1
Ultrafine Particle emission characteristics of diesel engine by on-board and test bench
2
measurement
3
4
HUANG Cheng1,2, LOU Diming1,*, HU Zhiyuan1, TAN Piqiang1, YAO Di1, HU Wei1, LI
5
Peng1, REN Jin1, Chen Changhong2
6
1. School of Automobile Studies, Tongji University, Shanghai 201804, China. E-mail:
7
[email protected]
8
2. Shanghai Academy of Environmental Sciences, Shanghai 200233, China
9
10
Abstract: This study investigated the emission characteristics of ultrafine particles
11
respectively based on the test bench and on-board measurement. The bench test results show
12
the ultrafine particle number concentration of the diesel engine is in the range of
13
(0.56--8.35)×108 cm-3. The on-board measurement results illustrate the ultrafine particles are
14
strongly correlated with the change of real-world driving cycles. The particle number
15
concentration would be low to 2.0×106 cm-3 and 2.7×107 cm-3 under decelerating and idling
16
operations and high to 5.0×108 cm-3 under accelerating operation. It is also indicated that the
17
particle number measured by two methods both increases with the growth of engine load at
18
each engine speed. The particle number presents “U” shape distribution with the change of
19
speed at high engine load situation, which implies the particle number will reach a low level
20
at medium engine speed. The particle sizes of both measurements all show single mode
21
distributions. The peak of particle size is located at about 50--80 nm in the accumulation
22
mode particle range. Nucleation mode particles will significantly increase caused by high
23
concentration of unburned organic compounds at low engine load operations like idling and
24
decelerating.
25
Key words: ultrafine particle emission; emission factor; diesel engine; on-board emission
26
measurement; TSI EEPS
27
28
29
-------------------------------* Corresponding author. E-mail: [email protected].
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1
1
Introduction
2
Diesel vehicle have greatly developed in the past decade since its advantage of energy
3
saving and low carbon potential. However, particle emission from diesel engine have been
4
the focus of debate due to its environmental impact (Lloyd and Cackette, 2001; Robinson et
5
al., 2007) and adverse health effects (Reed et al., 2004; Nel et al., 2005; Pope et al., 2006).
6
Therefore, particle emission characteristics, especially for ultrafine particle emission, have
7
been paid more and more attention in these years. Particle emissions in diesel engine exhaust
8
have been proved with ultrafine particles smaller than 0.1µm mainly. The particle size
9
distributions are often measured to be bimodal in the previous study, which are composed of
10
nucleation and accumulation modes (Kittelson, 1998). Based on some test bench studies,
11
particles in nucleation mode are usually smaller than 50nm in diameter and in liquid form
12
(Shi et al., 1999; Shi et al., 2000; Harris and Maricq, 2001). Nucleation mode particles
13
formed by the nucleation of semivolatile organic and sulfur compounds during the dilution
14
and cooling processes (Wong et al., 2003; Giechaskiel et al., 2005; Rönkkö et al., 2006).
15
Sakurai et al. (2003) furtherly reported the composition of organic compounds and sulfur acid
16
in the nanoparticles and indicated that sulfur contents in fuel and lube oil play important roles
17
in the formation of nanoparticles, which has been confirmed by other researches (Maricq et
18
al., 2002; Lehmann et al., 2003). Accumulation mode particles in diameter range of 50-100
19
nm consist of solid carbonaceous agglomerates with adsorbed and condensed semivolatile
20
species.
21
Test bench is the major method for emission measurement since its accuracy and
22
repeatability. However, real-world emission test based on on-board emission measurement
23
system was gradually applied as an important supplement of the test bench, especially on
24
heavy duty vehicle emission measurement, in recent years (Chen et al., 2007; Durbin et al.,
25
2007; Zhang and Frey, 2008). Some studies have shown that the bench test cycle has big
26
difference compared with real-world driving condition (Bergmann et al., 2009; Liu et al.,
27
2011).
28
To control the particle emissions from diesel vehicles, more stringent emission standards
29
on diesel engine are gradually implemented in most of the countries. With the implementation
30
of Euro Ⅴ and Euro Ⅵ standards, ultrafine particle number concentration measurement is
2
1
currently of great concern. The emission characteristics of ultrafine particles from in-use
2
diesel vehicles still remain unclear in real-world situation, especially for the nanoparticles.
3
For this purpose, this study conducted particle number emission measurements by both
4
portable emission measurement system and test bench in Shanghai, China. Ultrafine particle
5
emission characteristics of diesel engine in test cycle and real-world driving conditions will
6
be discussed in this paper.
7
8
1 Materials and methods
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1.1 Tested engine and vehicle
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The test engine is a common rail direct-injection inline 4 cylinder diesel engine
11
turbocharged intercooler, equal to Euro Ⅲ emission standard. The test bus is an in-use
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vehicle which meets the requirement of Euro Ⅳ emission standard. The bus is randomly
13
selected from the bus fleet in Shanghai. The accumulative mileage reached 53550 km. Table
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1 shows the technical specifications of the tested engine and bus.
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Table 1 Technical specifications of the test engine and vehicle
Parameters
Engine
In-use bus
Engine Type
common rail direct-injection inline 4
common rail direct-injection inline 4
cylinder diesel engine turbocharged
cylinder diesel engine turbocharged
intercooler
intercooler
Displacement (L)
5.308
7.1
Maximum Power at rpm
132 kW at 2300 rpm
213 kW at 1800-2100 rpm
Maximum torque at rpm
660 N∙m at 1400-1600 rpm
1200 N∙m at 1050-1650 rpm
Emission Level
Euro Ⅲ
Euro Ⅳ
After treatments
None
Selective Catalytic Reduction (SCR)
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Diesel fuel used in the test was directly from market. Fuel quality meets the requirement
17
of the local standard equal to Euro Ⅳ. The sulfur content should be controlled below 50 ppm.
18
19
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1.2 Test cycles
The test cycle for the engine was operated on the external characteristic curve. The
3
1
speeds were in the range from idle speed (700 r·min-1) to the rated speed (2300 r min-1). Each
2
200 r·min-1 set one test point. At maximum torque speed (1500 r min-1) and rated speed (2300
3
r min-1), the engine loads of 10%, 25%, 50% and 75% were tested, respectively. Each test
4
point was operated in 1 min to measure the ultrafine particle number concentration and size
5
distribution.
6
The driving route for on-board measurement was designed in the real traffic covering the
7
elevated roads, arterial roads, and residential roads. Total distance of the route is
8
approximately 22 km. The length of elevated roads, arterial roads and residential roads takes
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up 26%, 38%, and 35%, respectively. Tested bus run 2 times a day, once in the peak hour
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during 7:00 to 9:00, another in the non-peak hour at noon.
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Based on the real-world driving cycle data of on-board measurement system, we
12
simulate the engine map of the test bus. The methodology of engine operation simulation is
13
referenced from the vehicle dynamics analysis, including rolling resistance, grade resistance,
14
aerodynamic resistance, and acceleration resistance, as shown in the following equation.
15
16
Pt  Ft  v  Ff  Fw  Fi  Fj   v


CD  A  air  v 2
 Mg  f 
 Mg  sin(  )    M  a   v
2


(1)
17
Where, Pt represents for vehicle traction power (kW); Ft for the traction of a vehicle (N); M is
18
the total mass of the bus (kg); V (m·s-1) for speed; a for acceleration (m·s-2); G is the
19
acceleration of gravity (9.81 m·s-2); A is the frontal area of bus (m2); ρair represents for air
20
density, 1.207 kg·m-3 (20 ℃); α for road slope angle (this study assume the flat topography is
21
0°); CD is air resistance coefficient, heavy duty bus generally takes 0.65; f is vehicle rolling
22
resistance coefficient, which can be calculated by vehicle speed. The engine speed and torque
23
also can be simulated according to the transmission ratio of the bus. See the detailed
24
calculation method from reference of Huang et al. (2010). Fig. 2 shows the comparison of
25
engine performance between the test bench and on-board measurement studies. It is indicated
26
from Fig. 2 that the engine performance of real-world driving condition has large difference
27
compared with the bench test cycle. The engine was mainly operated on medium and low
28
engine speed and load in real-world driving condition, as discussed by Liu et al. (2011).
4
1
2
Fig. 1 comparison of the engine performance of test bench (a) and on-board measurement (b).
3
4
1.3 Emission measurement system
5
With the growing attentions on the particle number and size distribution from diesel
6
engine, kinds of real time particle measurement devices were employed on both test bench
7
and on-board measurement (Vogt et al., 2003; Durbin et al., 2007; Liu et al., 2009; Johnson
8
et al., 2011). In this study, a fast scanning particle size spectrometer (EEPS 3090, TSI Inc.)
9
was used during the test. The EEPS system has been successfully applied in the previous
10
on-board measurement studies (Bergmann et al., 2009; Merkisz et al., 2009). In order to
11
simulate the actual dilution process of engine exhaust in real-world situation, a rotaring disc
12
and thermo-diluter was installed before the EEPS system to remove the large particles above
13
the instrument’s size range and diluted the sample flow to 500 times. The EEPS spectrometer
14
counts particles in the range from 5.6 to 560nm in 10Hz data collection frequency and
15
operates over a wide particle concentration range down to 200 particles·cm3 and at ambient
16
pressure to prevent evaporation of volatile and semi-volatile particles (TSI, 2004). The
17
equipment was installed around some buffers to reduce the vibration during the real-world
18
test.
19
The on-board measurement system was installed in the bus and warmed up before each
20
test. A barometer was also mounted to measure ambient humidity and temperature. Vehicle
21
speed, altitude, latitude, and longitude were logged continuously by a GPS device. Water tank
22
was loaded to simulate the heavy load condition. Total weight of the devices, batteries, and
23
water tanks accounted approximately for 60% of the GVWR of the bus. The condensation of
24
water and high molecular weight hydrocarbon was prevented by heating the sampling tube up
25
to190 ℃ during the test. Fig. 2 shows the schematic diagram of the installation of on-board
26
measurement system.
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28
Fig. 2 Schematic diagram of the installation of on-board measurement system.
29
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2 Results and discussion
31
2.1 Ultrafine particle emission characteristics of diesel engine by bench test
5
1
Fig. 3(a) and (d) show the ultrafine particle number and size distribution of each speed on
2
the external characteristic curve. The particle number concentration under full load condition
3
is in the range of (2.00--8.35)×108 cm-3 and show “U” shape distribution. The particle number
4
concentration under low speed (700--1100 r·min-1) and maximum power speed (2300 r·min-1)
5
achieve the highest, while the particle number under medium speed (1500--1900 r·min-1)
6
reach the lowest. Particle size under each speed appears single mode distribution. The peak
7
size is about 50 nm in the accumulation particle range. Fig. 3(b) and (e) indicate the particle
8
number and size distribution of different loads under the speed of 1500 r·min-1. Particle
9
number concentration is in the range of (0.56--3.07)×108 cm-3. Except for 10% load, the
10
particle number under other loads is in the same emission level. The particle size under the
11
loads from 25% to 100% is also in single mode distribution. For 10% load, particle number
12
shows bimodal distribution. The ratio of nucleation mode particle increases compared with
13
other engine loads. The particle number under max. power speed (2300 r·min-1) generally
14
shows growth trend with the increase of engine load. But the particle size distribution under
15
low load (≤50%) condition has significant difference with high load condition. Nucleation
16
mode particles increase to 3.0×108 cm-3 at 10 nm diameter.
17
Generally, ultrafine particle emission under medium speed is relatively low. Lower or
18
higher speed operations will promote the formation of ultrafine particles. Particle number
19
concentration shows growth trend with load. In addition, the nucleation mode particles tend
20
to occur under low engine load. Nucleation mode particles are formed by the carbon particle
21
and the nucleation of sulfuric acid or other gaseous precursors such as HC. Under low load
22
condition, HC and other gaseous precursor are relatively high to promote the generation of
23
nucleation mode particles. Especially for high speed situation, high concentration of carbon
24
particles and unburned gaseous emissions will contribute to nucleation of nano-particles.
25
26
Fig. 3 particle number (a, b, c) and size distributions (d, e, f) under different test cycle point by bench test.
27
28
2.2 Instantaneous ultrafine particle emission change under on-road driving condition
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The ultrafine particle number emission factor of the whole real-world test cycle is about
30
7.6×1014 km-1. The emission factor of ultrafine particle mass is 0.42 g·km-1 based on the
6
1
conversion of particle number in each size by assuming the density of 1 g·cm-3. In this study,
2
we mainly focus on the emission characteristics of ultrafine particle number on the real-world
3
driving cycles. Fig. 4 shows a whole driving cycle fragment of test bus in real-world driving
4
condition. Its engine power, nucleation and accumulation mode particle number, and size
5
distribution are also described in the figure. Compared with the bench test, the particle
6
emission under real-world driving condition is more complex. The real time on-board
7
measurement instrument could accurately capture the particulate emission characteristics with
8
the change of driving cycles. Particle number concentration during idling operation kept on
9
2.7×107 cm-3, and the particle size is mainly concentrated in the accumulation mode. During
10
acceleration operation, the engine power sharply increased, and particle number
11
concentration simultaneously rised to 5.0×108 cm-3 or higher level. Acceleration operation
12
increased the fuel injection and decreased the air fuel ratio. High temperature and hypoxic
13
condition produced higher amount of nanoparticles and also enhance their chance of collision
14
and conglomeration to form more accumulation mode particles. In the deceleration processes,
15
the engine operated at low load situation and the particle number rapidly decreased to about
16
2.0×106 cm-3, even lower than idling operation. While the particle emission at deceleration
17
operation mainly composed of nucleation mode particles. It could be the reason that more HC
18
and other unburned organic compounds emitted and were furtherly nucleated to nanoparticles
19
at low engine load condition (Mathis et al., 2004).
20
21
Fig. 4 Segments of instantaneous particle number and size distribution emission under variable driving
22
conditions. Deeper color in the last chart means more particle number.
23
24
2.3 Comparison of ultrafine particle emission characteristics by test bench and
25
on-board measurement
26
According to the simulation results of engine performance of real-world driving
27
conditions, particle number concentration distribution under different engine speeds and loads
28
of the test bus, as shown in Fig. 5(a). Fig. 5(b) indicates the particle number concentration
29
under each engine speed and load of the engine. By comparison, the ultrafine particle
30
emission level of the test Euro Ⅳ bus is relatively lower than the Euro Ⅲ engine. However,
7
1
the distributions of particle number concentration with the change of engine speeds and loads
2
are basically consistent with each other. The real-world test results show that particle number
3
generally increased with the growth of engine load at each speed. At high load condition, the
4
particle number concentration shew “U” shape distribution with the engine speed.
5
6
7
Fig. 5 Particle number concentration distribution of each engine speed and load based on real-world (a)
and test bench studies (b)..
8
9
Fig. 6 selects the particle size distribution of different engine loads at the maximum
10
torque speed (1300 r·min-1) and 1700 r·min-1. The particle size distribution under different
11
engine conditions generally remains the same and appears single mode distribution. It is
12
similar to the engine bench test that nucleation mode particles show low proportion in the
13
total particles, which could be induced by the reduction of diesel sulfur content from 2009 in
14
Shanghai. In addition, since we didn’t capture the max. power speed under real-world driving
15
condition, it can not be proved if the nucleation mode particles would increased a lot under
16
high engine speed operation.
17
18
19
Fig. 6 particle size distribution at different engine loads under the speeds of 1300 r·min-1 (a) and 1700
r·min-1 (b) based on real-world measurement.
20
21
3 Conclusions
22
This study discussed the ultrafine particle emission characteristics by engine bench and
23
real-world diesel bus measurement, respectively. The bench test results show the emission
24
characteristics of ultrafine particles under different engine operations. Correspondingly, the
25
on-board measurement results also illustrate that the ultrafine particle emission is strongly
26
correlated with the change of real-world driving cycle. In addition, the engine performance
27
simulation results based on on-board measurement data present great difference compared
28
with the bench test cycle. However, the results both indicate that the particle numbers in the
29
range of 5.6--560 nm measured by diesel engine and on-road diesel bus were basically at the
30
same level, about 107--108 cm-3. Particle number concentration of Euro Ⅲ diesel engine was
31
relatively higher than that of Euro Ⅳ diesel bus. However, since the large differences of
8
1
experiment method and test cycle of the bench and on-board measurements, it is hard to
2
accurately demonstrate the difference of particle emission level between these two engine
3
types. It can also be concluded that the distributions of particle number concentration under
4
different engine speeds and loads are basically consistent with test bench and on-board
5
measurement. The particle number results measured by two methods both show growth trend
6
with the increase of engine load at each engine speed. The particle number presents “U”
7
shape distribution with the change of speed at high engine load driving condition, suggesting
8
that the particle number emission are relatively lower at medium speed operation. The
9
particle sizes generally show single mode distributions by both test methods maybe due to the
10
reduction of diesel sulfur content in Shanghai. The peak of particle size is located at about
11
50--80 nm in the accumulation mode particle range. At low engine load operations like idling
12
and decelerating, the proportion of nucleation mode particles is significantly higher than the
13
other diving conditions. However, the experiment sample in this study is relatively small and
14
some analysis is based on simulation data, which exists certain errors with the actual situation.
15
Therefore, more experiment studies need to be carried out to accurately explain the ultrafine
16
particle emission characteristics of diesel engine and its impact factors.
17
18
Acknowledgments
19
This work was supported by the projects of “Instantaneous emission and environmental impact study
20
on vehicle alternative fuel (10231201902)” and “Study and demonstration of real time on-road vehicle
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emission and pollution warning (10231201700)” from Shanghai science and Technology Commission. We
22
are grateful to Shanghai Bus Company for their technical support on test bus.
23
24
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List of figure captions
20
Fig. 1 comparison of the engine performance of test bench (a) and on-board measurement (b).
21
Fig. 2 Schematic diagram of the installation of on-board measurement system.
22
Fig. 3 particle number (a, b, c) and size distributions (d, e, f) under different test cycle point by bench test.
23
Fig. 4 Segments of instantaneous particle number and size distribution emission under variable driving
24
conditions. Deeper color in the last chart means more particle number.
25
26
Fig. 5 Particle number concentration distribution of simulated engine speed and load under real-world
driving conditions.
27
28
Fig. 6 particle size distribution at different engine loads under the speeds of 1300 r·min-1 (a) and 1700
r·min-1 (b) based on real-world measurement.
29
12
250
160
External characteristic curve
140
100
Power (kW)
Power (kW)
200
Test cycles
120
80
60
150
100
40
50
20
0
0
500
700
900
1100 1300 1500 1700 1900 2100 2300 2500
500
700
900
1100
-1
1500
1700
1900
2100
2300
Speed (r·min )
(a) Engine bench test
1
1300
-1
Speed (r·min )
(b) On-board measurement
Fig. 1 comparison of the engine performance of test bench (a) and on-board measurement (b).
2
GPS
Environment
Vent
Laptop
Flow meter
TSI
EEPS
Spectrometer
Water tank
Engine
+SCR
Exhaust
3
Fig. 2 Schematic diagram of the installation of on-board measurement system.
1E+09
1E+08
(d) Speeds on100% load
dN/dlogDp(#·cm-3)
dN/dlogDp(#·cm-3)
(a) Speeds on100% load
1E+08
Total particle
1E+07
1E+06
1E+05
Nucleation particle
Accumulative particle
1E+07
700
900
1100
1300
1500
1700
1900
2100
2300
1E+04
500
1000
1500
-1
Speed (r·min )
2000
2500
1
10
100
1000
100
1000
Dp (nm)
1E+09
1E+08
(b) loads on 1500 r·min -1
(e) loads on 1500 r·min -1
dN/dlogDp(#·cm-3)
dN/dlogDp(#·cm-3)
4
5
1E+08
1E+07
1E+06
1E+05
10%
25%
50%
75%
100%
1E+07
1E+04
0%
20%
40%
60%
Load (%)
80%
100%
120%
1
10
Dp (nm)
13
1E+09
1E+08
(f) loads on 2300
r·min -1
dN/dlogDp(#·cm-3)
dN/dlogDp(#·cm-3)
(c) loads on 2300 r·min -1
1E+08
1E+07
1E+06
1E+05
10%
25%
50%
75%
100%
1E+07
1E+04
0%
40%
60%
Load (%)
80%
100%
120%
1
10
100
1000
Dp (nm)
Fig. 3 particle number (a, b, c) and size distributions (d, e, f) under different test cycle point by bench test.
50
45
40
35
30
25
20
15
10
5
0
180
Velocity
160
Engine power
140
120
100
80
60
Power (kW)
v (km·h-1)
1
2
20%
40
20
0
1.E+08
-3
dN/dlog(Dp) (#·cm )
1.E+09
1.E+07
1.E+06
Total particle
1.E+05
Nucleation particle
1.E+04
Accumulative particle
1.E+03
339.8
107.5
60.4
34.0
Dp (nm)
191.1
19.1
10.8
6.0
1
3
4
5
6
6
11
16
21
26
31
36
41
46
51
56
61
66
71
76
81
86
91
96
Fig. 4 Segments of instantaneous particle number and size distribution emission under variable driving
conditions. Deeper color in the last chart means more particle number.
14
8.5E+08
2.25E+08
2.00E+08
PN (#·cm-3)
PN (#·cm-3)
1.75E+08
1.50E+08
1.25E+08
1.00E+08
1100
1300
6.5E+08
1500
1700
5.5E+08
1900
2100
4.5E+08
2300
3.5E+08
2.5E+08
100%
75%
50%
7.50E+07
5.00E+07
1100
1.5E+08
5.0E+07
25%
1300
-1
1500
1700
Speed (r·min )
1
2
3
7.5E+08
10%
Load
0%
20%
60%
80%
100%
120%
Load
(a) on-board measurement result
(b) test bench result
Fig. 5 Particle number concentration distribution of each engine speed and load based on real-world (a)
and test bench studies (b)..
1E+08
1E+08
-1
-1
(b) Loads on 1700 r·min
1E+07
dN/dlogDp(#·cm-3)
dN/dlogDp(#·cm-3)
(a) Loads on 1300 r·min
1E+06
10%
25%
50%
75%
1E+05
1E+07
1E+06
10%
25%
50%
75%
1E+05
100%
100%
1E+04
1E+04
1
10
100
Dp (nm)
4
5
6
7
40%
1000
1
10
100
1000
Dp (nm)
Fig. 6 particle size distribution at different engine loads under the speeds of 1300 r·min-1 (a) and 1700
r·min-1 (b) based on real-world measurement.
15