Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine

doi:10.1016/j.proci.2018.09.013

Proceedings of the Combustion Institute

Volume 37, Issue 4, 2019, Pages 4839-4846

https://doi.org/10.1016/j.proci.2018.09.013 Get rights and content

Abstract

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.

Introduction

High-pressure fuel injection is widely used to reduce engine-out particulate emissions in modern diesel engines. Previous studies reported that high pressure injection suppresses soot formation due to better fuel-air mixing [1], [2], which is related to enhanced air-entrainment, increased lift-off length, decreased mixture stoichiometry at the flame base, and shorter soot residence time [3], [4], [5]. However, given the in-cylinder soot process involves both the formation and oxidation occurring simultaneously [6], [7], [8], a recent study by Gallo et al. [9] suggested that the soot oxidation rate can make a more significant impact on the overall in-cylinder soot than the formation rate does. For instance, the higher injection pressure leads to enhanced late-cycle mixing due to stronger turbulent flows, which causes increased soot oxidation that can outperform soot formation [10]. Since the enhanced mixing caused by higher injection pressure results in both reduced soot formation and increased soot oxidation, however, their role in overall soot reduction is straightforward. A more complex factor influencing soot formation and oxidation simultaneously is the flame temperature [11]. For instance, the entrainment of higher temperature ambient gases leads to higher flame temperature, which promotes not only the soot oxidation but also the soot formation [2], [6], [11], [12]. In this regard, our previous studies conducted in the same optical engine of the present study showed that higher injection pressure causes increased high-temperature reaction [4], [13], which raises an interesting question about its influence on soot formation and oxidation. This question, to the best of our knowledge, has not been addressed clearly in the literature.

To bridge this gap, the present study shows not only the overall distribution of soot during the soot formation/oxidation process based on planar laser-induced imaging of soot incandescence (PLII) but also high-temperature reaction using planar laser-induced fluorescence of hydroxyl radical (OH-PLIF) imaging to better understand the correspondence between the soot and high-temperature flame when the fuel injection pressure is varied. To analyse the soot formation and oxidation status, the thermophoretic soot particle sampling using a transmission electron microscope (TEM) grid is also conducted for various in-bowl locations in the same engine. There are many fine papers about the injection pressure effects on in-cylinder soot but the present work combines the PLII images and TEM images of soot particles at fixed operating conditions for the first time.

Section snippets

Optical engine setup and operating conditions

The OH-PLIF/PLII imaging and in-bowl soot sampling experiments were performed in a single-cylinder, small bore optical diesel engine as shown in Fig. 1. Table 1 shows the optical engine specifications and operating conditions, which are almost identical to the paper presented in the 36th meeting [14] except the significant improvement made for the location of soot sampling probes. As shown in Fig. 1, the soot particle sampling was conducted within the piston-bowl where the sooting flame

Heat release rates and in-cylinder soot distributions

Shown at the top of Fig. 2 are the ensemble-averaged in-cylinder pressure traces of motoring and firing cycles for three different injection pressures. The corresponding apparent heat release rate (aHRR) traces are plotted with actual injection timing and duration measured using a Bosch tube-type injection rate meter. The figure shows that the aHRR traces are almost identical for all the three injection pressures but start to deviate upon the start of combustion occurring near TDC. The first

Conclusion

The in-cylinder soot reduction through high fuel injection pressure has been studied in a small-bore optical diesel engine by performing PLII imaging and the structure analysis of sampled in-flame soot particles. To better understand the correspondence between the soot and high-temperature reaction distributions, OH-PLIF imaging was also performed. The engine was operated with methyl decanoate, a selected surrogate fuel for its low-sooting propensity and thus minimal beam attenuation issue in

Acknowledgment

The experiments were conducted at the UNSW Engine Research Laboratory, Sydney, Australia. The financial support was provided by the U.S. Army International Technology Center Pacific (ITC-PAC) under Contract no. FA5209-17-P-0060.

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Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine

Abstract

Introduction

Section snippets

Optical engine setup and operating conditions

Heat release rates and in-cylinder soot distributions

Conclusion

Acknowledgment

Combust. Flame

Combust. Flame

Prog. Energy Combust. Sci.

Symp. Combust.

Symp. Combust.

Proc. Combust. Inst.

Fuel

Prog. Energy Combust. Sci.

Combust. Flame

Combust. Flame

Prog. Energy Combust. Sci.

Combust. Flame

Fuel

Combust. Flame

SAE Technical Paper 1999-01-0523