BIOGAS DUAL FUEL ENGINE EXPERIMENTS
4.2 Performance Parameters
At an individual tested engine loads, both shaft power and torque outputs were kept equal in magnitude for both diesel (baseline) and dual fuel operations. From Fig. 4.1, it can be seen that the brake power and torque increased gradually with an increase in load. It is obvious because, at constant engine speed, as load increases both torque and power output increase.
For a safe operation, the diesel engine was run to maximum power output and torque of 4.5 kW and 3.0 kg-m respectively, which are about 90% of maximum engine ratings.
Figure 4.1 Variation of shaft output and torque with engine load
The brake thermal efficiencies of both the dual fuel operations were lower than that of diesel mode as shown in Fig. 4.2. A considerable reduction in thermal efficiency (about 19% to 40%) was observed under dual fuel modes as compared to diesel mode for the entire load range. This is mainly due to the lower heating value of biogas. At lower loads of 0 to 40%, there were maximum reductions of about 30 to 40% for both the dual fuel modes. This is due to the poor mixing of air-biogas and inadequate combustion rate of this mixture in the lower temperature zone of 0 to 40% loads. The trend of dual fuel mode efficiency matches closely to that of diesel mode when the load was increased. The thermal efficiency was found maximum at slightly lean mixture condition of 80% engine load for all the tested fuel conditions. Among all the test modes, the best efficiency point is achieved at this load.
Mixtures richer or leaner than this point caused incomplete combustion or slow the burning rate, and hence led to a drop in thermal efficiency. The maximum thermal efficiencies with the dual fuel mode were 16.8% and 16.1% for diesel and JOME pilot ignition respectively.
While a maximum of 20.9% thermal efficiency was found for the diesel mode. At the higher temperature zone of 100% load, the higher self ignition temperature property bearer biogas combusted better to give an improved efficiency at the minimum oxygen operation. At this condition, the dual fuel modes have an equal efficiency reductions range at 80% load as compared to diesel mode. At best efficiency point, the efficiency with the biogas-JOME mode was observed only 4% lower as compared to biogas-diesel mode. There were no substantial differences in thermal efficiency found in between both dual fuel modes. This is because of the matched properties of calorific value and density for both the pilot fuels.
Including this, the presence of oxygen molecules in the bio-diesel helped a better combustion in the engine combustion chamber.
Figure 4.2 Variation of brake thermal efficiency with load
The lubrication regime of the mating surface affects the efficiency, durability and exhaust emissions of the engine. In general, most friction occurs at, or near, top dead centre where low piston velocity is low, and where the temperature is higher, due to the proximity of the combustion chamber (Masjuki et al., 1999). Biogas contains lower energy than that of diesel oil. So, the biogas dual fuel mode generates lower combustion pressure and hence, their expansion ratio is less. Including this, biogas dual fuel mode produced less combustion temperature compared to diesel mode. Again, low viscous biogas does not mix with lubricating oil during engine operation. All these factors collectively reduced the friction power (FP) consumption of the both the dual fuel operations as compared to diesel mode (Fig. 4.3). Jatropha bio-diesel contains oxygenated moieties and double bonds. This improved the overall lubricity of bio-diesel over diesel fuel and resulted lower friction coefficient (Bhale et al., 2008). This led to lower FP consumption of about 0.5 to 1.5 kW for biogas- JOME mode over biogas-diesel operation. This is one of the advantages of using bio-diesel as pilot over diesel fuel. In addition, as a whole, the dual fuel operations are able to save about 0.5 to 2 kW of FP than that of diesel mode. This benefits a dual fuel mode over a diesel mode. In general, the IMEP method to calculate FP is based on the concept of force balance.
Therefore, this method can be prone to large errors if the cylinder pressure measurements are inaccurate. Looking at the quantity of FP saved (about 0.5 to 1.5 kW), it seems that these values are at the higher side due to errors in the calibration of the pressure transducer which causes substantial difference in the calculated IMEP and hence FP. Also, the uncertainty of the pressure transducer used in this work is 2% as per the Supplier’s manual.
Figure 4.3 Comparison of friction power saved during dual fuel operations with load
Brake specific energy consumption (BSEC) of a dual fuel mode was calculated from the data of fuel consumption rate and LHV of pilot liquid fuel (diesel or JOME) and biogas. The BSEC of the engine was higher at part loads (up to 40%) irrespective of the fuel used shown in Fig. 4.4. This is due to the poor combustion efficiency of biogas fuelled dual fuel modes.
The BSECs of both the dual fuel modes were higher than that of diesel mode for the entire
load range. The pilot fuel had less influence, and there were no differences in the BSEC with the type of pilot fuel used. This indicates that JOME can be used as an alternative to diesel fuel as the pilot ignition for a dual fuel operation. The presence of carbon dioxide in the biogas cut down the burning velocity and thereby resulted in incomplete combustion that increased the BSECs of both the dual fuel modes. In the process, both the dual fuel modes established higher exhaust gas temperatures (about 40 to 700 C) than that of diesel mode (Fig 4.5). The maximum exhaust temperatures for diesel and JOME pilot ignited dual fuel modes were found as 880 K and 895 K respectively as compared to 846 K of diesel mode.
Figure 4.4Variation of brake specific energy consumption with load
Figure 4.5Variation of exhaust gas temperature with load
The variation of volumetric efficiency with power output is shown in Fig. 4.6. The volumetric efficiencies with dual fuel operations were found lower than diesel mode. The higher temperature of the retained exhaust gas (Fig. 4.5) preheats the incoming fresh air and lowered the volumetric efficiency (Kumar et al., 2003a). In addition, at higher power outputs higher biogas substitution displaces a greater proportion of air. Hence, the volumetric efficiency of dual fuel operations decreased further to result its minimum value at the maximum power output condition. Among the dual fuel operations, biogas-JOME fuelling
JOME and hence, higher biogas flow rate during its operation. However, the differences in volumetric efficiencies between these two dual fuel modes were not substantial. The maximum and minimum volumetric efficiencies found for diesel mode was about 85% and 81% at 0 and 100% respectively. These values were 82 and 78%, and 82 and 77%, for diesel and JOME pilot ignition dual fuel modes respectively.
Figure 4.6 Volumetric efficiency variation with load
Comparing both the dual fuel operations, the biogas flow rates under biogas-JOME mode were found little higher than the other mode for the entire load range (Fig. 4.7). At maximum load, due to the presence of oxygen in the bio-diesel, biogas-JOME mode was able to accommodate more volume of biogas combustion still in the rich mixture zone. Under diesel- biogas mode, a possible diesel fuel replacement of 22, 36, 45, 54, 60 and 69% was determined at 0, 20, 40, 60, 80 and 100% engine load respectively (Fig. 4.7). On the other hand, the biogas-JOME operation showed a maximum of 22 to 66% bio-diesel substitution for the same loading combinations. However, considering the fossil fuel replacement, the biogas-JOME mode was able to save 100% diesel oil. At lower loads of 0 to 40%, due to a lower overall engine temperature, the inability of easy auto-ignition of biogas enabled a low gas flow into the combustion chamber. In the process, it provided less energy for the dual fuel operation. Therefore, to provide a higher temperature zone to the biogas for its auto-ignition and also, to maintain a constant power output at individual loads, a larger amount of liquid pilot fuel was supplied at lower loads. However, this lowered the pilot fuel replacement at lower loads. Due to its high self ignition temperature (about 580°C), biogas burned better at higher load temperature zone. Hence, the liquid fuel replacements improved at higher loads and also, showed its maximum value at 100% load for the dual fuel modes.
Figure 4.7 Variation of liquid fuel substitution and gas flow rate with load