Energy Fuels 2010, 24, 2465–2469 Published on Web 03/10/2010
: DOI:10.1021/ef9016043
Regulated and Nonregulated Emissions from a Dimethyl Ether Powered Compression Ignition Engine Liu Jie, Liu Shenghua,* Li Yi, Wei Yanju, Li Guangle, and Zhu Zan School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Received December 28, 2009. Revised Manuscript Received February 19, 2010
Dimethyl ether (DME) is widely studied to reduce diesel engine emissions, especially the NOx and smoke emissions. However, the DME and aldehyde emissions are seldom studied. In this paper, a two-cylinder, direct injection diesel engine was applied. Pure diesel and DME were used to study the engine emissions, especially the exhaust DME and formaldehyde emissions. Experimental results indicate that DME fuel averagely reduces the CO emissions by 59% and 23.7% at two engine speed. NOx emissions were reduced by 45.2% and 57.9%, separately, in comparison to diesel at two engine speeds. Smoke is almost zero for DME under all the engine operation conditions. The NOx/smoke trade-off correlation was broken. The formaldehyde exhaust concentration from the engine with DME operation is just a little higher than that with diesel fuel. Both engine load and engine speed have little effect on the DME emissions. Because the response of the flame ionization detector to DME is 0.5 times that of C1 and it has no response to formaldehyde, the total hydrocarbon emission of the engine is revised accordingly.
with diesel fuel.6,7 Further, NOx reduction can be achieved with higher EGR rate.8-10 The data of CO emissions show some contradictions depending on the engine system and operating conditions. DME has good mixing characteristics and higher oxygen content so that the locations of the fuel-rich regions in the combustion period could be reduced, resulting in lower CO emissions.11 On the other hand, higher emissions of CO and HC have been reported along with lower NOx in many investigations.12,13 As the oxygen content is 35% and there is no C-C bond in DME, smoke emission is almost zero. However, the nonregulated emissions are rarely mentioned, especially the formaldehyde and CH3OCH3 emissions. Formaldehyde is an intermediate product of the combustion of diesel fuel14 and DME.15 Formaldehyde can be toxic,
1. Introduction The recent increase in the world oil prices and some concern over their energy security, coupled with the growing awareness of the environmental problems associated with the use of petroleum fuels, has led to the renewed interest in alternative fuel.1-4 These alternative fuels should be nontoxic, renewable sources of energy, which should not lead to a secondary pollution. Dimethyl ether (DME) is one of the most promising alternative automotive fuel solutions among the various ultra clean, renewable, and low-carbon fuels under consideration worldwide. DME can be derived from many sources, including renewable materials (biomass, waste, and agricultural products) and fossil fuels (natural gas and coal). Only modest modifications are required to convert a diesel engine to run on DME.5 As an automotive fuel, when compared with petroleumderived diesel, DME’s emission performance demonstrates a number of significant benefits, according to various sources in the literature. NOx was found to be lower with DME than
(6) Fleisch, T. H.; Meurer, C. DME, The diesel fuel for the 21st Century, AVL conference engine and environment, 1995. (7) Kapus, P; Ofner, H. Development of fuel injection equipment and combustion system for DI diesels operated on di-methyl ether. SAE Paper 950062, SAE Trans. J. Fuel Lubr., 1995, 104 (4), 54-9. (8) Kinoshita, K.; Oguma, M.; Goto, S.; Sugiyama, K.; Mori, M.; Watanabe, T. Effects of fuel injection conditions on driving performance of a DME diesel vehicle. SAE Paper, 2003-01-3193, 2003. (9) Lim, O. T.; Sato, Y.; Oikawa, H.; Nozaki, S.; Noda, T.; Ushiyama, D.; Ishikawa, T. Development of dimethyl ether engines for light-duty trucks using a large exhaust gas recirculation system, Proc. Inst. Mech. Eng., Part D: J. Automobile Eng., Volume 222, Number 1, 2008. (10) Lee, S.-W.; Murata, Y.; Daisho, Y. Spray and combustion characteristics of dimethyl ether fuel. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng., Volume 219, Number 1, 2005. (11) Teng, H.; McCandless, J. C.; Schneyer, J. B. Thermo-chemical characteristics of di-methyl-ether an alternative fuel for compressionignition engines. SAE Paper 2001-01-0154, SAE Trans. J. Fuel Lubr. 2001, 110 (4), 96-106. (12) Mingfa, Y.; Zunquing, Z.; Sidu, X.; Maoling, F. Experimental study on the combustion process of dimethyle ether (DME). SAE Paper 2003-01-3194, SAE Trans. J. Fuel Lubr. 2003, 112 (4), 2422-9. (13) Sorenson, S. C.; Mikkelsen, S. Performance and emissions of a 0.273 l direct injection diesel engine fuelled with neat dimethyl ether. SAE Paper 950064, SAE Trans. J. Fuel Lubr. 1995, 104(4), 80-90. (14) Curran, H. J.; Pitz, W. J.; Westbrook, C.K.; Callahan, C.V.; Dryer, F.L. Oxidation of Automotive Primary Reference Fuels at Elevated Pressures. Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998, pp 379-387.
*To whom correspondence should be addressed. E-mail: shenghua@ mail.xjtu.edu.cn. (1) Arul Mozhi Selvan, V.; Anand, R. B.; Udayakumar, M. Combustion Characteristics of Diesohol Using Biodiesel as an Additive in a Direct Injection Compression Ignition Engine under Various Compression Ratios. Energy Fuels 2009, 23 (11), 5413–5422. (2) Lapuerta, M.; Armas, O.; Garcia-Contreras, R. Effect of Ethanol on Blending Stability and Diesel Engine Emissions. Energy Fuels 2009, 23 (9), 4343–4354. (3) Zhu, R.; Wang, X.; Miao, H.; Huang, Z.; Gao, J.; Jiang, D. Performance and Emission Characteristics of Diesel Engines Fueled with Diesel-Dimethoxymethane (DMM) Blends. Energy Fuels 2009, 23 (1), 286–293. (4) Ren, Y.; Huang, Z.; Miao, H.; Jiang, D.; Zeng, K.; Liu, B.; Wang, X. Combustion and Emission Characteristics of a Direct-Injection Diesel Engine Fueled with Diesel-Diethyl Adipate Blends. Energy Fuels 2007, 21 (3), 1474–1482. (5) Wang, H. W.; Zhou, L. B.; Jiang, D. M.; Huang, Z. H. Study on the performance and emissions of a compression ignition engine fuelled with dimethyl ether. Proc. Inst. Mech. Eng., Part D: J. Automobile Eng., Volume 214, Number 1, 2000. r 2010 American Chemical Society
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: DOI:10.1021/ef9016043
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Table 1. Engine Specifications item engine type combustion chamber bore (mm) stroke (mm) displacement (cm3) compression ratio rated power (kW)/rated speed (rpm) max torque (N 3 m)/speed (rpm) nozzle number diameter (mm) fuel delivery angle inject open pressure (Mpa) plunger diameter (mm)
diesel engine
Table 2. Physical and Chemical Properties of DME and Diesel DME engine
2102QB diesel engine ω type 102 115 1880 17.5:1 26.1/2700 26.8/2800 110.2/1400 108.9/2000 4 0.27 5 0.35 25 °CA BTDC 22 °CA BTDC 19.1 18 8.5 10.5
allergenic, and carcinogenic. However, it is proven that the hydrocarbon detector, flame ionization detector (FID), hardly has a response on CH2O.16 The CARB method 1004 (ASTM method D5197, U.S. EPA methods TO-11A and 8315) is commonly used to monitor aldehyde emissions from engines. However, except for the sampling time, it takes over 1 h for the operation of derivation, elution, extraction, and detection of the DNPH hydrazone. The complex sample pretreatment is not endurable, especially when a lot of testing points are needed. Over the past few years, a fast chromatographic method with the pulsed discharge helium ionization detector (PDHID) has been developed, which is capable of monitoring low levels of formaldehyde with a simple sampling process in several minutes.17 The presence of PDHID makes it much easier to investigate engine-out HCHO and CH3OCH3 emission characteristics in a wide range of engine speeds and torque. As the response of DME on a flame ionization detector (FID) is 0.5 times that of the HCs, the HCs are evaluated by equivalent methane, so the measured value of DME emission is less than it is theoretically. FID has no response on the formaldehyde emission. When running on pure DME, the measurements of total HC (THC) will inevitably result in errors with an FID without special calibration. However, most literature reports use an FID directly without attention to this. In this study, the effects of DME fuel on engine power performance and emissions were investigated. The nonregulated emissions including formaldehyde and DME were detected by PDHID in selective Ar-PDHID mode to improve the response and selectivity by eliminating the interference of water and permanent gases.18
properties
DME
chemical formula molecular weight (g) boiling point (°C) Reid vapor pressure (MPa) liquid density (g/cm3) liquid viscosity (cP) low heat value (MJ/kg) explosion limit air (vol %) ignition temperature (°C) Cetane number stoichiometric air-fuel ratio (kg/kg) latent heat of evaporation (kJ/kg) carbon content (wt %) hydrogen content (wt %) oxygen content (wt %)
CH3-O-CH3 46.07 -24.9 0.51(20 °C) 0.668 0.15 28.43 3.4-17 235 55-60 9 460(20 °C) 52.2 13 34.8
diesel CxHy
0.826 4.4-5.4 42.5 0.6-6.5 250 45 14.6 290
nozzle with five holes of 0.35 mm diameter is adopted to ensure a sufficient flow area, the fuel delivery advance angle is reduced from 25° crank angle (CA) before top dead center (BTDC) to 22 °CA BTDC, and a low injector opening pressure (18 MPa) is used for DME engine operation. The regulated emissions of carbon monoxide (CO), total hydrocarbon (THC), and nitric oxide (NOx) emissions were detected by HORIBA MEXA7100DEGR analyzer (Horiba Ltd., Japan), and smoke emission was tested by AVL DiSmoke 4000. Nonregulated emission as CH2O and CH3OCH3 were detected by GC (GC-2010, Shimadzu, Japan) consisting of a Gs-OxyPLOT capillary column (10 m 0.53 mm inner diameter 10 μm film, Agilent Technologies) and a pulsed discharge ionization detector [PDHID, model D4-I-SH-17R, VICI (Valco Instrument Co., Inc.)] performing in Ar-PDHID mode. See details in our previous work.19,20 Two Siemens SITRANS FC300 mass flow meters were used to measure engine fuel consumption. One was set prior to and the other after the injection pump. The difference of the mass flow rates was the DME consumption rate. The engine was coupled to a electrical dynamometer (type CW500, powerlink, China) to control engine speed and load. The diesel fuel used in this study is 0# diesel oil, which was obtained from China petroleum and chemical corporation. And Dimethyl ether fuel was obtained from China Jutai chemical corporation. The physical and chemical properties of DME compared with diesel are shown in Table 2. DME will be in the gaseous state even at -20 °C and ambient pressure, and its vapor pressure varies with temperature. In this study, the fuel delivery pressure was increased to 1.7-2.0 MPa to prevent vapor lock in the fuel system. DME has a low viscosity. To ensure reliability and durability of the parts of the fuel system, 2 wt % of castor oil is added to DME. The engine was warmed up until the cooling water temperature varied from 80 to 85 °C while the lubricating oil temperature was over 60 °C and, then, was loaded to the test points. Each measurement was repeated three times, and the average data were used for analysis. If the deviation of the measurement data is beyond the predefined limit, the measurement should be conducted again to avoid measurement errors. At each mode of operation, the engine was allowed to run for a few minutes until the exhaust gas temperature, the cooling water temperature, and the lubricating oil temperature stabilized before data were measured. Before running the engine with a new blended fuel, the new fuel was used to clean out the remaining fuel from the pipeline of the engine to avoid contamination with the leftover fuel. In each tested condition, the variation of speed was controlled within (5 r/min, the controlled precision of torque was 0.5%.
2. Experimental Apparatus and Test Fuels In this study, experiments were conducted on a direct-injection (DI), four-stroke, two cylinder, water-cooled diesel engine. The specifications of the test engine are listed in Table 1. The low heat value of DME is only 64.7% of that of diesel. In order to retain engine’s power output, according to the equal heating value principle, it is necessary to increase the amount of fuel injection per cycle. The amount of DME supplied per cycle is approximately 1.6 times that of diesel fuel in this experiment. These modifications include the following: the plunger diameter is enlarged from 8.5 mm for the diesel engine to 10.5 mm for the DME engine to maintain the equivalent amount of heat of fuel, a (15) Curran, H. J.; Pitz, W. J.; Westbrook, C. K.; Dagaut, P.; Boettner, J-C; Cathonnet, M. A Wide Range Modeling Study of Dimethyl Ether Oxidation. Int. J. Chem. Kinet. 1998, 30, 229–241. (16) Hunter, M. C.; Bartle, K. D.; Lewis, A. C.; McQuaid, J. B.; Myers, P.; Seakins, P. W. High Resolut. Chromatagr. 1998, 21, 75–80. (17) Hopkins, J. R.; Still, T.; Al-Haider, S.; Fisher, I. R.; Lewis, A. C.; Seakins, P. W. Atmos. Environ. 2003, 18, 2557–2565. (18) Dojahn, J. G.; Wentworth, W. E.; Stearns, S. D. J. Chromatogr. Sci. 2001, 39, 54–58.
(19) Wei, Y. J.; Liu, J.; Zhu, Z.; Li, G. L.; Liu, S. H. Tans. Chin. Soc. Intern. Combust. Engines 2008, 26, 533–537. (20) Wei, Y.; Liu, S.; Liu, F.; Liu, J.; Zhu, Z.; Li., G. Formaldehyde and Methanol Emissions from a Methanol/Gasoline-Fueled SparkIgnition (SI) Engine. Energy Fuels 2009, 3313–3318.
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Figure 2. NOx emissions.
Figure 1. CO emissions.
The fuel consumption uncertainty was less than 2%. The relative experimental error is less than 2% for the CO emission and 3% for the THC and NOx emissions.
3. Results In this paper, engine power was first studied under full load operating conditions. For emission studies, experiments were executed at two engine speeds of 1400 and 1870 r/min, which refers to ESC test engine speed nA and nB. The brake mean effective pressures (load) were set to be 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 MPa. At each test condition, experiments were carried out with pure diesel and DME. 3.1. Regulated Emissions. When DME is used as fuel, unburned DME will emit, and it impacts the formaldehyde emission as the change of fuel oxidation mechanism. Although the two emissions are not restricted in vehicle emission regulations in many countries, it is significant to investigate their emission characteristics. In this paper, the emission characters of CH3OCH3 and CH2O are presented. Moreover, THC emissions were revised by the CH3OCH3 and CH2O emissions. The regulated emissions are also presented. 3.1.1. CO Emission. Figure 1 presents variations of the CO emissions for diesel and DME at engine loads and speeds. Compared with diesel, CO emissions for DME averagely decrease 59% at 1400 rpm, and DME fuel averagely reduces the CO emissions by 23.7% at 1870 rpm. CO emission is the product of incomplete combustion, and it is controlled primarily by the fuel/air equivalence ratio. DME has good mixing characteristics and higher oxygen content so that the locations of the fuel-rich regions in the combustion period could be reduced, resulting in lower CO emissions. 3.1.2. NOx Emission. The variations of NOx emissions for diesel and DME at engine loads and speeds are shown in Figure 2. NOx emissions for the two fuels increase significantly when the engine load increases. DME shows greater NOx emission reductions in comparison to diesel at all the load conditions under two engine speeds. DME fuels averagely reduce NOx emissions by 45.2% and 57.9%, separately, in comparison to diesel at two engine speeds, because the fuel delivery advance angle is retarded and the larger amount of heat absorption caused by DME evaporation will delay the ignition timing. Therefore, when an engine is running on pure DME, the peak cylinder pressure decreases, because more fuel burns after TDC. Lower peak cylinder
Figure 3. Smoke emissions.
pressures results in lower peak temperatures. As a consequence, the NOx concentration starts to diminish. 3.1.3. Smoke Emission. Figure 3 gives the variations of the smoke emissions in terms of an extinction coefficient (unit, m-1) for diesel and DME at engine loads and speeds. As the load increases, the smoke emissions for diesel increase gradually. Smoke is almost zero for DME under all the engine operation conditions. It is well-known that soot is formed in fuel-rich regions under high temperature conditions. The proportion of fuel carbon forming soot precursors has been found to decrease with increased oxygen content in the fuel21 and with a decreased number of C-C bonds.22 It can be concluded that the soot formed in DME combustion should be almost zero at an oxygen content of 35% and no C-C bonds. The trade-off relationship between NOx and smoke emission is well-known for general diesel engines. Generally, if one technical means leads to the reduction of NOx emission, it will also cause smoke emission to increase simultaneously. In this study, the trade-off correlation between NOx and smoke emission is broken for the DME engine. (21) Westbrook, C. K. Chemical kinetic modeling of oxygenated diesel fuels in advanced petroleum-based and alternative fuels. DOE Report, 1999. (22) Ogawa, H.; Miyamoto, N.; Yagi, M. Chemical-kinetic analysis on PAH formation mechanisms of oxygenated fuels. SAE Paper 2003-01-3190, SAE Trans. J. Fuel Lubr. 2003; 112 (4), 2413-2421.
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: DOI:10.1021/ef9016043
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Figure 4. CH2O emissions.
Figure 6. THC emissions.
Figure 5. DME emissions.
Figure 7. Revised THC emissions.
3.2. Nonregulated emissions. Formaldehyde is an intermediate product of the combustion of diesel fuel and DME. For the diesel fuel oxidation process, formaldehyde is mainly produced from the decomposition reactions of methoxy.14 For DME, the predominant route to formaldehyde formation is the β-scission of the methoxy-methyl radicals.15 With the change in generation path, formaldehyde emission may have different characters, and when DME is used as fuel, unburned DME will be emitted. 3.2.1. Formaldehyde Emission. Figure 4 shows the experimental results of formaldehyde emissions at engine loads and speeds. As seen in these two figures, the formaldehyde content in the emission from the engine with DME operation is just a little higher than that with diesel fuel operation at 1400 r/min and 1870 r/min. As we know, formaldehyde is not only an intermediate in combustion but also a partial oxidation product in exhaust gas. For intermediate products, it depends on in-cylinder mean temperature, mixture composition, and residence time. The outlet temperature and stay time in the exhaust pipe affect the partial oxidation products. At low and medium temperature conditions, formaldehyde concentration is relatively constant. 3.2.2. DME Emission. The DME emissions are shown in Figure 5. Both engine load and engine speed have little effect on the DME emissions. As there is no DME emission from
diesel fuel, it can be concluded that DME is only from unburned DME fuel. Unburned DME will exist at a too lean or too rich region of the spray. At lower load, the ignition delay is longer; more fuel will mix with air to equivalence ratios lower than the lean limit of combustion. At higher load, the ignition delay is shorter and more fuel is injected after the ignition delay period. Slow mixing of fuel with air results an over-rich mixture, so more unburned fuel will be presented in the exhaust. 3.2.3. THC Emission. Figure 6 illustrates variations of the HC emissions for diesel and DME at engine loads and speeds. As shown in the Figure 7, load shows no significant effect on the HC emissions for both of the fuels. However, when comparing Figure 5 with Figure 6, DME emission is a little higher than THC emission. It is known that FID response differs from the kind of HC, especially if the HC coexists with oxygen. The response of CH3OCH3 is 0.5 times that of CH4, and FID has no response on CH2O. Therefore, the real value of THC emission must be revised by the data measured by both the Horiba device and the GC device. THC emission should be THCreal ¼ THCHoriba þ 0:5CCH3 OCH3 þ CCH2 O The revised THC emissions are presented in Figure 7. It is shown that with this revision, the THC emissions from DME fuel are very close to that from the diesel fuel. 2468
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: DOI:10.1021/ef9016043
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HC emissions consist of completely or partially unburned fuel, mainly produced in locations where combustion takes place under fuel-rich or fuel-lean conditions. These fuel/air mixture can become too lean to autoignite, or the mixture may be too rich to ignite or support a flame.23 The other two sources of fuel which enter the cylinder during combustion and which result in HC emissions is due to slow or under mixing with air and the wall quenching of the flame. Although the oxygen content in DME will reduce the fuelrich region, the good mixing characteristics of DME will enlarge the fuel lean region.
investigated and compared to the baseline diesel fuel. The main results can be obtained as follows: (1) Compared with diesel, CO emissions for DME averagely decrease 59% at 1400 rpm, and DME fuel averagely reduces the CO emissions by 23.7% at 1870 rpm. (2) DME shows greater NOx emission reductions in comparison to diesel at all the load conditions. DME fuels averagely reduce NOx emissions by 45.2% and 57.9%, separately, in comparison to diesel at two engine speeds. Smoke is almost zero for DME under all the engine operation conditions. The NOx/smoke trade-off correlation was broken. (3) Our measurements indicate that the HCHO exhaust concentration from the engine with DME operation is 50-100% higher than that with diesel fuel. Both engine load and engine speed have little effect on the DME emissions. (4) The THC emissions from the DME engine are very close to that from the diesel engine.
4. Conclusions The effects of DME on performance and emissions of a 2-cylinder compression ignition (CI) engine have been
Acknowledgment. This work was supported by National Natural Science Foundation of China.
(23) Heywood, J. B. Internal combustion engine fundamentals; McGraw-Hill Book Company: New York, 1988.
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