Injection strategy for simultaneous reduction of NOx and soot emissions using two-stage injection in DME fueled engine

Applied Energy 143 (2015) 262–270

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Injection strategy for simultaneous reduction of NOx and soot emissions using two-stage injection in DME fueled engine Su Han Park a, Seung Hyun Yoon b,⇑ a b

School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 500-757, Republic of Korea Division of Automotive Engineering, Yeungnam College of Science & Technology, 170 Hyeon chung-ro, Nam-gu, Daegu 705-703, Republic of Korea

h i g h l i g h t s  The two-stage injection strategy is applied to reduce emissions.  NOx and soot can be reduced by multiple injection below single injection mode.  The pilot injection strategy with advanced 2

a r t i c l e

i n f o

Article history: Received 25 September 2014 Received in revised form 8 January 2015 Accepted 12 January 2015

Keywords: DME (dimethyl ether) Multiple injection strategy NOx (nitrogen oxides) emission Pilot injection strategy

nd

injection is best to the reduction of emissions.

a b s t r a c t The ultimate purpose of this study is the reduction of exhaust emissions from a dimethyl ether (DME) fueled diesel engine without deterioration of engine performance, such as indicated mean effective pressure (IMEP) and indicated specific fuel consumption (ISFC). In this study, we applied multiple injection strategies to achieve the research goal. In a comparison between diesel and DME single injection combustion, the IMEP in both fuels was similar around the top dead center (TDC) injection condition (this is the typical injection timing). However, the nitrogen oxide (NOx) emission in DME was higher than that in diesel. The single injection combustion in DME and diesel was compared on the basis of the same energy input condition. When the injection timing was advanced in order to reduce the DME NOx emission, the IMEP in DME decreased below the diesel level. Therefore, multiple injection strategies, including pilot injection, split injection, and advanced + post injection, were applied in this study. In the experimental results, the pilot injection strategy with advanced main injection (2nd injection) resulted in the lowest NOx, HC, and CO emissions. In the case of soot emission, DME itself has soot free combustion, and emitted an ignorable amount of soot. Moreover, the pilot injection strategy showed the highest IMEP level and the lowest ISFC level in the test conditions. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In order to replace depleting fossil fuels and satisfy the strengthened emission regulation, the research and development of various environmentally-friendly alternative fuels is critically needed. Dimethyl ether (DME) has attracted attention as a substitute for conventional diesel fuel in a compression ignition (CI) diesel engine with high thermal efficiency. Hence, over the past few decades, many researchers have attempted to apply DME to the CI engine [1–4]. Because DME has a smaller lower heating value (LHV), it should be supplied at a greater amount than diesel by 1.67 times in order to supply DME corresponding to diesel with the same energy density [5]. In addition, the low viscosity and bad lubricity of DME induce wear of moving part and fuel leakage ⇑ Corresponding author. Tel.: +82 53 650 9231; fax: +82 53 625 0863. E-mail address: [email protected] (S.H. Yoon). http://dx.doi.org/10.1016/j.apenergy.2015.01.049 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

in the engine. Therefore, some modifications in the fuel supply system are needed. On the other hand, DME has a higher cetane number than diesel, which is a very important factor for determining the combustion quality. Moreover, soot free combustion is possible because DME has no carbon-to-carbon bond (CH3–O–CH3). DME can be produced from various sources, such as natural gas, crude oil, propane, residual oil, coal and biomass waste products [6]. Therefore, DME is sufficiently suitable as a substitute of diesel. Due to these characteristics of DME, the spray, atomization, combustion and exhaust emissions of the DME fueled diesel engine system, as well as the applicability and properties of DME, have been investigated by many researchers [7–9]. Park and Lee [6,10], Thomas et al. [11], and Arcoumanis et al. [12] summarized the overall characteristics of DME combustion and emissions in a diesel engine, including fuel properties, spray and atomization. In addition, Teng et al. [5,13–16] spent several years analytically studying DME fuel in terms of its properties,

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263

Nomenclature BTDC CA CO DME HC IMEP ISISFC mdiesel

before top dead center (°) crank angle (°) carbon monoxide dimethyl ether hydrocarbon indicated mean effective pressure (bar) indicated specificindicated specific fuel consumption (g/kW h) injection quantity of diesel (mg)

spray characteristics, combustion and emissions characteristics. In order to improve the combustion performance and reduce emissions from the DME fueled engine, various new injection technologies have been applied to the DME engine. Suh et al. [17] studied the effect of multiple injection strategies on the neat dimethyl ether (DME) fuel atomization and reduction of exhaust emission characteristics in a compression ignition (CI) engine. Pilot and split injections under various injection mass and timing conditions as a multiple injection strategy were applied to reveal their effect on the improvement of spray atomization and the reduction of exhaust emissions. In their research, it was revealed that multiple injection strategies for DME fuels lead to poor atomization characteristics, and can achieve a simultaneous reduction of NOx and soot emissions in comparison to single injection results. Suh et al. [17] also reported that the advance of the first injection timing caused the gradual decrease of NOx emission without increasing soot emissions. Yoon et al. [18] researched the effect of spray angle and injection strategy on DME combustion, emissions characteristics, and particle size distribution characteristics in a common-rail diesel engine. They revealed that the combustion pressure from single combustion for narrow-angle injectors (60° and 70°) increased, compared to the results of the wide-angle injector (156°) with advanced injection timing. DME combustion for all test injectors indicated low levels of soot emissions. NOx emissions for narrow-angle injectors simultaneously increased in proportion to the advance in injection timing up to BTDC 25°, whereas this was BTDC 20° for the wide-angle injector. For multiple injections, the combustion pressure and rate of heat release (ROHR) of the first injection with narrow-angle injectors are combusted more actively, and the ignition delay of the second injected fuel is shorter than with the wide-angle injector. In addition, they reported that the particle numbers of narrow-angle injectors (60° and 70°) were similar between multiple and single injections. The injector with a 60° angle showed a smaller total particle volume than the injector with a 70° angle, despite the higher total particle number. However, the injector with a wide-angle (156°) revealed the higher total number and total volume of particles due to the increase in particle size. As mentioned in various studies regarding DME combustion, the weakest aspect of DME fuel in a diesel engine is the high NOx emission. In order to reduce the NOx emission, the exhaust gas recirculation (EGR) [19] method and an after-treatment device have usually been used [20,21]. However, the disadvantage of EGR is the decrease of engine performance and an increase of other emissions (soot, HC, and CO) [22–24]. Furthermore, after-treatment devices, such as lean NOx trap (LNT), Urea selective catalytic reduction (SCR) incur costs. Therefore, the authors have attempted to decrease the NOx emission without deterioration of soot emission and engine performance in a common-rail diesel engine without using an after-treatment device and EGR system. Zheng et al. [25] have investigated that the effect of two-stage injection on combustion and emissions characteristics in a diesel engine. They

mDME NOx Pinj ROPR Sengine TDC

s u

injection quantity of DME (mg) nitrogen oxides injection pressure (bar) rate of pressure rising (bar / °) engine speed (rpm) top dead center (°) injection timing (°) equivalence ratio

revealed that the application of pilot injection close to main injection caused the reduction of the peak of premixed heat release rate and maximum pressure rise rate. Also, they reported that the interval between pilot and main injections is important to reduce the smoke emission. Zhuang et al. [26] also studied the effect of injection strategy (single injection and multiple injection) on combustion performance and emission characteristics in a direct injection diesel engine. They reported that the application of pilot injection can reduce NOx emission with an acceptable level of soot emission. Besides above two studies, there are more literatures about multiple injection combustion strategy including two-stage injection [27–29]. The purpose of this study is to find optimal injection strategies for the simultaneous reduction of NOx and soot emission without the deterioration of engine performance, as well as the achievement of low HC and CO emission. In order to achieve this purpose, DME was used in this study, and three types of injection strategies with multiple injections were introduced. 2. Experimental setup and procedure 2.1. Experimental setup A modified single-cylinder diesel engine with a common-rail direct injection system was used in this study as shown in Fig. 1. The fuel supply and return system were modified for DME. As shown in Fig. 1, the DME fuel tank was pressurized over 6 bar (0.6 MPa) by nitrogen gas for supplying a liquid phase DME. The fuel return part in test injector was also modified to keep pressure over 6 bar. The compression ratio and displacement volume of the test engine are 17.8 and 373.3 cc, respectively. Table 1 shows the detailed engine specifications. In order to control the DME engine, a DC dynamometer with 55 kW was used. The combustion pressure from the engine combustion chamber was measured using a piezoelectric pressure transducer (6057A80, Kistler) coupled to a charge amplifier (5018A, Kistler), and was acquired using a PCbased data acquisition system. In addition, the injection timing, fuel injection quantity, and injection strategies were controlled by an injector driver (TDA-3300, TEMS). Exhaust emissions, such as HC, CO, NOx, and soot, were measured and analyzed using an emission analyzer (Horiba, MEXA-554JK) and soot analyzer (AVL, AVL-415-S) with a filter paper method. In order to supply a liquid-phase DME fuel, a gas-phase DME was pressurized at over 6 bar with nitrogen gas. The detailed specifications for the emission analyzer, and the properties information for DME and diesel are listed in Tables 2 and 3, respectively. 2.2. Experimental procedure In this study, the injection pressure and engine speed were fixed at 600 bar and 1200 rpm, respectively. In a single injection mode, the injection timing, which is called ‘start of energizing’

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Pressure controller

High pressure pump

Common-rail

N2 tank DME Fuel tank Injector

Exhaust Pre-mixed chamber

Smoke meter Exhaust gas analyzer (NOx, HC, CO)

Pressure sensor Air flow meter

Encoder Charge amplifier Timing pulse generator Air compressor

Data acquisition system

Experimental engine

DC Dynamometer

Fig. 1. Schematic of experimental apparatus.

Table 1 Specifications of single-cylinder diesel engine.

Table 3 Physical and chemical properties of diesel and DME.

Item

Specification

Property

DME

Diesel

Engine type Number of cylinder Bore  stroke Displacement volume Fuel injection system Valve type Compression ratio

Direct injection diesel engine 1 75.0 mm  84.5 mm 373.3 cc Bosch common rail DOHC 4 valves 17.8

Injector Number of hole Hole diameter Spray angle

6 0.128 mm 156°

Chemical formula Molar mass Vapor pressure at 20 °C, bar Boiling temperature, °C Liquid density at 20 °C, kg/m3 Liquid viscosity at 25 °C, kg/ms Gas specific gravity (vs air) Lower heating value, MJ/kg Cetane number Stoichiometric A/F ratio, kg/kg Enthalpy of vaporization at NTPa, kJ/kg

CH3OCH3 46.07 5.1 25 660 0.12–0.15 1.59 28.43 55–60 9.0 460 ( 20 °C)

C8–C25 200 <0.01 150–380 800–840 2–4 – 42.5 40–55 14.6 250a

a

Table 2 Specifications of the exhaust emission analyzer.

NOx HC CO

Item

Specification

Model Principle of measurement

Response

MEXA-554JKNOx (Horiba) CO, HC: non-dispersive infrared rays NOx: chemical method (ECS sensor) HC: 0–10,000 ppm vol. CO: 0–10 vol.% NOx: 0–4000 ppm HC: ±12 ppm vol. CO: ±0.06 vol.% NOx: less than ±1.0% 90% response within 10 s

Model Principle of measurement Measuring range Repeatability Response

AVL-415S Filter paper method 0–10 FSN (0–32,000 mg/m3) 0.005 FSN + 3% 0.001 FSN/0.01 mg/m3

Measuring range

Measuring accuracy

Soot

(SOE) in this study, was changed from TDC to BTDC 40° as an interval of 5°. The SOE was determined as the time when the injection signal was conducted to the injector. For single injection mode,

Normal temperature and pressure.

each injection amount of diesel and DME is 10 mg and 16.4 mg, respectively, because the LHV of DME is 1.4-fold lower than that of diesel fuel. On the other hand, three types of injection strategies were applied in this study, including pilot injection (mode 1), split injection (mode 2), and advanced + post injections (mode 3). Generally, the pilot injection strategy (mode 1) is used to reduce the engine noise and NOx emissions [30,31], and the split injection strategy (mode 2) was used in order to reduce both particulate matter and NOx emission from the diesel engine [32–34]. In the advanced + post injections strategy (mode 3), the first injection with a relatively large injection amount was injected at the advanced injection timing (generally, around BTDC 30°), and the second injection with a relatively small injection amount was injected at around TDC. The diagram for these injection strategies is shown in Fig. 2. On the other hand, the combustion pressure data was ensemble-averaged at over 300 cycles in order to compensate for the cycle-to-cycle variations. The combustion pressure data was analyzed to calculate the rate of heat release (ROHR) using the first

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Fig. 2. Injection strategies for the simultaneous reduction of NOx and soot emissions from DME combustion.

law of thermodynamics. Detailed test conditions are listed in Table 4.

3. Results and discussions 3.1. Comparison of combustion and emission characteristics of diesel and DME in a single-injection combustion mode Fig. 3 shows a comparison of the IMEP and ISFC characteristics of diesel and DME fuels in a single injection mode. As shown in Fig. 3(a), the IMEPs in diesel and DME fuels showed decreasing trends with the advance of injection timing. In contrast to the IMEP in conventional injection timings (around TDC), the IMEP at BTDC 30° crank angle (CA) has a decreased value of about 62.7%. This explains why the advance of injection timing induced the ignition before TDC, which caused the increase of negative work. With the advance of injection timing, the IMEP in DME combustion showed a lower value than that in diesel combustion. This is why the DME with a high cetane number ignited earlier than diesel; consequently, it affected the increase of the negative work of DME combustion although diesel and DME were supplied to the combustion cylinder as the same energy density. On the other hand, the slightly

high IMEP of DME can be found around the injection conditions of BTDC 5° and TDC, compared to that of diesel. The reasons for high IMEP of DME can be explained by non-luminous flame [35] and higher combustion efficiency of DME [36]. The non-luminous flame implies a lower heat radiation; consequently, the cooling loss can be reduced through the cylinder wall [35]. In addition, the higher combustion efficiency means the low fraction of incomplete combustion product such as HC and CO [36]. From the results of Fig. 3(a), it can be seen that the excessive advance of injection timing for the reduction of exhaust emissions may induce the significant deterioration of combustion performance. Fig. 3(b) shows the fuel consumption characteristics of diesel and DME fuels. With the advance of injection timing, the ISFC generally increased. This result concurs closely with the IMEP results, and it commonly has an inverse proportion to IMEP. Fig. 3(b) shows the high fuel consumption in DME combustion. This result can be explained as follows. As shown in Table 3, DME has a lower LHV than diesel. Therefore, DME was supplied by as much as 1.64 times compared to diesel in order to supply the same energy per injection. In Fig. 3(b), the average fuel consumption of DME is larger than that of diesel at about 71% of all the injection conditions. This ratio of fuel consumption in diesel and DME is the same as the ratio of the amount of fuel supply in both fuels. Fig. 4 shows the combustion characteristics such as combustion pressure, rate of heat release (ROHR), and rate of pressure rise (ROPR) characteristics of diesel and DME in a single injection mode. The combustion characteristics of diesel and DME fuels were compared at two injection timings of TDC and BTDC 30° CA. The primary combustion characteristics (premixed combustion duration and ignition delay) are listed in Table 5. As shown in Fig. 4, the ignition in DME occurred in advance because of the high cetane number (at TDC: about 0.5 CA degrees, at BTDC 30°: about 1.5 CA degrees). In the TDC injection case, the ignition in both fuels was started in the expansion stroke, and the peak combustion pressure in DME is higher than that in diesel because DME was ignited under a higher ambient temperature and ambient pressure. The other reason for the high peak combustion pressure of DME is the greater fuel injection of DME. These combustion characteristics in TDC affected a slightly high IMEP of DME as shown in Fig. 3(a). In the ROHR and ROPR curves, the peaks of DME at TDC injection were slightly lower than those of diesel. This can be explained by the fact that DME combustion has an insufficient air–fuel mixing period due to the short ignition delay. On the other hand, the diesel combustion with the relatively sufficient premixing period rapidly progressed immediately after ignition, and was then represented as a higher peak value in ROHR and ROPR. In addition, the combustion period of DME is longer than that of diesel in the comparison of the ROHR curves of both fuels due to the longer injection duration. These phenomena also affected the lower peak of ROHR in DME combustion.

Table 4 Experimental conditions. Engine speed (rev/min) Intake temperature (K) Mass flow rate of intake air (kg/h) Overall equivalence ratio Injection pressure (bar)

1200 320 ± 5 12.34 0.43 600

Injection timing and injection quantity Single injection Multiple injection

Pilot injection (mode 1) Split injection (mode 2) Advanced + post injection (mode 3)

Injection timing BTDC 40°–TDC (5° step)  1st injection: BTDC 30°  2nd injection: BTDC 5°, TDC, ATDC 5°

Injection quantity mfuel,diesel = 10 mg mfuel,DME = 16.4 mg 4.9 mg + 11.5 mg 8.2 mg + 8.2 mg 11.5 mg + 4.9 mg

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120

6 5

mfuel,diesel=10mg, mfuel,DME=16.4mg

IMEP (bar)

diesel

DME

4

-62.7%

3 2

100

τ=TDC

τ=BTDC 30

400

o

diesel DME

350

80

300 250

60

200 40

150

20

100 50

0

1

0 -40

-20

0 -40

-30

-20

-10

25

1200

20

m fuel,diesel =10mg,mfuel,DME =16.4mg DME

600

ROPR (bar/degree)

Pinj=600bar, Sengine =1200rpm diesel

20

40

(a) Combustion pressure and ROHR

(a) IMEP

900

0

Crank angle (deg. ATDC)

0

Injection timing (deg. ATDC)

ISFC (g/kWh)

450

Pinj=600bar, Global φ: 0.43, single injection

ROHR (J/degree)

Combustion pressure (bar)

Pinj=600bar, Sengine=1200rpm

Pinj=600bar, Global φ: 0.43, single injection τ=TDC

τ=BTDC 30o

diesel DME

15 10 5 0

300

-40

-20

0

20

40

Crank angle (deg. ATDC)

0 -40

-30

-20

-10

0

(b) ROPR

Injection timing (deg. ATDC)

(b) ISFC

Fig. 4. Combustion characteristics of diesel and DME fuels in TDC and BTDC 30° injection timings.

Fig. 3. Comparison of IMEP and ISFC characteristics of DME and diesel fuels in various injection timing conditions.

In the condition of the BTDC 30° injection, DME also showed higher peak combustion pressure due to the larger fuel injection amount. In contrast to the TDC condition, the peaks of ROHR and ROPR in DME combustion are higher than those in diesel combustion. The significant reason for this result is the difference of process; BTDC 30° injection is under a compressed process, while TDC injection is under an expanded process. Although DME has short ignition delay, its low boiling temperature provides better fuel/air mixing and superior atomization due to the flash boiling effect. Hence, DME combustion in the BTDC 30° injection condition has a shorter combustion period and a higher peak ROHR and ROPR than diesel combustion. Fig. 5(a) compares the ISNOx emission characteristics of diesel and DME are shown in wide injection conditions from BTDC 40° through to TDC with the interval of 5°. Fig. 5(b) shows the spray targeting points for various injection timings, which were extracted from Ref. [37] in order to enhance understanding. As shown in Fig. 5(a), the comparison of ISNOx emissions characteristics in diesel and DME showed different results according to the injection timings. Hence, the regions of injection timings were divided into three parts, including Region A, Region B, and Region C, for convenience of explanation. The spray targeting characteristic of each region corresponded to the region shown in Fig. 5(b). For example, when the fuel was injected at Region A, most of the injected spray reached the deepest points in the piston bowl. In the case of Region B, the injected spray was targeted to the upper side of the piston bowl, then progressed along to the wall with

tumble flow. Most of the injected spray at Region C flows into the crevice or squish areas. Detailed information with spray images can be found in Ref. [37]. In the comparison of ISNOx emission in Region A, DME emitted larger NOx than diesel. This can be explained by the oxygen effect in DME. When both fuels are injected in Region A, the injected spray forms a mixture with air for ignition, and can utilize most of the oxygen in the combustion chamber. Hence, both fuels actively combust using oxygen in the piston bowl. In the case of DME, combustion that is more active is possible because DME can use more of its oxygen. Consequently, the cylinder temperature in the DME combustion increased more than that in diesel combustion, and it then provided more appropriate environment for the formation of NOx. Therefore, the ISNOx from DME is higher than that from diesel. On the other hand, when the injection timing is advanced to Region B, the ISNOx emission from DME is lower than that from diesel. In Region B, the injected spray forms a more uniform mixture than in Region A and Region C. Due to the oxygen contents of DME, a leaner air/fuel mixture is formed in DME combustion. In addition, the superior evaporation characteristics of DME decrease the in-cylinder temperature. Therefore, for the above reasons, DME combustion emitted lower ISNOx emission than diesel combustion. In addition, the temperature reduction due to the evaporation of DME explains the lower ISNOx than diesel. Finally, in the section for NOx emission in the single combustion mode, the ISNOx from DME seems higher than that from diesel in Region C. This is due to the incomplete combustion of diesel. This result can be explained in more detail using soot, HC, and CO emission as shown in Fig. 6.

S.H. Park, S.H. Yoon / Applied Energy 143 (2015) 262–270 Table 5 Ignition delay and premixed combustion duration (CA10–CA50) in Fig. 4.

Diesel (s = TDC) DME (s = TDC) Diesel (s = BTDC 30°) DME (s = BTDC 30°) * **

Premixed combustion duration (CA10*–CA50**) (°)

Ignition delay (°)

1.4 2.3 1.6 1.1

6.6 6.1 12.8 11.3

CA10: the crank angle at which 10% of the total heat release has occurred. CA50: the crank angle at which 50% of the total heat release has occurred.

60 Pinj=600bar, Sengine=1200rpm mfuel,diesel=10mg, mfuel,DME=16.4mg

ISNO x (g/kWh)

diesel

DME

40

267

diesel is lower than that from DME in Region C. This is because diesel spray is mostly emitted as soot after combustion reaction as shown in Fig. 6(a). However, unburned DME emitted as HC because DME does not form soot emission due to the absence of C–C bond. Therefore, it seems that DME emitted higher ISHC than diesel in Region C. In the injection timings corresponding to Region A and Region B (after BTDC 25°), the ISSoot, ISCO, and ISHC emissions from DME are very low, and close to zero emission, or these emissions are much lower than those from diesel. Figs. 5 and 6 illustrate that DME showed lower HC, CO, and Soot emissions than diesel, while NOx in DME combustion showed higher emission than that in diesel at the same energy supply conditions. These results are almost similar to those reported in other literatures. In order to minimize the ISNOx emissions of DME below diesel emissions, the injection timing should be advanced as shown in the results in Fig. 5 (such as Region B). However, the advance of injection timing induced some deterioration of combustion performance, such as the decrease of IMEP and the increase of ISFC. Therefore, various multiple injection strategies are applied in order to reduce the exhaust emissions while maintaining the combustion performance in this study.

20

Region C

Region B

3.2. Two-stage injection strategies for emission reduction from DME combustion

Region A

0 -40

-30

-20

-10

0

Injection timing (deg. ATDC)

(a) ISNOx

(b) Spray targeting for injection timings [38] Fig. 5. ISNOx emission and spray targeting points of DME and diesel in various injection timings. ((b) Was cited from Ref. [30]).

Fig. 6 shows the ISSoot, ISCO, and ISHC emissions characteristics of diesel and DME in a single injection mode. In the injection timings before BTDC 25° (corresponding to Region C), DME showed very low ISSoot, ISCO emission, and high ISHC emission results. As mentioned in the above section (Fig. 5), these results can be explained as the incomplete combustion of diesel fuel. When the injection timing advanced before BTDC 25°, the injected spray targeted toward the piston lib or squish or ultimately to the cylinder wall. The diesel spray then reached the wall of the piston or cylinder because the diesel spray has a longer spray tip penetration and worse evaporation characteristics than the DME spray [38–41]. The impinged spray formed a thin fuel film on the wall. These fuel films create incomplete combustion and a locally fuel-rich region. The incomplete combustion provides more CO emission and the fuelrich region create soot emission. In addition, the incomplete combustion induced a low in-cylinder temperature; consequently, the ISNOx emission from diesel combustion is lower than the DME combustion as shown in Fig. 5. On the other hand, the ISHC from

Three types of multiple injection strategies are introduced in this study. The results from multiple injection were compared to the results from single combustion mode at TDC and BTDC 30°. In multiple injection strategies, the injection pressure was fixed at 600 bar, and the first injection timing was also fixed at BTDC 30°. Fig. 7 shows a comparison of the ISNOx emissions from various multiple injection strategies and single combustion mode of diesel and DME. First, the ISNOx emission from the three combustion modes with multiple injections showed lower than single combustion modes in diesel and DME when the second injection timing was TDC. When the second injection timing was advanced, the ISNOx in mode 1 and mode 2 significantly decreased, and that in mode 3 increased compared to the single combustion results at the TDC injection timing in diesel and DME. In addition, the retarded second injection timing induced the increase of ISNOx emission in mode 1 and mode 2, while the ISNOx in mode 3 decreased. The main feature in the three combustion modes with multiple injection strategies is the ratio of injection quantity in the first and second timings. The first injection quantity increases according to the change from mode 1 through to mode 3. As shown in Fig. 7, the combination of the pilot injection and advanced main injection resulted in the lowest ISNOx emission in all test conditions. The higher IMEP characteristics of mode 1 with advanced main injection, is one of the reasons for the lowest ISNOx emission. Fig. 8 shows the ISsoot emission results from the various multiple injection strategies. As shown in the figure, the soot emission values from DME combustion are very low compared to the diesel soot emission. Although little difference is shown between the injection strategies, they cannot be compared because they are significantly low. In the same injection mode, the advance and retardation of the main injection caused an increase of the ISSoot emission. The comparison of ISHC and ISCO emissions in the single and multiple combustion strategies of diesel and DME are shown in Figs. 9 and 10. The change of modes from mode 1 through to mode 3 caused the increase of ISHC and ISCO in all test cases. The change of mode implies an increase of injection quantity at first timing, which then causes an increase of incomplete combustion fraction. On the other hand, the change of the second injection timing has little influence on the ISHC and ISCO emissions at each combustion mode. Therefore, it can be said that the ISHC and ISCO emissions

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1.2

Pinj=600bar, Global φ: 0.43

Pinj=600bar, Sengine=1200rpm

25

m fuel,diesel =10mg, mfuel,DME=16.4mg

0.9

diesel

st

o

1 injecon ming: BTDC 30 (fix) Pilot injecon

Split injecon

Adv. + post injecon

DME

ISsoot (g/kWh)

20

ISNOx (g/kWh)

DME (τ=BTDC 30o)

0.6

0.3

15

Diesel (τ=TDC)

5

-30

-20

-10

DME (τ=TDC)

10

0.0

-40

Diesel (τ=BTDC 30o)

0

Change of 2nd injection timing

0 TDC injection --

Injection timing (deg. ATDC)

-- Adv. injection -- Ret. injection

Injection strategy

(a) ISsoot

Fig. 7. ISNOx emission characteristics of DME fueled diesel engine in various twostage injection strategies (change of 2nd injection timings and injection strategy).

120 Pinj=600bar, Sengine =1200rpm mfuel,diesel =10mg, m fuel,DME=16.4mg diesel

DME

Pinj=600bar, Global φ: 0.43

2.0

1st injecon ming: BTDC 30o (fix) Pilot injecon

60

30

0 -40

-30

-20

-10

0

ISsoot (g/kWh, *10-3)

ISCO (g/kWh)

90

1.5

Split injecon

Adv. + post injecon

Diesel soot emission τ=BTDC 30o : 0.1743g/kWh τ=TDC : 0.0055g/kWh

1.0 DME (τ=TDC)

0.5

Injection timing (deg. ATDC)

Change of 2nd injection timing DME (τ=BTDC 30o)

(b) ISCO

0.0 TDC injection --

10

-- Adv. injection -- Ret. injection

Injection strategy Pinj=600bar, Sengine =1200rpm mfuel,diesel =10mg, m fuel,DME =16.4mg

ISHC (g/kWh)

8

diesel

DME

Fig. 8. ISsoot emission characteristics of DME fueled diesel engine in various twostage injection strategies (change of 2nd injection timings and injection strategy).

6 Pinj=600bar, Global φ: 0.43

4

4

1st injecon ming: BTDC 30 o (fix) Pilot injecon

Split injecon

DME (τ=BTDC

2

Adv. + post injecon

30o)

0 -40

-30

-20

-10

0

Injection timing (deg. ATDC)

(c) ISHC Fig. 6. Comparison of ISsoot, ISCO, and ISHC emissions in DME and diesel combustion.

ISHC (g/kWh)

3

2 Change of 2 nd injection timing

1

Diesel (τ=BTDC 30o)

DME, diesel (τ=TDC)

0 TDC injection --

-- Adv. injection -- Ret. injection

Injection strategy were mainly affected by the first injection strategies. In terms of the ISHC and ISCO emissions, combustion mode 1 is superior to the other combustion modes. Combustion mode 1 showed the lower ISHC and ISCO than diesel and DME combustion at BTDC 30°. Fig. 11 shows the combustion performance including IMEP and ISFC in diesel and DME combustion. In a single combustion mode of diesel and DME, IMEP at the injection timing of TDC is approximately 4.6–4.8 bar. As shown in Fig. 11(a), combustion mode 1

Fig. 9. ISHC emission characteristics of DME fueled diesel engine in various twostage injection strategies (change of 2nd injection timings and injection strategy).

showed almost 4.8 bar, regardless of the second injection timings. The IMEP of combustion modes 2 and 3 is lower than that of combustion mode 1. However, these are higher than IMEP at BTDC 30° in diesel and DME single combustion. Generally, as shown in

S.H. Park, S.H. Yoon / Applied Energy 143 (2015) 262–270

4. Conclusions

Pinj=600bar, Global φ: 0.43

40

1st injecon ming: BTDC 30o (fix) Pilot injecon

Split injecon

ISCO (g/kWh)

Adv. + post injecon

30o)

DME (τ=BTDC

30

Diesel (τ=BTDC 30o) : 88.92g/kWh

20 Change of 2nd injection timing

10 Diesel (τ=TDC) DME (τ=TDC)

0

TDC injection --

-- Adv. injection -- Ret. injection

Injection strategy Fig. 10. ISCO emission characteristics of DME fueled diesel engine in various twostage injection strategies (change of 2nd injection timings and injection strategy).

7

6

269

P ni j=600bar, Global φ : 0.43 1st injection timing: BTDC 30o (fix) DME injection strategy Pilot injection Split injection

1. By advancing the injection timing, the ISNOx in DME can be reduced below that in diesel. However, this induces the deterioration of IMEP. In addition, the adjustment of the injection timing (e.g., BTDC 25°–BTDC 20°) can reduce the emission region in a DME single injection combustion mode. 2. ISNOx and ISSoot emissions can be reduced below the levels of the diesel single injection combustion mode due to the multiple injection. In addition, ISNOx emission was show when the 2nd injection was triggered in advance at the pilot injection mode. ISHC and ISCO emissions increased with the increase of the 1st injection quantity. The injection timing of the 2nd injection has little influence on the ISHC and ISCO emissions. 3. The IMEP showed the highest level and the ISFC showed the lowest level at the pilot injection combustion mode. The 2nd injection timing has little effect on IMEP and ISFC characteristics. 4. Synthesizing the experimental results of this study, the pilot injection strategy with advanced 2nd injection timing is optimal to achieve the simultaneous reduction of ISNOx and ISSoot emission without deterioration of engine performance (IMEP and ISFC), with low ISHC and ISCO emissions.

Adv. + post injection

IMEP (bar)

Acknowledgement 5

This study was financially supported by Chonnam National University, 2014. 4

References 3

2 -6

-3

0

3

6

nd

2 injection timing (deg. ATDC)

(a) IMEP 700

ISFC (g/kWh)

600

P ni j=600bar, Global φ : 0.43 1st injection timing: BTDC 30o (fix) DME injection strategy Pilot injection Split injection

Adv. + post injection

500

400

300

200 -6

-3

0

3

6

nd

2 injection timing (deg. ATDC)

(b) ISFC Fig. 11. IMEP and ISFC characteristics of DME combustion in various two-stage injection strategies (change of 2nd injection timings and injection strategy).

Fig. 11(a), the increase in first injection quantity caused the decrease of IMEP because of the combustion reaction before TDC and the increase of negative work. On the other hand, the ISFC characteristics according to the combustion modes showed a counter-trend compared to IMEP as shown in Fig. 11(b).

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