In-cylinder Pressure Analysis

The indicated mean effect pressure (IMEP) is calculated by the measured combustion pressure, derived from Eq. (3.18) in Sec. 3.2.4, for the different biogas supply rates as a function of excess air ratio with different spark timings. Figure 4.14 shows the the in-cylinder pressure as a function of crank angle degree with 240L/min biogas supply rate and λ=1.0 at different spark timings. It can be found that the peak pressure occurs later in the expansion stoke when the spark timing is delayed.

Moreover, the faster spark timing has a higher value of maximum pressure than the slower one; with the advanced spark timing, the peak pressure occurs closer to the top-dead-center. Figure 4.15 shows the calculated IMEP with 240L/min biogas supply rate and λ=1.0 at different spark timings during 200 combustion cycles. Obviously, the IMEP of spark timing BTDC13 is greater than the others as a result of the fact that

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the power outputs of the spark timing BTDC13 is higher.

Figures 4.16a~c show the CoVIMEP, derived from Eqs. (3.19) in Sec.

3.2.4, as a function of excess air ratio with different biogas supply rates and different spark timings. It can be seen from these figures that CoVIMEP has a minimum value near stoichiometric point, where the engine performs the more stable IMEP during combustion with the higher work output, and it increases toward fuel-rich and fuel-lean regions.

When the engine is operated at fuel-rich or fuel-lean region, the fluctuaions of in-cylinder pressure become unsteady during combustion process. The CoVIMEP is also affected by the spark timing. The spark timing of BTDC13 provides a lower CoVIMEP than the advanced and delayed spark timing with the biogas supply rates of 220, 240 and 260L/min. In other words, the cycle-by-cycle in-cylinder pressure variations of spark timing BTDC13 are smaller than the other two spark auto-ignition of unburnt mixture gases due to the heat radiation from the flame front of spark plug. The in-cylinder pressure fluctuates severely because two flames develope in the cylinder at the same time. Besides, the advanced spark timing makes the knock of the engine easily. The knock oringinates in the extremely rapid release of abundant energy contained in the end-gas before the propagating flame, leading to a high

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local pressure. Such pressure generates shock waves propagating through the cylinder that may weaken the materials and cause the resonance of the cylinder at its natural frequency.

From Fig 4.13a~c and Fig 4.16a~c, it can be found that there are opposite trends between the CoVIMEP and the estimated CH4 consumption ratio. The relationship between the CoVIMEP and the estimated CH4

consumption ratio under different spark timings is shown in Fig 4.17. It reveals that the lower CoVIMEP, the in-cylinder pressure fluctuates more slightly, making a higher CH4 consumption ratio.

In summary, the lower CoVIMEP not only makes the consumption of CH4 better but also reduces the probability of knock in the cylinder. In addition, the delayed spark timing offers the lower CoVIMEP than the advanced spark timing.

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Table 4.4a Power Generation Rates as A Function of Excess Air Ratio at BTDC13 at Biogas Volume Flow Rate 220L/min

Biogas supply at 220L/min (BTDC13)

Air flow rate (L/min) 1847 1692 1482 1348 1211 Excess air ratio 1.23 1.12 0.98 0.90 0.80 Power generation (kW) 4.04 7.10 12.50 11.31 9.56 Thermal efficiency 0.166 0.167 0.182 0.185 0.179 Combustion efficiency 0.530 0.664 0.755 0.672 0.588 Fuel conversion efficiency 0.088 0.111 0.137 0.124 0.105 Waste gas temperature (°C) 495 498 512 507 505

Table 4.4b Power Generation Rates as A Function of Excess Air Ratio at BTDC13 at Biogas Volume Flow Rate 240L/min

Biogas supply at 240L/min (BTDC13)

Air flow rate (L/min) 1663 1463 1345

Excess air ratio 1.01 0.89 0.82

Power generation (kW) 15.13 14.76 13.68 Thermal efficiency 0.160 0.194 0.181 Combustion efficiency 0.950 0.767 0.761 Fuel conversion efficiency 0.152 0.149 0.138 Waste gas temperature (°C) 517.6 517 516

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Table 4.4c Power Generation Rates as A Function of Excess Air Ratio at BTDC13 at Biogas Volume Flow Rate 260L/min

Biogas supply at 260L/min (BTDC13)

Air flow rate (L/min) 1660 1586 1419

Excess air ratio 0.93 0.89 0.80

Power generation (kW) 16.06 15.76 14.53 Thermal efficiency 0.186 0.183 0.183 Combustion efficiency 0.828 0.799 0.737 Fuel conversion efficiency 0.154 0.146 0.135 Waste gas temperature (°C) 521.25 516 509

Table 4.4d Power Generation Rates as A Function of Excess Air Ratio at BTDC17 at Biogas Volume Flow Rate 220L/min

Biogas supply at 220L/min (BTDC17)

Air flow rate (L/min) 1806 1686 1491 1324 1215 Excess air ratio 1.20 1.12 0.99 0.88 0.81 Power generation (kW) 4.84 5.44 6.16 6.02 5.28 Thermal efficiency 0.123 0.147 0.152 0.147 0.133 Combustion efficiency 0.430 0.407 0.446 0.450 0.437 Fuel conversion efficiency 0.053 0.059 0.067 0.066 0.058 Waste gas temperature (°C) 469 472 466 456 456

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Table 4.4e Power Generation Rates as A Function of Excess Air Ratio at BTDC17 at Biogas Volume Flow Rate 240L/min

Biogas supply at 240L/min (BTDC17)

Air flow rate (L/min) 1735 1667 1470 1307

Excess air ratio 1.06 1.02 0.89 0.80

Power generation (kW) 7.58 8.5 7.6 7.18

Thermal efficiency 0.140 0.155 0.153 0.153 Combustion efficiency 0.545 0.552 0.501 0.471 Fuel conversion efficiency 0.076 0.085 0.076 0.072 Waste gas temperature (°C) 475 475.6 477 463

Table 4.4f Power Generation Rates as A Function of Excess Air Ratio at BTDC17 at Biogas Volume Flow Rate 260L/min

Biogas supply at 260L/min (BTDC17)

Air flow rate (L/min) 1689 1580 1449

Excess air ratio 0.95 0.89 0.81

Power generation (kW) 8.78 7.98 7.42 Thermal efficiency 0.147 0.139 0.131 Combustion efficiency 0.553 0.534 0.526 Fuel conversion efficiency 0.081 0.074 0.069 Waste gas temperature (°C) 478 479 467

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Table 4.4g Power Generation Rates as A Function of Excess Air Ratio at BTDC9 at Biogas Volume Flow Rate 220L/min

Biogas supply at 220L/min (BTDC9)

Air flow rate (L/min) 1749 1642 1470 1340 1200 Excess air ratio 1.16 1.09 0.98 0.89 0.80 Power generation (kW) 1.92 2.6 4.14 2.96 2.2

Thermal efficiency 0.058 0.071 0.103 0.080 0.063 Combustion efficiency 0.362 0.401 0.440 0.407 0.383 Fuel conversion efficiency 0.021 0.028 0.045 0.032 0.024 Waste gas temperature (°C) 540 566 569 563 540

Table 4.4h Power Generation Rates as A Function of Excess Air Ratio at BTDC9 at Biogas Volume Flow Rate 240L/min

Biogas supply at 240L/min (BTDC9)

Air flow rate (L/min) 1740 1645 1434 1316

Excess air ratio 1.06 1.00 0.87 0.80

Power generation (kW) 4.66 5.94 7.38 6.98

Thermal efficiency 0.080 0.104 0.142 0.127 Combustion efficiency 0.582 0.576 0.524 0.553 Fuel conversion efficiency 0.047 0.060 0.074 0.070 Waste gas temperature (°C) 570 570 561.8 560

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Table 4.4i Power Generation Rates as A Function of Excess Air Ratio at BTDC9 at Biogas Volume Flow Rate 260L/min

Biogas supply at 260L/min (BTDC9)

Air flow rate (L/min) 1630 1591 1428

Excess air ratio 0.92 0.89 0.80

Power generation (kW) 9.6 9.14 8.9

Thermal efficiency 0.139 0.134 0.133 Combustion efficiency 0.639 0.633 0.620 Fuel conversion efficiency 0.089 0.085 0.082 Waste gas temperature (°C) 587 581 570

Figure 4.18 shows comparison of the thermal efficiency between Lin’s[4], Huang’s[5] researches and this study with biogas supply rate of 200L/min.

In the research of Lin[4], he used a 30kW-generator by fueling the biogas with 60% of CH4 without removing the water vapor in biogas. The highest thermal efficiency with biogas supply rate at 200, 220 and 240L/min are 0.231, 0.243 and 0.283 respectively. Huang [5] also operated the same generator as Lin[4], and she used 73% of CH4 without removing the water vapor in biogas. The highest thermal efficiency with biogas supply rate at 200, 220 and 240L/min are 0.27, 0.25 and 0.254, respectively. In this study, the corresponding maximum thermal efficiencies are 0.285, 0.298 and 0.304 with biogas supply rate at 200,