Fossil fuel consumption is increasing rapidly every year as demand is directly proportional to the total population growth. Billions of litres are consumed every year, leading to scarcity of diesel fuel in few decades . Though the entire world is trying to move towards alternative power sources, diesel power is mandatory and should be existed for many decades, especially for heavy vehicles. The complete replacement for fossil fuel and its commercialization in a prescribed time limit is not worth considering . Hence, alternative fuel sources need the present scenario to take off the diesel fuel crisis. Apart from that, the developed alternative diesel fuels should meet the stringent emissions regulation developed by the Board of National Emission Regulation Authorities (BNERA) to save the environment .
Biodiesel is preferred as a potential alternative source for existing diesel fuel as the biodiesel can be partially or fully replaced instead of diesel. Plenty of valuable studies have been conducted by many researchers in the domain of first-second-and third-generation biodiesels. The main sources for first-generation biodiesel are edible oils such as palm, soybean, corn, and rapeseed oils [4, 5]. Though these oils are renewable and environmentally friendly, oil production competes with food crops that lead to food-energy conflict. The non-edible oils are primary sources of second-generation biodiesels that have some potential benefits compared to first-generation biodiesels, such as effective land utilization and does not affect the environmental food chain. Jatropha, Karanja, Rubber seed, Pine, Eucalyptus, and Mahua oils are common second-generation biodiesels [6–8]. These biodiesels required sophisticated downstream processing techniques that lead to high production costs. The additional challenges in second-generation biodiesels are the water and land-use competition and uncertainty for long-term production . Micro-algae are identified as third-generation biodiesel, which is having a distinctive yield compared to others. Though the micro-algae do not have any conflict in the environmental food chain, the algae biomass production is insufficient for commercialization, and the production cost is not economically viable .
Waste-to-energy is an interesting concept for biodiesel preparation as the raw materials are derived from organic and inorganic compounds. The engine researchers have made several attempts to derive biodiesel from the waste and developed some interesting products like waste plastic oil, waste cooking oil, waste food oil, lemon peel oil, etc. [6, 10, 11]. Out of these, lemon peel oil (LPO) recommends for this study as it has some unique characteristics such as low viscosity, low carbon chain length, high heating value, and innate oxygen content compared to other biodiesels [12, 13]. These properties promote a better mixing rate with air and improved atomization that results in complete combustion. Apart from that, the lemon peel is widely available throughout the world and will not lead to an additional cost for raw material .
Ashok et al.  have conducted a study using LPO in the diesel engine and reported that LPO could be replaced for diesel fuel. They have improved 12% in brake thermal efficiency (BTE) and 9% in brake specific fuel consumption (BSFC) for LPO compared to base diesel. They also pointed out that due to the complete combustion of LPO, the emission parameters such as hydrocarbon (HC), carbon monoxide (CO), and smoke are significantly reduced. In contrast, the oxides of nitrogen (NOx) emission are increased by 55% due to longer ignition delay period (IDP). The same authors reported in another study that 20% LPO in diesel fuel substantially reduces the BSFC and emissions level due to the enriched oxygen content . The report concludes that 20% of LPO in diesel fuel can be used as a regular fuel for common rail diesel injection system engine at 600 bar injection pressure and 30% pilot injection rate. Velavan et al.  experimented with visualizing the in-cylinder combustion flame and evaluating the engine behaviour fuelled with LPO and gasoline. The experimental report stated that the presence of LPO in base fuel reduces the flame diffusion and increases the premixed combustion rate. The report also concludes that up to 10% LPO in base fuel significantly reduces emissions and improves engine performance. Aruna Kumari et al.  have examined the emission characteristics of LPO blended with base diesel and concluded that the presence of LPO (20%) in diesel fuel reduces the HC, CO, and smoke by 25%. 25.6% and 15.4%, respectively, whereas the NOx emission, is increased by 22%.
The above discussions related to the experimental results of LPO clearly show that the LPO is a suitable alternate for diesel fuel, and the overall engine performances are pretty good except for NOx emission. The NOx emission, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), is of serious concern to the environment and human life. These emissions have been identified as ozone layer depletion, lung cancer, and eye irritation. Hence, the removal of tailpipe NOx emission from the diesel engine is essential for the present scenario. Selective catalytic reduction, selective non-catalytic reduction, and exhaust gases scrubbed with chemicals are represented as some post-processing NOx emission reduction techniques . Water in base fuel, a pre-processing emission control technique, is a much interesting concept, as the NOx emissions can be regulated without any engine modification in water emulsion fuel. Additionally, water in base fuel significantly improves the engine performance and reduces the other emissions level .
Many researchers have conducted plenty of valuable research using water in base fuel (for both diesel and biodiesels) by many researchers and depicted interesting results. The experimental results of Vellaiyan et al. show that 10% water in diesel fuel reduces the NOx, HC, CO, and smoke emissions by 32.5%, 9.9%, 12.8%, and 17.8%, respectively, at peak engine load conditions . The same authors reported in another study that 10% water in diesel fuel improves the BSFC and BTE by 6.6% and 12%, respectively. They also pointed out that the IDP is continuously increasing with increased water concentration . The possible causes for the improvements in engine behaviour are enhanced air-fuel mixing, micro-explosion phenomena, and high premixed combustion rate associated with water emulsion fuel [19, 20]. Many researchers report similar experimental results for the biodiesel water emulsion fuels such as soybean biodiesel, lemon peel oil, palmyra palm, waste cooking oil, and lemongrass oil. [12, 21, 22]. The collective results of the above experiments show that the water in base fuel is a suitable NOx emission control technique that can improve the performance and other emissions. Meanwhile, the improvement in engine behaviour can be derived for limited water concentration, and normally for the water concentration above 10%, the negative impacts are derived due to longer IDP associated with emulsion fuel [12, 21].
From the discussions related to low viscous LPO and water emulsion fuel, it is noted that the IDP has a vital role in engine performance. The IDP value should be reduced to improve the overall engine performances and obtain smooth operation. The IDP value is directly related to a cetane number, and an increase in the cetane number of the fuel is significantly reducing the IDP . To improve the cetane number of the fuel, nano additives and other chemical agents are recommended by many researchers. Though the presence of the nanoparticles significantly improves the engine characteristics, the nanoparticle emissions to the atmosphere and the fuel cost are identified as major concerns [4, 19]. The cetane improver is a suitable option to improve the fuel properties. It contains carbon, nitrogen, and hydrogen as a major composition and can be blended with base fuel without any difficulty. From several cetane improvers, 2-Ethylhexyl nitrate (EHN) is preferred for this study as it is a commercial product that is readily available in the market at a low cost. The free radicals promoted by the EHN during the fuel combustion enhances the acceleration rate of the combustion process that can reduce the IDP [24, 25].
In light of the above deliberations, it is noted that the LPO can replace diesel fuel, and the water inclusion in LPO has a significant impact on engines' emissions and performance. To overcome the negative impact, i.e., the longer IDP associated with low viscous LPO and water emulsion fuels, a suitable cetane improver can be added to improve the fuel properties and enhance the cetane number. As far as the authors' knowledge is concerned, no efforts have been taken on the concept described above. This attempt will promote an efficient engine operation with a common environmental effect for high water concentrated emulsion fuels. Taking as an inventiveness, the current study has been carried out to examine the engine behaviour fuelled with 10% water in LPO (10WLPO), 20% water in LPO (20WLPO), and EHN blended 20WLPO (20WLPOE), and the assessments are compared with neat diesel (ND) and LPO. The cost analysis for various fuels is also carried out for the test fuels.