Results and Discussion: Preheated Biodiesel Blends Run Engine
6.3 Result and Discussion
6.3.3 Effect of Preheated Biodiesel Blends on Combustion Characteristics
6.3.2.4 Effect of Preheated Biodiesel Blends on Volumetric Efficiency
Figure 6.7(a) illustrate the effect of preheated POME biodiesel/diesel blends on the volumetric efficiency with variation of loads under no-EGR. For all fuels, it drops slightly with increase of loads (Rambabu et al., 2013). This is because of increased in-cylinder gas temperature at higher engine load leading to higher gas pressure in the cylinder. Hence, it restricts the air quantity at inlet of the engine cylinder resulting slightly drop of the volumetric efficiency of the engine with increasing load. The effect of preheating POME helps to improve volumetric efficiency of diesel engine. The heated POME blend fuels causes improvement in fuel injection characteristics and combustion efficiency leading to increased amount of air intake. Due to this fact, there is a slight increase of volumetric efficiency, but it is still somehow lower than the mineral diesel fuel. At 100% loading condition, the average value of volumetric efficiency is about 80 % for all test fuels including diesel. The variations of volumetric efficiency for different fuels with EGR rates at full engine loading condition is shown in Figure 6.7(b). All the test fuels show marginal drop (2.5 %) with increasing the EGR rates which is in line with reported literatures (Agarwal et al., 2011). It is mainly due to the reduction of amount of intake air mass flow into engine cylinder for combustion and supply of exhaust gases to in the engine cylinder through EGR system.
(a) (b)
Figure 6.7: Variation of volumetric efficiency for fuels: (a) With loads (no-EGR), (b) With EGR rates at full load.
6.3.3.1 Effect of Preheated Biodiesel Blends on PCP
Figure 6.8(a) demonstrate the variation of peak cylinder pressure (PCP) for different test fuels with engine load without considering EGR rate. It is seen that the PCP all tested fuels were increased with increasing load (Rambabu et al., 2013). At lower loads up to 40%, the PCP values of preheated biodiesel (PPBD100) are highly 8.9% as compared to diesel (PBD0). All test fuels have identical values (71 bar) of PCP at higher loads. It is because of a shorter ID for biodiesels, thereby injection timing (IT) is advanced because of a higher bulk modulus and density of POME.
Even the preheated POME (high density) has lower ID, the oxygenated nature of the fuel improves ignitability and produced maximum cylinder gas temperature even within the existing delay period (Qi et al., 2009). The PCP for diesel fuel is less at lower engine load due to a longer ID as compared to biodiesel and occurred in working stroke far from the position of top dead center (TDC).
However, at higher load, PCP occurred at near to TDC at same crank angle position with biodiesel.
It is realized in Figure 6.9(a) that, when the load increases, the ID for test fuels (PPBD100, PPBD80, PPBD60 and PPBD40) is shortened that lead to start of ignition before TDC and PCP increases more rapidly. The preheating and blending POME improved the fuel properties of biodiesel resulting marginal increase of ID compared to preheated PPBD100.
(a) (b)
Figure 6.8: Variations of PCP for fuels: (a) With loads (no-EGR), (b) With EGR rates at full load.
The effect of EGR rate on PCP for different test fuels at full load is shown in Figure 6.8(b).
The PCP for all fuels decreases with higher EGR rate. This because of the shortage of oxygen in combustion chamber because of occurrence of higher CO2 in the exhaust gas. These gases can simple dilute the intake air availability in combustion chamber and affects air-fuel mixing process and combustion characteristics. With respect to diesel (PBD0), the test fuel PPBD20 to PPBD60 at EGR30% rate, the drop in PCP is 1.04%, 3.11% and 8.73 %, respectively with same 30% of EGR rate, and (6.95%, 8.13 % and 12.73%) as compared to the engine operated without EGR (no- EGR).
6.3.3.2 Effect of Preheated Biodiesel Blends on ID
In diesel engine, the time lag between fuel injection and combustion initiation is called “ignition delay” (Heywood, 1988). Figure 6.9(a) shows the variation of ignition delay (ID) for all preheated POME biodiesel/diesel blends with reference to diesel for entire load spectrum of the engine with no-EGR. It can be seen that ID gets longer with increasing preheated POME biodiesel content in the blend fuel. The increase is obviously due to the decrease in Cetane number of diesel brought by the addition of preheated POME biodiesel. It is also seen that the ID for all tested fuels decreases with increasing of load. This is as a result of the increase in-cylinder gas temperature with increasing load ensuing a shorter ID for tested fuels. For PPBD100, the IDs was shorter compared
to other test fuels over the operating loading conditions (Rajasekar and Selvi, 2014). In this case, the higher Cetane number of POME plays a vital role on affecting the chemical delay period for ignition compared to other tested fuels properties. At full load, the IDs for preheated POME (PPBD100) and diesel fuel (PBD0) was 13° CA and 18° CA respectively. The combined effect of fuel preheating and blending casued to decrease the ignition delay as compared to preheated neat POME biodiesel “PPBD100” (Figure 6.9-a). At full engine load, the IDs for PPBD20, PPBD40, PPBD60 and PPBD80 were less by 5.56%, 11.11%, 16.67% and 22.22%, respectively as compared to diesel fuel (PBD0). This is due to a decrement of kinematic viscosity and density of POME with the effect of heating and blending.
Figure 6.9(b) illustrates the effect of increasing EGR rates on the ignition delay of preheated POME biodiesel/diesel blends at full engine load. The variation of IDs for test fuels shows increasing trends with EGR rate. This due to the supply of more exhaust gas to the engine cylinder that decreases the quantity oxygen needed for combustion. Hence, ID increases causing to drop in combustion efficiency. One of the important effects of EGR is prolongation of ID (Chen et al., 2014). Hence, increase in EGR rates increased the ignition delay for all blends. With respect to the test fuel PPBD20, PPBD40 and PPBD60 at EGR30% rate, the drop in ID is 9.52%, 14.2%
and 14.3% as compared diesel (PBD0) with same EGR rate, and increased by 11.76%, 12.5% and 20% as compared to the engine operated without EGR.
(a) (b)
Figure 6.9: Variations of ID for fuels: (a) With loads (no-EGR), (b) With EGR rates at full load.