration recorded with a digital oscilloscope at 1/us intervalsduring the glow discharge of each engine cycle.Images of the spark-discharge plasma channel andthe subsequent combustion were recorded using ahigh-speed framing camera. Images could be re-corded only every third or fourth cycle. Therefore, sixtests of 300 engine cycles/test were recorded at eachoperating condition to sample 480 imaged cycleswith corresponding heat-release data. The equip-ment, procedures, and the measured parameters aregiven in this section. 3.1 Engine configuration and operation The engine used in this study, illustrated in Fig. 5, isan optical single-cylinder SIDI engine with a circularpiston bowl [12]. This is5Ca four-valve pent-roofengine with centre injector and centre spark pluglocated in a spray-guided SIDI configuration (SG/SIDI). The geometric characteristics and operatingparameters are listed in Table 1. The photographand schematic in Fig.5 show one of the twowindows at either end of the pent-roofed combus-tion chamber, a window in the piston bowl, and theextended Bowditch-type piston. For the tests described in this paper, an all-steelpiston top and a water-cooled chrome-plated steelcylinder were used. A single one-piece Rulon pistonring was used as suggested by Miles and Steeper [13]but with the addition of a felt oiler ring where the second compression ring would otherwise have beenlocated. This ring arrangement provided blow-bythat was less than 1 per cent of the trapped chargeand allowed consistent steady state engine operationin excess of 3 h. CI combustion was achieved using an exhaust-rebreathing strategy, where high in-cylinder gastemperatures are realized at intake-valve closing,(IVC), by reopening the exhaust valve during theintake stroke (Fig. 6). In these tests 60 per centresidual mass was realized. In addition, the intake Fig.6 Intake- and exhaust-cam profiles for rebreath-ing operation Fig.7 Field of view (a) back lit and (b) back lit with spray and spark discharge plenums and runners were heated to the desiredintake temperature to minimize thermal gradients inthe fresh charge. The exhaust runner and plenumwere electrically heated to 400 C to minimize heatloss from the burned gases during exhaust andreingestion. An 8-hole, 60° included cone angle (see Table 1)injector was used with one spray jet targeted at, butjust below, the spark plug gap (Fig.7(b)). A high-energy ignition system was used, which producedon average 67 mJ to the plug. Two different sparkplugs were used here, both with a 9 mm, centre-electrode reach. A spark plug with traditional flatelectrodes was used in the initial tests; data fromthis spark plug is used in Figs 9 and 10 to demon-strate the correspondence between the imaging andelectrical characteristics. A spark plug with smalliridium pins on both the centre electrode andground strap was used for all other tests presentedhere. Both are 5 k resistive plugs and thus 52 mJ ofthe 67 mJ was delivered, on average, across thespark-plug gap. 3.2 Pressure-based combustion analysis andemissions The cylinder pressure was measured with a Kistler6125 piezoelectric transducer with a slotted flamearrestor and fitted through the water jacket into theridge of the pent-roofed head. The pressure wasrecorded once per CA. The pressure-based combus-tion analysisis(HRR versus CA and mass-burnedfraction, (mbf), versus CA) was computed for thepressure record from each cycle and then averagedover alll cycles, but modified for the CI andrebreathing operation. To compensate for the heat-loss for the CI operation, the Woschni heat-loss correlation constant C2 was set to zero assrperChang [14], to suppress the pressure correctionnormally used for analysis of SI combustion withflame propagation. The heat-loss totalled over allcycles was adjusted to balance with the work plusfuel energy delivered over all cycles. The deliveredfuel was measured with a positive-displacementmeter to 0.1 mg/cycle, and the average deliveredfresh air measured with choked-flow orifices to1 per cent. To compute the trapped-residual-massfraction with the rebreathing cam, the volume ofthe fresh charge at IVC is computed assuming all ofthe delivered average fresh charge is trapped and atthe average intake temperature and MAP. Theremaining cylinder volume is assumed to be filledwith residual that is at the same average tempera-ture, pressure, and composition measured in theexhaust port. Since the exhaust temperature andcomposition are averaged over 300 cycles and thepressure of each cycle waspegged(in-cylinderpiezoelectricpressuree:transducerreadjustedttointake manifold absolute pressure to account forcharge drift) at -180 ATDC compression, the onlymodelled cycle-to-cycle variations in the residualmass fraction are those due to differences in thepressure at IVC. Finally, the mbf computed bythe heat release code is less than that indicated bythe combustion efficiency from the exhaust emis-sions; the combustion efficiency is based on thedelivered fuel whereas the final mass computedin the heat release is based on the trapped fuel(delivered plus rebreathed). A standard engine-emissions bench was used for..exhaust-gasmeasurements and analysed asperStivender [15]. This provided the steady-state en-gine-out emissions averaged over the duration of the300 cycle tests, including hydrocarbons, O2, CO2,CO, NO. The air-fuel ratio (AF), based on oxygen and carbon balance were computed. All three valuesof AF (based on emissions oxygen balance, emis-sions fuel balance, and the previously describedmeasured fuel and air flowrates)s)were 20:1 andagreed to within a 0.5 ratio. The single-cycleheat-release analysis was used tocompute five one-per-cycle parameters for charac-terizing cyclic variability. In particular, CA10 crank angle ATDC of 10 per cent mass-burned fractionCA50 crank angle ATDC of 50 per cent mass-burned fractionCA90 crank angle ATDC of90 per cent mass-burned fractionFmbf the final mass-burned fractionCATrans crank angle the combustion transi-tioned to CI CATrans was computed as the CA of maximumsecondderivative ofthe.eHRR(d²HRR/dCA2:justified based on the arguments given in section2. These parameters were tabulated and cross-correlated with the image and spark-dischargeparameters discussed in the following sections.CA50 is used to quantify the combustion phasingof the main portion (CI) of the heat release. It isalso used conditionally to sample cycles withinthe heat-release analysisIstoDiaverage data fromcycles with like phasing. Conditionally sampledensemble averages of the cycle-resolved measuresHRR(CA), mbf(CA), and flamee area,Aflm(CA)were computed for the SACI operation also (e.g.Fig. 1). The heat release code was modified to identifycycles that had image and spark-discharge recordsand to register these cycles withi the pressurerecords. Cycle identification was necessary becauseimages and spark-discharge records were not avail-able every engine cycle. Specifically, each engine testrecorded 300 cycles of pressure data; however, thefirst image did not occur on the first recordedpressure cycle and images could be recorded onlyevery third or fourth engine cycle, randomly.Furthermore, although the oscilloscope could recordspark-discharge data on every cycle, it experiencedrandom drop-out also. Thus, cycle identification wasrequired and accomplished by recording the oscillo-scope-time-base gate and the image-intensifier gatealong with the in-cylinder pressure. The conditional-sampling algorithm was used to compile a table thatrelated the pressure-data-cycle number with theimage-record number, and spark-discharge-recordnumber. 3.3 Imaging A high-speed framing camera (LaVision Speed Star)with a lens-coupled intensifier (LaVision IRO) wasused for recording both the spark-discharge plasmachannel and the combustion chemiluminescence.The camera has a dynamic range of 12 bits with anoverall gain of approximately 6. The spatial resolu-tion here was 1.9 pixel/mm. For these tests theframes were recorded at up to 1 per CA (167 us)per image with 40 us gated-intensifier exposuretimes. The camera records one 16-frame image percycle at approximately 3/s; this was imposed by thetransfer time of the images to the random-accessmemory (RAM) of the host computer. The limitationof 80 images per test, noted in the previous section,was imposed by the 2 Gbyte RAM on the hostcomputer. A Halle 50mm f/2.5 UV lens with a 375 +25nmoptical bandpass filter was used to image either thespark-discharge or the combustion chemilumines-cence. This filter was used for imaging the spark-discharge as it is centred about the cyanide (CN)emission wavelength; this was used for spark-discharge AF measurements as described in the nextsection. The same filter was used for imaging the CIcombustion chemiluminescence, as it removed Cchemiluminescence from rich burning as well asincandescence. The filter resulted in imaging of boththe flame-kernel and HCCI combustion chemilumi-nescence; this filter is presumed to capture chemi-luminescence from both HCO* and the CO+0→CO2+hv continuum during combustion [16]. The spark discharge and flame propagation couldnot be imaged during the same tests for two reasons.Firstly, all 16 frames were required to resolveadequately either the spark-discharge (1 image/CAfor 15°) or the flame propagation (1 image/2.5 CA for35°) but were insufficient to do both. Secondly, theearly kernel imaging required 100 per cent gain toassure that the earliest kernels were detected. Thehigh intensity of the spark discharge would havedamaged the intensifier. For both reasons, the sparkdischarge and early kernel development had to berecorded during separate tests. Because the spark plug gap was in the bowl atTDC, it was not possible to image perpendicular tothe window in the head. Instead, the camera wasangled down 14±1° as shown in Fig. 8. Thisprovided a view of the spark-plug electrodes albeitdistorted. It caused a further complication; lightemitted or scattered from the top of the combustionchamber and above the direct field of view wasreflected into the imaged field of view via the bottom Fig.8Camera field of view and reflections from the bottom of the window surface of the window. This is illustrated by thedotted line in Fig. 8. An image of the bowl, sparkparflug, and combustion chamber are showninFig. 7(a) while illuminated with a white light. Thedouble images shown in Fig.7(a) are a result ofreflections illustrated in Fig. 8. The ramifications ofthe reflected images on the image processing will bediscussed. 3.4 Spark-discharge measurements and analysis The spark discharge event was recorded both bymeasuring the electrical discharge and by imagingthe plasma channel. Several metrics were computedfrom the spark discharge secondary-voltage andsecondary-current measurements and correlatedwith CA50. Plasma-channel excursion, measuredfrom the images, was correlated with CA50. 3.4.1 Electrical measurements The secondary voltage and current were recordedduring each engine cycle with a digital oscilloscope(time-based) sampling once per us. A Tektronix 60151high-voltage probe and a IPearson1100 currentmonitor (with a 100×1Mhz amplifier) were usedfor this purpose. An example of the record from adischarge is shown in Fig. 9(a). This shows anunresolved breakdown and arc discharge followedby the long glow discharge. The beginning of thedischarge, SOSpk, is measured as the time of thecurrent rise, and is set equal to the spark angle onthe CA encoder based engine-controller spark angle.The end of discharge, EOSpk, was defined as thetime at which the current returned to zero andgenerally was coincident with the disappearance ofthe plasma channel images. The voltage signal after Ispk=0 is the dissipation of the residual charge in thecoil. The following spark-discharge parameters werecomputed for each cycle from the spark-dischargecurrent and voltage records. Egap total energy delivered to the gap Pgap total powertspk spark discharge durationIcN/E; spark AF dV/dtspk voltage-decay rate Egap and Pgap were computed by integrating theproduct of the voltage and current between thebeginning (breakdown) and end of the discharge(I=0) minus the energy lost to the plug resistance.The duration is the time from SOSpk to EOSpk. Icn/E; was used as a measure of the AF in the spark-pluggap at SOSpk, AF as described by Fansler et al. [17],where Icn is the intensity of the CN emissionsrecorded by the intensified camera averaged over thefirst image after the spark breakdown and E; is thedischarge energy delivered to the gap during thecamera exposure.dV/dtdur was used in an attempt tocharacterize the velocity attheie spark plugasdescribed by Maly [18, 19]. This was found to beinadequate. Instead, imaging was used to record themaximum plasma-channel excursion during eachcycle as described in the next section. 3.4.2Imaging of the plasma channel To indicate the effect of the flow on the discharge,the spark-discharge plasma channel was imaged andthe maximum excursion of each cycle computed.A single parameter was used to correlate the cycle-to-cycle spark-dischargeplasma-channel excur-sions with cycle-to-cycle variations in combustion Fig. 9 (a) Current and voltage record and (b) binarized image of a plasma discharge form thesame cycle. The x’marks the c.o.g. position computed from each image phasing, namely, MaxXcog,=maximum distance (during a cycle)from the centre of the electrode gapto the plasma centre of gravity. To measure this, one 16-frame image per cycle wasrecorded of the spark-discharge channel taken at aframing rate of one per CA (0.167 ms) for 15 CAs(2.5ms). The 2.5 ms range was adequate for deter-mining the maximum excursion, in spite of the factthat there was a small percentage of cycles thatlasted up to 3 ms; 2.5ms was adequate becauselonger duration discharges occur in cycles with littleflow and therefore little or no plasma-channelexcursion. The voltage and current traces from acycle are shown in Fig. 9(a) along with the intensi-fier-gate signal. Processed images of the spark-discharge plasma-channel in Fig. 9(a) are presentedin Fig. 9(b). The outline of the spark-plug electrodesis superimposed for the purpose of illustration. Theoriginal images of plasma intensity were binarized atan intensity value of 0.03 times the maximumintensity within each image; thus, the thresholdchanges from frame to frame. This frame-to-framethreshold variation1 compensatesfor thelargechange in dynamic range during the dischargeof each cycle and from cycle to cycle. Note that the lastt two imagesdisappeared becausetheplasma chemiluminescence intensity falls belowthe threshold. To quantify the excursion of the spark channelfrom the gap, the centre of gravity (average centre ofarea), Xcog=(x"cog+y cog)1/2 was computed for eachframe and tabulated for each imaged cycle. The Xcogfor each frame is shown by the green ‘x'in Fig. 9(b).The maximum excursion of the channel, MaxXcogduring each cycle was then tabulated and correlatedwith other metrics in the spark and combustionprocesses. It is assumed that the impedance varia-tions due to temperature and composition variationsare small, and thus the cyclic variation of the sparkchannel excursion is dominated by the momentumof the velocity field. 3.5 Chemiluminescence-image analysis The combustion chemiluminescence was imagedfrom the CA immediately after EOSpk and thentaken every 2.5 CA thereafter. Thus, each 16-frameimage started at a different CA from engine cycle toengine cycle depending on the duration of theparticular discharge. Thiss is illustrated for twodifferent cycles in Figs 10(a) and (b), which showsthe first three intensifier-gate timings for a short and Fig. 10Spark voltage and current records from two cycles illustrating the variable start-of-imageacquisition long spark discharge respectively. The first imageframe was triggered 40 us (1/4°CA) after the spark-discharge current fell to zero to assure the dischargeplasma luminescence was over and thereby avoidintensifier damage. Since the spark-discharge dura-tion range was from 1.15 to 2.5 ms, the first frame ona given cycle was from -38 to -28 ATDC. Sixteen-frame images from the same SACI cycleare shown in Fig. 11, where Fig. 11(a) is displayedwith a linear scaleand 11(b) is displayed with a non-linear scale to enhance the low-intensity earlykernel. Each image was processed to remove therandom-pixel background noise, which was presentdue to the 100 per cent intensifier-gain setting. Also,a mask (Appendix 2) was applied as evident by theboundary in the last frame((+3.7ATDC). Thedynamic range is large because the early kernel hascounts as low as 1 and as large as approximately 4kin the most intense cycles. The one count isconsidered significant during the EKG period, sincethe image-processing noise-rejection procedures(masking and shot-noise removal procedures de-scribed in Fig. Al of Appendix 2) were sufficientlysevere that images with no early kernel (the weakestof the intensity) had no pixels above 0 count. Thus,the algorithm erred on the side of missing imageswith extremely weak chemiluminescence to assurethat one count was valid. The goal of this severecount removal was to assure that little or no noisecontributed to the flame area at the risk of losingvery weak early kernels. As the transition to flame was a robust intensity change, this does not affectthe computation of EOEkg,which isthe onlvquantitative metric used here. 3.5.1 Definition offlame parameters The images of the flame chemiluminescence wereanalysed to determine the following parameters. Aflm flame/kernel area in each frame 1flm average flame intensity in each frame EOEkg CA at the end of EKG Theparameterscomputeddffor the imagesinFigs 11(a)and (b)aare plotted in Fig. 11(C). Tocompute the flame area, the images were binarized(O's and l's,1=flame area, threshold to be dis-cussed) and the pixels summed to give the flamearea in that image. The average flame intensity wasdetermined by multiplying the intensity of eachframe by the binarized flame area in that frame andaveraging over that area. EOEkg was determinedfrom Aim(CA) as the crank angle where the secondderivative of Aflm(CA) is maximum. This is justifiedby the observation that the early kernels and thesubsequent iflame had distinctly different area-growth rates, as shown in the individual cycle ofFig. 11(c) and the conditionally-averaged curve inFig.1. Two different thresholds were used for binarizingeach image frame, a fixed ‘2-count’threshold was crank angle,aTDC Fig.11 Flame images from one cycle presented with (a) a linear intensity scale and (b) a non-linear scale. The flame areas computed using the two-count and 3 per cent filters areshown in (c) used to determine the EOEkg and a‘3 per cent-of-max’was used to define flame areas during the flamepropagation period. The two-count threshold wasfound to be best for identifying the end of the EKG event since the EKG had weak luminosity and nobackground noise. Also, it provided a single max-imum second derivative on the Aflm(CA) curve (c.f.the curves in Fig. 11(c)); which is unambiguous for detection of the second derivative. However, thetwo-count threshold overestimated the area duringthe flame propagation period because of the muchbrighter background scattering; thus the two-countthreshold would binarize most of the masked region.The high background light intensity could be fromeither or both of the (a) scattered chemilumines-cence from the combustion chamber walls, or (b)early, low-intensity CI chemiluminescence. The ‘3 per cent-of-max'binarization threshold setequal to one, all pixels whose intensities in a framewere greater than 3 per cent of the maximumintensity. This approach compensated for the verylarge dynamic range (>10’) of the chemilumines-cence intensity during the flame propagation period,since it automatically tracks the flame-intensityincrease. Furthermore, it rejects the increased back-ground noise that occurred with increased intensity,and thereby produces a flame area that agreed withwhat could intuitively be seen by eye.* The 3 percent-of-max threshold could be used to determinethe EOEkg, but produced noisier results. A directcomparison of the two-count and 3 per cent-of-maxthresholds revealed a 1:1 correlation with about 5 percent outliers (Appendix2). When cycles correspond-ing to these outliers were investigated, it was foundthat the 3 per cent-of-max threshold would producetwo maximum second derivatives because the max-imum-intensity increase from one frame to the nextcould sometimes produce a smaller area. This can beseen by comparing the two Anm(CA) curves inFig. 11(C). In any case, the major use of the thresholdwas to define the end of the EKG and estimate theobservable flame area; the use of one threshold overthe other does not alter the conclusions in this study. 4 RESULTS 4.1 Discrimination of flame and CIchemiluminescence 4.1.1 Flame discrimination in single cycles withhomogenous mixtures Discriminating between a flame and CI is difficultbecause chemiluminescence images from both typesof combustion are spatially inhomogeneous [20, 12].Also, imaging is a volume integration; thus, at anygiven time both processes may be occurring indifferent regions of the volume. CI and SACIcombustion could not be separated with spectral ( *The u se of intensity t o d iscriminate b etween f l ame and CI chemiluminescence is fully discussed in section 4. ) Fig.12(CI of homogeneous mixture (a) with and (b)without SI filters because the spectral signatures overlap. In thisstudy the intensity was used to define the spark-assisted combustion, which have here been pre-sumptuously labelled as early (flame) kernels andflame propagation leading to CI. Of course, this isnot rigorous but is based on empirical observationsmade in both homogeneous and fuel-stratified in-cylinder charges. The use of intensity to discriminatebetween ‘flame’and CI combustion is justified asfollows. First, consider the case of spark ignition in ahomogeneous mixture that creates a reaction kernelwith greater intensity than the CI but does notcontribute to the heat release. This is demonstratedin Fig. 12, which shows images from two differenthomogeneous cycles (single-early injection, 7.5 mg/cycle) at different intensifier gains, one cycle withand one cycle without spark discharge. These imageswere taken in this engine using a multi-framing CCDhaving a direct-coupled intensifier (LaVision, StreakStar) with an optical bandpass filter at 375 ±25 nm,which has higher sensitivity than the lens-coupledframing camera (LaVision, Speed Star). Figure 12(a)shows pure CI operation, which produces chemilu-minescence that is spatially uniform. Figure 12(b)shows the same operation with the spark turned on;the spark created a localized high-intensity chemi-luminescence region that grew in spatial extent,albeit slowly, and was eight times more intense thanthe HCCI chemiluminescence at the same CA. Thelatter image is interpreted as chemiluminescencefrom a reacting kernel initiated by the spark. Theengine-combustion phasing based on LPP and heatrelease analysis was unaffected by the spark atthis high-dilution and low fuelling rate. The pointhere is that even in a very homogeneous highlydilute environment, where the spark assist did not contribute to compression ignition (i.e. was notdetectable with heat-release analysis and did notalter the combustion phasing), the reaction kernelcould be discriminated from the CI combustionbased on intensity. As noted in the introduction, it isnot possible to proclaim that this is a true flame, butit is reasonable to assume the chemiluminescence-intensity increase isaan indication of a reactingkernel. 4.1.2 Flame discrimination in individual cycles withstratified mixtures Next, consider the applicability of flame discrimina-tion based on intensity with fuel-stratified engineoperation, where spark-assisted combustion does affect combustion phasing. Here, the intensity of thespark-ignited reaction kernels from three individualcycles during the fuel-stratified SACI engine opera-tion characterized in Table 1 is compared. Thepurpose is to demonstrate, in detail, SACI combus-tion evolution during three individual cycles withearly to late phasing. Figure 13 showss116-frameimages from three different cycles corresponding toCA50=-2,+2, and +8 ATDC. The 16 frames on theleft of each image were binarized with a 3 per cent-of-maximum value as described previously; theframes to the right are the original images displayedwith the non-linear scale of Fig. 11(b). The EOSpk,EOEkg, and CATrans are indicated based on thespark-dischargemeasurements,, image analysis,and heat-release analysis respectively. The heat- Fig. 13Binarized images and intensity images of cycles with (a) early, (b) late, and (c) no EKGafter spark xc Fig.14HRR: (a) flame area and intensity; (b) for thethree cycles shown in Fig. 13 release curves, flame area, and spatially-averagedflame intensity corresponding to these three cyclesare presented in Fig. 14. In all three cycles a reactionkernel existed at the end of the spark discharge, EOSpk,as indicated by the binarized images where a kernelexists in the first frame of all three images. Consider-able variation exists after this first image. In Fig. 13(a), the cycle with the earliest CA50, thereaction kernel chemiluminescence waspresentimmediately after EOSpk, remained visible as it grewin spatial extent until it filled the entire field of view.Comparison of Fig. 13(a) with Fig. 14 reveals thethree periods of combustion following spark-dis-charge. During the EKG period (EOEkg-EOSpk) thearea and intensity of the reaction kernel change littleand no significant heat release occurs. During theflame period (CATrans-EOEkg) the reaction kernelarea and intensity grow significantly along withsignificant heat release. During the CI period (CA>-CATrans) the area cannot grow since the chemilumi-nescence fills the field of view. However,the HRR andintensity grow at a significantly faster rate. Figure 13(b) shows a spark-assisted cycle that,although delayed, still advanced the combustioncycle earlier compared to a pure CI cycle. In thiscycle, the kernel that existed at EOSpk almostdisappeared until -18.5 ATDC. The disappearanceoccurs because either the kernel was behind the plug, )the kernel intensity became low and undetect-able, and/or (c)kernel moved into the bowl and returned The description of the three combustion periodsgiven for the cycle of Fig. 13(a) apply identically toFig. 13(b) except that the duration of the EKG periodis longer for the latter. Figure 13(C) shows a very late burning cycle wherethe initial kernel disappeared, and there was noindication of flame propagation in either the imageor heat-release analysis, essentially a pure CI cycle. Achemiluminescence image appeared only after theHRR transitioned to high values consistent with CIcombustion at CATrans=-1ATDC. No chemilumi-nescence was detected prior to CATrans where there isan almost constant near-zero HRR. This low HRRprior to CATrans is always observed in late SACI cyclesand is always observed in pure CI engine operation(early-injection). Although this could be due toinaccuracy of the heat-release/heat-transfer model,it is reasonable that a small amount of heat releasedoes exist due to the hydrocarbons and CO in the 60per cent rebreathed residuals. There could in fact be aconstant HRR release but no chemiluminescence wasrecorded because it did not exist, the intensity was tooweak to be detected by this camera, or the chemilu-minescence was hidden from the field of view. The images of the three example cycles in Fig. 13demonstrate that the intensity-based discriminationof the flame is reasonable both in the homogeneousand the fuel-stratified spark-ignited mixtures, at leastto the end of the EKG period. The images alsodemonstrate that there is a time between the endof the spark discharge and the beginning of thedetectable heat release (beginning of the flame) whena reaction kernel exists, as evidenced by the chemi-luminescence, that may grow quite slowly or dis-appear. The end of this EKG period is followed by amore rapidly growing reaction kernel (labelled here asa flame) that shows both higher HRR and increasedchemiluminescence.Thiscomparisonbetweenimages and the analysis also demonstrates that (a) EOEkg and the beginning of the heat-releaseagree; (b)CATrans occurs at the same time that thechemiluminescence fills the field of view andhas an intensity that begins to exceed that of theflame.0. The importance of (a) is that EOEkg is a validmetric to describe the beginning of the flame, whichwill be used to determine the phase delay between the end of spark and beginning of flame propagationwith measurable heat release. The EOEkg based onreaction-kernel area rather than the HRR is betterbecause dAfm(CA)/dCA is a larger and moredefinitive change compared to either dP(CA)/dCAor dHRR(CA)/dCA. Point (b) provides a visualcorroboration that CATrans, which is determinedby the heat release analysis, occurs at the beginningof the spatially distributed compression-ignitionevent. These conclusions have been demonstratedhere for only three example cycles as it is impracticalto publish many more. However, these conclusionshave been observed to be true for many cycles frommany different tests. 4.2Definition of EKG and flame combustion In this section the image-processing techniquesdemonstrated in Fig. 13 are used to show that the Alm(CA) trends in Fig. 14 are applicable beyondthese three cycles. Figure 15 shows the flame area growth, Aflm(CA),of many cycles conditionally sampled on CA50; eachcycle-resolved plot begins at the EOSpk for itsrespective cycle. Figure 15 demonstrates that thefeatures of Aflm(CA) observed in the three cycles inFig. 14 can be generalized. Namely, there exists (a)a slow or no area-growth-rate EKG period afterEOSpk, followed by (b)a relatively linear area-growth rate that achieves(c)a maximum and constant area once the com-bustion image fills the field of view. Figure 15 also demonstrates that (using the heavyline located at 10 ATDC as a guide) the duration ofthe EKG increases with CA50 delay. The success ofthe conditional sampling in classifying the early andlate Aflm(CA) curves provides confidence that the Crank Angle deg aTDC Fig. 15Cyclic variability of the engine cycles conditionally sampled on CA50 NEE 之工 ensemble averages conditionally sampled on CA50can be used to represent the EKG and flame periodsof the individual cycles with like phasing (CA50). Figure 16 plots conditionally sampled ensembleaverages
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