Thermodynamic Potential Study
8.3 Thermodynamic Potential of Emulsified POME
8.3.2 Exergy Analysis
The performance of WIP at higher CR can be analyzed with respect to the peak pressure and peak heat release rate curve (Figure 8.13). The CR rise from 17 to 18 increases the average value of peak pressure by 18.66% for the WIP run engine. It is reported that a rise in CR increases temperature at the end of compression stroke (Aziz et al., 2005). Both of these facts cause 34.04% rise of peak heat release rate near to TDC for WIP. However, for neat POME, the rise of peak pressure and peak heat release rate, both were approximately of 15%. This high increase of peak heat release rate shows that WIP emulsion burns with almost double intensity than neat POME near to TDC. For a constant speed engine, the presence of “micro- explosion” of emulsified POME is probably the better way to explain this fact, since at each load relative air fuel ratio should be fixed to produce same power. Alongside, it is seen that, IT advancement elevations peak pressure. Advancement of IT means that fuel is supplied at cooler atmosphere to that of retardation. Hence, fuel consumption becomes more for the same BP. It is also observed from Fig. 8.11, that the increase in CR decreases exhaust heat loss.
This is also proved from the curves of EGT (Figure 8.14). The increase of CR from 17 to 18 has reduced EGT around 15% by average. It is realized that, a fall of uncounted heat loss takes place for WIP run engine. The loss of energy per unit time (% of fuel input) which cannot be trapped are 17.6%, 14.6% and 13.4% for CR of 17, 17.5 and 18 and 13.14%, 17.80% and 14.72% for ITs of 20°,23°and 28ºBTDC. This is a reasonably definite drop, which can be achieved with the increase of exhaust energy loss per unit time.
with it are calculated by using Eqs. (C7) through (C13). The variation of availability parameters as a function of CR and IT are elaborated in Figs. 8.15 and 8.16.
Figure 8.14 Influence of compression ratio and injection timing on exhaust gas temperature for emulsified POME run engine
Figure 8.15 Influence of compression ratio on exergy distribution for emulsified POME run engine
Figure 8.16 Influence of injection timing on exergy distribution for emulsified POME run engine
270 285 300 315 330 345 360
16.8 17.0 17.3 17.5 17.8 18.0 18.3
Exhaust Gas Temperature (ºC)
Compression Ratio
IT (WIP): 20ºBTDC IT (WIP): 23ºBTDC IT (WIP): 28ºBTDC
LOAD:100%
SPEED:1500 RPM FUEL:WIP
0 10 20 30 40 50 60 70
16.8 17.0 17.3 17.5 17.8 18.0 18.3
Exergy (% Fuel Input)
Compression Ratio
As Aw Ae Au LOAD: 100%
SPEED: 1500 RPM FUEL: WIP
0 10 20 30 40 50 60 70
18 20 22 24 26 28 30
Exergy (% Fuel Input)
Injection Timing (ºCA)
As Aw Ae Au LOAD: 100%
SPEED: 1500 RPM FUEL: WIP
The fuel energy inputs for WIP run engine for the CRs of 17, 17.5 and 18 are 12.19 kW, 11.46 kW, and 11.65 kW respectively. With increase of CR, the shaft availabilities are found to be 26.59%, 27.46% and 28.07% of fuel energy input, respectively. During the engine operation, as the CR increases, the cylinder volume reduces. This increases the temperature and hence the peak pressure (verified from Fig. 8.13) of combustion. This intensifies the combustion and improves the shaft availability. It is also found that an increase in CR from 17 to 18 reduces the exhaust availability by around 11%. This is the consequence of the reduction of EGT as discussed in the earlier section (Figure 8.14). It has a significant effect on the cooling water availability that got increased by around 50%. However, the availability destruction has hardly a variation (1% drop) with the increase of CR. For the ITs of 20°,23°and 28ºBTDC, the fuel availability values are 11.42 kW, 12.11 kW and 11.76 kW.
Definitely, the fuel input value for the WIP run engine is the lowest for retarded IT. It is also found that the retardation of IT provides higher shaft availability. The values of shaft availabilities are 28.56%, 27.16% and 26.39% of fuel input. As IT is retarded from 23ºBTDC (standard IT for diesel) to 20ºBTDC, the piston is actually pushed towards the TDC and thereby the swept volume of the cylinder is reduced at the instant of fuel injection. This causes the WIP to be injected at a crank angle nearest possible from TDC. As a result, at the start of fuel injection, the environment inside the cylinder gets mechanically warmer. This probably increases the shaft availability percentage. The change in cooling water availability is almost negligible. However, the exhaust availability is increased by 23% with the advancement of IT (at 28ºBTDC). This is because, at 28ºBTDC, the EGT is highest among all the other ITs (Figure 8.14). As a result, at 28ºBTDC uncounted availability is reduced.
This has finally increased the exergetic efficiency as seen from Fig. 8.17.
Figure 8.17 Effect of injection timing and compression ratio on exergy efficiency for emulsified POME run engine
36 38 40 42 44 46
16.8 17.0 17.3 17.5 17.8 18.0 18.3
Exergetic Efficiency (%)
Compression Ratio IT (WIP): 20ºBTDC
IT (WIP): 23ºBTDC IT (WIP): 28ºBTDC
LOAD:100%
SPEED:1500 RPM FUEL:WIP
Table 8.4 Results of exergy analysis for emulsified POME run engine
(± standard deviation)
Fuel CR IT Ain (kW) As (kW) Aw (kW) Ae (kW) Ad (kW) II Diesel 17.5 23 13.01 ±(0.09) 3.49 ±(0.07) 0.08 ±(0.34) 1.45 ±(0.01) 7.99 ±(0.11) 38.62±(0.04)
W
I
P
17 20 11.31 ±(0.05) 3.16 ±(0.03) 0.05 ±(0.02) 1.69 ±(0.13) 7.33 ±(0.03) 43.34±(0.02) 17.5 20 11.08 ±(0.07) 3.16 ±(0.03) 0.03 ±(0.96) 1.34 ±(0.09) 6.56 ±(0.09) 40.83±(0.05) 18 20 11.87 ±(0.00) 3.47 ±(0.07) 0.08 ±(0.34) 0.99 ±(0.48) 6.41 ±(0.11) 38.26±(0.11) 17 23 13.38 ±(0.11) 3.48 ±(0.07) 0.03 ±(0.76) 1.33 ±(0.10) 8.53 ±(0.16) 36.21±(0.00) 17.5 23 11.53 ±(0.03) 3.15 ±(0.03) 0.05 ±(0.04) 1.44 ±(0.01) 6.89 ±(0.03) 40.24±(0.01) 18 23 11.42 ±(0.04) 3.21 ±(0.01) 0.08 ±(0.38) 1.26 ±(0.16) 6.86 ±(0.04) 39.90±(0.01) 17 28 11.87 ±(0.00) 3.06 ±(0.06) 0.03 ±(0.70) 1.61 ±(0.09) 7.17 ± (0.01) 39.61±(0.03) 17.5 28 11.76 ±(0.01) 3.12 ±(0.04) 0.03 ±(0.56) 1.72 ±(0.15) 6.88 ±(0.04) 41.51±(0.06) 18 28 11.65 ±(0.02) 3.12 ±(0.04) 0.06 ±(0.07) 1.80 ±(0.19) 6.67 ±(0.07) 42.72±(0.07)
It is also observed that, the trend of exergy efficiency for the IT of 20ºBTDC is different from the other two cases. The reason is the surge of uncounted availability at this IT. It is jointly responsible for a 4% and 50% drop of shaft and cooling water availability at CR=17.5 and a 36% drop of exhaust availability at CR=18 (at 20ºBTDC). These facts are responsible for a drop of almost 3% and 5% of second law efficiency by value. The variations of entropy generations for various CR-IT combinations are shown in Fig. 8.18. The plots of entropy generation suggest that at a lower CR, the entropy generation increases. The mean value of availability destruction and entropy generation for WIP run engine are 7.03 kW and 0.024 kW/K. The trends of entropy generations reduce with the increase of CR and retardation of IT. Therefore, from thermodynamic point of view, the emulsified fuel has to be run in a diesel engine at a higher CR and at a retarded IT.
Figure 8.18 Effect of injection timing and compression ratio on entropy generation for emulsified POME run engine