Experimental study on the effects of spray- wall interaction on partially premixed combustion (PPC) and engine emissions

Item Type Article

Authors Tang, Qinglong; An, Yanzhao; Raman, Vallinayagam; Shi, Hao; Sim, Jaeheon; Chang, Junseok; Magnotti, Gaetano; Johansson, Bengt

Citation Tang Q, An Y, Raman V, Shi H, Sim J, et al. (2019) Experimental study on the effects of spray-wall interaction on partially premixed combustion (PPC) and engine emissions. Energy & Fuels. Available: http://dx.doi.org/10.1021/ acs.energyfuels.9b00602.

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DOI 10.1021/acs.energyfuels.9b00602

Publisher American Chemical Society (ACS)

Journal Energy & Fuels

Rights This document is the Accepted Manuscript version of a Published Work that appeared in final form in Energy & Fuels, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/pdf/10.1021/ acs.energyfuels.9b00602.

Download date 24/09/2021 19:52:45 Link to Item http://hdl.handle.net/10754/652851 Subscriber access provided by King Abdullah University of Science and Technology Library Combustion Experimental study on the effects of spray-wall interaction on partially premixed combustion (PPC) and engine emissions Qinglong Tang, Yanzhao An, Vallinayagam Raman, Hao Shi, Jaeheon Sim, Junseok Chang, Gaetano Magnotti, and Bengt Johansson Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00602 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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1 2 Experimental study on the effects of spray-wall interaction on partially 3 4 premixed combustion (PPC) and engine emissions 5 6 Qinglong Tanga, Yanzhao Ana,*, Vallinayagam Ramanb, Hao Shia, Jaeheon Simb, 7 8 Junseok Changb, Gaetano Magnottia, Bengt Johanssona 9 a 10 Clean Combustion Research Center (CCRC), King Abdullah University of Science and Technology 11 (KAUST), Thuwal, Saudi Arabia 12 bFuel Technology Division, R&DC, Saudi Aramco, Dhahran 31311, Saudi Arabia 13 14 15 Abstract 16 17 We investigated the detailed spray-wall interaction in partially premixed combustion (PPC) in an 18 19 optical engine with a wide-angle injector at low engine load. The fuel-trapping effect of the top- 20 21 land crevice was visualized by fuel-tracer planar laser-induced fluorescence (PLIF) for the first time. In 22 23 24 agreement with the fuel distribution shown by PLIF, the combustion region first moves into the squish 25 26 region and then tends to move back to the piston bowl when advancing the injection timings. Results 27 28 indicate that a considerable portion of the fuel is trapped in the piston top-land crevice for the cases with 29 30 31 injection timings at -30° (SOI-30 case) and -40° (SOI-40 case) CA aTDC. The SOI-40 case with earlier 32 33 injection timing presents a backflow process of the trapped fuel from the piston top land crevice to the 34 35 squish region. However, there is not enough time for the fuel vapor to flow back before the start of 36 37 combustion for the SOI-30 case, resulting in a relatively lower fuel-air equivalence ratio in the squish 38 39 40 region and thus higher CO emission than the SOI-40 case. This study provides insights into the fuel 41 42 distribution characteristics in piston crevice and the potential effects on the engine emissions under PPC 43 44 condition. 45 46 47 Keywords: PPC; Spray-wall interaction; PLIF; Fuel-trapping effect; Piston top-land crevice 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 2 of 19

1 2 1. Introduction 3 4 Conventional diesel compression-ignition engine (CI) has high efficiency and excellent durability 5 but it suffers from increased NOx and soot emissions due to the feature of mixing-controlled combustion. 6 7 Homogeneous charge compression ignition (HCCI) can achieve lower NOx and soot emissions through 8 9 premixed fuel-air charge and low-temperature combustion. However, the combustion phasing of HCCI, 10 dominated by the chemical kinetics, is difficult to control. Besides, the excessive pressure rise rate (PRR) 11 12 at medium-high engine load limits the upper operating range of HCCI [1-3]. As an intermediate process 13 14 between HCCI and CI, partially premixed combustion (PPC) adopts earlier timings than CI. 15 16 Therefore, the fuel has more time for pre-mixing before ignition, which favors the low-temperature 17 combustion. Meanwhile, a certain level of fuel stratification is maintained so that the combustion phasing 18 19 can be well controlled with a restrained peak PRR [4]. 20 21 The decoupling of the fuel injection and the ignition process, i.e. a positive ignition dwell, is crucial 22 23 for PPC. It is a sign of sufficient fuel-air mixing and normally could lead to low-temperature combustion. 24 Using fuels like diesel and n-heptane, PPC can be easily implemented with an early start-of-injection (SOI) 25 26 timing and a moderate exhaust gas recirculation (EGR) at low-medium engine load [5, 6]. However, at 27 28 high engine load, it requires too much EGR to get a positive ignition dwell. This results in lower 29 combustion efficiency and fuel penalty with a sharp increase in the unburned hydrocarbon (UHC) and 30 31 carbon monoxide (CO) emissions [7, 8]. On the other hand, for fuels like gasoline and iso-octane, a 32 33 positive ignition dwell can be realized at the medium-high engine load with a moderate EGR rate [9, 10]. 34 35 These fuels have a higher resistance to auto-ignition and prolong the ignition delay to enhance the fuel- 36 air mixing process. Engine experiments on gasoline PPC showed that an indicated engine efficiency higher 37 38 than 50% could be achieved [11]. However, a major challenge to PPC using gasoline-like fuels comes 39 40 from the combustion stability at low engine load when the overall fuel-air charge is overly lean [12, 13]. 41 The best fuel for PPC operation seems to be in between that of diesel and gasoline. Hildingsson et al. [14] 42 43 investigated the PPC engine performance and emissions using gasoline fuels with different research 44 45 octane number (RON). They observed that the optimum RON range was between 75 and 85 in the 46 47 conditions studied. Recently, PPC operated with low-octane fuel like dieseline (a mixture of the diesel 48 and gasoline) and naphtha at low load condition gained more attention due to the improvement of the 49 50 combustion stability and engine efficiency [15, 16]. 51 52 PPC adopts early fuel injection strategy that produces longer fuel spray penetration length due to the 53 54 relatively lower in- pressure and temperature at the time of injection [17]. This could result in an 55 interaction between fuel sprays and walls, especially when using a traditional diesel injector with a wide 56 57 spray angle [18, 19]. Herein, the “walls” refers to the surfaces of the piston top, cylinder wall and piston 58 59 60 ACS Paragon Plus Environment Page 3 of 19 Energy & Fuels

1 2 top-land. Many studies considered the spray-wall interaction to be closely related to the high emissions of 3 4 UHC and CO in PPC mode at low load [20, 21]. Peterson et al. [22] explored the fuel distribution in PPC 5 mode at a low engine load of 3 bar gross indicated (IMEP) using quantitative 6 7 fuel-tracer planar laser-induced fluorescence (PLIF) imaging in an optical engine. The fuel injection 8 9 timing was at -23° angle after top dead center (CA aTDC) with a global fuel-air equivalence ratio 10 of 0.3. They found that a portion of the fuel was directly injected into the squish region forming a local 11 12 overly rich mixture with equivalence ratio from 1.0 to 1.4. Most of the fuel was directed into the lower 13 14 part of the piston bowl prior to ignition. It forms an overly lean mixture with equivalence ratio from 0.1 15 16 to 0.5 in the middle-upper part of the piston bowl near the injector nozzle. They predicted that most of the 17 UHC and CO emissions originated from the overly lean mixture (0.1< 휙 <0.5) in the piston bowl and 18 19 squish region, and a small part of CO derived from the overly rich mixture (1.0 < 휙 <1.4) in the squish 20 21 region. Wang et al. [23] studied the spray-wall interaction under different fuel injection timings by 22 23 qualitative fuel-tracer PLIF imaging. They believed that the fuel-trapping effect in the squish region was 24 responsible for the increased UHC and CO emissions when using early fuel injection timings. 25 26 In the above-reviewed literature, the piston top-land crevice was not visualized due to the limited 27 28 view field in the optical engine setups. Actually, the fuel trapped in the piston top-land crevice under early 29 injection condition has long been suspected to be an important source of UHC and CO emissions under 30 31 PPC mode [24-26]. However, few studies have been concentrated on the spray-wall interaction involved 32 33 with the piston top-land crevice. The understanding of the distribution characteristics of the fuel trapped 34 35 in the piston top-land crevice is quite limited. Besides, the effects of different fuel injection timings on the 36 in-cylinder fuel distribution situations and the following combustion process at PPC mode still need 37 38 further investigation. 39 40 Therefore, in the present study, the detailed spray-wall interaction under different fuel injection 41 42 timings was studied in an optical engine under PPC mode at low engine load. The behavior of the fuel 43 trapped in the piston top-land crevice was visualized by fuel-tracer PLIF imaging for the first time. A low- 44 45 octane fuel of PRF77 (a mixture of 77% iso-octane and 23% n-heptane by volume) was directly injected 46 47 at different timings. Toluene of 1% by volume served as the tracer for the fuel vapor under non- 48 combusting condition. A high-speed camera captured the natural flame luminosity (NFL) images from 50 49 50 consecutive fired cycles. A flame probability distribution index was calculated and linked to the fuel 51 52 distribution obtained by the PLIF imaging. The effects of fuel injection timings on the engine-out UHC 53 54 and CO emissions were analyzed combining the fuel distribution information. This work aims to gain 55 insight into the in-cylinder spray-wall interaction and combustion characteristics of PPC at low engine 56 57 load. It also contributes valuable experimental data for the validation of the three-dimensional simulation 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 4 of 19

1 2 for the spray-wall interaction under PPC condition. 3 4 2. Experimental setup and optical diagnostic techniques 5 2.1 Optical engine and operating conditions 6 7 A light-duty, single-cylinder AVL optical engine was employed in this study. The main engine 8 9 specifications are listed in Tab. 1. The piston of the engine has a quartz piston crown and an extended 10 piston sleeve that allows for the optical access. The piston crown has a ω -shape bowl with a curved bottom 11 12 surface, as shown in Fig. 1a. It enables the entrance of the vertical laser sheet into the 13 14 and made the whole squish region visible near the top dead center (TDC). The diameter of the combustion 15 16 chamber is 50.8 mm. A quartz liner with an outer diameter of 135 mm and a height of 29 mm is aligned 17 into the upper part of the cylinder wall by a hydraulic system. The laser sheet passes through the quartz 18 19 liner and the in-cylinder process is visible from the side view. The engine is equipped with a Bosch 20 21 common-rail injector. The injector nozzle is located in the center of the cylinder and had eight orifices 22 23 with a spray angle of 162°. The eight orifices are evenly spaced with a diameter of 0.18 mm. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 (a) Side view (b) Top view 41 42 Fig. 1. Schematic of the optical engine and the laser location relative to the fuel sprays and combustion chamber. Note that the 43 natural flame luminosity imaging and PLIF imaging were carried out separately and the laser was turned off when using the 44 45 high-speed camera. 46 47 The fuel injection timing, pressure and duration were controlled through the AVL FI2RE software. 48 49 A heater was installed in the manifold to preheat the supplied air to support the auto-ignition of the 50 low-octane fuel. A pressure transducer measured the in-cylinder pressure during the engine combustion 51 52 process with a resolution of 0.2° CA. The pressure data was processed to get the averaged heat release 53 54 rate. The NOx, CO and UHC emissions in the exhaust pipe were measured by the AVL AM i60 test bench. 55 The engine was run at a speed of 1200 revolutions per minute (rpm) without EGR. The intake pressure 56 57 and temperature were 1.5 bar and 413 K, respectively, producing a motored TDC pressure of 35 bar and 58 59 60 ACS Paragon Plus Environment Page 5 of 19 Energy & Fuels

1 2 TDC temperature of 872 K. The engine coolant and lubrication temperature were kept at 333 K. The 3 4 injected PRF77 mass for each cycle was 14 mg and the global equivalence ratio was 0.28. The fuel 5 injection duration was about 8° CA for all cases with the start of injection (SOI) timings from -20° to -60° 6 7 CA aTDC. The cases with SOI timings at -20°, -30°, -40° and -60° CA aTDC were denoted as SOI-20, 8 9 SOI-30, SOI-40 and SOI-60, respectively. 10 Tab. 1. Optical engine specifications. 11 12 85 mm 13 90 mm 14 Displacement 0.51 L 15 length 138 mm 16 9.5 17 Combustion chamber shape ω 18 Intake valve number 2 19 Intake valve open at 30° CA bTDC 20 Intake valve closed at 45° CA aBDC 21 Exhaust valve number 2 22 Exhaust valve open at 50° CA bBDC 23 Exhaust valve closed at 25° CA aTDC 24 25 26 Tab. 2. Engine operating conditions 27 Engine speed 1200 rpm 28 Intake pressure 1.5 bar 29 Intake temperature 413 K 30 Coolant temperature 333 K 31 Lubricant temperature 333 K 32 Fuel Injection pressure 600 bar 33 34 Fuel PRF77 35 Fuel mass 14 mg/cycle 36 Start of injection (SOI) -20°~-60° CA aTDC 37 Global equivalence ratio 0.28 38 Air was supplied in the intake stroke for the combusting engine cycles during the high-speed NFL 39 40 experiment. While the engine was fired for 150 consecutive cycles, the cylinder pressure and NFL images 41 42 of the last fired 50 cycles were used for post-processing. In the fuel-tracer PLIF experiment, the intake air 43 44 was replaced by pure nitrogen to eliminate the oxygen quenching effect on toluene PLIF signal. Tab. 2 45 summarizes the main engine operating conditions. 46 47 2.2 Fuel-tracer PLIF imaging 48 49 The fuel-tracer PLIF imaging was carried out under non-combusting condition by supplying the 50 51 intake with pure nitrogen. A10 Hz crank signal was produced by the engine to trigger the Nd:YAG laser 52 (Q-smart 850, Quantel) through a pulse delay generator (DG535, Stanford Research). The fourth harmonic 53 54 (266 nm, 60 mJ/pulse) was chosen for the toluene tracer excitation. The laser beam was formed into a 55 56 vertical laser sheet less than 1 mm thick and 20 mm high by the LaVision sheet and collimator optics. The 57 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 6 of 19

1 2 laser sheet was located just below the bottom of the . It passed through the center of the 3 4 cylinder head, intersecting two of the fuel sprays, as shown in Fig. 1b. 5 A crank signal from the engine triggered the ICCD camera (PI-MAX3, Princeton Instruments) to 6 7 capture the toluene fluorescence signal after the start of fuel injection. The ICCD camera, equipped with 8 9 a 105 mm, f/4.5 UV-lens, was placed on the side of the quartz liner and perpendicular to the laser sheet. 10 A band-pass filter (FF01-292/27, Semrock) with a transmission range of 278.5-305.5 nm was coupled to 11 12 the lens to collect the toluene PLIF signal. The gate-width and gain level were set to 100 ns and 90, 13 14 respectively. The pulse delay generator synchronized the timing sequence of the signals for the laser, 15 16 engine and camera. 17 18 19 20 21 22 23 24 25 26 (a) Single-shot fuel-tracer PLIF image in false color 27 28 29 30 31 32 33 34 35 36 37 38 39 (b) Single-shot NFL image in true color 40 41 Fig. 2. View field of the ICCD camera for the fuel-trace PLIF from the side of the piston at -30° CA ATDC for the SOI-40 42 case (a) and the high-speed camera for the natural flame luminosity (NFL) imaging from the bottom of the piston at -2.7° CA 43 44 ATDC for the SOI-40 case (b). The squish region and piston top-land crevice are marked by two dashed rectangles in Fig. 2a. 45 Thirty PLIF images were acquired from 30 consecutive engine cycles at each specified crank angle. 46 47 The image distortion caused by the optical liner and piston bowl was corrected. More detail about the 48 49 image distortion correction is in the Appendix. The correlation coefficient between each single-shot image 50 and the averaged image was calculated. The one with the maximum correlation coefficient was chosen as 51 52 a representative. 53 54 The view field of the ICCD camera is illustrated in Fig. 2a by a single-shot PLIF image of the fuel 55 56 spray on the right-hand side shown in Fig. 1b. The PLIF image presents a clear view of the interaction 57 among the fuel spray, piston bowl and squish region. The fuel distribution in the piston top-land crevice 58 59 60 ACS Paragon Plus Environment Page 7 of 19 Energy & Fuels

1 2 (about 0.8 mm in width) can also be clearly visualized. This provides important information about the fuel 3 4 trapped in the top-land crevice. Note that the height of the top-land crevice, i.e., the distance between the 5 piston top and the first piston ring, was about 40 mm but only the region swept by the laser sheet (20 mm 6 7 high) was visualized. 8 9 2.3 High-speed natural flame luminosity (NFL) imaging 10 When the high-speed NFL images were measured, the laser and ICCD camera were shut down and 11 12 the intake gas was changed to air. A high-speed CMOS camera (Photron Fastcam SA4) was used to record 13 14 the NFL during the combustion from the bottom view of the piston through a UV-mirror. One example of 15 16 the NFL image is shown in Fig. 2b. A 50 mm, f/1.4 lens was coupled to the high-speed camera. The frame 17 rate and exposure time of the camera were set to 10000 frames per second and 100 μs, respectively. Thus, 18 19 the temporal resolution of the acquired images was 0.72° CA at the engine speed of 1200 rpm. 20 21 During the experiment, the NFL image sequences for 50 consecutive fired engine cycles were 22 23 recorded simultaneously with the cylinder pressure so that the gross IMEP can be correlated with the NFL 24 intensity. In order to minimize the interference from soot radiation, the blue channel of the RGB image 25 26 was extracted to represent the chemiluminescence in the early stage of the combustion. The distortion 27 28 caused by the curved surfaces of the piston crown window was corrected as shown in the Appendix. 29 A flame probability distribution index (PDI) was calculated to describe the flame distribution in the 30 31 cylinder. PDI describes the probability of flame occurrence at any specified pixel (x, y). To do this, we 32 33 need to know how often the flame appears at the pixel (x, y) in each image. So a binarization process was 34 35 carried out to answer yes or no at pixel (x, y) for each image. A threshold of 10% of the maximum signal 36 intensity, 퐼푡ℎ푟푒푠ℎ표푙푑 , for each image was employed. If the signal intensity at pixel (x, y) is lower than 37 38 퐼푡ℎ푟푒푠ℎ표푙푑, the intensity of the binarized image at pixel (x, y), 푖(푥, 푦) , is assigned to 0, indicating no flame in 39 40 this location. Otherwise, it is assigned to 1. Finally, the PDI at location (x, y), PDI(푥,푦) , was obtained by 41 42 statistics on the 50 binarized images using the following equation. PDI of 0 means there is no flames in 43 44 the specified region, while PDI of 1 means flames always appear in location (x, y). This procedure was 45 applied to every pixel of the image to get the PDI distribution in the whole cylinder. 46 47 50 48 푃퐷퐼(푥, 푦) = ( 푖(푥, 푦))/50 49 ∑ 1 50 51 3. Results and discussions 52 53 3.1. Analysis of engine-out emissions, rate of heat release and natural flame luminosity 54 The engine-out emissions of CO, UHC, and NO under different SOI timings from -10° CA aTDC 55 x 56 to -120° CA aTDC are shown in Fig. 3. NOx emission reduces gradually to near zero while UHC and CO 57 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 8 of 19

1 2 increase to a plateau with advanced SOI timings from -10° to -60° CA aTDC. There is no big change in 3 4 emissions at SOI earlier than -60° CA aTDC. Note that the CO emission of the SOI-30 case is even higher 5 than that of the SOI-40 case. This similar CO emission jump for the SOI-30 case was also observed when 6 7 using fuels of PRF60 and naphtha under similar engine operating conditions in the author’s previous works 8 9 [16, 27]. This interesting phenomenon of the CO emission trend in PPC range could possibly be explained 10 by the differences in spray-wall interaction situations. Thus, PPC cases with SOI from -20° to -60° CA 11 12 aTDC are chosen for analysis of rate of heat release, NFL intensity and spray-wall interaction. 13 14 15 1200 300

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20 n 400 100 N O a 21 200 50 22 C O 23 0 0 24 -120 -100 -80 -60 -40 -20 0 25 SOI (CA ATDC) 26 Fig. 3. Engine-out emissions of CO, UHC and NOx under the different start of injection (SOI) timings. 27 28 29 150 60 SOI-20 30 SOI-30 120 ) SOI-40 C A )

31 40 SOI-60  r 90 a J / ( 32 b (

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n 0.5 40 e t n I 41 0.0 L 42 -20 -15 -10 -5 0 5 10 15 20 N F 43 Crank Angle (ATDC) 44 45 Fig. 4. Cylinder pressure (P), rates of heat release (RoHR) and NFL intensity averaged from 50 engine cycles of the PPC 46 cases with different SOI timings. The two circles on the RoHR curves show the CA10 and CA50. 47 48 The peak cylinder pressures and rates of heat release become lower with the advancing of the SOI 49 timing, as shown in Fig. 4. The CA10 (the crank angle when 10% of the total combustion heat is released) 50 51 and CA50 for each case are marked by two circles on the rates of heat release curves. The combustion 52 53 phasing, indicated by CA50, is first advanced and then retarded. The SOI-30 case presents the earliest 54 combustion phasing compared to all other cases. Figure 4 also shows the NFL intensity with error bars 55 56 averaged from 50 fired engine cycles for different SOI cases. The overall NFL intensity decreases with 57 58 59 60 ACS Paragon Plus Environment Page 9 of 19 Energy & Fuels

1 2 the advancing of the SOI timings. Typical NFL images at CA50 for all four cases showing blue flames 3 4 are presented in the upper panel of Fig. 5. The NFL intensity of the SOI-60 case is almost negligible 5 compared to that of the SOI-20 case. The NFL signal is captured by the high-speed color camera in the 6 7 spectral range of 350-700 nm and the chemiluminescence in the ultraviolet range like OH is not included. 8 9 That’s why the NFL intensity curves slightly lag behind the RoHR curves at the beginning of the 10 combustion. Before the peak of the NFL intensity, most of the natural flame comes from the blue 11 12 chemiluminescence derived from the band spectra of C2, HCO, CH2O and CH, and the continuum spectra 13 14 of CO oxidation [28, 29]. After the peak of the NFL signal, soot radiation begins to emerge and gradually 15 16 prevails. The heat release is negligible for all four cases at 5.2° CA and the NFL images, shown in the 17 lower panel of Fig. 5, are dominated by soot radiation. This explains why the curves of the NFL intensity 18 19 in Fig. 4 persist even after the end of the main heat release. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Fig. 5. Typical NFL images at CA50 and 5.2° CA aTDC for all cases. The NFL magnification factor is shown on the upper 34 right of each image. 35 36 4 37 S =132.8, S =11.9, S =6.0, S =5.6 38 -60 -40 -30 -20 )

39 r 3 a b

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41 E M 42 I 2 SOI-20 SOI-30 43 SOI-40 44 SOI-60 45 1 0.0 0.5 1.0 1.5 2.0 2.5 46 7 NFL Intensity at CA50 (10 a.u.) 47 48 Fig. 6. Relationship between the gross IMEP and NFL intensity at CA50. The slops of each fitted curves are marked out. 49 Considering that the flame chemiluminescence during the main combustion is closely related to the 50 51 heat release process, the images of the NFL at CA50 for all cases are chosen to evaluate the cycle-to-cycle 52 53 variation in PPC mode. The plot of gross IMEP versus NFL intensity at CA50 in Fig. 6 essentially presents 54 55 a linear relationship, i.e., the higher the NFL intensity, the larger the IMEP. The slopes of the linear fitting 56 curves increase with the advancing of the SOI timings. For example, the slope of the SOI-20 case is about 57 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 10 of 19

1 2 5.6, while it is 132.8 for the SOI-60 case. This significant difference is because the overall NFL intensity 3 4 variation for cases with different SOI timings is much larger than that of IMEP. 5 The evolution of flames during the combustion process for all four cases are presented in terms of 6 7 flame PDI, as shown in Fig. 7. The higher the flame PDI index, the higher the probability of flame 8 9 occurrence and vice versa. The images at CA10 show that the initial ignition kernels for all cases are 10 located in the squish region closer to the cylinder wall. Flames emerge in the piston bowl later on for the 11 12 SOI-20 case, and eight fuel jets can be clearly recognized by the separated flame regions at -1.3° CA. The 13 14 combustion is mainly distributed in two regions at CA50 (-0.6° CA), the squish region and the piston 15 16 bowl, which are separated by the bowl rim. The flames in the squish region gradually fade away after 17 CA50 and the main flame region is confined in the piston bowl at 5.2° CA. As mentioned in Fig. 5, the 18 19 NFL before CA50 mainly comes from the blue chemiluminescence and the NFL after 5.2° CA from soot 20 21 radiation. The soot radiation due to the injector dribbling can be seen at 10.2° CA, as shown by the white 22 23 arrows in Fig. 7. In the following 10° CA, the soot radiation in the piston bowl gradually decreases. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Fig. 7. The flame probability distribution index (PDI) of PPC cases with different SOI timings. The crank angle is marked on 48 the upper left of each image. The first and fourth images of each sequence correspond to the CA10 and CA50, respectively. 49 The white arrows mark the soot radiation of the injector dribbling. 50 51 In comparison to the SOI-20 case, a major difference in the flame PDI for SOI-30 case is that most 52 53 of the flame is confined in the squish region and cannot be seen in the piston bowl before CA50. The soot 54 radiation due to injector dribble emerges late after 5.2° CA but shows a lower flame PDI. For cases of the 55 56 SOI-40 and SOI-60, the overall flame PDI is decreased with the advancing of SOI timings. The lower 57 58 59 60 ACS Paragon Plus Environment Page 11 of 19 Energy & Fuels

1 2 flame PDI essentially means a higher engine cycle-to-cycle variation and lower combustion stability. The 3 4 variation of PDI at CA50 along a distance from the center of the bowl (a center line marked out in Fig. 7 5 for reference) is illustrated in Fig. 8. It shows that most of the flame region for the cases of SOI-30, SOI- 6 7 40 and SOI-60 is located in the squish region and the flame region tends to move into the piston bowl with 8 9 the advancing of the SOI timings. This trend is will be further discussed in section 3.2 combining the fuel 10 distribution information. 11 12 1.0 DI-20 13 DI-30 0 DI-40 14 5 0.8 DI-60 C A

15 t

a 0.6 16 L 17 N F 0.4

f

18 o 0.2

19 D I P 20 0.0 Combustion Chamber Squish 21 0 5 10 15 20 25 30 35 40 22 Distance from Center (mm) 23 24 Fig. 8. The flame probability distribution index (PDI) along the center line at CA50 shown in Fig. 7a. The positions of the 25 piston bowl and squish region are marked out. The piston bowl wall chamfer region is shown by a dashed black rectangle. 26 3.2 The spray-wall interaction under the different start of injection (SOI) timings 27 28 The averaged fuel-tracer PLIF images at different SOI timings are shown in Fig. 9. The overall fuel 29 30 distribution in the piston bowl, squish region and piston top-land crevice can be visualized and linked to 31 32 the flame PDI in Fig. 7. The single-shot images are presented in Fig. 10 to show the detailed local fuel 33 spray characteristics. The false-color images later than -20° CA have the same scale of 0 to 5000 and other 34 35 images have the scale from 0 to 7000. Note that the fuel injection duration is about 8° CA, so the first 36 37 images of the SOI-20 case and SOI-60 case, as well as the first two images of the SOI-30 case and SOI- 38 39 40 case, are during the fuel injection process. 40 For the SOI-20 case in Fig. 9a, the fuel spray first impinges on the piston bowl rim at -15° CA, and 41 42 a large part of the fuel spray moves into the upper part of piston bowl and a small part into the squish 43 44 region. When the fuel injection ends at -10° CA, the fuel in the upper bowl region goes into the lower part 45 of the bowl while the fuel in the squish region penetrates deeper into the squish region, forming two 46 47 relatively fuel-rich regions near the side wall of the bowl and piston top. When the heat release initiates 48 49 at -5° CA, the fuel in the bowl occupies most of the bowl region and the fuel in the squish is very close to 50 51 the cylinder wall but has not yet gone into the piston top-land crevice. Note that a higher PLIF intensity 52 at the same crank angle (in-cylinder pressure and temperature) indicates a higher fuel-air equivalence 53 54 ratio. Thus, the peak equivalence ratio in the squish regions is relatively higher than that in the piston bowl 55 56 at -5° CA. This distribution can explain why the initial flame kernels first appear in the squish region at 57 CA10 in Fig. 7. Subsequently, auto-ignition happens in the bowl region where most of the fuel resides in. 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 12 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Fig. 9. The spray-wall interaction processes shown by the averaged PLIF images under different SOI timings: (a) SOI-20, (b) 26 SOI-30, (c) SOI-40 and (d) SOI-60. The measurement crank angles (CA aTDC) are marked on the right of each averaged 27 image. (D: downstream flow, B: backflow, O: overflow) 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Fig. 10. The spray-wall interaction processes shown by the single-shot PLIF images under different SOI timings: (a) SOI-20, 53 (b) SOI-30, (c) SOI-40 and (d) SOI-60. The measurement crank angles (CA aTDC) are marked on the right of each averaged 54 image. 55 56 57 58 59 60 ACS Paragon Plus Environment Page 13 of 19 Energy & Fuels

1 2 For the SOI-30 case in Fig. 9b, most of the fuel spray first touches the piston top at -25° CA and only 3 4 a very small part of the spray spreads into the rim of the bowl. The fuel penetrates deeper into the squish 5 region along the piston top surface at -22.5° CA, because the fuel injection has not ended at this time and 6 7 the spray has very high momentum. It is very interesting to note that the single-shot PLIF images in Fig. 8 9 10 presents a clear view of the corrugated plumes inside the fuel spray. When the fuel injection ends at - 10 20° CA, the spray head in the squish region hits on the cylinder wall, forming a relatively fuel-rich region 11 12 wherein some part of fuel squeezes into the piston top-land crevice. The fuel in the crevice still has a 13 14 certain momentum and thus it moves deeper downstream (shown by the white arrow “D”) into the crevice. 15 16 Most of the top-land crevice in the diagnostic region has been filled with the fuel-air mixture by -10° CA, 17 which has a relatively higher equivalence ratio than that in the squish region, and very little fuel is 18 19 distributed in the piston bowl region. The fuel distribution corresponds well with the flame PDI in Fig. 8 20 21 in which most of the flames are confined in the squish region for the SOI -30 case. 22 23 For SOI-40 case in Fig. 9c, the fuel spray has a longer penetration length at 5° CA after the start of 24 injection (-35° CA aTDC) compared to the SOI-30 case due to lower in-cylinder pressure and temperature. 25 26 The spray head just hits on the corner between the piston top and cylinder wall at -32.5° CA. The fuel 27 28 with a higher momentum due to the lower in-cylinder pressure moves downstream into the top-land 29 crevice with a faster speed. The top-land crevice in the view field has been completed distributed with the 30 31 fuel-air charge at -30° CA when the fuel injection ends. When the piston moves up, the mixture in the 32 33 squish region and top-land crevice is squeezed, and some part of the mixture in the squish region overflows 34 35 into the piston bowl (shown by the white arrow “O”). At -10° CA, it is noteworthy to see a charge backflow 36 process in the top-land crevice (shown by the black arrow “B”) that results in a relatively lower 37 38 equivalence ratio in the upper part of the crevice. Due to the backflow of the charge, the fuel distribution 39 40 is also relatively richer in the squish region as against the SOI -30 case at the same crank angle. 41 As shown in Figure 9d, the SOI-60 case has the longest spray penetration length. The spray head 42 43 almost touches the cylinder wall at 5° CA ASOI (-55° CA aTDC). The spray impinges on the cylinder 44 45 wall without the interference of the piston top by the time of 10° CA ASOI (-50° CA aTDC), showing 46 47 obvious two lateral vortexes as illustrated by the two white arrows. The spray momentum can be dissipated 48 in these two vortexes, producing a wide fuel distribution area close to the cylinder wall. The fuel 49 50 penetration into the piston top-land crevice impelled by the injection momentum, as shown in the SOI-30 51 52 and SOI-40 cases, cannot be seen here. When the piston moves up at -40° CA, the fuel-air charge in the 53 54 squish region is compressed and the charge overflow into the piston bowl is more distinct at -30° CA. 55 Meanwhile, the mixture closer to the cylinder wall stays and falls into the top-land crevice when the piston 56 57 gradually approaches TDC. The backflow in the piston top-land crevice can be seen at -20° CA. Finally, 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 14 of 19

1 2 more fuel-air mixture diffuses into the bowl at -10° CA, resulting in a relatively lower equivalence ratio 3 4 in the squish region and piston crevice compared to the other three cases at the same crank angle. This 5 leads to the lower flame PDI in the squish region at CA50 but the flames are closer to the piston bowl 6 7 center, as shown in Fig. 8. 8 9 In short, the spray-wall interaction locations move from the piston bowl wall to piston top and finally 10 to the cylinder wall with the advancing of the SOI timings. A considerable portion of the fuel is trapped 11 12 in the piston top-land crevice in cases of SOI-30 and SOI-40, forming relatively local fuel-rich regions 13 14 there. The behavior of the fuel-air charge in the piston top-land crevice is different for the cases of SOI- 15 16 30, SOI-40 and SOI-60. With much earlier SOI timing in the cases of SOI-40 and SOI-60, a backflow 17 process from the crevice into the squish region can be seen after the fuel moving downstream into the top- 18 19 land crevice. However, there is no time for the fuel-air charge to flow back before the start of combustion 20 21 in the SOI-30 case, resulting in a relatively lower local fuel-air equivalence ratio in the squish region 22 23 compared to that of the SOI-40 case. These different mechanisms of the fuel-air charge distribution in the 24 top-land crevice could affect the engine combustion and emissions. 25 26 3.3 Discussion on the impact of the spray-wall interaction on the UHC and CO emissions of PPC at 27 28 low load 29 As mentioned in the introduction section, Peterson et al. [22] found that most of the UHC and CO 30 31 emissions came from the overly lean mixture (0.1< ϕ <0.5) in the piston bowl and squish region. 32 33 Subsequently, Ekoto et al. [30, 31] evaluated the UHC and CO emissions of PPC under the same optical 34 35 engine with similar operating conditions. They reconstructed the late-cycle UHC and CO distribution in 36 the upper part of the piston bowl and squish region combining PLIF imaging and multidimensional 37 38 simulations. It showed that most of the UHC originated from the middle-upper part of the piston bowl 39 40 close to the injector nozzle while the CO is mainly from the squish region at a load of about 3 bar IMEP, 41 which is close to the engine load in the present study. 42 43 The UHC emission in Fig. 3 increases with the advancing of the SOI timings. It is easy to understand 44 45 this trend based on Ekoto’s finding because more overly lean mixture is generated in the main UHC 46 47 formation region (middle-upper part of the piston bowl) due to a longer premixing time with earlier SOI 48 timings. Normally, this trend also applies to the CO emission because the fuel-air equivalence ratio in the 49 50 main CO formation region (squish region) should be decreased with the advancing of the SOI timings. 51 52 However, Figure 3 presents a higher CO emission for the SOI-30 case compared to that of the SOI-40 53 54 case. In order to explain this anomalous phenomenon, we need to compare the fuel distributions in Fig. 9 55 for the SOI-30 case and SOI-40 case at -10° CA that is just before the initiation of the combustion. It 56 57 shows that the overall fuel-air equivalence ratio in the squish region of the SOI-30 case is lower than that 58 59 60 ACS Paragon Plus Environment Page 15 of 19 Energy & Fuels

1 2 of the SOI-40 case due to the fuel trapping effect of the piston top-land crevice. Thus, more overly lean 3 4 mixture formed in the squish region could results in a higher CO emission for the SOI-30 case. This means 5 that the fuel trapping effect of the piston top-land crevice has a great impact on the CO emission for the 6 7 SOI-30 case, in which there is not enough time for the trapped fuel-air mixture to backflow into the squish 8 9 region. 10 4. Conclusions 11 12 We studied the detailed spray-wall interaction under different SOI timings in an optical engine under 13 14 PPC mode at low engine load. The distribution characteristics of the fuel trapped in the top-land crevice 15 16 were visualized by the fuel-tracer PLIF imaging for the first time. The effects of the SOI timings on the 17 combustion stability and engine-out emissions were analyzed. Some conclusions can be drawn as follows. 18 19 The gross IMEP and the NFL intensity at CA50 present a linear relationship under different SOI 20 21 timings for PPC. The slopes of the linear fitting curves increase with the advancing of the SOI timings. 22 23 The flame probability distribution index shows that the main combustion region first moves into the squish 24 region and then tends to move back to the piston bowl with the advancing of the SOI timings. This trend 25 26 is verified by the fuel distribution shown by the PLIF imaging. 27 28 The locations of the spray-wall interaction move from the piston bowl wall to piston top and finally 29 to the cylinder wall with the advancing of the SOI timings. A considerable portion of the fuel is trapped 30 31 in the piston top-land crevice for the SOI-30 case and SOI-40 case, forming relatively fuel-rich region 32 33 there. The cases with much early SOI timings like SOI-40 and SOI-60 present a backflow process of the 34 35 trapped fuel vapor from the top-land crevice to the squish region. However, there is not enough time for 36 the fuel vapor to flow back before the start of combustion for the SOI-30 case, resulting in a relatively 37 38 lower fuel-air equivalence ratio in the squish region than that of the SOI-40 case. This can account for the 39 40 increase in the engine-out CO emission for the SOI-30 case compared to that of the SOI-40 case because 41 studies in the literature showed that the CO emission mainly came from the overly lean mixture in the 42 43 squish region at low engine load. 44 45 Acknowledgments 46 47 This work was funded by competitive research funding from King Abdullah University of Science 48 and Technology (KAUST) and Saudi Aramco under the FUELCOM2 program. The authors would like 49 50 to express our gratitude to our research Technicians, Riyadh Jambi and Adrian. I. Ichim for their support 51 52 during the experiment at KAUST engine lab. 53 54 Appendix: image distortion correction 55 The image distortion from the side view comes from two parts: optical liner distortion and piston 56 57 bowl distortion. The view fields in the squish region and piston bowl were corrected separately and then 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 16 of 19

1 2 put together. Take the optical liner distortion, for example, a target with a square grid was imaged by the 3 4 ICCD camera with and without installing the liner. The grid images with and without distortion were used 5 to produce a two-dimensional piecewise linear transformation matrix that can correct the distorted image, 6 7 as shown in Fig. A1. We can see that the main image distortion is located in the squish region close to the 8 9 liner wall on the right side from line 24 to 28. 10 11 12 13 14 15 16 17 Fig. A1 Side view correction: optical liner distortion 18 19 20 21 22 23 24 25 26 27 28 29 30 Fig. A2 Bottom view correction 31 Similarly, another transformation matrix derived from a polar coordinates target, as shown in Fig. 32 33 A2 corrected the bottom view distortion produced by the piston. The main image distortion is in the region 34 35 around the piston bowl from line 3 to 8 that has large curvature. Note that the corner between piston bowl 36 wall and piston top has a small chamfer that results in excessive distortion. The signal in this region is 37 38 unusable even after the mentioned correction, as shown in Fig. 7 and Fig. 8. 39 40 References 41 42 [1] Dec J. Advanced compression-ignition engines—understanding the in-cylinder processes. P Combust 43 Inst. 2009;32(2):2727-42. 44 45 [2] Lu X, Ji L, Ma J, Zhou X, Huang Z. Combustion characteristics and influential factors of isooctane 46 47 active-thermal atmosphere combustion assisted by two-stage reaction of n-heptane. Combust Flame. 48 49 2011;158(2):203-16. 50 [3] Liu P, Zhang Y, Wang L, Tian B, Guan B, Han D, et al. Chemical mechanism of exhaust gas 51 52 recirculation on polycyclic aromatic hydrocarbons formation based on laser-induced fluorescence 53 54 measurement. Energ Fuel. 2018;32(6):7112-24. 55 [4] Yao M, Zheng Z, Liu H. Progress and recent trends in homogeneous charge compression ignition 56 57 (HCCI) engines. Prog Energ Combust. 2009;35(5):398-437. 58 59 60 ACS Paragon Plus Environment Page 17 of 19 Energy & Fuels

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