SYNGAS–DIESEL DUAL FUEL ENGINE EXPERIMENTS
6.3 Combustion Characteristics
comparatively lesser than that of syngas with 25 and 50% CO fraction. At 100% load, the lowest volumetric efficiency for 100% CO syngas mode was found to be 75.4%.
Figure 6.6 Variation of syngas flow rate with engine load
Figure 6.7 Variation of ignition delay with engine power output
Figure 6.8 Variation of peak cylinder pressure with engine load
For diesel mode, at maximum load (100%), the peak cylinder pressure was found to be highest (69.5 bar) within the entire load range. The peak cylinder pressure were also observed maximum at 100% load for dual fuel operations, and found to be 73.6, 66.5, 56.2 and 59.7bar for syngas fuels with H2 fraction of 100, 75, 50 and 0%, respectively. For syngas dual fuel operations, shorter combustion duration and an increased ignition delay shifted the overall combustion to expansion stroke, and hence, a reduction of cylinder pressure was resulted.
After 40% load, the increase in peak cylinder pressure was observed more intense for the dual fuel modes due to their better combustion at higher temperature zones. Therefore, for the dual fuel modes, the trends of increase in peak pressure prior to 40% load were noticed different than that of after the same load. Among all dual fuel operations, 100% H2 syngas mode resulted a higher cylinder pressure at all loads because of its shortest combustion duration and highest LHV. Again, at medium and high loads, the peak cylinder pressure of 100% H2
fraction syngas mode was higher than that of diesel mode due to its high energy release rate.
The syngas fuels which contain H2 and CO i.e., 75 and 50% H2 fraction fuels, the combustion retarded further and the cylinder pressure reduced as compared to 100% H2 fraction syngas mode. Compared to the 100% H2 case, beyond 60% load, the combustion of 75% H2 syngas was slightly longer; however, the ignition delay was not affected significantly (Fig. 6.7).
Thus, at medium and high loads, a higher increase rate of cylinder pressures for 75% H2 operation was observed. The peak cylinder pressure of 100% CO syngas was lowest among all tested syngas fuels. This is due to the poor combustion and low energy release rate of fuel.
However, for the better combustion of CO gas at maximum temperature region of 100% load, the lower heat content 100% CO syngas mode produced higher peak cylinder pressure than the 50:50 H2:CO syngas mode.
The trend of maximum rate of pressure rise (MRPR) or combustion noise was similar to that of peak cylinder pressure as shown in Fig. 6.9. At lower engine loads, the cylinder pressure and temperature of dual fuel modes were reduced due to retarded combustion and inefficient oxidation of the gaseous fuels. As the pilot quantity was low, the lean gas-air mixture and thereby, a weak ignition source resulted slower combustion rates. This dropped the peak cylinder pressure and the MRPR. This may be one of the reasons for the reduced thermal efficiency under dual fuel operations at lower loads. Except for 100% H2 syngas mode (beyond 60% load), all other tested dual fuel modes the MRPR were lower than that of diesel mode for the entire load range. Similar to biogas operations, the syngas dual fuel operations are also established safe operation in the base engine due to their lower MRPR.
Figure 6.9 Variation of MRPR with engine load
The average data of pressure-crank angle
(
P-θ)
recording at different portion of engine loadsnormal combustion curves occurred for the dual fuel modes. This was because of the limited syngas flow (up to the engine misfire condition) for the dual fuel combustion. The rise in the peak pressure depends on the energy release rate of fuels. Due to this, the peak pressure was very high for 100% H2 syngas mode, and also, occurred earlier irrespective of increased ignition delay as compared to other syngas fuels. This is because of higher flame velocity of H2 gas (2.7 m/s). It was observed from P-θ diagrams that the attainment of dual fuel peak pressures lag by 6 to15°CA as compared to diesel mode. This is due to the release of syngas energy in the expansion stroke which also lowers peak pressure for dual fuel modes as compared to that of diesel mode. At higher loads of 80 and 100%, the attainment of peak pressure for higher H2 content syngas (100 and 75%) operations were observed to be minimum of about 6 to 8° CA. Where as, for other dual fuel modes, this value was increased to about 10 to 15° CA due to their lower flame velocity and poor combustion rate.
Figure 6.10 Variation of cylinder pressure with crank angle at 60% engine load
Figure 6.11 Variation of cylinder pressure with crank angle at 80% engine load
Figure 6.12 Variation of cylinder pressure with crank angle at 100% engine load
The corresponding heat release rate diagrams of pressure crank-angle data are indicated in Figs. 6.13 to 6.15. In general, during a dual fuel operation, the initial heat release is due to the combustion of pilot fuel and then inducted syngas dominates the combustion phase. It can be seen from the figures that there are two pressure peaks for the tested dual fuel modes, one for pilot fuel (diesel) combustion, and the other for the primary (syngas) combustion. It was observed that the peak heat release reaches high values for the higher energy content fuels.
Similar trends were exhibited with theP-θ variations also. The heat release rates of dual fuel modes were higher than that of diesel mode for the entire load range. The magnitude of heat release rates increased for an increase in load due to the combustion of higher amount of syngas at higher loads. At 100% load, the maximum heat release rates were found as 215, 166, 133 and 125 J/ deg.CA for the 100, 75, 50 and 0% H2 content syngas operations, respectively. However, this value was 111 J/ deg.CA for diesel mode, and it occurred at 80%
load. The heat release of H2 content syngas combustion is completed in a shorter period of time than that of diesel combustion because of the higher burning velocity of hydrogen.
Looking at the heat release diagrams, it can be seen that there were negative apparent heat release rates after the end of combustion for all the tested H2 content syngas operations. This is because of the formation of H2O molecules or moisture by H2 gas present in syngas composition. A higher amount of negative heat release rate was observed for high H2 content syngas. The amounts of negative heat release rate of syngas operations (about 30 J/ deg.CA) are much higher than that of the earlier described biogas operations (about 10 J/ deg.CA).
This is due to the presence of huge amount of H2 content combustion during syngas operations. In the process, the negative heat release have resulted a higher cooling loss during
content syngas dual fuel modes produced about 0.5 to 1.5 kW higher cooling losses to that of diesel mode. However, 100% CO dual fuel mode did not follow this cause because it was free from any H2 content in its composition, and also, it did not produced any negative heat release rates in its combustion.
Figure 6.13 Variation of net heat-release rate as a function of crank angle at 60% load
Figure 6.14 Variation of net heat-release rate as a function of crank angle at 80% load
Figure 6.15 Variation of net heat-release rate as a function of crank angle at 100% load
Figure 6.16 Variation of kW cooling loss as a function of engine load