Literature Review
2.3 Effects of Preheated Neat VO in CI Engines
According to Devan and Mahalakshmi (2009), the higher viscosity of VO is responsible for higher HC emissions. The higher fuel viscosity may lead to higher fuel spray droplet size which affects the fuel spray quality. In some exceptional investigations, HC emissions is found to be lower when VO is used. However, they have not substantiated any support for their findings.
(a) Jatropha oil (b) Poon oil
(c) Jatropha oil (d) Palm oil
Figure 2.12 Trends of HC emissions for vegetable oil against diesel in CI engine (Chauhan et al. 2010; Devan and Mahalakshmi 2009; Agarwal and Agarwal 2007; Almeida et al.
2002)
respectively when the oil is heated from 40 to 100 ᵒC. However, diesel shows a little variation in the same range of temperature. In the most of investigations, the VOs have been preheated in the range of temperature from 70 to 90 ᵒC. However, the optimal value of this preheated temperature range could not have been ascertained as mostly the studies have been done on a single fixed preheated temperature.
Figure 2.13. Effect of preheating temperature on the kinematic viscosity of the fuel (Sonar et al. 2015; Agarwal and Agarwal 2007; Hazar and Aydin 2010)
2.3.1 Brake Thermal Efficiency
The BTE of engine is found to improve with the use of preheated VO in comparison to the one without preheating. However, it remains to be lower as compared to diesel. In Figure 2.14, the results of neat and preheated karanja and jatropha oil are illustrated (Agarwal and Rajamanoharan 2009; Singh 2013). Preheating shows improved results in the intermediate and higher engine loads. Preheating of karanja and jatropha oil respectively results in 11 to 24% and 2 to 4% higher BTE as compared to their respective neat oil. The preheating of oil reduces the viscosity which facilitates better atomization of fuel particles ensuring better combustion and improved BTE (Pugazhvadivu and Jeyachandran 2005; Agarwal and Agarwal 2007; Yilmaz and Morton 2011; Singh 2013; Sonar et al. 2015). In the studies of Agarwal and Rajamanoharan (2009), the preheating of VO leads to higher BTE than that of diesel. This is because of the reduction in the viscosity and increase in volatility along with the oxygen content of VO that gives better fuel combustion which improves the BTE.
However, in the study of Nwafor (2003), the use of preheated oil deteriorates the engine BTE.
(a) Karanja oil (b) Jatropha oil
Figure 2.14 Effects of preheated vegetable oil on BTE of the engine. (Agarwal and Rajamanoharan 2009; Singh 2013)
2.3.2 Brake Specific Fuel Consumption
The preheating of VO results in the lower BSFC as compared to neat VO (Agarwal and Rajamanoharan 2009; Singh 2013; Agarwal and Agarwal 2007; Sonar et al. 2015). The typical results when jatropha and mahua oil have been used is shown in Figure 2.15 (Sonar et al. 2015; Singh 2013). The reduction in BSFC is in the range of 3 to 8% and 4 to 7%, respectively when preheated jatropha and mahua oil are used in the engine. This reduction in BSFC is due to preheating which reduces the oil viscosity leading to better combustion in the engine (Sonar et al. 2015). However, it is found to be higher in the study of Nwafor (2003) when preheated rapeseed oil is used. Pradhan et al. (2014) have found it to be lower at 25 and 50% of engine load and relatively higher at engine load of 75 and 100%.
(a) Jatropha oil (b) Mahua oil
Figure 2.15 Effects of preheated vegetable oil on BSFC of the engine (Singh 2013; Sonar et al. 2015)
2.3.3 Exhaust Gas Temperature
The EGT is generally found to be higher when preheated VO is used in the engine as compared to the neat VO (Nwafor 2004; Pugazhvadivu and Jeyachandran 2005; Agarwal and Agarwal 2007; Yilmaz and Morton 2011; Sonar et al. 2015; Hazar and Sevinc 2019). The typical results of preheated jatropha and rapeseed oil against the neat oil is shown in Figure 2.16 (Nwafor 2004; Agarwal and Agarwal 2007). The EGT increases by an average of around 31 ᵒC when preheated jatropha oil is used in the engine, while preheated rapseed oil shows a very little variation. The increase in combustion gas temperature due to preheating of oil (as it increases the fuel temperature) is the probable cause for the increase in EGT (Pugazhvadivu and Jeyachandran 2005; Sonar et al. 2015).
(a) Jatropha oil (b) Rapeseed oil
Figure 2.16 Effects of preheated vegetable oil on EGT of the engine (Agarwal and Agarwal 2007; Nwafor 2004)
2.3.4 Carbon Monoxide Emissions
The effect of preheated VO, jatropha and waste frying oil, vis-à-vis neat oil on the CO emissions has been illustrated in Figure 2.17 (Pugazhvadivu and Jeyachandran 2005;
Agarwal and Agarwal 2007). It indicates a reduction in the CO emissions with the preheating especially at higher engine loads (Hazar and Sevinc 2019). The preheating reduces the oil viscosity that results in a better fuel atomization. This improves the spray characteristics and an improved fuel air mixing. This ultimately leads to better combustion and reduced CO emissions (Pugazhvadivu and Jeyachandran 2005; Pradhan et al. 2014).
(a) Jatropha oil (b) Waste frying oil
Figure 2.17 Effects of preheated vegetable oil on CO emissions of the engine (Agarwal and Agarwal 2007; Pugazhvadivu and Jeyachandran 2005)
2.3.5 Oxides of Nitrogen
The effect of preheated canola and waste frying oil on NOX emissions is presented in Figure 2.18 (Yilmaz and Morton 2011; Pugazhvadivu and Jeyachandran 2005). It reflects a relatively higher NOX in the exhaust when preheated VO is used. Similar results have also been reported (Hazar and Sevinc 2019). Agarwal and Rajamanoharan (2009) also reported a higher NO emissions with the preheated karanja oil. However, the mass of NO emissions decreases with the increased engine load with lowest emissions at the highest load of the engine. The increase in combustion gas temperature with the preheated fuel may be attributed to the increase of NOX (Pugazhvadivu and Jeyachandran 2005).
(a) Canola oil (b) Waste frying oil
Figure 2.18. Effects of preheated vegetable oil on NOX emissions of the engine (Yilmaz and Morton 2011; Pugazhvadivu and Jeyachandran 2005)
In the investigation of Pradhan et al. (2014), NOX emissions is found to be higher at 0 and 25% of engine loads; while it is found to be lower at loads of 50, 75 and 100% with preheated jatropha oil as compared to the neat oil. This lower NOX emissions at the higher engine load has been attributed to the instantaneous chemical reaction and low air-fuel ratio of preheated oil.
2.3.6 Hydrocarbon Emissions
In comparison to the neat VO, the engine driven on the preheated VOs releases lower unburned HC (Agarwal and Agarwal 2007; Pradhan et al. 2014; Sonar et al. 2015; Hazar and Sevinc 2019). Typical results illustrated in Figure 2.19 (Agarwal and Agarwal 2007; Pradhan et al. 2014) indicate that preheating effectively mitigates the HC emissions at higher engine loads. It reduces by up to 34% when preheated jatropha oil is used. This reduction in HC emissions may be due to more complete and cleaner combustion with better atomization of fuel molecules because of the preheated oil (Pradhan et al. 2014). Nwafor (2004) reported a higher HC emissions with the preheated oil as compared to the neat oil while using rapeseed oil.
(a) Jatropha oil (b) Jatropha oil
Figure 2.19 Effects of preheated vegetable oil on HCemissions of the engine (Pradhan et al. 2014; Agarwal and Agarwal 2007)
2.3.7 Smoke Emissions
As compared to neat VO, the use of preheated VO reduces the smoke emissions (Pugazhvadivu and Jeyachandran 2005; Agarwal and Agarwal 2007; Agarwal and
Rajamanoharan 2009; Singh 2013; Hazar and Sevinc 2019). The preheating results in the reduction of fuel viscosity which subsequently improves spray, mixing of air-fuel and combustion characteristics (Hazar and Aydin 2010). Figure 2.20 (Agarwal and Rajamanoharan 2009; Singh 2013) illustrates the comparative smoke emissions of neat and preheated jatropha and karanja oil. It indicates a higher degree of reduction at the intermediate loads while lower variation in the higher engine loads.
(a) Jatropha oil (b) Karanja oil
Figure 2.20 Effects of preheated vegetable oil on smokeemissions of the engine (Agarwal and Rajamanoharan 2009; Singh 2013)