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Energy & Fuels 2007, 21, 1454-1458
Performance and Emissions of a Compression-Ignition Engine Fueled with Dimethyl Ether and Rapeseed Oil Blends Wang Ying* and Zhou Longbao School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, P. R. China ReceiVed January 16, 2007. ReVised Manuscript ReceiVed March 8, 2007
Dimethyl ether (DME) and rapeseed oil are two kinds of promising alternative fuels for compressionignition (CI) engines. Performance and emissions of a CI engine fueled with different proportions of DME/ rapeseed oil blends are investigated in this paper. The results prove that an engine can run well with DME/ rapeseed oil blends under all operating conditions. With an increase of the rapeseed oil concentration in blends, the power and torque of an engine can be improved. However, NOx emission becomes worse, simultaneously. Smokeless combustion can be realized with less than about a 6% rapeseed oil mass fraction in the blend. Above more than about a 6% rapeseed oil concentration, smoke emission occurs and becomes more and more severe with an increase of the rapeseed oil mass fraction. An increase in the rapeseed oil mass fraction will also result in an increase of the ignition delay, the fraction of fuel burned in the premixed combustion phase, and the maximum cylinder pressure.
1. Introduction In response to the greater tightening of standards for exhaust emissions and the depletion of conventional petroleum fuel, the search for new solutions has intensified all over the world. On the one hand, research work toward reducing emissions leads to improved fuel injection equipment or combustion systems for engines; on the other hand, alternative fuels are increasingly pursued because of their potential to cut down emissions drastically while keeping engine equipment costs acceptable. Up to now, many kinds of alternative fuels have been investigated. Dimethyl ether (DME) is found to be one of the most promising alternative fuels for diesel engines. DME, mainly made from coal, is an oxygenate fuel and has no carboncarbon bonds, and it exhibits a higher cetane number than diesel fuel and has excellent autoignition characteristics. Many investigations indicate that high power output, smokeless combustion, and low NOx emissions can be realized in DME engines. Flesich et al.1 performed emission tests on a Navistar T444E 7.3-liter turbocharged diesel engine and demonstrated that the results are below 1998 California Ultralow Emission Vehicle regulations for medium-duty vehicles without the use of exhaust aftertreatment. The results can also meet EURO III heavy-duty truck emission standards. Sorenson and Mikkelsen2 had studied DME in a modified diesel engine, and their results showed that the engine could achieve ultralow-emission prospects without a fundamental change in combustion systems. Oguma et al.3 studied atomization characteristics for various ambient pressures * Corresponding author e-mail:
[email protected]. (1) Fleisch, T.; McCarthy, C.; Basu, A.; Udovich, C.; Charboneau, P.; Slodowske, W.; Mikkelsen, S.-E.; McCandless, J. A New Clean Diesel Technology: Demonstration of ULEV Emissions on a Navistar Diesel Engine Fueled with Dimethyl Ether. SAE Tech. Pap. Ser. 1995, 950061. (2) Sorenson, S. C.; Mikkelsen, S.-E. Performance and Emissions of a 0.273 Liter Direct Injection Diesel Engine Fuelled with Neat Dimethyl Ether. SAE Tech. Pap. Ser. 1995, 950064. (3) Oguma, M.; Hyun, G.; Goto, S.; Konno, M.; Kajit, S. Atomization Characteristics for Various Ambient Pressure of Dimethyl Ether (DME). SAE Tech. Pap. Ser. 2002, 2002-01-1711.
of dimethyl ether and its influence on emission. Eirich et al.4 introduced the development of a DME-fueled shuttle bus. Wang et al.5 investigated the combustion and emission characteristics in a vehicle engine with DME and determined that the DME engine had a high thermal efficiency, short premixed combustion, and fast diffusion combustion. Wang et al.6 also carried out research on diesel emission improvements by the use of oxygenated DME/diesel blend fuel and found that the smoke emission of engines fueled with DME/diesel blends can be reduced drastically at high loads. Although DME fuel has some excellent properties suitable for diesel engines as mentioned above, many experiments prove that the sliding parts in the fuel system have a high wear rate when pure DME is used as a fuel due to its low viscosity (about 1/20 that of diesel in a laboratory environment) and poor lubricity. Kajitani et al.7 found that the engine ran quite smoothly for a period of about 30 min from the start, but thereafter some unstable changes in speed and load were observed. This was soon followed by a severe stall. A careful inspection of the engine revealed serious wear on the needle of the injector nozzle, and a rather remarkable amount of scoring on the injector plunger. Thus, special treatment on the surface of sliding parts of the fuel injection system must be made or a certain proportion of lubricant is added into DME to avoid their abnormal wear. Sivebake and Sorenson8 investigated the lubricity of DME by MFPRR2 and also found that the wear scar diameter of DME (4) Eirich, J.; Chapman, E.; Glunt, H.; Klinikowski, D.; Boehman, A. L.; Hansel, J. G.; Heydorn, E. C. Development of a Dimethyl Ether (DME)Fueled Shuttle Bus. SAE Techn. Pap. Ser. 2003, 2003-01-0756. (5) Ying, W.; Longbao, Z.; Zhongji, Y.; Hongyi, D. Study on Combustion and Emission Characteristic of a Vehicle Engine Fuelled with DME. Proc. Inst. Mech. Eng., Part D 2005, 219 (D2), 263-269. (6) Ying, W.; Longbao, Z.; Hewu, W. Diesel Emission Improvements by the Use of Oxygenated DME/Diesel Blend Fuels. Atmos. EnViron. 2006, 40 (13), 2313-2320. (7) Kajitanl, S.; Chen, Z. L.; Konno, M. Engine Performance and Exhaust Characteristics of Direct-Injection Diesel Engine Operated with DME. SAE Tech. Pap. Ser. 1997, 972973. (8) Sivebaek, I. M.; Sorenson, S. C. Dimethyl Ether (DME)sAssessment of Lubricity Using the Medium Frequency Pressurized Reciprocating Rig Version 2 (MFPRR2). SAE Tech. Pap. Ser. 2000, 2000-01-2970.
10.1021/ef0700230 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007
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Table 1. Specifications of CA498 Engine bore × stroke/mm × mm cylinder number displacement/L compression ratio rated power/kW rated speed/rpm nozzle opening pressure/MPa plunger diameter /mm
98 × 105 4 3.168 18.5 62 3200 15 9.5
is larger than that of diesel. Teng et al.9 found that the liquid with long-chain, polar-headed molecules is a of good additive for DME operation. It is well-known that rapeseed oil has long-chain, polarheaded molecules and has a high viscosity. Rapeseed oil is renewable and appears to be an alternative green fuel substitute for diesel. Bao and He,10 Labeckas and Slavinskas,11 Tsolakis and Megaritis,12 and Nwafor et al.13,14 studied the performance and emissions of compression-ignition fueled with rapeseed oil. They found that physical and chemical characteristics of rapeseed oil are similar to those of diesel fuel and good performance can be achieved with the engines powered by rapeseed oil. In addition, DME and rapeseed oil have good cosolubility above 273 K at normal temperature and pressure when the rapeseed oil mass fraction is below 10% on the basis of our experiments. So it may be wise to use a DME/rapeseed oil blend as alternative fuel for combustion-ignition (CI) engines. Good performance and improvement of the poor lubricity of DME fuel can be expected when the engine is fueled with a blend of DME/rapeseed oil. The objectives of this study are to formulate the blend of DME/rapeseed oil and study the characteristics of performance and emissions in a compression ignition engine. 2. Experimental Setup and Fuel Properties Experiments are conducted on a four-cylinder, naturally aspirated, four-stroke, water-cooled, direct-injection diesel engine manufactured by FAW Jiefang Automotive Company, Ltd., Dalian Diesel Engine Company. Engine specifications are shown in Table 1. The schematic diagram of the experiment setup is illustrated in Figure 1. In this study, an electric dynamometer assembled on a fourcylinder and four-stroke direct-injection diesel engine has been used. There are different thermo junctions and electrical units on the dynamometer and the engine. Circuits in all units have been connected to each other, and they have been controlled by a computer. As DME has a high vapor pressure in the laboratory environment, the fuel system of the four-cylinder engine is pressurized at 1.8 MPa by an electrical pump in order to prevent excessive leakage and vapor lock in the fuel system. On the basis of our experiments, DME and rapeseed oil have good cosolubility, and their stratified temperatures are below 273 K at normal temperature and pressure when the rapeseed oil mass (9) Teng, H.; McCandless, J. C.; Schneyer, J. B. Viscosity and Lubricity of (Liquid) Dimethyl EthersAn Alternative Fuel for Compression-Ignition Engines. SAE Tech. Pap. Ser. 2002, 2002-01-0862. (10) Bao, Y. D.; He, Y. Study on Noise of Rapeseed Oil Blends in a Single-Cylinder Diesel Engine. Renewable Energy 2006, 31 (11), 17891798. (11) Labeckas, G.; Slavinskas, S. The Effect of Rapeseed Oil Methyl Ester on Direct Injection Diesel Engine Performance and Exhaust Emission. Energy ConVers. Manage. 2006, 47 (13-14), 1954-1967. (12) Tsolakis, A.; Megaritis, A. Exhaust Gas Assisted Reforming of Rapeseed Methyl Ester for Reduced Exhaust Emission of CI Engine. Biomass Bioenergy 2004, 27 (5), 493-505. (13) Nwafor, O. M. I. Emission Characteristics of Diesel Engine Operating on Rapeseed Methyl Ester. Renewable Energy 2004, 29 (1), 119129. (14) Nwafor, O. M. I.; Rice, G.; Ogbonna, A. I. Effect of Advanced Injection Timing on the Performance of Rapeseed Oil in Diesel Engines. Renewable Energy 2000, 21 (3-4), 433-444.
Figure 1. Schematic diagram of experimental equipment.
fraction is below 10% in blends. The properties of test fuels are given in Table 2. It can be seen that the oxygen mass fraction in the five kinds of blends decreases from 34.31% to 32.36%. The cetane number of blends decreases and the viscosity increases with the addition of rapeseed oil by estimation. The fuel properties also show that DME has a high oxygen content and cetane number, while the calorific value is less and the viscosity is low compared to those of pure diesel fuel and rapeseed oil. In the experiments, the above five fuel blends with different rapeseed oil mass fractions are tested on the CA498 engine. Meanwhile, these parameters are compared with those of pure diesel fuel in order to clarify the effect of blends on the combustion and emission of engines (a CI engine cannot run for much longer of a period with pure DME fuel, so a comparison is only made with pure diesel fuel). Gaseous emissions are measured by an AVL exhaust gas analyzer, in which NOx is analyzed with a chemiluminescent detector and CO and CO2 are analyzed with a nondispersive infrared analyzer. HC is analyzed with a flame ionization detector. CO, HC, CO2, and NOx emissions are average values of the acquired data at each steady-state operating condition. Smoke is measured by a part-flow smoke opacimeter (AVL Dismoke 4000). All of the tests are repeated.
3. Results and Discussion 3.1. Power and Torque Output. Figure 2 illustrates the power and torque output for different fuels at various speeds (speed characteristics at full load). It can be seen that the power and torque output of the engine increases with an increase of the rapeseed oil mass fraction in blends as the calorific value of DME is about 71% of that of rapeseed oil, and the calorific value of blends decreases from 27 826 kJ/kg to 28 730 kJ/kg. Moreover, owing to the lower calorific value of the blend compared to diesel fuel, the fuel supply amount per cycle for blend operation is enlarged by increasing the plunger stroke of the fuel pump in order to make the power and torque output of the blends approach those of the corresponding diesel engine. 3.2. Combustion Characteristic. Figure 3 shows the indicator diagrams of the compression-ignition engine fueled with blend 1, blend 3, blend 5, and diesel. The diagrams are obtained for the same speed (1800 rpm) and brake mean effective pressure (BMEP; 0.71 MPa) for different fuels. It can be seen that the value of maximum cylinder pressure increases with the addition of the rapeseed oil mass fraction. The reason for this is that the temperature and pressure in the cylinder increase owing to the rise in temperature of the blend with the reduction of the amount of DME evaporation. In addition, the longer ignition delay of rapeseed oil will increase the amount of fuel being injected into the cylinder prior to combustion, which would result in the higher peak cylinder pressure. It can also be expected that the rate of maximum cylinder pressure of each blend is lower than that of diesel fuel. Figure 4 exhibits the curves of the heat release for blend 1, blend 3, blend 5, and diesel at conditions of 1800 rpm and a 0.71 MPa BMEP. The results show that an increase in the
1456 Energy & Fuels, Vol. 21, No. 3, 2007
Ying and Longbao
Table 2. Physical and Chemical Properties of Test Fuelsa properties
diesel
DME
rapeseed oil
blend 1
cetane number low calorific value (kJ/kg) density (kg/m3) viscosity (cP) stoichiometric air/fuel ratio (kg/kg) wt % of carbon wt % of hydrogen wt % of oxygen C/H
40-55 42 500 840 3.61 14.6 86.0 14.0 0 6.14
>55 27 600 660 0.15 9.0 52.2 13.0 34.8 4.02
32.2 38 900 920 23.6 12.56 77.1 12.1 10.4 6.37
>32.2 27 826 665.2 0.15∼23.6 9.07 52.698 12.982 34.312 4.06
blend 2
blend 3
blend 4
blend 5
28 052 670.4
28 278 675.6
28 504 680.8
28 730 686
9.14 53.196 12.964 33.824 4.10
9.21 53.694 12.946 33.336 4.15
9.28 54.192 12.928 32.848 4.19
9.356 54.69 12.91 32.36 4.24
a Blend 1: 2% rapeseed oil + 98% DME. Blend 2: 4% rapeseed oil + 96% DME. Blend 3: 6% rapeseed oil + 94% DME. Blend 4: 8% rapeseed oil + 92% DME. Blend 5: 10% rapeseed oil + 90% DME. The low calorific values of DME/rapeseed oil blends are estimated by theoretical calculation.
Figure 4. Heat release of different fuels.
Figure 2. Torque and power output for different fuels. Figure 5. Ignition delay of different fuels.
Figure 3. Indicator diagrams of different fuels.
rapeseed oil mass fraction will result in an increase of the heat release and the fraction of fuel burned in the premixed combustion phase. The figure also shows a delay of the heat release initiating timing in the case of DME/rapeseed oil blends compared with pure diesel fuel, and this delay would decrease with the addition of rapeseed oil. An increase of the rapeseed oil fraction will result in a decrease of the cetane number of
blends, which in turn leads to the long ignition delay, and eventually brings out an increase of the heat release rate and the fraction of fuel burned in the premixed combustion phase. More fuels are needed in the DME/rapeseed oil blend for getting the same BMEP as diesel, and this causes an increase in the duration of fuel injection; furthermore, the oxygen-containing fuel blends will improve combustion in the diffusive phase and shorten the combustion duration of the diffusive burning phase as shown in Figure 4. The heat release process almost completes at the same crank angle, and this and another aspect provided evidence to support the fast diffusive burning phase in the case of DME/rapeseed oil operation. Figure 5 gives the ignition delay of blend 1, blend 3, blend 5, and diesel at conditions of 1800 rpm and a 0.71 MPa BMEP. Generally speaking, the ignition delay shows an increase with an increase of the rapeseed oil mass fraction in the blend. As explained above, the lowering cetane number with increased rapeseed oil addition may be responsible for the increase of ignition delay. 3.2. Brake Thermal Efficiency. Figure 6 gives the brake thermal efficiencies of different fuels. It is obvious that the brake
Performance of a Compression Ignition Engine
Figure 6. Comparisons of the brake thermal efficiency of different fuels.
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Figure 8. CO emissions of different blends at 1800 rpm.
Figure 9. HC emissions of different blends at 1800 rpm. Figure 7. NOx emission of different blends at 1800 rpm.
thermal efficiency of the engine is enhanced slightly with an increase of the rapeseed oil concentration in blends. The reason for this is that the decrease in cetane number of the fuel blends also results in a long ignition delay, which in turn brings a high combustion rate in the premixed burning phase and a high cylinder pressure due to the faster heat release rate. In addition to this, the higher calorific value of the blend leads to the release of more heat during the same time period. It is also found that the brake thermal efficiency of the DME/rapeseed oil blend is a little lower than that of the conventional diesel engine because the postcombustion period becomes a little longer and a secondary injection may exist in the fuel system for blend operation. 3.3. NOx Emission. Figure 7 shows NOx emission curves of different blends at 1800 rpm. It can be seen that the NOx emission becomes worse with an increase of the rapeseed oil concentration due to the increase in the fraction of fuel burned in the premixed combustion phase and the presence of a high combustion temperature. The results also show that the addition of rapeseed oil in DME fuel has a strong influence on NOx emission at high engine loads, while it has little influence at low engine loads. Previous experiments show that, due to a larger amount of heat absorption caused by DME evaporation and a smaller amount of the premixing mixture of DME fuel, NOx emission for DME operation is about 40% of that for diesel operation.5 So it would be also expected that NOx emission of DME/ rapeseed oil blends is much lower than that of diesel. 3.4. CO Emission. Figure 8 illustrates CO emission curves of different blend fuels at 1800 rpm. It is clear that the CO emission decreases with an increase of rapeseed oil. This may be attributed to two facts about DME fuel: (1) The lower
modulus of elasticity of DME can easily result in a secondary injection in the fuel system and leads to insufficient combustion and higher CO emission finally. (2) The low combustion temperature and thick quenching layer caused by the high latent heat of DME result in a low oxidation rate of CO. Figure 8 also shows that CO emission increases with an increase of BMEP. The higher excess air ratio of the mixture may be accounted for by the increase of CO emission at a high BMEP. 3.5. HC Emission. Figure 9 illustrates HC emission curves of different blends at 1800 rpm. Unlike the behavior of NOx and CO emissions versus the mass fraction of rapeseed oil, the exhaust HC concentration shows less variation with the addition of rapeseed oil. It is suggested that adding a small proportion of rapeseed oil almost has no remarkable influence on the characteristics of quick evaporation of fuels and no increment impact on HC formation. Figure 9 also shows that HC emission reduces with an increase of BMEP. It is possible that, at the light-duty level, the amount of fuel injected into the cylinder per cycle is small and the temperature of the fuel/air mixture and the surface of the cylinder is low, which makes the flame front of the spray abruptly quench on the surface and produces HC. At a high load, due to a high cylinder temperature, the blend evaporates quickly and the mixture becomes more homogeneous so that the HC emissions are reduced. 3.6. Smoke Emission. Figure 10 exhibits how smoke varies at an intermediate speed of 1800 rpm. It can be found that, under about a 6% rapeseed oil concentration, almost no smoke emission can exist because the oxygenate blend and the fast diffusion combustion suppress smoke formation. The results also show that the existence of rapeseed oil in blends has a large influence on smoke emission when the
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Figure 10. Smoke emission of different blends at 1800 rpm.
rapeseed oil mass concentration is above 6% or so. Above a 6% rapeseed oil concentration in the blend, smoke emission becomes worse with an increase of the rapeseed oil mass (the decrease of oxygen content as shown in Table 2), as expected. 4. Conclusion Although there are some practical obstacles (such as a complicated fuel system and a higher price of DME) for the application of DME/rapeseed oil blend fuels, it is still one kind of good alternative fuel for compression-ignition engine. Exhaust emissions of an engine fueled with DME/rapeseed oil are much lower than those of diesel engines, excluding CO emission. The rate of peak cylinder pressure and the rate of maximum pressure rise of the blend fuel are lower than those of diesel fuel, which means combustion noise of the blend fuel is lower than that of diesel. Furthermore, when a large-scale DME production is developed, the cost of blend fuel may be lower than that of diesel. The study on performance and emissions of a compression ignition engine fueled with DME and rapeseed oil blends has been conducted, and the main results are summarized as follows:
Ying and Longbao
A careful examination of the fuel system is carried out after all experiments. Especially, the plunger surface and injector surface are examined by scanning microscopy. There is no obvious wear on the fuel system of the CI engine. The engine operates well with blends of rapeseed oil and DME under all conditions. The power and torque output of the CI engine can be improved with an increase of the rapeseed oil concentration in the blend. An increase in the rapeseed oil mass fraction will result in an increase of the heat release, the fraction of fuel burned in the premixed combustion phase, and the rate of maximum cylinder pressure. Smokeless combustion can be realized in the engine with less than about a 6% rapeseed oil mass fraction in the blend. However, when the mass fraction of the rapeseed oil in the blend is above about 6%, smoke will occur and become more and more severe with an increase of the rapeseed oil mass fraction. NOx emission increases and CO emission decreases with an increase of the rapeseed oil mass fraction in the blend. HC emission shows less variation with the addition of rapeseed oil. Lots of studies on rapeseed oil and DME blends as fuel in the CI engine should be carried out in the future. In our studies, experiments are made with a traditional CI engine, and experiments must be extended to the common-rail CI engine in the next step. Optimization on parameters of the fuel and combustion system must be carried out to improve the performance and economics. A numerical simulation calculation of the DME/rapeseed oil blend should be conducted. An endurance experiment must be continued to examine the performance of the engine with DME and rapeseed oil blends. Acknowledgment. The authors wish to express their deep thanks to Project of National Science Foundation of Xi’an Jiaotong University (Approval No.: xjj2004009). EF0700230
