Energy Fuels 2010, 24, 1323–1332 Published on Web 01/04/2010
: DOI:10.1021/ef9010143
Effect of Multiple Injection Strategies on the Spray Atomization and Reduction of Exhaust Emissions in a Compression Ignition Engine Fueled with Dimethyl Ether (DME) Hyun Kyu Suh,† Seung Hyun Yoon,‡ and Chang Sik Lee*,§ ‡
† Research Institute of Industrial Science, Hangyang University, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, Graduate School of Hanyang University, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, and § School of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea
Received September 11, 2009. Revised Manuscript Received November 16, 2009
This experimental research paper presents the effect of multiple injection strategies on the neat dimethyl ether (DME) fuel atomization and reduction of exhaust emission characteristics within a compression ignition (CI) engine. Pilot and split injections under various injection mass (minj) and timing (tinj) conditions as a multiple injection strategies were applied to reveal its effect on the improvement of spray atomization and the reduction of exhaust emissions in terms of spray tip penetration, Sauter mean diameter (SMD), rate of heat release (ROHR), indicated mean effective pressure (IMEP), and generation of exhaust emissions (such as CO, HC, NOX, and soot). These experimental results were then compared with those from diesel fuel cases. It was revealed that multiple injection strategies for DME fuels lead to poor atomization characteristics, because the second injection of fuel influences both the density and velocity of spray droplets at the points of measurement. This issue can be resolved by controlling the injection mass and the second injection timing, or by increasing the injection pressure. However, multiple injection strategies can achieve a simultaneous reduction of NOX and soot emissions in comparison to single injection results, and NOX emissions gradually decreased with the advance of the first injection timing without increasing soot emissions due to a lower C-H ratio. It was also observed that the concentrations of HC and CO emissions for DME are influenced by the first injection timing. For a retarded first injection timing (BTDC 20°), HC and CO emissions for DME indicated relatively low levels, in comparison to the single injection case.
engines with DME fuel are quieter, in comparison to a conventional diesel engine.2,3 Despite the extremely low exhaust emission performance of DME fuel, relatively higher NOX emissions have been reported by many researchers.4-6 To reduce the NOX emissions from the compression ignition engine, several after-treatment devices such as NOX particle filters (NPFs), NOX storage catalysts (NSCs),7 lean NOX traps (LNTs), and selective catalyst reduction (SCR) 8 have been used. On the other hand, DME requires a higher injected mass to obtain the same
1. Introduction Recently, dimethyl ether (DME) as an alternative fuel has been actively discussed for clean combustion and ultralow exhaust emission in a compression ignition engine, because of its unique ignition properties, in comparison to other candidates.1 At first, the low boiling point of DME fuel leads to quick evaporation when a liquid-phase DME spray is injected into the engine cylinder and, as a result, it can lead to better mixing with the air in the engine cylinder. The high cetane number that results from the low ignition temperature and almost instantaneous vaporization can result in a shorter total ignition period. For combustion performance of the DME, smokeless combustion can be expected, because of the high oxygen content in the fuel, which is ∼34.8% by mass, and this induces the low formation and high oxidation rate of particulates in a compression ignition engine. Therefore, the advantages of DME over conventional diesel include decreased emissions of hydrocarbon and carbon monoxide, and DME combustion does not generally produce particulate matter (PM) or soot. In this point of view, these decreased pollutant emissions observed with DME will contribute to clean air. At the same time, it was also reported that compression ignition (CI)
(2) Verbeek, R. P.; Van, D. A.; Van, W. M. Global Assessment of Dimethyl Ether as an Automotive Fuel, Second Edition; 96.OR.VM.029.1/RV, TNO Road-Vehicles Research Institute, Delft, The Netherlands, 1996. (3) Tsuchiya, T.; Sato, Y. Development of DME engine for heavy-duty truck, SAE Technical Paper 2006-01-0052, 2006. (4) Kim, M. Y.; Bang, S. H.; Lee, C. S. Experimental investigation of spray and combustion characteristics of dimethyl ether in a common-rail diesel engine. Energy Fuels 2007, 21, 793-800. (5) Kajitani, S.; Chen, Z. L.; Konno, M.; Rhee, K. T. Engine performance and exhaust characteristics of direct injection diesel engine operated with DME, SAE Technical Paper 972973, 1997. (6) Alam, M.; Fujita, O.; Ito, K.; Kajitani, S.; Oguma, M.; Machida, M. Performance of NOX catalyst in a DI diesel operated with neat dimethyl ether, SAE Technical Paper 1999-01-3599, 1999. (7) Ranalli, M. NOX-Particulate Filter (NPF) Versus NOX Storage Catalyst (NSC): Evaluation of an After-Treatment Concept to Meet Future Diesel Emission Standards, SAE Technical Paper 2005-01-1087, 2005. (8) Theis, J.; Gulari, E. A LNTþSCR System for Treating the NOX Emissions From a Diesel Engine, SAE Technical Paper 2006-01-0210, 2006. (9) Sorenson, S, C.; Glensvig, M.; Abata, D. Di-metyl ether in diesel fuel injection systems, SAE Technical Paper 981159, 1998.
*Author to whom correspondence should be addressed. Tel.: þ82-22220-0427. Fax: þ82-2-2281-5286. E-mail:
[email protected]. (1) Choi, C. Y.; Reitz, R. D. An experimental study on the effects of oxygenated fuel blended and multiple injection strategies on DI diesel engine emissions. Fuel 1999 78, 1303-1317. r 2010 American Chemical Society
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: DOI:10.1021/ef9010143
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amount of energy as the diesel fuel, because of its lower density and combustion enthalpy.10 Investigations have recently been conducted on the effect of multiple injections on the mixture formation of fuel spray, because it could lead to a solution for the reduction of NOX emissions.11-18 Li et al.19 found that splitting the fuel injection can offer some benefits to the mixture preparation of DI engines. Zhang et al.20 examined the characteristics of the fuel distributions in the split injection of impinged diesel sprays on a flat wall. They also examined the effect of injection mass ratio and dwell between fuel injections on the overall air ratio excess in the double pulse of diesel injection. The laser absorption scattering (LAS) technique was applied to the study on the process of mixture formation in a combustion chamber under split diesel injection.21 From this point of view, it is expected that the application of multiple injection strategies can reduce the NOX emissions from a CI engine fueled with DME. Moreover, it is true that most research done on the multiple injections has been focused on the combustion and emission characteristics, rather than the spray. Considering that the CI engine performance is strongly dependent on the atomization characteristics of the fuel spray, it is necessary to investigate the relationship between spray atomization and combustion emission performance in a CI engine fueled with DME fuel. In the present study, the effect of multiple injection strategies of DME fuel on the fuel atomization and reduction of exhaust emission were investigated under various injection mass and engine operating conditions. The effect of multiple injection strategies on the spray atomization characteristics of DME fuel was analyzed in terms of spray evolution from the spray images, and Sauter mean diameter (SMD) of injected droplets obtained from the droplet measuring system, phase Doppler particle analyzer (PDPA). At the same time, the effect of multiple injection strategies for the DME fuel on the
Figure 1. Schematics of the test nozzle and spray analysis system.
combustion and reduction of exhaust emissions was revealed by analyzing the combustion pressure, the rate of heat release (ROHR), the indicated mean effective pressure (IMEP), and the generation of exhaust emissions (such as CO, HC, NOX, and soot). Moreover, the effect of the injection timing of DME fuel on the spray atomization and combustion performance under multiple injection conditions was also studied. The results of multiple injection strategies were compared with the results from the single injection condition and diesel fuel in a CI engine. 2. Experimental Apparatus and Procedure
(10) Arcoumanis, C. The second European Auto-Oil programme (AOLII). European Commission, Vol. 2, Alternative Fuels for Transportation, 2000. (11) Tennison, P. J.; Reitz, R. D. An experimental investigation of the effect of common-rail injection system parameters on emissions and performance in a high-speed direct injection diesel engine. J. Eng. Gas Turbine Power 2001, 123, 167-174. (12) Badami, M.; Mallamo, F.; Millo, F.; Rossi, E. E. Influence of Multiple Injection Strategies on Emissions, Combustion Noise and BSFC of a DI Common Rail Diesel Engine, SAE Technical Paper 2002-01-0503; Society of Automotive Engineers: Warrendale, PA, 2002. (13) Tanaka, T.; Ando, A.; Ishizaka, K. Study on pilot injection of DI diesel engine using common-rail injection system. JSAE Rev. 2002, 23, 297-302. (14) Montgomery, D. T.; Reitz, R. D. Six-mode cycle evaluation of the effect of EGR and multiple injections on particulate and NOX emissions from a D.I. diesel engine, SAE Technical Paper 960316, 1996. (15) Tow, T. C.; Pierpont, D. A.; Reitz, R. D. Reducing particulate and NOX emissions by using multiple injections in a heavy duty D.I. diesel engine, SAE Technical Paper 940897, 1994. (16) Minami, T.; Takeuchi, K.; Shimakezi, N. Reduction of Diesel Engine NOX Using Pilot Injection, SAE Technical Paper 950611, 1995. (17) Carlucci, P.; Ficaralla, A.; Laforgia, D. Effect of Pilot Injection Parameters on Combustion for Common Rail Diesel Engines, SAE Technical Paper 2003-01-0700, 2003. (18) Zhang, L. A Study of Pilot Injection in a DI Diesel Engine, SAE Technical Paper 1999-01-3493, 1999. (19) Li, T.; Nishida, K.; Zhang, Y.; Yamakawa, M.; Hiroyasu, H. An Insight into Effect of Split injection on Mixture Formation and Combustion of DI Gasoline Engines, SAE Technical Paper 2004-01-1949, 2004. (20) Zhang, Y.; Ito, T.; Nishida, K. Characterization of Mixture Formation in Split-Injection Diesel Sprays via Laser Absorption-Scattering (LAS) Technique, SAE Technical Paper 2001-01-3498, 2001. (21) Zhang, Y.; Nishida, K. Vapor/Liquid Behaviors in Split-Injection D.I. Diesel Sprays in a 2-D Model Combustion Chamber, SAE Technical Paper 2003-01-1837, 2003.
2.1. Experimental Setup. The flow and spray characteristics of injectors are strongly influenced by liquid fuel properties such as density (F), viscosity (μ), and surface tension (σ). To investigate this relationship, a test involving both a single-hole nozzle with an internal hole diameter (D) of 0.3 mm and a hole length (L) of 0.8 mm was used for the atomization measurement of the fuel, as shown in Figure 1. The influence of multiple injections on the overall diesel and DME spray structure, including spray tip penetration, can be obtained from a spray visualization system. The spray visualization system was composed of a high-speed camera (Photron, Fastcam-APX RS) with two metal-halide lamps (Photron, HVC-SL) as the light source, a signal synchronization system that is injector-driven (TEMS, TDA-3200H), a digital pulse/ delay generator (Berkeley Nucleonics Corp., Model 555), an image grabber installed within a personal computer (PC), and a high-pressure injection system operated by two high-pressure pumps (Haskel, HSF-300), as illustrated in Figure 1. In the case of DME fuel, nitrogen gas was used to pressurize the fuel tank to 1 MPa, to avoid vaporization in the fuel supply line. The effect of multiple injections on the mean droplet size (SMD) was analyzed by PDPA, which is composed of an Ar-ion laser with wavelengths of 514.5 nm and 488 nm, a transmitter, a receiver, and a data acquisition system, as also shown in Figure 1. On the basis of the data rates and the signal intensity, the output power of the Ar-ion laser and the photomultiplier tube (PMT) voltage were determined to be 700 mW and 500 V, respectively. At each measurement point, ∼20 000 droplets were captured and averaged. To obtain the time-resolved data, the signal analyzer was synchronized with the injector driver using 1324
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Table 1. Criteria for Droplet Measurement parameter
value
burst threshold mixer frequency filter frequency PMT voltage signal-to-noise ratio diameter subrange
0.5 mV 36, 40 MHz 40 MHz 500 V 65 2-50 μm
the digital pulse/delay generator. At a specific time, the representative SMD was determined by averaging the captured droplets at each of the measured points. The droplet measurement is conducted based on numerous criteria, such as the threshold voltage and signal-to-noise ratio (SNR). When these conditions are adjusted, the data rate and the results accuracy can be improved. In this case, the criteria on the droplet measurements were set to optimum values for the high-speed fuel spray, as listed in Table 1. A schematic of the experimental engine system is illustrated in Figure 2. The test engine is a direct-injection, four-stroke single cylinder diesel engine with a 17.8:1 compression ratio. The valve train had an overhead-cam-type valve mechanism with two intakes and two exhaust valves. The specifications of the test engine are listed in Table 2. The test engine was controlled by a direct current (DC) dynamometer of 55 kW, and the in-cylinder pressure was measured using a piezoelectric pressure transducer (Kistler, 6055B) and a glow plug adapter (Kistler, 6535Q) coupled to a charge amplifier (Kistler, 5011B). For each test case, the combustion pressure was sampled over 1000 cycles with a sampling interval of 0.1° crank angle (CA, θ). Next, the obtained pressure data in the cylinder with a crank angle (CA, θ) were calculated relative to the rate of heat release (ROHR) during the combustion period of the engine. A universal pressure controller (TEMS, TDA-1100) maintained the injection pressure in the common rail as the target pressure. At the same time, an injector driver (TEMS, TDA3300) and a crank angle sensor controlled the injection timing (tinj) and the energizing duration (teng) to the injector. The exhaust emissions measurement was conducted by an NOX analyzer (Yanaco, BCL-511), soot analyzer (AVL, AVL-407), and an HC-CO analyzer (Horiba, MEXA-554JK). Measurements of the exhaust emissions were achieved after the engine operation had sufficiently warmed and stabilized for each test case. 2.2. Experimental Procedures. To clarify the effect of multiple injection strategies on the DME fuel spray atomization, combustion and emission characteristics, different injection mass of fuel (minj) and injection timing (tinj) were selected. The multiple injection strategy divides the pulse signal from a single injection into two pulse signals, with a dwell time of 1.11 ms, to correspond to a 10° crank angle, although a total injection amount is maintained constant at 10 mg per stroke for a single injection case. The injection timing for a single injection was varied from BTDC 40° to TDC in step with 10°. For the case of multiple injection strategy, the first injection timing occurred from BTDC 40° to BTDC 20° in step with 10°, and a first injection mass varied from 2 mg to 5 mg to form a premixed charge in the cylinder. The second injection timing is injected at TDC and the injection mass is 8 mg and 5 mg when the energizing duration of the test injector is controlled. The detailed test conditions as well as the concept of injection strategy are shown in Table 3. This work analyzes the visualization under an ambient pressure of 2 MPa and the droplet analysis measurements were performed under an ambient pressure of 0.1 MPa, considering that the floating droplets under high ambient pressure conditions can interrupt the measurement. At the same time, simultaneous measurement of the DME droplet size under different injection conditions were conducted in 10-mm intervals in the axial direction from the nozzle tip. To examine the
Figure 2. Schematics of the test engine and data acquisition system. Table 2. Specifications for the Test Engine System parameter type bore stroke displacement compression ratio combustion chamber type number of valves fuel injection system intake valve IVO IVC exhaust valve EVO EVC
comment/value DI diesel engine (NA) 75.0 mm 84.5 mm 373.3 cc 17.8 re-entrant 2 intake and 2 exhaust common-rail type BTDC 8° ABDC 52° BBDC 8° ATDC 38°
Table 3. Experimental Test Conditions parameter engine speed injection pressure coolant temperature oil temperature injection timing (deg. ATDC) single multiple injection mass single multiple
value/comment 1500 rpm 60 MPa 70 °C 70 °C -40, -30, -20, -10, TDC -40 þ TDC, -30 þ TDC, -20 þ TDC 10 mg/stroke 2 - 8 mg/stroke (1st - 2nd injection, pilot condition), 5 - 5 mg/stroke (1st - 2nd injection, split condition)
time-dependent development of spray, a time-resolved analysis of the acquired data was performed during the many injection events. All tests of engine performance were conducted under a constant engine speed of 1500 rpm (25 Hz), and a fixed injection pressure (Pinj) of 60 MPa. To investigate the effect of injection strategy on the combustion and exhaust emissions, two multiple injection strategies were investigated, as stated previously.
3. Results and Discussions 3.1. Effect of Multiple Injection Strategies on the Spray Atomization Characteristics of DME Fuel. Figure 3 illustrates the spray evolution process between the diesel and DME fuel with both single and pilot injection under roomtemperature ambient conditions (20 °C). Both of the fuels’ sprays progressed downstream as time elapsed after the start of injection (tasoi) and were diluted from the outer region of 1325
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Figure 4. Spray tip penetration of diesel and DME under single and pilot injection conditions (Pinj = 60 MPa, Pamb = 2 MPa, Tamb = 293 K, and tadv = 20° CA).
the same injection pressure are equal at the moment of injection start, the fuel momentum to ambient gas affected by the first injection mass (mp = 2 mg) must be decreased; consequently, the relative velocity between the spray and the ambient gas would be decreased in the pilot injection case. However, the spray tip penetration of the main injection event is similar in comparison to the single injection condition. From these results, it can be deduced that the pilot spray of DME influences the relative velocity between the spray and the ambient gas, as well as improving upon the main injection velocity. To analyze the effect of pilot fuel injection on the droplet atomization efficiency, the local and overall Sauter mean diameter (SMD) of the DME was measured at different pilot advance times (tadv) and compared to the single injection case of diesel fuel. The local SMD indicates that the mean droplet size was measured at a specific measurement point for all measurement durations. On the other hand, the overall SMD indicates the averaged value for all droplets captured at all measurement points for a specific time interval (Δt). The results of a single injection show that diesel fuel has a larger droplet size and it decreases gradually, because of the atomization of droplets when the spray moves to the downstream, as illustrated in Figure 5a. For the case of pilot injection, the local SMD of pilot injection is only slightly smaller than of a single injection of DME near the nozzle. However, it shows only a slight increase as the spray moves downstream and, finally, it has a value similar to that of the single injection case for DME fuel. The overall SMD of diesel and DME fuel are investigated in Figure 5b. From previous results, in the injection and spray evolution characteristics, a lower viscosity and surface tension of DME can become a factor in a lower SMD, because it enhances the break-up process of the fuel droplet. In the case of the single injection, the SMD of diesel fuel has a higher value over the entire injection time. The SMD of the spray droplet decreased as the time elapsed from the start of the injection. However, for a pilot injection event, the overall SMD suddenly increased 2.8 ms after the start of injection, because of the main spray injection. It may influence the density and velocity of injected fuel droplets at the measuring points by the increasing droplets number. Therefore, it can
Figure 3. Comparison of diesel and DME spray evolutions under single and pilot injection conditions (Pinj = 60 MPa, Pamb = 2 MPa, Tamb= 293 K, and tadv = 20° CA).
spray. The second injection of fuel spray can be observed 2.8 ms after the start of energizing (tasoe), because the pilot advance time was set to a crank angle (CA) of 20°. As many previous researchers have already reported,22-24 diesel fuel is denser and developed faster than DME fuel, because DME has faster evaporation and vaporization characteristics. From the spray images of the DME fuel, the spray tip penetration, which is defined as the maximum distance that the spray can reach from the nozzle tip, are measured and compared in Figure 4. In this figure, the lines and symbols indicate the single and pilot injection conditions, respectively. It can be observed that the start of the diesel fuel injection is a little faster than the DME fuel (∼0.1 ms), regardless of injection conditions, because of a higher bulk modulus of diesel fuel, and diesel fuel has a longer tip penetration during an entire injection period, in comparison to DME fuel. It has been concluded that the reason for the shorter DME penetration is due to the lower density and viscosity of DME, which may induce the deceleration of fuel droplet momentum. In addition, the faster vaporization, which is due to the higher evaporating rate, and the higher latent heat value cannot lead to full development when the DME fuel injection occurs. In the case of pilot injection, the spray tip penetration of the fuel injection is shorter than that of the main injection, because of the shorter energizing duration and small injected mass which induces a low droplet momentum and injection velocity. Basically, the momentum transfer of injected fuel to the stagnant ambient air is greatly dependent on the injection mass of fuel. Therefore, even the injection velocities under (22) Suh, H., K.; Park, S. W.; Lee, C. S. Atomization Characteristics of Dimethyl Ether Fuel as an Alternative Fuel Injected through a Common-Rail Injection System, Energy Fuels 2006, 20, 1471-1481. (23) Suh, H. K.; Lee, C. S. Experimental and analytical study on the spray characteristics of dimethyl ether (DME) and diesel fuels within a common-rail injection system in a diesel engine. Fuel 2008, 87, 925-932. (24) Suh, H. K.; Park, S. H.; Kim, H. J.; Lee, C. S. Influence of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME). Fuel 2009, 88, 1070-1077.
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Figure 6. Comparison of diesel and DME spray evolutions under split injection conditions (Pinj = 60 MPa, Pamb = 2 MPa, Tamb = 293 K, and tadv = 20° CA).
Figure 5. Sauter mean diameter of diesel and DME under single and pilot injection conditions: (a) local SMD and (b) overall SMD. (Pinj = 60 MPa, Pamb = 2 MPa, Tamb= 293 K).
be concluded that the main injection event after a pilot injection may affect the increase in droplet size. Figure 6 illustrates the spray progress of diesel and DME fuel under split injection conditions, according to the elapsed time after the injection start. During an entire injection period, the low concentration of DME spray was observed. The second injection of fuel spray was started at 3.0 ms after the start of energizing (tasoe). Approximately 3.0 ms after the injection start, diesel spray injected in the first stage still existed, while the DME spray disappeared. It can be also observed that the second fuel injection penetrates the first fuel injection. As mentioned previously, because of this spray behavior, fuel droplets at the measuring volume must be denser. It can lead to a higher frequency of droplet collision and coalescence in the measuring volume; consequently, the droplet atomization efficiency would be worse than in the single injection case. Comparison of diesel and DME spray tip penetration under the split injection case are conducted as shown in Figure 7. Different from the pilot injection, the diesel and DME spray tip penetrations of the first and second injections are almost the same, because of the sufficient energizing
Figure 7. Spray tip penetration of diesel and DME under split injection conditions (Pinj = 60 MPa, Pamb = 2 MPa, Tamb = 293 K, and tadv = 20° CA).
duration (teng) of the first injection. These sufficient energizing durations lead to a rich mixture formation within a combustion chamber. Approximately the same trend of overall SMD distributions shown with the pilot injection is expected with split injection conditions, as illustrated in Figure 8. For the case of split injection, the SMD of diesel and DME fuel varied with almost the same pattern; however, the diesel fuel has a slightly larger SMD value after the second injection. From these results, it can be summarized that the fuel properties of DME (such as lower density, viscosity, and surface tension) can become a factor in a shorter spray tip penetration with slow development, a lower SMD value than diesel, because these fuel properties enhance the droplet break-up process of the fuel droplet. It can be also concluded that multiple injection strategies do not improve the DME fuel atomization, because the second injection influences the density and velocity of droplets in the measuring points and, 1327
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Figure 8. Overall SMD distributions of diesel and DME spray under split injection conditions (Pinj = 60 MPa, Pamb = 2 MPa, Tamb = 293 K, and tadv = 20° CA).
therefore, leads to the poor and similar atomization of multiple injection strategies. Basically, in a dense spray, there is a high probability for the occurrence of droplet collisions. These collisions can result in a change of droplet velocity and size. Droplets can break up into smaller droplets, but they can also combine to form larger drops, which is called droplet coalescence. Therefore, it was concluded that the SMD (D32) increases as the gas density increases, because of the higher number of collisions (coalescence).25 This can be solved by controlling the injection mass, the second injection timing, or by increasing the injection pressure. On the other hand, it is estimated that first injection fuel may form a flame core in a combustion chamber, and this flame core, which is generated by the first injection of fuel, may influence the second injection of fuel, with regard to combustion. 3.2. Comparisons of the Combustion and Emission Characteristics for Single Injection. The combustion pressure, the rate of heat release (ROHR), and the indicated mean effective pressure (IMEP) of DME and diesel fuel for a single injection condition varies with the injection timing start at TDC and BTDC 30° are shown in Figure 9. For the case of the TDC injection timing, as shown in Figure 9a, the results of DME fuel indicate lower premixed spikes and a relatively short ignition delay, compared to diesel fuel combustion. Moreover, despite the longer injection duration, the total combustion duration of DME is considerably shorter than that of diesel fuel. These results can be explained by the fact that DME fuel has a tendency to autoignite easily, because of its active low-temperature of oxidation reactions, higher evaporating rate, and cetane number. Moreover, DME fuel showed a lower peak combustion pressure and lower heatrelease rates than those of diesel fuel. At the same time, a lower indicated mean effective pressure (IMEP) was also indicated for the lower low heating value (LHV) of the DME, which is only ∼65% of that from diesel fuel. In addition, the LHV of DME reduced the accumulated energy during the ignition delay period. Consequently, the premixed spike of DME decreased. Therefore, the larger amount of DME fuel per cycle is needed to provide the same combustion
Figure 9. Effect of injection timing on the combustion characteristics of diesel and DME fuel under single injection conditions: (a) SOE = TDC and (b) SOE = BTDC 30°. (Pinj = 60 MPa, minj = 10 mg).
performance, and these can be obtained from the increase in the nozzle hole diameter or from the longer energizing duration of the test injector. At the advanced injection timing of BTDC 30°, as shown in Figure 9b, the ignition delays and combustion durations of DME and diesel fuels are prolonged; the peak combustion pressures and heat releases for test fuels are increased, in comparison to the results of the TDC case. However, it can observed that the values of IMEP are considerably reduced, because the earlier combustion during the compression stroke increased negative work and the decrease of IMEP in the DME combustion is reduced according to the advance of the injection timing. Figure 10 shows the effect of injection timings on the concentrations of soot, NOX, HC, and CO emissions for DME and diesel fuels. In general, the gaseous exhaust emissions during combustion period in a diesel engine are mainly influenced by the chemical and physical fuel properties, such as structure of fuel composition, cetane number, oxygen content, and the aromatic content of the fuel. DME is well-known as a nontoxic alternative fuel with an implied higher oxygen content of ∼34.8% by mass. In addition, DME has a low C-H and C-O ratio, because of the absence of C-C bonds in its chemical structure. Furthermore, the superior evaporation characteristics
(25) Baumgarten, C. Mixture Formation and Internal Combustion Engines; Springer: New York, 2005; pp 11-15. (ISSN 1860-4846.)
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Figure 10. Effect of injection timing on the exhaust emissions (CO, HC, NOX, and soot) of diesel and DME fuel under single injection conditions (Pinj = 60 MPa, minj = 10 mg).
reduce the wall wetting problem with the impinged fuel spray. Because of these characteristics, it is widely reported that the combustion characteristics of DME fuel generally induces a remarkable decrease of unburned HC, CO, and soot emissions, when compared to conventional diesel combustion. As shown in Figure 10, DME combustion exhibits almost a zero level of soot emissions at all test ranges; because of the high oxygen content, the absence of soot precursors and the short diffusion combustion phase suppressed soot formation. However, in the case of diesel fuel, the concentration of soot emissions is considerably increased as the injection timing advances. The concentrations of NOX emissions for DME and diesel fuels eventually increase in proportion to the advance of injection timing up to BTDC 10° for DME and BTDC 20° for diesel, respectively. However, at further advanced injection timings, NOX emissions for test fuels indicated lower concentration levels. In particular, DME combustion emitted significantly lower NOX emission, in comparison to diesel fuel for all test conditions. These results can explain the higher latent heat of evaporation, the lower stoichiometric air requirement, and the lower LHV of DME, which are attributed to the reduction of NOX emissions due to the decrease of peak combustion pressure, heat release, and combustion temperature, as shown in the combustion characteristics of Figure 9.
Figure 11. Effect of multiple injection strategies on the combustion pressure and rate of heat release of diesel and DME fuel: (a) pilot injection strategy and (b) split injection strategy. (Pinj = 60 MPa, minj = 10 mg).
It can be seen that the concentration of HC and CO emissions fueled with DME are generally lower at retarded injection timing conditions than those of diesel fuel. As the injection timing is retarded, the temperature and pressure of cylinder are rapidly increased and most of the injected fuel is directed toward into piston bowl without fuel impingement on the cylinder wall and piston head. The reason for lower HC and CO emissions can also be said to be that DME spray has relatively better fuel atomization and air-fuel mixing, as well as a more complete combustion characteristic, which can be explained by the faster evaporation and vaporization of fuel spray droplets.22-24 These results can also be attributed to the shortened diffusion combustion phase and high oxygen content of the DME. The oxygen content and lower C-H ratio definitely resulted in an improved complete combustion and, thus, the unburned hydrocarbon and carbon monoxide emissions can be reduced. 3.3. Effect of Multiple Injection Strategies on the Combustion and Exhaust Emissions Reduction. The effect of variation in the injection mass of the first and second injection on the combustion pressures and the rate of heat releases of DME and diesel are illustrated in Figures 11a and 11b, respectively. 1329
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Figure 12. Effect of multiple injection strategies on the IMEP for diesel and DME fuel (Pinj = 60 MPa, minj = 10 mg).
For the case of pilot injection strategies in which the first injection timing is BTDC 30° and the second injection is TDC, the total injection quantity was kept at 10 mg per stroke. As seen in Figure 11, the maximum combustion pressures and heat-release rates of the multiple injection strategy for both fuels were lower than those of the single injection results, as shown in Figure 9. It is widely reported that the combustion of single injection caused the rapid premixed combustion phases, because most fuel is injected during the ignition delay period under high ambient pressure and temperature conditions and, thus, is combusted immediately. For this reason, undiluted air-fuel mixtures and fuel-rich region exist locally in the combustion chamber, which usually causes the formation of harmful exhaust emissions and combustion noises. Conversely, two combustion phases occur, because of the double injection that occurs in the case of the multiple injection strategy. The first combustion and heat release generated according to the first fuel injection, as well as the high temperature and pressure generated from the first combustion, influenced the rapid combustion, with an extremely short ignition delay of the second fuel injection, as shown in Figure 11. The first injection of 2 mg for DME, which is a relatively small amount of fuel, is burned more actively and the first ignition delay is shorter, in comparison to diesel fuel. The ignition delay of the second injection of DME fuel is also shortened and the peak combustion pressure of the second injection is all very much the same, despite the lower LVH, as shown in Figure 11a. In Figure 11b, the split injection strategies for DME fuel indicate that the peak heat release of the first injection is much lower than that for diesel fuel. However, the peak heat release of the second injection of DME occurred more actively in both cases, although the combustion of the first injection already occurred and thus created an undiluted mixture with insufficient oxygen levels, which may then cause deterioration or suppress the ignition and performance of the second injection combustion. The improvement in performance of the second combustion of DME is thought to be the effect of fuel properties such as larger oxygen content and lower stoichiometric air requirements, although they are reasons for the good ignition ability.
Figure 13. Effect of multiple injection strategies on the concentrations of soot emissions (top panel) and NOX emissions (bottom panel) for diesel and DME fuel (Pinj = 60 MPa, minj = 10 mg).
Figure 12 shows the comparison of IMEP values, according to the injection strategy (single and multiple injections), as well as the variation of the first injection mass and timing. In this figure, the IMEP values of the DME fuel revealed it to be relatively lower, in comparison to diesel fuel in all test ranges. Therefore, considering the low LHV and combustion enthalpy, DME fuel needs greater injection mass to achieve the same combustion performance of diesel fuel. For the cases of split injection strategy, as the first injection timing advanced, the IMEP of both fuels decreased. However, the pilot injection strategies of DME fuel generated almost the same performances in comparison to single injection results. Figure 13 shows the effect of injection strategy varied with injection timing and mass on the concentrations of soot and NOX emissions. In this figure, multiple injection strategy of DME achieved simultaneous reduction of NOX, and soot emissions, compared to single injection results, despite the lower IMEP values, and NOX emissions gradually decreased with the advance of first injection timing without increasing soot emissions, because of a lower C-H ratio. Moreover, the oxygen content in DME promotes a more complete combustion, reduction of soot formation, and also enhances the oxidation of the soot emissions during the combustion process. In contrast, the reduction of NOX emissions can be achieved in diesel combustion, because of the low combustion temperature and much larger soot emissions with the adoption of a multiple injection strategy. The effect of multiple injection strategy on HC and CO emissions are shown in Figure 14. The concentrations of HC 1330
Energy Fuels 2010, 24, 1323–1332
: DOI:10.1021/ef9010143
Suh et al.
density and viscosity of DME fuel and it may induce the deceleration of fuel droplet momentum. In addition, the faster vaporization, because of the higher evaporating rate and latent heat value, cannot lead to full development when the DME fuel is injected. (2) The multiple injection strategies cannot improve the DME fuel atomization, because the second injection influences the spray density at each measuring point. A solution can be attained by controlling the injection mass and the second injection timing, as well as by increasing the injection pressure. (3) Soot emissions of DME combustion is shown to be almost at the zero level for all test ranges, because the high oxygen content, the absence of soot precursors, and the short diffusion combustion phase suppressed the soot formation. (4) The emission concentration of NOX for DME and diesel fuels eventually increased in proportion to the advance of injection timing up to BTDC 10° for DME and BTDC 20° for diesel, respectively. However, at further advanced injection timings, the NOX emissions for both test fuels indicated lower concentration levels. (5) Even multiple injection strategies of DME fuel cannot enhance fuel atomization, but it achieved simultaneous reduction of NOX and soot concentration, in comparison to single injection results, and NOX emissions gradually decreased with the advance of the first injection timing without an increase in soot emissions, because of a lower C-H ratio. The concentrations of HC and CO emissions for DME are influenced by the first injection timing for both test fuels. With a retarded first injection timing (BTDC 20°), HC and CO emissions for DME indicated relatively low levels, in comparison to the single injection case. However, at advanced first injection timing over BTDC 30°, the HC and CO emissions are increased.
Figure 14. Effect of multiple injection strategies on the concentrations of HC emissions (top panel) and CO emissions (bottom panel) for diesel and DME fuel (Pinj = 60 MPa, minj = 10 mg).
Acknowledgment. This study was supported by the CEFV (Center for Environmentally Friendly Vehicle) of the Eco-STAR Project from MOE), Republic of Korea (Ministry of Environment). Also, this work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2009-352-D00035).
and CO emissions for DME are influenced by the first injection timing and injection mass for both test fuels. At retarded first injection timing (BTDC 20°), HC and CO emissions for DME indicated relatively low levels, compared to the single injection levels. However, at advanced first injection timing over BTDC 30°, HC and CO emissions showed rapidly increased, especially in the split injection (5 mg þ 5 mg) case. These results are mainly caused by too early combustion of first injected fuel at compression stroke, which deteriorated the oxygen in the charge and complete combustion phase; thus, the second combustion produced the unburned gas emissions, especially in diesel fuel. In addition, relatively low levels of IMEP can be a reason for the increase in emission concentrations, as shown in Figure 12.
Nomenclature Acronyms and Abbreviations ATDC = After top dead center BTDC = Before top dead center CA = Crank angle CI = Compression ignition DME = Dimethyl ether EVC = Exhaust valve close EVO = Exhaust valve open IMEP = Indicated mean effective pressure IVC = Intake valve close IVO = Intake valve open LHV = Lower heating value LNTs = Lean NOX traps NPFs = NOX particle filters NSCs = NOX storage catalysts PDPA = Phase Doppler particle analyzer ROHR = Rate of heat release SCR = Selective catalyst reduction SMD = Sauter mean diameter SOE = Start of energizing
4. Conclusions The effect of multiple injection strategy of dimethyl ether (DME) fuel on the improvement of fuel atomization and the reduction of exhaust emission characteristics was analyzed under the various engine operating conditions in this work. These results compared with those from the single injection condition in a compression ignition (CI) engine. The conclusions are summarized as follows: (1) The start of diesel fuel injection is slightly faster than ∼0.1 ms, in comparison to DME fuel, because of a higher bulk modulus of fuel, and diesel fuel has a longer tip penetration in an entire injection period, in comparison to DME fuel. The reason for the shorter DME penetration is caused by the lower 1331
Energy Fuels 2010, 24, 1323–1332
: DOI:10.1021/ef9010143
Suh et al.
μ = viscosity [Ns/m ] F = density [kg/mm3] 2
TDC = Top dead center ULSD = Ultra low surfer diesel
Subscripts
Variables
adv = advance amb = ambient asoe = after start of energizing asoi = after start of injection eng = energizing inj = injection m = main p = pilot
D = hole diameter [mm] L = length [mm] m = fuel mass [mg] P = pressure [MPa] t = time [ms] Greek Symbols σ = surface tension [N/m]
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