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Zare, Ali, Bodisco, Timothy, Nabi, Nurun, Hossain, Md. Farhad, Rahman,Md Mahmudur, Ristovski, Zoran, & Brown, Richard(2017)The influence of oxygenated fuels on transient and steady-state engineemissions.Energy, 121, pp. 841-853.
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https://doi.org/10.1016/j.energy.2017.01.058
Accepted Manuscript
The influence of oxygenated fuels on transient and steady-state engine emissions
Ali Zare, Timothy A. Bodisco, Md Nurun Nabi, Farhad M. Hossain, M.M. Rahman,Zoran D. Ristovski, Richard J. Brown
PII: S0360-5442(17)30058-0
DOI: 10.1016/j.energy.2017.01.058
Reference: EGY 10193
To appear in: Energy
Received Date: 11 May 2016
Revised Date: 24 November 2016
Accepted Date: 10 January 2017
Please cite this article as: Zare A, Bodisco TA, Nabi MN, Hossain FM, Rahman MM, Ristovski ZD,Brown RJ, The influence of oxygenated fuels on transient and steady-state engine emissions, Energy(2017), doi: 10.1016/j.energy.2017.01.058.
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The influence of oxygenated fuels on transient and steady-state engine
emissions
Ali Zarea,*, Timothy A. Bodiscob, Md Nurun Nabia,c, Farhad M. Hossaina, M.M. Rahmana,c, Zoran D.
Ristovskia,c, Richard J. Browna
aBiofuel Engine Research Facility, Queensland University of Technology (QUT), QLD, 4000 Australia
bSchool of Engineering, Deakin University, VIC, 3216 Australia
cInternational Laboratory for Air Quality and Health, Queensland University of Technology (QUT), QLD, 4000 Australia
Abstract
This research studies the influence of oxygenated fuels on transient and steady-state engine
performance and emissions using a fully instrumented, 6-cylinder, common rail turbocharged
compression ignition engine. Beside diesel, the other tested fuels were based on waste cooking
biodiesel (primary fuel) with triacetin (highly oxygenated additive). A custom test was designed in
this study to investigate the engine performance and emissions during steady-state, load acceptance
and acceleration operation modes. Furthermore, to study the engine performance and emissions
during a whole transient cycle, a legislative cycle (NRTC), which contains numerous discrete
transient modes, was utilised. In this paper, the turbocharger lag, engine power, NOx, PM, PN and PN
size distribution were investigated. During steady-state operation, compared to diesel, the oxygenated
fuels showed lower indicated power, while they showed higher values during turbocharger lag and
acceleration. Also, during acceleration and load increase modes, NOx, PM and PN peaked over the
steady-state counterpart, also, the accumulation mode count median diameter moved toward the
larger particle sizes. Increasing the fuel oxygen content increased the indicated specific NOx and PN
maximum overshoot, while engine power, PM, PN and PM maximum overshoot decreased. Also, the
accumulation mode count median diameter moved toward the smaller particle sizes.
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Keywords: Turbocharger lag; Fuel Oxygen; NOx; PM; PN; Particle size distribution.
1. Introduction
Despite only a low fraction of engine operation, in vehicles, being at a steady-state condition [1],
to date, most studies about internal combustion engines have focused on steady-state operation
[2]. However the results from transient operation studies are more likely to reflect the reality.
Here transient operation can be defined as any operation in which the engine speed or fuel
injection change frequently; these parameters remain relatively unchanged during steady-state
operation [2].
In general, combustion products under transient operation are more readily produced when
compared to steady-state operation. For example, a report showed that during acceleration, the
NO concentration of a diesel engine peaked around 800 ppm, while its steady-state counterpart
was around 600 ppm [3]. Also shown in this study was an order of magnitude overshoot in
smoke opacity during acceleration when compared to steady-state operation. Another report
showed a 3.6 times increase in diesel PN during transient operation, compared to its steady-state
counterpart [4].
Currently, the turbocharged diesel engine is one of the most preferred engines [5] due to its high
fuel efficiency and subsequent low CO2 emission [6]. However, the transient operation of
turbocharged diesel engines has been associated with slow acceleration rates, subsequently poor
drivability and overshoots in combustion products [7]. This could be due to the most notable off-
design parameter, turbocharger lag, which is a mismatch between the supplied air and the
injected fuel [8].
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The European Union issued a directive to increase the use of renewable biofuels to 10% by 2020
in order to offset fossil fuel (EU Directive 2009/28/EC). Among different biofuels introduced by
industry and research groups, waste cooking biodiesel has attracted attention owing to some
advantages such as global availability, its low price and close properties to diesel [9]. Also,
disposal issues could be another motivation to utilise it as a fuel [10]. In terms of the effect of
using waste cooking oils as a fuel on engine performance and exhaust emissions in the literature,
there are some advantages and disadvantages [11]; for example, reducing the PM [11, 12], CO
and HC emissions [12-15], increasing the NO2 [14], NOx [11] and BSFC [12-14], and
decreasing the brake power [13, 15].
The presence of oxygen in the fuel is a significant factor differentiating biofuels from
conventional fossil fuels. Fuel oxygen content, which depends on the fatty acid ester profile such
as carbon chain length and unsaturation level [16], is known as a major factor in reducing the
emission [17-19]. Therefore, it can be concluded that in order to boost the emission reduction a
low volume of highly oxygenated fuel additives can be used.
Triacetin [C9H14O6] which is the product of the acetylation process of glycerol and acetic acid
can be used as a highly oxygenated fuel additive [20]. Glycerol is a byproduct of the biodiesel
transesterification process and therefore by producing more biofuels the availability of this
product increases proportionally [21]. The huge quantity of glycerol may drop its price to a level
that can justify the use of glycerol as a burner fuel in combustion, however there are some
limitations due to its chemical and physical properties [22]. According to this, triacetin which is
the production of glycerol-derived fuels could be a solution [23]. However, a limited number of
studies in the literature (mostly from our research group) have focused on using triacetin as an
additive [21, 24-27]. It was reported that blending biofuels with this highly oxygenated fuel
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additive increases the kinematic viscosity and density, however, it decreases the heating value
and the cetane number of the blend [20].
To-date investigation on transient engine operation has been limited due to factors such as the
complexity of the experiment and the availability of automatically controlled test-beds with high-
tech fast response measuring instruments. A thorough search in the literature showed only a
limited number of publications studying the influence of biofuels on exhaust emissions during
transient operation compared to the studies on steady-state operation [2]. Also, it was found that
most transient operation studies focused on continuously transient cycles using a quasi-steady-
state manner of analysis, presenting the engine performance and exhaust emissions results with
mean and cumulative values over either the entire cycle or segments of the cycle. However, this
method of analysis conceals the effect of individual load acceptance and engine speed changes.
Hence in order to study the transient operation mechanism, it is important to evaluate the
transient engine performance and emissions separately for each engine speed and load change.
However, only a small portion of the publications on transient operation studied the acceleration
and load acceptance [2]. For example, Dimitrios C. Rakopoulos et al. [3] studied different
exhaust emissions during transient and stead-state conditions. The fuels used in this work were
diesel, biodiesel and n-butanol. The results showed that during turbocharger lag NO and smoke
opacity peak over their steady-state counterpart. Similarly, another study by Constantine D.
Rakopoulos et al. [1] evaluated NO and smoke opacity during 3 different acceleration and load
increase modes using a turbocharged diesel engine. This study showed the turbocharger lag as
the reason of overshoots in NO and smoke opacity. In addition, the maximum global gas
temperature in the cylinder during acceleration for the tested fuels were showed and used to
interpret the NOx behaviour. Tan et al. [28] studied the particle number emissions from a diesel
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engine under load increase operation mode indicating that the total particle number increases
with the engine torque. The tested fuels in this study were diesel, pure Jatorpha biodiesel and two
biodiesel blends with diesel fuel. The particle number emission in nucleation and accumulation
modes were investigated showing that by increasing the biodiesel blend ratio the particle number
in accumulation mode decreases, while the particle number in nucleation mode increases. To
study more about the research done about different transient discrete modes readers can refer to
Ref. [1, 4-6, 8, 28-34].
This paper studies the influence of oxygenated fuels on engine performance and emissions
during transient and steady-state operations using a range of fuels with 0 to 14.23% oxygen
content. The oxygenated fuels were based on waste cooking biodiesel (primary fuel) with
triacetin (highly oxygenated additive). A custom test was designed in this study to investigate the
exhaust emissions during steady-state, load acceptance and acceleration operation modes. In
addition, to evaluate the exhaust emissions during a whole transient cycle, a legislative cycle
(NRTC), which contains numerous discrete transient modes, was utilised. The results can also
correlate against the discrete transient modes in custom test. A comprehensive search in the
literature could not find any investigation studying the influence of triacetin as a fuel additive
with waste cooking biodiesel on diesel engine exhaust emissions under transient engine
operation.
2. Material and methods
2.1 Engine specification and test setup
The engine used in this study was a fully instrumented, 6-cylinder turbocharged, aftercooled,
common rail compression ignition engine, as described in Table 1. The engine was coupled to an
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electronically controlled water brake dynamometer to control the engine load during steady-state
and transient operation. In addition, this engine research facility has the ability to program
different test cycles that can be run automatically.
Figure 1 illustrates the schematic of the experimental setup. Details of exhaust sampling systems
can be found in Ref. [17, 21, 25]. Also, readers can refer to Ref. [35, 36] for more specific
information about the in-cylinder pressure, crank angle and fuel injection data collection in our
engine laboratory.
2.2 Fuel selection
The tested fuels can be seen in Table 2. This study used diesel (D100), waste cooking biodiesel
(B100), a blend of D100, B100 and triacetin (T100), and three blends of waste cooking biodiesel
(primary fuel) and triacetin (additive). The 6 tested fuels showed in Table 2 are named according
to the portion of each fuel in the final fuel. As an example, T4B96 contains 4% (by volume) of
triacetin and 96% (by volume) of waste cooking biodiesel. The stability and miscibility test of
blends was conducted at room temperature and no phase separation occurred during 96 hours of
the test.
The tested fuel oxygen content ranged from 0 to 14.23%. As can be seen, the lower heating value
(LHV) decreases with oxygen content. This has a negative effect on engine power. Also, it can
be seen from the table that the kinematic viscosity increases with oxygen content. This fuel
property is related to the degree of unsaturation and has an adverse effect on the evaporation
characteristics of the fuel and fuel spray atomisation during combustion [2].
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In Table 2, the listed values for the blends are calculated based on pure substance compositions.
More information about waste cooking biodiesel and triacetin can be found in Ref. [20] and [37],
respectively.
2.3 Design of experiment
Driving cycles, which can be used to model the exhaust emissions are split into two categories.
The first category, modal, contains various quasi steady-state modes of load and speed, and the
second category, real world driving cycles, is based on actual driving data [38]. This study used
Non-road Transient Cycle (NRTC) to study the influence of fuel oxygen on engine performance
and exhaust emissions during a transient cycle. Among all the transient cycles, NRTC was
selected due to the high frequency of abrupt speed and load changes, as shown in Figure 2. This
cycle is required by different emission standards for non-road engines, including the US EPA
Tier 4 rule, EU Stage III/IV regulation and Japanese 2011/13 regulation in order to regulate the
emission from mobile non-road engines. NRTC was developed by a cooperation of the EU
authorities and the US EPA [39].
Transient driving cycles are composed of frequent engine speed and load changes, hence limiting
their research value into the fundamentals of transient response. Therefore, designing a custom
quasi-steady-state test containing acceleration and load increase discrete modes, and steady-state
modes aids to investigate the transient and steady-state operation fundamentally. However, a
limited number of studies in the literature used controllable transient conditions [2, 3].
In order to do a fundamental investigation about the transient operation, using a driving cycle
from the literature is more preferable; however, no appropriate driving cycle was found to fulfil
the research goals of this study. From the literature, only a limited number of research groups
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designed a test to study the acceleration and load acceptance transient modes. Some examples of
these custom-made tests can be seen in Ref. [1, 3, 4, 31, 40].
In this study, in reference to the concept of custom transient tests used in the literature [21], a
custom test based on a driving schedule of modal cycles with some added transient load increase
and acceleration modes was designed to investigate the transient and steady-state engine
operating characteristics and exhaust emissions. To design this custom test, as the tested engine
in this research had a Euro III emission certification, the engine speed and load were selected
from a legislated test cycle in the Euro III legislation, European Stationary Cycle (ESC) [39].
Regarding the engine speed used in the custom test, in addition to the idle condition at
approximately 700 rpm, the other two speeds from ESC were Speed A and Speed B calculated
from Equation (1) [39].
A = nlo + 0.50(nhi - nlo) (1)
B = nlo + 0.75(nhi - nlo),
where nlo is the lowest speed at which 50% of maximum power occurs, and nhi is the highest
speed in which 70% of the maximum power occurs [39]. The nhi and nlo can be found by
mapping the engine speed and power.
According to the Supplemental Emissions Test (SET) introduced in the US EPA 2004 emission
standards, instead of sharp changes between steady-state modes used in ESC, some controllable
transient ramps were added in the custom test to enable investigation on the exhaust emissions
during acceleration and load acceptance.
The custom test designed for this study is shown in Figure 3. In this designed test, there are two
main parts each related to one speed, (a) 1864 or (b) 2257 rpm. In each engine speed there are 4
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loads involved, 0, 25, 50 and 75%. Each part includes 3 load increase modes, in addition to one
acceleration mode. Each part starts with the 0% load and 700 rpm (idle condition), then over 5 s
the engine accelerates from 700 rpm to the targeted speed, 1864 or 2257 rpm. At this point the
engine speed remains unchanged until the end. The load increase modes activate after reaching
the targeted speed. In the first load increase mode, over 5 s the engine load increases from 0 to
25% and remains constant for 30 s. Then the second load increase mode activates. Over 5 s the
engine load increases from 25 to 50% and remains unchanged for 30 s. Then in the last load
increase mode, over 5 s the engine load increases from 50 to 75% and remains constant at 75%
for 30 s.
2.4 Experimental procedure
During the experiments, two gas analysers and three particle measuring instruments were used to
minimize the faulty measurement. To ensure the reliability of the results, the engine was fully
warmed up for at least one hour before each test and the experiment with each fuel was repeated
at least two times. The calculated coefficient of variation parameter confirmed the repeatability
of the results. No further preconditioning was necessary to ensure repeatable initial conditions as
the test engine used in this investigation does not have after-treatment systems. It should be
noted that after each fuel change the fuel lines were cleaned.
3. Result and discussion
This section studies the influence of oxygenated fuels on exhaust emissions during turbocharger
lag, acceleration, load increase, steady-state and a transient cycle. In the analyses, in addition to
the effect of fuel properties, the transient engine performance and exhaust emissions mechanism
is also discussed. The method of analysis in this study is to investigate different engine
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performance and exhaust emissions over the custom test and the NRTC transient cycle. Then, it
studies the parameters in the first part (first 20 s) of the custom test which contains turbocharger
lag, acceleration, load increase from 0 to 25% load and steady-state mode at 25% load. During
the first 20 s of the custom test at (a) 1864 and (b) 2257 rpm, for the first 2 s the engine runs at
the idle condition (0% load and 700 rpm), then over 5 s the engine accelerates from 700 rpm to
the targeted speed, 1864 or 2257 rpm. At this point the engine speed remains unchanged until the
end. The load increase modes starts after reaching to the targeted speed. In the first load increase
mode, over 5 s the engine load increases from 0 to 25% and remains constant for 30 s. It should
be mentioned that in all the figures, the fuels are named according to their oxygen content and
the illustrated data for the 6 fuels are differentiated by colour.
3.1 Turbocharger lag
Figure 4 shows the intake air and injected fuel during the first 20 s of the custom test for D100. It
is shown that after the initial 2 s, when the engine accelerates from 700 rpm to (a) 1864 or to (b)
2257 rpm, the injected fuel increases, while the intake air remains constant for some seconds and
then it begins to increase. This delay time, which is highlighted in Figure 4, is called
turbocharger lag and it occurs during load (or engine speed) increase owing to a mismatch
between the rapid response of the fuel pump and the slower response of the turbocharger
compressor to supply air due to the turbocharger moment of inertia. This delay time occurs due
to the fact that the engine crank shaft and turbocharger shaft are not mechanically connected.
During turbocharger lag, the AFR (air to fuel ratio) reduces, hence rich combustion impacts the
torque build-up and also causes an overshoot in exhaust emissions, compared to their steady-
state counterparts [1]. This is discussed in greater detail in the next sections.
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3.2 Engine power
Figure 5 and 6 show the engine power during custom test and NRTC, respectively. It is shown in
Figure 5 that D100 has the highest indicated power at all modes of the custom test and this
parameter decreases with fuel oxygen content. For example, at 2257 rpm under 75% engine load,
D100 (0% oxygen content) has the highest indicated power (124.7 kW) and T10B90 (14.23%
oxygen content) has the lowest indicated power. A similar trend can be seen during NRTC
shown in Figure 6, as the brake power with oxygenated fuels were lower than D100. The lower
engine power with the oxygenated fuels could be due to the heating value of the fuels [21, 25,
41]. For example, D100, which had the highest heating value between the tested fuels, showed
the highest mean brake power over the NRTC (30.8 kW). As shown in Table 2, the lower heating
value (LHV) of the tested fuels decreased with the fuel oxygen content.
The influence of oxygenated fuels on engine power during transient discrete operating modes
shows a different trend when compared to steady-state condition within which the oxygenated
fuels had lower engine power than D100. Figure 7 illustrates the brake power during the first 20
s of the custom test at (a) 1865 rpm and (b) 2257 rpm. The figure shows that after the initial 2 s,
when the engine accelerates from 700 rpm to (a) 1864 or (b) 2257 rpm, the oxygenated fuels had
a relatively higher brake power than D100. This result was confirmed by a further analysis on the
indicated torque corresponding to the same test and duration. This observed trend could be
explained by the fact that during this transient mode the AFR decreases to lower than its
respective steady-state value (leading to a rich combustion), and the presence of oxygen in the
oxygenated fuels aids the rich combustion to move it toward the stoichiometric condition in
which a higher engine power is produced.
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3.3 Nitrogen oxides
Figure 8 illustrates the indicated specific NOx during custom test, and Figure 9 illustrates NOx
emission during NRTC. It can be seen in Figure 8 that the indicated specific NOx increases with
engine load. The reason is that increasing the engine load decreases the AFR and increases the
injected fuel quantity, which leads to a higher combustion temperature, and consequently to
higher NOx formation. The figure also shows that the indicated specific NOx emission increases
moderately with fuel oxygen content. For example, under 50 and 75% engine load at both speeds
the indicated specific NOx with B100, T4B96, T8B92 or T10B90 is higher than D100. The
maximum increase of 20.7% was related to T892 at 2257 rpm under 50% engine load. However,
at 25% engine load, the maximum increase of 5.8% was related to B100 at 1865 rpm. The results
coincide with Ref. [21].
It has been frequently reported in the literature that using oxygenated fuels increases the NOx
formation [21, 25, 30, 42]. This could be due to different reasons such as higher oxygen content,
viscosity, density, cetane number and bulk modulus of the biofuels [30].
In comparison to D100, with oxygenated fuels, owing to the presence of oxygen in the fuel the
local conditions during combustion are closer to the stoichiometric condition leading to a better
and more complete premixing during the ignition delay. This results in a higher fraction of heat
release during the premixed phase of combustion and increases the in-cylinder temperature and
residence time at high temperature. The explained mechanism increases the NOx formation. [2,
21, 42, 43].
The higher adiabatic flame temperature with oxygenated fuels owing to the presence of double
bonds in their molecules could be another reason for their higher NOx formation, when
compared to D100 [42]. Also, it has been reported that the adiabatic flame temperature can be
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influenced by the in-cylinder radiative heat transfer from the formation of particles [30, 42], as a
lower PM formation leads to a lower radiative heat transfer, and consequently results in a higher
flame temperature and a higher NOx [21]. Similar to this study (Section 3.4), the lower PM
formation with oxygenated fuels has been reported in the literature [2, 42, 43].
Figure 9 illustrates that the NOx emission during NRTC decreases with fuel oxygen—the reason
being that the illustrated NOx is not normalised with the engine power. Since the engine power
decreases with the fuel oxygen, as shown in Figure 6, a further analysis showed that the power
normalised NOx emission mean value over the cycle increases with the fuel oxygen content.
This also happens with the un-normalised NOx emission during the custom test, as it decreases
with the fuel oxygen content. While, the normalised value (indicated specific NOx emission) has
an increasing trend as shown in Figure 8.
In Figure 10, the insets relate to the first 20 s of the custom test and show that during acceleration
the NOx emission peaks over its respective steady-state counterpart due to the drop in AFR, as
discussed in Section 3.1. Insufficient air during turbocharger lag, which decreases the AFR, leads
to NOx emission overshoot. This increase is expected because of the local high temperatures due
to the nearness of the air and fuel mixture during combustion to its stoichiometric condition
caused by low supplied-air response [3, 21]. However, after some engine cycles, where the AFR
becomes stable, the NOx emission will also be steady. Regarding the effect of fuel oxygen on
NOx during acceleration mode, as can be seen in Figure 10, NOx emission decreases with fuel
oxygen content. As this figure shows the un-normalised NOx emission, the trend is opposite to
the indicated specific NOx emission shown in Figure 8 illustrating the increasing trend of NOx
emission with fuel oxygen.
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3.4 Particulate matter
Figure 11 and 12 illustrate the indicated specific PM for the tested fuels under the custom test
and PM emission during NRTC, respectively. As can be seen in Figure 11, the indicated specific
PM emission increases with engine load. This is due to the lower equivalence and higher injected
fuel quantity at high loads, which leads to higher combustion temperature, longer diffusion
combustion duration (hence less time after diffusion combustion) and lower oxygen
availability—all of which reduce the soot oxidation process in the expansion stroke leading to
higher PM formation [7]. This trend is most noticeable when the load increases from 50% to
75% it can be observed that the PM emission increase is much more at 1865 rpm than that at
2257 rpm (this can be also seen in Figure 13). The most likely reason is that at 75% load and
1865 rpm fuel injected per stroke is higher (1.02e-8 g/stroke) and equivalence ratio is lower
(3.02) than that of 75% load and 2257 rpm (0.94e-8 g/stroke and 3.43, respectively). The effect
of engine speed and load on PM emission has been discussed in greater detail in Ref. [21] with
respect to AFR, equivalence ratio and oxygen ratio.
Figure 11 also shows that PM emission decreases with fuel oxygen content. As can be seen,
D100 (with the lowest fuel oxygen content) has the highest PM and T10B90 (with highest fuel
oxygen content) has the lowest PM. The highest reduction of 89.8% can be seen at 1865 rpm
under 50% engine load related to T10B90. The results coincide with Ref. [21]. A similar trend
can be seen during NRTC in Figure 12, where D100 has the highest PM emission during the
cycle. The mean value of PM emission over the cycle shows that using oxygenated fuels can
decrease the PM emission up to 90%.
It has been frequently reported in the literature that using oxygenated fuels leads to PM reduction
[2, 42, 43]. It was shown that compared to the physical properties of fuel such as cetane number
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and viscosity, the fuel chemical properties, particularly the oxygen content, is more influential in
reducing the PM emission [17]. This is owing to PM formation during combustion in the fuel-
rich zone under high temperature decomposition. Here, the fuel oxygen content aids the soot
oxidation process as it reduces the local fuel-rich zone within the core region of the sprayed fuel,
which prevents higher PM formation. Therefore, increasing the oxygen content of the fuel can
decrease PM formation [2, 21, 42-45].
In Figure 13, the sub-diagrams related to the first 20 s of the custom test show that during
acceleration, the PM emission increases sharply and peaks over the respective steady-state
counterpart due to the drop in the AFR. Insufficient air during turbocharger lag, consequently
decreasing the trend of the AFR, causes an overshoot in the PM emission. A rapid increase in
fuel injection pressure at the start of the transient mode increases the liquid fuel jet penetration,
and since this higher fuel jet momentum is not accompanied by equally enhanced gas motion, the
liquid fuel impingement on the cool combustion wall increases. This leads to a lower mixture
preparation rate and a greater heterogeneity of the mixture. Also, this phenomenon prolongs the
combustion and decreases the soot oxidation process time and causes an overshoot in PM
emission [1, 3, 46].
As shown in Figure 13, in addition to the steady-state modes of the custom test, the PM
emissions and their peak values during acceleration with the oxygenated fuels are lower
compared to D100 due to the presence of oxygen in the fuel. This can be explained by the fact
that during this transient condition, in which the combustion suffers from the lack of air, the
oxygen molecules in the oxygenated fuels move the combustion toward the stoichiometric
condition and this aids the soot oxidation process, hence reduce the PM emission.
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3.5 Particle number
Figure 14 and 15 illustrate the indicated specific PN at all modes of the custom test and PN
emission during NRTC, respectively. As can be seen in the figures, during both the custom test
and NRTC, PN emission with oxygenated fuels are lower, when compared to D100. For
example, during the custom test, at 1865 rpm under quarter load, D100 showed the highest value
and using T10B90 reduced the indicated specific PN by 84%. The results coincide with Ref.
[21].
Similar to PM, it can be seen that PN at modes of the custom test and during NRTC decreases
with fuel oxygen. The effect of fuel oxygen content on PN reduction has been reported in the
literature [21, 25, 47]. The presence of oxygen promotes combustion in the core region of the
sprayed fuel and within the fuel-rich diffusion-flame region, and aids the oxidation of the already
formed soot [21]. In addition to the fuel oxygen, another reason could be a lack of aromatic
hydrocarbons, which results in less soot formation [40].
In Figure 16, the sub-diagrams related to the first 20 s of the custom test show that during
acceleration the PN emission increases sharply and peaks over the respective steady-state
counterpart, this is owed to the drop in the AFR. Insufficient air during turbocharger lag,
consequently decreasing the AFR, worsens the soot oxidation process and causes an overshoot in
the PN. However, after some engine cycles, where the AFR becomes stable, the PN emission
will be steady. Also, as can be seen in the custom test sub-diagrams in both Figures 14 and 16,
the same as PM, PN increases with engine load due to the higher quantity of injected fuel and
lower AFR. Increasing the engine load leads to a greater quantity of injected fuel, higher
combustion temperature and longer diffusion combustion duration, hence less time after
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diffusion combustion which reduces the soot oxidation process in the expansion stroke and also
lowers oxygen availability.
As shown in Figure 16, in addition to the steady-state modes of the custom test, the PN
emissions and their peak values during acceleration with D100 are higher when compared to the
oxygenated fuels. The reason could be the presence of oxygen in the fuel. This can be explained
by the fact that during the transient condition, in which the combustion suffers from the lack of
air, the oxygen molecules in the oxygenated fuels move the combustion toward the
stoichiometric condition, which aids PN emission reduction.
3.6 Particle size distribution
Particle number (PN) emission standard has been recently introduced in the Euro 5b (2011) and
Euro 6 (2014) for diesel engines and petrol engines, respectively. However, there is still a lack of
regulation on the size of emitted particles. It has been shown that the toxicity of particles
increases when the size of the particles decreases [48].
In particle size distribution, accumulation and nucleation modes are the main categories.
Particles with diameter sizes between 3 to 30 nm are classified as nucleation mode particles
forming only a small portion of the total particle mass. These particles consist of soluble organic
fraction (SOF) and sulphates. The other category, which has a superior share in the total particle
mass, contains the particles with a diameter size of 30 to 1000 nm. These particles are mainly
agglomerated soot and adsorbed materials [40].
Figure 17 illustrates the particle size distribution during the custom test at two engine speeds,
1865 and 2257 rpm. In each sub-diagram, solid lines and dotted lines show the particle size
distribution during the steady-state and at the PN peak point of the acceleration mode,
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respectively. The solid lines in Figure 17, which correspond to steady-state operation, shows that
the size of the particles decreases with engine speed. For example, under 75% engine load, by
increasing the engine speed from 1865 to 2257 rpm, the accumulation mode count median
diameter, which is the peak in the PN size distribution graph in the accumulation part, for all the
tested fuels with 0, 6.02, 10.93, 12.25, 13.57 and 14.23% oxygen content, decreases from 75 to
65, 75 to 56, 65 to 56, 65 to 49, 56 to 49, and 56 to 49 nm, respectively.
The figure shows that the size of the emitted particles increases with engine load. For example, at
1865 rpm under 25, 50 and 75% engine loads, the accumulation mode count median diameters
are 56, 65 and 75 nm (for D100), and 49, 56 and 65 nm (for B100). This behavior can be
explained as increasing the engine load leads to a greater quantity of injected fuel, which favours
the formation of larger particles due to the higher combustion temperature, longer diffusion
combustion duration and hence less time after diffusion combustion, which in turn reduces the
soot oxidation process in the expansion stroke and also lowers oxygen availability [2].
The presence of oxygen in the fuel could be a reason for the decrease in the accumulation mode
peak size as it can promote combustion in the core region of the sprayed fuel and within the fuel-
rich diffusion-flame region, and helps with the oxidation of the already formed soot [21]. Also,
lack of aromatic hydrocarbons in biodiesels has been reported to be another reason for a
decreased in the size of accumulation mode particles [40]. As illustrated in Figure 17, by
increasing the oxygen content (shown with both the solid and dotted lines), the accumulation
mode count median diameter moves toward the smaller particle sizes. For example, under 50%
engine load at 1865 rpm, the accumulation mode count median diameters during steady-state
operation (solid lines) for all the tested fuels with 0, 6.02, 10.93, 12.25, 13.57 and 14.23%
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oxygen content are 65, 56, 56, 49, 49 and 49 nm, respectively. This also can be seen in dotted
lines, which show the particle size distribution at the PN maximum overshoot point.
Figure 18 shows the diesel PN size distribution during the first 20 s of the custom test at both
engine speeds illustrating from the transient start point until the maximum overshoot point, and
the steady-state averaged value. As can be seen, the size of particles from the start point until the
maximum PN overshoot decreases toward the smaller particles, while PN increases. A reason
could be a rapid increase in fuel injection pressure at the start of the transient mode, which leads
to an increase in the liquid fuel jet penetration. Since this increased liquid fuel jet is not
accompanied with the equally enhanced gas motion, it causes an increase in liquid fuel
impingement on the cool combustion wall, hence a lower mixture preparation rate and lower soot
oxidation—all of which lead to larger emitted particles [1, 3, 46]. However, by increasing the
supplied air and recovering the AFR after its drop, the emitted particles will be smaller and
finally the smallest particles emitted during steady-state, as shown by a solid line in Figure 18.
Figure 17 compares the PN size distribution curves during the steady-state and its respective
maximum overshoot for all the tested fuels at all modes of the custom test. As can be seen, for
the oxygenated fuels, the PN peaks over the steady-state value in both nucleation and
accumulation modes, while for diesel it mainly occurs in accumulation mode. This could be due
to the presence of oxygen in the fuel which aids combustion during transient operation.
The particle size distribution in Figure 17 and 18 are presented in lognormal distribution to be
the best fit for signal source aerosols and in keeping with practice in this journal. The lognormal
distribution is used instead of normal distribution, as the difference between the lowest and
highest values in X-axis is high (0 to 1000 nm), and the range of 0 to 300 nm in X-axis has a
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significant importance. For more specific information about use of lognormal distributions in
aerosol science refer to Ref. [49].
3.7 Maximum overshoot in PM and PN under load increase modes
The maximum overshoot (Mp) is defined as the maximum peak value of the response measured
from unity [50]. This can be defined as:
Mp = [(P-S)/S]*100% (2)
where P is the maximum value and S is the final steady-state value on the response curve.
Apart from the PM and PN overshoots observed during the first 20 s of the custom test at both
engine speeds in Figures 13 and 16, load increase modes from 25 to 50% and 50 to 75% at both
engine speeds can cause overshoots in PM and PN emissions. These overshoots can be observed
visually in the custom test sub-diagrams of Figures 13 and 16, however, Figure 19 shows the
numerical overshoots analysis.
Figure 19 shows that the Mp for PM emission with oxygenated fuels is relatively lower when
compared to D100. In Section 3.4, it was explained how the fuel oxygen content can improve the
decreased soot oxidation process during overshoots. In contrast to PM emission, the Mp for PN
emission increases with fuel oxygen, as shown in Figure 19. This can be explained by comparing
the difference between maximum values in the nucleation and accumulation modes on the
steady-state curve and its respective maximum overshoot curve at each mode for each fuel in
Figure 17. The difference between these curves in the accumulation mode corresponds to the PM
overshoot, as PM is mostly formed from the particles in the accumulation mode. In PN formation
both the accumulation and nucleation modes are responsible, however the nucleation mode is
superior. As can be seen, for D100 the difference mainly occurs at the accumulation mode and
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there is no sizeable difference in the nucleation mode between the steady-state and maximum
overshoot curves, while due to the presence of oxygen, for the other tested fuels, during
overshoot the PN peaks over the steady-state value in both the nucleation and accumulation
modes. As can be seen for the oxygenated fuels, the proportional overshoot (difference between
steady-state and its maximum overshoot curves) in the nucleation mode is higher compared to
the proportional overshoot in the accumulation mode. Hence the oxygenated fuels Mp for PN
emission is higher than that of diesel.
4. Conclusion
This paper studied the effect of oxygenated fuels on transient and steady-state engine
performance and emissions. The engine used in this study was a fully instrumented, 6-cylinder,
common rail turbocharged compression ignition engine. This study used a range of fuel oxygen
content (0 to 14.23%), based on waste cooking biodiesel (primary fuel) with triacetin (highly
oxygenated additive). A custom test was designed in this study to investigate the engine
operation characteristics and exhaust emissions during steady-state, load acceptance and
acceleration operation modes. In addition, to evaluate the engine operation characteristics and
exhaust emissions during a whole transient cycle, a legislative cycle (NRTC), which contains
numerous discrete transient modes, was utilised. In this paper, different engine performance and
exhaust emission parameters were investigated and the following conclusions were drawn:
• During transient operation the AFR decreased to lower than its respective steady-state
value.
• During steady-state operation the oxygenated fuels showed lower indicated power, while
they showed higher values during turbocharger lag and acceleration.
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• The brake power during NRTC decreased with fuel oxygen content.
• During steady-state modes of the custom test, NOx, PM and PN increased with engine
load. And, the size of emitted particles decreased with engine speed and increased with
engine load.
• During transient operation, NOx, PM and PN peaked over their steady-state counterparts
due to a drop in the AFR. And the accumulation mode count median diameter moved
toward the larger particle sizes compared to the steady-state.
• By increasing the fuel oxygen content, the indicated specific NOx increased, while the
indicated specific PM and PN decreased. And, the accumulation mode count median
diameter moved toward the smaller particle sizes.
• Oxygenated fuels showed a relatively lower PM-Mp, a relatively higher PN-Mp during
load increase modes of the custom test.
5. Acknowledgements
This research was supported by the Australian Research Council’s Linkage Projects funding
scheme (project number LP110200158). The author would also like to acknowledge Mr. Andrew
Elder from DynoLog Dynamometer Pty Ltd, Mr. Noel Hartnett and Dr. Md. Mostifizur Rahman
for their laboratory assistance, Dr. Michael Cholette and Dr. Meisam Babaie for their guidance,
Peak3 Pty Ltd for assistance with measuring instruments, Eco Tech Biodiesel for the fuel supply.
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7. Tables
Table 1 Engine specifications ....................................................................................................... 27
Table 2 Fuel properties ................................................................................................................. 28
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Table 1 Engine specifications
Model Cummins ISBe220 31 Emission standard Euro III Cylinders 6 in-line Aspiration Turbocharged Capacity 5.9 L Compression ratio 17.3:1 Bore x stroke 102 x 120 (mm) Maximum power 162 kW @ 2500 rpm Maximum torque 820 Nm @ 1500 rpm Fuel injection High pressure common
rail Dynamometer type Electronically controlled
water brake dynamometer
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Table 2 Fuel properties
Fuel name D100 B100 T100 D60B35T5 T4B96 T8B92 T10B90 O (wt%) 0 10.93 44.00 6.02 12.25 13.57 14.23 H (wt%) 14.8 12.21 6.42 13.47 11.97 11.74 11.63 C (wt%) 85.1 76.93 49.53 80.46 75.81 74.73 74.19 Kinematic viscosity@40°C (mm2/s)
2.64 4.82 7.83 3.66 4.94 5.06 5.12
Density@15°C (g/cc) 0.84 0.87 1.159 0.866 0.882 0.893 0.898 Cetane number 53.3 58.6 15 53.24 56.86 55.11 54.24 Lower heating value (MJ/kg) 41.77 37.2 16.78 38.92 36.38 35.57 35.16 Higher heating value (MJ/kg) 44.79 39.9 18.08 41.74 39.02 38.15 37.72
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8. Figures
Figure 1 Schematic of the experimental setup .............................................................................. 31
Figure 2 NRTC (Non-road Transient Cycle) schedule ................................................................. 32
Figure 3 Custom transient test at (a) 1865 rpm and (b) 2257 rpm ................................................ 33
Figure 4 Intake air and injected fuel during the first 20 s of the custom test at (a) 1865 rpm and
(b) 2257 rpm, with D100 .............................................................................................................. 34
Figure 5 Engine power at all modes of the custom test ................................................................ 35
Figure 6 Engine power during NRTC ........................................................................................... 36
Figure 7 Brake power during first 20 s of the custom test at (a) 1865 rpm and (b) 2257 rpm ..... 37
Figure 8 NOx emission at all modes of the custom test ............................................................... 38
Figure 9 NOx emission during NRTC .......................................................................................... 39
Figure 10 NOx emission during first 20 s of the custom test at (a) 1865 rpm and (b) 2257 rpm . 40
Figure 11 PM emission at all modes of the custom test ............................................................... 41
Figure 12 PM emission during NRTC .......................................................................................... 42
Figure 13 PM emission during first 20 s and all modes of the custom test at (a) 1865 rpm and (b)
2257 rpm ....................................................................................................................................... 43
Figure 14 PN emission at all modes of the custom test ................................................................ 44
Figure 15 PN emission during NRTC ........................................................................................... 45
Figure 16 PN emission during first 20 s and all modes of the custom test at (a) 1865 rpm and (b)
2257 rpm ....................................................................................................................................... 46
Figure 17 PN size distribution for oxygenated fuels during the custom test at (a) 1865 rpm and
(b) 2257 ......................................................................................................................................... 47
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Figure 18 Diesel PN size distribution during the first 20 s of the custom test at (a) 1865 rpm and
(b) 2257 rpm, showing from the start point until the maximum overshoot point, and steady-state
averaged value .............................................................................................................................. 48
Figure 19 Mp for PM and PN emissions at load increase modes of the custom test .................... 49
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Figure 1 Schematic of the experimental setup
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Figure 2 NRTC (Non-road Transient Cycle) schedule
0
25
50
75
100
0 400 800 1200
No
rmal
ised
val
ue
(%)
Time (s)
Speed % Torque %
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Figure 3 Custom transient test at (a) 1865 rpm and (b) 2257 rpm
700
1100
1500
1900
2300
0
25
50
75
100
0 25 50 75 100
Sp
eed
(rp
m)
Load
(%
)
Time (s)(a)
700
1100
1500
1900
2300
0
25
50
75
100
0 25 50 75 100
Sp
eed
(rp
m)
Load
(%
)
Time (s)(b)
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Figure 4 Intake air and injected fuel during the first 20 s of the custom test at (a) 1865 rpm and (b) 2257 rpm, with D100
0
0.1
0.2
0.3
0 5 10 15 20
Time (s)
Injected fuel (L/m)
Intake air (kg/s)
(a)
Turbocharger lag
0
0.1
0.2
0.3
0 5 10 15 20
Time (s)
Injected fuel (L/m)
Intake air (kg/s)
(b)
Turbocharger lag
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Figure 5 Engine power at all modes of the custom test
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
30
60
90
120
150
0 3 6 9 12 15
Ind
icat
ed p
ow
er (
kW)
Oxygen content (%)
1865@25 2257@25 1865@50
2257@50 1865@75 2257@75
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Figure 6 Engine power during NRTC
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
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Figure 7 Brake power during first 20 s of the custom test at (a) 1865 rpm and (b) 2257 rpm
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
10
20
30
40
0 5 10 15 20
Bra
ke p
ow
er (
kW)
Time (s)(a)
0
10
20
30
40
0 5 10 15 20
Bra
ke p
ow
er (
kW)
Time (s)(b)
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Figure 8 NOx emission at all modes of the custom test
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
2.5
3
3.5
4
4.5
5
0 3 6 9 12 15
Ind
icat
ed s
pec
ific
NO
x (g
/kW
h)
Oxygen content (%)
1865@25 2257@25 1865@50
2257@50 1865@75 2257@75
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Figure 9 NOx emission during NRTC
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
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Figure 10 NOx emission during first 20 s of the custom test at (a) 1865 rpm and (b) 2257 rpm
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
50
100
150
200
0 5 10 15 20
NO
x (p
pm
)
Time (s)(a)
0
50
100
150
200
0 5 10 15 20
NO
x (p
pm
)
Time (s)(b)
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Figure 11 PM emission at all modes of the custom test
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
0.05
0.1
0.15
0.2
0 3 6 9 12 15
Ind
icat
ed s
pec
ific
PM
(g
/kW
h)
Oxygen content (%)
1865@25 2257@25 1865@50
2257@50 1865@75 2257@75
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Figure 12 PM emission during NRTC
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
1
2
3
4
0 400 800 1200
PM
(m
g/m
^3)
Time (s)
Mean value over cycle:
0.4030.1550.0790.0500.0430.040
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Figure 13 PM emission during first 20 s and all modes of the custom test at (a) 1865 rpm and (b) 2257 rpm
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
0.2
0.4
0.6
0 5 10 15 20
PM
(m
g/m
^3)
Time (s)(a)
0
0.2
0.4
0.6
0 5 10 15 20
PM
(m
g/m
^3)
Time (s)(b)
0
1
2
3
4
0 25 50 75 100
PM
(m
g/m
^3)
Time (s)(a)
0
1
2
3
4
0 25 50 75 100
PM
(m
g/m
^3)
Time (s)(b)
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Figure 14 PN emission at all modes of the custom test
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0.E+00
4.E+14
8.E+14
1.E+15
2.E+15
0 3 6 9 12 15
Ind
icat
ed s
pec
ific
PN
(#
/kW
h)
Oxygen content (%)
1865@25 2257@25 1865@50
2257@50 1865@75 2257@75
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Figure 15 PN emission during NRTC
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
20
40
60
80
0 400 800 1200
PN
(#
/cm
^3)
Time (s)
Millions Mean valueover cycle:
3.0942.5462.7752.6502.2672.261
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Figure 16 PN emission during first 20 s and all modes of the custom test at (a) 1865 rpm and (b) 2257 rpm
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
1.75
3.5
5.25
7
0 5 10 15 20
PN
(#
/cm
^3)
Time (s)
Millions
(a)
0
1.75
3.5
5.25
7
0 5 10 15 20
PN
(#
/cm
^3)
Time (s)
Millions
(b)
0
2
4
6
8
10
0 25 50 75 100
PN
(#
/cm
^3)
Time (S)
Millions
(a)
0
2
4
6
8
10
0 25 50 75 100
PN
(#
/cm
^3)
Time (s)
Millions
(b)
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Figure 17 PN size distribution for oxygenated fuels during the custom test at (a) 1865 rpm and (b) 2257
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions 25% Load
(a)
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions 25% Load
(b)
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions 50% Load
(a)
0
3
6
9
12
1 10 100 1000d
N/d
logD
p (
#/c
m^3
)
Particle size (nm)
Millions 50% Load
(b)
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions 75% Load
(a)
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions 75% Load
(b)
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Figure 18 Diesel PN size distribution during the first 20 s of the custom test at (a) 1865 rpm and (b) 2257 rpm, showing from the start point until the maximum overshoot point, and steady-state averaged value
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions PeakSteady-State7654321
(a)
0
3
6
9
12
1 10 100 1000
dN
/dlo
gDp
(#
/cm
^3)
Particle size (nm)
Millions PeakSteady-State987654321
(b)
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Figure 19 Mp for PM and PN emissions at load increase modes of the custom test
0% O2 6.02% O2 10.93% O2 12.25% O2 13.57% O2 14.23% O2
0
10
20
30
40
0 3 6 9 12 15
PM
-Mp
(%
)
Oxygen content (%)
1865@50 2257@50
1865@75 2257@75
0
10
20
30
40
50
0 3 6 9 12 15
PN
-Mp
(%
)
Oxygen content (%)
1865@50 2257@50
1865@75 2257@75
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1. A custom test was designed to investigate transient engine operation.
2. Waste cooking biodiesel (primary fuel) and triacetin (highly oxygenated additive) were used.
3. Turbocharger lag caused an overshoot in NOx, PM and PN emissions.
4. Oxygenated fuels showed higher PN and lower PM overshoots compared to diesel.
5. Oxygenated fuels emitted smaller particles during transient compared to diesel.
