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Performance and Emissions of a Diesel Engine Fuelled with Methanol Ruizhi Song, Jie Liu, Lijun Wang, and Shenghua Liu* School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, People’s Republic of China ReceiVed June 22, 2008. ReVised Manuscript ReceiVed August 27, 2008
Methanol and diesel are not very miscible, which makes it difficult to mix them together as a diesel engine fuel. Dual-fuel operation is favored, and there is potential to reduce particulate matter (PM) and NOx emissions simultaneously. In this work, an electronically controlled low-pressure common rail system was employed to deliver methanol to the inlet port, while the engine’s original high-pressure diesel injection system was used to deliver a suitable quantity of diesel fuel for ignition. The experimental results show that the full-load power of the dual-fuel engine can reach or even exceed that of the original diesel engine when a suitable minimum pilot diesel quantity is used. Under dual-fuel conditions, smoke is reduced significantly, while a modest reduction in NOx is observed. The equivalent brake-specific fuel consumption is improved under high-load operating conditions. Especially, the dual-fuel engine shows a better fuel economy when run at a high rate of methanol addition. However, unburned hydrocarbon (HC) and carbon monoxide (CO) emissions for dual-fuel operation increase when methanol is added.
Methanol, as a kind of promising oxygenated alternative fuel, has cleaner burning characteristics and can reduce NOx and PM from diesel engines when it is mixed with diesel fuel. Because of the miscibility problems and the decrease of the cetane number, the ratio of methanol/diesel is not so high but the combustion changes a lot, even with relatively small additions of methanol. Moreover, the formation of diesel NOx and PM emissions is promoted by the local high-temperature and fuelrich conditions that occur in diesel combustion. If methanol and
diesel are used in a dual-fuel mode, then NOx and PM can be reduced simultaneously without encountering the miscibility problem. Shi et al.5 used an emulsified mixture of methanol and diesel to study the engine performance and found that the engine gave good performance when the amount of methanol in the mixed fuel did not exceed 30% by weight. Huang et al.6 investigated engine combustion characteristics using methanol/diesel blends, and the maximum methanol mass fraction was 20% by weight. Bayraktar7 studied experimentally the performance parameters of methanol-diesel-dodecanol blends, and the methanol concentration in the blends was merely 15% by volume. Popa et al.8 conducted experimental investigations using a methanol in-cylinder injection system. The engine combustion and emission characteristics were studied by using this dual-fuel injection system. However, a substantial modification of the diesel engine was performed, and the system was a little complex because the methanol injector was placed at the top of combustion chamber. Yao et al.9 developed a diesel-methanol compound combustion system. The engine control strategy and the influence of the methanol mass fraction on the engine emissions and performance were not reported. In this study, a dual-fuel system was employed. The dualfuel compression ignition (CI) engine can run on pure diesel by using its original diesel injection system. Methanol is delivered by an electronically controlled low-pressure common rail system, similar to a port fuel injection gasoline engine. It
* To whom correspondence should be addressed. E-mail: shenghua@ mail.xjtu.edu.cn. (1) Zhang, Y.; Boehman, A. L. Energy Fuels 2007, 21, 2003–2012. (2) Shi, X. Y.; Yu, Y. B.; He, H.; Shuai, S. J.; Dong, H. Y.; Li, R. L. J. EnViron. Sci. 2008, 20, 177–182. (3) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33, 233–271. (4) Ribeiro, N. M.; Pinto, A. C.; Quintella, C. M.; Rocha, G. O.; Teixeira, L. S. G.; Guarieiro, L. L. N.; do Carmo Rangel, M.; Veloso, M. C. C.; Rezende, M. J. C.; Serpa da Cruz, R.; de Oliveira, A. M.; Torres, E. A.; de Andrade, J. B. Energy Fuels 2007, 21, 2433–2445.
(5) Shi, S. X.; Zhao, K. H.; Fu, M. L.; Wang, S. K.; Sun, Z. Y. An investigation of using methanol as an alternative fuel for diesel engines. In Proceedings of the 15th International Congress on Combustion Engines, 1983. (6) Huang, Z. H.; Lu, H. B.; Jiang, D. M.; Zeng, K.; Liu, B.; Zhang, J. Q.; Wang, X. B. Bioresour. Technol. 2004, 95, 331–341. (7) Bayraktar, H. Fuel 2008, 87, 158–164. (8) Popa, M. J.; Negurescu, N.; Pana, C. Results obtained by methanol fuelling diesel engine. SAE Tech. Pap. 2001-01-3748, 2001. (9) Yao, C. D.; Cheung, C. S.; Cheng, C. H.; Wang, Y. S. Energy Fuels 2007, 21, 686–691.
1. Introduction Diesel vehicles save about 20% or more of fuel as compared to those powered by spark-ignition (SI) engines, which is helpful to ease the problem of petroleum shortage and CO2 emission. However, diesel engines suffer from higher emissions of nitrogen oxides (NOx) and particulate matter (PM). To meet the tightening regulations on exhaust emissions, especially to reduce NOx and PM simultaneously, very complicated technologies have been applied, such as an electronically controlled highpressure common rail injection system or a selective catalytic reduction (SCR) postcombustion treatment system. On the other hand, investigations have been carried out by using oxygenated alternative fuels and the new concept of engine combustion technology. Previously published results showed that diesel NOx and PM emissions can be controlled to meet Euro IV or higher standards.1-4
10.1021/ef800492r CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
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Figure 1. Engine test bench setup.
is injected during the induction process. Methanol is thus premixed with air to form a homogeneous mixture in the cylinder. The pilot diesel is directly injected into the cylinder prior to the end of the compression stroke. Once the autoignition of pilot diesel occurs, the premixed homogeneous methanol/air mixture will be ignited and the flame propagation starts at the same time. Because of the lean-burning characteristics and less fuel-rich diffusion combustion, NOx and PM emissions can be reduced. The purpose of this paper is to identify the strategy of dualfuel operation and investigate the effects of the methanol rate on the dual-fuel engine performance, emission characteristics, as well as the engine combustion characteristics.
Figure 2. Comparisons of engine performance under full-load operating conditions. Table 1. Test Engine Specifications specification
2. Test Engine Experimental Setup The experiments were conducted using a direct-injection, fourstroke, single-cylinder, water-cooled diesel engine. The major specifications of the engine are listed in Table 1, and the setup of the test bench is shown in Figure 1. An electronically controlled low-pressure common rail system was employed to deliver methanol. The methanol injector is fixed to the intake manifold about 250 mm ahead of the intake valve. Methanol is injected during the induction process, and its quantity is controlled by the width of the injection pulse. The cylinder pressure was sensed by a water-cooled piezoelectric pressure transducer (Kistler6125A). Data of 100 cycles were averaged to calculate the heat release rate and other combustion characters. Engine emissions were measured online by the exhaust analyzer (Horiba MEXA 7100DEGR), in which HC was analyzed with a flame ionization detector (FID), CO was analyzed with a nondispersive infrared analyzer (NDIR), and NOx was measured with a chemiluminescent detector (CLD). Smoke was measured with a smoke meter (AVL Dismoke4000). In the experiments, the methanol quantity is controlled by the pulse width and diesel is controlled by the fuel pump rack position. The methanol mass fraction φM is defined to be
φM )
BM × 100 (%) BM + BD
(1)
3. Results and Discussion 3.1. Dual-Fuel Engine Power and Fuel Economy Performance. 3.1.1. Dual-Fuel Control Strategy. The combustion of methanol/diesel in a dual-fuel engine combines the characteristics of a CI engine operating with pure diesel and a SI engine
bore × stroke displacement compression ratio combustion chamber rated power/speed nozzle hole diameter number of nozzle holes
100 × 115 mm 903 cm3 18 ω type 11 kW/2300 rpm 0.3 mm 4
operating with pure methanol. The autoignition of diesel will ignite the methanol/air mixture within the cylinder; however, there are some differences. The introduction of methanol into the charge changes the charge properties, and diesel autoignition will thus be affected. Of course, methanol combustion will be affected under diesel ignition and high temperature and pressure conditions, too. On one hand, the methanol mixture should be within the flammability limit or the engine will misfire. This limit is based on the combustion efficiency in this paper, because the poor combustion will result in high HC and CO emissions. On the other hand, because of the change of mixture components and temperature, diesel autoignition is a little different from the pure diesel operating condition when methanol is added in a dualfuel mode. If the pilot diesel is too little, ignition will become unstable. In this case, the engine will lose its power and fuel economy and emissions will be rather poor. Therefore, there exists a minimum diesel quantity to ignite the methanol charge to obtain the same full-load engine power output as under pure diesel operating conditions. To do this, the engine was first run to full-load condition with pure diesel, and then diesel was reduced while methanol was added until the engine could not keep its power. Parts a and b of Figure 2 give the full-load
Diesel Engine Fuelled with Methanol
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Figure 4. Dual-fuel engine performance map under constant pilot diesel quantity.
Figure 3. Comparison of dual-fuel engine equivalent brake-specific fuel consumption.
engine power and the corresponding diesel equivalent fuel consumption, respectively. Figure 2c is the measured minimum pilot diesel. As shown in Figure 2, the pilot diesel quantity per cycle increases with the increase of the dual-fuel engine speed. However, under higher engine speed conditions, the increasing trend becomes flat. Therefore, the methanol/diesel dual-fuel engine can not only obtain better power output but can also operate with a high methanol rate under higher speed conditions. 3.1.2. EquiValent Brake-Specific Fuel Consumption. The equivalent brake-specific fuel consumption (beq) is used to compare the economy of the dual-fuel engine. For convenience, methanol is converted to be equivalent diesel according to its lower heating value. It is evaluated by the following formula: beq )
HLdBD + HLmBM × 1000 HLdpe
(2)
Figure 3 shows the effect of the methanol fraction on equivalent brake-specific fuel consumption. As shown, under high-load operating conditions, beq for dual-fuel operation is remarkably lower compared to that under normal diesel operation at high load. Moreover, with the increase of the methanol fraction, the improvement of beq is becoming more pronounced. As is well-recognized, methanol has a higher flame speed. The addition of methanol can result in a faster combustion rate; therefore, the engine fuel economy is improved significantly, especially with a high methanol fraction. However, under lowload operating conditions, beq for dual-fuel operation is slightly higher compared to that under normal diesel operation. Under low-load conditions, even with a higher ratio of methanol/diesel, the methanol/air mixture is lean; therefore, the burning velocity becomes slower. Additionally, diesel ignition is postponed, and the temperature of the mixture is lower because of the lower engine load. All of the reasons above make the dual-fuel heat release far away from the top dead center (TDC), which means a poorer thermal efficiency. Thus, for better fuel economy, it is better for the dual-fuel engine to run with a high methanol fraction under high-load operating conditions and with pure diesel under low-load operating conditions.
3.1.3. Performance Map. Figure 4 gives the results under the condition of constant pilot diesel quantity mp ) 17 mg/ cycle. As observed, to obtain better fuel economy, the engine should run at a high rate of methanol under high-load operating conditions. As can be seen from Figure 4, the best fuel economy is sited in the region with higher load and speed, when fuelled with a methanol/diesel dual fuel. Meanwhile, the region of best fuel economy extends over a wide range of engine speeds at high load. Under high-load operating conditions, more methanol is supplied to maintain the engine power output. The methanol/ air mixture is rich, and the temperature is high, which cause the combustible mixture to burn rapidly. Moreover, the suitable pilot fuel quantity can lead to the successful ignition and flame propagation and, consequently, lead to increasing the output power and an improvement in fuel economy. 3.2. Engine Combustion and Emissions. To study the effects of the methanol rate on dual-fuel engine combustion and emissions, three loads at two engine speeds were selected. They are designated low load (bmep ) 0.35 MPa), middle load (bmep ) 0.56 MPa), and full load (bmep ) 0.77 MPa), and the engine speeds are 1600 and 2000 r/min, respectively. The cylinder pressure, exhaust HC, CO, and NOx, and exhaust smoke were measured under stable operation at each condition. On the basis of the data, the following effects were analyzed. 3.2.1. Effect of the Methanol Rate on Engine Combustion. In a conventional direct-injection diesel engine, the combustion process can be divided into two phases: premixed combustion and diffusion combustion, which is determined by the flammable mixture preparation and combustion. The amount of fuel consumed in the premixed combustion phase has a positive correlation to NOx emission, while diffusion combustion has a significant effect on the formation of PM.10,11 However, when methanol is co-combusted with diesel as diesel engine fuel, the mixture preparation as well as charge properties change. The ratio of methanol/diesel plays an important role.12 To study the combustion characteristics for the dual-fuel engine, the cylinder pressures are sampled according to crank angle. On the basis of the data, the heat release rate is calculated by the following equation10 cp dV cv dp dQW dQB )p +V + dφ R dφ R dφ dφ
(3)
(10) Heywood, J. B. Internal Combustion Engine Fundamentals, 1st ed.; McGraw-Hill: New York, 1988. (11) Matsui, Y.; Kamimoto, T.; Matsuoka, S. Formation and oxidation processes of soot particulates in a DI diesel enginesAn experimental study via the two-color method. SAE Tech. Pap. 820464, 1982.
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Figure 5. Time histories of the rate of heat release.
where p is the measured cylinder pressure and V, dV/dφ, and dp/dφ can be calculated from the crank angle and cylinder pressure. Heat transfer through the chamber wall is estimated by Woschni’s formula10 dQW ) hcA(T - Tw) dφ
(4)
The surface areas and temperatures of the piston, cylinder wall, and cylinder head exposed to the gases are considered, respectively. The average temperature of the cylinder gas can be computed by the gas state equation pV ) mRT (5) Figure 5 shows a group of calculated heat release rates at three engine loads. Under each of the same engine load and speed conditions, ignition delay and the maximum rate of heat release increase with the increase of the methanol mass fraction. In addition, the shape of the rate of heat release changes from (12) Wang, L. J.; Song, R. Z.; Zou, H. B.; Liu, S. H.; Zhou, L. B. Proc. Inst. Mech. Eng., Part D 2008, 222, 619–627.
dual-peak mode of pure diesel to single-peak mode, especially under middle- and high-load conditions. Another interest phenomenon is that the combustion is almost finished at the same crank angle. Dual-fuel combustion is more complicated. Methanol changes the gaseous properties of cylinder gas. More methanol results in a lower temperature of the cylinder gas, which means the ignition delay will be longer, although the total combustion phase becomes shorter. Otherwise, more diesel will lead to a longer diffusion combustion phase. These two competitive factors control the dual-fuel engine combustion processes. In this study, experiments show that the methanol rate should be less than 70%. Although a large fraction of methanol will result in a higher peak of heat release rate, it is within the reasonable range because it occurs later during the fast downward movement of the piston. 3.2.2. Effect of the Methanol Rate on Emissions. 3.2.2.1. NOx and Smoke Emission Characteristics. Figure 6 compares NOx emissions when the engine is fuelled with diesel alone and with dual fuel. NOx emissions from the dual-fuel engine are lower to some extent, as compared to that of pure diesel operation at
Diesel Engine Fuelled with Methanol
Figure 6. NOx emissions versus the methanol mass fraction.
the same operating condition, and tend to be reduced with the increase of the methanol mass fraction. According to the thermal NO mechanism, NO emission is favored by a higher oxygen concentration, temperature, and longer residence time.10,13 Under dual-fuel operating conditions, ignition delays and the peak heat release rate occur later. Although the peak heat release rate is higher, because of fast expansion of the volume as a result of the fast downward movement of the piston, the highest temperature is lower. Therefore, the fact that the highest temperature is lower and its region lasts for a shorter time suppresses the formation of NO. The measured smoke emissions are shown in Figure 7. There is a sharp decrease of engine smoke when the engine runs with a higher methanol fraction, especially under high-load operating conditions. Diesel engine smoke results from the pyrolysis of heavy hydrocarbons. Methanol contains no carbon-carbon bonds; thus, there is no pyrolysis. The application of methanol will reduce diesel quantity injected into the cylinder. Because of the longer ignition delay and the higher latent heat of vaporization of methanol, more diesel will be evaporated, which reduces diesel pyrolysis.14,15 Otherwise, the presence of methanol will act to enhance soot oxidation by the increased concentration of oxygen atoms.16 The relationship between NOx and smoke emissions under different load operating conditions is plotted in Figure 8. Unlike a normal diesel engine, a simultaneous reduction in both smoke and NOx is achieved with the addition of methanol. There is no tradeoff between NOx and smoke emissions as observed in conventional diesel engines. 3.2.2.2. CO and HC Emissions. Figure 9 gives the curves of CO emissions versus the methanol mass fraction. CO is an intermediate product of dual-fuel combustion. Because of a (13) Hill, S. C.; Smoot, L. D. Prog. Energy Combust. Sci. 2000, 26, 417–458. (14) Tree, D. R.; Svensson, K. I. Prog. Energy Combust. Sci. 2007, 33, 272–309. (15) Jacobs, T. J.; Assanis, D. N. Proc. Combust. Inst. 2007, 31, 2913– 2920. (16) Stanmore, B. R.; Brilhac, J. F.; Gilot, P. Carbon 2001, 39, 2247– 2268.
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Figure 7. Smoke emissions versus the methanol mass fraction.
Figure 8. Relationship between NOx and smoke emissions.
higher excess air ratio in diesel engine combustion compared to SI engine operation, CO emission is generally lower in a CI engine. When methanol is introduced into the cylinder under dual-fuel operating conditions, CO emissions seem minimally affected under low- and middle-load conditions. However, there is a sharp increase under full-load conditions. This can be considered to be the postcombustion oxidation of unburned methanol. Figure 10 shows HC emissions under different loads and engine speeds. It can be seen that the HC emission increases significantly when methanol is added under the same load and speed conditions. HC emission is mostly due to the retention of unburned fuel in crevices in the cylinder. Methanol mixed
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Figure 9. CO emissions versus the methanol mass fraction. Figure 10. HC emissions versus the methanol mass fraction.
with air is compressed into the crevices during the compression stroke and is released during the expansion process. Most of it is exhausted in the unburned form. 4. Conclusions In comparison to pure methanol operation, the methanol/diesel dual-fuel operation has obvious advantages. In this study, the engine has a higher compression ratio (CR) of 18, which is beneficial to the engine fuel economy. The performance and emissions of a compression ignition engine fuelled with methanol/diesel dual fuel were investigated, and the main results can be concluded as follows: (1) Methanol can be used as the fuel of the diesel engine by means of pilot diesel ignition. The dual-fuel operation can produce the equivalent or even more power than the original engine with a better fuel economy. (2) Under dual-fuel operating conditions, ignition delay becomes longer and the maximum rate of heat release becomes larger; however, the total engine combustion phase is similar to pure diesel operation. (3) There is a significant reduction in smoke emissions and a modest reduction in NOx emissions. The tradeoff relationship between NOx and smoke emissions disappears; that is, they are reduced simultaneously when under methanol/diesel dual-fuel operating conditions. However, HC and CO emissions increase. (4) It is better for the dual-fuel engine to run with a high rate of methanol under high-load operating conditions and with pure diesel under low-load
operating conditions. In this case, an improved thermal efficiency as well as an increased alternative ratio can be reached. (5) For methanol/diesel dual-fuel operation, the diesel system plays important roles of cold start, ignition, and so on. It is suitable for the dual-fuel operation for some fixed conditions, such as a diesel generator, because the best operating is limited under high-load conditions. Acknowledgment. This study is supported by the National 863 Research Project (2006AA11A1A4). The authors acknowledge the colleagues of Xi’an Jiaotong University for their help with the experiment and preparation of the manuscript.
Nomenclature bmep ) brake mean effective pressure (MPa) BD ) diesel consumption (kg/h) BM ) methanol consumption (kg/h) beq ) diesel equivalent brake-specific fuel consumption (g kW-1 h-1) Bq ) diesel equivalent fuel consumption (kg/h) CA ) crank angle (deg) HLd ) lower heating value of diesel (MJ/kg) HLm ) lower heating value of methanol (MJ/kg) mp ) pilot diesel quantity (mg/cycle) Pe ) brake power of the engine (kW) φ ) crank angle (deg) φM ) methanol mass fraction (%) EF800492R
