Effect of EGR and injection timing on combustion and emission characteristics of split injection strategy DI-diesel engine fueled with biodiesel

Abstract

In this study, the effect of injection timing and EGR rate on the combustion and emissions of a Ford Lion V6 split injection strategy direct injection diesel engine has been experimentally investigated by using neat biodiesel produced from soybean oil. The results showed that, with the increasing of EGR rate, the brake specific fuel combustion (BSFC) and soot emission were slightly increased, and nitrogen oxide (NO_x) emission was evidently decreased. Under higher EGR rate, the peak pressure was slightly lower, and the peak heat release rate kept almost identical at lower engine load, and was higher at higher engine load. With the main injection timing retarded, BSFC was slightly increased, NO_x emission was evidently decreased, and soot emission hardly varied. The second peak pressure was evidently decreased and the heat release rate was slightly increased.

Introduction

Today, air pollutants emitted from diesel vehicles, such as NO_x and particulate matter (PM), have created serious air pollution problems in major cities around the world. Regulations and control measures aimed at lowering exhaust emissions from truck and bus diesel engines have been adopted in an effort to improve air quality in cities. However, the degree of improvement seems to be not very satisfactory, mostly because of the difficulties in removing NO_x and the compromise between PM and NO_x emissions.

Over the past years, the investigations on diesel engines have expanded in the area of alternative fuels, among which biodiesel represents a very promising fuel. Pure biodiesel is a fuel composed of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats. Biodiesel is a sulfur-free, nontoxic, biodegradable, oxygenated, and renewable fuel and can be used in standard diesel engines with little or no engine or fuel system modifications. In comparison to diesel fuel, biodiesel has comparable energy density and cetane number, little sulfur and much oxygen [1], [2], [3], [4], [5]. However, high viscosity, high molecular weight, low volatility, etc. of biodiesel may in some cases lead to problems such as severe engine deposits, injector cooking, and piston ring sticking [6], [7], [8], [9]. In general, biodiesel provides comparable fuel efficiency and horsepower. Using biodiesel instead of diesel fuel reduces emissions such as the overall life cycle of carbon dioxide (CO₂), PM, carbon monoxide (CO), sulfur oxides (SO_x), volatile organic compounds (VOCs), and unburned hydrocarbons (HC). However, despite its potential benefits of lowering these pollutants, experiments indicated that the NO_x emission will be increased with the use of biodiesel [10], [11], [12], [13], [14], [15], [16], [17].

The above comparisons between diesel and biodiesel were based on the original setting of an engine using diesel fuel. The formulation of fuel composition can improve the biodiesel combustion performance and exhaust emissions. However, the results showed that it was difficult to acquire NO_x emission neutral while improving other pollutant emissions simply by fuel reformulation [18], [19], [20], [21], [22]. Therefore, modification of engine parameters may be feasible to optimize the engine emissions due to the difference in chemical composition and combustion characteristics between diesel and biodiesel. These were related, for example, to the injection strategies, or to exhaust gas recirculation (EGR). Recently, a split injection strategy has been proposed as a means to reduce NO_x emissions, and this allows the injection to be retarded to reduce NO_x emissions without a significant penalty in soot levels. Kim et al. [23] studied the effect of split injection on exhaust emissions, soot particulates, and engine performance using an electrically controlled direct injection diesel engine fueled with neat biodiesel derived from soybean. The results showed that split injection reduces NO_x emissions significantly without a significant increase in soot emissions. Decreasing soot, median particle diameter, and particle number concentration were seen in accordance with retarded injection timing for split injection. Zhang and Boehman [24] studied the impact of biodiesel on NO_x emissions in a 2.5 L common rail DI-diesel engine with different fuel injection strategies. They found that retarding injection timing under single injection conditions was the more effective approach to reduce the NO_x emissions than using pilot injection with retarded main injection in terms of NO_x and fuel consumption tradeoff. Under the low engine load condition, the pilot injection strategy led to substantially reduced NO_x emissions. Others have studied the effects of combining biodiesel and EGR. The general conclusion from these studies was that combining EGR and biodiesel was an effective way to decrease NO_x and/or PM. The majority of these studies have been performed on relatively small diesel engines. Tsolakis et al. [25] found that the use of biodiesel fuel could decrease the smoke and NO_x from a single-cylinder engine equipped with EGR under certain engine conditions when compared to diesel. The retardation of the injection timing could result in further reduction of NO_x at a cost of small increases of smoke and fuel consumption. Prandeep and Sharma [26] varied EGR levels and engine load on a single-cylinder engine and found that biodiesel produced more smoke at lower loads and less smoke at higher loads when compared to diesel fuel. Agarwal et al. [27] tested a two-cylinder engine equipped with EGR and biodiesel fuel and found that the 15% EGR and 20% biodiesel blend was the optimum combination. Saleh [28] studied the effect of EGR on the NO_x emissions of two-cylinder, naturally aspirated direct injection diesel engine fueled with jojoba methyl ester. They concluded that EGR is an effective technique for reducing NO_x emissions with JME fuel especially in light-duty diesel engines. For all operating conditions, a better trade-off between HC, CO and NO_x emissions can be attained within a limited EGR rate of 5–15% with very little economy penalty.

The work in this paper was a continuation of our experimental and numerical investigations of the influence of neat biodiesel from soybean oil on the characteristics of a diesel engine [29]. Attention was focused on the determination of the injection timing and EGR rate for biodiesel with the aim to reduce all engine harmful emissions, especially NO_x and PM.