Elsevier

Fuel

Volume 182, 15 October 2016, Pages 272-283
Fuel

Full Length Article
Optical study of throttleless and EGR-controlled stoichiometric dual-fuel compression ignition combustion

https://doi.org/10.1016/j.fuel.2016.05.021Get rights and content

Highlights

  • We studied the in-cylinder flame of SDCI combustion.

  • The premixed blue flame is identified using RGB magnitudes.

  • The effects of EGR and injection strategy studied.

  • The blue flame is significantly faster than in SI combustion.

Abstract

This paper investigates using EGR instead of throttle to control the load of a stoichiometric dual-fuel dieseline (diesel and gasoline) compression ignition (SDCI) engine for reducing fuel consumption and emissions in future internal combustion engines. To investigate the combustion process in the SDCI mode, the flame development in a single-cylinder optical engine with a dual injection (PFI + DI system) has been recorded by high-speed imaging. The images are processed and analyzed, and the premixed blue flame is identified according to their RGB magnitudes. The effects of EGR and load conditions, injection pressure, injection timing, and split injection strategy on SDCI combustion are studied. An earlier injection strategy is found to be ideal for soot reduction; however, the ignition–injection decoupling problem results in difficulties in combustion control. A split injection strategy offers a flexible solution as simultaneous optimization for control of pressure rise rate, reduction of soot and increase of thermal efficiency. The premixed blue flame propagation speeds are correlated to the heat release data, and it is found that the flame propagation speeds are significantly higher than those in conventional spark ignition (SI) engines even with an EGR ratio higher than 40%.

Introduction

Automotive industry is driven by more and more stringent emission legislations but they are more concerned with fuel economy nowadays because of the CO2 target is also becoming very demanding. Many technologies that can combine the advantages of diesel engines and gasoline engines are under development to address the challenge. Previous dual fuel combustion studies have covered a wide range of potential fuels, including natural gas, bio-fuel, and hydrogen [1], [2], [3], [4], [5]. Concepts such as HCII, HCCI, PCCI, dieseline/PPCI and RCCI [6], [7], [8], [9], [10] have also been studied widely. Using diesel with direct injection (DI) to ignite premixed gasoline mixtures by port fuel injection (PFI) is considered a potential way to realize clean and high efficiency combustion [10], [11], [12], [13]. Some previous researches have investigated stoichiometric combustion strategies with dual fuel at high loads [14], [15]. The authors of this paper proposed the concept of stoichiometric dual-fuel compression ignition (SDCI) combustion [16], which uses EGR to control the fresh intake charge quantity and thus the load, while maintaining the total equivalence ratio at 1.0 and using gasoline as much as possible. As such, three-way catalysts (TWCs) can be used in the after-treatment system to deal with NOx, HC and CO emissions across a wide range of operating conditions while soot formation is minimized in the combustion system [17], [18]. TWCs offer high conversion ratios of harmful gaseous species in the exhaust and are also a lower-cost solution compared to conventional diesel particulate filters (DPF) or selective catalytic reduction (SCR). The thermal engine study by the authors [16] has revealed that SDCI combustion can achieve high thermal efficiencies that are close to or higher than that of commercial diesel engines even at low loads due to significantly shorter combustion durations. The large percentage of premixed combustion in the SDCI mode results in significant soot reduction compared to the conventional diesel combustion, thus, even if an upstream DPF is needed to ensure the performance of TWC, it can be much smaller and cheaper than current commercial DPF systems for the same engine displacement. Other previous studies [19], [20] pointed out that there is a risk of strong knocking in dual fuel combustion at very high loads, which may result in serious engine damage in some conditions [21], [22], [23]; however, with retarded DI timing, it is possible to find the compromise between soot emissions and the pressure rise rate [16]. Furthermore, SDCI combustion also provides flexible use of alternate fuels as PFI premixed gasoline can be easily replaced by other fuels without changing the combustion mode significantly because combustion phasing is generally controlled by DI timing in the dual fuel mode. Moreover, with respect to tailpipe emissions, stoichiometric combustion with TWC after-treatment leads to lower sensitivity to fuel properties and injection strategy than conventional diesel combustion.

Although previous studies have investigated dual fuel combustion using a variety of approaches, EGR-controlled SDCI combustion is relatively a new research area, especially concerning high EGR ratios. To gain a comprehensive understanding of the in-cylinder combustion process and soot formation in SDCI engines, in-cylinder optical studies appear to be useful.

Optical diagnostics can apply transient and non-intrusive techniques for study of combustion study [24], [25]. It can realize measurements which cannot be achieved by conventional engine testing [26], [27], [28]. In recent decades, high-speed flame imaging has been one of the most widely used optical methods for in-cylinder combustion research [29]. Combined with other information, such as in-cylinder pressure traces, combustion processes can be investigated in greater details through the study of flame propagation images [30], [31]. Premixed gasoline combustion often has a very low luminance blue flame while diesel combustion often shows a bright red or yellow flame due to high soot concentrations. The previous work [32] used flame luminance to indicate the soot concentration distributions in a diesel flame semi-quantitatively, and the two-color method has also been directly applied based on RGB channel signals [33]. These studies by the authors demonstrated that it is possible to separate blue and yellow flame signals using RGB channels of the images, and thus to distinguish the main areas of premixed gasoline combustion and diesel–gasoline mixed combustion. An evaluation of soot distribution can also be realized.

In this paper, high-speed imaging was used to investigate SDCI combustion in a single-cylinder optical engine. The flame was separated into blue and yellow flame areas based on the RGB signal. Flame development and simultaneously recorded heat release data were combined to analyze the combustion at different EGR ratios, injection pressures, DI timing, and gasoline-to-diesel ratios. Different pilot injection strategies are also compared.

Section snippets

Experimental setup

Fig. 1 shows the experimental setup. The single-cylinder optical engine has a cylinder head modified from a commercial 4-cylinder diesel engine equipped with a common rail injection system. A PFI injector is mounted in the intake manifold to supply gasoline. The high pressure pump, DI, and PFI injection systems are controlled by an open ECU that is connected to the control PC. A quartz window is mounted in the extended piston, and a 45° incline mirror is fixed below the window. A dynamometer is

Image processing

RGB data from the images were used to separate the yellow flames and blue flames, with the advantage of using the colored camera taken. In fact, the yellow flame is much brighter than the premixed blue flame; it is very difficult otherwise to distinguish the flames using monochrome images. Fig. 2 shows the process of using different color channels to measure the area of the flame. The RAW colored images were read into a computer and separated into R, G, and B channels; each channel can be

Results and discussion

Unless specifically noted, results presented in the current paper are averages. Fig. 4 shows the sequential single-shot flame images for different typical running conditions with each individual combustion pressure trace for the cycle used for averaging representative to the corresponding averages.

The flame luminance for different running conditions is significantly different for different in-cylinder temperature, mixture strength, and soot formation. The lower luminance of the yellow flame is

Conclusions

SDCI combustion is studied using a combination of optical techniques with thermal analyses in an optical engine. High-speed color imaging results are analyzed using the RGB channel signals separately. The blue flame and yellow flames in SDCI combustion are compared under different fuel supply strategies. The effects of EGR ratio, injection pressure, injection timing, and split injection strategy are discussed. The main conclusions drawn from the study are as follows.

  • (1)

    EGR ratio in SDCI combustion

Acknowledgements

This research is supported by China NSFC under the grant no. 51506111. The authors would like to thank Jaguar Land Rover and Shell Global Solutions UK for their generous support and contributions to the ongoing research at the University of Birmingham. The authors also thank Prof. Gautam Kalghatgi of Saudi Aramco for useful discussions.

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