Investigation on the combustion and emission characteristics of diesel engine fueled with diesel/methanol/n-butanol blends

Highlights

• Different diesel/methanol/n-butanol blend fuels are employed in this work.

• Effects of fuel additive rate on spray, combustion and emission characteristics are investigated.

• A computational modeling is developed by the chemical kinetics mechanism.

• Methanol additive and n-butanol play very important roles.

Abstract

In this work, the diesel engine fueled with diesel/methanol/n-butanol blended fuel was employed to investigate the effects of different fuel blending ratios on the spray, combustion, and emission characteristics of diesel engine in term of cylinder pressure, cylinder temperature, heat release rate, brake specific fuel consumption, brake power, brake thermal efficiency, NO_x emission, soot emission, CO emission, and HC emission. The model was developed by a three-dimensional CFD model in CONVERGE and was verified by the experiment results. In addition, an improved chemical kinetic mechanism including 359 reactions and 77 species was employed to simulation the combustion processes. The results showed that diesel/methanol/n-butanol blends played an essential role in the fuel spray and combustion processes. The blended fuel had longer ignition delay, higher cylinder pressure, and higher peak heat release rate compared with diesel. Moreover, the occurrence of micro-explosion improves the blending of fuel and air. More specifically, the diesel/methanol/n-butanol blends could reduce NO_x, CO, soot, and HC emissions. It can be found that the best blending ratio of blended fuel is 70%diesel + 20%methanol + 10%n-butanol. Therefore, the diesel/methanol/n-butanol blended fuel can improve the combustion and emission characteristics of the engine.

Introduction

The diesel engine has the advantages of high thermal efficiency [1], low fuel consumption [2], high output power, and good safety [3], and is widely used in automobiles, agriculture, and industry [4]. Similarly, CO, PM, and NO_x emissions from diesel engines threaten public health and the ecosystem [5]. In order to meet this challenge and the requirements of resource shortage and environmental problems, many automobile manufacturers and researchers have no choice but to improve engine technology and find alternatives to fossil energy [6]. To support this, the governments and institutions should encourage and implement national energy policies and reduce dependence on traditional fuels [7]. A practical solution is to add oxygenated fuel or additives to diesel. In diesel engines, oxygenated fuel can reduce engine emissions of soot, nitrogen oxides (NO_x), carbon monoxide (CO), carbon dioxide (CO₂), and unburned hydrocarbons (HC), and improve combustion characteristics [8]. Alcohol is an oxygenated fuel. Its combustion performance is different from diesel. It has fast flame propagation speed, high efficiency, good quality, low CO and HC emissions, and low engine temperature [9]. Therefore, the alcohol fuel has attracted more and more researchers' attention in recent years. Among all oxygenated biofuels, alcohol has been widely studied in the past few decades. Alcohol molecules contain hydroxyl (OH), so it can reduce engine soot emission during diesel blended combustion, which is helpful to achieve cleaner combustion.

At present, the research on alcohol fuels mainly focuses on methanol [10], ethanol [11], propanol [12], butanol [13] and pentanol [14]. Among all alternative fuels, the short-chain alcohol-based biofuels such as methanol and ethanol have attracted much attention because of their mature production process and high oxygen content [15]. In addition, the combustion and emission characteristics of engine can be improved due to the advantages of high octane number, high latent heat of vaporization, and good lean combustion [16]. For example, methanol is a relatively clean liquid fuel that can be converted from coal and natural gas. It can effectively use coal, coal bed methane, and coke oven gas. The utilization rate of inferior coal can be improved while reducing crude oil consumption. Methanol is a liquid fuel, and its storage and transportation are similar to diesel [17]. The cost of methanol is about one-third of that of gasoline and diesel, which makes the market prospect of methanol quite broad. Yao et al. [18] studied diesel-methanol compound combustion (DMCC) and found DMCC reduced soot and NO_x emissions. Berber [19] studied the effect of adding methanol to diesel on the engine. The results showed that the addition of methanol reduced the emissions of CO₂ and CO. In addition, Dou et al. [20] showed that the number of particles gradually decreased with the increasing methanol ratio. However, methanol and diesel differ significantly in polarity and are challenging to be miscible [21], and the fuel stability after blending methanol and diesel is poor [22]. Compared with pure diesel, methanol/diesel blended fuels reduce engine brake thermal efficiency (BTE) and increases HC and CO emissions [23]. Therefore, these problems greatly limit the application of methanol as an alternative fuel for diesel engines.

In recent years, the long-chain alcohols containing four or more carbons have had significant advantages over short-chain alcohols in diesel engines. Long-chain alcohols have higher energy density and higher cetane numbers than short-chain alcohols. They also have good blending stability and can be blended with diesel at a large blending ratio. In addition, long-chain alcohols have low moisture absorption and are easy to store and transport [24]. N-butanol is a four-carbon alcohol that has been widely studied as an alternative fuel or fuel additive in recent years [25], [26]. N-butanol can be produced from food waste, biome, eukaryotes, and other plant components containing cellulose [27]. It is a potential alternative, which can be used alone or blended with traditional fuels to reduce dependence on fossil fuels and reduce particulate emission (PM) without significant impact on NO_x or cetane number [28]. Compared with methanol and ethanol, n-butanol has a higher calorific value and viscosity, lower volatility and latent heat of vaporization, and lower hygroscopicity [29] and corrosivity [30]. In addition, n-butanol has better miscibility with diesel than methanol or ethanol [31]. Because n-butanol has its advantages, more and more studies on n-butanol as a fuel additive have been carried out in recent years. These studies generally showed that using n-butanol petroleum-based diesel blends could achieve cleaner emission without significantly reducing engine performance [32]. Satsangi et al. [33] studied the effects of n-butanol/diesel blends on an internal combustion engine's combustion noise, performance, and emission characteristics. The results showed that n-butanol/diesel blended fuel had higher pressure and heat release rate (HRR) than pure diesel fuel. Studies have shown that [34] n-butanol is a high-quality renewable alternative fuel and can be used as a cosolvent of diesel and methanol blended fuel. Zhang et al. [35] studied n-butanol added to diesel/biodiesel blends on engine performance and particulate emission. The results showed that the addition of n-butanol could reduce PM emission and improve engine performance characteristic. Sharon et al. [36] studied the effects of palm oil, n-butanol, and diesel mixture on the performance and emission characteristics of diesel engine. The results showed that adding n-butanol could reduce CO and NO_x emissions, reduce flue gas opacity and improve BTE. Tüccar et al. [37] had carried out similar research and obtained a similar conclusion. In addition, Chen et al. [38] studied the combustion and emission characteristics of diesel, n-pentanol, and methanol blends on a common rail diesel engine. The results showed that the opacity and soot emission of diesel/n-pentanol/methanol blends were lower than pure diesel. However, there are few studies on diesel/methanol/n-butanol blends.

With the developments of the economic and technology, the computational fluid dynamics (CFD) simulation technology has been widely used. Compared with the traditional production technology, the CFD simulation technology has the advantages of low cost and short cycle. It can use the data information generated in the product's whole life cycle to simulate the working state of the product in the virtual environment and finally achieve the optimization of the product by modifying the parameters. For example, CFD based on the electronic computer can simulate automobile design, manufacturing, and optimization with a small error using a discrete mathematical algorithm, significantly shortening the research and development cycle. At present, the popular CFD software includes ANSYS Fluent, AVL-Fire, CONVERGE, etc. For example, Fan et al. [39] studied the mixture formation and combustion of direct hydrogen injection and natural gas rotary injection engines through ANSYS Fluent software. The results showed that hydrogen stratification became more and more evident with the delay of injection time. Luo et al. [40] established the CFD model to study the influence of injection strategy on diesel engine combustion and emission characteristics in AVL-Fire environment. Kattela et al. [41] had developed a CFD model to investigate the engine performance and emission characteristics of diesel engine fueled with butanol/diesel blends as fuel. The results showed that the blending of butanol could reduce engine emission. Chen et al had developed a CFD model and investigated the air–fuel interactions in SIDI optical engine. They found that fuel injection led to air entrainment, which affected the air flow between fuel plumes and the fuel spray speed was slightly correlated with this air flow enhancement [42]. In addition, a 3-CFD model had been developed and verified by experimental result by Chen et al [43]and they found that the expectations were in full agreement with the 3D images [44]. Thus, the simulation can be widely used in the design of diesel engine.

Above all, the behaviors of diesel/methanol/n-butanol blended fuel are different due to the different thermos-physical properties. In this paper, a CFD simulation model of diesel engine was established by CONVERGE 3.0 coupled with CHEMKIN Ⅱ and verified by the experimental results. In addition, the effects of different blending ratios on the spray, combustion and emission characteristics of diesel engine fuel with diesel/methanol/n-butanol blend fuels were investigated. This study has specific practical and academic significance for reducing diesel vehicle pollution emission and realizing source emission reduction.