Literature Review
2.5 Recent Work on VCR Engine
The numerous work performed using VCR engine in research and development field throughout the globe are basically on performance enhancement by parametric optimization.
Various alternative liquid and gaseous fuels are also used for this purpose. Some of these important contributions are described in the following subsections.
2.5.1 Effect of Load
The study performed in Kirloskar TV1 VCR engine (Jindal et al., 2010b) showed for 100%
Jatropha methyl ester that the increase in load reduces BSFC and increases BTHE. This is the result of reduction in losses and increase in power at higher loads. Laguitton and his coworkers reported that at lower loads slower rate of combustion at lower CR results reduction in maximum rate of pressure change. However, NOX and smoke formation increases with the increase in load and fuel consumption remains unaffected (Laguitton et al., 2007). Experimental works performed for Mahua biodiesel (B100) and its four blends (B20, B40, B80, and B100) with diesel in VCR engine showed the similar results as reported by earlier researcher (Raheman and Ghadge, 2008). For constant CR and speed, the surge in load in a VCR diesel engine in dual fuel mode will lead to the increase in the mass of gaseous fuel to be admitted. Hence, the BTHE of the engine will increase for constant pilot fuel supply.
Later, when it auto ignites, the gaseous fuel burns with a higher rate of pressure rise (Selim, 2004).
2.5.2 Effect of Compression Ratio
The experimental works of Jindal and his coworkers (Jindal et al., 2010b) show that increase in CR increases BTHE. This is because of the combination of higher power output at higher CR, better combustion and improved lubricity of biodiesel. Side by side, HC emissions tend to increase with the increase in CR from 17 to 18. At lowest CR, insufficient heat of compression delays ignition and hence HC emission reduces. However, at higher CR, CO and smoke opacity is lower. This is because; at lower CR, temperature attained inside cylinder is low resulting incomplete combustion, which reverses in higher loads. As a result higher CR increases CO2 and exhaust gas temperature (EGT) and subsequently NOX emission.
Raheman and Ghade, (2008) reported that, B100 provides 19.3% and 11.5% reduction in BSFC when CR is increased from 18 to 19 and 19 to 20 respectively; whereas for diesel the corresponding values are 10.7% and 8.0%. The increase in BTHE are 23.1%, 29.5%, 32.5%, 37.8%, 40.6% and 41.7% for diesel, B20, B40, B60, B80 and B100 when CR is increased from 18 to 20. That is running the engine at higher CR is more beneficial with biodiesel comparable to diesel. This is because; biodiesels provide lower volatility and better viscosity than diesel that results improved combustion characteristics at higher CR. Laguitton et al.
(2007) varied the CR (18.4:1 and 16.0:1) by reducing the bowl pip size, while maintaining the same squish height. Their study showed that at lower CR the combustion starts very late even at higher load also. As a result, combustion remains incomplete; hence, temperature falls resulting drop in the NOX. This fact also cuts smoke formation at high load and low CR.
Selim (2004) reported that LPG has most nonuniform behavior as its knock onset was observed very early (8.1 Nm torque) for a CR of 22. However, at these lower CR engine can withstand more amount of gaseous fuel. For N2 and methane, the onset behaviors of knock and ignition failure are almost similar with a tendency to burn a little more quantity. He also reported that increasing the CR increases the combustion noise due to the higher self-ignition possibility of the gaseous fuels at higher pressures and temperatures. Selim and his coworkers reported that, pressure rise rate increases as the CR increase due to increase in temperature (Selim et al., 2008). This consequently increases the susceptibility of the gaseous fuel to auto ignite. This fact also causes the increase in the maximum pressure. The increase in CR increases the expansion ratio thereby increases in power output and thus the fuel intake falls.
LPG-diesel is the most effective dual fuel composition. Their study showed that increase in CR cuts HC emissions for all dual fuel compositions studied. However, dual fuel composition
with Jojoba methyl ester yields more CO and HC compared to diesel. This is because of the higher kinematic viscosity (3 to 10 times) of that particular biodiesel than diesel.
2.5.3 Effect of Injection Timing
The effect of ethanol fuels on the spray behavior, combustion and emission characteristics of a common rail four cylinder diesel engine were studied by Park et al., (2011) at 1,500 rpm of engine speed and various ITs. Both the ethanol blended fuel compositions are showing unstable combustion characteristics at the lowest and highest load (0 and 60 Nm). However, at 30Nm load fuel IT of 6° and 9°BTDC provides the best combustion characteristics comparable to TDC and 3°BTDC. The ethanol-blended fuels offer larger ID than diesel due to low BTHE of the ethanol. The peak combustion pressure and heat release rate is also increased with the advance of IT because of the shift of combustion towards TDC from expansion stroke. Alternatively, IT retardation reduces the peak pressure and temperature inside cylinder, which consequently drop in NOX formation.
As observed in Fig. 2.14 (Sayin et al., 2010), both advancing (25°BTDC) and retarding (15°BTDC) the IT result rise in the BSFC, BSEC and drop in BTHE comparable to original IT (20°BTDC). This is due to the joint effect of low density, low viscosity, and lower BTHE of methanol. Therefore, more fuel consumption takes place and efficiency falls. However, all the three blending of methanol are producing affective drop in CO, HC and smoke opacity compare to diesel. Further, IT advancement causes additional drop of the above emissions.
The IT advancement causes increase in the NOX and near TDC peak cylinder pressure and hence maximum combustion temperature increases (Sayin et al., 2009) and reflected in the EGT. Advancing IT surges the CO2 and is expected to be the combined effect of high fuel intake and ample time access to complete the combustion. Same group of authors (Sayin et al., 2008) reported fall of CO and HC emissions due to the IT advance. CO2 and NOX
emissions are increased with IT advance. Raheman and Ghade (2008) have studied the effect of IT of Mahua methyl ester and its blend with diesel. The IT advancement causes BSFC to drop for almost all the fuel blends except 20CR and 45°IT. At this advanced IT, it is difficult to attain ample temperature to auto ignite the fuel. Further, IT advancement from 35° to 40°
and 40° to 45°BTDC causes surge in BTHE from 12.9% to 18.1% and 2.9% to 7.5% for the fuel blends studied. Therefore, advancing IT from 35° to 40° is more useful because further advancement marks miss matching of the peak pressure growth. This is the result of partial burning of fuel due to lower temperature caused at that IT.
Papagiannakis et al. (2007) studied the effect of pilot fuel injection advance (4°, 6° and 8°
before normal IT) on performance and emission of a NG-diesel dual fuel engine by using numerical methods. Their study showed that advancing IT reduces the BSFC and increases maximum cylinder pressure for (Figure 2.15). The advancement of IT of pilot fuel compensates its increase in ignition lag in dual fuel mode resulting maximum combustion pressure to shift towards the TDC. As a result, less amount of fuel is required to produce same amount of pressure. Side by side increase in nitric oxide and reduction in carbon monoxide and soot emission results as reported by the other researchers (Sayin et al., 2008).
Figure 2.14 Performance variation with blends compared to diesel fuel at different ITs and ORG IP (Sayin et al., 2010)
Figure 2.15 Brake specific fuel consumption versus engine load for various Its (Papagiannakis et al., 2007)