Lower Alcohol Additives
Biodiesel is derived from vegetable oil, animal fat, and WFO. The properties of these fuels are similar to those of petroleum diesel fuel. Biodiesel is a promising substitute for petroleum diesel fuel in diesel engines. However, the obstacles to biodiesel use in diesel engines are the low-temperature properties (Kannan and Anand 2011;;Huang and Wu 2008) and inferior engine’s performance, combustion, and emission characteristics (Huang and Wu 2008). Biodiesel has the drawbacks of poorer low-temperature properties, greater NO emissions, and inferior engine characteristics compared to conventional diesel. These must be rectified before the commercialization of biodiesel. Researchers have been making efforts from this perspective to improve fuel properties to attain better performance and lower emissions in diesel engines through fuel modification like biodiesel being blended with higher alcohol and conventional diesel or ternary blends.
Various additives such as alcohols, cetane improvers, water emulsions, oxygenates, and nano-additives were blended with biodiesel to reduce exhaust emission in diesel engines (Qi et al. 2011). Boretti (2012) reported that alcohol blended with biodiesel reduced exhaust emissions like smoke, unburned hydrocarbon, and carbon monoxide. Agarwal (2007) mentioned that alcohols are renewable energy sources with merits like lower viscosity and density and higher oxygen content. These are highly miscible with biodiesel and diesel fuel and enhance the properties of biodiesel blends. Alcohol blended with biodiesel improves the cold flow properties and reduces poisonous gas emission. The addition of more alcohol in biodiesel blends increased the latent heat of evaporation (LHE) of the fuel, causing reduction the in-cylinder temperature, leading to lower NO emission formation. Biodiesel and biodiesel blends produced lower NO emissions due to their lower iodine values related to neat diesel fuel. Carbon monoxide, sulfur oxide (SO), NO, and PM emissions significantly decreased in biodiesel-alcohol blends. Alcohol blends improve the cloud and pour point properties and reduce the viscosity and density of the blends (Ren et al. 2008).
Nadir and Sanchez (2012) investigated the effect of biodiesel-diesel-ethanol and -methanol blends on diesel engine emission characteristics under various load conditions. Biodiesel-diesel-methanol blends have higher CO and UBHC emissions compared to biodiesel-diesel-ethanol blends due to the higher latent heat of vaporization of methanol compared to ethanol, which suppressed in-cylinder temperature and pressure due to a cooling effect caused by adding more methanol into biodiesel blends. Mwangi et al. (2015) evaluated the influence of green fuel in diesel engines. Blends of biodiesel or acetone-butanol-ethanol solution reduced uncontrolled emissions like polycyclic aromatic hydrocarbons, persistent organic components, carbonyl and CO, UBHC, and particulate matter, whereas NO emission increased.
Hwanam and Byungehul (2008) confirmed that an additive like ethanol improved chemical properties; however, it negatively affected other properties like the flash and fire points, calorific value, and cetane number. So it is difficult to prepare a perfectly suitable blend to replace conventional diesel for use in diesel engines. Yao et al. (2010) observed that the drawbacks like poor blending solubility and poor miscibility of adding lower alcohols such as methanol and ethanol with diesel at low temperatures. Similarly, Moreau et al. (2012) noticed that the alcohols containing more carbons could be used as a resource because of their physical and thermodynamic properties. Sivalakshmi and Balusamy (2011) concluded that higher alcohols have higher cetane numbers and better miscibility with diesel. However, Giakoumis et al. (2013) found that using butanol used as a fuel additive in diesel engines resulted in favorable exhaust emission changes.
Arbab et al. (2015) investigated the effect of diesel-n-butanol binary blends on engine performance and emissions. The experimental study showed that diesel-n- butanol blends will be a next-generation fuel. Atmanli et al. (2015) optimized the diesel-n-butanol-cotton seed oil ternary blends to better emission and performance. The optimum concentration was 65.5% vol. diesel, 23.1% vol. n-butanol, and 11.4% vol. cotton seed oil. The results show that the BTE, braking power, and braking torque decreased, while BSFC increased for the above-mentioned blends compared to conventional diesel. Nitric oxide, carbon monoxide, and unburned hydrocarbon emissions decreased for the corresponding blends as 11.33%, 45.17%, and 81.45%, respectively, compared to conventional diesel. Sharon et al. (2013) examined diesel- palm oil-n-butanol blends for diesel engine as an alternative fuel. Increase of butanol in ternary blends resulted in a reduction of NOs, CO, C02, and smoke emissions while UBHC emission and BTE increased.
Ileri (2016) investigated n-butanol-diesel-vegetable oil blends in diesel engine. It was noticed that blends having 60% of n-butanol content reduced CO and UBHC emissions. Kumar et al. (2013) examined alcohol properties compared to those of conventional diesel and gasoline fuel, and also evaluated the influence of bio- diesel-diesel-alcohol blends on diesel engine performance characteristics. Engine studies clearly showed that butanol was superior to methanol and ethanol due to its superior fuel properties. Experimental results showed that the alcohol blends along with a cetane improver reduced engine emissions. Jeevahan et al. (2018) studied the influence of 1-butanol-biodiesel blends on diesel engine characteristics. The results revealed that biodiesel-1-butanol blends reduced engine emissions like CO, UBHC, and NO and increased engine performance. They reported that biodiesel-1-butanol blends could replace conventional diesel.
Higher Alcohol Additives
Nour et al. (2019) carried out research on a common rail direct injection (CRDI)- assisted diesel engine fueled with biodiesel-higher alcohol blends. The study reported that biodiesel-higher alcohol blends had high stability with no phase separation. NO and smoke emissions were reduced for higher alcohol biodiesel blends, whereas CO and UBHC emissions increased for biodiesel-higher alcohol blends. Ashok et al. (2019) investigated the effect of n-biodiesel-n-pentanol blends on diesel engine performance, combustion, and emission characteristics. The addition of n-pentanol to biodiesel improved thermal efficiency and reduced UBHC and CO emission by about 15-43%, or 33-50% when compared to pure biodiesel. The addition of n-pentanol above 40% had a negative effect on engine performance. The study concluded that higher alcohols were better alternatives than lower alcohols.
Yilmaz et al. (2018) examined the effect of ternary blends on diesel engine characteristics. This study exposed that biodiesel, diesel, and alcohol are the most important fuel sources for diesel engines. The direct use of biodiesel in diesel engines is limited due to its inferior characteristics like higher viscosity and density and lower heating value, which leads to poor atomization and air-fuel mixture, in turn resulting in partial combustion, lower BTE, and higher exhaust emission. Zhi-Hui and Rajasekahar (2016) noticed that the higher percentage of biodiesel in diesel may cause poor atomization, carbon deposits, clogging of fuel lines, and thickening of lubricating oil due to its poor volatility and higher viscosity. In order to reduce the disadvantages of biodiesel, it is blended with alcohol and diesel. Popa et al. (2001) reported that the alcohols are extracted from renewable sources using fermentation process of biomass feedstock like corn and sugar. Atmanli et al. (2015) reported that alcohols are clean fuel because of the hydroxyl (OH) presence in the molecular structure. They can be easily mixed with organic components like vegetable oil and biodiesel, which reduces the viscosity and improves the phase stability at low temperatures. While alcohol can’t be used directly in diesel engines, some of their properties make them suitable additives for biodiesel and diesel as noticed by Kumar and Saravanan (2016).
When Zaharin et al. (2017) reported that biodiesel and alcohol are promising alternative fuels for diesel engines, it gained much attention from researchers and scientific community. This study concluded that alcohol is a potential additive that can be blended with biodiesel and diesel fuel. The additives can improve the spray characteristics of biodiesel-diesel blends and also the air-fuel mixing process, which promotes complete combustion. The lower viscosity and higher volatility of alcohol when blended with biodiesel reduces the viscosity of biodiesel. Also, with the addition of alcohol to biodiesel, the oxygen content is further improved, which improves the combustion efficiency and lowers engine emissions. The low viscosity of alcohol improves the spray characteristics of blends and decreases fuel resistance during fuel injection as noticed by Popa et al. (2001). Lower alcohols allow biodiesel to be used in diesel engines, improving microemulsion, but by themselves they have limited use in diesel engines due to their inferior fuel properties like lower cetane numbers and calorific values. Especially, methanol and ethanol are not good alternative fuels for diesel engine.
Lower alcohol can be effortlessly mixed with diesel and biodiesel, which enhances the combustion efficiency and decreases emissions. Even though lower alcohols can increase the oxygenation of fuels, they have some limitations such as poor miscibility, lower heating values, lower cetane numbers, poor lubrication, and stability problems when mixed with biodiesel and conventional diesel as noticed by Yilmaz et al. (2014). Lower alcohols having low flashpoints are considered as class I fuel, while conventional diesel fuel is classified as class II fuel. Lower alcohol blended with diesel falls down to class I. Higher alcohol has been gaining much attention from researches due to its fuel properties like cetane number, less hygroscopic nature, higher energy density, and better blending stability when compared to lower alcohols like ethanol and methanol as noticed by Ren et al. (2008). Kumar and Saravanan (2016) observed that higher alcohol-biodiesel-diesel blends have the potential to replace fossil fuels wholly or partially. Higher alcohol can mix with biodiesel-diesel without any phase separation. Therefore, no co-solvent or emulsifying agent is required to maintain blend stability when higher alcohol is blended with biodiesel-diesel.
Higher alcohols exhibit less corrosion on materials and have high flashpoints; these properties make them safe to store, handle, and transport. Also, alcohol fuel reduced the delay period and improved the air-fuel mixing process, which contributed to clean combustion as observed by Xing-Cai et al. (2004). Balamurugan and Nalini et al. (2014) noticed that alcohol with increased carbon number can be mixed with organic components more easily and also reach a high cetane number and heat of combustion. Thus, higher alcohols have higher potential to mix with biodiesel than lower alcohols. Biodiesel-diesel-alcohol blends improve diesel engine characteristics and diminish hazardous exhaust emissions by physicochemical properties compared to biodiesel-diesel and biodiesel-alcohol blends. Biodiesel-diesel-alco- hol blends can meet energy demands and regulate emissions as noticed by Zaharin et al. (2017).
Imdadul et al. (2016) reported that alcohol-diesel blends reduce viscosity, lubricity, cetane number, and ignitability. However, alcohol blended with biodiesel can provide higher oxygen content and increase the LHE, resulting in reduced NO and PM emissions and improved cold flow properties of the blends. The combination of biodiesel-diesel-alcohol blends can improve the cold flow properties, cetane number, and calorific value compared to raw biodiesel, and enhance the combustion efficiency, resulting in lower exhaust emissions and better thermal efficiency. The above literature studies clearly stated that a biodiesel-diesel-alcohol blend is a better option as fuel in diesel engines compared to biodiesel-diesel, biodiesel-alcohol, and diesel-alcohol blends. It can be a promising alternative fuel for diesel engines.
