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Energy & Fuels 2007, 21, 686-691
Reduction of Smoke and NOx from Diesel Engines Using a Diesel/Methanol Compound Combustion System Chunde Yao,*,† C. S. Cheung,‡ Chuanhui Cheng,†,‡ and Yinshan Wang† State Key Laboratory of Engines, Tianjin UniVersity, Tianjin 300072, China, and Department of Mechanical Engineering, The Hong Kong Polytechnic UniVersity, Hong Kong ReceiVed June 15, 2006. ReVised Manuscript ReceiVed December 5, 2006
This paper presents the concept of a diesel methanol compound combustion (DMCC) system and some experimental results on the application of the system to two diesel engines. In the DMCC system, diesel fuel is used for engine starting and for low load operation. At medium to high load, a fixed amount of diesel fuel is maintained while extra energy is acquired by injecting methanol into the intake manifold to form a homogeneous methanol/air mixture. The system was tested on two 4-cylinder diesel engines: one naturally aspirated and the other turbocharged. In both cases, the DMCC is found to reduce brake specific equivalent fuel consumption, reduce smoke emission, and reduce NOx emission but increase CO and HC emissions.
1. Introduction Worldwide, diesel engines are preferred to petrol engines for commercial applications because of their higher thermal efficiency. However, the high particulate and nitrogen oxides emissions of diesel engines are major air pollution problems. Hence, there is a need to develop strategies to address these problems in relation to diesel engines. It is well-known that the improvement of fuel properties can reduce pollutant emissions. There have been plenty investigations on the application of nonalcoholic oxygenates to diesel fuel for reducing pollutant emissions.1-4 The oxygenates used include esters, ethers, and carbonates. They are found to have a beneficial effect on reducing smoke and particulate emissions. It is believed that the oxygen contents of the oxygenate helps in reducing particulate formation during the combustion process. However, there are associated problems with the fuel additives including a probable increase in hydrocarbon and NOx emissions. Moreover, in most cases, the nonalcoholic fuel additives are more expensive and less readily available. Alternative fuels such as alcohol fuels have also been applied to reduce emissions. Ethanol has been applied to spark ignition engines with success5 while its application to the diesel engine is also a current topic of research.6 Methanol has been extensively investigated as a transportation fuel because it is readily available from the conversion of biomass, coal, and * Corresponding author. Tel.: +86-2227406649. Fax: +86-2227383362. E-mail:
[email protected]. † Tianjin University. ‡ The Hong Kong Polytechnic University. (1) Zhu, J.; Cao, X. L.; Pigeon, R.; Mitchell, K. J. Air Waste Manage. Assoc. 2003, 53, 67-76. (2) Xiao, Z.; Ladommatos, N.; Zhao, H. Proc. Inst. Mech. Eng. Part D-J. Automob. Eng. 2000, 214, 307-332. (3) Tsurutani, K.; Takei, Y.; Fujimoto, Y.; Matsudaira, J.; Kumamoto, M. The effects of fuel properties and oxygenates on diesel exhaust emissions. SAE 952349, 1995. (4) Cheung, C. S.; Liu, M. A.; Lee, S. C.; Pan, K. Y. Clean Air 2005, 6, 239-253. (5) Brusstar, M.; Stuhldreher, M.; Swain, D.; Pidgeon, W. SAE 200201-2743, 2002. (6) Bilgin, A.; Durgun, O.; Sahin, Z. Energy Sources 2002, 24, 431440.
natural gas and its application in fuel cell vehicles.7,8 However application of methanol to diesel engines is difficult. The low cetane number of methanol makes autoignition difficult. A methanol fuel compression ignition engine received active research in the 1980s and the early 1990s,9-12 and methanolfueled buses had once been introduced to reduce pollutant emission.13 Wang et al.14 compared the emissions from diesel buses with alternative fuel buses. Their results show a reduction of particulate and NOx emissions but an increase in CO and HC emissions from the methanol fueled (M100) buses. However, operational problems have prohibited the development of M100 diesel vehicles. Of recent interest is the combined use of diesel and methanol. Chao et al.15 investigated the emission characteristics of a 6-cylinder natural-aspirated direct-injection diesel engine using diesel blended with up to 15% by volume of a methanolcontaining additive. They also found a decrease in PM and NOx emissions but an increase in CO and HC emissions. Huang et al.16,17 also conducted similar research on a single-cylinder direct-injection diesel engine with a stabilized diesel-methanol (7) Allard, M. Issues associated with widespread utilization of methanol. SAE 2000-01-0005, 2005. (8) Weimer, T.; Schaber, K.; Specht, M.; Bandi, A. Energy ConVers. Manage. 1996, 37, 1351-1356. (9) Heinrich, W.; Marquardt, K. J.; Schaefer, A. J. Methanol as a fuel for commercial vehicles. SAE 861581, 1986. (10) Hikino, K.; Suzuki, T. Development of methanol engine with autoignition for low NOx emission and better fuel economy. SAE 891842, 1989. (11) Richards, B. G. Methanol-fueled Caterpillar 3406 engine experience in on-highway trucks. SAE 902160, 1990. (12) Savonen, C. L.; Miller, S. P. Development status of the Detroit Diesel Corporation methanol engine. SAE 901564, 1990. (13) The Los Angeles County Metropolitan Transportation Authority (LACMTA). Methanol buses return to Los Angeles. Alternative Fuel Data Centre of U.S.A., 1998; http://www.afdc.org. (14) Wang, W. G.; Clark, N. N.; Lyons, D. W.; Yang, R. M.; Gautam, M.; Bata, R. M.; Loth, J. L. EnViron. Sci. Technol. 1997, 31, 3132-3137. (15) Chao, M. R.; Lin, T. C.; Chao, H. R.; Chang, F. H.; Chen, C. B. Sci. Total EnViron. 2001, 279, 167-179. (16) Huang, Z. H.; Lu, B. H.; Jiang, D. M.; Zeng, K.; Liu, B.; Zhang, J. Q.; Wang, X. B. Proc. Inst. Mech. Eng. Part D-J. Automob. Eng. 2004, 218, 435-447. (17) Huang, Z. H.; Lu, B. H.; Jiang, D. M.; Zeng, K.; Liu, B.; Zhang, J. Q.; Wang, X. B. Bioresour. Technol. 2004, 95, 331-341.
10.1021/ef0602731 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/27/2007
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Table 1. Basic Properties of Diesel and Methanol Fuel fuel type
diesel
methanol
cetane number density (kg/m3) @20°C lower heating value (MJ/kg) boiling point (°C) latent heat of vaporization (MJ/kg) fraction of oxygen/wt% autoignition temp. (°C) viscosity (MPa‚S) @20 °C
55 840 42.7 195 0.28 0 308 2.8
4 792 19.9 64.5 1.09 50 470 0.61
blend with up to 18% by weight of methanol. In these two cases, the methanol and diesel fuels have to be premixed. Popa et al.18 applied two fueling methods in their investigation. The first method takes in methanol through a carburetor while diesel is supplied through the normal fuel injector. The second one uses a double injection method. In both methods, there is no need to premix the two fuels. The first method, the fumigation approach which has also been reported in ethanol applications,6,19 is unfavorable for starting and low load operation. When methanol is applied to a diesel engine, its high heat of evaporation will lead to difficulty in cold start and emission of a large quantity of aldehydes when the engine is operating at light loads. The aldehydes play a major role in the formation of photochemical smog and are strong oxidants which can irritate the respiratory system of human beings. The high aldehydes emissions coupled with ignition difficulties have limited the development of methanol-fueled diesel vehicles, not to mention its application to in-service diesel vehicles. To solve these problems, a diesel/methanol compound combustion (DMCC) system is proposed and investigated. Using the DMCC, methanol will be injected into the manifold to form a homogeneous mixture with air for combustion, while the original diesel fuel injection system will be retained but slightly modified to limit diesel fuel injection. At engine start and light loads, the engine will operate on diesel alone to ensure cold starting capability and to avoid aldehydes production. At medium to high loads, the engine will operate on a homogeneous air/methanol mixture ignited by pilot diesel to reduce particulate and NOx emissions. At medium to high load, methanol is used to provide part of the required engine power and hence reduces the reliance on diesel and also reduces CO2 emission. The system thus developed can be retrofitted on in-use diesel engines and vehicles. The DMCC system is developed taking into consideration properties of the two fuels. The major properties of diesel and methanol used in the present experimental investigation are given in Table 1. It is well-known that methanol has a higher autoignition temperature than diesel, resulting in difficulty of compression ignition. For this reason, diesel fuel is used for generating a pilot flame at medium to high loads. On the other hand, methanol has a higher latent heat of vaporization and thus it absorbs a great deal of heat upon vaporization and helps to reduce combustion temperature and, consequently, NOx emission. Furthermore, its C/H ratio is low and also it contains oxygen, both of which are favorable for complete combustion, lower smoke emission, and a reduction of the specific CO2 emission. Thus, the DMCC is designed to take advantage of each fuel by providing a different proportion of diesel and methanol to the engine based on the loading requirement. Thus, (18) Popa, M. G.; Negurescu, N.; Pana, C.; Racovitza, A. Results obtained by methanol-fueling diesel engine. SAE 2001-01-3748, 2001. (19) Abu-Qudais, M.; Haddad, O.; Qudaisat, M. Energy ConVers. Manage. 2000, 41, 389-399.
Table 2. Specifications of Baseline Engines Engine model engine type bore/stroke (mm) displacement (cc) aspirated method compression ratio max. torque/speed (Nm/rpm) max. power/speed (kW/rpm) injection pressure (MPa) injection nozzle nozzle diameter (mm)
485QDI
4102BZQ
water-cooled, in-line, 4-stroke, 4-cylinder, DI, diesel engine 85/95 102/118 2156 3856 natural-aspirated turbocharged 18 16.5 110/2000 343/1600 31/3000 88/2800 20 19.5 4-hole nozzle 4-hole nozzle 0.32 0.28
it adopts diffusion combustion using diesel fuel alone at low load as well as at starting and warm-up stages and uses quasiHCCI (homogeneous charging compression ignition) formed by a methanol/air mixture combined with diesel fuel at medium and high load conditions. 2. Experimental Setup Experiments were carried out on two diesel engines to investigate the performance of the engines using the DMCC system. The two engines include a natural-aspirated engine and a turbocharged engine. The specifications of the engines are shown in Table 2 and the experimental setup is shown in Figure 1. The engines were tested on a CW260 electrical vortex dynamometer, and the exhaust was measured using a FQD-102A Bosch digital smoke meter and a HORIBA MEXA-7100 exhaust gas analyzer. The fuel consumptions were measured by two FCM-05 instantaneous automatic fuel consumption meters. Figure 1 shows that methanol is injected into the intake manifold through an electronically controlled fuel injection system. The injection pressure is about 0.3 MPa. The quantity of methanol injected is controlled by the methanol injector operating time which is controlled by the engine control unit (ECU) based on the conditions as shown in Figure 2. In Figure 2, T is the cooling water temperature, n is the engine speed, and Ttq is the engine torque, while T0, n0, and Ttq0 are the corresponding minimum value of each parameter set for injecting methanol. When the values of T0, n0, and Ttq0 are not reached, the engine will operate on pure diesel. When all parameters exceed the minimum setting, methanol will be injected to supplement the extra power requirement. Some investigators applied heating to intake air18,20 because the injected methanol tends to cool down the intake air temperature which would then affect subsequent combustion. However, with heated air, there is a reduction of volumetric efficiency and a
Figure 1. Schematic of the experimental setup.
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Figure 2. Flowchart of the DMCC control system.
Figure 4. Comparison of smoke opacity emissions (D for baseline diesel, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
Figure 3. Comparison of brake specific equivalent fuel consumption (D for baseline engine, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
reduced efficiency in reducing smoke and NOx emissions. Since this project aims at investigating the effect of the DMCC system on smoke and NOx reduction, the intake air was not preheated.
3. Results and Discussion In this paper, experiments were carried out for each engine at its maximum torque speed. Thus, the natural-aspirated engine was tested at 2000 rev/min for a range of engine torques while the turbocharged engine was tested at 1600 rev/min at different (20) Udayakumar, R.; Sundaram, S.; Sivakumar, K. Engine performance and exhaust characteristics of dual fuel operation in DI diesel engine with methanol. SAE 2004-01-0096, 2004.
engine torque. For the natural-aspirated engine, the experiments were conducted with engine loads ranging from 60 Nm to the rated torque of 110 Nm. For the engine with the DMCC system, when the engine load reached 60 Nm, the diesel fuel flow rate was fixed and then methanol was injected to increase the loading to the desired value. Due to deterioration of the engine, the maximum torque reached with diesel alone was slightly less than 110 Nm, while with diesel and methanol the maximum torque achieved was slightly higher than 110 Nm. For the turbocharged engine, the experiments were carried out with engine loads ranging from 235 to 343 Nm. For the engine with the DMCC system, when the engine load reached 235 Nm, the diesel fuel flow rate was fixed and methanol was injected to increase the loading to the desired value. Due to an engine knocking problem, the maximum torque achieved with the DMCC system was limited to 313 Nm in this investigation. The fuel economy and the emissions of the DMCC system were investigated and are discussed in the following sections. Experiments were first conducted on the baseline engines using diesel fuel for all tests and then repeated on the modified engines using a combination of diesel and methanol. In the figures presented, “D” refers to the baseline engine using only diesel fuel and “D+M” refers to the modified engine using the DMCC system. 3.1. Comparison of Equivalent Brake Specific Fuel Consumption. The equivalent brake specific fuel consumption (beq) is used to compare the advantages of using the DMCC engine. The concept of beq is to convert the consumption of both methanol and diesel into the equivalent diesel fuel based on
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Figure 5. Comparison of NOx emissions (D for baseline engine, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
their lower heating values, as shown in the following equation:
beq )
HLd × Gd + HLm × Gm × 1000 HLd × Pe
(1)
Where, HLd and HLm are the lower heating value of diesel fuel and methanol, respectively, in megajoules per kilogram; Gd and Gm are the consumption of diesel and methanol, respectively, in kilograms per hour; and Pe is the brake power of the engine in kilowatts. The comparison of the equivalent brake specific fuel consumptions are shown in Figure 3a and b for the natural-aspirated engine and the turbocharged engine, respectively. In Figure 3a, the engine with DMCC started to consume methanol at a torque of 60 Nm. There is lower brake specific equivalent fuel consumption using DMCC. The difference is more significant when the engine torque is more than 90 Nm. The maximum reduction is 11.6% in this case. In Figure 3b, there is also a reduction of the brake specific equivalent fuel consumption upon using DMCC but the reduction is not significant throughout the tests. The maximum reduction is about 2.8% in this case. The injection of methanol has two effects. First, it will increase the volumetric efficiency and the enhanced rate of heat release due to the homogeneous methanol/air mixture. Second, the cooling effect will induce ignition delay which might not be beneficial to combustion efficiency. For the natural-aspirated engine, the former effect seems to be dominating which leads to increased improvement in fuel economy with the increasing amount of methanol burned. For
Figure 6. Comparison of intake air temperature (D for baseline engine, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
the turbocharged engine, the second effect is also significant, as evidenced by the engine knocking effect observed, resulting in only a marginal improvement in fuel economy. 3.2. Comparison of Emissions. (a) Smoke Opacity Emissions. The influence of DMCC on exhaust smoke opacity is shown in Figure 4a and b, from which it can be observed that the smoke opacity in general increases with engine load for both baseline engines. However, the smoke opacity hardly increases with the engine load when operating with the DMCC system. The reduction is more significant with the natural-aspirated engine than the turbocharged engine. When the DMCC is introduced, methanol is injected and mixes with intake air to form a homogeneous mixture. A fixed amount of diesel fuel is still injected into the cylinder to burn there and to ignite the compressed methanol/air mixture. The ignited diesel fuel will burn together with the methanol/air mixture. Smoke formation and oxidation mechanisms have been proposed by many investigators.21,22 There are several reasons leading to the reduction of smoke emission after injecting methanol. The first one is that the quantity of diesel fuel involved in burning is now less than when pure diesel fuel burns in diffusion mode. Second, the smaller amount of diesel fuel burning in diffusion mode is now combusting together with the (21) Haynes, B. S.; Wagner, H. G. Prog. Energy Combust. Sci. 1981, 7, 229-273. (22) Toshiyuki, Y.; Nobuyuki, F.; Shigeki, T.; Yasuhiro, F. Analysis of diesel smoke emission at low engine speed. SAE 950084, 1995.
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Figure 7. Comparison of CO emissions (D for baseline engine, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
homogeneous methanol/air mixture which will help to burn faster and with a high availability of oxygen supply, because of the presence of methanol. Third, the C/H ratio of methanol is lower than that of pure diesel fuel and thus there is a reduction of the total carbon proportion in the whole fuel which limits the production of smoke. Fourth, there is a reduction of the maximum combustion temperature which is also beneficial for reducing smoke. The turbocharged engine has methanol injection starting from 235 Nm. Thus, the methanol replacement is low and there is a high percentage of diesel fuel even at the maximum torque of 313 Nm, leading to a lower smoke reduction. On the other hand, the natural-aspirated engine has methanol injection starting from 60 Nm while the maximum torque is up to 110 Nm, indicating that there is a more significant methanol replacement and thus a higher level of smoke reduction. (b) NOx Emissions. The influence of DMCC on NOx emissions is shown in Figure 5a and b. It can be observed that in general there is an increase of NOx emission with engine load for both baseline engines and for the engines with the DMCC system. Comparing the baseline engine with the DMCC engine, there is about 8% reduction in NOx when operating with the DMCC. For the turbocharged engine, the reduction is more than 10% and reaches about 16% at high loading conditions. NOx formation depends on gas temperature as well as the oxygen concentration. Methanol injection helps to cool down the intake air temperature while its oxygen content will increase oxygen availability, coupled with other effects, such
Figure 8. Comparison of HC emissions (D for baseline engine, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
as combustion efficiency, it can produce different effects on NOx formation. For example, in Huang et al.,16 there was both increase and decrease of NOx emission; in Chao et al.,15 there was a reduction, and in Popa et al.,18 there was an increase. The intake air temperature measured at a position after the methanol injectors of the two diesel engines is shown in Figure 6a and b. After injecting methanol, the intake air temperature significantly reduces and the reduction of intake air temperature of the turbocharged diesel engine is higher than that of the natural-aspirated one. This is one of the reasons that NOx reduction of the turbocharged engine is greater than that of the natural-aspirated engine. The temperature of intake air of the turbocharged engine is higher than that of the natural-aspirated engine, which will increase the charge temperature toward the end of the compression stroke, resulting in an even higher gas temperature upon combustion. Thus, the NOx of the turbocharged engine is always higher than that of the natural-aspirated one at the corresponding engine load, for both the baseline case and for the DMCC case. Figure 6b shows that the intake air temperature at the maximum load reduces from 91.7 to 69.2 °C after injecting methanol into the intake manifold. This helps to reduce the maximum in-cylinder gas temperature and hence the NOx formed. The same is true for the natural-aspirated engine. Thus, the reduced intake air temperature is the major reason for the reduced NOx emission after using the DMCC system.
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8 also show that when the combustion temperature is increased at higher engine loads and the premixed methanol/air mixture is rich enough, the combustion conditions improve and the HC and CO emissions are reduced. Figure 7a shows that for the natural-aspirated engine, there is lower CO emission with the DMCC system at brake mean effective pressure (BMEP) values below 0.52 MPa but higher CO emission at higher power output. Thus at low concentrations of methanol in the intake air, the CO deterioration is overcome by the improved combustion conditions as indicated by the improved brake specific equivalent fuel consumption; while at high methanol concentration in the intake air, the CO deterioration cannot be compensated by the improved combustion conditions. Figure 9a and b shows the exhaust gas temperatures. The lowest exhaust gas temperature of the natural-aspirated engine is more than 250 °C and that of the turbocharged engine is more than 400 °C, both of them increase with engine load. The temperatures are high enough to light off an oxidation catalyst. This means that, even under the DMCC system, the exhaust gas temperature is high enough to ensure lighting off of an oxidation catalyst to remove CO and HC as well as aldehydes from exhaust emissions, which helps in alleviating the increased CO and HC emissions in the system. 4. Conclusions
Figure 9. Comparison of exhaust gas temperature (D for baseline engine, D+M for DMCC): (a) natural-aspirated engine; (b) turbocharged engine.
(c) CO and HC Emissions. The emissions of CO and HC are shown in Figures 7 and 8, respectively, from which it can be observed that both CO and HC increased after using the DMCC system. The extent of CO increase is more obvious for the turbocharged engine than for the natural-aspirated engine, while the HC increase is obvious for both engines. HC and CO are mainly products of incomplete combustion. Using the DMCC system, the injected methanol tends to reduce the combustion temperature, which makes it difficult to burn the homogeneous methanol/air mixture completely, especially when the amount of methanol injected is small and its mixture is too lean for sustaining combustion. Moreover, as in the HCCI engine, some of the methanol fuel is stored in crevices during the compression stroke and escapes combustion. Figures 7 and
This paper presents the concept of a diesel/methanol compound combustion (DMCC) system and some experimental results on the application of the system to two diesel engines. The following conclusions can be drawn from the current investigation: (1) The DMCC concept can be applied to both naturalaspirated and turbocharged diesel engines with reduced brake specific equivalent fuel consumption. (2) There is a significant reduction in smoke emissions from both engines using the DMCC system. (3) There is a reduction of NOx emission from both engines using the DMCC system due to the reduced intake air temperature and consequently the combustion temperature. (4) There is an increase in CO and HC emissions due to the lower combustion temperature as well as the homogeneous methanol/air charge formed. However, an oxidation catalyst can be applied to reduce these pollutants because when the DMCC system is in operation, the exhaust gas temperatures are high enough for lighting off. Acknowledgment. This project is supported by the Natural Science Foundation Committee of China (Contract No. 50576064) and a research grant from The Hong Kong Polytechnic University (G-YE33). EF0602731
