Elsevier

Fuel

Volume 246, 15 June 2019, Pages 79-92
Fuel

Full Length Article
Testing of anisole and methyl acetate as additives to diesel and biodiesel fuels in a compression ignition engine

https://doi.org/10.1016/j.fuel.2019.02.079Get rights and content

Abstract

This paper investigates the effects of anisole and methyl acetate (as fuel additives) on the performance and emission characteristics of a compression-ignition (i.e., diesel) engine. Anisole and methyl acetate can be obtained from methylation of phenol and acetic acid, respectively. Phenol and acetic acid are compounds which are abundant in bio-oil derived from pyrolysis of wood and is thus renewable in nature. Using methyl acetate as a diesel fuel additive in compression-ignition engines has rarely been reported in the literature. The objective of the current work is to provide testing results of methyl acetate and perform comparisons with anisole as fuel additives for both diesel and biodiesel fuels. The effects of loads, additive type, and base fuels were tested. The tested loads include 0, 1.26, 2.52, and 3.78 bar brake mean effective pressure (BMEP) and the base fuels include No.2 diesel and biodiesel from waste cooking oil. The additive concentrations were kept at 10% by volume. Engine performance, exhaust emissions, and in-cylinder combustion were measured and analyzed. For diesel-anisole (DA) blends, it was seen that the blends were comparable to diesel in terms of performance but with slightly higher fuel consumption rates. HC and CO emissions reduced slightly, however, NOx and soot concentration increased. Diesel-methyl acetate (DM) blends were comparable to diesel in terms of performance with a slight increase in the fuel consumption rates. HC and CO emissions decreased with added methyl acetates. NOx and soot concentration increased. Both anisole and methyl acetate of 10% by volume in biodiesel were tested and it was observed that both blends were slightly better than pure biodiesel in terms of performance. HC and CO emissions reduced for both blends. NOx and soot concentration however increased as compared to pure biodiesel. From the experiments, it is believed that both anisole and methyl acetate can be used as suitable additives to diesel and biodiesel in terms of performance; however, the emissions of NOx and soot can pose a challenge.

Introduction

Conventional fossil fuels are limited and are depleting. It is necessary to find renewable alternatives to fossil fuels. Bio-oil is one such alternative derived from biomass. Biomass or organic waste matter is renewable and is also favorable for agrarian economies. It already contributes almost 13% of the energy consumption around the world [1]. Biomass based fuels are being encouraged around the world [2]. Biomass can be converted to biofuels by various techniques such as pyrolysis, dry combustion, solvent extraction hydrolysis, anaerobic digestion, and liquefaction [1]. Pyrolysis decomposes biomass into solid, liquid, and gaseous products in a high temperature anaerobic environment [3]. Bio-oil obtained from wood pyrolysis is known as Wood Pyrolysis Oil (WPO). WPO cannot be used directly in engines as it has a tendency to form solid precipitates. Bio-oil obtained from pyrolysis also contains numerous oxygenated compounds which make them unsuitable to use as direct replacements of conventional fuels [4], [5]. In order to use them, they must be upgraded to remove the oxygenated compounds which increases their costs as compared to conventional fuels [6], [7]. It is also acidic in nature and causes corrosion to engine parts in the long term. The higher oxygen content and lower calorific value prevents it from being used in engines directly without system modification [8]. Even with modifications to diesel engine as were done by [9], [10] the direct use of bio-oil could cause damage to the engine. To solve this problem, it is necessary to look at individual components of bio-oil and determine their suitability as components of fuel.

Bio-oil is rich in acid and phenolics as a mixture of 30% water, 10% acids, 20% aldehydes/ketones, 15% alcohols, 30% phenolics and other miscellaneous compounds [1]. Of these compounds, acids and phenolics have relatively higher oxygen content. Research has been conducted in the field of oxygenated additives to diesel fuel and it has been found that oxygenated additives tend to decrease the hydrocarbon (HC) and carbon monoxide (CO) emissions due to enhanced combustion of the fuel [11], [12], [13], [14], [15], [16]. A study done on the combustion characteristic of lignocellulosic biomass based oxygenated compounds in compression ignition engines found that the oxygenated blends had overall positive effect on emissions at lower concentration (2% by vol) but negative effects were observed at higher concentration [17]. These oxygenated compounds are found in large quantities in some essential oils. For example, eugenol is found in large quantities in clove oil. Studies have been done to test the efficacy of clove stem oil in engines [18]. Studying constituent compounds and their derivatives rather than essential oils yield more conclusive results as was done by [19], [20] with regards to eugenol and eugenyl acetate.

As mentioned earlier one of the major problems of bio-oil is its lower calorific value. The lower calorific value is due to lower carbon content of the oxygenated compounds making bio-oil. One of the ways to solve this problem is to find individual compounds that exist in bio-oil in large quantities and increase their carbon content. Methylation is a technique whereby a methyl chain is added to organic compounds. Bio-oil contains large quantities of acids and phenolics. Out of these, two compounds that are of interest in this research are phenol and acetic acid. Phenol is a crystalline solid at room temperature with a molecular formula C6H5OH. Being solid it cannot be used directly in engines. By a two-step methylation process phenol can be converted to anisole (C6H5OCH3) [21]. Anisole, or methoxybenzene, is a potential surrogate compound for lignocellulosic biomass pyrolysis and combustion. Under pyrolysis and oxidative conditions, unimolecular decomposition of anisole to phenoxy radicals and methyl radicals was found to be important due to the relatively low bond strength between the oxygen and methyl group, ∼65 kcal/mole [22]. In a study done on heavy duty engines, it was found that under cooled EGR-like conditions, the ignition delay of anisole is so large that it would result in soot-free operation up to medium load [23]. Another study done on anisole, benzyl alcohol and 2-phenyl ethanol showed that the farther the oxygen group was from the aromatic ring, the better was the overall emission behavior [24]. The properties of anisole are as summarized in Table 1. Anisole has higher carbon content than phenol and thus has higher calorific value. Phenol has a measured solubility of 0.02 g/ml in dodecane whereas anisole is completely soluble making it ideal to be used as an additive in diesel. Acetic acid is an organic acid with a chemical formula CH3COOH. It is a weak acid but can be corrosive to the engine over the long run. It has only two carbon atoms per molecule and has lower calorific value. By a two-step methylation process it can be converted to methyl acetate (CH3COOCH3) [21]. It is liquid at room temperature and has the properties also listed in Table 1. It has higher carbon content than acetic acid and thus it has higher calorific value. The solubility of acetic acid in dodecane is 0.11 g/ml whereas methyl acetate is completely miscible. The methylation process thus makes acetic acid more suitable to be used as an additive in diesel. Anisole and methyl acetate have high research octane number (RON). Anisole has RON greater than 120 whereas methyl acetate has a RON of 103 [25]. Higher RON corresponds to lower cetane number. Diesel engines run on fuels which have high cetane number. Thus adding too much anisole or methyl acetate would be detrimental to the engine as it would result in longer ignition delay or possibly even misfire.

Apart from conventional diesel, biodiesel is being used as an alternative fuel. Biodiesel is a vegetable or animal fat-based diesel fuel consisting of long-chain alkyl esters. So far a lot of progress has been made with respect to developing biodiesel and testing its performance and emission characteristics with respect to that of diesel [26], [27], [28], [29], [30], [31], [32]. But biodiesels have few drawbacks, such as higher viscosity, higher molecular weight, lower volatility and higher pour point compared with the diesel. These drawbacks cause poor atomization and lead to incomplete combustion [33], [34]. By adding oxygenated additives, the ignition temperature of biodiesel can be shortened and also reduction in smoke emission is observed in the diesel engine [35]. Based on the composition of diesel and biodiesel, the oxygenated additives can affect the properties such as cetane number, density, viscosity, volatility, flash point and calorific value [36]. Based on these studies it would be interesting to see the effects of oxygenated additives like anisole and methyl acetate as biodiesel fuel additives on performance and emission characteristics of diesel engines.

Based on the brief literature survey, it is seen that although there are some works on using anisole as fuel additives in diesel fuel, there are very few works on applying methyl acetate as fuel additives in compression-ignition engines burning diesel and/or biodiesel fuels. Therefore, the objective of this work is to investigate the effects and provide comparative data of the addition of anisole and methyl acetate to diesel and biodiesel fuels in diesel engines. The performance and emissions of the engines under different operating conditions are tested and analyzed. Moreover, the in-cylinder combustion pressure and heat release rates are also measured. Results can provide useful information for the determination of using these two compounds as viable fuel additives in diesel engines.

Section snippets

Experimental setup and procedure

The experiments were carried out in a 10 hp (7.45 kW) single cylinder air-cooled compression-ignition (CI) engine. The specifications of the engine are as listed in Table 2. The engine has a bore of 86 mm and stroke of 72 mm with a displacement of 418 cc. Fig. 1 shows the schematic diagram of the experimental setup. The engine was loaded using a GO Power DY-7D water brake dynamometer. The dynamometer is coupled to the engine using 1:2 timing belt pulleys such that the dynamometer operates at

Fuels and blends tested

Two groups of fuel blends were tested. The first group is diesel-anisole and diesel methyl acetate blends. Commercial No. 2 diesel fuel was used for this experiment as a base fuel. Commercially available anisole and methyl acetate of 99% purity (from Fisher Scientific) were used for preparing the blends. The diesel blends tested were 10% anisole – 90% diesel (DA) and 10% methyl acetate – 90% diesel (DM) by volume. The biodiesel used in this experiment was obtained from Piedmont Biofuels (North

In-cylinder combustion

Fig. 2 shows the in-cylinder pressure of the four load conditions for the three fuels/blends. As the load increases, the amount of fuel burnt per cycle increases resulting in higher peak in-cylinder pressure and higher work output. It can be seen that at no load the peak pressure is around 5.0 MPa for the three fuels with diesel slightly lower. For 1.26 bar BMEP, the peak pressure rises to about 5.5 MPa and the difference between DA/DM and diesel becomes a little larger. The pressure increase

In-cylinder combustion

Fig. 7 shows the plots of pressure vs. crank angle for different load conditions respectively for the three fuels. At no load, it can be observed that pure biodiesel reaches peak in-cylinder pressure later than both BA and BM. This indicates slower heat release and longer time for biodiesel to burn off completely. BA and BM have lower boiling points resulting in better mixing than biodiesel. The peak in-cylinder pressure is maximum for BA at 5.15 MPa. It is almost same for biodiesel and BM. At

Conclusion

Two compounds, namely anisole and methyl acetate, as fuel additives blended with both diesel and biodiesel fuels were tested in this study. Performance and emission characteristics of the blended fuels were compared with baseline pure diesel and biodiesel. The findings can be summarized as follows:

  • 1.

    For diesel fuels/blend, DA had the highest peak in-cylinder pressure and peak heat release rate at lower loads. At higher loads, the performance was almost the same for the three diesel fuels/blends.

Acknowledgments

This work was supported in part by the IBSS project funded by Agriculture and Food Research Initiative Competitive Grant no. 2011-68005-30410 from the USDA National Institute of Food and Agriculture, by the NCSU Research & Innovation Seed Funding Program (RISF), and by the North Carolina Biotechnology Center Grant no. 2016-BIG- 6514.

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