Results and Discussion: Preheated Biodiesel Blends with Intake Air Preheating Mode Run Engine
7.3 Results and Discussion
7.3.2 Effect of Fuel and Intake Air Preheating on Engine Combustion Characteristics The primary purpose of preheating intake air is to improve combustion efficiency with shorter
ignition delay. Combustion characteristics of diesel engine running with diesel fuel and three preheated biodiesel blends of fuels are explained in this section.
7.3.2.1 Cylinder Pressure
The variations of cylinder gas pressure with crank angle for all tested fuels at 90% of engine load, are shown Figure 7.2(a). Cylinder pressure characterizes the ability of the fuel to burn with proper mixing with air. The pressure waves in the cylinder during combustion shows sudden rise in pressure indicating the effects engine noise with biodiesel operations. The results show that the peak cylinder pressure of the engine running with biodiesel is slightly higher than the engine running with diesel. The main cause for higher peak in-cylinder pressure in the CI engine running with biodiesel is because of the advanced combustion process initiated by easy flow-ability of bio-
diesel due to the physical properties of biodiesel. In addition, the presence of oxygen molecule in biodiesel, the hydrocarbons achieve complete combustion resulting in higher in-cylinder pressure (Gumus, 2010). The cylinder pressure of preheated biodiesel blends of fuel (PPBD20, PPBD40 and PPBD60) follows a similar trend to that of diesel (PBD0). It is evident that higher percentage fractions of biodiesel exhibited higher in-cylinder pressure as compared to diesel baseline (PBD0).
The peak in-cylinder pressure of PPBD60 fuel is higher than that of baseline diesel. This is due to higher cetane number that tends to lower ignition delay period of biodiesel resulting improved combustion. It can be seen from Figure 7.2(a) that the peak cylinder pressure occurred in the range of 5-8° CA aTDC for all test fuels. It resembles the reported data in the literature (Rao et al., 2008).The effect of fuel preheating (POME at 114 °C) and blend with diesel fuel has offered maximum peak cylinder pressure at 90% load conditions. This is due to a better fuel atomization and improved combustion process.
(a) (b)
Figure 7.2: Comparisons of combustion parameters with crank angle for test: (a) Cylinder pressure, (b) Net heat release rate.
7.3.2.2 Heat Release Rate
Figure 7.2(b) shows the heat release rate (HRR) with crank angle for diesel engine running with different test fuels at 90% load conditions. It can be seen that the engine running with biodiesel
blends of fuel have a higher peak in the heat release rate diagram with respect to the baseline diesel fuel. This phenomenon can be explained on the basis of the presence of the oxygen molecule in biodiesel fuel that results in the air-mixed fuel in the cylinder to burn completely and increase the heat release rate. It can be observed that the NHRR curve shifts towards right with increase in volume fraction of biodiesel in the test fuel, which indicates shorter ignition delay. In the same figure, the heat release rate initially goes in negative due to endothermic reaction of the charge and it becomes positive when combustion starts. It is obvious that, engine running with different blends of fuel (preheated and blend of POME) caused to increases in peak heat release rate at 90% load condition. This is due to the reduction in viscosity and density with heating and blending. When the viscosity is lowered, the fuel gets atomized finer and enhance the combustion process, especially the diffusion combustion by capitalizing the oxygen presence in POME.
7.3.2.3 Peak Cylinder Pressure
Figure 7.3(a)shows the peak cylinder pressure (PCP) for different test fuels with intake air preheating. It showed that the PCP all preheated biodiesel blends was slightly higher than diesel.
The result showed that increased percentage of biodiesel in blend ratio caused to increase the PCP of the engine (Kumar and Dixit, 2014). These may be due to a higher cetane number of biodiesel that shorten ignition delay and advance injection timing than diesel fuel. It results early combustion of biodiesel blends fuels and its PCP attain amaximum. The more oxygen content in POME help to burn more carbon fuel in combustion chamber that leads to improve combustion and increased PCP (Kumar and Dixit, 2014). In addition, preheating intake air causes higher PCP of all tested fuels. It is observed in Figure 7.3(a) that the PCP for all tested fuels increases at higher intake air preheating temperature. Thus, the air molecules in combustion chamber enhances molecular collisions at elevated temperature resulting improved reaction rate, short reaction time, early combustion with higher PCP all tested fuels. For 90% load, the maximum PCP for PBD0, PPBD20, PPBD40 and PPBD60 test fuels is attained at a preheating of 61°C as, 66.7, 67.7, 69.4 and 70.7 bar, respectively, whereas the PCP was seen minimum at 33 °C and the value being 59.2, 60.16, 61.3 and 62.1 bar. With intake air preheating, the average percentage increased in PCP for 61°C is about 15%, intake air at ambient conditions (33 °C).
7.3.2.4 Ignition Delay
The variation of ignition delay (ID) for various test fuels with intake air preheating engine is illustrated in Figure 7.3(b). The result revealedthat the ignition delay period for all tested fuel slightly decrease with increasing of intake air preheating temperature which is of similar trend as reported in the literature (Lapuerta et al., 2008). The overall combustion efficiency improved due to better mixing characteristics of high-temperature air with inducted vaporized fuel. It is seen in Figure 7.3(b) that, preheated biodiesel blended fuels have lower delay period than diesel fuel. The reason it may be due to oxygenated nature of biodiesel that helps to start early combustion and influence the delay period. Biodiesel has lower compressibility and higher viscosity compared to diesel fuel, which leads an early fuel injection, which is usually happening as soon as the injector needle lift up from its seat and affects adversely the combustion characteristics. The higher cetane number with relatively higher density and kinematic viscosity of biodiesel oil causes a smaller delay period than fossil diesel fuel. The result showed that, increasing biodiesel blend ratio to PPBD60 lowers the delay period significantly at varying intake air temperature. The reason may be due to an increase percentage of biodiesel in blend ratio that causes increased content of oxygen in the blended fuel, resulting in improved reaction rate and combustion, and shorten delay period.
However, the ignition delay of neat diesel fuel (PBD0) was longer as compared to all preheated biodiesel blended fuels. This is because of a simple and slow pre-flame chemical reaction takes place at low temperatures. For 90% load, ignition delay for PBD0, PPBD20, PPBD40 and PPBD60 test fuels at a preheating of 61°C were found minimum with 14, 13, 11 and 10 °CA, whereas the ignition delay was seen maximum at 33°C and the values as 18, 17, 15 and 14 °CA, respectively.
The overall percentage reduction of ignition delay for 61°C is in the range of 22-28%, when compared to without preheating intake air at 33 °C.