Effect of Multiple Injection Strategies on the Emission Characteristics

Energy & Fuels 2007, 21, 2673-2681

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Effect of Multiple Injection Strategies on the Emission Characteristics of Dimethyl Ether (DME)-Fueled Compression Ignition Engine Myung Yoon Kim, Seung Hyun Yoon, and Ki Hyoung Park Graduate School of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong Sungdong-gu, Seoul 133-791, Republic of Korea

Chang Sik Lee* Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong Sungdong-gu, Seoul 133-791, Republic of Korea ReceiVed April 13, 2007. ReVised Manuscript ReceiVed June 13, 2007

An experimental investigation was conducted to evaluate the effect of multiple injection strategies on the performance of DME-fueled engine and the reduction of exhaust emissions. In this work, three types of multiple injection strategiessincluding split injection, pilot injection, and partial premixed charge compression ignition (PPCCI)sare introduced, and the results are compared with those obtained by single injection under optimal operating conditions. To analyze the injection characteristics of dimethyl ether (DME) fuel, the injection rate profiles for multiple injections were investigated, in terms of their energizing current and injection rate. The result of an engine test reveals that multiple injection strategies of split and pilot injection showed a reduction in NOx emissions of ∼75% with a minor reduction in the indicated mean effective pressure (IMEP). PPCCI combustion offered a reduction in NOx emissions of ∼97.1%, whereas the IMEP was reduced by 23%. Under all operating conditions, no measureable soot emission was observed in the DME engine.

1. Introduction Dimethyl ether (DME) is a promising alternative fuel for various applications, including diesel engines, turbine power generation, fuel cells, and residential fuel for heating and cooking. It can be manufactured from various sources, such as natural gas, coal, methanol, and biomass.1 When used for compression ignition (CI) engines as a substitute for diesel fuel, it combusts without forming particulate matter (PM). Moreover, a DME-fueled engine provides higher power performance, compared to diesel fueling, because it requires less air to produce the same heat produced by diesel fuel. Using existing engine technology, DME produces less well-to-wheel greenhouse gas emissions than does Fischer-Tropsch (FT) diesel, biodiesel, ethanol, and methanol.2 DME consists of two methyl groups bonded to a central O atom and can be expressed by its chemical formula, CH3OCH3. It can be easily handled and stored as liquefied gas, in a manner similar to that for liquefied petroleum gas. Because of its high cetane number, DME can be used as fuel for a CI engine, and, consequently, the engine can be operated more quietly, because rapid premixed burning is reduced.3 Despite the many advantages of DME as a fuel for CI engines, several challenges are * Author to whom correspondence should be addressed. Tel.: +82-22220-0427. Fax: +82-2-2281-5286. E-mail : [email protected]. (1) Sorenson, S. C. Dimethyl Ether in Diesel Engines: Progress and Perspectives. J. Eng. Gas Turbines Power 2001, 123, 652-658. (2) Semelsberger, T. A.; Borup, R. L.; Greene, H. L. Dimethyl ether (DME) as an alternative fuel. J. Power Sources 2006, 156, 497-511. (3) Teng, H.; McCandless, J. C.; Schneyer, J. B. Compression Ignition Delay (Physical + Chemical) of Dimethyl EthersAn Alternative Fuel of Compression-Ignition Engines, SAE Paper No. 2003-01-0759, Society of Automotive Engineers, Warrendale, PA, 2003.

Table 1. Physical Properties of Dimethyl Ether (DME) property

value

formula molecular weight density lower heating value, LHV stoichiometric air:fuel ratio boiling point (at atmospheric pressure) sulfur content

CH3OCH3 46.07 g/mol 660 kg/m3 28.6 MJ/kg 9.0 -24.9 °C 0 ppm

exposed, because of its low viscosity and high vapor pressure. Because DME provides insufficient lubricity in the fuel injection system, including the fuel pump and injector, several types of lubricity improver were added to the neat DME for engine operation by previous researchers. The representative physical properties of DME are provided in Table 1. Despite the extremely low exhaust emission characteristics of PM and unburned hydrocarbons, relatively high NOx emissions were reported by several researchers.4-6 To reduce NOx emissions from a CI engine, expensive after-treatment devices (such as a lean NOx trap (LNT) or selective catalyst reduction (SCR)) is needed. Moreover, the conversion efficiency for reduction NOx emissions is limiting. (4) Kim, M. Y.; Bang, S. H.; Lee, C. S. Experimental Investigation of Spray and Combustion Characteristics of Dimethyl Ether in a CommonRail 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 Paper No. 972973, Society of Automotive Engineers, Warrendale, PA, 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 Paper No. 1999-01-3599, Society of Automotive Engineers, Warrendale, PA, 1999.

10.1021/ef0701844 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007

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Table 2. Specifications of Test Engine specification

value

bore stroke displacement volume compression ratio piston

75 mm 84.5 mm 373.3 cm3 17.8:1 re-entrant

Table 3. Specifications of Fuel System specification injector type injection pressure number of nozzle holes nozzle hole diameter included spray angle

value electronically controlled common-rail injector 50 MPa 6 0.128 mm 156°

Recently, multiple injection strategies (such as pilot injection, split injection, and partial premixed charge compression ignition (PPCCI)) were investigated to inhibit NOx emissions from a direct injection compression ignition engine. Despite the advantages of multiple injection strategies, previous research on the DME engine was limited to single injection applications. In this work, the effect of multiple injection strategies on the reduction characteristics of exhaust emission, in terms of NOx, hydrocarbon (HC), and carbon monoxide (CO), was investigated. At the same time, the engine performance (including engine output and cycle-to-cycle variation) was evaluated and injection rate profiles of the multiple injection rate were also discussed. As an alternative fuel of diesel fuel in a CI engine, a multiple injection strategy was applied to determine the possibility of improvement of exhaust emission characteristics and combustion performance of DME-fueled diesel engine. Several injection strategies (such as split injection, pilot injection, and injection for PPCCI) were introduced, and the effects of multiple injections were evaluated by comparing the results of engine performance and exhaust emission characteristics with those obtained by single injection under the optimal operating conditions. 2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus. The engine used for this work is a single-cylinder, direct-injection diesel engine that has been equipped with a common-rail injection system. The detailed specifications of this engine and fuel system are listed in Tables 2 and 3, respectively. The experimental apparatus for the DME-fueled engine study was composed of a fuel injection system with a timing controller, an exhaust emission analyzer, a dynamometer, and a test engine, as shown in Figure 1. Using a direct current (DC) dynamometer system with a maximum braking power of 55kW, the engine speed and torque were controlled. Because DME has a high vapor pressure, the fuel system was pressurized by nitrogen gas, to prevent vapor-lock in the fuel system. The fuel pressure in the common-rail was pressurized using a couple of air-driven pumps (HSF-300, Haskel) and maintained at a constant level. In this work, all experiments were performed at a fixed injection pressure of 50 MPa. The injection duration of the common-rail injector was controlled by an injector driver (TDA 3300, TEMS) and a timing pulse generator was coupled with a crank angle sensor and camshaft position sensor. A water-cooled-type heat exchanger was installed to refrigerate the fuel that was passing through the injector. The injection mass of the fuel was measured using a precision electronic balance (GP-30K, AND).

Figure 1. Experimental apparatus for engine test.

The cylinder pressure was monitored by a piezoelectric pressure sensor (6052B1, Kistler) coupled with a charge amplifier (5011B, Kistler). The pressure data was acquired using a DAQ board (PCI-MIO-16E-1, NI) with a maximum sampling rate of 1 mega-samples/s and a combustion analysis program. Exhaust gas component measurements from the engine were conducted with a NOx, HC, and CO analyzer. A smoke meter (415S, AVL) measured the soot and particulate in the exhaust gas emitted from the engine. To enhance the durability of the fuel injection system of the engine, 1000 ppm of a lubricity improver (Lubrizol, 539M) was added into the neat DME. 2.2. Experimental Conditions. All engine tests were conducted at an engine speed of 1500 rpm and an injection pressure of 50 MPa. The total amount of fuel injected was fixed at 12 mg for both single and multiple injections. An injection rate measurement system based on the Bosch method7 was used to measure the injection rate profile. In this method, the highpressure DME is injected into a long tube filled with the fuel, and the fuel injection rate can be determined by measuring the pressure of fuel in the tube. During injection rate measurement, the fuel pressure within the tube was held constant at 4 MPa. This pressure condition is similar to the charge pressure near top dead center (TDC) in the combustion chamber of the engine where the injection event occurs. The experimental investigations were performed at 1500 rpm, with cooling water at 70 °C and an intake air pressure of 0.1 MPa. The study on the multiple injection on the exhaust emissions and combustion was analyzed at the same equivalence ratio (Φ ) 0.3). 2.3. Injection Strategies. To achieve clean exhaust emission characteristics, especially in NOx emissions, three types of multiple injection strategiessincluding split injection, pilot injection, and PPCCI combustionswere introduced for a DMEfueled engine, and their combustion and exhaust emission characteristics were compared with those obtained under single injection conditions. The injection timing was defined as the timing when the energy for injection is first applied, which is called the start of energizing (SOE), and the injection strategies introduced for this work are detailed in Figure 2. Actual injection was initiated after the given injection timing, because of the injection lag, as will be discussed later in Figure 4 of this paper. 2.3.1. Split Injection. Recently, a split injection strategy has been proposed as a method to reduce both particulate and NOx emissions from a diesel engine.8,9 A split injection strategy is (7) Bosch, W. The Fuel Rate Indicator: A New Measuring Instrument for Display of the Characteristics of Individual Injection, SAE Paper No. 660749, Society of Automotive Engineers, Warrendale, PA, 1966.

Emission Characteristics of DME-Fueled CI Engines

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a lower peak temperature, reducing NOx formations. A pilot fuel mass of 2 mg/cycle, which is relatively large, was used for this work, to maximize the effect of pilot injection. 2.3.3. Early Injection for Partial Premixed Charge Compression Ignition (PPCCI). PPCCI combustion13-15 is a combination of premixed charge compression ignition and conventional diesel combustion. In this concept, a portion of fuel is premixed by early pilot injection and the remainder of the fuel is injected near TDC. The premixed fuel is burned at a low temperature and under lean conditions; it demonstrates very low NOx emissions levels. The secondary injection can control the combustion and offers more engine output. In this work, half the fuel was premixed, and the remainder was injected near TDC. 3. Experimental Results and Discussion

Figure 2. Definitions of injection timings for multiple injection strategies: (a) single injection, (b) split injection, (c) pilot injection, (d) partial premixed charge compression ignition (PPCCI).

introduced to determine whether using a split injection strategy with a DME-fueled engine would produce a reduction in emissions while maintaining engine performance. To prevent interaction between the first and second injections, a crank angle (CA) of 10° was chosen as the interval between the two injections, based on injection characteristics, including injection delay and injection duration, as measured from the injection rate profile. The split injection timing (τsplit) indicates the center between the SOE of the first and second injections. Therefore, the SOE of first injection was 5° CA before τsplit, and the SOE of the second injection was 5° CA after τsplit. 2.3.2. Pilot Injection. The main purpose of pilot injections is to reduce the engine noise, as well as to reduce NOx emissions emitted from the CI engine.10-12 When a small portion of the fuel is injected during a compression stroke prior to the main injection, it burns prior to the main injection. Because of the combustion, the pressure and temperature of gases in the combustion chamber increase, thus reducing the ignition delay of the main injection. As a result, most of the fuel combusts at (8) Montgomery, D. T.; Reitz, R. D. Effects of Multiple Injections and Flexible Control of Boost and EGR on Emissions and Fuel Consumption of a Heavy-Duty Diesel Engine, SAE Paper No. 2001-01-0195, Society of Automotive Engineers, Warrendale, PA, 2001. (9) Choi, C. Y.; Reitz, R. D. An experimental study on the effect of oxygenated fuel blends and multiple injection strategies on DI diesel engine emissions. Fuel 1999, 78, 1303-1317. (10) 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 Turbines Power 2001, 123, 167-174. (11) 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 Paper No. 2002-01-0503, Society of Automotive Engineers, Warrendale, PA, 2002. (12) 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.

3.1. Single Injection. For a CI engine, retarded injection timing has been a common means of controlling NOx emissions, by reducing the combustion pressure and temperature. However, excessively retarded injection timing causes increasing engine instability; occasionally, the engine can misfire. Therefore, a suitable injection timing that simultaneously satisfies exhaust emission and engine performance limitations should be chosen for the engine. In this study, to evaluate the effect of multiple injection strategies on the DME-fueled engine, the experimental results of multiple injection strategies were compared with those obtained by single injection with the best injection timing that simultaneously satisfies exhaust emission and engine performance requirements. To determine the best injection timing for a single injection, the accumulated combustion pressures during 30 consecutive cycles and the engine output with varying injection timing in step with 2° CA are determined; these values are shown in Figure 3. The injection rate profiles for the single injection case are provided in Figure 4a (shown later in this work). As can be seen in Figure 3a, the injection timing at TDC produces the lowest NOx emissions (44 ppm). Because of the late injection timing, deteriorated engine performance, including a reduced indicated mean effective pressure (IMEP) and an increased coefficient of variation of the IMEP (COVIMEP), as well as increased HC and CO emissions, was observed. However, the injection timing at 2° BTDC (where BTDC denotes “before top dead center”), as shown in Figure 3b, indicates more stable combustion features with increased NOx emissions (243 ppm) and reduced HC and CO emissions. As the injection timing advanced to 4° BTDC, NOx emissions increased further (to 306 ppm) while HC and CO emissions continued to decrease. At the same time, the engine performance was equal to that observed during the SOE at 2° BTDC. Summarizing the engine performance and exhaust emission characteristics of single injection with various injection timings, 2° BTDC was determined to be the optimum injection timing for single injection, and this result was used for the baseline to evaluate the effects of multiple injection strategies. (13) Neely, G. D.; Sasaki, S.; Leet, J. A. Experimental Investigation of PCCI-DI Combustion on Emissions in a Light-Duty Diesel Engine, SAE Paper No. 2004-01-0121, Society of Automotive Engineers, Warrendale, PA, 2004. (14) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combustion and Emission Characteristics of a Partial Homogeneous Charge Compression Ignition Engine When Using Two-Stage Injection. Combust. Sci. Technol. 2007, 179, 531-551. (15) Kim, M. Y.; Lee, C. S. Effect of a Narrow Fuel Spray Angle and a Dual Injection Configuration on the Improvement of Exhaust Emissions in a HCCI Diesel Engine. Fuel, in press (DOI: 10.1016/j.fuel.2007.03.016).


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Figure 4. Profiles of injection rate and energizing current for (a) single injection and (b) split injection.

Figure 5. Effect of split injection (τsplit ) 1° BTDC) and single injection (τmain ) 4° BTDC) on the combustion characteristics.

Figure 3. Effect of injection timing on the combustion characteristics with single injection: (a) τmain ) TDC, (b) τmain ) 2° BTDC, and (c) τmain ) 4° BTDC.

3.2. Split Injection. The profiles of injection rate and energizing current for single injection at 4° BTDC and split injection at 1° BTDC are shown in Figures 4a and 4b, respectively. For the two injection strategies, injection delay, which is defined as the time interval between the SOE signal and the actual injection was 2.7°CA, as shown in Figure 4a. A

lower peak injection rate was observed with the split injection case, as shown in Figure 4b, because the injection event ended before reaching the peak injection rate. A comparison of the combustion pressure and the rate of heat release for split injection with the single injection case was done to clarify the effect of split injection; this comparison is shown in Figure 5. In the case of split injection, the split injection timing (τsplit) was 1° BTDC. Under that split injection condition, the first injection was performed at 6° BTDC and the second injection was performed at 4° ATDC (where ATDC denotes “after top dead center”). When the injection event was divided into two parts, the maximum combustion pressure and the rate of heat release were reduced. In particular, the combustion



Figure 6. Effect of split injection timing on the indicated mean effective pressure (IMEP).

Figure 7. Effect of split injection timing on the NOx emissions.

during the second injection is significantly affected by the combustion during the first injection and shows a longer combustion duration with a lower peak of heat release, because burn during the first injection increased the charge temperature and shortened the ignition delay of the second injection. Moreover, the products of prior combustion have a role that is similar to that of internal exhaust gas recirculation (EGR). Figures 6 and 7 show the effect of split injection timing on the IMEP and NOx emissions, respectively. The split injection timing at 1° ATDC produced an IMEP that was similar to the single injection baseline condition. More retarded or advanced injection timing slightly reduced the IMEP. As the split injection timing was retarded, NOx emissions decreased linearly; however, the engine had a tendency to misfire when the split injection timing was retarded beyond 3° ATDC. Three reasons can explain the low NOx emissions of split injection. First, during the pause between the first and second injections, the burned gas cools and the brisk reaction of the DME mixture with air can be controlled. Second, the division of the injection event suppresses the potential for high-temperature and high-pressure conditions where NOx are actively formed. Third, in the case of split injection, more fuel can be burned at a retarded CA where the charge temperature is low, compared to that of single injection. The effect of split injection timing on the HC and CO emissions is shown in Figure 8. The HC and CO emissions with a split injection strategy were generally observed at lower levels than those with the single injection at 2° BTDC. 3.3. Pilot Injection. Injection rate profiles for a single injection case and a multiple injection case with pilot injection and main injection are shown in Figure 9. For both cases, the main injection timing was fixed at TDC. To match the fuel quantity of the pilot injection case, the single injection had a

Figure 8. Effect of split injection timing on (a) the hydrocarbon (HC) emission and (b) the acrbon monoxide (CO) emissions.

Figure 9. Injection rate profiles for the single-injection only case and for the pilot-injection case (τpilot ) 20° BTDC, τmain ) TDC).

longer injection duration than the main injection of the pilot injection case. The effect of pilot injections (τpilot ) 20° BTDC) on the combustion characteristics with different main injection timings (τmain ) 5° BTDC and TDC) are shown in Figure 10. As shown in Figure 10a for the main injection timing of 5° BTDC, the pilot injection burn causes a noticeable increase in cylinder pressure at the end of the compression stroke, and it shortened the ignition delay of the main injection. Consequently, the premixed spike with a single injection condition was significantly reduced. This is caused by the shortened ignition delay, which reduces the amount of accumulated energy by supplying the fuel into the combustion chamber during the period between the start of injection and the start of combustion of the main injection. As a result, the premixed spike is reduced. In the case of the diesel engine, some researchers10,12 have reported that pilot injection increases the main combustion duration, because pilot combustion consumes the oxygen in the charge. Moreover, the combustion product with high thermal capacity is entrained


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Figure 11. Effect of pilot injection timing (τpilot) on the combustion characteristics (τmain ) 5° ATDC).

Figure 10. Effect of main injection timing (τmain) on the combustion characteristics (τpilot ) 20° BTDC): (a) τmain ) 5° BTDC and (b) τmain ) TDC.

Figure 12. Effect of pilot injection timing (τpilot) on the IMEP emission with different main injection timings (τmain).

into the main injection spray, and it has a role similar to that of internal EGR. As mentioned previously, the retarded injection timing is effective for reducing the maximum combustion temperature. As a result, the NOx emissions can be reduced. However, immoderately retarded injection timing creates unstable combustion with increasing HC and CO emissions. Therefore, restricted retarding of injection timing is possible due to an unstable combustion in the case of single injection, as shown in Figure 3. Figure 10b reveals that pilot injection is an effective method to retard the timing of the main injection for suppression of NOx formation. The main injection at TDC is too slow to maintain stable engine operation without pilot injection. However, in the case of pilot injection, pilot burn reduced the ignition lag and combustion duration of main injection. As a result, stable engine operation is achieved under this condition. The effect of pilot injection timing on the combustion characteristics with a fixed late main injection timing (τmain ) 5° ATDC) is shown in Figure 11. In this figure, the pilot timing was varied in 10° CA increments, while the main injection timing was held constant at 5° ATDC. Under this injection condition, if the main injection timing is extremely retarded, the engine cannot be operated without pilot injection, because of misfire. As the pilot injection timing (τpilot) is advanced, the heat release rate of the pilot burn is decreased. On the other hand, as τpilot is retarded, more combustion heat is released. It seems that the ignition timing of pilot injection is closely related to the injection timing, and more-advanced τpilot caused advanced combustion of the pilot injection. As one might expect, τpilot showed a close relationship with the combustion of the main injection. The most retarded pilot timing is observed at 10°

BTDC, where τpilot is closest to the main injection, which apparently has a most significant effect on the combustion of the main injection fuel; therefore, the shortest ignition delay of the main burn is generated. This can be explained as follows, in terms of pilot injection effect. First, as the pilot timing is advanced, the charge temperature at the main injection event is reduced, because the peak heat release rate of the pilot injection is reduced. At the highly advanced pilot injection timing, the fuel-air mixture could be overmixed and become too lean to burn, because the pilot injection quantity is too small. Second, as τpilot is advanced, locally, the high-temperature charge, which has a role in kindling the main injection, is well-mixed with relatively cold air. The effect of τpilot with three different main injection timings of 5° BTDC, TDC, and 5° ATDC on the IMEP and on NOx emissions is shown in Figures 12 and 13, respectively. As the pilot injection timing is advanced, the IMEP was reduced for all main injection timings, and the main injection timing at TDC offered the highest IMEP for all τpilot. In particular, at the main injection timing at 5° ATDC, where the timing is most retarded, the IMEP showed close dependence on τpilot. It rapidly decreased as τpilot advanced. The NOx emissions are observed to be dependent on both τpilot and τmain, and the level decreases as τpilot advances and τmain slows. However, the effect of pilot timing was determined to be relatively small, compared to the effect of main injection timing. As can be expected, the most retarded main injection timing at 5° ATDC resulted in the lowest NOx emissions. At this τmain value, with a τpilot of 10° BTDC, a reduction in NOx of 76.1% was observed, compared to single injection at 2° BTDC with an IMEP penalty of 3.1%. As the pilot timing advanced to 40° BTDC with the same τmain, a


Figure 13. Effect of pilot injection timing (τpilot) on the NOx emissions with different main injection timings (τmain).


Figure 15. Injection rate profile for PPCCI combustion (τ1st ) 40° BTDC, τ2nd ) TDC).

Figure 16. Effect of second injection timing of PPCCI combustion on the rate of heat release (τ1st ) 40° BTDC).

Figure 14. Effect of pilot injection timing (τpilot) on (a) the HC emissions and (b) the CO emissions with different main injection timings (τmain).

reduction in NOx of 95% was achieved while the decrease in IMEP was 37%. The formation of HC emissions is observed to be affected by both τpilot and τmain as shown in Figure 14a. Both τmain values of 5° BTDC and TDC indicated similar HC emission for all τpilot and the level increased as τpilot increased above 20° BTDC. However, a τmain value of 5° ATDC showed rapidly increased HC emission. An especially dramatic increase in HC emission was observed at a τpilot value of 40°BTDC, because of misfire and incomplete combustion of the main injection fuel. On the other hand, CO emission shows a different trend in Figure 14b. At a τmain value of 5° BTDC, with τpilot values of 30 and 40° BTDC, CO emissions were 0.08% and 0.14%, respectively. They are notably higher levels, compared to single injection at 2° BTDC. The misfire due to the overmixing of the pilot injection, which is estimated to contribute to the high CO emission of the early pilot timing. At the same time, the

dominant contributor to increase in HC and CO emissions by retarding main injection timing with fixed pilot timing, especially in the case of 40° BTDC of pilot timing, is the formation of incomplete combustion products during main injection combustion due to misfire. (See Figure 14.) 3.4. Early Injection for Partial Premixed Charge Compression Ignition. PPCCI was introduced to achieve lean NOx emissions characteristics of a DME-fueled engine. The injection rate profile for PPCCI combustion is shown in Figure 15. To elongate the premixing period and achieve lean and lowtemperature combustion, extremely early timing injection at 40° BTDC (first injection) was performed. For early timing injection, an immoderate advance in the pilot burn during the compression stroke increased negative work and heat loss through the combustion chamber during the compression stroke; however, the combustion by the advanced pilot injection is expected to generate negligibly low NOx emissions, because of the long period of mixing time before the combustion. Several methods (such as reducing compression ratio and EGR) have been proposed to reduce the negative work that originated from the fast burn by retarding the start of combustion.16 The effect of the second injection timing on the PPCCI combustion heat release curve is shown in Figure 16. Regardless of varying second injection timing, PCCI combustion, which consists of a sequential combination of a low-temperature reaction (LTR) and a high-temperature reaction (HTR), is observed. The second injection fuel was burned with short ignition delay. Compared with the 2-mg pilot injection, as previously discussed, a larger portion of fuel is injected during the first injection, and it produces more heat prior to the second injection. Consequently, the heat release peak of the second injection combustion decreased more. (16) Kim, M. Y.; Kim, J. W.; Lee, C. S.; Lee, J. H. Effect of Compression Ratio and Spray Injection Angle on HCCI Combustion in a Small DI Diesel Engine. Energy Fuels 2006, 20, 69-76.


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Figure 17. Effect of second injection timing of PPCCI combustion on the IMEP.

Figure 18. Effect of second injection timing of PPCCI combustion on the NOx emissions.

The IMEP results of PPCCI combustion can be observed in Figure 17. The PPCCI combustion produced lower IMEP by ∼22%-27%, compared to single injection at 2° BTDC. This is expected, because of the pilot combustion during the compression stroke, as mentioned previously. Moreover, the increased heat loss during the compression stroke also may be expected to decrease the engine output. In Figure 18, it may be seen that the greatest improvement in NOx was observed with the second injection timing of 5° ATDC, while the IMEP was decreased by 23%. Because of the low-temperature combustion of the first injection, HC and CO emissions were dramatically increased for all second injection timings, as shown in Figure 19. Considering the superior evaporation characteristics of DME, the high HC and CO emissions are not seriously affected by wall wetting but, instead, are a result of the combustion itself, because of lean and low-temperature combustion. As the second injection timing was retarded, the HC emission decreased slightly and the CO emission increased. 3.5. Comparisons of Exhaust Emission between Three Injection Strategies. To summarize and evaluate the effect of multiple injection strategies on exhaust emissions and engine performance, the results obtained from the best operating condition with multiple injection strategies were compared with those of the baseline single injection condition (τmain ) 2° BTDC). No soot emission was measured for all operating conditions, and, because HC and CO emissions from a compression ignition engine are easily removed using a diesel oxidation catalyst (DOC), the operating conditions that best reduce NOx emissions while maintaining a high IMEP were chosen as the best operating conditions. The operation conditions are τsplit ) 3° ATDC for the split injection strategy, τpilot ) 10° BTDC with τmain ) 5° ATDC for the pilot injection strategy, and τ1st ) 40° BTDC with τ2nd ) 5° ATDC for PPCCI

Figure 19. Effect of second injection timing of PPCCI combustion on (a) the HC emissions and (b) the CO emissions.

Figure 20. Effect of multiple injection strategies of the engine on the IMEP and exhaust emission characteristics, compared to the baseline single injection condition.

combustion. The results of the three injection strategies are compared in Figure 20. As the figure shows, the IMEP was reduced to achieve lower NOx emissions. In both split and pilot injections, a slight deterioration of IMEP (1.6 and 3.1%) was observed; however, the PPCCI injection strategy reduced the IMEP by 23%. Despite the reduction in the IMEP, substantial reductions in NOx emissions were achieved. For both split and pilot injections, HC and CO emissions were reduced or maintained, compared with those obtained under baseline conditions. However, the case of PPCCI injection showed a remarkable increase in the HC and CO emissions, compared to the other injection events.



4. Conclusions An experimental investigation was conducted to evaluate the effect of multiple injection strategies on dimethyl ether (DME)fueled engine performance and exhaust emissions. The main conclusions from this work are summarized as follows: (1) A split injection strategy with a pause between the two injections reduced NOx emissions by 74.5% at the slight expense of the indicated mean effective pressure (IMEP). The main reasons for NOx reduction are reduced brisk reaction, reduced combustion temperature and pressure, and retarded combustion event. (2) When conducting the pilot injection evaluation, retarding the main injection timing reduces the NOx emissions. As a result, the NOx emissions were reduced by 76.1% at an IMEP cost of 3.1% under injection conditions of τpilot ) 10° BTDC and τmain ) 5° ATDC. Moreover, the pilot injection strategy demonstrated simultaneous reduction in HC and CO emissions. (3) Early injection for partial premixed charge compression ignition (PPCCI) combustion reduces the IMEP by ∼22%27%, in contrast to the maximum NOx emissions reduction of 97.1%. (4) No soot emission was measured for any operating condition of the DME-fueled compression ignition (CI) engine. 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 CA ) crank angle CO ) carbon monoxide CI ) compression ignition COV ) coefficient of variation DME ) dimethyl ether EGR ) exhaust gas recirculation HC ) hydrocarbon PCCI ) premixed charge compression ignition HTR ) high-temperature reaction IMEP ) indicated mean effective pressure LNT ) lean NOx trap LTR ) low-temperature reaction NTC ) negative temperature coefficient PPCCI ) partial premixed charge compression ignition PM ) particulate matter SCR ) selective catalyst reduction SOE ) start of energizing Greek Symbols τ ) injection timing Subscripts main ) main injection pilot ) pilot injection split ) split injection EF0701844

Effect of Multiple Injection Strategies on the Emission Characteristics

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