Chapter 3 Experimental Apparatus and Procedures
3.4 The Effect of Spark Timing
The spark timing adjustment is an important parameter for engine performance. The optimum spark timing gives a maximum brake-torque, and leads to the maximum power output. In this study, the maximum power output of spark timing can be found. The advance or delay from the optimum spark timing lead to improper performance of engine.
The experimental parameters are spark-timing, biogas flow rate and excess air ratio. The biogas flow rates are 220, 240 and 260 L/min, and excess air ratios are ranged from 0.8 to 1.2. The optimum spark timing is adjusted in this study. Besides, the advance and delay of the optimum spark timing are investigated as well. At each specific spark timing, it tests different biogas flow rates and each flow rate is accompanied with different excess air ratios. The collected data include biogas flow rate, air flow rate, resultant power generation, pressure of in-cylinder, and concentrations of methane, oxygen, carbon dioxide and NOx. The experimental procedure is as follows:
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1. Dehumidify water vapor of intake biogas.
2. Measure the relative humidity, temperature and pressure of biogas.
3. Measure the intake biogas constitutes and their concentrations.
4. Operate the engine at least 20 minutes for warm up.
5. Control the spark timing at a fixed degree.
6. Fix the biogas flow rate at demanded quantity.
7. Control the air flow rate at specific excess air ratio.
8. Collect the corresponding data, mentioned above.
9. Repeat the procedure for different excess air ratio.
10. Repeat the above procedure for different spark timing.
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Chapter 4
Results and Discussion
This study is a continuous effort of Lin’s[4] and Huang’s[5] works. It carries out with three major modifications, which are the measurements of the detailed intake biogas constitutes and their concentrations, dehumidification of the water vapor in intake biogas and installation of the complete ignition system, consisting spark-plug pressure sensor, and rotary encoder to record the crank angle of piston cylinder.
The biogas used in this research was supplied from the anaerobic tank made of red plastic bag. The original biogas from the tank contains high concentration of H2S, around 4000ppm. It would corrode the engine severely if without proper treatment. Therefore, an H2S removal system, was built up by using biological process, which is environment and cost friendly. The removal rate of screened micro-organism could remove H2S of biogas up to 99%. In other words, the H2S concentration in the biogas was effectively reduced from 4000ppm to 50ppm.
4.1 Effect of Water Vapor of Intake Fuel
The desulfurized biogas passed a methane concentration analyzer, temperature with humidity transmitter and gas analyzer that the concentrations of CH4, O2, and CO2, temperature and relative humidity of biogas can be measured. From the measurements, it found that the biogas comprised O2. It is impossible for the biogas containing O2 after the anaerobic process, so the existence of O2 must be from the air, leaking from atmosphere to the storage tank. According to the concentration of O2, the corresponding N2 concentration can be deduced. In addition, the water
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vapor of biogas with and without dehumidification can be derived by using Eq. (3.2) in Sec. 3.2.1. Since the temperature of intake biogas was 30°C, therefore, the relative humidities of intake biogases without and with dehumidification were 85.2% and 52.7%, respectively. Table 4.1 shows the details of biogas compositions.
Table4.1 Compositions of Biogas with and without Dehumidification without Dehumidification with Dehumidification
CH4 72% 72.2%
CO2 18.6% 17.8%
O2 1.09% 1.39%
N2 4.1% 5.23%
H2O 3.14% 1.9%
Residues 1.07% 1.48%
From above table, there are two kinds of biogases due to an addition of dehumidifier (Sec. 3.1.4). Therefore, the respective stoichiometric air-fuel ratios based on the measured or deduced data are 5.57 (without dehumidification) and 5.31 (with dehumidification). Note that the maximum allowable total volume flow rate (sum of biogas and air flow rates) into the engine is about 2000L/min, therefore, the maximum air supply rate is limited by the biogas one. In other words, the experiments with the higher biogas flow rates carried out with a narrower range of air flow supply rates. So it restricted the maximum excess air ratio for each biogas supply rate.
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Figure 4.1 is the power generation rates as a function of excess air ratio with and without dehumidification. The detailed experimental values are given in the Table 4.2 a~f. In these tables, the measured results include the power generation, waste gas temperature and waste gas species concentrations (O2, CO2 and NOx). Besides, the thermal efficiency deduced from measurements, described in Sec.3.2.3, is provided as well.
It can be seen from Fig. 4.1 that the power outputs of dehumidified biogas are higher than those without dehumidification. Apparently, the power generations by dry intake biogas are better than those by untreated one. In general, the maximal power outputs occur at the excess air ratio approximately equal to 1.0 (stoichiometric condition). The maximum power outputs of biogas supply at 200, 220 and 240L/min after temperature. It is because more heat can be released during combustion as the biogas supply rate increases. The exhaust temperatures of dry biogas are higher than those of wet biogas at a specified excess air ratio. Also the maximum waste gas temperatures for the different biogas flow rates occur at λ~1.00.
There are two main reasons that power output is increased after dehumidification. First, from the composition of biogas (see Table 4.1.),
36 concentration and the avertion of energy absorption by water vapor, then the biogas flow enthapy increasing rate can be defined as:
(4.1) where , and can be measured and they refer to biogas flow rate, exhaust gas temperature and intake biogas temperature respectively.
Thus, the biogas flow enthapy increase rates of biogas supply at 200, 220 and 240L/min after dehumidification with an excess air ratio of 1.00 are 0.79%, 1.17% and 1.27% respectively. Obviously, the higher biogas supply flow provides more enthapy after the dehumidification.
Figures 4.3a~c show the thermal, fuel conversion and combustion efficiencies as a function of excess air of 240, 220 and 200L/min biogas supply rates with and without dehumidification. These are deduced by Eqs. (3.12), (3.13) and (3.14) given in section 3.2.3. It can be seen from these three figures that the thermal efficiency is higher than fuel conversion efficiency. This is because not all the fuel energy supplied to the engine is released by the combustion process since the combustion usually is incomplete. Therefore, it is necessary to cosider the combustion efficiency for evaluating the thermal efficiency. The average of combustion efficiency of the engine is about 0.85. In other words, about
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15% of energy is lost by the form of heat during combustion process.
Besides, it is obvious that the dry intake biogas has a better fuel conversion efficiency and thermal efficiency than the wet one at each biogas flow rate. However, the maximum power output corresponding to thermal efficiency for the biogas supply rate at 200 L/min and 220 L/min do not locate at λ=1.0. This is because the maximal fuel conversion efficiencies, displayed in Fig 4.3b and 4.3c, for both biogas supply rates are higher at λ=1.2 rather than at λ=1.0.
According to these data, it might conclude that the the engine can produce greater power and higher thermal efficiency by removing the water vapor in the intake biogas.
Figures 4.4, 4.5 and 4.6 are the O2 , NOx and CO2 concentrations in the exhaust gas at different biogas supply rates as a function of excess air ratio with and without dehumidification. These data are also listed in the row 8, 9 and 10 in Table 4.1 a~f as well. Figure 4.4 shows that O2
concentration in waste gas increases with increasing excess air ratio, because more O2 is left during combustion as the air is over supplied. In Fig. 4.5, NOx concentration reaches to a peak value in the range around λ=0.9~1.1 (near stoichiometric condition), coincident with the maximum waste gas temperature in Fig. 4.2. It indicates that the main source of NOx
is formed through high temperature oxidation of N2 in the air during combustion. Generally speaking, CO2 concentration in Fig. 4.6 decreases with excess air ratio when λ>0.9. All of them have a peak values at λ=0.9, except for the case 240L/min with dehumidification. Note that the dry and wet biogases have already contained about 18% of CO2, the extra CO2 is from the combustion. When the combustion is more completed,
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the generated CO2 can outweigh the dilution effect by other combustion product gases, leading to a peak appearance near the stoichiometric condition.
This study applies the measured O2 data in the waste gas to estimate the corresponding CO2 concentration and to calculate the mole number of waste gas compositions during calculation of combustion efficiency. The estimated CO2 is derived by using Eq. (3.10) given in section 3.2.2. The corresponding estimated CO2 concentrations are presented in the last row of Table4.2a~f. The maximum discrepancy of CO2 concentration between the estimations and the ones measured by the gas analyzer is within 5%, showing that both agree quite well.
To sum up, it can conclude that the dehumidified biogas provides up to 1.17% extra enthalpy and enhances the power output up to 5.9% with respective to the humid biogas at biogas supply rate of 220L/min at λ=1.0.
Besides, the fuel conversion efficiency and thermal efficiency of dehumidified biogas are higher than the ones without dehumidifying.
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Table 4.2a Power Generation Rates as A Function of Excess Air Ratio without Dehumidification at Biogas Volume Flow Rate
200L/min
Biogas supply at 200L/min (without dehumidification)
Air flow rate (L/min) 1359 1236 1128 1024 957 Excess air ratio 1.21 1.10 1.01 0.91 0.86 Power generation (kW) 19.53 20.30 20.59 17.59 14.11
Thermal efficiency 0.285 0.274 0.267 0.226 0.212 Combustion efficiency 0.825 0.870 0.848 0.903 0.771 Fuel conversion efficiency 0.235 0.239 0.226 0.204 0.163 Waste gas temperature (°C) 482 484.6 505.7 491.4 477.4
Table 4.2b Power Generation Rates as A Function of Excess Air Ratio with Dehumidification at Biogas Volume Flow Rate 200L/min Biogas supply at 200L/min (with dehumidification)
Air flow rate (L/min) 1306 1250 1076 938 839 Excess air ratio 1.22 1.17 1.01 0.88 0.79 Power generation (kW) 20.51 21.24 21.55 18.33 13.15
Thermal efficiency 0.274 0.275 0.276 0.251 0.223 Combustion efficiency 0.897 0.905 0.860 0.843 0.681 Fuel conversion efficiency 0.246 0.249 0.237 0.212 0.152 Waste gas temperature (°C) 484 498 509.5 497.4 487
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Table 4.2c Power Generation Rates as A Function of Excess Air Ratio without Dehumidification at Biogas Volume Flow Rate
220L/min
Biogas supply at 220L/min (without dehumidification)
Air flow rate (L/min) 1540 1335 1233 1096 986 Excess air ratio 1.25 1.08 1.00 0.89 0.80 Power generation (kW) 22.10 22.96 23.41 19.71 15.25
Thermal efficiency 0.286 0.298 0.295 0.244 0.241 Combustion efficiency 0.847 0.827 0.821 0.849 0.847 Fuel conversion efficiency 0.243 0.247 0.242 0.207 0.161 Waste gas temperature (°C) 493.5 498.2 515.8 505.4 500.3
Table 4.2d Power Generation Rates as A Function of Excess Air Ratio with Dehumidification at Biogas Volume Flow Rate 220L/min Biogas supply at 220L/min (with dehumidification)
Air flow rate (L/min) 1360 1263 1178 1007 942 Excess air ratio 1.16 1.08 1.00 0.86 0.80 Power generation (kW) 23.74 24.33 24.78 19.90 16.96
Thermal efficiency 0.313 0.312 0.302 0.256 0.252 Combustion efficiency 0.832 0.820 0.825 0.817 0.707 Fuel conversion efficiency 0.261 0.256 0.250 0.209 0.178 Waste gas temperature (°C) 501 506.4 521.5 506 504
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Table 4.2e Power Generation Rates as A Function of Excess Air Ratio without Dehumidification at Biogas Volume Flow Rate
240L/min
Biogas supply at 240L/min (without dehumidification)
Air flow rate (L/min) 1481 1336 1195 999
Excess air ratio 1.10 0.99 0.89 0.74
Power generation (kW) 25.54 25.65 22.95 16.46 Thermal efficiency 0.266 0.304 0.274 0.220 Combustion efficiency 0.926 0.815 0.810 0.723 Fuel conversion efficiency 0.248 0.247 0.222 0.159 Waste gas temperature (°C) 517.9 524.7 518.8 512.9
Table 4.2f Power Generation Rates as A Function of Excess Air Ratio with Dehumidification at Biogas Volume Flow Rate 240L/min Biogas supply at 240L/min (with dehumidification)
Air flow rate (L/min) 1400 1296 1159 1000
Excess air ratio 1.09 1.01 0.90 0.78
Power generation (kW) 26.05 26.35 24.89 19.71 Thermal efficiency 0.277 0.309 0.311 0.245 Combustion efficiency 0.907 0.822 0.770 0.774 Fuel conversion efficiency 0.251 0.254 0.240 0.190 Waste gas temperature (°C) 520 531 525.8 516
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4.2 Addition of Ignition System
In this study, the ignition system is installed on the engine to enhance its performance and combustion stability. The detailed compositions of biogas tested are shown in Table4.3. Remind that the biogas contains air and the water vapor has already dehumidified before it is sucked into the engine. The intake temperature of biogas is 27°C with a relative humidity of 53%.
Table4.3 Composition of Biogas with Dehumidification
CH4 CO2 Air H2O Residues
69% 13.3% 12.38% 1.99% 3.33%
The detailed collected and deduced data are listed in Table 4.4 a~i. In these table, under each fixed biogas flow rate, the combustion stability (CoVIMEP), derived from Eq. (3.19) in Sec. 3.2.4, is calculated by the measured combustion pressure during 200consecutive cycles. In addition, the metal tubes are installed on the top of cylinders in order to set up the spark-plug pressure sensor. The optimum spark timing gives the maximum brake-torque, and leads to the maximum power output. It is found that the spark timing for maximum power output is 13 degree (BTDC13) before top-dead-center. Moreover, the results of advance and delay of the best spark timing are provided as well.
4.2.1 Power Generation, Thermal Efficiency and Waste Gas Analysis The power outputs with different excess air ratios are shown in Fig 4.7a~c for the different spark timings. The excess air ratio for 260L/min
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biogas suply rate cannot reach 1.0 because the limitation of maximum allowable total volume flow rate into the engine, mentioned previously.
There is a drop on the fuel rich side except the biogas supply rate at 240L/min with spark timing of BTDC9, whose peak locates at λ=0.9.
Note that the biogas supply rate at 260L/min with spark timing of BTDC9 provides a better power output than BTDC17; as the biogas supply rate decreases, the spark timing of BTDC17 indicates greater power output than BTDC9. There are two factors, the density of mixture gases and the burning angle, to affect the relationship between the biogas supply rate and spark timing. The advanced spark timing has a longer duration of the overall burning process, but the delayed spark timing provides a smaller burning angle due to the exhaust valve opening. On the other hand, the delayed spark timing supplies a higher mixture gases density, whereas the mixture gases density of advanced spark timing is smaller due to the longer stroke. So the effect of mixture gases density is larger than the one of burning angle with biogas supply rate at 260L/min. As the biogas supply rate decreases, the effect of burning angle is more obvious than the effect of mixture gases density.
In addition, both the faster and slower ones than the optimum spark timing of BDTC13 lead to the decrease of power output. This is because the mixture gases, intake biogas and intake air, are not pushed into a proper position in the piston to ignite. The work transfer from the pistion to the mixture gases in the cylinder at the end of the compression stroke is too large if the combustion starts too early in a cycle. It results in auto-ignition of unburnt mixture gases due to the heat radiation from the flame front of spark plug. In contrast, if the spark timing starts too late,
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the peak in-cylinder pressure is reduced and the expansion stroke work transferring from the mixture gases to the piston decreases.
Figures 4.8a~c show the thermal efficiency for the different biogas supply rates as a function of excess air ratio with different spark timings.
The spark timing of BTDC13 provides better thermal efficiency than the others, whose thermal efficiencies are below 0.155. It can be seen from these firgues that the spark timing of BTDC17 has a better thermal efficiency than that of BTDC9, the increasing rate is more obvisouly as the biogas supply rate is decreased.
The waste gas temperature for the different biogas supply rates as a function of excess air ratio with different spark timings is shown in Fig 4.9. It reveals that the maximum temperature occurs around the stoichiometric point (λ=0.9~1.0) and it is decreasing toward the fuel-rich and fuel-lean regions for the each different spark timing. Futhermore, the exhaust temperature is also affected by the different spark timings. The
Therefore, the waste gas temperature is smaller than the others due to the heat losses.
The waste gas concentrations, including O2, NOx and CO2, are shown in Figs. 4.10, 4.11 and 4.12, respectively. These data are also presented in row 8, 9 and 10 in Table 4.4 a~i. In Fig. 4.10, the mixture gas of spark timing at BTDC13 leaves less O2 concnetration in the waste gasthan the
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others under the same biogas supply rate. Apparently, BTDC13 is the optimal spark timing, so the exhaust gas will contain lower levels of O2.
In Fig 4.11, the NOx concentration reaches to a peak value in the range near the stoichiometric condition. The spark timing at BTDC9 generate less NOx than the others under the same biogas supply rate. The spark timing significantly affects NOx emission level. This is because the higher peak in-cylinder pressures results in a higher peak burnt gas temperatures; therefore, the higher NOx are generated. Advancing the spark timing so that combustion occurs earlier in the cycle increases the peak in-cylinder pressure, because more biogas is burnt before top-dead-center, and the peak pressure comes closer to top-dead-center as the cylinder volume becomes smaller. On the contrary, the delayed spark timing reduces the peak in-cylinder pressure because more biogas burns after top-dead-center as the cylinder volume becomes larger. So, the delayed spark timing can decrease the NOx emission. Figure 4.12 shows the CO2 concentrations in waste gas for the different biogas supply rates as a function of excess air ratio with different spark timings. Apparently, the spark timing BTDC13 provides higher CO2 concentration than the delayed and advanced ones. The combustion becomes more completed as the spark timing of engine operates at BTDC13 or operates near the stoichiometric ratio, therefore, more CO2 is generated.
Figures 4.13a~c show the estimated CH4 consumption ratios as a function of excess air ratio with different biogas supply and different spark timings. It can be found that the maximum consumption ratio occurs in the neighborhood of stoichiometric condition (0.9<λ<1.1), then it decreases toward both the fuel-rich and fuel-lean regions. In addition,
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CH4 consumption ratio is higher when the engine operates at optimal spark timing with a fixed biogas flow supply. Comparinh with the advanced spark timing, the delayed spark timing performs higher CH4
consumption ratios.
To sum up, the optimum spark timing of the present engine locates at BTDC13, where provide the highest power generation, thermal efficiency and CH4 consumption ratios; delaying or advancing the spark timing leads to a poorer power outputs. Note that the delayed spark ignition from the optimum one can reduce NOx emission. Besides, delaying timing increases the waste gas temperture, and the heat losses to the combustion chamber wall are decreased.
4.2.2 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
When the engine is operated at fuel-rich or fuel-lean region, the fluctuaions of in-cylinder pressure become unsteady during combustion