Jet Interaction in a Small-Bore Optical Diesel Engine

The shortening of lift-off length associated with jet-wall and jet- jet interaction in a small-bore optical diesel engine.

Alvin M. Ruslya, Minh K. Lea, Sanghoon Kook*, a, Evatt R. Hawkesa, b, aSchool of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney NSW 2052, Australia

bSchool of Photovoltaic and Renewable Engineering, The University of New South Wales, Sydney NSW 2052, Australia

* Corresponding Author: Phone: +61 2 9385 4091, Fax: +61 2 9663 1222, E-mail: [email protected]

Abstract

Jet-wall and jet-jet interactions are important diesel combustion phenomena that impact fuel- air mixing, flame lift-off and pollutant formation. Previous studies to visualise a wall- interacting jet in heavy-duty diesel engines suggested that the shortening of lift-off length could occur due to the recirculated hot combustion products that are entrained back into incoming diesel jet. The significance of this effect, known as re-entrainment, can be higher in small-bore engines due to shorter nozzle-to-wall distance and increased wall curvature. In this study, we performed hydroxyl chemiluminescence imaging using an intensified CCD camera and high-speed imaging of natural soot luminosity using a CMOS camera in an automotive-size optical diesel engine. To provide detailed understanding of the reacting jet under the influence of re-entrainment as well as jet-jet interaction, various jet trajectories were investigated using one and two-hole injectors coupled with a modified piston that allows 1) the identification of the shortening of lift-off length, 2) the measurements of lift-off lengths for varying degrees of jet-wall interactions and 3) the clarification on inter-jet spacing effects on the lift-off lengths. Findings from the measurements support the re-entrainment theory because the shortening of lift-off length occurs only before the end of injection when the strong jet momentum induces the ambient gas entrainment. The shortening also shows good correspondence with the penetration of the recirculated jet head back towards the centre of the combustion chamber. Simultaneous imaging of the bowl-wall-interacting jet and cylinder-liner-wall-interacting jet depicts a shorter lift-off length for the bowl-wall- interacting jet, which further supports the importance of re-entrainment on diesel combustion.

Inter-jet spacing effects on the lift-off length are also studied utilising two-hole injectors with two inter-jet spacing angles (51.4o and 102.8o). A narrower spacing between the jets results in the shorter lift-off length of the primary jet due to the higher jet-jet interaction even if short injection duration is used to suppress the re-entrainment. For tested conditions of this study,

the shortening of lift-off length appears to be more sensitive to jet-jet interaction than to the re-entrainment caused by the jet-wall interaction.

Highlights

• Effect of re-entrainment is studied in a realistic small-bore diesel environment.

• Both jet-wall and jet-jet interaction induce shortening of lift-off length.

• Upstream lift-off movement correspond well with end-of-injection timing.

• Wider inter-jet angle caused longer lift-off due to reduced re-entrainment.

• Weaker re-entrainment may suppress downstream soot formation.

Keywords: Small-Bore Diesel, Jet-Wall, Jet-Jet, Re-Entrainment.

1. Introduction

The interaction between a reacting diesel jet and bowl (or cylinder liner) wall has been of great interest due to its influences on mixing and pollutants formation [[1]-[4]]. It is understood that limited mixing and hence locally rich mixtures in the near-wall central region can increase soot formation [[5]-[7]]. On the other hand, planar laser-induced exciplex fluorescence (LIEF) measurements of high-pressure diesel jets suggest that the wall-impinged jet has higher overall fuel-air mixing rate than the free jet [[5]]. This is because a turbulent vortex is formed at the jet head as the reacting jet impinges on the wall and propagates along the wall [[2],[5]]. The increased mixing in the jet head region is believed to outperform the limited mixing near the impingement point. However, increased soot due to very high soot formation near the impingement point cannot be fully resolved [[6],[8]]. In addition, soot deposition on the wall and valve surfaces originates from the jet-wall impingement

[[7],[9],[10]]. Deposited soot on the wall can then make its way to the exhaust or deteriorate the quality of engine oil.

In addition to these direct influences of jet-wall interaction on pollutants formation, it is recognised to have significant influence on the flame lift-off length, which in turn impacts downstream combustion and pollutant formation. In a constant-volume combustion chamber,

Pickett and Lopez [[8]] attempted to simulate jet-wall and jet-jet interactions by confining a jet to a box structure. The results of this experiment showed continual shortening of the lift- off length caused by wall-redirected combustion gases and thereby increased soot luminosity.

In later work focused on free-jets [[11]], a conceptual picture was drawn to depict the existence of a high-temperature product reservoir, which is suggested to play a role in stabilisation of diesel flame base. This high-temperature, low-density product reservoir, located on the edge of the vapour boundary, may explain and further empathise importance of jet-wall interaction. This is because the existence of the bowl-wall would potentially enable

hot combustion products at the periphery of fuel-air mixture to be redirected back towards the centre of the combustion chamber and possibly entrained into the incoming diesel jet. This entrainment of hot combustion products would cause shortening of the lift-off length, thereby increasing soot formation. This phenomenon, named as re-entrainment, was also observed in heavy-duty diesel engines [[12],[13]] under the influence of realistic jet-wall and jet-jet interactions.

The impact of re-entrainment on small-bore diesel engine combustion would be of higher significance than that on heavy-duty diesel engines. This is due to enhanced jet-wall interaction [[14]] associated with a shorter distance between the injector and the bowl wall as well as increased wall curvature. However, the details of the re-entrainment and its effect on the lift-off length in small-bore diesel engines are not well understood. Moreover, the small- bore design causes increased jet-jet interaction as neighbouring jets and their combustion products interact with each other at closer distance from the nozzle. It is well known that narrower inter-jet spacing causes decreased lift-off length [[15],[16]]; however, correspondence between jet-jet interaction and re-entrainment has not been discussed in details, particularly in small-bore diesel engines.

In the present study, jet-wall and jet-jet interaction is investigated with great emphasis on the re-entrainment and lift-off length in a small-bore diesel engine. To allow measureable jet- wall interaction, a long injection duration of 2.36 ms (actual) was implemented using a single-hole injector. We also used a modified piston where a section of the bowl-rim was removed to allow a different jet configuration. This achieved two different jet targeting approaches: one aiming at the bowl wall to create a near-axisymmetric wall-interacting jet and the other travelling towards the edge of the bowl-rim cut-out so that a half of the jet impinges on the bowl wall while the other half penetrates further into the bowl-rim cut-out region. The latter arrangement was for simultaneous visualisation of a liner-wall-interacting

jet (“free-jet”) and bowl-wall-interacting jet (“wall-jet”), enabling a direct comparison of jets with a different level of wall-interaction. Moreover, two two-hole nozzles of different inter- jet spacing (51.4o and 102.8o) were used to investigate the effect of jet-jet interaction and its influence on re-entrainment and lift-off length. Imaging of OH* chemiluminescence was performed for various conditions mentioned above as well as for various times after the start of injection so that transient behaviour of the flame base could be captured during and after the injection event. High-speed imaging of hot soot luminosity was also performed to ensure that the OH* chemiluminescence signal was not interfered by the soot incandescence signal as well as to observe trends in downstream soot concentration.

2. Experimental Setup

2.1 Optical diesel engine

A single-cylinder automotive-size diesel engine with optical access via a quartz piston window was used in this study as shown in Fig. 1. The bottom-view imaging of the combustion chamber was performed using a 45o angled mirror at the void of the extended piston assembly. The design of this engine using a hydraulic-ram-operated drop-down liner allows the cleaning of the chamber between engine runs, which is needed to minimise signal attenuation due to soot deposition on the surface of the quartz piston window.

2.2 Operating conditions and engine specifications

Specifications of facilities shown in Fig. 1 and selected operating conditions are listed in

Table 1. The single-cylinder optical engine was run by an AC motor at a fixed engine speed of 1200 rpm and was skip-fired at every 10th cycle to minimise thermal loadings on optical pieces and to remove residual gases for minimal cycle-to-cycle fluctuations. The base engine has 83 mm bore and 92 mm stroke with a geometric compression ratio of 17.7. As previously

mentioned, however, a portion of the bowl rim was removed resulting in a lower compression ratio of 15.2 for both jet targeting configurations used in this study. . The conventional engine head was equipped with an intake port throttle for variable swirl ratios. This was left fully open in this study, resulting in a swirl ratio of 1.4. Heated coolant water with constant temperature of 363 K was fed into the engine head, engine block and cylinder liner to simulate warmed-up engine operating conditions. The temperature of the naturally aspirated intake air was measured at 303 K throughout the experiments.

The fuel used was a conventional ultra-low sulphur diesel (ULSD) fuel with cetane number of 51. A second-generation Bosch common-rail injector equipped with a single-hole nozzle of 134 µm nominal diameter was employed. The original nozzle has 7 holes with even inter- jet spacing (51.4o) and 150o included angle. In this study, a laser-welding technique was applied to block the unused holes. A single-hole nozzle was used to isolate the target jet from interactions with adjacent jets as well as to allow for a long injection duration, within the safety limit of the optical parts of the engine. In addition to the single-hole nozzle, two two- hole nozzles with different inter-jet spacing of 51.4o and 102.8o were tested to study the jet- jet interaction. A long injection duration (2.36 ms for a single-hole nozzle) was chosen to extend the jet-wall interaction event and re-entrainment period so that a full history of the flame base movement during the injection and after the end of injection can be obtained. This actual injection rate was measured (setup not shown in Fig. 1) using a Bosch-type injection rate meter. Due to the long injection, the engine was operated at negative ignition dwell conditions, i.e., the combustion starts before the end of injection, representing upper-mid to high-load conditions. When two-hole nozzles were used, the injection duration was reduced to 1.53 ms to fix the injected fuel mass (10 mg per injection). The common-rail pressure was held constant at 70 MPa and the injection timing of 7oCA bTDC (crank angle degree before top dead centre) was fixed throughout the study.

2.3 Data acquisition and diagnostics

In-cylinder pressure: In-cylinder pressure was measured using a piezoelectric pressure transducer (Kistler type 6056A with glow plug adapter) as shown in Fig. 1. To address issues of the inherent cycle-to-cycle variations and also signal noise, the data were ensemble- averaged for a total of 30 fired cycles. The averaged in-cylinder pressure trace was used to calculate apparent heat release rate.

OH* chemiluminescence: The exothermic reaction of hydrocarbons during high temperature, stoichiometric combustion conditions (e.g., CH + O2 → CO + OH*) allows OH radicals to be chemically excited (OH*) during combustion [[17]]. The OH* signals are found in near- stoichiometric regions where high-temperature reaction occurs and therefore are useful to find the flame base in a reacting diesel jet [[18],[19]]. OH* chemiluminescence imaging essentially captures the emission of excited OH* as it returns back to the ground state. An intensified charged-couple-device (ICCD) camera (Lavision Nanostar), CoastalOpt UV

105mm f/4.5 lens, and a 310 nm wavelength band-pass filter [[19],[20]] were used for this imaging as shown in Fig. 1. The lens had quantum efficiency of 85% or higher at 250 ~ 650 nm range. While a range of ultraviolet wavelengths (306 ~ 310 nm) also contains fluorescence from CH and luminosity of soot, the signal from OH* is known to be dominant at 310 nm [[17],[19]] hence allowing this procedure to obtain OH* chemiluminescence with minimal interference from soot luminosity and other radicals. Obtained OH* chemiluminescence images were linearly averaged from a total of 30 images from 30 individual firing cycles at a selected crank angle degree. This averaging process was to address issues of cycle-to-cycle variations. Samples were taken at every 0.5 crank angle degrees from the first detectable instance of OH* signal to the end of main heat release.

Hot soot luminosity: High-speed natural soot luminosity imaging was also performed at the

same operating conditions of OH* imaging diagnostics. The soot luminosity is a mix of incandescence emitted by hot soot particles and chemiluminescence from electronically and chemically excited gas radicals (e.g., OH*, CH*, and C2) [[17]] and therefore does not represent in-cylinder soot concentration. However, the soot luminosity images help understand the progression of high soot regions and identify potential overlap with the OH* signal. For this diagnostic, a high-speed complementary metal-oxide-semiconductor (CMOS) camera (VisionResearch Phantom v7.3) coupled with a Nikkor 105mm f/2.8 lens was used at

36,000 frames-per-second. The resulting movie was used to reproduce still images at every

0.2oCA. These images were ensemble averaged over 20 cycles, similar to OH* chemiluminescence images.

Boundary detection: The boundaries of averaged OH* chemiluminescence and hot soot luminosity images were detected using a MATLAB code. The definition of the boundaries can be arbitrary since the threshold image counts or intensity differ from one crank angle degree to another. To address this issue, Otsu's method [[21]] was used in choosing an optimal threshold level specific to each individual image with the main objective of minimising the image’s intraclass variance resulting in minimal combined spread of black and white pixels during the binarisation process. Details of the boundary detection and averaging process will be discussed along with cycle-to-cycle variation in the results section.

2.4 Jet trajectory configurations and piston modification

A hot soot luminosity sample image of a conventional 7-hole combustion is shown in Fig. 2

(left) to identify the three jets (Jet A, B and C) used in this study (image taken from ref.

[[22]]). Shown on the right side of Fig. 2 are four different bottom-view schematics of the piston and various jet trajectories. The dashed boxes indicate the post-processed display view used in the following sections. The two single-jet trajectories (Jet A and Jet B) were used to

investigate the effects of jet-wall interaction (Fig. 2, top-right) with different jet targeting with respect to the bowl-rim cut-out region. For instance, Jet A impinges only on the bowl wall (the combustion event occurs near top dead centre and therefore a majority of the jet penetrates into the bowl) and therefore an axisymmetric jet was expected. By contrast, a half of Jet B is directed to the bowl-rim cut-out region resulting in an asymmetric jet. As mentioned previously, a portion of the bowl rim was removed to simultaneously visualise a jet interacting with cylinder-liner wall (“free-jet”) and another jet interacting with bowl wall

(“wall-jet”). Jet B shows that half of the jet impinges on the piston bowl wall while the other half of the jet enters the cut-out region and interacts with the cylinder-liner wall, with delayed bowl-wall/jet interaction. It is stressed that the “free-jet” of Jet B and any other jets entering the cut-out volume does have the aforementioned wall-interaction despite its naming convention but it is intended for the two jets to be distinguished more clearly in the text. In addition to these two single-hole nozzles, two two-hole nozzles (Jet B-C and Jet B-A) were tested to further investigate the lift-off length of the primary jet (Jet B) for different jet-jet spacing. For Jet B-C and Jet B-A configurations, two holes were left open while the rest were laser-welded. From Fig. 2, it may be noticed that the same modified piston was used for all the hole configurations. It was necessary to use the same piston for all jet arrangements to maintain the same in-cylinder pressure and temperature conditions. Since conventional seven-hole nozzles were laser-welded in the present study, there was possibility that hole-to- hole and nozzle-to-nozzle variations caused uncertainty. These variations, however, are dependent upon the manufacturing precision and hence unavoidable. Indeed, a soot luminosity image in Fig. 2 shows noticeable hole-to-hole variations. Throughout the experiments, however, observed trends and differences due to changes in operating conditions appeared to exceed this error. Also, errors due to injection-to-injection variations, which presumably were comparable to hole-to-hole and nozzle-to-nozzle variations, were

less than the differences caused by variations in operating conditions.

2.5 Definition of variables

A schematic diagram of Jet A is shown in Fig. 3 with multiple variables listed for discussion purposes. The sketch depicts a cropped section of the piston as previously annotated in Fig. 2 by a dashed rectangular box. The trajectory of Jet A is symmetric along the vertical axis drawn as a dotted line through the middle of the piston, which is also used to define separation point between the upstream swirl (upswirl) and downstream swirl (downswirl) side of the jet. It is noted that both sides of the jet are wall-jets by the definition of this study.

The resulting combustion zone is depicted with the red and green coloured fills representing hot soot luminosity and OH* chemiluminescence regions, respectively. The notation of “w” on the variables detailed at the bottom of Fig. 3 symbolises the contact between the jet to the inner bowl wall (wall-jet) while the subsequent notation after a hyphen of “u” and “d” is used to distinguish whether the jet is in the upswirl side or downswirl side, respectively.

The base heights of hot soot luminosity (SL) are defined as the point nearest to the nozzle baseline within a horizontal band (with reference to the baseline) of 0 to 5 mm, where 0 mm is the position of the injector. The nozzle baseline is defined as the plane perpendicular to the jet trajectory that cuts through the injector nozzle position. Similarly, the lift-off lengths notated with a prefix of “OH” are defined as the distance between the flame base and the nozzle baseline. Red and green bars are used to illustrate the hot soot base height and lift-off length, respectively, with solid and stripe used to distinguish upswirl and downswirl for Jet A.

The selection of the 5-mm range is adapted from Ref. [[13]], where 2-mm distance from the jet axis was used in a multi-hole injection arrangement. The wider band is to accommodate the wider spray angle associated with the larger nominal hole injector used in the current study (134 µm compared to 110 µm) and also the lower injection pressure used in this study

(70 MPa compared to 140 ~ 240 MPa). The OH region near the flame base is further illustrated in the magnified image in Fig. 3. From the tip of OH region that is used to determine the lift-off length, a rich partially-premixed reaction zone exists on the left that is delineated by a cone-shaped boundary in the jet core. On the right, a diffusion flame is formed near the stoichiometric location at the jet edge [[23]]. The OH* chemiluminescence images in this study is line-of-sight integrated and therefore cannot provide a cross-sectional view as in ref. [[23]]; however, early works conducted by Siebers and Higgins [[24]] showed a similar OH* chemiluminescence structure in the line-of-sight-integrated images with low intensity in the core and a thick layer at the jet edge. The vertical distance from the nozzle baseline to the head of the wall-reflected combustion gases is similarly defined within a range of 15 to 20 mm away from the jet trajectory axis to measure the penetration extent of the reacting jet after the wall impingement. A hyphen of “r” is used for this distance between the nozzle baseline and the wall-reflected jet head. For example, decreasing OHrw-d means the wall-reflected downswirl side of the jet penetrates further along the curvature of the bowl wall. Further details and samples of the acquisition and post-processing of the OH* chemiluminescence images will be shown in Section 3.2 along with Figs. 5 and 6.

Figure 4 shows the schematic of the Jet B configuration. The most noticeable difference between Jet B and Jet A is the introduction of cut-out volume on the left hand side of the piston bowl rim. The same notations are used for Jet B except for the omission of the “-d” and “-u”. Instead, “f” is used to denote a “free” side of the jet or more accurately, a half of the jet penetrating into the cut-out volume with delayed liner-wall/jet interaction compared to the bowl-wall/jet interaction. Analysis of jet-jet interaction of Jet B-C and Jet B-A was conducted using an identical field of view of Jet B with the presence of a neighbouring jet.

3. Results and Discussions

3.1 In-cylinder pressure traces and apparent heat release rate of Jet A and Jet B

Figure 5 shows 30-cycle ensemble-averaged in-cylinder pressure traces with Jet A (grey) and

Jet B (red) along with its corresponding motored cycle (black), measured injection rate and apparent heat release rate (aHRR). This direct comparison shows great similarity in the in- cylinder pressure and aHRR traces indicating that the two jet arrangements experience similar in-cylinder conditions during combustion. The measured injection rate also shows near identical profiles although different nozzle holes were used. The injection rate profile shows that ignition (i.e., crank angle degree location at which the fired pressure exceeds the motored pressure) occurs prior to the end-of-injection (EOI) at 10oCA aTDC (after top dead centre).

This confirms a negative ignition dwell condition where the fuel injection continues for some time after the start of combustion.

3.2 Cycle-to-cycle variations

During the imaging diagnostics, cycle-to-cycle variations of flame structures were measured.

To ensure that the acquired results are statistically meaningful, both pressure traces and flame images were ensemble averaged for 30 different fired cycles and an uncertainty range was calculated. Figure 6 shows an example of the post-processing of OH* chemiluminescence images. Two leftmost images are raw OH* images from two different cycles (cycle #10 and cycle #21) taken at 9oCA aTDC. The white dot at the centre illustrates the position of nozzle tip and a circle outlines the piston bowl wall. The OH* chemiluminescence signals appear as white clouds at upper half of the piston bowl and near the bowl wall. It is clear that two different cycles exhibit measurable differences in intensity and structure of OH* chemiluminescence. The averaged image of 30 individual cycles is shown next to raw images.

A simple linear averaging was used in this study i.e., the summation of each pixel intensity value and division with number of image samples. The grey-coloured triangle is used to

illustrate liquid and vapour fuel upstream of the OH* chemiluminescence boundary. This is to denote whether the image was taken during the injection or after the end of injection and is not from measurements.

Figure 7 displays OH* boundaries for three different individual cycles and the averaged image together with the lift-off length (OHw-d) and distance from the nozzle baseline to the wall-reflected jet head (OHrw-d). The OHw-d and OHrw-d from the averaged images are drawn across the individual cycle images for comparison purposes. The cycle-to-cycle variations are evident for both OHw-d and OHrw-d. A quick visual inspection suggested that the lift-off length variations were up to ±3 mm (~30% of 11 mm lift-off length), close to the variability range found in literature [[13],[15],[16]].

To provide further clarity on cyclic variations, average OHw-d and OHrw-d are plotted at various oCA aTDC together with error ranges (95% confidence) in Fig. 8. The error margins were calculated by measuring and comparing OHw-d and OHrw-d from individual images to its mean. It is shown that errors are in a range of 1~3 mm depending on crank angle locations. It

o is noticeable that the OHw-d shows a decreasing trend before the end of injection at 10 CA aTDC until it settles down and then slowly increases after the end of injection, opposed to a constant lift-off length of quasi-steady free jet (without wall interaction and fixed ambient temperature and pressure as in ref. [[2]]). Figure 8 suggests that such systematic variations of the lift-off length exceed the cycle-to-cycle variations and therefore are real. Similarly, penetration of the wall-reflected jet head back towards the nozzle (OHrw-d) is captured, which also exceeds the cycle-to-cycle variations.

A similar image processing procedure (Figs. 6~8) was also applied to hot soot luminosity images. Figure 9 shows two individual greyscale hot soot incandescence and the averaged image. The hot soot base heights (defined as SLw-d and SLw-u in Fig. 3) of this averaged

image are drawn across the individual cycle images for comparison. It is observed that cycle- to-cycle variations of soot luminosity images are higher than that of OH* chemiluminescence images, which is expected considering complex nature of soot formation process. The hot soot base heights together with an error range are plotted for various oCA aTDCs in Fig. 10.

It is notable that the hot soot base heights are mostly higher than the lift-off lengths (c.f. Fig.

8) at a fixed oCA aTDC even if the cycle-to-cycle variation is considered. This is consistent with the current understanding of the diesel jet structure for the presence of soot precursor formation region upstream of the soot formation region [[23]]. Although the hot soot luminosity signal cannot represent quantitative soot distribution within the reacting diesel jet, these hot soot base heights will provide complementary information to the lift-off length trends in the following discussions.

3.3 The shortening of lift-off length of wall-jets of Jet A

Figure 11 shows cycle-averaged OH* chemiluminescence (green) and hot soot luminosity

(red) boundaries for Jet A. A blue horizontal line shown at the bottom of each image corresponds to the nozzle baseline and a half circle illustrates the bowl wall, which equals to a field of view through the quartz piston window. Horizontal bars annotate the flame base or wall-reflected jet head for the upswirl side of the jet on the left as well as for the downswirl side of the jet on the right. The swirl flow is clockwise. Three time-stamps are shown at the image top-left and top-right with respect to oCA aTDC, after the end of injection (aEOI), and after the start of injection (aSOI). The postulated liquid/vapour fuel distribution is also illustrated to denote whether the imaging timing is before EOI or after EOI. The images are presented for every 0.5oCA so that temporal development of the diesel flame can be understood. It is notable that early OH* chemiluminescence signals are detected at 8oCA aTDC, which corresponds to the peak aHRR location (see Fig. 5). It was expected since OH* radicals are representative of high-temperature reactions. The images are presented from this

early OH* chemiluminescence detection to the end of main heat release at 11.5oCA aTDC, beyond the end of injection at 10oCA aTDC.

The first noticeable trend from Fig. 11 is that OH* chemiluminescence is visible at ranges outside of hot soot luminosity. As a result, the measured lift-off length is shorter than the hot soot base height indicating that the flame base resides upstream of detectable hot soot location. OH* chemiluminescence is also detectable at earlier crank angles compared to hot soot suggesting a residence time for soot inception. The separation between OH* chemiluminescence and hot soot luminosity signals confirmed that the measured lift-off lengths using OH* chemiluminescence were not interfered by strong soot luminosity. As mentioned previously, the swirl flow was clockwise, which caused asymmetry between upswirl and downswirl jets, particularly near the bowl wall. Similar asymmetry was reported by previous studies [[15],[16],[25],[26]] suggesting a shorter lift-off length of the downswirl side of the jet. However, the swirl flow does not appear to impact the flame bases (OHw-u and

OHw-d) significantly in our results. Although the asymmetric jet structure is obvious for all images presented in Fig. 11, the flame bases are detected at nearly the same location. This was possibly due to a lower swirl ratio of 1.4 than other engines [[27]]. Another interesting finding from Fig. 11 is missing OH* signals near the bowl wall as depicted by black-coloured area. It should be noted again that these images were ensemble-averaged for 30 individual images from 30 engine runs and the cleaning of quartz window was conducted between all engine runs. A quick investigation reveals that these signal losses were present on all the images taken from the first firing cycles to the last. Therefore, it is highly unlikely that the disappearance of OH* signal was due to cycle-to-cycle variations or soot deposition on the quartz surface. The missing OH* signals near the bowl wall should be due to the flame quenching caused by flame-wall interaction as reported in refs. [[7],[14]]. By contrast, hot

soot luminosity is clearly seen near the bowl wall once again suggesting the soot deposition on the wall via thermophoresis [[7]].

An interesting finding from Fig. 11 is the upstream movement of the flame bases (or

o shortening of OHw-u and OHw-d) between 8 and 10.5 CA aTDC. To highlight this transient behaviour of the flame base, OHw-u and OHw-d together with a distance from the nozzle baseline to the wall-reflected jet head (OHrw-u and OHrw-d) are plotted in Fig. 12. The hot soot base heights (SLw-u and SLw-d) are also shown, which are mostly higher than the lift-off lengths. This confirmed that the shortening of OHw-u and OHw-d was not a result of signal interference. One consideration was increasing in-cylinder pressure for the same crank angle locations of the shortening as noticed from Fig. 5. One might think that a well-known trend of decreasing lift-off length with increasing ambient temperature [[19]] explains this shortening.

However, a recent study empathised on the lift-off length behaviour under the influence of various degrees of jet-jet interactions reported that the ambient temperature increase alone cannot explain the decreasing lift-off length during the injection [[26]]. Previously, a similar trend of the upstream movement of hot reaction zones was observed by Genzale et al. [[28]] from their planar laser-induced fluorescence (PLIF) measurement of OH. However, it was due to a rebound of jet head away from the bowl wall during the late-cycle oxidation stage of combustion (i.e., long time after the end of injection), whereas the upstream movement of the flame base in Figs. 11 and 12 occurs during the injection and hence against the incoming jet.

In Fig. 12, the data are plotted as a function of oCA aEOI to stress this pre-EOI behaviour.

One possible mechanism of the shortening of lift-off length is the entrainment of hot combustion products [[8],[13],[26]]. It was suggested that due to jet-wall and jet-jet interaction, the hot combustion products reflected off the bowl wall could be entrained into the incoming jet as they are redirected back towards the centre of the combustion chamber.

As a result, the temperature and equivalence ratio of fuel-air mixture at the flame base would

increase, which would cause the upstream movement of the flame base. This phenomenon, named as re-entrainment, is a likely cause for the shortening of OHw-u and OHw-d in this study.

Although no direct measurement (or visualisation) of re-entrainment flow was attempted in this study, there are some evidences that support the re-entrainment theory. For instance,

Figures 11 and 12 show that the shortening occurs during the injection only when the jet momentum is sufficiently high to cause high entrainment. After the end of injection, the decrease of OHw-u and OHw-d slows down and the flame base continues to move downstream.

To induce the re-entrainment of hot combustion products, the high jet momentum should be present upstream of the flame base and therefore the fact that the shortening of the flame base was observed only before the end of injection supported the re-entrainment. Furthermore, it is notable that the penetration of the wall-reflected jet head (i.e., decrease of OHrw-u and OHrw-d) shows a correspondence with the shortening of the lift-off length. Although the imaging setup used in this study did not allow visualisation of hot combustion products movement outside the hot reaction zone, it was likely that the hot combustion products would travel along the wall contour following the wall-reflected jet penetration and some of them would move towards the centre of the combustion chamber, increasing the chance of re-entrainment.

Certainly there was no jet-jet interaction for the single-hole jet presented in Figs. 11 and 12, which was suggested as a primary cause for the redirection of the hot combustion products and thereby re-entrainment at large-bore engine conditions [[8],[13],[26]]. However, the enhanced wall interaction due to a short nozzle-to-wall distance and high curvature of the wall in small-bore engines might be enough to cause a similar effect even for a single-hole jet.

The impact of jet-jet interaction on the flame base behaviour will be discussed later in details.

3.4 Simultaneous visualisation of free-jet and wall-jet of Jet B

As discussed previously in the Experimental Setup section, a portion of the piston bowl-rim was removed to allow a half of the jet to penetrate further without the jet-wall interaction while the other half impinges on the bowl wall. This approach allows for simultaneous visualisation of a free-jet (i.e., free from the jet/bowl-wall interaction resulting in delayed jet interaction with the cylinder-liner wall) and a wall-jet (as Fig. 11), which provides opportunity to directly compare jets with earlier and later jet-wall interactions and thereby further investigating the re-entrainment. Fig. 13 shows the results. The presentation format of images is identical to Fig. 11, with additional illustration of the cut-out volume as a white outline and white-filled section. Once again, images are shown from the early detection of

OH* chemiluminescence that corresponds to the peak aHRR location at 8oCA aTDC to the end of main heat release at 11.5oCA aTDC.

It is observed that the wall-jet shows similar movement of the flame base and wall-reflected jet head as the downswirl jet in Fig. 11. In other words, the shortening of the lift-off length and a high penetration of wall-reflected jet head during the fuel injection are repeated.

However, marked differences are found in the free-jet compared to the wall-jet. Firstly, the free-jet lift-off length (OHf) is consistently higher than the wall-jet lift-off length (OHw) throughout the jet development. The swirl effects cannot explain this difference because upswirl (OHw-u) and downswirl (OHw-d) wall-jets showed the same lift-off lengths in Figs. 11 and 12. During the experiments, the consistency between Jet A and Jet B was carefully considered. For example, Fig. 14 shows that not only OHw-d and OHw-u of Jet A are near identical but also they are the same with OHw of Jet B. Therefore, the lift-off length of the free-jet (OHf) is longer than the lift-off length of any wall-jets tested in this study at fixed in- cylinder conditions. Secondly, Fig. 13 does not show the wall-reflected jet penetration for the free-jet during the injection event. It is only 11.5oCA aTDC at which OH* chemiluminescence signal is detected, long time after the end of injection. While the flame

development within the cut-out volume was not visualised, it may be presumed that the bisected jet travelled along the contour of the cut-out volume and cylinder liner, which eventually was redirected back towards the bowl area in an anti-clockwise manner. Finally, the soot luminosity signal is much weaker in the free-jet side than that in the wall-jet side. In fact, there is no soot luminosity signal detected before the end of injection, which shows good correspondence with the lift-off length. These trends, i.e., longer lift-off length, higher distance between the nozzle baseline and wall-reflected jet head, and lower soot luminosity for the free-jet, suggested that the re-entrainment played a significant role in the wall- interacting diesel jet development. During the fuel injection when the jet momentum was high, the free-jet had less chance to entrain hot combustion products that were re-directed back towards the centre of the combustion chamber due to the jet-wall interaction. It happened only long time after the end of injection that the cylinder-liner wall-reflected jet travelled towards the centre of the combustion chamber, which could not cause the re- entrainment at the time because the injection momentum diminished. As the free-jet was not strongly influenced by the re-entrainment and therefore the lift-off length was longer, downstream soot formation was reduced.

To further discuss the re-entrainment effects on the free-jet and wall-jet, the lift-off lengths

(OHf and OHw), distances between the nozzle baseline and the wall-reflected jet head (OHrf and OHrw), and hot soot base heights (SLf and SLw) are plotted altogether in Fig. 15. As discussed previously in the cycle-to-cycle variations section, the differences in all parameters well exceeded the error range. While the same trends in Fig. 13 are clearly seen in the plot, it is noticeable that the variations in lift-off length show a close correlation with the soot luminosity. Lower SL (higher soot luminosity) is pretty evident for the wall-jet compared to free-jet, which is consistent with lower OHw than OHf (richer mixture at the flame base).

While this gap between SLw and SLf is maintained, decreasing OHw and OHf appears to

induce decreasing SLw and SLf during the injection event. It is also observed that SLw and

SLf slowly increase long time after the end of injection. This well-known correspondence between the lift-off length and downstream soot formation was confirmed again for the wall- interacting diesel jet.

3.4 Jet-jet interaction effects on re-entrainment

Previous discussions suggest that the re-entrainment is one possible explanation of transient behaviour of the flame base when the diesel jet interacts with the wall. However, in a more realistic engine environment with a multi-hole nozzle, the jet-wall interaction is not the only agent to cause such transience because the jet-jet interaction also impacts the jet penetration upon the wall impingement. The jet head impingement to the neighbouring jet head results in agglomeration of hot reaction zone where fuel-rich hot products merge together

[[6],[13],[15],[25]-[26],[28]]. Also, this effect of jet-jet interaction can potentially cause enhanced re-entrainment [[8],[13],[26]] and thereby negative impacts on downstream soot formation. To investigate this, we prepared two-holes nozzles with different hole spacing (see

Fig. 2).

Figure 16 shows in-cylinder pressure trace, injection rate, and apparent heat release rate of both Jet B-A (102.8o spacing) and Jet B-C (51.4o spacing). While great similarity between Jet

B-A and Jet B-C for all data is found, it should be noted that the injection rate shows a measurable difference compared to a single-hole Jet B (see Fig. 5). For example, the peak injection rate is doubled for two-hole nozzles simply because an additional hole with the same nominal diameter is used for Jet B-A and Jet B-C. Also, the rate shape is changed from a near-square to a triangular shape, which is because the injection duration is reduced from

2.36 to 1.53 ms to match the injected mass. In the injection rate profile, the actual start of fuel injection was measured at the same -7oCA aTDC for both single-hole and two-hole injectors

but the end of injection occurred at 10oCA aTDC for a single-hole nozzle (Fig. 5) and 4oCA aTDC for a two-hole nozzle (Fig. 16). One might notice that a very low injection rate is still detected after 4oCA aTDC for the two-hole nozzles. The source of this low injection rate is unclear but presumably it is due to the increased resistance to the needle closing associated with changed internal nozzle flow (Quick spray visualisation using light scattering confirmed no fuel injection during this closing period). Therefore, 4oCA aTDC is used as the actual end of injection in the following discussions.

The shorter injection duration of two-hole injectors changes the combustion regime significantly. For instance, the start of combustion is observed at 7.5oCA aTDC, some positive time after the end of injection, resulting in positive ignition dwell as opposed to the negative ignition dwell observed in the single-hole nozzle measurement. Since the combustion occurs a long time after the end of injection, diminished jet momentum would suppress the re-entrainment and therefore the shortening of lift-off length might not be present. OH* chemiluminescence images shown in Fig. 17 (top) agrees with this.

Before the results in Fig. 17 are discussed in details, it should be noted that the lift-off lengths are defined for a long-injection condition at which the fuel is still being injected and the flame is lifted off from the nozzle. As the condition with the two-hole configurations does not allow for the start of combustion to occur before the end of injection (due to short injection duration), the lift-off lengths might not be defined. However, the movement of the OH* base boundary are still of great interest because it shows a strong dependency on the angular spacing between the two jets. While the same connotations will be used for consistency, it is noted here that OHf and OHw of the two-jet arrangements (Jet B-A and Jet B-C) are indicative of the distance from the nozzle to the OH* base boundary rather than the lift-off length as previously used. The images in Fig. 17 show that OHf and OHw do not vary much between 8 and 9.5oCA aTDC (images at the left column). This was because the diesel jet

developed after the end of injection and therefore OHf and OHw followed similar trends of post-EOI OHf and OHw of a single-hole Jet B (Figs. 13 and 15). However, this trend was changed at 10oCA aTDC (image at top-right). Figure 17 shows that the jet volume expands

o and grows further until 11.5 CA aTDC and OHw moves closer to the nozzle baseline. It is seen that the neighbouring jet A after the impingement on the wall travels along the bowl wall and agglomerates with Jet B, resulting in the growth of OH* chemiluminescence boundaries in a location where the OHw was measured. The OHf and OHw of Jet B-A together with those of Jet B are plotted in Fig. 18 for further discussion. The plot shows that OHf and

o OHw are measured at two distinct CA aEOI depending on the jet arrangement, i.e., the Jet B data are shown at -2 to 3oCA aEOI while Jet B-A results are found after 4oCA aEOI.

Therefore, Jet B flame develops under the strong influence of jet-wall interaction and re- entrainment but no jet-jet interaction. By contrast, Jet B-A flame is not affected by the re- entrainment but jet-jet interaction. Since the flames are detected at a distinctively different oCA aEOI range, a direct comparison between Jet B and Jet B-A is not possible. However, the close inspection of Figs. 5 and 16 suggest that the flame development occurred at very similar in-cylinder conditions. Also, one can predict how OHf and OHw of Jet B would develop at later oCA aEOIs (beyond the imaged period) considering a slow increase after the end of injection. Considering all these, higher OHf and OHw of Jet B-A than those of Jet B suggests that the effect of re-entrainment on the lift-off length shortening was stronger than the influence of the neighbouring jet.

However, the significance of jet-jet interaction should not be underestimated. In Fig. 19, results are shown for Jet B-C, using the same two-hole nozzle with narrower inter-jet spacing of 51.4o. Figure 18 shows that the OH* chemiluminescence images of Jet B-C are largely different to the images of Jet B-A. The OH* signals are observed to occupy a large area between the trajectory lines of Jet B and C, and the OHw is found very close to the nozzle at

about 5 mm. The OHf also appears to be shorter than that in Figs. 13 or 17. Since the images are shown for 4 to 7.5oCA aEOI at which the injection-induced jet momentum is very weak, the decreased re-entrainment was expected. However, the narrower spacing of the holes of Jet

B-C would likely cause stronger jet-jet interaction than Jet B-A, which could outperform the reduced re-entrainment due to the shorter injection duration than Jet B. Also, the growth of

OH* boundary on the left half of Jet B axis is observed between 8oCA and 9.5oCA aTDC

(images at the left column). This was due to the strong influence of Jet C penetration on Jet B, which resulted in much shorter OHf than that of Jet B or B-A flame.

The observed trends in Fig. 19 are well-reflected in the OHf and OHw plots as shown in Fig.

20. The lowest OHf and OHw in the present study are seen for Jet B-C although they are measured long time after the end of injection with the limited re-entrainment. Therefore, the results suggest that the narrow inter-jet spacing can cause significant issues (e.g., increased soot formation) even if the injection duration is short enough to suppress jet-wall interaction- driven re-entrainment.

4. Conclusions

The influence of re-entrainment on the lift-off length and development of wall-interacting jets were investigated in a small-bore optical diesel engine. The OH* chemiluminescence and hot soot luminosity imaging was performed using a single-hole nozzle and long injection duration (i.e., negative ignition dwell condition). The importance of re-entrainment and its impact on transient behaviour of the flame base were discussed in details by comparing diesel jets with a different level of jet-wall interaction. Two-hole nozzles were also tested and the results were compared to the single-hole data to understand the effect of jet-jet interaction on the shortening of lift-off length. The following conclusions may be drawn from this study:

• The flame base moves towards the nozzle in a wall-interacting jet before the end of

injection. This raises an issue because the downstream soot formation can increase with

the decreasing lift-off length. The existing theory of re-entrainment is used to explain this

trend based on the findings that (1) the lift-off length shortening occurs only during the

injection when the jet momentum is strong and (2) the penetration of wall-reflected jet

head corresponds well with the upstream movement of the flame base.

• Further evidence supporting the presence of re-entrainment is provided by bisecting a

single jet using a cut-out on the piston bowl rim. This approach enables simultaneous

visualisation of a half of the jet with strong bowl-wall/jet interaction and the other half of

the jet with delayed liner-wall/jet interaction. It is found that the lift-off length is

consistently longer for the free-jet because liner-wall-reflected jet head is redirected back

towards the centre of the combustion chamber only after the end of injection, resulting in

lower or no re-entrainment.

• Utilising two-hole nozzles together with short injection duration can decrease the re-

entrainment because the end of injection occurs earlier. However, jet-jet interaction is

well known to increase soot formation due to rich mixture formed in-between the jets.

OH* chemiluminescence images show that indeed the impact of jet-jet interaction on the

shortening of lift-off length is more significant than that of the re-entrainment if narrow

inter-jet spacing is selected. On the other hand, the lift-off length shows the highest value

when shortest injection duration and largest inter-jet spacing is used among all tested

conditions of this study, because both re-entrainment and jet-jet interaction are suppressed.

• To avoid the increased soot formation due to the lift-off length shortening during the

injection, one can design the combustion chamber with larger bowl diameter and apply

split main injections. This approach will effectively suppress the re-entrainment. A higher

swirl ratio may also be beneficial in accelerating the mixing process during injection to

minimise or even prevent the shortening. At the same time, large inter-jet spacing is preferred as otherwise the impact of jet-jet interaction on the lift-off length and downstream soot formation will be significant.

Acknowledgements

Experiments were conducted at the UNSW Engine Research Laboratory, Sydney,

Australia. Support for this research was provided by the Australian Research Council.

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List of Captions

Table 1 Selected operating conditions and engine specifications.

Fig. 1 Experimental setup for the optical engine and imaging diagnostics used in this study.

Fig. 2 Soot luminosity image of a conventional 7-hole nozzle (left), Jet A and Jet B configuration showing the single-hole nozzle arrangement with respect to the bowl-rim cut- out (top-right), and Jet B-C and Jet B-A for two-hole nozzles with different inter-jet spacing. Dashed boxes indicate the field of view used for data analysis.

Fig. 3 Schematic representation of single-hole diesel jet with no bowl-rim cut-out (Jet A). Definitions for hot soot base heights (SLw), lift-off lengths (OHw), and distance from the nozzle baseline to the wall-reflected jet head (OHrw) are given at the bottom. A soot luminosity image of Jet A is shown in the top-left corner.

Fig. 4 Schematic diagram of single-hole diesel jet with bowl-rim cut-out (Jet B). Definitions for hot soot base heights, lift-off lengths, and distance from the nozzle baseline to the wall- reflected jet head are provided for both free-jet and wall-jet. A soot luminosity image of Jet B is shown in the top-left corner.

Fig. 5 Comparison of in-cylinder pressure, injection rate and apparent heat release rate between jet A (grey) and Jet B (red) configurations.

Fig. 6 Post-processing procedures for OH* chemiluminescence images of Jet A including linear averaging of individual raw images and boundary detection of the averaged image.

Fig. 7 Variations in individual images of OH* chemiluminescence and comparison with its averaged image. Images are captures for Jet A and at 9oCA aTDC.

Fig. 8 Average value and error range of lift-off length of downswirl side of wall-interacting jet (OHw-d) and distance from the nozzle baseline to the wall-reflected jet head (OHrw-d) for various crank angle degree after the top dead centre (oCA aTDC).

Fig. 9 Two soot luminosity images showing cycle-to-cycle variations and averaged image of 20 individual cycles for Jet A. The hot soot base heights are annotated by a horizontal bar.

Fig. 10 Average value and error range of hot soot base heights for various oCA aTDCs for both upswirl and downswirl sides of Jet A.

Fig. 11 Temporal development wall-interacting diesel jet illustrated by averaged flame boundaries of hot soot luminosity (red) and OH* chemiluminescence (green) from 8oCA to 11.5oCA aTDC of Jet A.

Fig. 12 The hot soot base height (SL), lift-off lengths (OHw), and distance from the nozzle baseline to the wall-reflected jet head (OHrw) for various crank angle degrees after the end of injections (oCA aEOI). The data are from the images in Fig. 11 for Jet A.

Fig. 13 Temporal development of Jet B illustrated by boundaries of hot soot luminosity (red) and OH* chemiluminescence (green) from 8oCA to 11.5oCA aTDC.

Fig. 14 Lift-off lengths of Jet A (grey) and Jet B (red) for both wall-interacting jets (OHw) o and free jet (OHf) for various CA aEOIs.

Fig. 15 The hot soot base height (SL), lift-off lengths (OH), and distance from the nozzle o baseline to the wall-reflected jet head (OHr) for various CA aEOIs. The data are from the images in Fig. 13 for Jet B.

Fig. 16 In-cylinder pressure, injection rate, and apparent heat release rate of two-hole nozzles for Jet B-A and Jet B-C.

Fig. 17 Temporal development of Jet B-A illustrated by boundaries of OH* chemiluminescence (green) from 8oCA to 11.5oCA aTDC. Dashed lines are shown for two jet trajectories with 102.8o inter-jet spacing.

Fig. 18 Comparison of lift-off lengths of free and wall jets (OHf and OHw) between jet B and jet B-A arrangements.

Fig. 19 Temporal development of Jet B-C illustrated by boundaries of OH* chemiluminescence (green) from 8oCA to 11.5oCA aTDC. Dashed lines are shown for two jet trajectories with 51.4o inter-jet spacing.

Fig. 20 Comparison of the lift-off lengths of free and wall jets (OHf and OHw) between Jet B and Jet B-C arrangement.

Table 1 Selected operating conditions and engine specifications.

Engine speed 1200 rpm Displacement 497.5 cc (single-cylinder) Bore 83 mm Stroke 92 mm Compression ratio 15.2 Swirl ratio 1.4 Wall (coolant) temperature 363 K Intake air temperature 303 K Second-generation Bosch Injector type common-rail Nozzle type Hydro-grounded, K1.5/0.86 Nozzle hole diameter 134 µm Included angle 150° Number of holes 1 and 2 2.36 ms for 1-hole nozzle Injection duration 1.53 ms for 2-hole nozzle Injected fuel mass 10 mg Rail pressure 70 MPa Injection timing 7°CA bTDC

Common-rail Fuel injection system Injector

Pressure sensor Intake air Cylinder head heater Quartz pieces

Wall temperature control unit Cylinder For liner 45° angled High-Speed Extended mirror Soot Luminosity piston For Nikon Micro 105mm OH Chemiluminescence Prime lens @f/11 JENOPTIK CoastalOpt Vision Research 105mm UV-VIS @f/4.5 Phantom v7.3 250nm – 650nm range CMOS Sensor + 310 nm bandpass filter 256x256 pixels @36 000 fps LaVision Nanostar Intensified CCD Sensor 416x416 pixels

Fig. 1 Experimental setup for the optical engine and imaging diagnostics used in this study.

Low Low Display Range Liquid Spray

Jet A Jet B

Jet C 1 Jet B Jet A 7 2

6 3 Jet B-C Jet B-A

Jet B Jet C Jet B 5 4

Jet A Distance from nozzle [mm] nozzle from Distance Distance from nozzle [mm]

Soot Luminosity Image

Jet A Centreline SL

SLw-u SLw-d

Nozzle Baseline OHw-u OHw-d OHrw-u OHrw-d Injector

SLw-u – Hot soot base height of wall-jet on upswirl side SLw-d – Hot soot base height of wall-jet on downswirl side OHw-u – Lift-off length of wall-jet on upswirl side OHw-d – Lift-off length of wall-jet on downswirl side OHrw-u – Vertical distance of wall-reflected jet head from nozzle baseline (upswirl) OHrw-d – Vertical distance of wall-reflected jet head from nozzle baseline (downswirl)

Soot Luminosity Image

Jet B Centreline SL

Cut-Out Volume

SLf SLw Nozzle Baseline OHrf OHf OHw OHrw Injector

SLf – Hot soot base height of free-jet SLw – Hot soot base height of wall-jet OHf – Lift-off length of free-jet OHw – Lift-off length of wall-jet OHrf – Vertical distance of liner-reflected free-jet head from nozzle baseline OHrw – Vertical distance of wall-reflected jet head from nozzle baseline

Jet A Jet B-A Jet B-A Jet B-A Jet B Jet B-C Jet B-C Jet B-C

5 mm3/s Injection Rate InjectionInjection Profile Profile Injection Profile Injection Profile End of Injection

Fig. 5 Comparison of in-cylinder pressure, injection rate and apparent heat release rate between Jet A (grey) and Jet B (red) configurations.

Fig. 6 Post-processing procedures for OH* chemiluminescence images of Jet A including linear averaging of individual raw images and boundary detection of the averaged image.

Fig. 7 Variations in individual images of OH* chemiluminescence and comparison with its averaged image. Images are captures for Jet A and at 9oCA aTDC.

Jet A

OHw-d

OHrw-d

Average of 20 cycles 2 individual2 individual cycles2 individual cycles (Hot (Hot Soot 2cycles Soot individual Luminosity) Luminosity)(Hot Soot cyclesLuminosity)Average (HotAverage of Soot20Average cycles Luminosity) of of 20 20 cycles

10 °aTDC 10 °aTDC Jet A Jet A 10 °aTDC 10 °aTDC Jet A Jet A OH SL SLw-u SL f OHf f SLf w-u SLw-u OH OHw OH OHf SLSLw SL SLw-uSLf f w SLw f SL w SLw-d SLw SLw SLw-d #2 #9 w Average of 20 cycles Average of 20 cycles #2 OHOHw #9 SL OHw OHw wOH SLOHww w SL wf OHw OH SLw SL SLw-d OHSLw wf SLw w-d OHr w OHwr Average of 20 cycles Average of 20 cycles #2 r #9#2 OHr OHwr #9 OHr OH OHOHw OHw w OHwf w OHwf OHr OH OHr OH OHr wr OHr wr

Fig. 9 Two soot luminosity images showing cycle-to-cycle variations and averaged image of 20 individual cycles for Jet A. The hot soot base heights are annotated by a horizontal bar.

Jet A

SLw-u

SLw-d

Fig. 10 Average value and error range of hot soot base heights for various oCA aTDCs for both upswirl and downswirl sides of Jet A.

8 ⁰ aTDC Jet A 15 ⁰ aSOI 10 °aTDC 17 ⁰ aSOI -2 ⁰ aEOI EOI

OHw-d swirl OHrw-d

8.5 ⁰ aTDC 15.5 ⁰ aSOI 10.5 °aTDC 17.5 ⁰ aSOI -1.5 ⁰2 aEOI individual cycles (Hot0.5 Soot °aEOI Luminosity) Average of 20 cycles OH 10 °aTDC w-u Jet A SL OHf SLf w-u 9 ⁰ aTDC 16 ⁰ aSOI 9.5 ⁰ aTDC 16.5 ⁰ aSOI 10 ⁰ aTDC 17 ⁰ aSOI 9 ⁰ aTDC OH 16 ⁰ aSOI 9.5 ⁰ aTDC SL 16.5 ⁰ aSOI 10 ⁰ aTDC 17 ⁰ aSOI -18.5 ⁰ aEOI ⁰ aTDC 9 °aTDCw15.5 ⁰ aSOI 9-0.5 ⁰ aTDC ⁰ aEOI 16 ⁰ aSOI 11 °aTDC 16w ⁰ aSOI 9.5 ⁰ aTDC 18 ⁰ aSOI 16.5 ⁰ aSOI -1 ⁰ aEOI -0.5 ⁰ aEOI SLSLw EOIEOI SLw-d 0.5 ⁰ aEOI -1 °aEOI 1 ⁰ aEOI 1 °aEOIw 1.5 ⁰ aEOI SLAverage of 20 cycles SL #2 #9 fSLf wSLw SLw-u OHw OH SL OHw 1 OHf f 2 w-d OHwf 3 OHOHOHw rw-u w OHr OH OHr wr OHf OH OHOHrw SLf OHf f OHw rw SLf OHrw OH OHrf SL OHw w SLw w 11.5 ⁰ aTDC 10.5 ⁰ aTDC 17.517.5 ⁰ aSOI ⁰ aSOISL11SLw ⁰ aTDC 18 18⁰ aSOI ⁰ aSOI 11.5 ⁰ aTDC 18.518.5 ⁰ aSOI ⁰ aSOI 1010.5 ⁰ aTDC ⁰ aTDC 9.5 °aTDC17 ⁰ aSOI 10.5SLSLw11 ⁰w aTDC⁰ aTDC 11.5 °aTDC17.5 ⁰ aSOI 11 ⁰ aTDC 18 ⁰ aSOI 0.5 ⁰ aEOI 1 ⁰ aEOIw 16.5 ⁰ aSOI 1.51.5 ⁰ aEOI ⁰18.5 aEOI °aSOI 0.5 ⁰ aEOI -0.5 °aEOI 2.51 ⁰ ⁰aEOI aEOI 1.5 °aEOI 3 ⁰ aEOI 2 ⁰ aEOI OHOHw OHOHw w OH 4 5w OHwf wf 6 OH OHOHr r OHOHwr OHr r wr

DistanceDistance from from Jet Jet Centreline Centreline (mm) (mm) Fig. 11 Temporal development wall-interacting diesel jet illustrated by averaged flame boundaries of hot soot luminosity (red) and OH* chemiluminescence (green) from 8oCA to 11.5oCA aTDC of Jet A.

Jet A SLw-u SLw-d OHw -u

OHw-d OHrw-u

OHrw-d

Cut-out volume 8 °aTDC Jet B 15 °aSOI 10 °aTDC 17 °aSOI Jet B -2 °aEOI EOI

OHf OHw swirl OHrw

8.5 °aTDC 15.5 ° aSOI 10.5 °aTDC 17.5 °aSOI -1.5 °aEOI 0.5 °aEOI

SLw

9 °aTDC 16 °aSOI 11 °aTDC 18 °aSOI -1 °aEOI 1 °aEOI

9.5 °aTDC 16.5 °aSOI 11.5 °aTDC 18.5 °aSOI -0.5 °aEOI 1.5 °aEOI

OHrf

Fig. 13 Temporal development of Jet B illustrated by boundaries of hot soot luminosity (red) and OH* chemiluminescence (green) from 8oCA to 11.5oCA aTDC.

Jet A OHw-u Jet B OHf

Jet A OHw-d Jet B OHw

Fig. 14 Lift-off lengths of Jet A (grey) and Jet B (red) for both wall-interacting jets (OHw) and o free jet (OHf) for various CA aEOIs.

Jet B

SLf OHf

OHw SLw OHrf

OHrw

Jet B-A

Jet B-A Jet B-A Jet B-A Jet B-C Jet B-A JetJet B- BC-C Injection Rate Jet B-C Jet B-C 10 mm3/s 5 mm3/s End of Injection InjectionInjection Profile Profile Injection Profile Injection Profile

Fig. 16 In-cylinder pressure, injection rate, and apparent heat release rate of two-hole nozzles for Jet B-A and Jet B-C.

8 °aTDC Jet B 15 °aSOI 10 °aTDC 17 ° aSOI 4 °aEOI 6 °aEOI OHf OHw

Jet B-A Jet A 8.5 °aTDC 15.5 ° aSOI 10.5 °aTDC 17.5 °aSOI 4.5 °aEOI 6.5 °aEOI

9 °aTDC 16 °aSOI 11 °aTDC 18 °aSOI 5 °aEOI 7 °aEOI

9.5 °aTDC 16.5 °aSOI 11.5 °aTDC 18.5 °aSOI 5.5 °aEOI 7.5 °aEOI

Jet B-A OHf

Jet B OHf

Jet B OHw Jet B-A OHw

Fig. 18 Comparison of lift-off lengths of free and wall jets (OHf and OHw) between jet B and jet B-A arrangements.

8 °aTDC Jet B 15 °aSOI 10 °aTDC 17 ° aSOI 4 °aEOI 6 °aEOI Jet C

OHw OHf Jet B-C

8.5 °aTDC 15.5 ° aSOI 10.5 °aTDC 17.5 °aSOI 4.5 °aEOI 6.5 °aEOI

9 °aTDC 16 °aSOI 11 °aTDC 18 °aSOI 5 °aEOI 7 ° aEOI

9.5 °aTDC 16.5 °aSOI 11.5 °aTDC 18.5 °aSOI 5.5 °aEOI 7.5 °aEOI

Jet B OHf

Jet B-C OHf

Jet B OHw

Jet B-C OHw

Fig. 20 Comparison of the lift-off lengths of free and wall jets (OHf and OHw) between Jet B and Jet B-C arrangement.