Literature Review
2.3.6 Effect on Engine Emission
emission. Low Cetane number leads the fuel to increase ID and greater rates of pressure rise, which results a higher peak cylinder pressure and high peak combustion temperature. This high peak temperature increases NOX emission (Ishida et al., 1997). As a result, more the oxygen-enriched, additive, higher is the value for Cetane number of emulsified fuels, lower is the NOX emission (Ashok, 2011a).
2.3.6.2 Oxides of Carbon
The formation carbon monoxide (CO) is due to the incomplete combustion of fuel. It has been seen that, if there are any chances of increase in combustion temperature, then the oxygenation of CO increases (Heywood, 1988). For example, Lin and his coworkers showed that, with the increase of load and with lower water quantity in water-diesel emulsion, CO emission reduces (Lin and Wang, 2004a; Lin and Chen, 2006b). It is a well-known fact that with an increase in load, an engine consumes more fuel, and its combustion increases the cylinder temperature. Alongside, for emulsions with higher water quantity, higher will be the water evaporation. This consumed considerable amount of sensible and latent heat, thereby reducing the combustion temperature and increasing CO emission (Figure 2.13) (Lin and Wang, 2004a). For high-speed diesel engines, the relative time required for a complete combustion of one cycle, reduces. In that case, the time consumed during micro-explosion becomes predominant that significantly increases CO emission, especially beyond 1800 rpm (Nadeem et al., 2006). Moreover, for three-phase (O/W/O) diesel-water emulsion, the burning rate is lower than two-phase W/O emulsion. The application of ultrasonic emulsifier has been found to disperse the water droplets with smaller diameter than the mechanical homogenizer. This is found responsible for an increase of burning gas temperature and consequently a reduction of CO emission (Lin and Chen, 2008). The application of nano- particles (CNT, alumina), of 50 ppm or more, is found to reduce CO emission for water diesel emulsion in a diesel engine. However, the exothermic reaction of carbon oxidation is probably responsible for larger CO reduction, while using CNT than alumina (Basha and Anand, 2011a; Basha and Anand, 2011b). Similarly, application of H2O2 as a fuel additive has been found to increase the molecular oxygen, which reduces CO emission for diesel- ethanol emulsion run engine (Ravikkumar et al., 2001; Ashok, 2011b).
With the increase in load, the fuel consumption is increased. This reduces the equivalence ratio, and results a fuel rich combustion. Hence, more fuel is reacted with air to increase the CO2 emission, irrespective of the fuel type (Lin and Chen, 2006b). However, engine run with
three-phase water-biodiesel emulsion with 10% water is found to produce lower CO2
emission than neat biodiesel (soybean) run. This is because, the water content might have soaked the partial enthalpy of reaction, which reduces the conversion rate of CO to CO2. However, with the increase of engine speed, the mixing of the atomized fuel particles with air might have improved. This has subsequently increased the CO2 emission of the three-phase biodiesel emulsion run engine (Lin and Lin, 2007b).
Figure 2.12 Reduction of NOX emission with the increase in water percentage in water-diesel emulsified fuel (Samec et al., 2000)
Figure 2.13 Variation of CO emission of water-diesel emulsion for various engine load and water percentage (Lin and Wang, 2004a;
Lin and Chen, 2006b)
2.3.6.3 Hydrocarbon and Smoke
The hydrocarbon (HC) emission from a diesel engine depends mainly on the mixing of fuel and air. In this regard, the over-mixing or under-mixing of fuel, the penetrating force of fuel spray, the atmosphere (pressure and temperature) inside the cylinder at the instant of fuel injection are vital points (Heywood, 1988). The earlier studies on emulsified fuel in a diesel engine show that, in one side, micro-explosion improves mixing of fuel and air. On the other side, it also prompts quenching of local flames owing to a lower temperature, an inhibitory consequence of water (Tsukahara et al., 1982a; Tsukahara et al., 1982b; Subramanian, 2011).
The effect is seen predominant in the work of Park and his coworkers (Park et al., 2001), where at each load, for water diesel emulsion, the increase in water percentage from 10% to 40%, with a step of 10%, have increased the HC emission almost linearly. However, preheating emulsified waste vegetable oil with 40% water by volume has been found to produce almost diesel equivalent HC emission, when run in a diesel engine (Reding et al., 2009). Alongside, to achieve a lower HC, the IT of the water emulsified diesel fuel has also to be optimized (18ºBTDC) rather than just retarding (15ºBTDC and 12ºBTDC) or advancing it (21ºBTDC and 24ºBTDC). The addition of methanol, ethanol and diesel with biodiesel
0 3 6 9 12 15 18
6 8 10 12 14 16
Reduction of NOXfrom diesel (%)
Brake mean ef f ective pressure (bar) W/O: W-5%
W/O: W-10%
W/O: W-15%
W/O: W-20%
W/O: W-25%
300 400 500 600 700 800 900
0 3 6 9 12 15
CO emission (ppm)
Brake mean ef f ective pressure (bar) Diesel W/O: W-10% [42]
O/W/O: W-10% [42]
W/O/W: W-15% [43]
O/W/O: W-15% [43]
water emulsion has also found to produce a higher HC emission than neat diesel or neat biodiesel (Kannan and Anand, 2001). Further, applications of CNT or alumina with water diesel emulsion have not reduced the smoke emission significantly, either (Basha and Anand, 2011a; Basha and Anand, 2011b). The effect of these nanoparticles in the particulate emission is also not explored by the researchers in respective works. The primary importance of using water-emulsified fuel may be the reduction in NOX. However, it can simultaneously reduce smoke emission too (Murayama et al., 1978). It is reported that, neither water fumigation, nor water direct injection can reduce both NOX and smoke emission together.
This is because of the water vapor from emulsion present in the fuel rich side and destroy soot precursors. However, this is not the case for water fumigation or direct injection (Greeves et al., 1976). Hence, with the increase in water in emulsion, the smoke emission reduces progressively (Park et al., 2001; Subramanian, 2011). The same trends of results are reported for water-animal fat emulsion with methanol. The presence of methanol oxygenates the charge. Alongside, the micro-explosion of water vapor eliminates the formation of rich pockets and dilutes the black smoke released (Kumar et al., 2005; Lin and Lin, 2007b).