Biodiesel is proven to be the best substitute for petroleum-based conventional diesel fuel in existing engines with or without minor engine modifications. The performance characteristics of biodiesel as a fuel in CI engine are slightly lower than that of diesel fuel. The emission characteristics of biodiesel are better than diesel fuel except NOX emission. The thermo-physical properties of biodiesel are improved by suspending the nano metal particles in the biodiesel, which make them an observable choice for the use of nanoparticles-added fuels in CI engine. High surface area of nanoparticles that promotes higher operating pressure and heat transfer rates that further quicken the combustion process by providing better oxidation. Thus, it has been inferred that addition of nanoparticles as an additive to biodiesel fuel blends in diesel engines and its effects on performance, combustion, and emission characteristics are discussed in this chapter.
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Every year there is a rise in energy demand due to the rapid development in industrialisation and automotive sector, demand and depletion of fossil fuels, fuel price instabilities, diminished energy security, ambiguity in oil supply to the consuming nations, fuel import costs, increased harmful environmental effects due to various pollutants are the important driving forces to search for new alternative fuels that are renewable, eco-friendly and harmless in now-a-days. This precarious situation has directed many researchers to go a better substitute for conventional fuels in engines.
In recent decades, the biodiesel has received significant consideration, because it is biodegradable, economically viable, non-toxic, environmentally acceptable, easily accessible and technically suitable and also it has received much attraction for conventional diesel engines and can noticeably reduce exhaust emissions from the engine (Balamurugan, Tamilvanan, Anbarasu, Akil, & Srihari, 2013). Biodiesel has numerous advantages compared to conventional diesel fuel. The major advantages are 10–11% oxygen content which results in better combustion characteristics and increase the engine efficiency and also high cetane number concerning 60–65% which reduces the ignition delay (Dinesh, Tamilvanan, Vaishnavi, Gopinath, & Mohan, 2019). Biodiesel reduces carbon dioxide emission by 78% on a life cycle basis while compared to fossil diesel fuel and reduces smoke by reason of free soot. Most of the studies has exposed that, make use of biodiesel in compression ignition (CI) engine can reduce emissions of hydrocarbon (HC), particulate matter (PM), carbon dioxide (CO2) and carbon monoxide (CO) but oxides of nitrogen (NOX) emissions are raised. Another issues like high consumption rate, lower performance, improper combustion, clogging and stability with long-term practice of biodiesel blends restricts their usage as a prospective replacement in diesel engine (Sakthivel, Ramesh, Purnachandran, & Shameer, 2018).
The research community claimed that enhancement in the performance, combustion and emission characteristics and improvement in fuel economy of the pure diesel, biodiesel and its blends in CI engines could be achieved either by modifying the engine design or fuel modification with the application of nano metal additives. Fuel additives are reviewed and classified into different categories such as nanoparticles-based additives, oxygen-rich additives, tocopherol additives, water, and polymer based additives (Khalife, Tabatabaei, Demirbas, & Aghbashlo, 2017). Compared to engine modification technique, fuel modification technique is one of the cheaper and easier which are normally favored in CI engine without any additional equipment and engine modifications (Ying, Longbao, & Hewu, 2006). On that basis, nano fuel techniques (application of nano scale metal particle additives in diesel and biodiesel fuel) have been employed for improving the biodiesel performance and reducing its emissions by using fuel additives. Such formulated fuels have a lot of advantages compared to neat biodiesel and its blends (Mehta, Chakraborty, & Parikh, 2014).
Nano fuels comprise a nano-sized metal particle having size ranging from 1 to 100 nm is mixed inside the base fluid (diesel/biodiesel) by means of ultrasonication process. The nano fuel additive has a higher surface to volume ratio and act as a catalyst that results in enhanced characteristics of fuels which leads to enhanced performance and combustion characteristics (Tamilvanan, Balamurugan, & Vijayakumar, 2019; Tomar & Kumar, 2019). Metallic particle additives like cerium (Ce), iron (Fe), platinum (Pt), manganese (Mn) barium (Ba), cerium oxide (CeO), aluminum (Al), aluminum oxide (Al2O3), copper (Cu) and titanium oxide (TiO2) are identified as combustion improving catalyst which could results in reduced fuel consumption and emissions (Hosseinzadeh-Bandbafha, Tabatabaei, Aghbashlo, Khanali, & Demirbas, 2018). The results obtained by adding metal additives into biodiesel fuel blends were promising as a result of improvement in chemical and thermo-physical properties of modified fuel (Saxena, Kumar, & Saxena, 2017).