Flame Front Propagation in an Optical GDI Engine Under Stoichiometric and Lean Burn Conditions

energies

Article Flame Front Propagation in an Optical GDI Engine under Stoichiometric and Lean Burn Conditions

Santiago Martinez 1 ID , Adrian Irimescu 2, Simona Silvia Merola 2,* ID , Pedro Lacava 1 and Pedro Curto-Riso 3 ID 1 Technological Institute of Aeronautics, São Jose dos Campos 12228-900, Brazil; santiagoﬁ[email protected] (S.M.); [email protected] (P.L.) 2 Istituto Motori, Consiglio Nazionale delle Ricerche, 80125 Napoli, Italy; [email protected] 3 Department of Applied Thermodynamics, University of La República, Montevideo 11200, Uruguay; pcurto@ﬁng.edu.uy * Correspondence: [email protected]; Tel.: +39-081-717-7224

Received: 13 July 2017; Accepted: 30 August 2017; Published: 5 September 2017

Abstract: Lean fueling of spark ignited (SI) engines is a valid method for increasing efficiency and reducing nitric oxide (NOx) emissions. Gasoline direct injection (GDI) allows better fuel economy with respect to the port-fuel injection configuration, through greater flexibility to load changes, reduced tendency to abnormal combustion, and reduction of pumping and heat losses. During homogenous charge operation with lean mixtures, flame development is prolonged and incomplete combustion can even occur, causing a decrease in stability and engine efficiency. On the other hand, charge stratification results in fuel impingement on the combustion chamber walls and high particle emissions. Therefore, lean operation requires a fundamentally new understanding of in-cylinder processes for developing the next generation of direct-injection (DI) SI engines. In this paper, combustion was investigated in an optically accessible DISI single cylinder research engine fueled with gasoline. Stoichiometric and lean operations were studied in detail through a combined thermodynamic and optical approach. The engine was operated at a fixed rotational speed (1000 rpm), with a wide open throttle, and at the start of the injection during the intake stroke. The excess air ratio was raised from 1 to values close to the flammability limit, and spark timing was adopted according to the maximum brake torque setting for each case. Cycle resolved digital imaging and spectroscopy were applied; the optical data were correlated to in-cylinder pressure traces and exhaust gas emission measurements. Flame front propagation speed, flame morphology parameters, and centroid motion were evaluated through image processing. Chemical kinetics were characterized based on spectroscopy data. Lean burn operation demonstrated increased flame distortion and center movement from the location of the spark plug compared to the stoichiometric case; engine stability decreased as the lean flammability limit was approached.

Keywords: spark ignition engine; direct injection; ﬂame imaging; spectroscopy; lean operation

1. Introduction The Paris Climate Agreement COP21 emphasized the urgency to accelerate climate action in order to achieve the global target to keep global temperatures well below 2 ◦C above pre-industrial levels, and pursue efforts to limit warming to 1.5 ◦C[1]. Independently of the possibility and effective aims of the different countries to achieve these commitments, all participants in COP21 agreed that transport and energy sectors can contribute to reducing greenhouse gas (GHG) emissions and building economy-wide resilience to the impacts of climate change. Even though there is a trend of shifting transportation to electric vehicles [2], internal combustion engines will remain the dominant propulsion system for the next decade [3,4] and an efﬁcient solution for distributed generation [5].

Energies 2017, 10, 1337; doi:10.3390/en10091337 www.mdpi.com/journal/energies Energies 2017, 10, 1337 2 of 23

With respect to gasoline engines, the development of direct injection (DI) technologies supported by downsizing, variable valves timing, and lift has allowed the industry to improve fuel economy and engine performance [6–9]. In order to further enable a reduction in fuel consumption and CO2 emissions of gasoline direct-injection (GDI) engines, lean burn strategies are often tested [10]. Low fuel consumption with high power output can be simultaneously attained by means of the GDI engine technology, since a lean overall fuel–air mixture is possible with precisely controlled fuel injection directly into the cylinder, which leads to highly efficient combustion. The significant reduction in fuel consumption is achieved as a result of the reduced pumping loss during the intake stroke; furthermore, when the engine is operating under low loads, cooling losses are diminished because of lower combustion temperature. Meanwhile, when the engine is operating under high loads, the volumetric efficiency improves by the latent heat of the fuel and the possibility of knocking is reduced [11]. However, direct fuel injection into the cylinder of a GDI engine leads to the formation of particulate matter (PM), similar to in a diesel engine. The size distributions are strongly affected by the degree of fuel–air mixing because particles are mainly formed in the fuel-rich mixture region as a result of the poor spray evaporation and wall wetting [12,13]. Several solutions for reducing PM emissions, mainly based on alternative fuel injection and ignition strategies, as well as the control of charge motion, have been proposed taking into account the dependence from the engine operating parameters [14–18]. However, most of the studies were focused on stoichiometric mixture combustion and not on lean burn GDI operation. The effect of air dilution on in-cylinder particle formation is controversial; it was demonstrated that the PM concentration at a relatively high load decreases only for optimized ignition conditions. The degree of lean operation in GDI engines is principally limited by increasingly unstable combustion, particularly for homogeneous approaches. Under highly dilute conditions, flame speeds decrease, resulting in poor fuel conversion efficiency due to prolonged combustion. The upper stability limit is generally recognized as 5% coefficient of variation (COV) of the indicated mean effective pressure (IMEP) [19,20]. Moreover, cycle-to-cycle variation plays a key role in combustion since it can skew the output power even up to 10% in GDI engines [21]. Innovative ignition systems [22,23] and injection strategies [24] have been developed in order to extend this lean limit and exploit the benefits of homogeneous lean (λ > 1.4) combustion in terms of fuel consumption and nitrogen oxide (NOx) emissions. Deeper insight into the effects of excess air ratio on in-cylinder processes can be a useful support in the development of advanced solutions for the optimization of GDI engines. To this aim, the combustion process in a wall guided DISI engine operating in lean burn homogenous charge conditions was investigated. The experiments were performed at fixed engine speed and wide open throttle (WOT) in an optically accessible single-cylinder engine fueled with commercial gasoline. The air–fuel ratio (λ) varied from stoichiometric to values close to the lean flammability limit. High spatial resolution cycle resolved digital imaging was used to characterize the flame front propagation by macroscopic flame parameters; natural emission spectroscopy in the UV-visible range was applied to follow the evolution of chemical species (such as OH) and soot formation. Optical results were correlated with thermodynamic data and exhaust emission measurements. The main goal of the work is to contribute to the understanding of fundamental in-cylinder processes and support computational fluid dynamics (CFD) modeling.

2. Experimental Setup and Methods The measurements were performed on an optically accessible single cylinder DISI engine. It was equipped with the four-valve cylinder head of a commercial SI power unit (Figure1), centrally located semi-surface discharge spark plug, and wall-guided injection. The fuel system featured a commercial six-hole solenoid injector installed in the lateral side of the cylinder head. Further details on the engine are reported in Table1. Energies 2017, 10, 1337 3 of 23 Energies 2017, 10, 1337 3 of 23

8 Exhaust valves Optical 7 window 6 5 Pressure sensor 4 3 2 Injector 1 Intake valves

(a) (b)

Figure 1. Schematic representation of (a) the experimental setup and (b) bottom field of view of the Figure 1. Schematic representation of (a) the experimental setup and (b) bottom ﬁeld of view of the combustion chamber with the locations of the spectroscopic region of interest. combustion chamber with the locations of the spectroscopic region of interest.

Table 1. Engine specifications. Table 1. Engine specifications. Parameter Description ParameterDisplacement Description399 cm3 DisplacementStroke 39981.3 cm 3mm StrokeBore 81.379.0 mm mm Bore 79.0 mm ConnectingConnecting Rod Rod 143 143 mm mm CompressionCompression Ratio Ratio 10:1 10:1 NumberNumber of Valves of Valves 4 4 IntakeIntake Valves Valves Opening Opening (IVO) (IVO) 363363 CAD CAD BTDC BTDC Intake Valves Closure (IVC) 144 CAD BTDC ExhaustIntake Valves Valves Opening Closure (EVO) (IVC) 153144 CAD CAD ATDC BTDC ExhaustExhaust Valves Valves Closure Opening (EVC) (EVO) 360153 CAD CAD ATDC ATDC ExhaustIgnition Valves System Closure (EVC) central360 sparkCAD plugATDC Ignition System central spark plug A conventional Bowditch design was used for optical accessibility, with an optical crown (18 mm thickA fused-silica conventional window, Bowditch which design ensured was a fieldused offor view optical 63 mm accessibility, in diameter), with which an optical was screwed crown onto(18 mm the thick piston. fused-silica Therefore, window the combustion, which ensured chamber a wasfield visible of view through 63 mm a in UV-enhanced diameter), which 45-degree was mirror,screwed mounted onto the within piston. the Therefore, hollow piston. the combus Slottedtion graphite chamber piston was rings visible were through used to a provide UV-enhanced oil-less lubrication45-degree mirror, with uninterrupted mounted within bronze–Teflon the hollow rings piston used. Slotted for sealing. graphite piston rings were used to provideEngine oil-less speed lubrication was fixed with at 1000 uninterrupted rpm in order bronze–Teflon to study lean rings burn used combustion for sealing. in low turbulence conditions.Engine The speed load was was fixed fixed at in1000 wide rpm open in order throttle to (WOT)study lean mode burn and combustion the start of injectionin low turbulence was also fixedconditions. during The the load intake was stroke, fixed atin 300wide crank open angle throttle degrees (WOT) (CAD) mode beforeand the top start dead of injection center (BTDC) was also to createfixed during a homogeneous the intake mixture stroke, insideat 300 thecrank combustion angle degrees chamber. (CAD) Coolant before and top lubricantdead center temperatures (BTDC) to werecreate maintained a homogeneous at 330–335 mixture K using inside a thermalthe combustion conditioning chamber. unit; Coolant intake air and temperature lubricant temperatures was around 300were K maintained and ambient at pressure 330–335 was K using approximately a thermal 1condit atm.ioning All measurements unit; intake wereair temperature performed atwas minimum around spark300 K timingand ambient that ensured pressure maximum was approximately brake torque 1 (MBT) atm. (TableAll measurements2). The air–fuel were ratio performed was set atat stoichiometricminimum spark conditions, timing that and ensured was thenmaximum increased brake until torque the (MBT) combustion (Table process 2). The air–fuel became ratio unstable was (COVset at IMEPstoichiometric > 6%). Table conditions,2 shows theand increase was then of sparkincreased timing until in fivethe CADcombustion steps, with process the lambdabecame valuesunstable augmented; (COV IMEP a relatively > 6%). Table coarse 2 shows crank anglethe increa (CA)se step of spark was chosen, timing given in five the CAD flat behaviorsteps, with of the IMEPlambda close values to the augmented; MBT crank a angle. relatively coarse crank angle (CA) step was chosen, given the flat behavior of the IMEP close to the MBT crank angle.

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Table 2. Operating points.

Lambda Rpm SA (CAD bTDC) SOI (CAD bTDC) DOI (CAD) Load 1.0 10 21.5 1.3 15 16.5 1.41000 20300 15.5 WOT 1.5 25 14.4 1.6 30 13.5

In-cylinder pressure was acquired with an accuracy of ±1%, and averaged over 200 consecutive engine cycles; the optical measurements were carried out for 25 cycles of each set of 200 cycles. The relative air–fuel ratio was measured using a wide band exhaust gas oxygen sensor, with an accuracy of ±1%. Injection pressure was maintained at 100 bar for all conditions. Pollutant species concentrations (CO, CO2, HC and NOx) were measured in the undiluted exhaust gas stream using an AVL Digas 4000 analyzer (AVL, Graz, Austria), with a resolution of 0.01% (CO), 0.1% (CO2) and 1 ppm for the other two chemical species, all within 3% accuracy; the measurement principle was an electrochemical sensor for NOx and non-dispersive infrared (NDIR) analyzer (AVL, Graz, Austria) for the other components. Lastly, smoke opacity was measured by an acimeter (AVL Opacimeter 439, AVL, Graz, Austria).

2.1. Thermodynamic Analysis Heat release analysis was performed with a simplified approach of the first law of thermodynamics, using Equation (1): γ 1 dQ = ·p·dV + ·V·dp, (1) γ − 1 γ − 1 where Q is the net heat released measured in J, p pressure in Pa, V is the cylinder volume in m3, and the ratio of speciﬁc heats γ was set to 1.35. Mass fraction burned (MFB) was calculated based on the integral heat release [20,25] via Equation (2):

Qk − Qign MFBk = , (2) QEOC − Qign where the subscript ‘k’ means the position (CAD) during the combustion process, ‘ign’ denotes the initiation of combustion (ignition), and ‘EOC’ the end of combustion that was considered equal to EVO [25,26]. The combustion process in SI engines can be divided into four main stages [20]: spark and flame initiation, initial flame kernel development, turbulent flame propagation, and flame termination. This process can be quantified with the MFB curve, in which the CA corresponds to 0–10%, MFB is the flame development angle, 10–90% MFB corresponds to the rapid burning angle, and 0–90% MFB is the overall duration of the process. Combustion stability is represented by the cyclic variability derived from pressure data (COVIMEP). Lastly, fuel conversion efficiency was calculated using Equation (3):

IMEP·Vd η f = , (3) m f ·LHV where η f is the fuel conversion efﬁciency, IMEP in Pa was calculated based on recorded in-cylinder 3 pressure curves, Vd in m corresponds to the displacement volume, and LHV is the lower heating value of gasoline, taken as 42.9 MJ/kg; the mass of fuel injected per cycle m f was measured in kg, with an accuracy of ±1%. Energies 2017, 10, 1337 5 of 23

2.2. Optical Set-Up and Measurement Procedure Early flame front propagation was investigated through cycle resolved digital imaging and UV-visible natural emission spectroscopy. Visualization was performed with a high-speed CMOS camera (Optronis CamRecord 5000, 8-bit, 16 µm × 16 µm pixel size) (Optronis, Kehl, Germany) equipped with a 50 mm Nikon objective. The camera worked in full chip configuration (512 × 512 pixel) detecting 5000 frame per second with 200 µs fixed as exposure time. The f-stop of the objective was set at 2.8 to improve the signal to noise ratio without extensive saturation effects. The set-up allowed the acquisition of the images with a dwell time of 200 µs, corresponding to 1.2 CAD at 1000 rpm, and a spatial resolution of 0.188 mm per pixel. The optical trials related to each engine operative condition consisted in the acquisition of 60 frames per cycle after spark timing, during 25 consecutive engine cycles. Natural emission spectroscopy was applied to follow the evolution of excited chemical species involved in the combustion process from SI until the late combustion phase. UV-visible radiation from the combustion chamber was focused by a 78-mm focal length, f/3.8 UV Nikon objective lens onto the 250 µm micrometric controlled entrance slit of a f/4 spectrometer (Acton SP2150i, Princeton Instruments, Acton, MA, USA) with 150 mm focal length and 300 groove/mm grating (with 300 nm blaze wavelength). The spectrometer was coupled with an intensified charge coupled device (ICCD) camera (Princeton Instruments PI-MAX3 Gen II–UV, Princeton Instruments, Trenton, NJ, USA) provided with a 1024 × 1024-pixel sensor, 16 bit–32 MHz readout, with 45 lp/mm resolution and 0.02 lm/cm2 equivalent background illumination. The central wavelength was fixed at 350 nm in order to obtain information in the spectral range of 250–470 nm. Spectroscopic investigations were carried out in the central region of the combustion chamber. ICCD acquired 1024 spectra that appeared as rows and covered ≈90 mm along the vertical direction. In order to improve the signal-noise ratio, the spectra were binned (128 rows were merged) in eight locations; these were rectangular (≈1.5 × 11.3 mm), as shown in Figure1. Location 4 corresponded to the spark plug region placed in the center of the combustion chamber, while Locations 2 and 6 corresponded to the intake and exhaust valves region, respectively. Location 8 was intentionally far from the optical limit and it was used as background reference. The ICCD camera operated in sequential mode; 100 acquisitions (eight spectra per frame) were carried out at fixed gate width (40 µs =∼ 0.24 CAD) and increasing delay from spark timing (83.3 µs/cycle =∼ 0.5 CAD/cycle). Synchronization of various control triggers for ignition, injection and cameras was achieved using the optical encoder mounted on the crankshaft as an external clock connected to an AVL Engine Timing Unit (AVL, Graz, Austria). In order to achieve quantitative information from the optical investigations, image processing of the flame visualization and retrieving procedure of the spectroscopic data were developed. Image processing allowed the evaluation of macroscopic parameters related to flame morphology by a routine developed in Vision of National Instruments (National Instruments, Austin, TX, USA) [27]. The raw CMOS acquisitions were exported as eight-bit grey-level images. After the extraction of the intensity level, the image processing sequence adjusted the contrast and brightness of the images with respect to the maximum intensity value in order to optimize the signal to noise ratio. Successively, an appropriate circular mask was fixed in order to cut light from reflections at the boundaries of the optical access. Finally, a threshold was applied to obtain binary images. The output of the thresholding operation is a binary image [28,29] whose gray level of 1 (white) will indicate a pixel belonging to the object and a gray level of 0 (black) will indicate the background. A fixed threshold value was chosen for each engine operative condition, since the image sequence maintained an approximately constant difference between the flame intensity and background value during the combustion process. After thresholding, morphological transformations were applied to fill holes and remove small objects that cannot be considered part of the flame and may bias the evaluation of morphological parameters. More details about the procedure are given in [30]. The results of image processing consisted in the flame area A, Waddel disk diameter WDD, circularity FC and flame centroid movement CDM [31]. The flame area A corresponded to the number of pixels included in the foreground of binary images. Energies 2017, 10, 1337 6 of 23

The Waddel disk diameter was the diameter of a disk with the same area as the binary flame, calculated with Equation (4): √ WDD = 4 × A/π, (4) where the flame area was obtained by scaling the result in pixels and normalized to the cylinder cross section. The two geometrical parameters provided data useful for a direct comparison with combustion vessel experiments, where similar methods of flame radius growth quantification have been employed for laminar and turbulent flame speed characterizations [32]. The equivalent flame diameter allowed the estimation of the flame speed Sti , calculated using Equation (5): 1 WDDi−WDDi−1 St = , (5) i 2 ∆t where WDDi is the equivalent flame diameter in correspondence of the frame i, ∆t is the dwell time between two frames. Even though the view from the bottom of the combustion chamber allows only a line-of-sight evaluation and ‘projected burnt area’ would be a more correct phrasing [33], reference will be made to ‘flame area’ and ‘flame speed’ throughout the text. Combined visualization from below and the side in other studies [34] has resulted in a comparable flame front propagation speed during the initial combustion stages; therefore, the methodology used in this study can be considered as representative for the overall propagation speed up to the optical limit. Actual flame front delimitation is a matter of debate, with no clear chemical species accepted as markers that identify reaction zones (even though CH is generally accepted as representative in this sense [35]), and even different types of camera can give close but nonetheless dissimilar flame areas [36]. Nonetheless, the chemiluminescence given by the chemical reactions can be considered as a good evidence of flame front evolution. Regarding its propagation velocity, no unified definition of turbulent flame speed can be found in the literature; in this work, flame speed refers to the measured displacement of the flame front, determined based on the optical data recorded from below the combustion chamber. The shape of flame fronts was evaluated in terms of Heywood circularity factor FC that corresponds to the ratio between the perimeter P of the image and the circumference of a circle with the same area, according to Equation (6):

Pi FCi = , (6) π × WDDi where perimeter Pi was obtained by scaling the result measured in pixels. Finally the centroid CDMi was the arithmetical center of the binary flame image of the frame i; it was identified by the x and y coordinates with respect to the Cartesian system fixed in the center of the combustion chamber, corresponding to the location of the spark plug. The procedure for retrieving the spectroscopic data was developed in LabView of National Instruments (National Instruments, Austin, TX, USA), following the approach reported in [37]. Specifically, the intensity level of an individual compound (such as OH or CN radicals) was calculated by considering the difference between the highest emission Imax in a selected range and the value Imin obtained by linearly interpolating the intensities of closest emission minimum, I(λmin1) and I(λmin2), according to Equations (7)–(9): I = Imax − Imin (7)

Imax = I(λmax) (8) λ − λ I − I(λ ) max min1 = min min1 (9) λmin2 − λmin1 I(λmin2) − I(λmin1) where local minimum and maximum wavelength values were determined for each component. Figure2 reports a sketch of the procedure applied for the emission bands related to the OH radical. In the case of broadband emissions, the average intensity in the selected spectral range was considered Energies 2017, 10, 1337 7 of 23

as representative of the compound’s emission. Speciﬁcally for CO2* the averaged emission from

370 nmEnergies to 4002017, nm 10, 1337 was considered; soot emission was evaluated from 460 nm to 470 nm. 7 of 23

Energies 2017, 10, 1337 7 of 23 5000 OH Imax 5000 Selected spectral range OH Imax Selected spectral range 4000 4000

3000 3000

2000 CO2* Imin1 Imin2 2000 CO2* Imin Imin1 Imin2 I ICO2* 1000min ICO2* 1000

min1 min2 0 min1 max min2 0250 300 350 400 450 max 250 300wavelength 350 [nm] 400 450 Figure 2. Schematization of the applied methodologywavelength for [nm] retrieving the emission intensity of Figure 2. Schematization of the applied methodology for retrieving the emission intensity of chemical chemical species from spectroscopic results. speciesFigure from 2. spectroscopic Schematization results. of the applied methodology for retrieving the emission intensity of chemical species from spectroscopic results. 3. Results and Discussion 3. Results and Discussion 3. ResultsIn-cylinder and Discussion pressure was analyzed as average traces for 200 consecutive acquisitions. As shown inIn-cylinder FigureIn-cylinder 3, the pressure IMEP pressure decreased was was analyzed analyzed as the as air–fuel as average average ratio traces tr wasaces foraugmented; for 200 200 consecutiveconsecutive this was acquisitions. acquisitions. to be expected, As As showngiven shown in Figurethatin3 ,Figure thethe IMEPinjected 3, the decreased IMEPfuel quantity decreased as the was air–fuel as reduced the air–fuel ratio compared was ratio augmented; was to the augmented; stoichiometric this was this towas case. be to expected, At be the expected, limit given of given lean that the injectedcombustionthat fuel the quantityinjected regime fuel was (λ quantity reduced= 1.6) a wasreduction compared reduced of tocomparedaround the stoichiometric 30% to inthe COV stoichiometric was case. recorded, At thecase. limit with At the of respect leanlimit combustion ofto leanthe condition of λ = 1.0. Figure 4 depicts the coefficient of variation for the IMEP, as a parameter regimecombustion (λ = 1.6) regime a reduction (λ = 1.6) of arounda reduction 30% ofin around COV 30% was in recorded, COV was with recorded, respect with to respect the condition to the of representative for combustion stability; this graph suggests an increase of cycle by cycle variations λ = 1.0.condition Figure 4of depicts λ = 1.0. the Figure coefﬁcient 4 depicts of the variation coefficient for theof variation IMEP, as for a parameterthe IMEP, as representative a parameter for withrepresentative the increase for of combustion lambda values. stability; Up to this λ = 1.5,graph stable suggests engine an operation increase wasof cycle maintained, by cycle with variations COV combustion stability; this graph suggests an increase of cycle by cycle variations with the increase of valueswith the below increase 4%. Afterof lambda this point,values. instability Up to λ = featur 1.5, stableed a steepengine increase operation with was a peak maintained, of around with 6% COV for lambda values. Up to λ = 1.5, stable engine operation was maintained, with COV values below 4%. thevalues ‘leanest’ below case. 4%. After this point, instability featured a steep increase with a peak of around 6% for Afterthe this ‘leanest’ point, instabilitycase. featured a steep increase with a peak of around 6% for the ‘leanest’ case.

Figure 3. IMEP evaluated as average on 200 consecutive engine cycles for different air fuel ratios. Figure 3. IMEP evaluated as average on 200 consecutive engine cycles for different air fuel ratios. Figure 3. IMEP evaluated as average on 200 consecutive engine cycles for different air fuel ratios.

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Figure 4. COVIMEP evaluated on 200 consecutive engine cycles for different air fuel ratios. Figure 4. COVIMEP evaluated on 200 consecutive engine cycles for different air fuel ratios. Figure 4. COVIMEP evaluated on 200 consecutive engine cycles for different air fuel ratios. Figure 5 shows the fuel conversion efficiency, calculated as the ratio between work output and chemicalFigure energy5 5 shows shows content thethe fuel fuelof the conversionconversion injected fuel. efficiency,efficiency, The stoi calculated calculatedchiometric as as case the the ratio wasratio betweenthe between ‘least’ work workefficient, output output further and and confirmingchemicalchemical energy the need contentcontent to study ofof thethe the injectedinjected dilute conditions fuel.fuel. The stoichiometricstoi to reducechiometric engine case fuel was consumption. the ‘least’ efficient,efficient, Moreover, furtherfurther the efficiencyconfirmingconfirming increased the need need toup to study studyto λ the= the 1.5dilute dilute and conditions conditionsreached toits to reducepeak reduce slightlyengine engine fuelover fuel consumption. 23%. consumption. To put Moreover, things Moreover, into the λ perspective,theefficiency efficiency increased the increased injected up fuel upto λ toquantity = 1.5= 1.5 and was and reachedreduc reacheded byits its overpeak peak 36% slightly slightly and theover over IMEP 23%. 23%. featured To To put a decrease things intointo of aroundperspective,perspective, 30%, thethewhen injectedinjected switching fuelfuel quantityquantity from stoichiometric waswas reducedreduced to byby λ overover = 1.6 36%36% operation; and the this IMEP explains featured the a decreaseincrease of of λ aroundaround 10% 30%,30%, in whenwhen efficiency switchingswitching when from lean stoichiometricair–fuelstoichiometric mixtures to wereλ == 1.61.6 employed. operation;operation; this explainsexplains the increaseincrease ofof around 10%10% inin efficiencyefficiency whenwhen leanlean air–fuelair–fuel mixturesmixtures werewere employed.employed.

Figure 5. Fuel conversion efficiency evaluated on 200 consecutive engine cycles for different air fuelFigure ratios. 5.5. Fuel conversion efficiency efficiency evaluated on 200 consecutive engine cycles for different airair fuelfuel ratios.ratios. As the relative air–fuel ratio was varied, two different behaviors become apparent in Figures 4 As the relative air–fuel ratio was varied, two different behaviors become apparent in Figures 4 and 5,As in theline relative with well-establish air–fuel ratioed was trends varied, [20]. twoMoving different along behaviors the x-axis become (i.e., lambda apparent values), in Figures the first4 and 5, in line with well-established trends [20]. Moving along the x-axis (i.e., lambda values), the first regime,and5, in which line with starts well-established at stoichiometric trends conditions [ 20]. Moving and ends along at the the locationx-axis (i.e., of peak lambda efficiency values), (λ the = 1.5), first regime, which starts at stoichiometric conditions and ends at the location of peak efficiency (λ = 1.5), isregime, characterized which starts by ata steadily stoichiometric increasing conditions engine and efficiency ends at theand location low variability. of peak efficiency Once over (λ = 1.5),the is characterized by a steadily increasing engine efficiency and low variability. Once over the thresholdis characterized characterized by a steadily by increasingpeak efficiency, engine efficiencythe second and lowregime variability. can be Once identified, over the which threshold is threshold characterized by peak efficiency, the second regime can be identified, which is characterized by decreasing efficiency and rapidly increasing COVIMEP. Moreover, in this region, the maximumcharacterized decrease by decreasing of the performance efficiency and (IMEP) rapidly wa increasings recorded, COV betweenIMEP. Moreover, two consecutives in this region, lambda the maximum decrease of the performance (IMEP) was recorded, between two consecutives lambda

Energies 2017, 10, 1337 9 of 23

Energiescharacterized 2017, 10, 1337 by peak efficiency, the second regime can be identified, which is characterized9 of by23 decreasing efficiency and rapidly increasing COVIMEP. Moreover, in this region, the maximum decrease values,of the performance with a reduction (IMEP) of was 6% recorded, of IMEP between for λ = two1.6, consecutiveswith respect lambda to λ = values,1.5. Figure with a6 reductionreports the of average6% of IMEP pressure for λ = traces 1.6, with of the respect five toairλ fuel= 1.5. ratios Figure that6 reports were tested. the average It is immediately pressure traces evident of the that five airfor stoichiometricfuel ratios that wereoperation tested. the It is highest immediately peak evident pressure that was for stoichiometric obtained, as operation well as theoverall highest higher peak in-cylinderpressure was pressure obtained, during as well expansion, as overall in higherline with in-cylinder the calculated pressure IMEP. during The fact expansion, that the inengine line withwas operatedthe calculated close IMEP. to the The MBT fact points that the in engineeach condition was operated is evidenced close to the by MBTthe location points in of each peak condition pressure, is withinevidenced the bysame the range location for ofall peak air–fuel pressure, ratios. within As lambda the same values range increase, for all air–fuel the main ratios. parameter As lambda that sufferedvalues increase, important the mainchanges parameter is the flame that suffered developmen importantt angle changes (i.e., the is theinitial flame part development of combustion), angle reflected(i.e., the initialin more part advanced of combustion), ignition reflected(e.g., start in of more spark advanced (SOS) needed ignition to (e.g.,be advanced start of sparkby 20 (SOS)CAD whenneeded switching to be advanced from stoichiometric by 20 CAD when to λ = switching 1.6). from stoichiometric to λ = 1.6).

Figure 6. 6. In-cylinderIn-cylinder pressure pressure traces traces averaged averaged over over 200 200 co consecutivensecutive cycles cycles for for each each tested tested air–fuel air–fuel ratio. ratio.

A more detailed analysis of the averaged pressure traces was performed using the heat release approachA more described detailed in analysis the previous of the averaged section. pressure Figure traces7 depicts was the performed rate ofusing heat therelease heat releasein the combustionapproach described process infor the the previous stoichiometric section. and Figure lean7 depicts burn conditions. the rate of heatAs expected, release in the the ‘fastest’ combustion heat releaseprocess was for theobtained stoichiometric with λ = 1, and while lean for burn lean conditions. operation ‘smoother’ As expected, traces the were ‘fastest’ recorded; heat releasethis is λ directlywas obtained correlated with to =lower 1, while laminar for leanflame operation speed, resulting ‘smoother’ in ‘slower’ traces wereentrainment recorded; and this oxidation is directly of thecorrelated fresh charge. to lower The laminar effect is flame most speed, evident resulting in the initial in ‘slower’ stages entrainment of combustion, and given oxidation that offlame the kernelfresh charge. development The effect is influenced is most evident to a large in thedegree initial by the stages air–fuel of combustion, ratio. On the given other that hand, flame turbulent kernel flamedevelopment speed was is influenced closer for to aall large cases, degree which by theis ex air–fuelplained ratio. by the On theeffect other of hand,turbulence turbulent on flame flame speed was closer for all cases, which is explained by the effect of turbulence on flame propagation propagation (e.g., St = SL +0 Cd · rf/lM · u’, with St as turbulent flame speed, SL laminar flame speed, Cd (e.g., St = SL + Cd · rf/lM · u , with St as turbulent flame speed, SL laminar flame speed, Cd dimensionless dimensionless constant, rf flame radius, lM turbulence microscale and u’ turbulence intensity [38]). A r l u0 commonconstant, characteristicf flame radius, for allM leanturbulence cases is more microscale significant and ratesturbulence of heat release intensity during [38 ]).the A late common stages ofcharacteristic combustion, for which all leanis well cases correlated is more to significant lower oxidation rates ofrates heat behind release the during flame front the late[36]. stages of combustion, which is well correlated to lower oxidation rates behind the flame front [36].

Energies 2017, 10, 1337 10 of 23 Energies 2017, 10, 1337 10 of 23

Figure 7. Rate of heat release (a) and integral heat released; (b) traces averaged over 200 consecutive Figure 7. Rate of heat release (a) and integral heat released; (b) traces averaged over 200 consecutive cycles for each tested air–fuel ratio. cycles for each tested air–fuel ratio. The MFB traces shown in Figure 8 emphasize the fact that each operating point was close to the MTBThe ignition MFB tracessetting; shown this effect in Figure is more8 emphasize evident thecompared fact that to each the pressure operating curves point was(Figure close 6), to given the MTBthat ignitionthe reference setting; point this was effect taken is more as evidentthe SOS. compared All cases tofeatured the pressure quite curvesclose evolutions (Figure6), givenduring that the thefinal reference phase of point combustion, was taken as asthis the part SOS. of Allthe casesprocess featured was controlled quite close by evolutions the mass transfer during thefrom final the phasetop-land of combustion, region to the as cylinder. this part of the process was controlled by the mass transfer from the top-land regionTable to the 3 cylinder.shows several interesting parameters calculated based on the traces shown in Figure 8, whichTable suggest3 shows an severalincrease interesting in the duration parameters of the calculated early stage based of flame on the development, traces shown as in the Figure air–fuel8, whichratio suggestwas augmented. an increase Moreover, in the duration the maximum of the early in stagecrement of flame was development, registered when as the passing air–fuel ratiofrom waslambda augmented. 1.5 to 1.6, Moreover, with a difference the maximum of 5.4 incrementCAD. This wasparameter registered could when be linked passing with from the lambda increment 1.5 to in 1.6,combustion with a difference instability of and 5.4 CAD. decrease This of parameter the fuel conv couldersion be linked efficiency with for the the increment same lambda in combustion variation. instabilitySimilar and values decrease of 50% of the MFB fuel angle conversion were registered efficiency because for the sameof the lambda advanced variation. spark timing used in leanSimilar burn conditions; values of 50% this MFBwas to angle be expected, were registered given becausethat ignition of the was advanced set for operation spark timing close used to the in leanpoint burn of MBT. conditions; this was to be expected, given that ignition was set for operation close to the point of MBT. Table 3. Burn duration. Table 3. Burn duration. Lambda CA 0–10% MFB CA 50% MFB 1.0 Lambda CA 0–10%16.4 MFB CA 50% MFB 377.2 1.3 1.022.8 16.4 377.2 382.0 1.4 1.325.4 22.8 382.0 379.8 1.5 1.429.0 25.4 379.8 380.0 1.6 1.534.4 29.0 380.0 382.4 1.6 34.4 382.4

Energies 2017, 10, 1337 11 of 23 Energies 2017, 10, 1337 11 of 23

Figure 8. Mass fraction burned traces averaged over 200 consecutive cycles for each tested air–fuel ratio. Figure 8. Mass fraction burned traces averaged over 200 consecutive cycles for each tested air–fuel ratio.

The analysis of exhaust gas measurements gave more information on the chemistry of combustionThe analysis for the of exhaustinvestigated gas measurements conditions (Figur gavee more9). Carbon information monoxide on the (CO) chemistry concentrations of combustion were forlow, the with investigated only one conditionscase above (Figure 0.5% in9). the Carbon stoichiometric monoxide condition; (CO) concentrations this was to were be expected, low, with given only onethe caseincreased above availability 0.5% in the of stoichiometric oxygen in lean condition; conditions. this wasUnburned to be expected,hydrocarbon given (HC) the increasedemissions availabilityfeatured shown of oxygen a flat trend, in lean with conditions. a slight increase Unburned for diluted hydrocarbon conditions. (HC) Given emissions that no featured misfires shown were adetected, flat trend, this with result a slightcan be increaserelated to for more diluted prominent conditions. phenomena Given thatof quenching no misfires near were the detected, cylinder thiswall result [20], as can the be lean related flammability to more prominentwas approached phenomena. Nonetheless, of quenching this mechanism near the cylindercan be considered wall [20], asto thehave lean a flammabilityrelatively reduced was approached. influence on Nonetheless, overall efficiency; this mechanism to put things can be consideredinto perspective, to have a aconcentration relatively reduced of 100 influence ppm of onn-hexane overall in efficiency; burned gas to put (the things basis into for perspective,HC as n-hexane a concentration equivalent ofmeasurements 100 ppm of nperformed-hexane in in burned this study) gas (the is basisequivalent for HC to as aroundn-hexane 0.5% equivalent of the fuel measurements content of performedstoichiometric in this air–fuel study) mixtures. is equivalent The evolution to around of 0.5%nitrogen of the oxides fuel can content be related of stoichiometric to the thermal air–fuel effect mixtures.[20,39,40] with The evolutionthe highes oft value nitrogen for the oxides stoichiometric can be related combustion to the thermal process. effect With [ 20lean,39 mixtures,,40] with thethe highesttwo competing value for effects the stoichiometric (i.e., higher combustion concentration process. of oxygen With lean and mixtures, lower burned the two gas competing temperature) effects (i.e.,featured higher different concentration weights, of with oxygen temperature and lower having burned the gas more temperature) prominent featuredinfluence. different For the weights,‘leanest’ with temperature having the more prominent influence. For the ‘leanest’ case, a reduction of over case, a reduction of over 95% in NOx concentration was recorded. This value was comparable to the 95%efficiency in NO ofx concentrationthree way catalytic was recorded.converters This as indi valuecated was in comparable[20]. This further to the emphasizes efficiency ofthe three potential way catalyticof lean operation converters as asan indicated option for in increasing [20]. This efficiency further emphasizes and reducing the emissions, potential of given lean operationthat catalytic as anconverters option for efficiency increasing for efficiencyreducing andCO and reducing HC is emissions, high even givenin lean that conditions. catalytic convertersWhen looking efficiency at the foropacity reducing data, CO further and HC confirmation is high even of in the lean benefits conditions. of lean When operation looking atcan the be opacity identified, data, furtherwith a confirmationsignificant reduction of the benefits when passing of lean from operation stoichiometric can be identified, to λ > 1 region, with a thus significant suggesting reduction low particle when passingemissions from for stoichiometricthe alternative to controlλ > 1 region, strategy. thus suggesting low particle emissions for the alternative controlEven strategy. if the in-cylinder pressure measurements, coupled with exhaust gas analysis, allow a comprehensiveEven if the analysis in-cylinder of the pressure combustion measurements, characteristics coupled and with pollutant exhaust formation, gas analysis, they allowhave aa comprehensiveglobal characteranalysis and do of not the furnish combustion detailed characteristics results of andthe pollutantlocal distribution formation, of theythe burned have a globalmass. characterIn this sense, and docycle not resolved furnish visualization detailed results represen of thets local a powerful distribution tool offor the quantitative burned mass. analysis In this of sense,flame cyclefront resolvedpropagation. visualization Figure represents10 shows a powerfulselection toolof images for quantitative detected analysisin the stoichiometric of flame front propagation.condition from Figure the first 10 shows evidence a selection of the spark of images until the detected late combustion in the stoichiometric phase. The condition spark plug from design the first(i.e., evidencesemi-surface of the discharge) spark until allowed the late us combustion to detect the phase. flame Theinception spark right plug designfrom the (i.e., glow semi-surface phase that discharge)can be easily allowed identified us toby detectthe luminous the flame bright inception spot in right the center from of the the glow combustion phase that chamber. can be easily identified by the luminous bright spot in the center of the combustion chamber.

Energies 2017, 10, 1337 12 of 23 Energies 2017, 10, 1337 12 of 23 Energies 2017, 10, 1337 12 of 23

Figure 9. Exhaust gas emissions for different air–fuel ratios. Figure 9. Exhaust gas emissions for different air–fuel ratios. Figure 9. Exhaust gas emissions for different air–fuel ratios.

Figure 10. Images of flame evolution for stoichiometric operations. Figure 10. Images of flame evolution for stoichiometric operations. The glow phaseFigure featured 10. Images radiative of flame emissions evolution of for CN stoichiometric and OH radicals operations. [41–44]. As shown in FigureThe 11, glow only phasethe highest featured emission radiative band emissions of the CN of violetCN and system OH Bradicals2 Σ+ → Χ[41–44].2 Σ+ at As388 shownnm were in TheFigureresolved glow 11, for only phase both the cases. featured highest The emissionweaker radiative lines band emissions at of421 the nm CN and of violet CN 358 andnmsystem were OH B well2 radicals Σ+ →distinguished Χ2 [Σ41+ –at44 388]. only Asnm in shownwere the in Figureresolvedstoichiometric 11, only for theboth case. highest cases. Similarly, The emission weaker the 309 bandlines nm at band of 421 the nmcharacteristic CN and violet 358 nm systemof werethe Α well2B Σ2 +Σ →distinguished+ Χ→2 ΠX 2transitionΣ+ at only 388 for in nm OHthewere resolvedstoichiometricradicals for was both identifiable cases.case. Similarly, The in all weaker thethe spectra,309 lines nm whileband at 421 characteristicth nme weaker and 358band of nmthe at 283Α were2 Σ nm+ → wellwas Χ2 Πresolved distinguished transition only for in OH the only in lean burn limit. Other OH lines (such as the band at 314 nm) were too weak to be detected in all the the stoichiometricradicals was identifiable case. Similarly, in all the the spectra, 309 nm while band the characteristicweaker band at of 283 the nmA was2 Σ+ resolved→ X2 Π onlytransition in the for leancases. burn Moreover, limit. Other the emission OH lines due (such to theas the N2 bandtransition at 314 C 3nm) Πu →were Β3 Πtoog [42,45,46] weak to be resulted detected sufficiently in all the OH radicals was identifiable in all the spectra, while the weaker band at 283 nm was resolved2 + only cases.intense Moreover, to interfere the with emission the CN due (0, 0)to band.the N 2An transition emission C band3 Πu → around Β3 Πg 336[42,45,46] nm due resulted to the NH sufficiently Α Σ → in the lean burn limit. Other OH lines (such as the band at 314 nm) were too weak to be detected intense to interfere with the CN (0, 0) band. An emission band around 336 nm due to the NH Α2 Σ+ → C3 → B3 in all the cases. Moreover, the emission due to the N2 transition Πu Πg [42,45,46] resulted sufficiently intense to interfere with the CN (0, 0) band. An emission band around 336 nm due to the Energies 2017, 10, 1337 13 of 23

Energies 2017, 10, 1337 13 of 23 NH A2 Σ+ → X2 Π electronic transition was also observed, but the intensity resulted too low for the Χ2 Π electronic transition was also observed, but the intensity resulted too low for the retrieving Energies 2017, 10, 1337 13 of 23 retrievingprocedure. procedure. Χ2 Π electronic transition was also observed, but the intensity resulted too low for the retrieving 16000 procedure. 1.0 1.6 16000 CN 1.0 12000 1.6 N2CN 12000

8000 N2

8000 CN

4000 OH CN NH N + 2 CN 4000 OH OH NH N + 0 2 CN OH 250 300 350 400 450 0 wavelength [nm] Figure 11. UV-visible spectra250 detected in 300 the region350 near the spark 400 plug (Loc. 450 4) at 2 CAD ASOS for Figure 11. UV-visible spectra detected in the region near the spark plug (Loc. 4) at 2 CAD ASOS for λ = 1.0 and λ = 1.6. wavelength [nm] λ = 1.0 and λ = 1.6. Figure 11. UV-visible spectra detected in the region near the spark plug (Loc. 4) at 2 CAD ASOS for CN emissions at 388 nm were observed until around 15 CAD ASOS, when the end of the glow λ = 1.0 and λ = 1.6. CNphase emissions occurred; atthis 388 result nm is were in agreement observed with until the around images of 15 Figure CAD 10 ASOS, that showed when the evidence end of of the the glow phasespark occurred;CN for emissions about this 15–20 result at 388CAD is nmin after were agreement ignition observed was with until initia the aroundted. images Starting 15 ofCAD Figurefrom ASOS, the 10 methodology whenthat showed the end developed of evidence the glow by of the sparkphaseother for about authorsoccurred; 15–20 [47–49] this CAD result and after using is in ignitionagreement the retrieving was with initiated. proced the imagesure Starting of ofspectroscopic Figure from 10 thethat data, methodology showed the air–fuel evidence developedratio of was the by correlated with background-corrected emission intensity ratio of CN (388 nm)/OH (309 nm) otherspark authors for about [47–49 15–20] and CAD using after the ignition retrieving was initia procedureted. Starting of spectroscopic from the methodology data, the developed air–fuel ratio by was reported in Figure 12a. The averaged values at 4 and 7 CAD ASOS are showed in Figure 12b; the correlatedother authors with background-corrected [47–49] and using the retrieving emission proced intensityure of ratio spectroscopic of CN (388 data, nm)/OH the air–fuel (309 ratio nm) was reported related uncertain was estimated <5%. Results demonstrated a good linear relationship between in Figurecorrelated 12a. with The averagedbackground-corrected values at 4emission and 7 CAD intensity ASOS ratio are of showed CN (388 in Figurenm)/OH 12 (309b; the nm) related reportedlambda valuesin Figure and 12a. CN/OH The averaged ratios. In values agreement at 4 and wi 7th CAD the literatureASOS are [49,50],showed thein Figurechange 12b; in thethe uncertain was estimated <5%. Results demonstrated a good linear relationship between lambda values relatedpressure uncertain near the sparkwas estimated plug determined <5%. Results a change demon in thestrated linear atendency. good linear relationship between and CN/OH ratios. In agreement with the literature [49,50], the change in the pressure near the spark lambda5 values and CN/OH ratios. In agreement with the literature [49,50], the change in the plugpressure determined nearCN/OH athe change spark plug in the determined linear tendency. a change in the linear tendency. 1.6 54 1.3 CN/OH 1.0 1.6 4 1.3 3 1.0

10 0 5 10 15 20 25 CAD ASOS 0 (a)(b) 0 5 10 15 20 25 Figure 12. Time CADevolution ASOS of CN (388 nm)/OH (309 nm) emission intensity ratio (a) and average value estimated at( 4a )(and 7 CAD ASOS (b) at different air–fuel ratios. b)

Figure 12. Time evolution of CN (388 nm)/OH (309 nm) emission intensity ratio (a) and average Figure 12. Time evolution of CN (388 nm)/OH (309 nm) emission intensity ratio (a) and average value value estimated at 4 and 7 CAD ASOS (b) at different air–fuel ratios. estimated at 4 and 7 CAD ASOS (b) at different air–fuel ratios.

Energies 2017, 10, 1337 14 of 23 Energies 2017, 10, 1337 14 of 23

As observed in previous works [[25,39,40],25,39,40], flameflame kernel emission could be well distinguished at around 1010 CADsCADs after after spark spark timing timing due due to theto the luminosity luminosity of plasma of plasma induced induced between between the spark the plug’sspark electrodes.plug’s electrodes. After thisAfter time, this the time, flame the front flame propagated front propagated from the from center the of center the combustion of the combustion towards thetowards cylinder the cylinder walls. As walls. evidenced As evidenced in Figure in Figure10, around 10, around 15–20 15–20 CAD CAD ASOS, ASOS, a slight a slight asymmetry asymmetry and significantand significant displacement displacement of the of flamethe flame were were observed observed towards towards the the exhaust exhaust valves. valves. The The effect effect can can be appreciatedbe appreciated also also in in terms terms of of spectroscopic spectroscopic results, results, as as shown shown in in Figure Figure 13 13a.a. Specifically,Specifically, atat 1515 CAD ASOS, no evidence of flamesflames could be revealedrevealed in thethe regionregion ofof thethe intakeintake valvesvalves (Loc.(Loc. 2); radiativeradiative emissions werewere detecteddetected only only in in the the center center of theof th combustione combustion chamber chamber (Loc. (Loc. 4) and 4) inand the in region the region close toclose the to exhaust the exhaust valves valves (Loc. 6).(Loc. 6).

1600 1600 Intake valves Intake valves Spark plug OH Spark plug OH Exhaust valves Exhaust valves 1200 1200

800 800

CO2* emission intensity [a.u.] emission intensity 400 [a.u.] emission intensity 400 HCO+HCOH CH

HCO+HCOH

0 0 250 300 350 400 450 250 300 350 400 450 wavelength [nm] wavelength [nm] (a) (b)

Figure 13. UV-visible spectra detected in three regions of the combustion chamber at 15 CAD ASOS Figure 13. UV-visible spectra detected in three regions of the combustion chamber at 15 CAD ASOS (a) andand 1818 CADCAD ASOSASOS ((b)) forfor stoichiometricstoichiometric operation.operation.

Moreover, no more evidence of CN emission was observed near the spark plug and the spectrumMoreover, featured nomore high evidenceintensity offor CN the emission OH band was at observed 309 nm. nearA weak the sparkband plugat 431 and nm the due spectrum to the featured high intensity for the OH band at 309 nm. A weak band at 431 nm due to the A2∆ → X2Π Α2Δ → Χ2Π CH transition [44] was distinguished. As observed in previous works [37,51], during CHkernel transition growth [ 44the] wasemission distinguished. spectra presented As observed a continuum in previous background works [37,51 ],due during to the kernel overlapped growth the emission spectra presented a continuum background due to the overlapped emissions of HCO emissions of HCO and H2CO [52,53]. The formyl radical spectrum featured a Vaidya band system andthat Hextended2CO [52 ,from53]. The 280 formylnm to radical410 nm, spectrum with high featureder heads a in Vaidya the range band of system 310–340 that nm extended [54]. Excited from 280formaldehyde nm to 410 nm,emission with higher consisted heads of indiffuse the range band ofs 310–340between nm 300 [ 54nm]. Excitedand 500 formaldehyde nm (Emeléus emission system) consisted[55]. The ofbalance diffuse between bands between the two 300 species nm and depended 500 nm (Emel on differentéus system) effects, [55]. Theincluding balance local between fuel theconcentration two species and depended pressure, on which different influenced effects, includingthe efficiency local of fuel radiative concentration reactions and of pressure,formaldehyde which: influenced the efficiency of radiative reactions of formaldehyde: HCOH + hν → H + HCO, HCOH + hν → H +HCO. HCOH + hν → H + HCO, HCOH + hν → H2 + HCO. Formyl radicals successively interacted with oxygen compounds to form CO2 and H2O [54]. Spectroscopic investigations demonstrated that the flame front reached the intake valves region Formyl radicals successively interacted with oxygen compounds to form CO2 and H2O[54]. (Loc.Spectroscopic 2) around 18 CAD investigations ASOS. As demonstrated shown in Figure that 13b, the the flame corresponding front reached spectrum the intake maintained valves region the (Loc.features 2) aroundpreviously 18 CAD observed. ASOS. At As the shown same in time, Figure th 13e b,burned the corresponding mass at the center spectrum of the maintained combustion the featureschamber previouslywas characterized observed. by At a thehigh-intensity same time, OH the burnedband superimposed mass at the centeron a broadband of the combustion due to chamberexcited CO was2 molecules characterized [56,57]. by The a high-intensityemissions were OH exte bandnded superimposedfrom approximately on a broadband300 nm to 500 due nm, to with a peak around 400 nm and were most likely induced by the radiative reactions: excited CO2 molecules [56,57]. The emissions were extended from approximately 300 nm to 500 nm, with a peak around 400 nm and were most likely induced by the radiative reactions: Σ+ Π→Β

1 3 3 CO Σ + O Π → CO 2 B Β →Β 2

Β →Σ +ℎ.

Energies 2017, 10, 1337 15 of 23

3 1 CO2 B2 → CO2 B2

1 1 + Energies 2017, 10, 1337 CO2 B2 → CO2 Σg + hν. 15 of 23

FlameFlame displacement displacement was was partially partially due due to to fluid fluid mo motion,tion, basically basically induced induced by by tumble, tumble, and and to to the differentdifferent thermalthermal regimeregime between between the the intake intake and and exhaust exhaust side side of theof combustionthe combustion chamber. chamber. On the On other the otherhand, hand, the evidence the evidence of high-intensity of high-intensity flames inflames the intake in the valve intake region, valve near region, the injector,near the confirmed injector, confirmedthe presence the of presence fuel deposits of fuel that deposits burned that when burned they when interacted they withinteracted the spark with ignited the spark flame ignited front flame(Figure front 10). (Figure In agreement 10). In agreement with other with studies, other the studies, deposits the deposits determined determined fuel-rich fuel-rich zones that zones slowed that sloweddown the down flame the propagation flame propagation [40]. In order[40]. In to betterorder investigateto better investigate the effect, the the imageeffect, processingthe image processingpreviously previously described wasdescribed performed was performed considering cons theidering sequences the sequences related to 25rela consecutiveted to 25 consecutive cycles for cyclesall lambda for all values lambda (from values 1.0 (from to 1.6). 1.0 Figure to 1.6). 14 Figure shows 14 the shows evolution the evolution of the average of the luminousaverage luminous centroid centroidwith respect with to respect the geometrical to the geometrical center of the center combustion of the combustion chamber, corresponding chamber, corresponding to the spark plug’sto the sparkcentral plug’s electrode. centralX electrode.–Y centroid X– coordinatesY centroid coordinates are biased are by abiased percentage by a percentage variation <8%.variation Data <8%. are Datareferred are toreferred the period to the of combustionperiod of combustion processes betweenprocesses spark between timing spark and timing the point and where the point flame where fronts flamereached fronts the opticalreached limit. the optical As evidenced limit. As in evidenced Figure 15 a,in thisFigure duration 15a, this is stronglyduration relatedis strongly to the related air–fuel to theratio. air–fuel Even thoughratio. Even the overallthough propagation the overall wasprop longeragation for was lean longer mixtures for lean compared mixtures to stoichiometric compared to stoichiometricoperation, given operation, that the sparkgiven timingthat the was spark advanced timing forwas the advanced former cases, for the the former actual cases, crank the angle actual that crankcorresponded angle that to flamescorresponded reaching to theflames optical reaching limit was the withinoptical alimit relatively was within narrow a rangerelatively (i.e., narrow around range10–15 CAD(i.e., around ATDC). 10–15 ‘Slower’ CAD lean ATDC). flames ‘Slower’ were found lean to flames ‘touch’ were the opticalfound limitto ‘touch’ mainly the in optical the exhaust limit mainlyvalves region,in the while exhaust stoichiometric valves region, combustion while featured stoichiometric a more evencombustion distribution featured around a the more periphery even distributionof the cylinder. around the periphery of the cylinder.

Figure 14. Evolution of the luminous centroid with respect to geometrical center of the combustion Figure 14. Evolution of the luminous centroid with respect to geometrical center of the combustion chamber; each step of the path is obtained by averaging data over 25 consecutive engine cycles. chamber; each step of the path is obtained by averaging data over 25 consecutive engine cycles. The centroid path demonstrated an increasing displacement of the flame as air–fuel mixtures were Theleaner. centroid This confirmed path demonstrated a more relevant an increasing effect of displacement tumble as laminar of the flame flame speed as air–fuel was lower; mixtures the effectwere leaner.of ignition This timing confirmed also needs a more to relevantbe consid effectered, of given tumble that as fluid laminar motion flame was speed stronger was earlier lower; duringthe effect the of compression ignition timing stroke, also needstherefore to be more considered, likely to given convect that the fluid flame motion kernel was away stronger from earlier the sparkduring plug’s the compression electrodes stroke,[34]. A thereforecounterclockwise more likely centroid to convect motion the flamewas observed kernel away for all from conditions, the spark dueplug’s to a electrodes slight swirl [34 present]. A counterclockwise during combustion, centroid as identified motion wasby the observed relatively for reduced all conditions, displacement due to alonga slight the swirl x-axis. present The duringpath that combustion, resulted was as identified slightly wider by the for relatively leaner reducedconditions, displacement suggesting along that flamethe x-axis. front The progression path that resultedwas biased was slightlyby the widercoupling for leanerof the conditions,in-cylinder suggesting flow and thatthe flamefuel concentrationfront progression field. was Figure biased 15a,b by the shows coupling the ofevol theution in-cylinder of flame flow area and and the fuelspeed. concentration The area field.was calculated as the average of 25 consecutive sequences and the speed was evaluated by the numerical incremental ratio of the Waddell diameter. The percentage error of flame areas was estimated around 7%. As expected, flame kernel inception (5% of normalized area) was prolonged at increasing lambda values; switching from the stoichiometric ratio to the lean burn limit, this initial combustion phase was longer, 25 CADs for the latter compared 10 CADs for the first case. The effect

Energies 2017, 10, 1337 16 of 23

Figure 15a,b shows the evolution of flame area and speed. The area was calculated as the average of 25 consecutive sequences and the speed was evaluated by the numerical incremental ratio of the Waddell diameter. The percentage error of flame areas was estimated around 7%. As expected, flame kernel inception (5% of normalized area) was prolonged at increasing lambda values; switching Energies 2017, 10, 1337 16 of 23 from the stoichiometric ratio to the lean burn limit, this initial combustion phase was longer, 25 CADs directlyfor the latter influenced compared flame 10 CADs front forpropagation the first case. and The determined effect directly an influencedincrease in flame the time front necessary propagation to reachand determined the optical an limit increase at increasing in the time lambda necessary values; to reach this thecorresponded optical limit to at a increasing decrease lambdain the laminar values; flamethis corresponded speed. In terms to a of decrease average in values, the laminar the lean flame burn speed. limit In was terms characterized of average by values, a 20% the decrease lean burn in thelimit highest was characterized flame speed by(Figure a 20% 15b), decrease in line in thewith highest the previous flame speedassertion (Figure with 15 regardb), in lineto turbulent with the previousflame speed. assertion with regard to turbulent flame speed.

(a) (b)

Figure 15. Evolution of ( a) the ﬂameflame front area and (b) related speed, both obtained by average data over 25 consecutive engine cycles.

The air–fuel ratio not only influenced the displacement of the flame but also the shape and The air–fuel ratio not only influenced the displacement of the flame but also the shape and wrinkling, as it can be appreciated in Figure 16 by the evolution of the flame front contours as they wrinkling, as it can be appreciated in Figure 16 by the evolution of the flame front contours as they approached the optical limit. To obtain more details on flame morphology, the Heywood Factor was approached the optical limit. To obtain more details on flame morphology, the Heywood Factor was evaluated in each operative condition. Results reported in Figure 17 suggest that for stoichiometric evaluated in each operative condition. Results reported in Figure 17 suggest that for stoichiometric combustion flame propagation spread more uniformly in all directions. On the other hand, in case of combustion flame propagation spread more uniformly in all directions. On the other hand, in case of diluted operation the simultaneous action of the flow field and fuel distribution determined an diluted operation the simultaneous action of the flow field and fuel distribution determined an increase increase in the flame distortion. At the lean burn limit, the effect was less evident most likely due to in the flame distortion. At the lean burn limit, the effect was less evident most likely due to the the concomitance of the convection velocity in the swirl plane and tumble motion. This fully fitted concomitance of the convection velocity in the swirl plane and tumble motion. This fully fitted the the observations reported in [34]. As a consequence of these concurrent phenomena, the highest observations reported in [34]. As a consequence of these concurrent phenomena, the highest values of values of the Heywood factor were found for the case of λ = 1.4. the Heywood factor were found for the case of λ = 1.4. Regarding the evidence of more wrinkled flames in the diluted cases shown in Figure 16, the Regarding the evidence of more wrinkled flames in the diluted cases shown in Figure 16, results were in good agreement with the analysis performed in [34,58]; these demonstrated that lean the results were in good agreement with the analysis performed in [34,58]; these demonstrated flames are more destabilized at fixed turbulence intensity than stoichiometric ones. Vortices in the that lean flames are more destabilized at fixed turbulence intensity than stoichiometric ones. Vortices turbulence field caused flame strain and curvature, ultimately leading to the onset of flame in the turbulence field caused flame strain and curvature, ultimately leading to the onset of flame wrinkling and formation of flame pockets [59]. wrinkling and formation of flame pockets [59].

Figure 16. Typical evolution of the flame front contours in the early combustion stages.

Energies 2017, 10, 1337 16 of 23 directly influenced flame front propagation and determined an increase in the time necessary to reach the optical limit at increasing lambda values; this corresponded to a decrease in the laminar flame speed. In terms of average values, the lean burn limit was characterized by a 20% decrease in the highest flame speed (Figure 15b), in line with the previous assertion with regard to turbulent flame speed.

(a) (b)

Figure 15. Evolution of (a) the flame front area and (b) related speed, both obtained by average data over 25 consecutive engine cycles.

The air–fuel ratio not only influenced the displacement of the flame but also the shape and wrinkling, as it can be appreciated in Figure 16 by the evolution of the flame front contours as they approached the optical limit. To obtain more details on flame morphology, the Heywood Factor was evaluated in each operative condition. Results reported in Figure 17 suggest that for stoichiometric combustion flame propagation spread more uniformly in all directions. On the other hand, in case of diluted operation the simultaneous action of the flow field and fuel distribution determined an increase in the flame distortion. At the lean burn limit, the effect was less evident most likely due to the concomitance of the convection velocity in the swirl plane and tumble motion. This fully fitted the observations reported in [34]. As a consequence of these concurrent phenomena, the highest values of the Heywood factor were found for the case of λ = 1.4. Regarding the evidence of more wrinkled flames in the diluted cases shown in Figure 16, the results were in good agreement with the analysis performed in [34,58]; these demonstrated that lean flames are more destabilized at fixed turbulence intensity than stoichiometric ones. Vortices in the Energiesturbulence2017, 10field, 1337 caused flame strain and curvature, ultimately leading to the onset of 17flame of 23 wrinkling and formation of flame pockets [59].

Figure 16. Typical evolution of the flame front contours in the early combustion stages. Energies 2017, Figure10, 1337 16. Typical evolution of the ﬂame front contours in the early combustion stages. 17 of 23

Figure 17. Evolution of the Heywood Circularity Factor, obtained by averaging data over 25 consecutive Figure 17. Evolution of the Heywood Circularity Factor, obtained by averaging data over 25 consecutive engine cycles. engine cycles. Following the combustion process throughout its progression, from around 20–35 CAD ASOS, the flamesFollowing reached the the combustion optical limit process in all throughout investigated its conditions. progression, The from emission around spectra 20–35 related CAD ASOS,to the burnedthe flames gas reached evolved the only optical in intensity limit in and all investigated not in spectral conditions. behavior, The as emissioncan be observed spectra in related Figure to 18a the forburned the stoichiometric gas evolved only condition. in intensity After andthis nottime, in as spectral shown behavior,in Figure as10, can strong beobserved luminous in flames Figure were 18a observedfor the stoichiometric first systematically condition. in the After intake this valves time, asregion shown and in then Figure in random10, strong locations luminous near flames the walls. were Asobserved observed first in systematically previous works in the[13, intake39,51,60], valves these region flames and were then due in random to the diffusive locations combustion near the walls. of stratifiedAs observed fuel in in previous the injector works region [13, 39and,51 on,60 ],the these piston flames surfaces. were In due terms to the of diffusivespectroscopic combustion evidence of (Figurestratified 18b), fuel these in the flames injector featured region a and continuous on the piston signal surfaces. that increased In terms with of a spectroscopic wavelength similar evidence to the(Figure blackbody 18b), these curve flames described featured by Planck’s a continuous radiation signal law that[61]. increased The oxidation with of a wavelengthfuel deposits similar induced to the blackbodyformation curveof carbonaceous described by structures Planck’s that radiation formed law soot [61]. by The aggregation. oxidation of Soot-specific fuel deposits emission induced wasthe formationstill significant of carbonaceous at 70 CAD structures ASOS in that the formed region sootnear by the aggregation. intake valves Soot-specific for the stoichiometric emission was condition,still significant as shown at 70 CADin Figure ASOS 19. in These the region results near dem theonstrate intake valvesthat the for fuel the deposit stoichiometric flames condition,persisted wellas shown after the in Figurenormal 19 combustion. These results event. demonstrate Moreover, th thate spectral the fuel evidence deposit of flames OH confirmed persisted the well role after of the normal combustion event. Moreover, the spectral evidence of OH confirmed the role of this this radical as a fundamental oxidant of soot in turbulent combustion systems, with O2 being of secondaryradical as a importance fundamental [62]. oxidant of soot in turbulent combustion systems, with O2 being of secondary importance [62]. 5000 OH 5000 Intake valves Intake valves Spark plug OH Spark plug 4000 Exhaust valves 4000 Exhaust valves

3000 3000

OH 2000 2000 OH CO2* CO2* soot emission intensity [a.u.] emission intensity 1000 1000

0 0 250 300 350 400 450 250 300 350 400 450 wavelength [nm] wavelength [nm] (a) (b)

Figure 18. UV-visible spectra detected in three regions of the combustion chamber at 23 CAD ASOS (a) and 38 CAD ASOS (b) for λ = 1.0.

Energies 2017, 10, 1337 17 of 23

Figure 17. Evolution of the Heywood Circularity Factor, obtained by averaging data over 25 consecutive engine cycles.

Following the combustion process throughout its progression, from around 20–35 CAD ASOS, the flames reached the optical limit in all investigated conditions. The emission spectra related to the burned gas evolved only in intensity and not in spectral behavior, as can be observed in Figure 18a for the stoichiometric condition. After this time, as shown in Figure 10, strong luminous flames were observed first systematically in the intake valves region and then in random locations near the walls. As observed in previous works [13,39,51,60], these flames were due to the diffusive combustion of stratified fuel in the injector region and on the piston surfaces. In terms of spectroscopic evidence (Figure 18b), these flames featured a continuous signal that increased with a wavelength similar to the blackbody curve described by Planck’s radiation law [61]. The oxidation of fuel deposits induced the formation of carbonaceous structures that formed soot by aggregation. Soot-specific emission was still significant at 70 CAD ASOS in the region near the intake valves for the stoichiometric condition, as shown in Figure 19. These results demonstrate that the fuel deposit flames persisted well after the normal combustion event. Moreover, the spectral evidence of OH confirmed the role of thisEnergies radical2017, 10 as, 1337 a fundamental oxidant of soot in turbulent combustion systems, with O2 being18 of of 23 secondary importance [62].

5000 OH 5000 Intake valves Intake valves Spark plug OH Spark plug 4000 Exhaust valves 4000 Exhaust valves

3000 3000

OH 2000 2000 OH CO2* CO2* soot emission intensity [a.u.] emission intensity 1000 1000

0 0 250 300 350 400 450 250 300 350 400 450 wavelength [nm] wavelength [nm] (a) (b)

Figure 18. UV-visible spectra detected in three regions of the combustion chamber at 23 CAD ASOS Figure 18. UV-visible spectra detected in three regions of the combustion chamber at 23 CAD ASOS (a) and 38 CAD ASOS (b) for λ = 1.0. Energies(a )2017 and, 10 38, 1337 CAD ASOS (b) for λ = 1.0. 18 of 23

1000 50°ASOS 60°ASOS 800 70°ASOS

600

400

200

0 250 300 350 400 450 wavelength [nm] Figure 19. UV-visible spectra detected near the intake valves in the late combustion phase for λ = 1.0. Figure 19. UV-visible spectra detected near the intake valves in the late combustion phase for λ = 1.0.

Figure 20 shows the maximum emissions intensity recorded during combustion of OH, CO2, Figure 20 shows the maximum emissions intensity recorded during combustion of OH, CO , and soot for three air–fuel ratios. In agreement with results of Ciatti et al. [63], there is an exponential2 and soot for three air–fuel ratios. In agreement with results of Ciatti et al. [63], there is an exponential decrease of intensity for all three categories of species, when mixtures were leaner. This is correlated decrease of intensity for all three categories of species, when mixtures were leaner. This is correlated to to lower peak pressure values, as well as actual concentrations within the burned/reacting gas. The lower peak pressure values, as well as actual concentrations within the burned/reacting gas. The fact fact that for λ = 1.6 soot-specific emission was much lower compared to the other two cases that for λ = 1.6 soot-speciﬁc emission was much lower compared to the other two cases emphasizes emphasizes that particle formation was directly reduced during combustion; on the other hand, with that particle formation was directly reduced during combustion; on the other hand, with a higher a higher equivalence ratio, the actual measurement at the exhaust was the result of soot formation– equivalence ratio, the actual measurement at the exhaust was the result of soot formation–oxidation oxidation that most likely continued throughout expansion and even during the exhaust stroke. that most likely continued throughout expansion and even during the exhaust stroke. Given the ‘slower’ Given the ‘slower’ combustion behavior for lean air–fuel ratios, maximum intensity values were combustion behavior for lean air–fuel ratios, maximum intensity values were recorded later (at higher recorded later (at higher CAD values ASOS). For OH and CO2 these locations were practically the CAD values ASOS). For OH and CO these locations were practically the same for each air–fuel ratio, same for each air–fuel ratio, suggesting2 that these instances were characterized by burned gas with suggesting that these instances were characterized by burned gas with compositions close to chemical compositions close to chemical equilibrium. Practically, OH radicals can be considered as more equilibrium. Practically, OH radicals can be considered as more representative for intermediate representative for intermediate reactions, while CO2 was specific to completed oxidation, as reactions, while CO was speciﬁc to completed oxidation, as observed by Irimescu et al [36]. This is observed by Irimescu2 et al [36]. This is very likely given that the instances were around the points of peak pressure, and therefore after flame front propagation was completed. Being the result of diffusive flames induced by fuel deposits, maximum values of soot-specific emissions were recorded later during expansion.

(a) (b)

Figure 20. Maximum emission intensity (a) and delay from the spark timing of the maximum emission intensity (b) measured in the combustion chamber from spectroscopic investigations for

OH, CO2, and soot.

Energies 2017, 10, 1337 18 of 23

1000 50°ASOS 60°ASOS 800 70°ASOS

600

400

200

0 250 300 350 400 450 wavelength [nm] Figure 19. UV-visible spectra detected near the intake valves in the late combustion phase for λ = 1.0.

Figure 20 shows the maximum emissions intensity recorded during combustion of OH, CO2, and soot for three air–fuel ratios. In agreement with results of Ciatti et al. [63], there is an exponential decrease of intensity for all three categories of species, when mixtures were leaner. This is correlated to lower peak pressure values, as well as actual concentrations within the burned/reacting gas. The fact that for λ = 1.6 soot-specific emission was much lower compared to the other two cases emphasizes that particle formation was directly reduced during combustion; on the other hand, with a higher equivalence ratio, the actual measurement at the exhaust was the result of soot formation– oxidation that most likely continued throughout expansion and even during the exhaust stroke. Given the ‘slower’ combustion behavior for lean air–fuel ratios, maximum intensity values were recorded later (at higher CAD values ASOS). For OH and CO2 these locations were practically the same for each air–fuel ratio, suggesting that these instances were characterized by burned gas with

Energiescompositions2017, 10, 1337close to chemical equilibrium. Practically, OH radicals can be considered as 19more of 23 representative for intermediate reactions, while CO2 was specific to completed oxidation, as observed by Irimescu et al [36]. This is very likely given that the instances were around the points of verypeak likelypressure, given and that therefore the instances after flame were aroundfront propagation the points ofwas peak completed. pressure, Being and therefore the result after of flamediffusive front flames propagation induced was by fuel completed. deposits, Being maximu the resultm values of diffusive of soot-specific flames inducedemissions by were fuel recorded deposits, maximumlater during values expansion. of soot-specific emissions were recorded later during expansion.

(a) (b)

Figure 20. Maximum emission intensity (a) and delay from the spark timing of the maximum Figure 20. Maximum emission intensity (a) and delay from the spark timing of the maximum emission emission intensity (b) measured in the combustion chamber from spectroscopic investigations for intensity (b) measured in the combustion chamber from spectroscopic investigations for OH, CO2, andOH, soot.CO2, and soot. Energies 2017, 10, 1337 19 of 23

The evolution of emission intensity for OH and sootsoot shown in Figure 21 further emphasizes the effect ofof gasgas densitydensity on on this this type type of of data; data; the the location location of peakof peak OH OH emission emission is practically is practically the samethe same as that as ofthat the of in-cylinder the in-cylinder pressure pressure traces traces (see Figure (see 6Figu forre comparison). 6 for comparison). As evidenced As evidenced by the common by the common trend of steeptrend intensificationof steep intensification followed byfollowed a decrease by duringa decrease expansion, during concentrationsexpansion, concentrations for both categories for both of speciescategories are of related species to ongoingare related oxidation to ongoing reactions oxid duringation flamereactions front during propagation flame (infront the propagation case of OH) and(in the at thecase surface of OH) of and liquid at the fuel surface film (for of liquid soot). fuel film (for soot).

8000 OH

6000

4000

emission intensity [a.u.] emission intensity 2000

0 20 40 60 80 CAD ASOS

Figure 21.21. EvolutionEvolution of of emission emission intensity intensity for for OH OH ( leftleft)) and soot (right) determineddetermined based onon spectroscopic investigations.investigations.

4. Conclusions With the goal of providing detailed insight into the specifics of lean combustion in SI engines, the present study employed in-cylinder pressure measurements, determination of exhaust gas concentrations, flame imaging, and spectroscopy. Apart from confirming well-established results related to this engine control mode, the results showed several correlations that were established between physical and chemical phenomena and their evolution during combustion. As expected, the major conclusion of the thermodynamic analysis was that lean operation ensured an increase of around 10% in fuel conversion efficiency compared to stoichiometric fueling, and acceptable stability, with COVIMEP slightly over the 5% threshold, even close to the flammability limit of λ = 1.6. Another result was the identification of a prolonged flame kernel phase as the main reason for the requirement of advanced spark timing as air–fuel mixtures were leaner. The analysis of exhaust gas emissions confirmed the potential of lean operation for reducing NOx emissions, even at levels comparable to the use of three-way catalytic converters. Opacity measurements also showed that this control strategy ensures reduced emission of particles, a pressing issue for GDI engines. Flame imaging data showed lower overall intensity as the equivalence ratio was lowered, as was to be expected given the lower concentration of reacting species during combustion. Gas density, directly determined by in-cylinder pressure, was the other factor that was identified as a major influence on flame luminosity. Spectroscopy measurements shortly after ignition was initiated showed a very good correlation between the ratio of CN and OH emission intensity and the relative air–fuel ratio; this was determined based on spark characteristics and confirmed the differences in chemical kinetics for each air–fuel ratio setting. Flame area and propagation speed data were in good agreement with the thermodynamic parameters and confirmed the influence of laminar flame speed on overall turbulent flame propagation velocity. Tumble motion was identified as the main factor of influence on flame displacement, and this parameter was found to feature the highest values for λ = 1.6; swirl had only a minor influence overall, but the effect was more prominent for lean operation. An interesting result was that flame circularity showed a decrease as the equivalence ratio

Energies 2017, 10, 1337 20 of 23

4. Conclusions With the goal of providing detailed insight into the specifics of lean combustion in SI engines, the present study employed in-cylinder pressure measurements, determination of exhaust gas concentrations, flame imaging, and spectroscopy. Apart from confirming well-established results related to this engine control mode, the results showed several correlations that were established between physical and chemical phenomena and their evolution during combustion. As expected, the major conclusion of the thermodynamic analysis was that lean operation ensured an increase of around 10% in fuel conversion efficiency compared to stoichiometric fueling, and acceptable stability, with COVIMEP slightly over the 5% threshold, even close to the flammability limit of λ = 1.6. Another result was the identification of a prolonged flame kernel phase as the main reason for the requirement of advanced spark timing as air–fuel mixtures were leaner. The analysis of exhaust gas emissions confirmed the potential of lean operation for reducing NOx emissions, even at levels comparable to the use of three-way catalytic converters. Opacity measurements also showed that this control strategy ensures reduced emission of particles, a pressing issue for GDI engines. Flame imaging data showed lower overall intensity as the equivalence ratio was lowered, as was to be expected given the lower concentration of reacting species during combustion. Gas density, directly determined by in-cylinder pressure, was the other factor that was identified as a major influence on flame luminosity. Spectroscopy measurements shortly after ignition was initiated showed a very good correlation between the ratio of CN and OH emission intensity and the relative air–fuel ratio; this was determined based on spark characteristics and confirmed the differences in chemical kinetics for each air–fuel ratio setting. Flame area and propagation speed data were in good agreement with the thermodynamic parameters and confirmed the influence of laminar flame speed on overall turbulent flame propagation velocity. Tumble motion was identified as the main factor of influence on flame displacement, and this parameter was found to feature the highest values for λ = 1.6; swirl had only a minor influence overall, but the effect was more prominent for lean operation. An interesting result was that flame circularity showed a decrease as the equivalence ratio was lowered, but over a certain range the trend was reversed, resulting in more circular flames near the flammability limit. Spectroscopy gave more details of the chemical kinetics of combustion, and showed that burned gas composition was quite similar at different locations within the combustion chamber; this was to be expected, given the injection strategy that comprised of fuel delivery during the intake stroke. A more interesting result was related to the emission intensity recorded for soot precursors that showed particle formation near the injector. The fuel jet impingement sites determined by the injector–piston geometry during injection were coherent with these results and showed a good correlation between the soot-specific intensity recorded during combustion and the measured opacity at the exhaust. The maximum values recorded for the OH, CO2, and soot ‘spectroscopic signals’, as well as the crank angle of these values, suggested that burned gas composition was close to equilibrium after flame front propagation, and that lean operation ensured much less particle formation due to oxygen availability. All these findings confirmed the potential of lean fueling for ensuring high efficiency and reduced formation of pollutant species, with acceptable combustion stability. The complete characterization through combined thermodynamic and optical investigations gave more insight into the specifics of combustion process, chemical kinetics, interactions with fluid motion, and the timing of spark-ignited and diffusive flames at the surface of liquid fuel deposits.

Author Contributions: Santiago Martinez, Adrian Irimescu and Simona Silvia Merola conceived the experiments, designed the trials, performed the measurements, analyzed the data and wrote the paper, Pedro Lacava and Pedro Curto-Riso coordinated the activities of Santiago Martinez. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2017, 10, 1337 21 of 23

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