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Performance and Emission Characteristics of a Spark-Ignition (SI) Hydrogen-Enriched Compressed Natural Gas (HCNG) Engine Under Various Operating Conditions Including Idle Conditions Fanhua Ma,* Shangfen Ding, Yefu Wang, Mingyue Wang, Long Jiang, Nashay Naeve, and Shuli Zhao State Key Laboratory of Automobile Safety and Energy, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed February 19, 2009. ReVised Manuscript ReceiVed April 14, 2009
An experimental study focusing on the effects of different hydrogen enhancement levels on the engine performance and emission characteristics was conducted. The experimental data were compiled on a turbocharged lean-burn natural gas spark-ignition engine operated under various conditions, including idle. Data were taken for pure natural gas operation as well as hydrogen-enriched compressed natural gas (HCNG), including hydrogen volumetric fractions of 20, 30, and 40%. For comparison purposes, the lower heating value for each hydrogen/ natural gas ratio remained identical at each engine-operating condition. The results show that, under idle conditions, hydrogen addition can reduce the output of the engine of hydrocarbon and carbon monoxide emissions, which are the two largest problems associated with idle operation. It is also found that this reduction is more obvious at relatively delayed spark timings. Under normal operating conditions, hydrogen addition was found to be beneficial to simultaneously improving fuel economy and reducing HC emissions. Although hydrogen addition increases NOx emissions, this can be compensated for by retarding the spark timing and running leaner operation, which are allowed by the faster burning velocity and broader ignition limit of hydrogen.
1. Introduction Being considered a near perfect energy carrier, hydrogen has potential to supplement or replace traditional hydrocarbon fuels in the future. Hydrogen is currently a scarce commodity compared to hydrocarbon fuels, which is in part due to the lack of distribution infrastructure. Currently, the use of hydrogen is likely limited to the role of a fuel additive. At least in the short term, natural gas vehicles are good alternatives to gasoline vehicles because of the lower emission levels and economic feasibility. In an effort to further reduce pollutants, some natural gas vehicles have been run on hydrogen-enriched compressed natural gas (HCNG). When it comes to performance, HCNG combines the advantages of both hydrogen and natural gas. Hydrogen is an excellent additive to natural gas because of its unique characteristics, including the laminar burning velocity of hydrogen, which is nearly 7 times that of natural gas. This effect allows hydrogen addition to increase the burning velocity of the fuel blend. Higher burning velocity causes shorter combustion duration and a greater degree of constant volume, which results in higher efficiency. Research focusing on HCNG engine applications is being conducted all over the world.1-6 * To whom correspondence should be addressed: State Key Laboratory of Automobile Safety and Energy, Tsinghua University, Beijing 100084, People’s Republic of China. Telephone: +86-10-62785946. Fax: +86-1062785946. E-mail:
[email protected]. (1) Wang, J. H.; Huang, Z. H.; Fang, Y.; Liu, B.; Zeng, K.; Miao, H. Y.; Jiang, D. M. Combustion behaviors of a direct-injection engine operating on various fractions of natural gas-hydrogen blends. Int. J. Hydrogen Energy 2007, 32, 3555–3564. (2) Ma, F. H.; Wang, Y.; Liu, H. Q.; Li, Y.; Wang, J. J.; Zhao, S. L. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int. J. Hydrogen Energy 2007, 32, 5067–5075.
Wang et al. studied the combustion behaviors on a directinjection engine fueled with HCNG.1 The results show that the brake effective thermal efficiency increases with an increased percentage of hydrogen at low and medium load. Also, the rapid combustion duration decreases, while the heat release rate and exhaust NOx increase with an increased percentage of hydrogen. This study suggests that the optimum hydrogen volumetric fraction in natural gas/hydrogen blends be around 20%, which results in the best overall combination of emissions and engine performance. Wang et al. also investigated cycle-by-cycle variations of a spark-ignition engine fueled with natural gas/ hydrogen blends.3 The results show that cycle-by-cycle variations reduce with an increased percentage of hydrogen at leanburn operation. Collier et al.4 carried out an experimental study on the untreated exhaust emissions of a hydrogen-enriched CNG production engine. It was concluded that hydrogen addition increases NOx and reduces HC emissions and was beneficial to alleviate the trade-off relationship between them. It was also observed in this study that the combustion stability improves because of the addition of hydrogen to the fuel. Ortenzi et al. compared experimental investigation on the emission and control strategies on an IVECO daily CNG (3) Wang, J. H.; Chen, H.; Liu, B.; Huang, Z. H. Study of cycle-bycycle variations of a spark ignition engine fueled with natural gas-hydrogen blends. Int. J. Hydrogen Energy 2008, 33, 4876–4883. (4) Collier, K.; Hoekstra, R. L.; Mulligan, N.; et al. Untreated exhaust emissions of a hydrogen-enriched CNG production engine conversion. SAE Tech. Pap. 960858, 1996. (5) Ortenzi, F.; Chiesa, M.; Scarcelli, R.; Pede, G. Experimental tests of blends of hydrogen and natural gas in light-duty vehicles. Int. J. Hydrogen Energy 2008, 33, 3225–3229. (6) Nagalinyam, B.; Duebel, F.; Schmillen, K. Performance study using natural gas, hydrogen-supplemented natural gas and hydrogen in AVL research engine. Int. J. Hydrogen Energy 1983, 8 (9), 715–720.
10.1021/ef900144s CCC: $40.75 2009 American Chemical Society Published on Web 05/01/2009
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vehicle.5 There were three typical fuels chosen, including pure CNG and two different HCNG blend ratios (CNG/hydrogen volumetric ratios of 90:10 and 85:15). At ECE15 driving cycle operating conditions, it was concluded that the control strategies for the CNG vehicle must be modified to adapt the new HCNG fuels. The relationship between emission and spark timing using pure CNG, HCNG blends (CNG/hydrogen volumetric ratios of 80:20 and 50:50), and pure hydrogen was studied by Nagalingam in 1983. The result was that the maximum power output of the engine fueled by HCNG was less than that fueled by CNG because of the smaller volumetric lower heating value of the hydrogen. The maximum brake torque (MBT) using HCNG was less than that of CNG, which was caused by the high burning velocity of HCNG fuel.6 It can be concluded that retarding the spark timing is effective at reducing NOx emission.7 In 1990, in a study conducted by Yusuf, a four-cylinder engine was used to test the equivalence air/fuel ratio and the lean-burn limit using a HCNG blend with a 20% volume of hydrogen. The equivalent air/fuel ratio with HCNG fueling increased from 1.29 to 1.67 at the best thermal efficiency of the engine.8 In 1993, Yusuf found that, by using HCNG fuel, NOx emission was higher compared to that of CNG fuel but both HC and CO emissions were lower and the lean-burn limit of the engine at an equivalent air/fuel ratio increased from 1.64 to 1.85.9 In 1994, using a gasoline engine, Hoekstra researched HCNG fuel (CNG/HCNG volume ratios of 89:11, 80:20, 72:28, and 64:36).10,11 The conclusion was reached that NOx emission increases but HC emission decreases with the increase of the excess air ratio. Another conclusion reached was that NOx emission using HCNG with 28 and 36% hydrogen decreases considerably when the excess air ratio was 1.6. Sierens and Rosseel studied the emission characteristics of HCNG (CNG/HCNG volume ratios of 90:10 and 80:20) in 2000.12 This study concluded that only HCNG with a low percent of hydrogen could have low-emission characteristics and that after treatment must be taken to gain ultra-low emission levels. Huang and Wang performed the study on HCNG fuel (CNG/ HCNG volume ratios of 95:5, 90:10, and 82:18) using a directinjection engine in 2006.13 It was concluded that ignition timing has a significant influence on the performance of the directinjection engine and that HC emissions decrease and NOx emissions increase when the ignition timing was gradually (7) Huang, Z. H.; Wang, J. H. The review of research on using hydrogen in combustion engine. State Key Laboratory of Power Engineering, Xi’an Jiaotong University, China, 2005; p 6. (8) Yusuf, M. J. Cylinder flame front growth rate measurement of methane and hydrogen enriched methane fuel in a spark ignited internal combustion engine. University of Miami, Miami, FL, 1990. (9) Yusuf, M. J. Lean burn natural gas fueled engines: Engine modification versus hydrogen blending. University of Miami, Miami, FL, 1993. (10) Hoekstra, R. L.; Collier, K.; Mulligan, N. Demonstration of hydrogen mixed gas vehicles. 10th World Hydrogen Energy Conference, Cocoa Beach, FL, June 20-24, 1994. (11) Hoekstra, R. L.; Collier, K.; Mulligan, N.; Chew, L. Experimental study of a clean burning vehicle fuel. Int. J. Hydrogen Energy 1995, 20 (9), 737–745. (12) Sierens, R.; Rosseel, E. Variable composition hydrogen/natural gas mixtures for increased engine efficiency and decreased emissions. J. Eng. Gas Turbines Power 2000, 122 (1), 135–140. (13) Huang, Z. H.; Wang, J. H. Combustion characteristics of a directinjection engine fueled with natural gas-hydrogen blends under different ignition timings. Fuel 2007, 86, 381–387. (14) Munshi, S. R.; Nedelcu, C.; Harris, J.; et al. Hydrogen blended natural gas operation of a heavy duty turbocharged lean burn spark ignition engine. SAE Tech. Pap. 2004-01-2956, 2004.
Ma et al. Table 1. Engine Specifications item
value
displacement volume (L) compression ratio bore (mm) stroke (mm) rating power rating torque
6.234 10.5 105 120 154 kW/2800 620 N m/1600
advanced. The CO emission levels gave little variation under various ignition timings. 2. Experimental Section The engine used in this experimental study was an inline sixcylinder turbocharged engine, which is used in full-size passenger vehicles. The engine specifications are shown in Table 1. The engine was coupled to an eddy-current dynamometer to gain engine and load measurements and control. Engine control management was carried out with an ITMS-6F control system (DELPHI, Inc.), which provides access to all calibration parameters. The ITMS-6F system allows the user to set a desired equivalence ratio and spark advance of the engine. The air/fuel ratio measurement was performed by a HORIBA wide-range λ analyzer, and the exhaust concentration of methane hydrocarbons (MHC), NOx, and H2 were measured by a MRU SWG 300-1 emission monitoring system manufactured by MRU GmbH of Germany. Considering that the fuel used is gaseous and that the volumetric lower heating value of the fuel may vary with different hydrogen fractions, the engine speed and lower heating value of the input fuel were fixed rather than employing the conventional strategy of fixing engine speed and load. By fixing the engine speed and the lower heating value, it is easier to investigate the effects of different hydrogen fractions on the performance and emission characteristics of the engine using identical excess air ratios, spark advances, and fuel energy (the lower heating value of the fuel) for each cycle. To obtain identical lower heating values for each fuel blend ratio at idle conditions, the engine speed was fixed at 800 rpm without the automatic idling control device. The throttle was closed completely, and the mixture entering the engine was controlled by an idle bypass valve, also controlling the idle speed. If the engine is running too rich or too lean, it cannot maintain idle speed even if the idle bypass valve is totally open; therefore, λ was chosen as 1.1, which is controlled by the engine control system. In the online hydrogen/natural gas mixing system, the hydrogen flow was accurately controlled according to the CNG flow and the required hydrogen/natural gas volume ratio. On the basis of the lower heating value of pure CNG, the CNG and hydrogen flows are determined at the required hydrogen/natural gas volume ratio. The idle bypass valve is artificially adjusted to maintain the required CNG flow and idle speed of 800 rpm. Three different engine speeds (800, 1200, and 2400 rpm) were used in the test to investigate the effect of hydrogen addition on engine performance under various engine speeds. It should be noted that, under each operating condition of the engine, the lower heating value of the fuel injected during each cycle is always equal to that of pure CNG.
3. Results and Discussion 3.1. Engine Speed of 800 rpm (Simulation of Idle Operation). To simulate idle operation, the throttle of the engine was set completely closed at 800 rpm and the intake air was completely controlled by the bypass motor. The excess air ratio remained unchanged at 1.1, because when the excess air ratio is below 1.1, the engine cannot reach steady operation if the fuel is too lean. Figure 1 shows the engine torque output for different types of fuel. From Figure 1, it can be observed that hydrogen addition
Characteristics of a SI HCNG Engine
Figure 1. Torque output versus ignition timing at 800 rpm.
Figure 2. Brake-specific methane emissions versus ignition timing at 800 rpm.
can improve the torque output at relatively retarded spark timing. However, the positive effect of hydrogen addition on torque output becomes less and less visible as the delay in spark timing increases and around 20 °CA before top dead center (BTDC). As stated before, in comparison to natural gas, HCNG increases the burning speed of the charge in the combustion chamber of the engine. This results in an improvement of the torque output of the engine at relatively delayed spark timings. Also, as seen in Figure 1, the difference in torque output for different hydrogen fractions (excluding 0%) is quite small. Because the input energy of the fuel during each cycle is the same, higher torque output also results in lower brake-specific fuel consumption (BSFC). Figure 2 shows the specific brake emission of methane for different blend ratios of HCNG fueling versus spark timing. It is clear that the addition of hydrogen decreases the methane emission significantly, especially under overdelayed and overadvanced spark timings. Higher hydrogen/natural gas ratios contribute to lower HC emission, and hydrogen shortens the quenching distance and makes combustion much more complete. In the case of pure CNG, emission of methane becomes extremely high at over-retarded and overadvanced spark timings, which is due to the combustion deterioration under these conditions. On the contrary, HCNG is less dependent upon spark timing because of the ability of hydrogen to improve the quality of combustion at unfavorable spark timing. This gives HCNG a sharp edge against CNG at idle operation for much lower methane emission because the spark timing should be varied to keep a stable idle speed. Also, it can be seen that, at over-retarded spark timings, the emission of methane decreases with the increased ratio of hydrogen.
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Figure 3. Brake-specific CO emissions versus ignition timing at 800 rpm.
Figure 4. Brake-specific NOx emissions versus ignition timing at 800 rpm.
Figure 3 shows that the brake-specific CO emission decreases with the increased hydrogen ratio at over-retarded spark timing, which could be explained by higher hydrogen/natural gas ratios and the improved combustion quality caused by the addition of hydrogen. NOx emission verses ignition timing was explicitly plotted in Figure 4. NOx emissions increased as the hydrogen fraction increased at a spark advance greater than 10 °CA BTDC, which is caused by the elevated flame temperature in the cylinder. As the spark timing was retarded, the flame temperature in the cylinder gradually dropped, which resulted in fewer NOx emissions. Delaying spark timing is an effective way to decrease the NOx emission of a HCNG-fueled engine. NOx-specific emission bounced up again because of the increased engine power loss when the spark advance was less than 5 °CA BTDC. When the ignition timing is set too close to TDC, the fuel cannot burn completely and the combustion process mainly takes place in the expansion stroke with a relatively low-pressure environment. This greatly decreases the power output to reflect the increasing brake-specific NOx emission. Hydrogen addition sustains relatively stable power output while reducing the brakespecific emissions. This is caused by the excellent combustion characteristics of hydrogen. 3.2. Engine Speeds of 1200 and 2400 rpm. The operating conditions used in this section are 100 and 85 kPa manifold absolute pressure, which was achieved by adjusting throttle position for 1200 and 2400 rpm and setting the excess air ratio to 1.3 and 1.5, respectively.
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Figure 5. Torque output versus ignition timing at λ ) 1.3.
Figure 6. Torque output versus ignition timing at λ ) 1.5.
Figure 7. BSFC versus ignition timing at λ ) 1.3.
From Figures 5 and 6, it can be observed that, with identical fuel energy and equivalent excess air ratios, as the MBT
Figure 8. BSFC versus ignition timing at λ ) 1.5.
approaches TDC, the torque output increases. This can be explained by the fact that hydrogen addition increases the
Characteristics of a SI HCNG Engine
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Figure 9. Brake-specific methane emissions versus ignition timing at λ ) 1.3.
Figure 10. Brake-specific methane emissions versus ignition timing at λ ) 1.5.
Figure 11. Brake-specific NOx emissions versus ignition timings at λ ) 1.3.
burning speed of the flame causing the real engine cycle to be similar to the ideal constant volume cycle, thus improving the thermal efficiency of the engine. The torque drop caused by retarded spark timing was relatively smaller in the case of HCNG fueling compared to that in the case of CNG fueling, making it possible to decrease NOx by retarding spark timing in a HCNG-fueled engine. The lower heating values for both CNG and HCNG injected during each cycle were equal, therefore causing the fuel with higher torque output to have lower BSFC. Figures 7 and 8 show the BSFC with HCNG and CNG fueling. In accordance with the lower heating value equivalent transformation rules, the mass of H2 in the HCNG fuel can be converted to a CNG mass with an equal lower heating value; this mass can then be added to the mass of CNG in the HCNG blend, therefore calculating an equivalent CNG mass. Using this equivalent data, the BSFC of HCNG and CNG can be compared. It was found that BSFC of
the HCNG fueling can be reduced by increasing the ratio of hydrogen. The minimum BSFC was attained using 40% HCNG, which results in a 5.07% lower BSFC than that of CNG fueling at the same conditions. Figures 9 and 10 indicate the variation of specific brake methane emission versus spark timing for HCNG fuel at different ratios of hydrogen. As can be seen, methane emissions for HCNG fueling are greatly reduced as the hydrogen fraction increases and the spark timing is retarded. The main causes were that combustion quality was improved and quench distance was shortened as well as the natural gas/hydrogen ratio was decreased by the increasing fraction of hydrogen. Methane has a relatively stable chemical structure, therefore making it difficult to reduce emissions by after treatment. For this reason, the engine fueled with HCNG has more advantages regarding the methane emissions than that of CNG fueling.
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Figure 12. Brake-specific NOx emissions versus ignition timing at λ ) 1.3.
Figures 11 and 12 give the NOx emissions for both HCNG and CNG fueling. NOx emissions are heavily dependent upon the cylinder temperature, and an increase of hydrogen results in higher cylinder temperature, thus resulting in additional NOx emissions. This study shows that, for the same excess air ratios, delayed spark timing can effectively reduce NOx emissions in HCNG applications. 4. Conclusion The following conclusions can be drawn from the above experimental results and analysis: (1) Under idle operation conditions, hydrogen addition is an effective method for improving the power output of the engine and reducing both exhaust emissions and fuel consumption. However, there is not significant improvement when increasing the hydrogen ratio in the HCNG fuel. (2) Under normal operation conditions, the addition of hydrogen is effective at improving the power output
of the engine and reducing fuel consumption. Furthermore, these results improve as the ratio of hydrogen is increased. (3) The methane emissions using HCNG are comparatively less than the methane emissions of CNG. Methane emission can be reduced by increasing the hydrogen fraction and/or retarding spark timing. (4) NOx emissions are heavily dependent upon the cylinder temperature, which increases as the fraction of hydrogen is increased; thus, holding all other parameters constant, hydrogen addition increases NOx emission. An effective way to reduce NOx emission is to increase the excess air ratio and/or retard the spark timing. (5) The addition of hydrogen to the HCNG blend results in lower methane emissions as well as reduced fuel consumption, although it also results in additional NOx emissions, which must be considered when determining an appropriate hydrogen ratio in the HCNG blend. EF900144S
