Thermodynamic Potential Study
8.2 Thermodynamic Potential of Neat POME
8.2.2 Exergy Analysis
Figure 8.5 Effect of injection timing and compression ratio on exhaust gas temperature for neat POME run engine
Fuel injection advancement and retardation have a significant effect on the energy distribution (Figure 8.2). Advancing (28ºBTDC) and retarding (20ºBTDC), the IT has increased the shaft power (% of fuel input) and hence BTHE (Figure 8.4). This is because of the reduction of fuel supply during IT advancement or retardation than the standard IT. The mean values of fuel energy supplied per unit time during 20ºBTDC and 28ºBTDC are 12.75 kW and 13.14 kW which, in turn, are 6.3% and 3.2% lower than the rate of energy supplied during standard IT, respectively. There is a fluctuation of uncounted heat loss, considering CR and IT variation. However, if considered, the average values of rate of exhaust heat loss for all the CRs are 2.34 kW, 2.54 kW and 2.53 kW for 20ºBTDC to 28ºBTDC. This is because of the rise of EGT with IT advancement and increase in average cooling heat loss per unit time. These values are 4.15 kW, 4.27 kW and 4.41 kW for 20ºBTDC to 28ºBTDC. All these facts coupled with higher rate of fuel energy input at 23ºBTDC causes reduction in uncounted heat loss for IT retardation and advancement.
been converted into mechanical shaft work (3.43 kW). Some amount of exergy is flown through engine cooling water and exhaust gas. The rest amount of exergy has been lost due to friction, radiation, heat transfer to surroundings. However, with respect to cumulative availability, the exergy associated with the cooling water and exhaust gas come under consideration (Sahoo et al., 2011). Therefore, approximately 30% of input exergy is found which can be called as available energy from the thermodynamic viewpoint. As a result, 70%
of input exergy is destroyed from the system.
Figures 8.6 and 8.7 describe the variation of availability associated with shaft, cooling water, exhaust gas and availability destruction with respect to CR and IT separately. The trend of shaft availability for CR variation (Figure 8.6) is almost same to that of shaft power as described in Fig. 8.1. The shaft availabilities for the CRs of 16, 17, 17.5 and 18 are 23.8%, 24.3%, 24.5% and 24.7% of fuel input, respectively. The average input fuel availabilities are 14.47 kW, 14.15 kW, 13.99 kW and 13.91 kW for CRs of 16, 17, 17.5 and 18. This reduction of absolute value of fuel availability is responsible for the increase in shaft availability although it is considered as the shaft work or BP, which is maintained constant throughout.
The cooling water availabilities for all the CRs studied are very low. Only a maximum of around 0.5% of fuel availability is found to be associated with the cooling water. This is probably as a consequence of the lesser increase of engine cooling water temperature in the course of POME test. The variation of exergy flow through the exhaust gas also has a little effect on CR variation. On the other hand, the availability destruction trend has shown a lowering trend. The increasing trend of shaft availability coupled with diminishing of fuel availability are probably be the reason of the reduction of availability destruction at the circumstances of almost unchanged cooling water and exhaust gas availabilities for CR variation.
The effect of IT on availability balance demonstrates (Figure 8.7) that the shaft availability increases with IT advance and retardation. The mean shaft availabilities at 20ºBTDC and 28ºBTDC are 7% and 4% higher than 23ºBTDC. POME having a higher Cetane number than diesel delivers lower ignition delay (Aziz et al., 2005). As a result, the peak heat release rate point has a match with the top dead center point (TDC) for the IT retardation rather than advancement or the standard diesel IT. This is the reason of higher shaft availability at IT of 20ºBTDC. However, no significant variation is accomplished from cooling water availability because; the maximum cooling water availability is found as 0.5% of fuel input. The reason is discussed in the earlier paragraph. There is an insignificant increase in exhaust gas
availability encountered with the increase in IT. This is probably because of the increase in the EGT as observed from Fig. 8.5. The tendencies of availability destruction for IT retardation and advancement are found to be lower than the standard 23ºBTDC. The countable increase in shaft availability is attributable to the IT advancement and retardation comparable to other forms of exergy (cooling water and exhaust flow) is the reason of around 1.5% and 1% fall of exergy destruction.
Figure 8.6 Effect of compression ratio on exergy distribution for neat POME run engine
Figure 8.7 Effect of injection timing on exergy distribution for neat POME run engine
The variations of exergy efficiency with respect to CR and IT are included in Fig. 8.8. It is clear that POME provides a better second law (exergy) efficiency with advancement (28ºBTDC) and retardation (20ºBTDC) of IT rather than 23ºBTDC of IT at higher CR range.
0 15 30 45 60 75
15.5 16.0 16.5 17.0 17.5 18.0 18.5
Exergy (% Fuel Input)
Compression Ratio
As Aw Ae Ad
LOAD: 100%
SPEED: 1500 RPM FUEL: POME
0 15 30 45 60 75
17 20 23 26 29
Exergy (% Fuel Input)
Injection Timing (⁰CA)
As Aw Ae Ad LOAD: 100%
SPEED: 1500 RPM FUEL: POME
Comparing the mean values, it is seen that POME run engine offers around 31% of maximum exergy efficiency for CR of 17%, 17.5% and 29% for CR of 16. While IT advancement and retardation allow 31% of exergy efficiency, the standard IT of 23ºBTDC gives 29% of the same. That means a lower compression ratio with standard diesel IT is not much efficient while running POME in diesel engine as far as the paramount deployment of the rate of available energy of fuel is concerned.
Figure 8.8 Effect of injection timing and compression ratio on exergy efficiency for neat POME run engine
The variation of entropy generation with respect to CR and IT are shown in Fig. 8.9. The trend of entropy generation is found almost reciprocal of exergy efficiency as expected. The trend suggests that a decrease in the CR increases the entropy generation. Side-by side, 23ºBTDC is bestowed with higher entropy generation than 20ºBTDC and 28ºBTDC of IT.
The mean values of exergy destruction and entropy generation are 10.0 kW and 0.033 kW/K, respectively. Therefore, the exergy analysis shows that, for enhanced utilization of the available energy supplied by POME, every time the engine has to be run at higher CR with IT retardation.