Testing of formulated fuel with variable aromatic type and contents in a compression-ignition engine

Highlights

• Novel fuels with optimal selection of aromatic are proposed.

• Utilization of dearomatized hydrocarbon in different blend ratios with aromatics

• Effect of fuel composition on spray parameters, engine performance and emissions studied

• Monocyclic single ring aromatics significantly reduce the PM, gaseous and acoustic emissions.

Abstract

Fuels used in various combustion engines necessitate deep understanding of the fuel components before being used as a feedstock with present engine technology. Fuel components impact the efficiency of the engine along with the formation of pollutant emissions. Aromatics are important component of the fossil-based diesel fuel, which highly affects the tailpipe emissions. Therefore, the study of aromatics has a great significance for diesel engine. A comprehensive study with a total of 13 different aromatics was performed to investigate the impact of their structure and content on diesel engine characteristics. Diesel, pure higher chain alkanes and single added aromatics were blended as ternary mixture. Detailed spray investigations were carried out at an injection pressure of 200 bar using a three-hole nozzle of diesel engine. The aromatic structure and content highly influenced the emission and performance characteristics. A blending ratio of 10% aromatics in base fuel shows promising results compared to 5% and 15% blending content of aromatics in the base fuel. Higher aromatics blending in ternary mixture led to higher particulate matter, carbon monoxide, and unburnt hydrocarbon emissions. Due to higher degree of unsaturation, polycyclic single and double ring aromatics produce higher pollutant emissions compared to single ring monocyclic aromatics.

Introduction

The efficient combustion of fossil fuels has been vital to fulfill the global energy demands for several decades. However, the higher regulated and unregulated emissions from burning of these fossil-based fuels for energy are a major concern. Greenhouse gas (GHG) emissions have been continuously and drastically increasing due to excessive use of fossil fuels [1]. The regulatory bodies are continuously working on reducing the emission levels from the environment [2]. These forces have been motivating the researchers to work on various strategies to reduce the emissions from fossil fuel based combustion systems.

Direct injection (DI) diesel engines are widely used and diesel fuel is a workhorse throughout the world due to its higher efficiency, economy, higher power and reliability than gasoline spark ignition engines for heavy and medium-duty applications in transportation, construction, power generation, agriculture, and manufacturing applications [3]. Carbon dioxide (CO₂) emissions from diesel engines are relatively lower as compared to gasoline engines. Fuel economy improves with diesel-fuelled engines compared to gasoline engines that save about 20% or more fuel [4]. In most of the developing countries, the consumption of diesel is higher than gasoline. However, the engines fueled with diesel need to be modified because of their higher nitrogen oxides (NO_x) and particulate matter (PM) emissions [5]. Diesel engine emitted small particulates can penetrate human lung's respiratory tract and cause severe adverse health effects [6]. The emission regulations have become stringent for NO_x and PM emissions to preserve the environment. This has been continuously forcing the transportation sector to mitigate the problem of higher emissions from the combustion of diesel fuels. The manufacturing companies of diesel-fueled vehicles have been continuously working on the improvement of hardware or development of engine parts and parameters for better performance with fewer tailpipe emissions. The companies implemented exhaust gas recirculation, changes in the combustion chamber geometries, multiple injection strategies, etc. to reduce the exhaust emissions [[7], [8], [9]]. On the other hand, the oil firms have been continuously working to introduce better diesel surrogates or renewable fuels aimed at improved performance and lower emissions. Enhancing the efficiency of the energy release with lower pollutant emissions requires automobile manufacturers and oil companies to make an effort to determine the ideal solution with allied research programs [10].

Diesel is a complex mixture of hydrocarbons, which usually contains n-alkanes, isoalkanes, cycloalkanes and aromatics. Due to the higher degree of unsaturation, aromatics are the main contributors towards the PM emissions from the burning of diesel [11,12]. Earlier, surrogate fuels have been prepared through a combination of various alkanes to minimize the pollutants from diesel fuel combustion [13]. However, some authors in their recent studies reported that alkane blends alone do not provide the actual picture of fuel combustion. Hence, aromatics should be a part of the surrogate fuels [13]. Aromatics have gained attention due to their higher content (10–30% by mass) in diesel fuels and high tendency to form soot particles during combustion [14]. Thus, exploration has been continuously going on the impact of different aromatics on exhaust emissions in combustion engines.

Many researchers have studied the effects of fuel properties including the aromatics content and structure on the engine performance and emissions. Ryan et al. [15] reviewed the effect of aromatics content on exhaust emissions in a diesel engine. They summarized from the previous studies that low aromatics content reduces the NO_x emissions. They reported that a fuel with lower aromatics having low distillation endpoint and high flash point emits lower emissions as compared to hydrocarbon fuels alone. Fukuda et al. [16] studied the effect of aromatic structure and content on PM emissions in a single cylinder DI diesel engine. They compared the emissions of low sulfur diesel fuel with 10% (by volume) of single-ring aromatic (tetralin) and double-ring aromatic (naphthalene) components in the base fuel. They reported that the pollutant emissions are increased with aromatic contents. Single ring aromatics (tetralin) have higher NO_x and total PM emissions, and lower unburnt hydrocarbons (UHC) as compared to double ring aromatic compound (naphthalene). In another study, the effect of aromatic content was studied independent of density [17]. No significant impact on PM emissions was found [17]. The effect of molecular structure of hydrocarbon fuels on tailpipe emissions was studied by Miyamoto et al. [18]. They used a base fuel (n-tetradecane and heptamethylnonane mixture) blended with 0–60% (by volume) of mono-aromatic, with 0–40% (by volume) di-aromatic and non-aromatic as a feedstock for the diesel engine. The results revealed that NO_x and dry soot emissions linearly increase with aromatic content. They also concluded that the level of emissions is lower with mono-ring aromatics as compared to double-ring aromatics. They used DI and indirect injection engines for experiments and found similar trends with both the engines. Asuami et al. [19] used different fuels with varying aromatic and sulfur contents. They concluded that CO, NO_x, PMs and total UHC were increased with higher aromatic contents due to higher soluble organic fraction. Similar trends for NO_x and PMs were found by Tsurutani et al. [10]. They also studied the effect of aromatic structure on emissions and found significant impact of the structure of aromatics on PM emissions. The definite trend for higher NO_x with higher aromatic content was also reported by Neil et al. [20]. Mi et al. [21] discussed the impact of aromatic contents on polycyclic aromatic hydrocarbon (PAH) emissions in heavy duty diesel engines. They fueled the engine with the poly-aromatic (fluorene)-diesel and mono-aromatic (toluene)-diesel blends. They found that total PAH emissions increased by 2.6 and 5.7 times with the addition of 3% and 5% fluorene (by volume) as compared to diesel, while 10% of toluene blended fuel did not affect the PAH emissions as compared to diesel. Diesel blended aromatics (30% by volume fraction) were also tested in a common rail diesel engine [22]. The authors selected four aromatics i.e. toluene, n-butylbenzene, tetralin and 1-methylnaphthalene for their study. Results showed that CO and NO_x emissions were higher with diesel-aromatics blends at low load conditions as compared to pure diesel fuel. HC emissions were higher for double ring aromatics while lower for single ring aromatics as compared to diesel. Xiao et al. [23] conducted tests with heptane as a base fuel and its blend with toluene (mono-aromatic) and methylnaphthalene (di-aromatic) on a diesel engine. The results revealed that smoke, NO_x and UHC increased significantly for both the blends of toluene and methylnaphthalene as compared to the base fuel (heptane). N-heptane has been frequently used in diesel engine due to its low boiling point that eliminated the problem of fuel vaporization [24]. Kidoguchi et al. [25] assessed the emission characteristics of a DI diesel engine fueled with six fuels with varying cetane number and aromatic contents. NO_x and PM emissions were found to be higher for the fuel with high cetane number and high aromatic contents respectively. A study on the effect of injection pressure revealed that the PM emissions were marginally affected at high injection pressures [25]. Hellier et al. [26] studied the effect of 1-octene/n-octane and toluene/n-heptane binary blends in a DI diesel engine. They reported that NOx emissions increase with the increase of 1-octene and toluene content in the blend and simultaneously increase in adiabatic flame temperature where ignition delays have been equalized [26]. A binary fuel containing 70% n-decane and 30% methylnaphthalene (in volume fraction) has been especially formulated as a part of the Integrated Diesel European Action (IDEA). This binary fuel has a density and the cetane number in line with diesel and providing similar outcomes in terms of vaporization, ignition, and heat release [27].

The spray analysis for fuel atomization and development have a significant impact on diesel engine performance and emissions as the fuel spray influences the air-fuel mixing inside the cylinder [28]. Several studies on spray characterization at higher injection pressures have highlighted the effect of fuel properties and injection parameters on fuel sprays and found that preliminary spray tests provided significant information about the fuel performance in the engine [28,29].

Based on existing literature on the effect of fuel properties on compression ignition (CI) engine, it has been found that the aromatics content in base fuel significantly affects the fuel properties and hence, fuel atomization, combustion of the fuel, and emissions are also influenced. Various studies have reported on the effect of fuel properties on diesel engine characteristics. Few authors studied the effect of aromatic content and its structure on emissions. Not all aromatics contribute to the same levels of emissions at all conditions. Still, there is no particular study available in the literature that systematically analyzes the effect of different aromatics on diesel engine performance and emissions and their effects on fuel properties. It is therefore imperative to study the effects of aromatics content on base fuel properties that affect the engine performance and emissions and to analyze the data from various perspectives to improve the fuel quality for future applications. In the present study, 13 different aromatics in three blending ratios are studied. The effect of aromatics on base fuel properties, their performance and emission in a diesel engine are investigated. The spray characteristics is also tested for all the fuel blends for better understanding of spray behavior of fuel blends inside the combustion chamber. The present study mainly emphasizes on the effect of different aromatics on engine performance and emissions. Therefore, the engine parameters such as injection timing, injection pressure and compression ratio were not altered.