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Effects of various split injection strategies on combustion and emissions characteristics in a single-cylinder diesel engine
Accepted Manuscript Effects of various split injection strategies on combustion and emissions characteristics in a single-cylinder diesel engine Suhan Park, Hyung Jun Kim, Dal Ho Shin, Jong-Tae Lee PII: DOI: Reference:
S1359-4311(17)36206-3 https://doi.org/10.1016/j.applthermaleng.2018.05.025 ATE 12164
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
25 September 2017 7 February 2018 7 May 2018
Please cite this article as: S. Park, H. Jun Kim, D. Ho Shin, J-T. Lee, Effects of various split injection strategies on combustion and emissions characteristics in a single-cylinder diesel engine, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.05.025
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A submission to ”Applied Thermal Engineering” Submission date: September 25, 2017 R1 Submission date: February 7, 2018
Effects of various split injection strategies on combustion and emissions characteristics in a single-cylinder diesel engine
Suhan Parka,*), Hyung Jun Kimb), Dal Ho Shinc), Jong-Tae Leeb) a*)
School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of
Korea (*Corresponding author, Email: [email protected]) b)
National Institute of Environmental Research, 42 Hwangyeong-ro, Seo-gu, Inchon, 22689, Republic of Korea
c)
Powertrain Research Center, Korea Automotive Technology Institute, 55 Jingok-sandan-jungang-ro, Gwangsan-gu, Gwangju, 62465, Republic of Korea
Corresponding Author Professor Suhan Park School of Mechanical Engineering, Chonnam National University 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea Tel: +82-62-530-1674 Fax: +82-62-530-1689 Email: [email protected]
1
Abstract The purpose of this study is to investigate combustion and exhaust emissions characteristics by applying various split-injection strategies in order to determine the optimal injection that improves fuel efficiency and reduces exhaust emissions, compared with single-injection combustion. The split-injection strategies, such as the change of injection pressure, injection timing, and injection interval, were applied to single-cylinder diesel engine. From the analysis of the experimental results, it was revealed that, when the injection pressure was increased without changing the injection timing in split injection, indicated mean effective pressure (IMEP) and brake mean effective pressure (BMEP) decreased and brake specific fuel consumption (BSFC) deteriorated, owing to the increase of dwell duration. The increase of injection interval induced deterioration of BSFC and caused an increase in NOx and HC emissions with significant reduction in CO and soot emissions. On the other hand, the retardation of injection timing with a fixed injection interval caused an improvement of the BSFC, and a decrease in NOx, CO, and HC emissions. However, soot emission increased. According to the analysis, a short injection interval and injection timing around top dead center (TDC), with not too high injection pressure, improve the BSFC and emission characteristics in split-injection diesel combustion.
Keywords Split injection, injection interval, BSFC (brake specific fuel consumption), single-cylinder diesel engine, exhaust emissions
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Nomenclature ATDC
after top dead center
TDC
top dead center
BTDC
before top dead center
BSFC
brake specific fuel consumption, g/kWh-h
Pinj
injection pressure, bar
Pmax
maximum in-cylinder pressure, bar
mfuel
injection quantity, mg
ROHR
rate of heat release, J/deg.
HC
hydrocarbon, ppm
NOx
nitrogen oxides, ppm
CO
carbon monoxide, %
Sengine
engine speed, rpm
CA10
the point where the amount of heat corresponding to 10% of total heat generation occurs.
CA50
the point where the amount of heat corresponding to 50% of total heat generation occurs.
CA90
the point where the amount of heat corresponding to 90% of total heat generation occurs.
τ1st
1st injection timing
τ2nd
2nd injection timing
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1. Introduction Various studies are being conducted to satisfy the fuel efficiency and exhaust emission regulations by
improving the fuel consumption rate and reducing exhaust emissions. Because fuel in a diesel engine with high
thermal efficiency and power-output is directly injected into the combustion chamber at the end of the
compression stroke, a non-uniform fuel-air mixture is formed in that chamber. There is a problem of generation
of nitrogen oxides (NOx) in the high temperature region and formation of particulate matter in the fuel rich
region [1, 2]. Many researchers are actively studying the application of after-treatment equipment such as a
diesel particulate filter (DPF), selective catalyst reduction (SCR), and diesel oxidation catalyst (DOC) [3-5], or
combustion optimization techniques such as high-pressure injection and multi-stage injection [6-8] to reduce
diesel exhaust emissions. Multi-stage injection is being considered as a very attractive way to achieve
optimization of the combustion and exhaust characteristics, without adding more complexity and cost to the
engine. In general, multi-stage injection means a method in which the main injection is divided into two or more
injections, thereby widening the reaction region between the air in the combustion chamber and the periphery of
the combustion flame; thus, the distribution of the fuel rich region in the combustion chamber is reduced. In
addition, the application of multi-stage injection reduces the ignition delay of the main injection and confirms the
phenomenon of rapid combustion. Therefore, it is known that the combustion by multi-stage injection reduces
nitrogen oxides and particulate matter emission compared to single injection combustion [9, 10]. Therefore,
numerous researchers continue investigating the multi-stage injection combustion as one of the methods to
reduce exhaust emissions from diesel engines and to improve fuel efficiency. Roh et al. [11] presented a study on
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the effects of pilot injection on combustion and exhaust characteristics in a four-cylinder diesel engine. They
reported that the maximum pressure of combustion during pilot injection is significantly reduced and the rate of
increase of combustion pressure in the cylinder is also reduced. Li et al. [12] studied the effect of multi-stage
injection on particulate matter and reported that the amount of particulate matter increases with the increase in
the diffusion combustion zone when pilot injection is applied. Park et al. [13] also reported that when the pilot
injection strategy was applied in the idling state and low-rpm region, the high temperature combustion area is
formed in the combustion chamber during the main injection, and the particulate matter (PM) is increased owing
to decrease of air entrainment into the spray plume. Eom et al. [14], in their study on the effect of pilot injection
on nanoparticle emission characteristics in a passenger diesel engine,
reported that the number of nanoparticles
increased with the addition of pilot injection. Huang et al. [15] investigated the effect of pilot injection on low
temperature combustion in diesel engines. They reported that a smaller pilot-main interval could remarkably
reduce brake specific fuel consumption (BSFC) and NOx. Lee et al. [16] reported that the application of multi-
stage injection to a passenger diesel engine powered by JP-8 fuel, delayed ignition timing and reduced
particulate matter by 75 % without increasing nitrogen oxides. In this way, studies on the combustion and
emission characteristics according to the application of multi-stage injection in diesel engines are actively being
carried out. In particular, efforts are being made to reduce fuel consumption and exhaust emissions by applying
various pilot injection strategies, such as those involving injection timing and injection amount.
This study seeks the optimal injection strategy, based on the application of split-injection, for the
improvement of fuel efficiency and exhaust emission reduction. Split injection means that the amount of the fuel
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injected before the main injection is equal to the amount of fuel injected in the main injection. It is different from
pilot injection, in which a small amount of fuel is injected before the main injection. Therefore, the purpose of
this study is to analyze the characteristics of combustion and exhaust emissions according to the application of
various split-injection strategies, such as injection timing, injection interval, and injection pressure, in order to
find the optimal injection strategy to improve fuel efficiency and to reduce exhaust emissions, compared with
single-injection combustion.
2. Experimental apparatus and procedure Com mon-rail
Fuel High Conditioning Pressure System Pum p
ETAS
Fuel Tank
ETK 7.1
INCA
Injector
Exhaust
Intake Air
Soot m eter
MSS
AVL Exhaust gas analyzer (NOx, HC, CO)
Pressure sensor Micro IFEM
Encoder Data acquisition system
Dynam om eter Controller Experim ental engine
AC Dynam om eter
Fig. 1 Schematic diagram of test engine and measurement systems
Figure 1 shows a schematic diagram of the single-cylinder diesel engine used in this study in order to
measure and analyze combustion and emissions characteristics by various split-injection strategies. The
experimental apparatus consists of the single-cylinder diesel engine with common-rail injection, engine control,
fuel supply / injection, intake-air control, and data acquisition systems. The main features of the diesel engine
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used in this study are natural aspiration, compression ratio of 17.1, bore / stroke size of 85 mm / 90 mm, and displacement volume of 510.7 cm3. The test injector has a spray angle of 142o, a hole diameter of 0.180 mm, and
five-holes. A detailed engine specification was listed in Table 1.
Table 1
Specifications of single-cylinder diesel engine
Item
Specification
Engine type
Direct injection diesel engine
Number of cylinder
1
Bore × Stroke
80.0 mm × 90.0 mm
Displacement volume
510.7 cc
Fuel injection system
Bosch common rail (CP3)
Compression ratio
17.1
Engine management system (EMS)
AVL-RPEMS + ETK 7 (Bosch)
Number of hole
5
Hole diameter
0.180 mm
Spray angle
142o
Flow
375 ml / 30 sec
Injector
A prototype engine-control-unit (ETK 7.1, ETAS) controlled the fuel injection parameters. The injection
variables, such as injection pressure, injection timing, and injection quantity, were controlled by using the INCA
program in which the communication system was established with an ETAS system. The engine speed and
torque were controlled by an AC dynamometer (AMK DW 13-170-4, AVL), and the fuel injection timing was
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measured in units of 0.1o by the crank angle sensor (3,600 pulse/rev). The pressure in the combustion chamber
was measured using a piezo-electric pressure sensor (SL31D, AVL). The sensor signal was amplified and
converted by an AVL micro IFEM piezo module, and then averaged over 300 cycles to minimize the effect of the
inter-cycle variation. A fuel mass flow meter (AVL, 735S) was used to measure the fuel consumption of the
single-cylinder diesel engine according to the various injection strategies. The fuel temperature of the supplied
fuel was kept constant by using a fuel temperature controller (AVL, 753C). The emission analyzer (HORIBA,
MEXA-554JKNOx) measured the emissions of CO, HC, and NOx. CO and HC emissions were measured with
non-dispersive infrared rays and the NOx emissions were measured by means of a chemiluminescence detector
(CLD). The micro soot sensor (AVL), which is based on the photoacoustic measurement method, was used to
measure and analyze the soot emission. The detailed specifications for the emission analyzer are listed in Table 2. Table 2
Specifications of exhaust emission analyzer
Item
Specification Model
MEXA-554JKNOx (HORIBA)
Principle of
CO, HC: Non-dispersive infrared rays
measurement
NOx: Chemical method (ECS sensor) HC: 0.00 ~ 10,000 ppm
Gaseous emission
Measuring range
CO: 0.00 ~10.00% vol. NOx: 0.00 ~ 5,000 ppm
(NOx, HC, CO)
HC: ± 12 ppm vol. Measuring accuracy
CO: ± 0.06 % vol. NOx: ± 20 ppm vol.
Response
90 % response within 10 second
Model
AVL Micro Soot Sensor (MSS)
Principle of Soot
measurement
Photo-acoustics
Measuring range
0.001 ~ 50 mg/m3
Dilution ratio
Adjustable from 2 ~ 20
8
1μg/m3
Detection limit
Spl it Injection w/ Fixed Injection timing (Pres sure Variations) Interval (8degree)
1st
2nd
7mg (5.25 deg.)
7mg (5.25 deg.)
Inj. Pr. : 500 bar ~ 1200 bar Injection Timing: BTDC 11 o + BTDC 3 o Spl it Injection w/ Fixed 2nd Injection ti ming (Interval Variations) 18degree
Single Injection
14mg
Inj.Pr. : 500 bar TDC ~ BTDC 27 o
8degree
VS
1st 7mg (5.25 deg.)
∙∙∙
1st
2nd
7mg (5.25 deg.)
7mg (5.25 deg.)
2 nd injection timing: BTDC 3 o Interval: 8deg. ~ 18deg. Spl it Injection w/ Fixed Interval (Ti mi ng Variations) Interval (8degree)
1st
2nd
7mg (5.25 deg.)
7mg (5.25 deg.)
2 nd injection: TDC ~ BTDC 18 o
Fig. 2 Injection strategies used in this study
In general, the multi-stage injection can be divided into pilot injection and split injection. Pilot injection
means a method wherein a small amount of fuel is injected before the main injection. Split injection can be
defined as a multi-stage injection method that injects the same amount of fuel in each stage. In this study, the
injection strategy, as shown in Figure 2, was used to analyze the combustion and exhaust emissions
characteristics of the diesel engine according to various split-injection application. Three split-injection strategies were applied. In the case of Figure 2(a), the injection timing and injection interval are fixed as BTDC 11o+BTDC 3o and 8o, respectively, and the injection pressure is changed from 500 bar to 1200 bar. Figure 2(b) shows the
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case where the injection timing of the first injection is changed by keeping the second injection timing at BTDC 3o, i.e., the interval between two injections is changed from 8o to 18o. The minimum injection interval is fixed at 8o because the energizing duration for injection of 7 mg at the experimental condition (injection pressure 500 bar) is about 7.5o (crank angle, CA). Therefore, the dual injection is not realized at intervals smaller than 8 degree CA.
Figure 2(c) shows the case where the injection timing is changed by keeping the injection interval of 8 degrees.
The experimental results obtained by split injection application were compared to those obtained by single-
injection application. The detailed experimental conditions are shown in Table 3.
Table 3 Experimental conditions
Single injection
Split injection
Injection pressure
500 bar
Injection timing
TDC ~ BTDC 27o (3 degree interval)
Injection quantity
14 mg
Injection pressure
500 bar ~ 1200 bar
Injection timing
BTDC 11o + BTDC 3o
Injection quantity
7 mg + 7 mg
Injection pressure
500 bar
(Fig. 2(a))
Split injection
Injection timing
1st injection: variable 2nd injection: BTDC 3o
(Fig. 2(b)) Injection quantity
7 mg + 7 mg
Injection interval
8 degree ~ 18 degree
Injection pressure
500 bar
Split injection
Injection timing
2nd injection: TDC ~ BTDC 18o
(Fig. 2(c))
Injection quantity
7 mg + 7 mg
Injection interval
8 degree
10
3. Results and discussion 3.1 Combustion and emission characteristics with single-injection strategy
In-Cylinder Pressure [bar]
100 Single Injection Strategy Pinj=500bar, mfuel=14mg
80
60 BTDC 27 o
40
TDC
20
0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (a) Combustion pressure curves
Rate of heat release [J/deg]
300 Single Injection Strategy Pinj=500bar, mfuel=14mg
250 BTDC 27 o
200
TDC
150 100 50 0 -50 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (b) ROHR Fig. 3 Combustion pressure curves and ROHR characteristics of single injection combustion (Pinj=500bar, mfuel=14mg, Sengine=1600 rpm, Injection timing: TDC ~ BTDC 27o, 3 deg. Interval)
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The combustion and exhaust emissions characteristics were investigated according to the application of
single-injection strategy before testing various split-injection strategies. Figure 3 shows the characteristics of in-
cylinder combustion pressure and rate of heat release (ROHR) according to the change of single injection timing. The injection timing advanced from TDC to BTDC 27o at intervals of 3o. The ROHR was calculated by the first
law of thermodynamics and the state equation of ideal-gas based on the measurement of combustion pressure. As
shown in Figure 3(a), as the injection timing was advanced, the ignition timing is advanced and the maximum
combustion pressure was increased. It can be considered that ignition timing and ignition delay have influence,
as stated in existing literature [17-19]. When the ignition occurred before TDC, the advance of injection timing
caused increase of ignition delay. Then better air-fuel mixture is formed, which results in better combustion and higher in-cylinder pressure. When the injection timing was advanced more than BTDC 12o, it can be observed
that ignition was started before TDC. On the other hand, Figure 3(b) shows the ROHR according to the injection
timing. The peak ROHR decreased by advancing injection timing, but it started to increase again from the injection timing of BTDC 12o. This ROHR characteristic is also closely related to the ignition timing and
ignition delay. Hence, it is considered that the increase after decrease of ignition delay as the injection timing
advances affected the decrease and increase of the maximum ROHR. The quantitative analysis related to this is
presented in detail in Figures 4-6.
Figure 4 shows the mean effective pressures (MEPs), i.e., indicated mean effective pressure (IMEP) and
brake mean effective pressure (BMEP), and the brake specific fuel consumption (BSFC) characteristics
according to the ignition timing. In this study, the ignition delay is defined as the period between ignition timing
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(CA10) and injection timing. CA10 was calculated from the amount of heat release, and it means the point where
the amount of heat corresponds to 10% of total heat generation. As shown in the Figure 4, the MEPs tended to
increase as the ignition timing approached to the top dead center (TDC), and it was confirmed that the MEPs
decreased as the ignition timing advanced to before the TDC. The BSFC appeared to be contrary to the trend of
MEPs. This occurs because, when ignition occurs near the TDC, the negative work, which means minus work
owing to the ignition before TDC, is minimized [20, 21] and the explosive force by combustion is delivered to
the piston in the most efficient way, resulting in the highest MEPs; thus, the consumption of fuel is also
minimized. Therefore, the relationships between ignition timing and BSFC and between ignition timing and
MEPs show opposite correlation based on TDC. 260 Single Injection Strategy Pinj=500bar, mfuel=14mg
Indicated MEP
7.0
240
6.0 Brake MEP
5.0
220
4.0 200
BSFC
BSFC [g/kW-h]
Mean effective pressure [bar]
8.0
3.0 2.0 -15
180
-10
-5
0
5
10
15
20
Ignition timing [deg.ATDC] Fig. 4 Mean effective pressures (MEP) and BSFC characteristics of single injection combustion
Figure 5 shows various combustion characteristics, such as the maximum pressure (P max), maximum rate of
heat release (ROHRmax), ignition delay, and combustion duration, for the injection timing. As shown in Figure 3,
Pmax increased with the advance of injection timing, and ROHRmax increased after decreasing. This is closely
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related to the increase in ignition delay after advancing the injection timing. If the ignition delay is long, the
mixture of fuel and air is more uniformly mixed and the instantaneous ROHR is increased when the combustion is performed. Therefore, in the injection timing range (TDC ~ BTDC 12o) ignited after the TDC, the ROHR is
also decreased due to the reduction of the ignition delay according to the advance of the injection timing. On the other hand, in the region where the ignition started before TDC (before the injection timing of BTDC 12o), the
ignition delay gradually increased according to the advance of the injection timing, and the ROHR max was also
greatly increased. The overall combustion period (CA10-CA90) was also similar to the tendency of the ignition
delay.
P
[bar]
90
Single Injection ROHRmax Combustion duration
ma x 100
Ignition delay [deg.] 20 Ignition delay Pmax
ROHR [kJ/deg]
250
18
50
45
80 40
200
16
70
35
150
60
14 30
50
100
12 40
50
30 -30
10 -25
-20
-15
-10
-5
25
Combustion duration [degree]
300
20
0
Injection Timing [deg.ATDC] Fig. 5 Analytical combustion characteristics (maximum combustion pressure, maximum ROHR, ignition delay, and combustion duration)
14
10
CO [%]
HC [ppm]
0.10
40
0.09
8
35
0.08
2000 1800 1600
30
6
25
0.06 0.05
4
1400
20
CO HC
0.04
15
0.03
1200 1000 800
NOx [ppm]
Soot [mg/m3]
0.07
600 10
2
0.02
400
soot
NOx
0.01
0
0.00 -30
5 0
-25
-20
-15
-10
-5
200 0
0
Injection Timing [deg.ATDC] Fig. 6 Exhaust emission characteristics (NOx, soot, HC, and CO)
Figure 6 shows the exhaust emission characteristics according to the change of injection timing in a single-
injection combustion. The NOx tended to increase with the advance of injection timing, and HC and CO emissions were the lowest at the injection timing of BTDC 15o and then increased again. The injection timing condition of BTDC 15o, which showed the highest IMEP and BMEP results, is considered the most favorable
combustion condition in this test engine (see Figures 3 and 4). Soot emissions increased after decreasing to BTDC 25o. A clear trade-off relationship between NOx and soot was confirmed. In addition, it was confirmed
that extremely early injection and TDC resulted in abrupt increase of HC due to combustion instability. After
confirming these basic emission characteristics of single-injection combustion, we compared the results with the
exhaust emissions from split-injection combustion.
3.2 Combustion and emission characteristics with split-injection strategy
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In this study, we investigated the effects of three kinds of split-injection, namely, change of injection
interval, change of injection timing, and change of injection pressure, on the combustion and exhaust emission
characteristics of a single-cylinder diesel engine.
Figures 7-10 show the effect of injection pressure on the combustion and emission characteristics of splitinjection combustion. In this experiment, the injection interval was fixed to 8 o and the injection timing was set as BTDC 11o + BTDC 3o. The injection pressure was changed from 500 bar to 1200 bar in 100 bar increments. 350
9
Split Injection Strategy Pinj=500bar, mfuel=7+7mg
Indicated MEP
300
8 7 6
250 Brake MEP
200
5 4
BSFC [g/kW-h]
Mean effective pressure [bar]
10
150 BSFC
3 2
100
500
600
700
800
900
1000
1100
1200
Ignition timing [deg.ATDC] Fig. 7 Mean effective pressures and BSFC of split injection combustion at various injection pressures
Figure 7 shows the IMEP and BMEP, and the BSFC characteristics with varying injection pressures. As
shown in the figure, both MEPs decreased with the increase of injection pressure. The BSFC is increased up to
injection pressure of 800 bar, then decreased and increased repeatedly, and converged to about 250 g/kWh. The
BMEP converged to about 4 bar after an injection pressure of 800 bar. In addition, it was confirmed that the
FMEP, which is the difference between BMEP and IMEP, is increased by the increase in injection pressure.
Generally, it is known that when the injection pressure is increased, the atomization of the fuel is improved and a
16
uniform mixture is formed, thereby improving the combustion performance [22-23]. However, in this case of
split injection, it is difficult to expect combustion performance improvement simply by increasing the injection
pressure. This is because if the injection timing is fixed and the injection pressure is increased under the same
injection amount condition, the energizing duration of the electric current in each injection is shortened and the
intervals (dwell duration) between injections are distant from each other. The increase in the injection interval
causes the deterioration of combustion performance [24] (see Figure 11). In addition, the improvement of
atomization by the increase of injection pressure causes the spray droplet to evaporate rapidly, and the
combustion is quickly terminated because of the increase in burning speed and flame speed [25]. This leads to
low combustion pressure characteristics in the expansion stroke, resulting in a decrease in MEP owing to a
reduction in the burning work. A slight increase in the negative work caused by the advance in ignition timing (ATDC 1.8o → BTDC 1.6o) owing to the increase in injection pressure causes a decrease in the MEP.
Figure 8 shows the in-cylinder combustion pressure profiles and ROHR curves for the increase in injection
pressure. Figure 8(a) shows the advance of the ignition timing and the reduction in combustion pressure in the
expansion stroke with the increase in injection pressure. In addition, it can be seen that the maximum combustion
pressure increases because of the increase in injection pressure, which can attributed to the active combustion
reactions caused by the improved atomization characteristics. Figure 8(b) shows the ROHR characteristics with
increasing injection pressure. As can be seen in the figure, the maximum ROHR approximately doubled,
increasing from 93.8 J/degree to 184.9 J/degree, and the ignition timing advanced when the injection pressure
was increased. Although the first ROHR peak is increased with increasing injection pressure, the second peak is
17
maintained, and two peaks are clearly observed. It is confirmed that the maximum ROHR occurrence point is
slightly advanced, but it is almost the same. In Figure 9, it is possible to observe the reduction in energizing
duration and the increase in injection interval with increasing injection pressure. Figure 9 shows that the
energizing duration must be shortened for injecting the same amount at high injection pressure. It was also confirmed that the injection interval, which is the duration between the end of 1 st injection and the start of 2nd
injection, was increased because of the reduction in energizing duration. In addition, at the injection pressure of
800 bar or higher, it was confirmed that the duration of the electric current was gradually decreased and the
injection interval was gradually increased.
In-Cylinder Pressure [bar]
100 Split Injection Strategy Injection timing: BTDC 11o+BTDC 3o mfuel=7+7mg
80
500 bar 900 bar
700 bar 1100 bar
60
40
20
0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (a) Combustion pressure curves
18
250 1100 ba r
200 150
Max: 184.9 J/deg. @ BTDC 1 o
100 50 0
Rate of heat release [J/deg.]
-50 900 ba r
Max: 162.4 J/deg. @ BTDC 1 o
250 700 ba r
200 150 Max: 180.5 J/deg. @ TDC
100 50 0 -50
500 ba r
Max: 93.8 J/deg. @ ATDC 1 o
-20
0
20
40
60
80
Crank angle [deg.ATDC] (b) ROHR Fig. 8 Combustion pressure curves and ROHR characteristics of split injection combustion at various injection pressures (500bar ~ 1100bar) 8 mfuel=7+7mg, Sengine=1600rpm
5.5 deg.
*This is for one injection. *Single injection: 772s
550
Injection interval Energizing duration
500
6
4.3 deg.
450
4
400
3.0 deg.
350
2
3.5 deg.
Injection interval [deg.]
Energizing duration [s]
600
300 0 500
600
700
800
900
1000
1100
1200
Injection pressure [bar] Fig. 9 Energizing duration and interval between two injections* at various injection pressures (*The interval in this graph means the duration between the end of 1st injection and the start of 2nd injection.)
19
40
CO [%]
HC [ppm]
0.8
40
soot
35
35
2000 1800 1600
0.6
30
CO
20
1400
NOx
25
25 0.4
20
15
15
1200 1000 800
NOx [ppm]
Soot [mg/m3]
30
600
10
0.2
10 400
5 0
5
HC 0.0
0 500
600
700
800
900
1000
1100
200 0
1200
Injection Pressure [bar]
Fig. 10 Exhaust emission characteristics for increasing injection pressure in a split-injection combustion (injection interval: 8 degrees, injection timing: BTDC 11o+BTDC 3o)
Figure 10 shows the exhaust emission characteristics according to the variation of injection pressure when the injection interval is maintained at 8o and the injection timing is BTDC 11o + BTDC 3o. As the injection
pressure increased, soot and CO emissions decreased significantly. It can be judged that the fraction of
incomplete combustion decreased because combustion was actively occurring owing to the improvement of the
atomization performance with the increase of injection pressure. The NOx tended to increase with the active
combustion characteristics. It can be considered that the improvement in combustion performance by increasing
injection pressure induced the increase in the thermal NOx owing to high combustion temperature. The high
combustion temperature is directly related to the decrease in CO (high conversion from CO to CO2). HC showed a slight tendency to increase (2.1 ppm → 7.1 ppm) but it was negligible.
20
BSFC [g/kW-h]
270
11
Split Injection Strategy (Fixed 2nd Injection, BTDC 3 deg.) Pinj=500bar, Sengine=1600 rpm, mfuel=7+7mg BSFC
10
IMEP
240
9
210
8
180
197.6 g/kW-h Lowest BSFC in Single Injection
7
150
IMEP [bar]
300
6
120 5
8
10
12
14
16
18
Interval between injections [degree] Fig. 11 BSFC and IMEP of split injection combustion with fixed 2nd injection timing (Pinj=500bar, mfuel=7+7mg, 2nd Injection timings: BTDC 3o)
Figure 11 shows the effect of the injection interval change on the BSFC and IMEP with a fixed second injection timing at BTDC 3o. As shown in the figure, as the injection interval increased, BSFC increased and
IMEP decreased. When the second injection timing was fixed, the increase in the injection interval means that
the first injection gradually advances. This means that the ignition timing of the fuel / air mixture is advanced
and the combustion phase is advanced. As a result, the amount of combustion ignited prior to TDC increases, and
the IMEP decreases as the negative work increases. The decrease in IMEP leads to an increase in BSFC. In addition, as shown in the figure, when the injection interval is larger than 14 o, the BSFC in a split injection is increased more than the minimum BSFC of the single injection; thus, an injection interval of 14o or less in the
split injection is advantageous.
21
100
In-Cylinder Pressure [bar]
Split Injection Strategy (Fixed 2nd Injection, BTDC 3 deg.) Pinj=500bar, mfuel=7+7mg Inteval Varations (8 deg.
80
18 deg.)
60 Increase of injection interval
40 CA50 Advance with increase of interval
20
0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC]
Ignition timing (CA10) [deg.ATDC] Ignition delay [degree]
(a) In-cylinder pressure curves 15 Split Injection Strategy (fixed 2nd injection timing) Pinj=500bar, mfuel=7+7mg
10
Ignition delay
5 0
Ignition before TDC
Ignition timing
-5 -10 -15 8
10
12
14
16
18
Injection interval [degree] (b) Ignition timing and ignition delay Fig.12 Combustion characteristics (combustion pressure curves, ignition timing, and ignition delay) of split injection combustion with variable injection interval (8 deg. to 18 deg.)
Figure 12 shows the characteristics of the combustion pressure profile, ignition timing (CA10), and ignition
delay according to the injection interval. As described above, it can be seen that the ignition timing and CA50
(the crank angle at 50% of the cumulative heat release) advance because of the increase in injection interval. In
22
Figure 12(a), it can be seen that as the injection interval increases, not only the advance of the ignition timing but
also the combustion end are accelerated; the combustion chamber pressure is lower in the expansion stroke.
These characteristics (increased negative work owing to advancement of ignition timing and lower combustion
chamber pressure in expansion stroke) cause a decrease in IMEP and an increase in BSFC. Figure 12(b) shows
that the increase in the injection interval affects the advancement of ignition timing and the decrease / increase of
ignition delay. Especially, it was confirmed that ignition progresses before the TDC when the injection interval increased. However, it was confirmed that the ignition delay was longer when the injection interval was 12o or
more. This is because the temperature and pressure in the combustion chamber are low owing to the advance of
the first injection timing by the increase in the injection interval, and the sufficient energy for ignition is not
supplied. The increase of the ignition delay provides a condition in which the mixture of air and fuel becomes
uniform, thereby reducing the locally fuel-rich region in the combustion chamber, which is considered to have
the effect of suppressing the increase in nitrogen oxides, as shown in Figure 16(b). 200
10.5 Split Injection Strategy (Fixed Interval 8 deg.) Pinj=500bar, Sengine=1600 rpm, mfuel=7+7mg BSFC
10.0
IMEP
9.5 9.0
160
8.5
140
IMEP [bar]
BSFC [g/kW-h]
180
8.0 7.5
120
7.0
-21
-18
-15
-12
-9
-6
-3
0
3
2nd Injection Timing [deg.ATDC] Fig. 13 BSFC and IMEP characteristics of split injection combustion with fixed interval (8 degrees)
23
In-Cylinder Pressure [bar]
120 Split Injection Strategy (fixed interval, 8 degrees) Pinj=500bar, mfuel=7+7mg
100
55
80
50
45
60
40
35 21
24
27
30
33
Crank angle [deg.ATDC]
40 20 0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (a) Combustion pressure Split Injection Strategy (fixed interval) Pinj=500bar, mfuel=7+7mg
200 150 100 50 0 -50
BTDC 20 o + BTDC 12 o
BTDC 17 o + BTDC 9 o
200 150 100 50 0 -50
Start of combustion during 2nd injection
Rate of heat release [J/deg.]
BTDC 23 o + BTDC 15 o
Start of combustion after end of 2nd injection
200 150 100 50 0 -50
BTDC 14 o + BTDC 6 o
BTDC 11 o + BTDC 3 o
200 150 100 50 0 -50
BTDC 8 o + TDC
-20
0
20
40
60
80
Crank angle [deg.ATDC]
(b) ROHR Fig. 14 Combustion pressure and ROHR characteristics of split injection combustion with fixed interval
24
Ignition timing (CA10) [deg.ATDC] Ignition delay [degree]
15 Split Injection Strategy (fixed interval) Pinj=500bar, mfuel=7+7mg
10
Ignition delay
5 0 -5
Ignition before TDC
Ignition timing
-10 -15 -18
-15
-12
-9
-6
-3
0
Injection timing [deg.ATDC] Fig. 15 Ignition timing and ignition delay of split injection combustion with fixed interval
Figures 13-15 show the combustion characteristics of a diesel engine when the injection timing was changed while keeping the injection interval (8o). The reason for setting the injection interval of 8 o is that the
best BSFC characteristics are shown in the above results. As shown in Figure 13, as the injection timing is
advanced, IMEP decreases and BSFC increases. In addition, the IMEP was maintained while the injection timing was advanced from TDC to BTDC 9o, and the IMEP was significantly decreased when the injection timing was advanced more than BTDC 12o. A detailed analysis and discussion of these results is presented in Figures 14 and
15.
Figure 14 shows the combustion pressure profile and ROHR curves according to the injection timing. As
the injection timing is advanced, the maximum combustion pressure increases and the ignition timing advances
gradually. In addition, the maximum ROHR was increased with the advance of injection timing. As shown in Figure 14(b), two distinct ROHR curves were observed by split injection at the injection timing of BTDC 8 o +
TDC. However, as the injection timing was advanced, the peak of the first ROHR curve was increased and the
25
peak of the second ROHR curve was lowered. This is because the ignition delay becomes longer as the injection
timing advances, the premixing period becomes longer, and the mixed air and fuel in the combustion chamber
are burned at almost the same time.
Figure 15 shows the ignition timing and ignition delay characteristics. The ignition delay was defined as the
interval from the second injection timing to CA10 in a split injection case. The ignition timing advances
according to the advance of the injection timing, and it is confirmed that ignition timing is earlier than TDC when the injection timing is advanced to BTDC 6o. On the other hand, in the analysis of the ignition delay, when the injection timing is advanced more than BTDC 12o, the combustion is started through the premixing period
after the end of the second injection. However, in the other cases, there is no sufficient premixing process
because the combustion was started immediately after the end of second injection or during the injection process.
Hence, it can be considered that these characteristics have influenced the ROHR characteristics in Figure 14.
NOx
HC
CO
Soot
(a) (b)
(a) Emission characteristics in injection timing-interval map
26
40
(b) CO [%]
HC [ppm]
0.8
40
soot
35
35
2000
1.2
0.4
20
15
15
1200 1000 800
Soot [mg/m3]
25
NOx
HC [ppm] 20
NOx
1000
800
60
0.9
40
0.6
HC 10
0.2
10 400
5 0
HC
5
0.0
0 8
10
12
14
16
Interval between Injections [degree]
18
600
400
600
10
1200
15
CO
1400
25 20
80
30
CO
NOx [ppm]
Soot [mg/m3]
0.6
1.5
1800 1600
30
100
NOx [ppm]
(a) CO [%]
20
0.3
0
0.0
5
soot
200
200 0
0 -20
-15
-10
-5
0
0
Injection Timing [deg.ATDC]
(b) Emission characteristics in a line (a) and (b) Fig. 16 Exhaust emissions (soot, NOx, HC, CO) characteristics in split-injection strategies (injection timing variation and interval variation)
Figure 16 shows the characteristics of exhaust emissions with varying injection intervals and injection
timing. As shown in Figure 16(a), as the injection interval gets better and the injection timing approach TDC,
NOx and HC emissions are reduced. Regarding CO and soot, both of them showed low emission values except the interval of 8o. Figure 16(b) shows the exhaust emissions results at positions (a) and (b). As mentioned above,
the amount of nitrogen oxides increased with the injection interval, and the injection timing decreased with TDC.
The increase in injection interval caused a significant decrease in soot and CO and an increase in HC. On the
other hand, as the injection timing approaches TDC, CO and HC including NOx are decreased, and soot is
increased.
27
350
Pinj=500bar, mfuel=14mg (7+7mg), Sengine=1600 rpm Single Injection (s,inj=BTDC 12 deg.)
Ignition before 2 nd injection
Split Injection (1st=BTDC 11 deg., 2nd=BTDC 3 deg.) Injection pressure variations (500bar ~ 1200bar) Interval variations (8deg. ~ 18deg.) Injection timing variations (TDC ~ BTDC 18) with fixed interval
BSFC [g/kW-h]
300
250 Increase of Injection pressure
Single injection (197.6 g/kW-h)
200
150
Increase of Interval between injections
Advance of injection timing
Split injection (126.3 g/kW-h)
100 -6
-4
-2
0
2
4
6
8
10
12
14
16
Ignition delay [degree] Fig. 17 BSFC characteristics in single- and split-injection strategies
Figure 17 shows the BSFC characteristics for the ignition delay when applying the three kinds of split
injection strategies above discussed. The results of single injection are also shown for comparison with the split
injection results. As shown in the figure, when single injection was changed to split injection (injection interval 8o, injection timing: BTDC 11o+BTDC3o), ignition delay was shortened and BSFC was also improved. At this
time, when the injection interval was increased, the ignition delay was decreased, the fuel efficiency
characteristics were deteriorated (red square symbol). When the injection interval was kept at a minimum, the
ignition delay was increased when the injection timing is changed (green triangle symbol). In addition, it was
confirmed that the ignition delay was slightly reduced, and the fuel efficiency was worse when the injection pressure was increased by maintaining the injection timing (BTDC 11o + BTDC 3o) and injection interval (8o).
Based on these experimental results, the minimum fuel efficiency characteristics in this study were obtained
when the injection interval was kept to a minimum and the injection timing was brought near TDC. On the other
28
hand, in this experiment, it was confirmed that the fuel efficiency was good when the injection pressure was low
(500 bar). This is because the injection pressure was increased without adjusting the injection interval.
4. Conclusions In this study, the three kinds of split injection strategies (injection pressure, injection interval, injection
timing) were applied to investigate the combustion and exhaust emissions characteristics in a single-cylinder
diesel engine. The main findings from the experiments are summarized as follows.
1.
When the injection pressure was increased without changing the injection timing in a split injection, the
IMEP and BMEP decreased and the BSFC deteriorated owing to the increase in dwell duration. The
BSFC converged beyond the injection pressure of 800 bar. The increase in injection pressure induced
the significant reduction of soot and CO, while NOx increased.
2.
The increase in injection interval caused a deterioration of the BSFC with decrease of the IMEP. The
retardation of injection timing with fixed injection interval conditions caused the improvement of the
BSFC.
3.
The increase in injection interval caused the increase in NOx and HC emissions, while it caused
significant reduction in CO and soot emissions. In addition, the retardation of injection timing with a
fixed injection interval decreased of NOx, CO, and HC emissions, while soot emission was increased.
29
4.
From the analysis of the experimental results, it can be concluded that a short injection interval,
injection timing around TDC, and not too high injection pressure conditions are needed for the
improvement of fuel consumption and emission characteristics in a split-injection diesel combustion.
Acknowledgement This study was supported by Basic Science Research Program (2016R1D1A3B03935537) and Basic Research Laboratory Program (2015R1A4A1041746) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.
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[3] José Rodríguez-Fernández, Juan José Hernández, Jesús Sánchez-Valdepeñas. Effect of oxygenated and paraffinic alternative diesel fuels on soot reactivity and implications on DPF regeneration. Fuel 2016; 185: 460-7. [4] Sato S, Nakamura Y, Hirabayashi H, Sata S, Hosoya M. Characterization of emissions from UreaSCR and DPF system for heavy duty engine. SAE tech paper 2015; 2015-01-2016. [5] Shukla PC, Gupta T, Labhasetwar NK, Khobaragade R, Gupta NK, Agarwal AK. Effectiveness of non-noble metal based diesel oxidation catalyst on particle number emissions from diesel and biodiesel exhaust. Sci Total Environ 2017; 574: 1512-20. [6] Aalam CS, Saravanan CG, Anand BP. Impact of high fuel injection pressure on the characteristics of CRDI diesel engine powered by mahua methyl ester blend. Appl Therm Eng 2016; 106: 702-11. [7] Mathivanan K, Mallikarjuna JM, Ramesh A. Influence of multiple fuel injection strategies on performance and combustion characteristics of a diesel fueled HCCI engine – An experimental investigation. Exp Therm Fluid Sci 2016; 77: 337-46.
[8] Anand K, Reitz RD. Exploring the benefits of multiple injections in low temperature combustion using a diesel surrogate model. Fuel 2016; 165: 341-50. [9] Badami M, Mallamo F, Millo F, Rossi EE. Influence of Multiple Injection Strategies on Emissions, Combustions Noise and BSFC of a DI Common Rail Diesel Engine, SAE tech paper 2002; 2002-01-0503.
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[10] Park SH, Yoon SH. Injection Strategy for Simultaneous Reduction of NOx and Soot Emissions using TwoStage Injection in DME Fueled Engine. Appl Energ. 2015; 143, 262-70. [11] Roh HG, Lee D, Lee CS. Impact of DME-biodiesel, diesel-biodiesel and diesel fuels on the combustion and emission reduction characteristics of a CI engine according to pilot and single injection strategies. J Energy Inst 2015; 88: 376-85. [12] Li X, Guan C, Luo Y, Huang Z. Effect of multiple-injection strategies on diesel engine exhaust particle size and nanostructure, J. Aerosol. Sci. 2015; 89: 69-76. [13] Park C, Kook S, Bae C. Effects of multiple injections in a HSDI diesel engine equipped with common rail injection system. SAE tech paper 2004; 2004-01-0127. [14] Eom DS, Kang SH, Lee SW. Nanoparticle emission characteristics and reduction strategies by boost pressure control and injection strategies in a passenger diesel engine. Int J Auto Tech-Kor 2017; 18(1): 1-17. [15] Huang H, Wang Q, Shi C, Liu Q, Zhou C. Comparative study of effects of pilot injection and fuel properties on low temperature combustion in diesel engine under a medium EGR rate. Appl Energ 2016; 179: 1194-208. [16] Lee J, Lee J, Chu S, Choi H, Min K. Emission reduction potential in a light-duty diesel engine fueled by JP8. Energy 2015; 89: 92-9. [17] Gnanasekaran S, Saravanan N, Ilangkumaran M. Influence of injection timing on performance, emission and combustion characteristics of a DI diesel engine running on fish oil biodiesel. Energ 2016; 116: 1218-29. [18] Tumbal AV, Banapurmath NR, Tewari PG. Effect of injection timing, injector opening pressure, injector nozzle geometry, and swirl on the performance of a direct injection, compression-ignition engine fueled with honge oil methyl ester (HOME). Int J Auto Tech-Kor 2016; 17(1): 35-50. [19] Agarwal AK, Dhar A, Gupta JG, Kim WI, Choi K, Lee CS, Park S. Effect of fuel injection pressure and injection timing of karanja biodiesel blends on fuel spray, engine performance, emissions and combustion characteristics. Energ Convers Manage 2015; 91: 302-14. [20] Chumueang R, Laoonual Y, Chollacoop N. Effects of injection timing and injection pressure on combustion characteristics and emission of ethanol ED95 under partially premixed combustion condition. SAE tech paper 2015; SAE2015-32-0826. [21] Kook S, Park S, Bae C. Influence of early fuel injection timings on premixing and combustion in a diesel engine. Energ Fuel 2008; 22: 331-7. [22] Kastengren A, Ilavsky J, Viera JP, Payri R, Duke DJ, Swantek A, Tilocco FZ, Sovis N, Powell CF. Measurement of droplet size in shear-driven atomization using ultra-small angle x-ray scattering. Int J Multiphas Flow 2017; 92: 131-9. [23] Rollbusch C. Effects of hydraulic nozzle flow rate and high injection pressure on mixture formation, combustion and emission on a single-cylinder DI light-duty diesel engine. Int J Engine Res 2012; 13(4): 32339. [24] Herfatmanesh MR, Zhao H. Experimental investigation of effects of dwell angle on fuel injection and diesel combustion in a high-speed optical CR diesel engine. P I Mech Eng D- J Aut 2013; 227(2): 246-60. [25] Polymeropoulos CE, Das S. The effect of droplet size on the burning velocity of Kerosene-air spray. Combust Flame 1975; 25: 247-57.
31
S1359-4311(17)36206-3 https://doi.org/10.1016/j.applthermaleng.2018.05.025 ATE 12164
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
25 September 2017 7 February 2018 7 May 2018
Please cite this article as: S. Park, H. Jun Kim, D. Ho Shin, J-T. Lee, Effects of various split injection strategies on combustion and emissions characteristics in a single-cylinder diesel engine, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.05.025
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A submission to ”Applied Thermal Engineering” Submission date: September 25, 2017 R1 Submission date: February 7, 2018
Effects of various split injection strategies on combustion and emissions characteristics in a single-cylinder diesel engine
Suhan Parka,*), Hyung Jun Kimb), Dal Ho Shinc), Jong-Tae Leeb) a*)
School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of
Korea (*Corresponding author, Email: [email protected]) b)
National Institute of Environmental Research, 42 Hwangyeong-ro, Seo-gu, Inchon, 22689, Republic of Korea
c)
Powertrain Research Center, Korea Automotive Technology Institute, 55 Jingok-sandan-jungang-ro, Gwangsan-gu, Gwangju, 62465, Republic of Korea
Corresponding Author Professor Suhan Park School of Mechanical Engineering, Chonnam National University 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea Tel: +82-62-530-1674 Fax: +82-62-530-1689 Email: [email protected]
1
Abstract The purpose of this study is to investigate combustion and exhaust emissions characteristics by applying various split-injection strategies in order to determine the optimal injection that improves fuel efficiency and reduces exhaust emissions, compared with single-injection combustion. The split-injection strategies, such as the change of injection pressure, injection timing, and injection interval, were applied to single-cylinder diesel engine. From the analysis of the experimental results, it was revealed that, when the injection pressure was increased without changing the injection timing in split injection, indicated mean effective pressure (IMEP) and brake mean effective pressure (BMEP) decreased and brake specific fuel consumption (BSFC) deteriorated, owing to the increase of dwell duration. The increase of injection interval induced deterioration of BSFC and caused an increase in NOx and HC emissions with significant reduction in CO and soot emissions. On the other hand, the retardation of injection timing with a fixed injection interval caused an improvement of the BSFC, and a decrease in NOx, CO, and HC emissions. However, soot emission increased. According to the analysis, a short injection interval and injection timing around top dead center (TDC), with not too high injection pressure, improve the BSFC and emission characteristics in split-injection diesel combustion.
Keywords Split injection, injection interval, BSFC (brake specific fuel consumption), single-cylinder diesel engine, exhaust emissions
2
Nomenclature ATDC
after top dead center
TDC
top dead center
BTDC
before top dead center
BSFC
brake specific fuel consumption, g/kWh-h
Pinj
injection pressure, bar
Pmax
maximum in-cylinder pressure, bar
mfuel
injection quantity, mg
ROHR
rate of heat release, J/deg.
HC
hydrocarbon, ppm
NOx
nitrogen oxides, ppm
CO
carbon monoxide, %
Sengine
engine speed, rpm
CA10
the point where the amount of heat corresponding to 10% of total heat generation occurs.
CA50
the point where the amount of heat corresponding to 50% of total heat generation occurs.
CA90
the point where the amount of heat corresponding to 90% of total heat generation occurs.
τ1st
1st injection timing
τ2nd
2nd injection timing
3
1. Introduction Various studies are being conducted to satisfy the fuel efficiency and exhaust emission regulations by
improving the fuel consumption rate and reducing exhaust emissions. Because fuel in a diesel engine with high
thermal efficiency and power-output is directly injected into the combustion chamber at the end of the
compression stroke, a non-uniform fuel-air mixture is formed in that chamber. There is a problem of generation
of nitrogen oxides (NOx) in the high temperature region and formation of particulate matter in the fuel rich
region [1, 2]. Many researchers are actively studying the application of after-treatment equipment such as a
diesel particulate filter (DPF), selective catalyst reduction (SCR), and diesel oxidation catalyst (DOC) [3-5], or
combustion optimization techniques such as high-pressure injection and multi-stage injection [6-8] to reduce
diesel exhaust emissions. Multi-stage injection is being considered as a very attractive way to achieve
optimization of the combustion and exhaust characteristics, without adding more complexity and cost to the
engine. In general, multi-stage injection means a method in which the main injection is divided into two or more
injections, thereby widening the reaction region between the air in the combustion chamber and the periphery of
the combustion flame; thus, the distribution of the fuel rich region in the combustion chamber is reduced. In
addition, the application of multi-stage injection reduces the ignition delay of the main injection and confirms the
phenomenon of rapid combustion. Therefore, it is known that the combustion by multi-stage injection reduces
nitrogen oxides and particulate matter emission compared to single injection combustion [9, 10]. Therefore,
numerous researchers continue investigating the multi-stage injection combustion as one of the methods to
reduce exhaust emissions from diesel engines and to improve fuel efficiency. Roh et al. [11] presented a study on
4
the effects of pilot injection on combustion and exhaust characteristics in a four-cylinder diesel engine. They
reported that the maximum pressure of combustion during pilot injection is significantly reduced and the rate of
increase of combustion pressure in the cylinder is also reduced. Li et al. [12] studied the effect of multi-stage
injection on particulate matter and reported that the amount of particulate matter increases with the increase in
the diffusion combustion zone when pilot injection is applied. Park et al. [13] also reported that when the pilot
injection strategy was applied in the idling state and low-rpm region, the high temperature combustion area is
formed in the combustion chamber during the main injection, and the particulate matter (PM) is increased owing
to decrease of air entrainment into the spray plume. Eom et al. [14], in their study on the effect of pilot injection
on nanoparticle emission characteristics in a passenger diesel engine,
reported that the number of nanoparticles
increased with the addition of pilot injection. Huang et al. [15] investigated the effect of pilot injection on low
temperature combustion in diesel engines. They reported that a smaller pilot-main interval could remarkably
reduce brake specific fuel consumption (BSFC) and NOx. Lee et al. [16] reported that the application of multi-
stage injection to a passenger diesel engine powered by JP-8 fuel, delayed ignition timing and reduced
particulate matter by 75 % without increasing nitrogen oxides. In this way, studies on the combustion and
emission characteristics according to the application of multi-stage injection in diesel engines are actively being
carried out. In particular, efforts are being made to reduce fuel consumption and exhaust emissions by applying
various pilot injection strategies, such as those involving injection timing and injection amount.
This study seeks the optimal injection strategy, based on the application of split-injection, for the
improvement of fuel efficiency and exhaust emission reduction. Split injection means that the amount of the fuel
5
injected before the main injection is equal to the amount of fuel injected in the main injection. It is different from
pilot injection, in which a small amount of fuel is injected before the main injection. Therefore, the purpose of
this study is to analyze the characteristics of combustion and exhaust emissions according to the application of
various split-injection strategies, such as injection timing, injection interval, and injection pressure, in order to
find the optimal injection strategy to improve fuel efficiency and to reduce exhaust emissions, compared with
single-injection combustion.
2. Experimental apparatus and procedure Com mon-rail
Fuel High Conditioning Pressure System Pum p
ETAS
Fuel Tank
ETK 7.1
INCA
Injector
Exhaust
Intake Air
Soot m eter
MSS
AVL Exhaust gas analyzer (NOx, HC, CO)
Pressure sensor Micro IFEM
Encoder Data acquisition system
Dynam om eter Controller Experim ental engine
AC Dynam om eter
Fig. 1 Schematic diagram of test engine and measurement systems
Figure 1 shows a schematic diagram of the single-cylinder diesel engine used in this study in order to
measure and analyze combustion and emissions characteristics by various split-injection strategies. The
experimental apparatus consists of the single-cylinder diesel engine with common-rail injection, engine control,
fuel supply / injection, intake-air control, and data acquisition systems. The main features of the diesel engine
6
used in this study are natural aspiration, compression ratio of 17.1, bore / stroke size of 85 mm / 90 mm, and displacement volume of 510.7 cm3. The test injector has a spray angle of 142o, a hole diameter of 0.180 mm, and
five-holes. A detailed engine specification was listed in Table 1.
Table 1
Specifications of single-cylinder diesel engine
Item
Specification
Engine type
Direct injection diesel engine
Number of cylinder
1
Bore × Stroke
80.0 mm × 90.0 mm
Displacement volume
510.7 cc
Fuel injection system
Bosch common rail (CP3)
Compression ratio
17.1
Engine management system (EMS)
AVL-RPEMS + ETK 7 (Bosch)
Number of hole
5
Hole diameter
0.180 mm
Spray angle
142o
Flow
375 ml / 30 sec
Injector
A prototype engine-control-unit (ETK 7.1, ETAS) controlled the fuel injection parameters. The injection
variables, such as injection pressure, injection timing, and injection quantity, were controlled by using the INCA
program in which the communication system was established with an ETAS system. The engine speed and
torque were controlled by an AC dynamometer (AMK DW 13-170-4, AVL), and the fuel injection timing was
7
measured in units of 0.1o by the crank angle sensor (3,600 pulse/rev). The pressure in the combustion chamber
was measured using a piezo-electric pressure sensor (SL31D, AVL). The sensor signal was amplified and
converted by an AVL micro IFEM piezo module, and then averaged over 300 cycles to minimize the effect of the
inter-cycle variation. A fuel mass flow meter (AVL, 735S) was used to measure the fuel consumption of the
single-cylinder diesel engine according to the various injection strategies. The fuel temperature of the supplied
fuel was kept constant by using a fuel temperature controller (AVL, 753C). The emission analyzer (HORIBA,
MEXA-554JKNOx) measured the emissions of CO, HC, and NOx. CO and HC emissions were measured with
non-dispersive infrared rays and the NOx emissions were measured by means of a chemiluminescence detector
(CLD). The micro soot sensor (AVL), which is based on the photoacoustic measurement method, was used to
measure and analyze the soot emission. The detailed specifications for the emission analyzer are listed in Table 2. Table 2
Specifications of exhaust emission analyzer
Item
Specification Model
MEXA-554JKNOx (HORIBA)
Principle of
CO, HC: Non-dispersive infrared rays
measurement
NOx: Chemical method (ECS sensor) HC: 0.00 ~ 10,000 ppm
Gaseous emission
Measuring range
CO: 0.00 ~10.00% vol. NOx: 0.00 ~ 5,000 ppm
(NOx, HC, CO)
HC: ± 12 ppm vol. Measuring accuracy
CO: ± 0.06 % vol. NOx: ± 20 ppm vol.
Response
90 % response within 10 second
Model
AVL Micro Soot Sensor (MSS)
Principle of Soot
measurement
Photo-acoustics
Measuring range
0.001 ~ 50 mg/m3
Dilution ratio
Adjustable from 2 ~ 20
8
1μg/m3
Detection limit
Spl it Injection w/ Fixed Injection timing (Pres sure Variations) Interval (8degree)
1st
2nd
7mg (5.25 deg.)
7mg (5.25 deg.)
Inj. Pr. : 500 bar ~ 1200 bar Injection Timing: BTDC 11 o + BTDC 3 o Spl it Injection w/ Fixed 2nd Injection ti ming (Interval Variations) 18degree
Single Injection
14mg
Inj.Pr. : 500 bar TDC ~ BTDC 27 o
8degree
VS
1st 7mg (5.25 deg.)
∙∙∙
1st
2nd
7mg (5.25 deg.)
7mg (5.25 deg.)
2 nd injection timing: BTDC 3 o Interval: 8deg. ~ 18deg. Spl it Injection w/ Fixed Interval (Ti mi ng Variations) Interval (8degree)
1st
2nd
7mg (5.25 deg.)
7mg (5.25 deg.)
2 nd injection: TDC ~ BTDC 18 o
Fig. 2 Injection strategies used in this study
In general, the multi-stage injection can be divided into pilot injection and split injection. Pilot injection
means a method wherein a small amount of fuel is injected before the main injection. Split injection can be
defined as a multi-stage injection method that injects the same amount of fuel in each stage. In this study, the
injection strategy, as shown in Figure 2, was used to analyze the combustion and exhaust emissions
characteristics of the diesel engine according to various split-injection application. Three split-injection strategies were applied. In the case of Figure 2(a), the injection timing and injection interval are fixed as BTDC 11o+BTDC 3o and 8o, respectively, and the injection pressure is changed from 500 bar to 1200 bar. Figure 2(b) shows the
9
case where the injection timing of the first injection is changed by keeping the second injection timing at BTDC 3o, i.e., the interval between two injections is changed from 8o to 18o. The minimum injection interval is fixed at 8o because the energizing duration for injection of 7 mg at the experimental condition (injection pressure 500 bar) is about 7.5o (crank angle, CA). Therefore, the dual injection is not realized at intervals smaller than 8 degree CA.
Figure 2(c) shows the case where the injection timing is changed by keeping the injection interval of 8 degrees.
The experimental results obtained by split injection application were compared to those obtained by single-
injection application. The detailed experimental conditions are shown in Table 3.
Table 3 Experimental conditions
Single injection
Split injection
Injection pressure
500 bar
Injection timing
TDC ~ BTDC 27o (3 degree interval)
Injection quantity
14 mg
Injection pressure
500 bar ~ 1200 bar
Injection timing
BTDC 11o + BTDC 3o
Injection quantity
7 mg + 7 mg
Injection pressure
500 bar
(Fig. 2(a))
Split injection
Injection timing
1st injection: variable 2nd injection: BTDC 3o
(Fig. 2(b)) Injection quantity
7 mg + 7 mg
Injection interval
8 degree ~ 18 degree
Injection pressure
500 bar
Split injection
Injection timing
2nd injection: TDC ~ BTDC 18o
(Fig. 2(c))
Injection quantity
7 mg + 7 mg
Injection interval
8 degree
10
3. Results and discussion 3.1 Combustion and emission characteristics with single-injection strategy
In-Cylinder Pressure [bar]
100 Single Injection Strategy Pinj=500bar, mfuel=14mg
80
60 BTDC 27 o
40
TDC
20
0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (a) Combustion pressure curves
Rate of heat release [J/deg]
300 Single Injection Strategy Pinj=500bar, mfuel=14mg
250 BTDC 27 o
200
TDC
150 100 50 0 -50 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (b) ROHR Fig. 3 Combustion pressure curves and ROHR characteristics of single injection combustion (Pinj=500bar, mfuel=14mg, Sengine=1600 rpm, Injection timing: TDC ~ BTDC 27o, 3 deg. Interval)
11
The combustion and exhaust emissions characteristics were investigated according to the application of
single-injection strategy before testing various split-injection strategies. Figure 3 shows the characteristics of in-
cylinder combustion pressure and rate of heat release (ROHR) according to the change of single injection timing. The injection timing advanced from TDC to BTDC 27o at intervals of 3o. The ROHR was calculated by the first
law of thermodynamics and the state equation of ideal-gas based on the measurement of combustion pressure. As
shown in Figure 3(a), as the injection timing was advanced, the ignition timing is advanced and the maximum
combustion pressure was increased. It can be considered that ignition timing and ignition delay have influence,
as stated in existing literature [17-19]. When the ignition occurred before TDC, the advance of injection timing
caused increase of ignition delay. Then better air-fuel mixture is formed, which results in better combustion and higher in-cylinder pressure. When the injection timing was advanced more than BTDC 12o, it can be observed
that ignition was started before TDC. On the other hand, Figure 3(b) shows the ROHR according to the injection
timing. The peak ROHR decreased by advancing injection timing, but it started to increase again from the injection timing of BTDC 12o. This ROHR characteristic is also closely related to the ignition timing and
ignition delay. Hence, it is considered that the increase after decrease of ignition delay as the injection timing
advances affected the decrease and increase of the maximum ROHR. The quantitative analysis related to this is
presented in detail in Figures 4-6.
Figure 4 shows the mean effective pressures (MEPs), i.e., indicated mean effective pressure (IMEP) and
brake mean effective pressure (BMEP), and the brake specific fuel consumption (BSFC) characteristics
according to the ignition timing. In this study, the ignition delay is defined as the period between ignition timing
12
(CA10) and injection timing. CA10 was calculated from the amount of heat release, and it means the point where
the amount of heat corresponds to 10% of total heat generation. As shown in the Figure 4, the MEPs tended to
increase as the ignition timing approached to the top dead center (TDC), and it was confirmed that the MEPs
decreased as the ignition timing advanced to before the TDC. The BSFC appeared to be contrary to the trend of
MEPs. This occurs because, when ignition occurs near the TDC, the negative work, which means minus work
owing to the ignition before TDC, is minimized [20, 21] and the explosive force by combustion is delivered to
the piston in the most efficient way, resulting in the highest MEPs; thus, the consumption of fuel is also
minimized. Therefore, the relationships between ignition timing and BSFC and between ignition timing and
MEPs show opposite correlation based on TDC. 260 Single Injection Strategy Pinj=500bar, mfuel=14mg
Indicated MEP
7.0
240
6.0 Brake MEP
5.0
220
4.0 200
BSFC
BSFC [g/kW-h]
Mean effective pressure [bar]
8.0
3.0 2.0 -15
180
-10
-5
0
5
10
15
20
Ignition timing [deg.ATDC] Fig. 4 Mean effective pressures (MEP) and BSFC characteristics of single injection combustion
Figure 5 shows various combustion characteristics, such as the maximum pressure (P max), maximum rate of
heat release (ROHRmax), ignition delay, and combustion duration, for the injection timing. As shown in Figure 3,
Pmax increased with the advance of injection timing, and ROHRmax increased after decreasing. This is closely
13
related to the increase in ignition delay after advancing the injection timing. If the ignition delay is long, the
mixture of fuel and air is more uniformly mixed and the instantaneous ROHR is increased when the combustion is performed. Therefore, in the injection timing range (TDC ~ BTDC 12o) ignited after the TDC, the ROHR is
also decreased due to the reduction of the ignition delay according to the advance of the injection timing. On the other hand, in the region where the ignition started before TDC (before the injection timing of BTDC 12o), the
ignition delay gradually increased according to the advance of the injection timing, and the ROHR max was also
greatly increased. The overall combustion period (CA10-CA90) was also similar to the tendency of the ignition
delay.
P
[bar]
90
Single Injection ROHRmax Combustion duration
ma x 100
Ignition delay [deg.] 20 Ignition delay Pmax
ROHR [kJ/deg]
250
18
50
45
80 40
200
16
70
35
150
60
14 30
50
100
12 40
50
30 -30
10 -25
-20
-15
-10
-5
25
Combustion duration [degree]
300
20
0
Injection Timing [deg.ATDC] Fig. 5 Analytical combustion characteristics (maximum combustion pressure, maximum ROHR, ignition delay, and combustion duration)
14
10
CO [%]
HC [ppm]
0.10
40
0.09
8
35
0.08
2000 1800 1600
30
6
25
0.06 0.05
4
1400
20
CO HC
0.04
15
0.03
1200 1000 800
NOx [ppm]
Soot [mg/m3]
0.07
600 10
2
0.02
400
soot
NOx
0.01
0
0.00 -30
5 0
-25
-20
-15
-10
-5
200 0
0
Injection Timing [deg.ATDC] Fig. 6 Exhaust emission characteristics (NOx, soot, HC, and CO)
Figure 6 shows the exhaust emission characteristics according to the change of injection timing in a single-
injection combustion. The NOx tended to increase with the advance of injection timing, and HC and CO emissions were the lowest at the injection timing of BTDC 15o and then increased again. The injection timing condition of BTDC 15o, which showed the highest IMEP and BMEP results, is considered the most favorable
combustion condition in this test engine (see Figures 3 and 4). Soot emissions increased after decreasing to BTDC 25o. A clear trade-off relationship between NOx and soot was confirmed. In addition, it was confirmed
that extremely early injection and TDC resulted in abrupt increase of HC due to combustion instability. After
confirming these basic emission characteristics of single-injection combustion, we compared the results with the
exhaust emissions from split-injection combustion.
3.2 Combustion and emission characteristics with split-injection strategy
15
In this study, we investigated the effects of three kinds of split-injection, namely, change of injection
interval, change of injection timing, and change of injection pressure, on the combustion and exhaust emission
characteristics of a single-cylinder diesel engine.
Figures 7-10 show the effect of injection pressure on the combustion and emission characteristics of splitinjection combustion. In this experiment, the injection interval was fixed to 8 o and the injection timing was set as BTDC 11o + BTDC 3o. The injection pressure was changed from 500 bar to 1200 bar in 100 bar increments. 350
9
Split Injection Strategy Pinj=500bar, mfuel=7+7mg
Indicated MEP
300
8 7 6
250 Brake MEP
200
5 4
BSFC [g/kW-h]
Mean effective pressure [bar]
10
150 BSFC
3 2
100
500
600
700
800
900
1000
1100
1200
Ignition timing [deg.ATDC] Fig. 7 Mean effective pressures and BSFC of split injection combustion at various injection pressures
Figure 7 shows the IMEP and BMEP, and the BSFC characteristics with varying injection pressures. As
shown in the figure, both MEPs decreased with the increase of injection pressure. The BSFC is increased up to
injection pressure of 800 bar, then decreased and increased repeatedly, and converged to about 250 g/kWh. The
BMEP converged to about 4 bar after an injection pressure of 800 bar. In addition, it was confirmed that the
FMEP, which is the difference between BMEP and IMEP, is increased by the increase in injection pressure.
Generally, it is known that when the injection pressure is increased, the atomization of the fuel is improved and a
16
uniform mixture is formed, thereby improving the combustion performance [22-23]. However, in this case of
split injection, it is difficult to expect combustion performance improvement simply by increasing the injection
pressure. This is because if the injection timing is fixed and the injection pressure is increased under the same
injection amount condition, the energizing duration of the electric current in each injection is shortened and the
intervals (dwell duration) between injections are distant from each other. The increase in the injection interval
causes the deterioration of combustion performance [24] (see Figure 11). In addition, the improvement of
atomization by the increase of injection pressure causes the spray droplet to evaporate rapidly, and the
combustion is quickly terminated because of the increase in burning speed and flame speed [25]. This leads to
low combustion pressure characteristics in the expansion stroke, resulting in a decrease in MEP owing to a
reduction in the burning work. A slight increase in the negative work caused by the advance in ignition timing (ATDC 1.8o → BTDC 1.6o) owing to the increase in injection pressure causes a decrease in the MEP.
Figure 8 shows the in-cylinder combustion pressure profiles and ROHR curves for the increase in injection
pressure. Figure 8(a) shows the advance of the ignition timing and the reduction in combustion pressure in the
expansion stroke with the increase in injection pressure. In addition, it can be seen that the maximum combustion
pressure increases because of the increase in injection pressure, which can attributed to the active combustion
reactions caused by the improved atomization characteristics. Figure 8(b) shows the ROHR characteristics with
increasing injection pressure. As can be seen in the figure, the maximum ROHR approximately doubled,
increasing from 93.8 J/degree to 184.9 J/degree, and the ignition timing advanced when the injection pressure
was increased. Although the first ROHR peak is increased with increasing injection pressure, the second peak is
17
maintained, and two peaks are clearly observed. It is confirmed that the maximum ROHR occurrence point is
slightly advanced, but it is almost the same. In Figure 9, it is possible to observe the reduction in energizing
duration and the increase in injection interval with increasing injection pressure. Figure 9 shows that the
energizing duration must be shortened for injecting the same amount at high injection pressure. It was also confirmed that the injection interval, which is the duration between the end of 1 st injection and the start of 2nd
injection, was increased because of the reduction in energizing duration. In addition, at the injection pressure of
800 bar or higher, it was confirmed that the duration of the electric current was gradually decreased and the
injection interval was gradually increased.
In-Cylinder Pressure [bar]
100 Split Injection Strategy Injection timing: BTDC 11o+BTDC 3o mfuel=7+7mg
80
500 bar 900 bar
700 bar 1100 bar
60
40
20
0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (a) Combustion pressure curves
18
250 1100 ba r
200 150
Max: 184.9 J/deg. @ BTDC 1 o
100 50 0
Rate of heat release [J/deg.]
-50 900 ba r
Max: 162.4 J/deg. @ BTDC 1 o
250 700 ba r
200 150 Max: 180.5 J/deg. @ TDC
100 50 0 -50
500 ba r
Max: 93.8 J/deg. @ ATDC 1 o
-20
0
20
40
60
80
Crank angle [deg.ATDC] (b) ROHR Fig. 8 Combustion pressure curves and ROHR characteristics of split injection combustion at various injection pressures (500bar ~ 1100bar) 8 mfuel=7+7mg, Sengine=1600rpm
5.5 deg.
*This is for one injection. *Single injection: 772s
550
Injection interval Energizing duration
500
6
4.3 deg.
450
4
400
3.0 deg.
350
2
3.5 deg.
Injection interval [deg.]
Energizing duration [s]
600
300 0 500
600
700
800
900
1000
1100
1200
Injection pressure [bar] Fig. 9 Energizing duration and interval between two injections* at various injection pressures (*The interval in this graph means the duration between the end of 1st injection and the start of 2nd injection.)
19
40
CO [%]
HC [ppm]
0.8
40
soot
35
35
2000 1800 1600
0.6
30
CO
20
1400
NOx
25
25 0.4
20
15
15
1200 1000 800
NOx [ppm]
Soot [mg/m3]
30
600
10
0.2
10 400
5 0
5
HC 0.0
0 500
600
700
800
900
1000
1100
200 0
1200
Injection Pressure [bar]
Fig. 10 Exhaust emission characteristics for increasing injection pressure in a split-injection combustion (injection interval: 8 degrees, injection timing: BTDC 11o+BTDC 3o)
Figure 10 shows the exhaust emission characteristics according to the variation of injection pressure when the injection interval is maintained at 8o and the injection timing is BTDC 11o + BTDC 3o. As the injection
pressure increased, soot and CO emissions decreased significantly. It can be judged that the fraction of
incomplete combustion decreased because combustion was actively occurring owing to the improvement of the
atomization performance with the increase of injection pressure. The NOx tended to increase with the active
combustion characteristics. It can be considered that the improvement in combustion performance by increasing
injection pressure induced the increase in the thermal NOx owing to high combustion temperature. The high
combustion temperature is directly related to the decrease in CO (high conversion from CO to CO2). HC showed a slight tendency to increase (2.1 ppm → 7.1 ppm) but it was negligible.
20
BSFC [g/kW-h]
270
11
Split Injection Strategy (Fixed 2nd Injection, BTDC 3 deg.) Pinj=500bar, Sengine=1600 rpm, mfuel=7+7mg BSFC
10
IMEP
240
9
210
8
180
197.6 g/kW-h Lowest BSFC in Single Injection
7
150
IMEP [bar]
300
6
120 5
8
10
12
14
16
18
Interval between injections [degree] Fig. 11 BSFC and IMEP of split injection combustion with fixed 2nd injection timing (Pinj=500bar, mfuel=7+7mg, 2nd Injection timings: BTDC 3o)
Figure 11 shows the effect of the injection interval change on the BSFC and IMEP with a fixed second injection timing at BTDC 3o. As shown in the figure, as the injection interval increased, BSFC increased and
IMEP decreased. When the second injection timing was fixed, the increase in the injection interval means that
the first injection gradually advances. This means that the ignition timing of the fuel / air mixture is advanced
and the combustion phase is advanced. As a result, the amount of combustion ignited prior to TDC increases, and
the IMEP decreases as the negative work increases. The decrease in IMEP leads to an increase in BSFC. In addition, as shown in the figure, when the injection interval is larger than 14 o, the BSFC in a split injection is increased more than the minimum BSFC of the single injection; thus, an injection interval of 14o or less in the
split injection is advantageous.
21
100
In-Cylinder Pressure [bar]
Split Injection Strategy (Fixed 2nd Injection, BTDC 3 deg.) Pinj=500bar, mfuel=7+7mg Inteval Varations (8 deg.
80
18 deg.)
60 Increase of injection interval
40 CA50 Advance with increase of interval
20
0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC]
Ignition timing (CA10) [deg.ATDC] Ignition delay [degree]
(a) In-cylinder pressure curves 15 Split Injection Strategy (fixed 2nd injection timing) Pinj=500bar, mfuel=7+7mg
10
Ignition delay
5 0
Ignition before TDC
Ignition timing
-5 -10 -15 8
10
12
14
16
18
Injection interval [degree] (b) Ignition timing and ignition delay Fig.12 Combustion characteristics (combustion pressure curves, ignition timing, and ignition delay) of split injection combustion with variable injection interval (8 deg. to 18 deg.)
Figure 12 shows the characteristics of the combustion pressure profile, ignition timing (CA10), and ignition
delay according to the injection interval. As described above, it can be seen that the ignition timing and CA50
(the crank angle at 50% of the cumulative heat release) advance because of the increase in injection interval. In
22
Figure 12(a), it can be seen that as the injection interval increases, not only the advance of the ignition timing but
also the combustion end are accelerated; the combustion chamber pressure is lower in the expansion stroke.
These characteristics (increased negative work owing to advancement of ignition timing and lower combustion
chamber pressure in expansion stroke) cause a decrease in IMEP and an increase in BSFC. Figure 12(b) shows
that the increase in the injection interval affects the advancement of ignition timing and the decrease / increase of
ignition delay. Especially, it was confirmed that ignition progresses before the TDC when the injection interval increased. However, it was confirmed that the ignition delay was longer when the injection interval was 12o or
more. This is because the temperature and pressure in the combustion chamber are low owing to the advance of
the first injection timing by the increase in the injection interval, and the sufficient energy for ignition is not
supplied. The increase of the ignition delay provides a condition in which the mixture of air and fuel becomes
uniform, thereby reducing the locally fuel-rich region in the combustion chamber, which is considered to have
the effect of suppressing the increase in nitrogen oxides, as shown in Figure 16(b). 200
10.5 Split Injection Strategy (Fixed Interval 8 deg.) Pinj=500bar, Sengine=1600 rpm, mfuel=7+7mg BSFC
10.0
IMEP
9.5 9.0
160
8.5
140
IMEP [bar]
BSFC [g/kW-h]
180
8.0 7.5
120
7.0
-21
-18
-15
-12
-9
-6
-3
0
3
2nd Injection Timing [deg.ATDC] Fig. 13 BSFC and IMEP characteristics of split injection combustion with fixed interval (8 degrees)
23
In-Cylinder Pressure [bar]
120 Split Injection Strategy (fixed interval, 8 degrees) Pinj=500bar, mfuel=7+7mg
100
55
80
50
45
60
40
35 21
24
27
30
33
Crank angle [deg.ATDC]
40 20 0 -40
-30
-20
-10
0
10
20
30
40
50
60
Crank angle [deg.ATDC] (a) Combustion pressure Split Injection Strategy (fixed interval) Pinj=500bar, mfuel=7+7mg
200 150 100 50 0 -50
BTDC 20 o + BTDC 12 o
BTDC 17 o + BTDC 9 o
200 150 100 50 0 -50
Start of combustion during 2nd injection
Rate of heat release [J/deg.]
BTDC 23 o + BTDC 15 o
Start of combustion after end of 2nd injection
200 150 100 50 0 -50
BTDC 14 o + BTDC 6 o
BTDC 11 o + BTDC 3 o
200 150 100 50 0 -50
BTDC 8 o + TDC
-20
0
20
40
60
80
Crank angle [deg.ATDC]
(b) ROHR Fig. 14 Combustion pressure and ROHR characteristics of split injection combustion with fixed interval
24
Ignition timing (CA10) [deg.ATDC] Ignition delay [degree]
15 Split Injection Strategy (fixed interval) Pinj=500bar, mfuel=7+7mg
10
Ignition delay
5 0 -5
Ignition before TDC
Ignition timing
-10 -15 -18
-15
-12
-9
-6
-3
0
Injection timing [deg.ATDC] Fig. 15 Ignition timing and ignition delay of split injection combustion with fixed interval
Figures 13-15 show the combustion characteristics of a diesel engine when the injection timing was changed while keeping the injection interval (8o). The reason for setting the injection interval of 8 o is that the
best BSFC characteristics are shown in the above results. As shown in Figure 13, as the injection timing is
advanced, IMEP decreases and BSFC increases. In addition, the IMEP was maintained while the injection timing was advanced from TDC to BTDC 9o, and the IMEP was significantly decreased when the injection timing was advanced more than BTDC 12o. A detailed analysis and discussion of these results is presented in Figures 14 and
15.
Figure 14 shows the combustion pressure profile and ROHR curves according to the injection timing. As
the injection timing is advanced, the maximum combustion pressure increases and the ignition timing advances
gradually. In addition, the maximum ROHR was increased with the advance of injection timing. As shown in Figure 14(b), two distinct ROHR curves were observed by split injection at the injection timing of BTDC 8 o +
TDC. However, as the injection timing was advanced, the peak of the first ROHR curve was increased and the
25
peak of the second ROHR curve was lowered. This is because the ignition delay becomes longer as the injection
timing advances, the premixing period becomes longer, and the mixed air and fuel in the combustion chamber
are burned at almost the same time.
Figure 15 shows the ignition timing and ignition delay characteristics. The ignition delay was defined as the
interval from the second injection timing to CA10 in a split injection case. The ignition timing advances
according to the advance of the injection timing, and it is confirmed that ignition timing is earlier than TDC when the injection timing is advanced to BTDC 6o. On the other hand, in the analysis of the ignition delay, when the injection timing is advanced more than BTDC 12o, the combustion is started through the premixing period
after the end of the second injection. However, in the other cases, there is no sufficient premixing process
because the combustion was started immediately after the end of second injection or during the injection process.
Hence, it can be considered that these characteristics have influenced the ROHR characteristics in Figure 14.
NOx
HC
CO
Soot
(a) (b)
(a) Emission characteristics in injection timing-interval map
26
40
(b) CO [%]
HC [ppm]
0.8
40
soot
35
35
2000
1.2
0.4
20
15
15
1200 1000 800
Soot [mg/m3]
25
NOx
HC [ppm] 20
NOx
1000
800
60
0.9
40
0.6
HC 10
0.2
10 400
5 0
HC
5
0.0
0 8
10
12
14
16
Interval between Injections [degree]
18
600
400
600
10
1200
15
CO
1400
25 20
80
30
CO
NOx [ppm]
Soot [mg/m3]
0.6
1.5
1800 1600
30
100
NOx [ppm]
(a) CO [%]
20
0.3
0
0.0
5
soot
200
200 0
0 -20
-15
-10
-5
0
0
Injection Timing [deg.ATDC]
(b) Emission characteristics in a line (a) and (b) Fig. 16 Exhaust emissions (soot, NOx, HC, CO) characteristics in split-injection strategies (injection timing variation and interval variation)
Figure 16 shows the characteristics of exhaust emissions with varying injection intervals and injection
timing. As shown in Figure 16(a), as the injection interval gets better and the injection timing approach TDC,
NOx and HC emissions are reduced. Regarding CO and soot, both of them showed low emission values except the interval of 8o. Figure 16(b) shows the exhaust emissions results at positions (a) and (b). As mentioned above,
the amount of nitrogen oxides increased with the injection interval, and the injection timing decreased with TDC.
The increase in injection interval caused a significant decrease in soot and CO and an increase in HC. On the
other hand, as the injection timing approaches TDC, CO and HC including NOx are decreased, and soot is
increased.
27
350
Pinj=500bar, mfuel=14mg (7+7mg), Sengine=1600 rpm Single Injection (s,inj=BTDC 12 deg.)
Ignition before 2 nd injection
Split Injection (1st=BTDC 11 deg., 2nd=BTDC 3 deg.) Injection pressure variations (500bar ~ 1200bar) Interval variations (8deg. ~ 18deg.) Injection timing variations (TDC ~ BTDC 18) with fixed interval
BSFC [g/kW-h]
300
250 Increase of Injection pressure
Single injection (197.6 g/kW-h)
200
150
Increase of Interval between injections
Advance of injection timing
Split injection (126.3 g/kW-h)
100 -6
-4
-2
0
2
4
6
8
10
12
14
16
Ignition delay [degree] Fig. 17 BSFC characteristics in single- and split-injection strategies
Figure 17 shows the BSFC characteristics for the ignition delay when applying the three kinds of split
injection strategies above discussed. The results of single injection are also shown for comparison with the split
injection results. As shown in the figure, when single injection was changed to split injection (injection interval 8o, injection timing: BTDC 11o+BTDC3o), ignition delay was shortened and BSFC was also improved. At this
time, when the injection interval was increased, the ignition delay was decreased, the fuel efficiency
characteristics were deteriorated (red square symbol). When the injection interval was kept at a minimum, the
ignition delay was increased when the injection timing is changed (green triangle symbol). In addition, it was
confirmed that the ignition delay was slightly reduced, and the fuel efficiency was worse when the injection pressure was increased by maintaining the injection timing (BTDC 11o + BTDC 3o) and injection interval (8o).
Based on these experimental results, the minimum fuel efficiency characteristics in this study were obtained
when the injection interval was kept to a minimum and the injection timing was brought near TDC. On the other
28
hand, in this experiment, it was confirmed that the fuel efficiency was good when the injection pressure was low
(500 bar). This is because the injection pressure was increased without adjusting the injection interval.
4. Conclusions In this study, the three kinds of split injection strategies (injection pressure, injection interval, injection
timing) were applied to investigate the combustion and exhaust emissions characteristics in a single-cylinder
diesel engine. The main findings from the experiments are summarized as follows.
1.
When the injection pressure was increased without changing the injection timing in a split injection, the
IMEP and BMEP decreased and the BSFC deteriorated owing to the increase in dwell duration. The
BSFC converged beyond the injection pressure of 800 bar. The increase in injection pressure induced
the significant reduction of soot and CO, while NOx increased.
2.
The increase in injection interval caused a deterioration of the BSFC with decrease of the IMEP. The
retardation of injection timing with fixed injection interval conditions caused the improvement of the
BSFC.
3.
The increase in injection interval caused the increase in NOx and HC emissions, while it caused
significant reduction in CO and soot emissions. In addition, the retardation of injection timing with a
fixed injection interval decreased of NOx, CO, and HC emissions, while soot emission was increased.
29
4.
From the analysis of the experimental results, it can be concluded that a short injection interval,
injection timing around TDC, and not too high injection pressure conditions are needed for the
improvement of fuel consumption and emission characteristics in a split-injection diesel combustion.
Acknowledgement This study was supported by Basic Science Research Program (2016R1D1A3B03935537) and Basic Research Laboratory Program (2015R1A4A1041746) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.
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