Experimental Investigation of Spray and Combustion Characteristics of

Energy & Fuels 2007, 21, 793-800

793

Experimental Investigation of Spray and Combustion Characteristics of Dimethyl Ether in a Common-Rail Diesel Engine Myung Yoon Kim, Seung Hwan Bang, and Chang Sik Lee* Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-Dong, Sungdong-Gu, Seoul 133-791, Korea ReceiVed July 5, 2006. ReVised Manuscript ReceiVed December 27, 2006

Various characteristics of dimethyl ether (DME) as an alternative fuel for compression ignition engines were experimentally investigated including its spray characteristics, combustion performance, and emission reduction in a common-rail diesel engine. The spray behavior of DME was analyzed in terms of injection rate, spray development, and spray tip penetration, which were measured by an injection rate meter and a highpressure spray chamber equipped with a spray visualization system. In addition, the engine performance and indicated mean effective pressure (IMEP), as well as exhaust emissions, including oxides of nitrogen (NOx), soot, hydrocarbons, and carbon monoxide were measured at various injection and operating cycle parameters. The combustion characteristics of DME fuel were compared with those of conventional diesel fuel in a diesel engine. Experimental results show that DME has an injection delay 0.03 ms shorter and a maximum injection rate 21% higher than those of diesel fuel at a constant injection pressure of 50 MPa and an injection mass of 8 mg/cycle. At a fixed energizing duration and injection pressure, a greater mass of DME was injected than that of diesel fuel. The DME-operated engine produced almost negligible soot emissions but also considerably higher NOx emissions than the engine operated with diesel fuel at a fixed IMEP.

1. Introduction Recently, the increasing price of crude oil and legislation mandating sustainable resource management have made the use of clean and renewable fuel more desirable. To cope with the depletion of petroleum resources in the future and the problem of global warming, use of alternative fuels such as biodiesel, alcohols, natural gas, hydrogen, and dimethyl ether (DME) is under intense investigation. Compared to some other candidates, DME appears to have great potential and should be considered as the fuel of choice for eliminating dependency on petroleum. In particular, DME can be used as a clean, high-efficiency fuel for the compression ignition engine and exhibits reduced near zero particulate matter (PM) emission. The benefits of DME as a fuel for the internal combustion engine are significant. First, DME has a high cetane number and, therefore, is appropriate for compression ignition (CI) engines; in particular, its cetane number is higher than that of diesel fuel. Generally, it is known that CI engine exhibits higher thermal efficiency than that of spark ignition (SI) engines due to the higher compression ratio and absence of throttle loss. Therefore, DME-fueled CI engines exhibit high thermal efficiency. Second, DME is an oxygenated fuel with 35 wt % oxygen and does not have carbon-carbon bonds in its chemical structure,1 thus producing virtually no soot and exhibiting increased tolerance for EGR (exhaust gas recirculation), which can reduce NOx emissions.2,3 Furthermore, the excellent evaporation char* To whom correspondence should be addressed. Telephone: +82-22220-0427. Fax : +82-2-2281-5286. E-mail: [email protected]. (1) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Tech. Pap. Ser. 2004, 2004-01-0093. (2) Kajitani, S.; Chen, Z. L.; Konno, M.; Rhee, K. T. SAE Tech. Pap. Ser. 1997, 972973. (3) Teng, H.; McCandless, J. C. SAE Tech. Pap. Ser. 2006, 2006-010053.

Table 1. Experimental Conditions for the Measurements of Injection Rate injection pressure tube pressure injection mass energizing duration

DME diesel

50 MPa 3.5 MPa 8 mg 468 µs 598 µs

acteristics reduce wall wetting due to impinged fuel spray, allowing an early timing injection strategy for HCCI (homogeneous charge compression ignition) combustion that can be used without major modifications to the fuel injection system. Because of these characteristics, vehicles that use DME fuel can exhibit low emissions and high efficiency. As some research has shown, however, while DME burns well in a direct injection (DI) diesel engine at light and medium loads at all speeds, combustion deteriorates at high load and high speeds. This deterioration causes higher CO emissions and fuel consumption than when the engine is operated with diesel fuel. These phenomena can be explained by poor spray penetration and faster ignition which cause insufficient air entrainment in the DME spray.4 The spray characteristics of DME for DI diesel engines have been widely studied. It is reported that DME spray has a relatively large spray angle and a turbulent, jet-like boundary (which can be explained by the rapid evaporation of spray droplets).5,6 The characteristics of the fuel-air mixing process greatly affect the combustion and exhaust emission characteristics of a direct injection compression ignition engine. Moreover, differences between the fuel properties of DME and diesel fuel lead (4) Teng, H.; McCandless, J. C. SAE Tech. Pap. Ser. 2005, 2005-011723. (5) Teng, H.; McCandless, J. C. SAE Tech. Pap. Ser. 2003, 2003-010759. (6) Yu, J.; Lee, J.; Bae, C. SAE Tech. Pap. Ser. 2002, 2002-01-2898.

10.1021/ef060310o CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007

794 Energy & Fuels, Vol. 21, No. 2, 2007

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Figure 2. Schematic of combustion chamber and fuel spray. Figure 1. Schematic of test engine and fuel supply and injection system. Table 2. Specification of the Test Engine bore × stroke displacement volume compression ratio valve system fuel injection system injector

hole number hole diameter spray angle

75 × 84.5 mm 373.3 cm3 17.8 DOHC 4 valves Bosch common-rail 6 0.128 mm 156°

to different spray development and combustion characteristics. Comparative studies concerning spray and combustion characteristics of DME and diesel fuel in DI diesel engines are, thus, necessary for understanding in detail the relationship between spray and combustion characteristics. The purpose of this work is to investigate the spray characteristics of DME fuel and to compare the spray behavior and injection characteristics of the DME fuel with conventional diesel fuel, using measurements of those characteristics made in a high-pressure spray chamber. On the basis of the results of spray visualization and the injection rate for the DME fuel, the combustion characteristics and emission reduction for the DME fuel at various injection and cycle parameters in a high pressure injection direct injection diesel engine were discussed. 2. Experimental Apparatus and Procedure 2.1. Injection Rate and Spray Visualization. 2.1.1. Injection Rate of DME and Diesel Fuel. In a direct injection diesel engine, injection parameters, such as the injection rate profile, injection delay, and injection duration significantly influence the combustion and exhaust emission characteristics. In this study, the injection rate profile was measured with an injection rate measurement system based on the Bosch method.7,8 In this method, fuel was injected into a tube equipped with a sensor that measured pressure variation. The pressure profile could be converted into the injection rate by the correlations with the total injected mass measured by an electronic balance (GP-30K, AND) and fuel pressure in the tube. While the injection rate was measured, the pressure of the tube was set equal to 3.5 MPa to maintain the pressure condition similar to that of the charge at near top dead center (TDC) in the combustion chamber of the test engine. In this study, 300 continuous injections were averaged for a test case. The experimental conditions for these measurements are listed in Table 1. 2.1.2. Spray Visualization. In this work, macroscopic spray characteristics, such as spray tip penetration, spray cone angle, and spray development process, were investigated to analyze the differences in the spray development of DME and diesel fuel. Frozen spray images were acquired, using a spray visualization system, which uses a Nd:YAG laser (SL2-10, Continuum) with a (7) Bosch, W. SAE Tech. Pap. Ser. 1996, 660749. (8) Park, S. W.; Lee, C. S. Exp. Fluids 2004, 37, 745-762.

Table 3. Experimental Conditions for the Engine Test engine speed injection pressure injection mass intake pressure EGR rate coolant temperature oil temperature

1500 rpm 35, 50 MPa 4, 8, 12 mg/cycle 0.1 MPa (naturally aspirated) 0, 45% 70 °C 70 °C

Table 4. Properties of DME and Diesel Fuel properties (kg/m3)

liquid density stoichiometric A/F (kg/kg) cetane number sulfate contents (ppm) lower heat value (MJ/kg) autoignition temperature (°C) kinetic viscosity (kg/ms) sulfate contents (ppm)

DME

diesel fuel

660 9.0 68 0 28.9 350 0.25 ∼1

828.2 14.6 56 16.3 42.5 250 2-3 16.3

wavelength of 532 nm as a light source, an optical system for generating a sheet beam, a high-resolution ICCD (intensified charge coupled device) camera, and an image grabber. The laser system produced a sheet beam with a thickness less than 1 mm that illuminated the spray, and the ICCD camera captured the frozen spray image, which was then stored on a personal computer, using an image grabber. The high-pressure chamber was filled with nitrogen gas at a pressure of 3.5 MPa to form a similar pressure condition to an actual charge in the combustion chamber of the engine at the end of a compression stroke. 2.2. Engine Performance and Exhaust Gas Analysis. Figure 1 shows a schematic of the experimental system. In this work, the experimental apparatus was composed of the test engine, the fuel injection system, the exhaust emission analyzer, and the powerperformance measuring system. Engine starting and loading were accomplished, using a DC dynamometer with a maximum brake power of 55 kW. Fuel injection parameters, including the injection pressure, injection timing, and injection duration, were controlled with a timing pulse generator and a universal injection driver (TDA-3200, TEMS). An optical angular encoder allowed for the synchronization of the injection timing with a resolution of 0.1 crank angle degrees. To pressurize the fuel in the common-rail, two high-pressure pumps (HSF-300, Haskel) were used. Exhaust gas component measurements from the engine were conducted by a NOx, soot, and HC/ CO analyzer. The in-cylinder pressure was measured via a piezoelectric sensor (6052B2, Kistler), coupled with a charge amplifier (5011B, Kistler). The pressure history was recorded via a data acquisition board and a combustion analysis program. The engine tests were carried out with a single-cylinder DI diesel engine with a displacement volume of 373.3 cm3. The specifications of the test engine are listed in Table 2. Figure 2 shows the spray shape of the injected fuel according to the injection angle and the piston position corresponding to the crank angle. The engine was operated at a constant speed of 1500 rpm, and injection timing was varied within the range from -70

Spray/Combustion Characteristics of Dimethyl Ether

Energy & Fuels, Vol. 21, No. 2, 2007 795

Figure 3. Relation between energizing duration and injection mass.

to +2 ATDC (after top dead center). Detailed experimental conditions for the engine test are listed in Table 3. 2.3. Test Fuels and Fuel Additive. The physical properties of DME and the diesel fuel used in this analysis are compared in Table 4. To substitute for a diesel fuel, 1.47 kg of DME is required for every 1 kg of diesel fuel because the heating value of DME is only 68% of that of diesel. Considering the heating value and air fuel ratio of the two fuels, the air requirement by mass of DME in the diesel fuel equivalent to generate the same heat is 13.26 kg, which is less than the 14.6 kg of air requirement for 1 kg of diesel fuel. It reveals that a DME engine can generate more output than that operated with diesel fuel in the same engine with a fixed mass of intake charge. Previous research3 has proved the fact that a DME engine has potentials to generate more power under the same cylinder size and excess air coefficient (λ) compared to a diesel engine due to the lower air requirements for equivalent heating value. An et al.9 also proved that a DME fueled CI engine could generate more power than a diesel fueled engine by their engine test. It is known that the low lubricity of neat DME causes severe wear on the fuel injection components, including the fuel injection pump, injector needle, and nozzle; therefore, additives for lubricity are required to ensure the durability of the system.2 In this work, 1000 ppm of an additive for lubricity (LUBRIZOL 539M, LUBRIZOL) was added into the neat DME.

3. Experimental Results and Discussion 3.1. Injection Quantity and Injection Rate. To analyze the injection mass per injection event at a constant energizing duration, the injected fuel was measured for various energizing durations, and the results are shown in Figure 3. The mass of the injected fuel was determined by measuring the mass of the consumed fuel in the fuel tank using an electronic balance (GP30K, AND). The mass of injected fuel through a single injection event was calculated by measuring the accumulated mass of fuel through 1000 continuous injections. The diesel fuel was injected at a fixed pressure of 50 MPa, and DME was injected at pressures of 35 and 50 MPa. A greater mass of DME (than that of diesel fuel) was injected at a constant energizing duration and a constant injection pressure of 50 MPa. It was expected that the lower kinetic viscosity of DME increased fuel mass flow in the nozzle, despite the density of DME being 80% of that of diesel fuel. Figure 4 shows the injection rate profile and injection current for 8 mg injection masses of both DME and diesel fuel. To fix the injection mass between the different fuels, energizing durations of 468 and 598 µs were used for DME and diesel fuel, respectively, according to the results acquired in Figure 3. The injection pressure was held constant at 50 MPa, and the (9) An, B.; Sato, Y.; Lee, S.; Takayanagi, T. SAE Tech. Pap. Ser. 2004, 2004-01-1864.

Figure 4. Comparison of injection current and rate of injection for (a) DME and (b) diesel fuel for an injection pressure of 50 MPa and an 8 mg injection mass.

data was averaged from 300 samples. The injection delay, which can be defined as the time interval between the start of the energizing (SOE) signal and the actual start of injection (SOI) was reported by the injection rate meter as 0.26 ms for DME, which was shorter than that of diesel fuel by 0.03 ms. The DME fuel showed a higher maximum injection rate than did diesel fuel, because the low kinetic viscosity of DME promoted faster flow through the injector nozzle as mentioned previously. Due to the faster injection rate, the actual DME injection duration was shorter than that of diesel fuel. The spray development process, at a constant injection pressure of 50 MPa, an ambient pressure of 3.5 MPa, and an injection mass of 8 mg, is shown in Figure 5. The number at the bottom of each figure gives the time after SOI (start of injection). The SOI was defined as the first appearance of liquidphase fuel in each case. As above, DME and diesel fuel both exhibited an SOI time interval of 0.03 ms under this experimental condition. As Figure 4a illustrates, the actual injection of DME terminated 0.9 ms after the start of energizing (SOE). That timing is 0.74 ms after SOI, considering the 0.26 ms injection delay. As the actual injection ended (0.74 ms after SOI), DME spray vanished. The spray tip penetration of the two fuels (Figure 6) was determined using the frozen spray images (Figure 5). As the figure shows, the spray tip penetration of DME was shorter than that of diesel fuel. This occurred because the DME atomized faster due to its lower kinetic viscosity and evaporated quickly due to the high evaporation rate. Moreover, the DME droplet has lower density than diesel fuel, and the deceleration of the DME droplet is expected to be larger than that of diesel fuel while the DME droplet traveled downstream. As a result, DME has shorter spray tip penetration than diesel fuel under the same


Kim et al.

Figure 7. Comparison of combustion characteristics for DME and diesel fuel (Pinj ) 50 MPa, SOE ) 6° BTDC, mDME ) mdiesel ) 8 mg/ cycle).

Figure 5. Comparison of spray development process for (a) DME and (b) diesel fuel according to the time after start of injection (Pinj ) 50 MPa, Pamb ) 3.5 MPa).

Figure 8. Effect of injection pressure on the combustion characteristics for DME (mDME ) 8 mg/cycle).

Figure 6. Comparison of spray tip penetration for DME and diesel fuel (Pinj ) 50 MPa, Pamb ) 3.5 MPa).

injection condition. Suh et al.1010 also experimentally investigated the spray development process of DME fuel by using a laser diagnostics technique, and they revealed that DME spray has smaller droplets than that of diesel fuel due to faster breakup and evaporation of the fuel. 3.2. Combustion and Exhaust Emission Characteristics of DME and Diesel Fuel. The combustion characteristics of DME and diesel fuel were measured under test conditions of an engine speed of 1500 rpm, injection pressure of 50 MPa, and a supplied fuel mass of 8 mg/cycle. As Figure 7 illustrates, the maximum combustion pressure for diesel fuel was higher than that of DME due to the relatively high lower heating value as listed in Table 4. Although the SOE was fixed at a constant timing of 6° BTDC, the ignition occurred earlier for DME than for diesel fuel due to the higher centane number and shorter injection delay of DME. The combustion also finished more quickly with DME than with diesel fuel. This can be explained (10) Suh, H. K.; Park, S. W.; Lee, C. S. Energy Fuels 2006, 20, 14711481.

by the flash boiling effect providing better fuel/air mixing and atomization due to the lower boiling temperature of DME (248 K at atmospheric pressure and temperature). The effect of varying the injection pressure (Pinj ) 35, 50 MPa) on the combustion characteristics of DME is shown in Figure 8. The injection profiles, corresponding to the test condition, are given at the bottom of the figure. When the injection pressure was increased from 35 to 50 MPa, the injection duration shortened, and the fuel/air mixing process was promoted, reducing the combustion duration. Due to the shorter injection duration and faster combustion, the maximum combustion pressure was greater in the 50 MPa case. The measured exhaust gas temperatures for DME and diesel fuel, as a function of IMEP (indicated mean effective pressure) at SOE of 2° and 6° BTDC, are shown in Figure 9. The exhaust gas temperature of DME is as much as 15 °C lower than that of diesel fuel. Kajitani et al.2 also reported that the DME fueled engine exhibits a higher exhaust gas temperature than that of diesel at a same engine output. Retarded injection timing (SOE ) 2° BTDC) produced higher exhaust gas temperatures than advanced injection timing (6° BTDC) for both fuels, because the main heat release was retarded to the expansion stroke, and work conversion efficiency deteriorated at the later injection timing. Therefore, the retarded timing further increased the heat wasted in the exhaust gas.



Figure 9. Exhaust gas temperature for DME and diesel fuel as a function of IMEP (SOE ) 2, 6° BTDC).

Figure 12. HC and CO emission for DME and diesel fuel as a function of IMEP (SOE ) 2, 6° BTDC). Figure 10. NOx emissions for DME and diesel fuel as a function of IMEP (SOE ) 2, 6° BTDC).

Figures 10 and 11 show NOx and soot emissions for DME and diesel fuel as a function of IMEP at SOE of 2° and 6° BTDC, respectively. Slightly higher NOx emissions occurred when the engine was fueled by DME. It is reasonable that the earlier burn of DME spray during the compression stroke leads to an increase of the charge temperature and, consequently, higher NOx emission is formed as reported by several researchers.2,11 Therefore, the higher NOx emission from the DME fueled engine is expected because of the advanced ignition timing of DME spray in the engine. On the other hand, lower NOx emissions have been reported when the engine was fueled by DME by other researchers12,13 compared to that of diesel fuel,

and they explain the result is affected by shorter ignition delay and fast combustion duration. The soot emissions under DME operation were virtually zero, regardless of IMEP (Figure 11). As mentioned above, since DME is an oxygenated fuel and does not have C-C bonds in its chemical structure, therefore, it produces little soot during combustion. Conversely, soot emission with IMEP was higher when the engine was operated with diesel fuel. Previous research on the DME fueled CI engine have reported that the emission of soot was negligible with DME.2,11 The HC (hydrocarbon) emissions were lower under DME operation than under diesel operation as shown in Figure 12. This phenomenon may be explained in terms of several parameters, including the ignition delay, spray wall wetting, and evaporation rate. In a DI diesel engine, there are two primary ways that fuel can escape the normal combustion process unburned: the fuel/air mixture can become too lean to autoignite or the mixture may be too rich to ignite or support a flame. This fuel can then be consumed only by a slower thermal oxidation reaction; therefore, hydrocarbons remain unburned due to incomplete mixing or to quenching of the oxidation process.14 DME has shorter spray penetration, higher evaporation rate, and better ignition characteristics than that of diesel fuel. In particular, DME spray adequately mixes and burns before the mixture reaches the boundary of the combustion chamber, explaining the lower HC emissions. Figure 12 also shows that DME operation also exhibited lower CO emissions, which occurred because the oxygen content in the chemical structure of DME prevented the formation of the CO during the combustion process.2

(11) Alam, M.; Fujita, O.; Ito, K.; Kajitani, S.; Oguma, M.; Machida, H. SAE Tech. Pap. Ser. 1999, 1999-01-3599. (12) Oguma, M.; Goto, S.; Watanabe, T. SAE Tech. Pap. Ser. 2004, 200401-1863.

(13) Longbao, Z.; Hewu, W.; Deming, J.; Zuohua, H. SAE Tech. Pap. Ser. 1999, 1999-01-3669. (14) Duggal, V. K.; Priede, T.; Khan, I. M. SAE Tech. Pap. Ser. 1978, 780227.

Figure 11. Soot emission for DME and diesel fuel as a function of IMEP (SOE ) 2, 6° BTDC).


Kim et al.

Figure 13. Effect of SOE on the indicated thermal efficiency of the engine with DME and diesel fueling (Pinj ) 35 MPa, mass of fuel ) 8 mg/cycle).

Figure 15. Effect of injection timing on the IMEP with NOx emission (mDME ) 8 mg/cycle, Pinj ) 35 MPa).

Figure 14. Peaks of LTR and HTR and appearance timing of LTR and HTR for DME (Pinj ) 35 MPa, mDME ) 8 mg/cycle).

Figure 16. Effect of injection timing on the IMEP with HC and CO emissions (mDME ) 8 mg/cycle, Pinj ) 35 MPa).

The indicated thermal efficiencies of the engine when it was fueled with DME and diesel fuel are shown in Figure 13. According to the varying SOE, the result indicates similar values around 40% for the two fuels. At earlier SOEs than 12° BTDC, the DME fueled engine showed lower efficiency due to the faster combustion increased negative work during compression stroke. On the other hand, when the SOE moved to TDC, the DME fueled case showed slightly higher efficiency than that of diesel fuel, because DME ignited faster during the expansion stroke, and as a result, it contributed to generate more work during the expansion stroke. When the ignition delay of a fuel mixture is extended in the engine due to advanced injection timing, the system exhibited an HCCI combustion process that consisted of a sequential combination of a low-temperature reaction (LTR) and a hightemperature reaction (HTR). Therefore, to characterize the HCCI combustion, parameters giving these two combustion timings and the maximum rates of heat release were introduced. The start of LTR (θLTR) was defined as the time when the LTR increases at a rate of at least 1 J/deg; the start of HTR (θHTR) was also defined as the timing when the HTR rises through a magnitude of 1 J/deg. If these two reactions were very close in

time, the heat releases of the two reactions partially overlapped. In this case, the start of HTR (θHTR) could not be directly distinguished. Since the heat release rate rises just after the decrease of LTR, this time was used as the θHTR. A combustion was regarded as “two-stage” if a negative temperature coefficient (NTC) region was observed between LTR and HTR. Similarly, if the two humps of LTR and HTR were not observed in the heat release curve, the combustion was classified as a “singlestage” combustion. Figure 14 shows the peaks of LTR and HTR (LTRmax, HTRmax) and appearance timings of LTR and HTR (θLTR, θHTR) for various SOEs. The SOEs are divided into two regions by their combustion characteristics. At SOE timings before 30° BTDC, combustions exhibited two stages, as described above, and the general characteristics of HCCI combustion, due to a slow reaction rate, were observed. At SOE timings after 20° BTDC, only single-stage combustions (HTR) were observed. In this range, the premixed combustion began during fuel injection. This means that the ignition and combustion processes took place before the fuel and air were adequately premixed, yielding a high local equivalence ratio at the time of ignition. High levels of NOx emissions can also be expected at this region.



the HCCI combustion region (Figure 14) and incomplete combustion products (Figure 16), the excessive shifting of the combustion event to the earlier side caused the increase of negative work during the compression stoke, which explains the deterioration of IMEP in HCCI combustion. Also, the increase of incomplete combustion products such as HC and CO is believed to cause deterioration of IMEP in the HCCI combustion region with advanced injection timing under DME operation. The effects of the SOE and EGR on the rate of heat release under DME operation are given in Figure 17. When the SOE timings were near the TDC, the rate of heat release exhibited typical, premixed, dominant combustion of the CI engine. As SOE timing was advanced before 30° BTDC, the graph of the heat release rate generally exhibited two humps. The first hump was attributed to the LTR, after which there was a short delay due to the NTC ignition behavior of the mixture. The second hump was due to the HTR.15-17 As SOE timing was advanced, a leaner mixture was formed as ignition delays were prolonged (due to low charge temperature at the time of injection), thus decreasing the peaks of heat release. As can be seen in the figure, EGR dramatically affected ignition timing. This was caused by the dilution effect of intake charge. In addition, the greater heat capacity of the exhaust gas reduced the charge temperature at the end of the compression stroke and, thus, tended to retard ignition.6,9 These results suggest that EGR can be chosen as the controlling factor for the start of HCCI combustion in DME engines. The significant influence of EGR was observed at late SOE timings near the TDC; this occurred because the start of combustion was delayed to the expansion stroke, at which point the in-cylinder temperature rapidly decreased and the reaction rate slowed.16 Figure 17. Effect of SOE and EGR on the rate of heat release (Pinj ) 35 MPa, mDME ) 8 mg/cycle).

When SOE was retarded over TDC, the maximum heat release decreased rapidly, because combustion occurred during the expansion stroke, when the in-cylinder temperature dropped rapidly. As the SOE timings were advanced beyond 25° BTDC, HTRmax decreased due to the long period of ignition delay that allows sufficient mixing of fuel and air. In that case, θLTR and θHTR were almost wholly unaffected by SOE, and the appearance timing and θLTR varied between 21.6-18.1° BTDC. This early onset of combustion with DME resulted in a reduction of the engine output, as discussed below. The effects of SOE on IMEP and NOx, HC, and CO emissions are given in Figure 15 and 16, respectively. The greatest IMEP was produced when injection timing was between TDC and 8° BTDC; however, NOx emission gradually increased as the SOE was advanced beyond TDC. If the SOE was retarded more than TDC, IMEP was rapidly aggravated, because the fuel-air mixture was injected into the combustion chamber during the expansion stroke (when the in-cylinder temperature decreases rapidly), causing the mixture to misfire. On the other hand, when the SOE was advanced beyond 25° BTDC, NOx emissions were dramatically reduced by HCCI combustion, while the IMEP was lower than when the SOE was near the time of TDC injection. As shown in Figure 16, the emissions of HC and CO varied similarly with the SOE. Rapid increases of these emissions occurred when the SOE was advanced 30° for HC and 25° for CO emission. The increase of incomplete combustion products deteriorates combustion performance, reducing the engine output performance. To summarize the combustion characteristics of

4. Conclusions A common-rail diesel engine fueled with DME was experimentally investigated in terms of spray and combustion characteristics. The following conclusions can be drawn from the experimental results and analysis. (1) The injection rate of DME exhibited an injection delay 0.04 ms shorter and a maximum pressure 21% higher than those of diesel fuel at a constant injection pressure of 50 MPa. (2) The spray tip penetration of DME was shorter than that of diesel due to the faster atomization characteristics of the fuel. Also, the DME spray showed lower hole-to-hole variation than did diesel spray. (3) When the same amount of DME or diesel fuel was supplied to the CI engine, the maximum combustion pressure of diesel fuel was higher than that of DME due to the relatively high lower heating value. The combustion of DME showed faster ignition and shorter combustion duration. (4) The DME engine emitted higher NOx emissions than the diesel engine but achieved virtually zero soot emissions. (5) As SOE was advanced beyond 30° BTDC, the typical pattern of HCCI combustion was observed because the fuel/air mixture was adequately premixed before the combustion; ignition delay rapidly increased as the SOE was advanced. Under that condition, the appearance timing of LTR and HTR was almost unchanged with variation of SOE. (15) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combust. Sci. Tech. 2005, 177, 102-125. (16) Kim, M. Y.; Kim, J. W.; Lee, C. S.; Lee, J. H. Energy Fuels, 2006, 20, 69-76. (17) Kim, D. S.; Kim, M. Y.; Lee, C. S. ASME J. Eng. Gas Turbines Power 2006, 497-505.


(6) In the case of 45% of EGR, the combustion reaction retarded the crank angle by approximately 5°. Acknowledgment. This work is financially supported by the Ministry of Education and Human Resource Development (MOE), the Ministry of Commerce, Industry and Energy (MOCIE), and the Ministry of Labor (MOLAB) through the fostering project of the Lab of Excellency. Also, this study was supported by the CEFV (Center for Environmentally Friendly Vehicle) of the Eco-STAR project from the MOE (Ministry of Environment, Republic of Korea).

Nomenclature ATDC ) after top dead center BTDC ) before top dead center CI ) compression ignition DI ) direct injection DME ) dimethyl ether DOHC ) double overhead cam HCCI ) homogeneous charge compression ignition

Kim et al. HTR ) high-temperature reaction ICCD ) intensified charged coupled device IMEP ) indicated mean effective pressure (MPa) LTR ) low-temperature reaction NTC ) negative temperature coefficient m ) mass of fuel (mg/cycle) P ) pressure (MPa) PM ) particulate matter SI ) spark ignition SOE ) start of energizing SOI ) start of injection T ) temperature (°C) Greeks θ ) crank angle (degree) Subscripts amb ) ambient inj ) injection EF060310O

Experimental Investigation of Spray and Combustion Characteristics of

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