BIOGAS DUAL FUEL ENGINE EXPERIMENTS
4.3 Combustion Parameters
Figure 4.7 Variation of liquid fuel substitution and gas flow rate with load
Figure 4.9 Variation of cylinder pressure with crank angle at 80% load
Figure 4.10 Variation of cylinder pressure with crank angle at 100% load
Figure 4.11 Variation of ignition delay with power output
In general, when load increases, the addition of fuel chemical energy per cycle increases the cylinder peak pressure (Fig. 4.12). Normal diesel operation showed the highest peak pressure of 69.5 bar at maximum load. The effect of lower burning velocity and lower energy input
per cycle of biogas reduced the peak pressure for dual fuel operations. This also resulted in a slower rate of pressure rise. At 100% load, the highest peak pressure found as 57.2 bar for biogas-diesel mode followed by 52.3 bar for biogas-JOME mode. Looking at the Fig. 4.12 it can be realized that the trends of increase in peak pressure under the dual fuel operations are mild within 0 to 40% load. This is due to the poor combustion of biogas at lower load operating conditions. However, beyond 40% load, these trends showed an intense degree of pressure rise in the higher temperature operating zones. Comparing in between the dual fuel modes, the bio-diesel ignited dual fuel mode produced a little lower peak pressure than that of diesel ignited mode. This is mainly because of the lower chemical energy capacity of JOME than the diesel fuel.
Figure 4.12 Comparison of peak cylinder pressure with load
When the load increased, there was an increase in maximum rate of pressure rise (MRPR) or combustion noise of all the fuel modes examined (Fig. 4.13). As the engine output increased, the mass of fuel-gas admission increased. Ignition of more gaseous mass caused the higher pressure rise rate and noise. The diesel mode showed a highest MRPR due to its higher energy content in fuel. For the dual fuel modes, the MRPR was highest for biogas-diesel followed by biogas-JOME mode. This may be postulated to higher ignition delay of the liquid pilot fuel and dual fuel mode as well (Selim et al., 2008). The jatropha bio-diesel has a lower ignition delay period as compared to the diesel fuel (Sahoo and Das, 2009). Therefore, as expected, biogas-diesel combustion showed a longer delay due to the presence of pilot diesel as shown in Fig. 4.11. Thus, it confirmed the lower combustion noise for the JOME pilot ignited dual fuel mode as compared to the diesel ignited mode. The increase in ignition delay of the fuels decreased the net thermal efficiency, and energy generated from burning
fuel. Hence, it increased the likelihood of emitting some of the combustion energy as exhaust gas in the form of higher exhaust gas temperature as illustrated in Fig. 4.5. Of course, from engine material and assembly destructive viewpoint, both dual fuel modes have established the safe operation in the base diesel engine as their MRPR were below to that of diesel mode.
Figure 4.13 Comparison of MRPR with load
The maximum heat release rate with an uncertainty of 0.5 % of both the dual fuel modes was lower than that of diesel mode (Fig. 4.14 to 4.16). The longer ignition delay of pilot fuel progressed to the delay in the biogas combustion and this reduced the heat release rate of the dual fuel modes. At low combustion temperature conditions of 0 to 40% loads, the reductions in heat release rates of the dual fuel modes were higher. This is because of the presence of CO2 and methane (which has a high self ignition temperature) in biogas. However, as load increased, the dual fuel operation heat release rates was improved due to the accumulation of a relatively higher of biogas supply for the combustion in the high temperature operating conditions. At 100% load, the maximum heat release rate of biogas-diesel and biogas-JOME modes were found as 144 J/deg. CA and 136 J/deg. CA as compared to 162 J/deg. CA for the diesel mode. Because of the higher ignition delay, the occurrence of maximum heat release rates observed late in the expansion stroke for both the dual fuel modes in comparison with the neat diesel mode. Hence, the heat releases during the late combustion phase for both the dual fuel operations were higher than that of the diesel mode. This lowered the expansion ratio for both the dual fuel modes and hence, produced lower power output for the dual fuel modes. From the Figs. 4.14 to 4.16, it can be viewed that there is a negative net heat release rate under dual fuel operations. This is probably due to the presence of some H2 in the biogas composition which form water molecules or moisture during the biogas combustion. Bio-
diesel ignited dual fuel operation produced little higher negative heat release rate than that of other mode due to the higher biogas consumption. Among the dual fuel modes, due to its lower heat content, JOME ignited dual fuel mode resulted lower heat release than that of diesel ignited pilot mode.
Figure 4.14 Variation of net heat-release rate as a function of crank angle at 60% load
Figure 4.15 Variation of net heat-release rate as a function of crank angle at 80% load