Post on 09-May-2022
Review Article
Emissions of automobiles fueled with alternative fuelsbased on engine technology: A review
Yisong Chen, Jinqiu Ma, Bin Han, Peng Zhang, Haining Hua, Hao Chen*, Xin Su
School of Automobile, Chang'an University, Xi'an 710064, China
h i g h l i g h t s
� Emissions of automobiles fueled with main alternative fuels are reviewed.
� Emissions of NG/gasoline bi-fuel, NG and NG/diesel dual fuel engines are analyzed.
� Emissions of SI engines fueled with oxygenated fuels are analyzed.
� Emissions of CI engines fueled with oxygenated fuels are analyzed.
a r t i c l e i n f o
Article history:
Received 20 February 2018
Received in revised form
18 May 2018
Accepted 21 May 2018
Available online 29 July 2018
Keywords:
Natural gas
Methanol
Ethanol
Biodiesel
PODEnEmission
a b s t r a c t
Diversification of alternative fuels for automobiles is not only an actual situation, but also a
development trend. Whether the alternative fuels are clean is an important issue. Emis-
sions of automobiles fueled with natural gas (NG), methanol, ethanol, biodiesel, dimethyl
ether (DME) and polyoxymethylene dimethyl ethers (PODEn) are investigated and reviewed
based on engine technology and fuel properties. Compared to gasoline, NG/gasoline bi-fuel
and NG automobiles have higher brake thermal efficiencies (BTE) and produce less HC, CO
and PM emissions, while more NOx emission. Compared to diesel, NG/diesel dual fuel
automobiles have lower BTE and emit lower soot and NOx emissions, but higher HC and CO
emissions. Methanol and ethanol blending in gasoline can obviously reduce the HC, CO and
PM emissions of spark ignition (SI) automobiles. Methanol or ethanol blending in diesel
may prolong the ignition delay, shorten the combustion duration and improve the BTE,
resulting in lower soot emissions. However, the HC, CO and NOx emissions of methanol or
ethanol diesel blend fuels are uncertain due to low cetane number, high latent heat of
vaporization. Most biodiesel has higher viscosity, distillation temperature, cetane number
and oxygen content than diesel. Soot emission of biodiesel is lower than that of diesel,
while NOx emission is higher. Both DME and PODEn do not contain CeC bonds and their
blend with diesel can prohibit the formation of soot. PODEn has high cetane number and
low viscosity, resulting in better ignitability and spray quality respectively. PODEn blending
shortens both the ignition delay and the combustion duration, improves the BTE, and in-
creases the temperature in the diffusion combustion phase, leading to a higher NOx
emission.
© 2018 Periodical Offices of Chang'an University. Publishing services by Elsevier B.V. on
behalf of Owner. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
* Corresponding author. Tel.: þ86 29 82334471.E-mail addresses: chenyisong_1998@163.com (Y. Chen), colen7680@126.com (H. Chen).
Peer review under responsibility of Periodical Offices of Chang'an University.
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier .com/locate/ j t te
j o u r n a l o f t r a ffi c and t r an s p o r t a t i o n e n g i n e e r i n g ( e n g l i s h e d i t i o n ) 2 0 1 8 ; 5 ( 4 ) : 3 1 8e3 3 4
https://doi.org/10.1016/j.jtte.2018.05.0012095-7564/© 2018 Periodical Offices of Chang'an University. Publishing services by Elsevier B.V. on behalf of Owner. This is an openaccess article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Since opening-up policy was implemented, China has expe-
rienced dramatic development, with averaged 9.8% annual
growth rates of gross domestic product (GDP), in comparison
with the world's average of 3.3% (Xu et al., 2017). Problems of
the world environment like global warming and energy
resources will cause great constraint on the world economy
and may affect profoundly the basic condition of human
survival. As an important vehicle, automobile play an
important role in people's daily life and commercial
activities. Besides, automobile industry is in an up road of
the overall industry system (He et al., 2017).
Traditional automobile fuels derived from non-renewable
fossil oil. Automobiles emit huge amounts of pollutants and
greenhouse gases (GHG), such as NOx, PM, COHC and CO2, and
exert great influence on atmosphere environment and global
warming. To protect the environment and ease the climate
change, the automobile technology worldwide tends to
develop in the directions of energy diversification and power
electrification. Electrification of automobiles has been
considered as the most effective way and the ultimate solu-
tion. However, even if the technologies of power battery,
electric motor and electronic control continuously improve,
two significant problems still exist. One is that the power
structure is inadequate and the other is the power constraint
if electric automobiles are widely applied. Diversification of
clean alternative fuels for automobiles is regarded as the best
choice and path in the transitional period between petroleum
fuels and electrification.
In general, alternative fuels include fossil and renewable
fuels. They can also be classified into gaseous and liquid
alternative fuels. Specifically, liquefied petroleum gas (LPG),
natural gas (NG), alcohols mainly involving methanol and
ethanol, ethers mainly including dimethyl ether (DME) and
polyoxymethylene dimethyl ethers (PODEn), and biodiesel are
in localized or industrial application.
Gaseous fuels such as compressed natural gas (CNG) are
promising alternative fuels which receive more attention all
over the world. Natural gas is a very promising and highly
attractive fuel because of its domestic availability, widespread
distribution infrastructure, low cost, and clean-burning qual-
ities to be used as a transportation fuel (Wei and Peng, 2016).
In that case, CNG is considered to be a “cleaner” fuel
compared to other fossil fuels. Therefore, it is used as an
alternative fuel in motor vehicles to reduce emissions of air
pollutants in transportation (Wang et al., 2016a). Besides, it
is also applied in gasoline engine as a bi-fuel, added to diesel
fuel and mixed with hydrogen to make better engine and
emission performance. Generally, there are three application
modes of natural gas, which are NG (CNG/LNG/HCNG),
diesel/CNG dual fuel and CNG/gasoline bi-fuel.
Among alternative fuels for gasoline, methanol (CH3OH)
fuel has been considered to be one of the most favorable fuels
for internal combustion (IC) engines due to the high octane
number and the high intramolecular oxygen content (Agarwal
et al., 2014; Li et al., 2010a; Zhen et al., 2013). A large number of
domestic and foreign scholars have studied the application of
methanol on internal combustion engine: it can be used with
different application modes (mixed or pure methanol) in a
spark-ignition (SI) engine (Agarwal et al., 2014; Gravalos
et al., 2013; Lennox et al., 2014; Liu et al., 2007; Zhen and
Wang, 2015) or with dual-fuel mode in a compression-
ignition (CI) diesel engine (Pan et al., 2015; Park et al., 2017;
Wang et al., 2008a; Wei et al., 2015). Ethanol and methanol
have similar physical and chemical properties, is considered
to replace fossil fuels of environmentally friendly fuel,
which can be blended with other fuels at different
proportions to improve engine emissions (Battal et al., 2017;
Oh et al., 2010; Shi et al., 2015; Turner et al., 2011).
The chemical formula of dimethyl ether (DME) fuel is
CH3OCH3; it is the simplest ether compound. Among alter-
native fuels, the application of DME for diesel engines has
been discussed by many investigators because it has no
carbonecarbon bonds and excellent self-ignition character-
istics compared to other fuels. Meanwhile, the cetane number
of DME fuel is significantly higher than that of conventional
diesel fuel (Park, 2012; Park and Lee, 2013; Roh et al., 2015;
Semelsberger et al., 2006; Youn et al., 2011). Due to its fuel
characteristics, DME is particularly suitable to be used as a
complete substitute for diesel in compression ignition (CI)
engines. It can be used as a blend with diesel to overcome
limitations of using pure DME, such as poor viscosity, low
density and its after-effects. DME could also be used as an
additive or as an ignition promoter in conventional diesel
combustion and for dual fuel operation (Lee et al., 2011; Su
et al., 2016; Thomas et al., 2014; Wang et al., 2013, 2014). In
recent years, it has been found that the mixture of polyoxy-
methylene dimethyl ethers (PODEn) is a promising engine fuel.
Chemical expression of PODEn is CH3O(CH2O)nCH3. As an
oligomer of ether, PODEn has a higher cetane number and
oxygen content and does not contain CeC bonds, which has
the potential to reduce diesel smoke and PM emissions (Lei
et al., 2009; Liu et al., 2016b, 2017a; Pellegrini et al., 2012), and it
can bemixed with diesel in any proportion (Burger et al., 2010;
H€artl et al., 2015; Liu et al., 2016a; Pellegrini et al., 2013). In fact,
n from 3 to 8 of PODEn has an excellent performance for diesel
additives (Stroefer et al., 2010; Zhao et al., 2013). At room
temperature, the PODEn-diesel mixture has excellent stability
(H€artl et al., 2015; Pellegrini et al., 2012; Zhao et al., 2013).
Generally, it can be used as a clean diesel blending compo-
nents due to its diesel similar physical properties and do not
need transformation of vehicle engine oil supply system (Liu
et al., 2017b; Xie et al., 2017).
Biodiesel, also called fatty acid methyl esters, is mainly
made from vegetable oils or animal fats and is an ideal alter-
native fuel for diesel engines. Compared to petroleum diesel,
biodiesel has higher cetane number, about 10% intra-
molecular oxygen, almost no aromatic hydrocarbon and sul-
fur. This leads to different fuel injection and spray properties,
combustion characteristic, and exhaust emissions from pe-
troleum diesel fuel (Hasan and Rahman, 2017; He et al., 2007;
Lou and Tan, 2016; Miri et al., 2016; Xu et al., 2012). But there
are some disadvantages of biodiesel which restrain its wide
application and hinder its use as a complete replacement for
diesel, which include higher kinematic viscosity, freezing
temperature and density, as well as its low calorific value.
Viscosity of biodiesel not only affects flow at all temperatures
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334 319
a fuel may be exposed to but also strongly influences the at-
omization of a fuel upon injection into the combustion
chamber and ultimately the possible formation of engine de-
posits (Knothe and Gazon, 2017). That is why many scientists
and investigators have studied blends of biodiesel with diesel
by varying the proportions of biodiesel and diesel to
investigate their suitability as a fuel in existing diesel
engines. These problems associated with biodiesel can be
overcome by using biodiesel-diesel blends (Nair et al., 2016;
Sun et al., 2010; Yasin et al., 2015; Yusaf et al., 2011).
2. Emissions of NG automobiles based onengine technology
2.1. Pure NG (CNG/LNG)
In general, NG vehicles have lower emissions compared to
gasoline or diesel engines (Kakaee and Paykani, 2013).
Specifically, a decreasing trend is found for PAHs, SO2 and
CO concentrations, while the NOx level is increased in
comparison to those before the implementation of CNG
(Ravindra et al., 2006). Due to the high octane number of NG,
engines can be operated with a higher compression ratio
(CR) for a better thermal efficiency (Liu et al., 2012). CRs of
NG engines are commonly designed in the range of 11e13,
which are higher than those of gasoline engines. CRs and
application modes of NG (CNG/LNG/HCNG) sole fuel, diesel/
CNG dual fuel and CNG/gasoline bi-fuel engines are shown
in Fig. 1.
It has been reported that the engine performance and
emission are greatly affected by varying compositions of
natural gas (Fig. 2). Themost important NG fuel property is the
Wobbe number (WN). Generally, it was agreed by researchers
that the fuels with higher hydrocarbons, higher WN, and
higher energy content exhibited better fuel economy and
carbon dioxide (CO2) emissions. NOx emissions were also
increased for gases with higher levels of higher WN, while
total hydrocarbons (THCs) and CO showed some reductions
(Kakaee et al., 2014). The results indicate that higher WN
improves the combustion and the efficiency as well.
Lean burn is an effective way to decrease the NOx emis-
sions, while it results in high cyclic variation. Dilution is
another method to achieve lean burn and low NOx emissions.
Common dilution gases are N2, CO2 and Ar. The results show
that the thermal efficiency first increases and then decreases
as the dilution ratio (DR) of Ar increases and NOx emissions
decrease significantly (Li et al., 2015). Ar dilution is superior in
maintaining higher thermal efficiencies than CO2 and N2 for
NG engines (Li et al., 2015). Lean burn as well as EGR
successfully satisfied the legal emission regulation when the
level of dilution was increased to the dilution limit, although
there was a slight reduction in efficiency (Lee et al., 2014).
2.2. NG/diesel dual fuel
Natural gas/diesel dual fuel is an operation mode in which
natural gas is introduced into the intake air of the inlet
manifold and then ignited by the direct injected diesel in the
cylinder. This mode has both economic and environmental
benefits. Due to the high auto-ignition temperature, NG can
hardly be burned through compression on diesel engines. In
dual fuel mode, diesel acts as the ignition source and NG
provides themain energy if needed for combustion. Generally,
dual fuel engine exhibited longer ignition delay than diesel;
had lower thermal efficiency than diesel at low and partial
loads and higher at medium and high loads; emitted less NOx
emissions than diesel engine, while more HC and CO emis-
sions (Abdelaal et al., 2013; Abdelghaffar, 2011; Cheenkachorn
et al., 2013).
The application of dual fuel mode significantly decreases
the NOx, CO2 and PM emissions (Cheenkachorn et al., 2013; Liu
et al., 2013; Lounici et al., 2014; Meng et al., 2016), compared to
diesel engines. The trade-off relationship between NOx and
PM emission is solved. However, the hydrocarbon (HC) and
carbon monoxide (CO) emissions may increase by several
times in comparison to normal diesel combustion (Liu et al.,
2013). The brake thermal efficiency (BTE) of dual fuel mode
is lower at low and intermediate loads, while under high
engine load conditions it is similar or a little higher when
compared with normal diesel mode (Lounici et al., 2014).
Dual fuel mode showed a simultaneous reduction of soot
and NOx species over a large engine operating area (Lounici
et al., 2014; Meng et al., 2016). In sum, trade-off relationship
between soot and NOx emissions of diesel engines can be
solved by using dual fuel mode, whereas the BTE of engine
decreases and HC emission increases.
Nithyanandan et al. (2016) found that the use of CNG
affects the morphology and nanostructure of PM, and hence
Fig. 1 e CRs and application modes for NG automobiles
with different engine modes.
Fig. 2 e Typical natural gas composition by volume (Kakaee
et al., 2014).
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334320
the oxidation reactivity of the soot. In Singh's study (Singh
et al., 2016), CMD (count mean diameter) graph showed that
average size of particulate emitted CNG engines were much
smaller compared to mineral diesel particulate. The addition
of compressed natural gas lowers CO2 emission and
decreases opacity of exhaust gases in all load modes, the
best positive impact has been achieved with the highest
CNG portions (Vygintas et al., 2017). Liu et al. (2015a,b) found
that the developed dual fuel model is capable to predicate
the flame propagation and emissions formation process in
the dual fuel engine. Flame quench region of the fuel-lean
mixture within the squish volume is the dominant source of
CO emissions a low engine speed condition. However, bulk
gas complete oxidation is impeded by the failure to
transition into strong high temperature combustion in the
cylinder center region, which accounts for the majority of
CO emissions at high engine speed. NO formation region
follows the development of the high temperature field for
both low and high engine speed, which is generated by the
combustion of the pilot diesel. Therefore, the injection
strategy and quantity of the pilot fuel significantly
determine the final exhaust NOx emissions during dual fuel
operation conditions.
On the whole, NG/diesel dual fuel automobiles simulta-
neously reduce CO2, NOx and PM emissions compared to
diesel ones, while increase the HC and CO emissions.
2.3. HCNG
Laminar burning velocity of NG is lower than that of methane,
which is 48 cm/s (Korb et al., 2016). The velocity of H2 is
290 cm/s and accordingly its addition in NG will surely
accelerate the combustion speed, shorten the combustion
duration and hence improve the thermal efficiency. Higher
efficiency is generally correlated with higher NOx emission,
lower HC and CO emissions, and lower brake specific fuel
consumption (BSFC). Compared to NG, HNG present higher
peak combustion pressure and combustion temperature,
and more concentrated heat release as shown in Fig. 3 (Korb
et al., 2016).
Mathai et al. (2012) made a comparative evaluation of
performance, emission, lubricant and deposit characteristics
of spark ignition engine fueled with CNG and 18% hydrogen-
CNG and found that HCNG fueled engine decreased BSFC,
CO and HC emissions with the increase of NOx emission.
Another study showed the effects of 0, 5%, 10% and 15%
blends of hydrogen by energy with CNG on bi-fuel NA SI
engine using SPFIS (Nitnaware and Suryawanshi, 2016). MBT
spark timing shown improvement in performance
parameters with reduction in NOx emission. Carbon based
emission reduced and NOx emission increased with increase
in hydrogen addition. The optimum maximum brake torque
(MBT) spark timing of 25�CA BTDC and injection pressure
2.6 bar is observed for 5% hydrogen addition at 2500 rpm.
Zareei et al. (2012) indicated that thermal efficiency,
combustion performance, NOx emissions improved with the
increase of hydrogen addition level. The HC and CO
emissions first decrease with the increasing hydrogen
enrichment level, but when hydrogen energy fraction
exceeds 12.44%, it begins to increase again at idle and
stoichiometric conditions.
In sum, NG automobiles with different types of engines
have different environmental effects, summarized in Fig. 4. As
a whole, improvement of BTE brings high NOx emission and
low HC, CO and PM emissions for SI engines. NG/diesel dual
fuel engines are modified on diesel engines and the
condition is complicated. Both NOx and PM emissions of
dual fuel engines are reduced compared to diesel. The
decrease of BTE of dual fuel engine causes high HC and CO
emissions.
3. Emissions of automobiles fueled withalcohols based on engine technology
The most common alcohols used on automobiles are meth-
anol and ethanol. Althoughmethanol can be produced from a
wide variety of renewable sources and alternative fossil fuel
based feedstocks, in practice methanol is mainly produced
from natural gas and in China from coal (Chen et al., 2014;
Sayah and Sayah, 2011; Vancoillie et al., 2013). Fuel ethanol
can be produced from both edible feedstocks such as corn,
wheat and stale grain and non-edible crops such as cassava
and sweet sorghum.
Fig. 3 e Comparison of combustion characteristics between NG and HNG (Korb et al., 2016). (a) In-cylinder pressure and
mean temperature. (b) Mass fraction burned and ROHR.
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334 321
Methanol is a colorless, polar and flammable liquid which
has higher octane number and heat of vaporization values as
compared to gasoline (Zhen and Wang, 2015). It is known that
ethanol and methanol have higher laminar flame speed,
higher octane number and higher intramolecular oxygen
contents than those of gasoline. Accordingly, the combustion
duration will be surely shortened. Lennox et al. (2014) and
Agarwal et al. (2014) studied the effects of methanol/gasoline
blends on engine performance, combustion and emission
characteristics, and compared with pure gasoline. The result
showed that methanol/gasoline blends can reduce the
combustion duration and exhaust gas temperature, increase
the peak heat release rate (PHRR), and increase the BTE.
Wu et al. (2016) experimentally investigated the effects of
pure methanol on the combustion performance under idle
condition based on a SI engine. The results showed that the
SI engine fueled with methanol illustrated better lean burn
performance than the engine fueled with gasoline. Compared
with the engine fueled with gasoline, the indicated thermal
efficiency (ITE) of engine fueled with methanol was
increased; the flame development and propagation periods
were shortened. On the whole, the combustion duration is
shortened and the heat release is concentrated when
alcohols are blended in gasoline. As a result, the combustion
process is improved. Although the BTEs of alcohols/gasoline
blends increase, engine power and torque decrease with the
increase fraction of alcohol, due to the low heating value
(Liu et al., 2007). Using methanol/gasoline blends on a spark-
ignition engine can significantly reduce CO and HC emissions
(Agarwal et al., 2014; Gravalos et al., 2013; Lennox et al., 2014;
Liu et al., 2007; Wang et al., 2015). Battal et al. (2017), Turner
et al. (2011) and Oh et al. (2010) investigated the effects of
ethanol/gasoline mixtures on the performance and emissions
at a spark-ignition engine. It is shown that the benefits of
adding ethanol into gasoline are reduced combustion
duration and increased in-cylinder pressure and combustion
efficiency. Experiments and theoretical calculations showed
that ethanol added fuels show reduction in CO, CO2 and NOx
emissions without significant loss of power compared to
gasoline. But it was measured that the reduction of the
temperature inside the cylinder increases HC emission.
N2 þ O 4 NO þ N (1)
N2 þ O2 4 NO þ O (2)
N þ OH 4 NO þ H (3)
As for NOx emission, the condition is extremely compli-
cated. In general, thermal NOx, prompt NOx, and fuel NOx are
the three formation processes. Thermal formation is repre-
sentative when temperatures are high and the relative air to
fuel ratio is close to 1. The reactions that take part on this
mechanism were described firstly by Zeldovich (1946) and
later extended by Bowman et al. (1975), described from Eq.
(1) to Eq. (3). Agarwal et al. (2014) indicated that gasohol
produced lower mass emissions of NO and smoke opacity.
The result was attributed to the higher latent heat of
vaporization of methanol compared to gasoline. Particulate
size-number concentration was lower for gasohol blends in
comparison to gasoline at all engine operating conditions.
Wang et al. (2015) indicated that evident decrease in NOx
emission was noticed with M15 and M25 fueling, but in the
case of M40, NOx emissions were similar with gasoline.
Mustafa et al. (2013) investigated the combustion and
exhaust emissions characteristics of a SI engine fueled with
the ethanol/gasoline (E5, E10) and methanol/gasoline (M5,
M10) fuel blends. NOx emissions decreased for all wheel
Fig. 4 e Comparison of emissions for NG vehicles.
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334322
powers at the speed of 80 km/h. It has been also observed that
the usage of alcohol fuel instead of gasoline caused to
decrease the NOx, and to increase CO2 emission because of
the improved and completed combustion (Balki et al., 2014;
Mustafa and Balki, 2014; Wu et al., 2016).
The increase of methanol increases the formaldehyde
emissions andmethanol emission increases with the increase
of engine load under different speeds (Liu et al., 2007; Wang
et al., 2015). Injection and ignition parameters also have sig-
nificant influence on the combustion and emission of engines
fueled with alcohol/gasoline blends. Advancing methanol in-
jection timing decreased the HC and CO, while increased the
NOx emission (Qu et al., 2015). Retarding ignition timing
decreased the HC and NOx emissions and the effect of
ignition timing changes on CO emission is small (Qu et al.,
2015). The HC, CO, and NOx emissions of rich mixture are
higher than those of lean mixture. Increasing intake air
temperature decreased the HC and CO emissions. Retarding
methanol injection timing, advancing ignition timing, using
lean mixture and reducing intake air temperature can
decrease the formaldehyde emission (Li et al., 2010b).
Alcohols can also be applied on diesel engines through two
methods: dual fuel mode or emulsification/micro-emulsifica-
tion. For dual fuel mode, methanol is mainly used. Compared
with conventional diesel, the methanol/diesel dual fuel com-
bustion mode significantly reduces NOx and PM emissions
(Pan et al., 2015; Park et al., 2017; Wang et al., 2008a; Wei et al.,
2015). When the amount of methanol is increased, cylinder
pressure and temperature decreased. The resulting decrease
in the combustion efficiency lowered the NOx emission and
BTE of the engine. Wang et al. (2008a) found that the
increase in methanol mass fraction lowers the polytropic
index of compression and the temperatures at BDC and
TDC, as well as the oxygen concentration in the mixture.
This prolongs the ignition delay under the same engine load
and speed condition by comparison with diesel operation.
The heat release rate changes from dual-peak mode to
single-peak mode. The high methanol mass fraction will
realize a simultaneous reduction in both smoke and NOx
under all the operating conditions. Meanwhile, the NOx-
smoke trade-off curve disappears in combustion of the dual
fuel, but CO and HC increase. Wei et al. (2015) investigated
the combustion and emission characteristics of a dual fuel
diesel engine with high premixed ratio of methanol (PRM).
High PRM prolonged the ignition delay but shortened the
combustion duration and decreased the in-cylinder gas
temperature. Both NOx and dry soot emissions were
significantly reduced, while HC, CO, formaldehyde emissions
and NO2 in NOx increased significantly with the increase of
PRM (Wei et al., 2015). Pan et al. (2015) and Geng et al. (2014)
found that there was a strong coupling between the intake
air temperature and the methanol fraction to performance
and emissions of the engine. At dual fuel operation mode,
decreasing intake air temperature reduced the indicated
thermal efficiency (ITE) and exhaust gas temperature and
the trend was more evidently as methanol energy fraction
increased. Decreasing of intake air temperature also
prolonged the ignition delay, which caused a later
combustion phasing and smaller peak cylinder pressure. By
the induction of methanol, NOx, NO and smoke emissions
decreased markedly, while NO2, CO, THC, formaldehyde and
methanol emissions increased. However, increasing the
intake air temperature would inhibit the NO2, THC, CO,
formaldehyde and methanol emissions and increase NO,
NOx and soot emissions. Overall, methanol/diesel dual-fuel
combustion performs better in terms of engine performance
and emissions reduction under rich mixture conditions
(Amin et al., 2015). Chen et al. (2017b) and Li et al. (2016)
investigate the effects of diesel injection parameters on the
rapid combustion and emissions of the diesel-methanol
dual-fuel engine. The experimental results show that the
diesel injection parameters affect rapid combustion fraction
(a) greatly, which increases as the diesel injection pressure
rises while decreases as the diesel injection timing advanced
or diesel injection quantity increases. Liu et al. (2015a,b)
indicated that at low injection pressure, the IMEP of dual
fuel mode is lower than that of pure diesel combustion
mode. COVIEMP of dual fuel mode firstly decreases and then
increases with the increasing of injection pressure. Their
researches shows both of NOx and smoke emissions are
reduced while CO and HC emissions increased obviously in
dual fuel mode (Liu et al., 2015a, b). Smoke emission can be
further reduced by coupling the high diesel injection
pressure and the advanced diesel injection timing. However,
NOx emission may be increased in this case (Park et al.,
2017). The methanol co-combustion ratio (CCR) is defined as
the ratio of methanol energy to the total energy in the dual-
fuel mode. qin is the injection timing. Comparison between
emissions of diesel/methanol dual fuel (DMDF) engine and
normal diesel is presented in Fig. 5(a) and (b).
Fig. 5 e Comparison of emissions between diesel and DMDF (Li et al., 2016). (a) CO and HC emissions. (b) NOx and soot
emissions.
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334 323
Alcohols can hardly be mixed with diesel, because the al-
cohols are polar solvents and diesel is non-polar fuel. Cosol-
vents and surfactants must be used to help the mixing of
diesel and alcohols, forming emulsified fuel or micro-emul-
sified fuel. Emulsified fuel is in the condition of milkiness and
opaque, and the mixing state is unstable. Micro-emulsified
fuel is transparent and stable. Some studies confirmed that
diesel/biodiesel blend fuels canmixedwith a low volume ratio
alcohols, forming micro-emulsified fuel. Further studies
(Aydin et al., 2015, 2017; Shi et al., 2015) have found that bio-
diesel/ethanol/diesel fuel blends can be directly used on a
diesel engine for lower PM and THC emissions. However, a
major drawback is that ethanol is immiscible in diesel over a
wide range of temperatures and water content because of
their difference in chemical structure and characteristic,
these can result in fuel instability (Prommes et al., 2007).
Both methanol and ethanol can be applied on SI or CI en-
gines. The ultimate aim is to improve the oxygen content and
thus the combustion efficiency. As a result, HC, CO and PM
emissions are reduced. Methanol/diesel dual fuel engine is an
exceptional case, which is similar with NG/diesel dual fuel.
The BTE of methanol/diesel dual fuel engine declines
compared to diesel, producing lower NOx and soot emissions
and higher HC and CO emissions.
4. Emissions of automobiles fueled withbiodiesel based on engine technology
Most researchers reported that use biodiesel in a conventional
diesel engine brings about a considerable reduction in CO, CO2
and PM (Hasan and Rahman, 2017; He et al., 2007; Lou and Tan,
2016; Miri et al., 2016; Xu et al., 2012). Feedstocks of biodiesel
are very abundant and they can be classified into three gen-
erations. The first generation refers to the edible crops such as
soybean, rapeseed and palm oil, the second includes oil plants
such as jatropha curcas, pistacia chinensis and sapium sebi-
ferum, and the third is microalgae such as Scenedesmus
obliquus. Production principle can be summarized into a
common formula, shown in Fig. 6. Abundant feedstocks result
in significant difference in chemical composition of biodiesel
and thus its properties. Table 1 lists the composition and
properties of some typical biodiesel (Jain and Sharma, 2011;
Serrano et al., 2013; Wang et al., 2012). Biodiesel mainly
contained the same five components: methyl palmitate
(C17H34O2), methyl stearate (C19H38O2, C18:0), methyl oleate
(C19H36O2, C18:1), methyl linoleate (C19H34O2, C18:2) and
methyl linolenate (C19H32O2, C18:3) (Shi et al., 2018). The
molecular structure of the above components is exhibited in
Fig. 7. Cetane number of methyl palmitate, methyl stearate,
methyl oleate, methyl linoleate and methyl linolenate are
respectively 86, 101, 59, 38 and 23. It can be concluded that
the higher the mass fraction of saturated fatty acid methyl
esters (S-FAMEs) in biodiesel is, the higher the cetane
number is and the better the oxidative stability is, while the
worse the low temperature fluidity is. To ensure the basic
use on diesel engines, the low temperature fluidity of
biodiesel must be guaranteed and as a result the mass
fraction of unsaturated FAMEs (U-FAMEs) is generally higher
than 70% by weight. Consequently, cetane number of
biodiesel in common use is a little higher than that of diesel,
which contributes to a better ignitability. Fig. 8 presents the
heat release and in-cylinder temperature of a common rail
diesel engine fueled with biodiesel and diesel. Biodiesel has
higher viscosity and distillation temperature, leading to a
poor homogeneity of mixture gas and atomization quality.
Consequently, the HRR of biodiesel in the pre-mixed
combustion phase is lower than that of diesel, and so does
the PHRR. With the proceeding of combustion, biodiesel
produces more active radicals and accelerates the speed of
chemical reaction, resulting in higher HRR compared to
diesel in the diffusion combustion phase. As a result, the in-
cylinder temperature of biodiesel is higher and thus the NOx
emission.
Due to the high cetane number, SOC of biodiesel advances
and the ignition delay shortens. In-cylinder temperature of
biodiesel is slightly higher than that of diesel (shown in phase Ⅰ
from A to B, Fig. 8). Lower LHV, higher viscosity and lower
volatility commonly contribute to the slower combustion
and hence the lower PHRR and combustion temperature of
biodiesel, compared to diesel (shown in phase Ⅱ from B to C,
Fig. 8). It can be concluded that the intensity of diffusion
combustion for biodiesel is obviously higher than that of
diesel, leading to the higher PCT and combustion
temperature (shown in phase Ⅲ from C to D, Fig. 8). It is
mainly because that the intramolecular oxygen increases
the OH intensity of biodiesel along the injector axial line and
the active radicals surely accelerate the combustion speed.
Fast combustion promotes the complete combustion of
biodiesel and the combustion duration shortens. As a result,
the in-cylinder temperature of biodiesel is lower than that of
diesel, as shown in the phase Ⅳ (Fig. 8).
There aremany researches find that rapeseed biodiesel and
its blends have higher cetane number, increased oxygen con-
tent, higher density and viscosity, but inferior lower heating
value, and compressibility when compared to diesel fuel. Miri
et al. (2016), Aldhaidhawi et al. (2017) and Ismet et al. (2012)
investigated the performance, combustion and emission
Fig. 6 e Chemical reaction formula for biodiesel production.
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334324
Table
1e
Com
positionofbiodiese
lderivedfrom
threegenera
tionfeedstock
s.
Com
position
w(%
)Vern
icia
ford
iiIdesia
polyca
rpa
Sapium
sebiferu
mSce
nedesm
us
obliquus
Xanth
oce
ras
sorb
ifolia
Arm
eniaca
sibirica
Soybean
Jatropha
curcas
Pistacia
chinensis
Rapese
ed
Elaeis
guineen
sis
C14:0
//
//
/0.03
0.30
//
0.10
1.00
C16:0
3.56
15.50
7.21
18.42
5.27
3.79
10.90
19.75
23.14
5.10
44.80
C16:1
/6.65
/2.31
/0.67
//
0.99
/0.30
C16:2
//
/3.26
//
//
//
/
C18:0
2.62
1.39
2.28
3.43
1.92
1.01
3.20
4.63
1.18
2.10
3.80
C18:1
10.57
9.53
14.61
49.64
31.17
65.23
24.00
46.83
44.35
57.90
39.90
C18:2
14.64
64.81
31.72
11.30
44.47
28.92
54.50
28.50
28.51
24.70
9.28
C18:3
59.20
2.08
41.02
8.26
6.46
0.14
6.80
0.01
0.84
7.90
0.22
C20:0
//
//
0.20
0.09
0.10
0.17
0.10
0.20
0.35
C20:1
0.90
0.04
//
/0.12
//
/1
/
C20:3
//
//
7.27
//
//
//
Oth
ers
8.51
//
3.38
3.24
/0.20
/0.89
/0.03
S-FAMEs
6.18
16.89
9.49
21.85
8.00
4.92
14.50
24.66
24.42
7.50
50.27
U-FAMEs
93.61
83.11
90.51
74.77
92.00
95.08
85.30
75.34
74.82
91.50
49.70
Cetane
number
37.0
45.0
43.0
46.0
47.6
48.8
/51.0
51.30
/61.0
CFPP
�11
�5�1
1�6
�8�1
4/
0�3
/12
Oxidative
stability
0.4
0.7
0.8
1.2
1.7
2.7
2.9
3.3
4.2
4.6
7.7
Ref.
(Wangetal.,
2012)
(Wangetal.,
2012)
(Wangetal.,
2012)
(Serranoetal.,
2013)
(Wangetal.,
2012)
(Wangetal.,
2012)
(Serranoetal.,
2013)
(Jain
andSharm
a,
2011)
(Wangetal.,
2012)
(Serranoetal.,
2013)
(Wangetal.,
2012)
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334 325
characteristics of a diesel engine fueled with rapeseed
biodiesel and its blends and compared with pure petroleum
diesel fuel. Researches results reveal that rapeseed biodiesel,
either pure or blended with diesel, has lower heat release
rate, reduced ignition delay and lower thermal efficiency.
Meanwhile, effective power and torque decrease at all engine
loads. Regarding gaseous emissions, rapeseed biodiesel
increase NOx emissions, other emissions such as those of CO
and particulate matter PM are usually found to significantly
decrease with rapeseed biodiesel content.
Qi et al. (2009, 2010) and €Ozener et al. (2014) investigated
the effect of biodiesel produced from soybean crude oil on
the combustion characteristics, performance and exhaust
emissions of a diesel engine. The results showed that
biodiesel exhibited the similar combustion stages to that of
diesel, however, biodiesel showed an earlier start of
combustion. At lower engine loads, the peak cylinder
pressure, the peak rate of pressure rise and the peak of heat
release rate during premixed combustion phase were higher
for biodiesel than for diesel. At higher engine loads, the peak
cylinder pressure of biodiesel was almost similar to that of
diesel, but the peak rate of pressure rise and the peak of
heat release rate were lower for biodiesel. The power output
of biodiesel was almost identical with that of diesel. It is also
observed that there is a significant reduction in CO and
smoke emissions at high engine loads, while NOx emissions
increased. Moreover, biodiesel provided significant reduction
in CO, HC, NOx and smoke under speed characteristic at full
engine load (Qi et al., 2009). €Ozer et al. (2016) indicated that
the maximum heat release rate and maximum in-cylinder
pressure were mostly increased with the combined effects of
biodiesel fuel addition and EGR application. Improvements
on the THC emissions were obtained by the use of 20 vol.%
soybean biodiesel fuel blend and increase of EGR rate at the
low and partial engine loads, while deterioration occurred at
the high engine load when the EGR was increased to over 5%
rate.
Wei et al. (2017) investigated the influence of waste cooking
oil (WCO) biodiesel on the combustion, emissions
characteristics of a diesel engine. The increase of in-cylinder
pressure is mainly due to the advanced start of combustion
and more complete combustion when using biodiesel. Lower
maximum heat release rate is due to the less intense
combustion in the premixed combustion phase. Earlier start
of combustion is mainly attributed to the higher bulk
modulus and higher viscosity of biodiesel. Biodiesel reduces
the weighted particle mass concentration and the weighted
geometric mean diameter of the particles. Enweremadu and
Rutto (2010) found that the engine performance of the WCO
biodiesel and its blends were only marginally poorer
compared to diesel. From the standpoint of emissions, NOx
emissions were slightly higher while HC emissions were
lower for WCO biodiesel when compares to diesel fuel.
Compared with other vegetable oils and petroleum diesel
fuels, palm oil is associated with better engine performance
and shorter ignition delay. Use of palm oil also reduces
exhaust emission of HC, CO and smoke and exhaust gas
temperatures, while significantly improve levels of NOx
(Leevijit et al., 2017; Mosarof et al., 2015; Ndayishimiye and
Tazerout, 2011).
However, some researches showed that the addition of
biodiesel reduces NOx emissions and increases in soot
Fig. 7 e Chemical structure of five major components in
biodiesel.
Fig. 8 e Comparison of heat release and in-cylinder temperature between diesel and biodiesel. (a) Heat release. (b)
Temperature.
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334326
emissions (Nair et al., 2016; Yasin et al., 2015; Yusaf et al.,
2011). Sun et al. (2010) report that inconsistencies of NOx
emission appear among studies due to different with engine
type, engine technology, and fuel feedstock.
5. Emissions of automobiles fueled withethers based on engine technology
DME, CH3eOeCH3, has a higher cetane rating and oxygen
content thandiesel andhas good evaporation characteristics in
the combustion chamber. Meanwhile, it has no direct CeC
bonds which produces considerably less pollutants like HC,
smoke and particulate matter (PM) than conventional fuels.
This property makes it very attractive as a clean fuel for
transportation and domestic utilization. Therefore, it is an
excellent, efficient alternative fuel for diesel engines. At pre-
sent, most of investigations focused on the pure DME com-
bustionor effects ofDMEquality onCI engine fuel economyand
emissions (Hou et al., 2014; Su et al., 2014; Su and Chang, 2014;
Park and Lee, 2013; Youn et al., 2011). DME has good solubility
with diesel (Wang et al., 2008b) (Fig. 9(a)). It is also found that
compared with diesel, BSECs of blend fuels firstly decrease
and then increase, as shown in Fig. 9(b). Consequently, the
sequence of BTE is diesel < DME10 < DME15 > DME20. This
condition is similar with PODEn. Adequate addition of DME
can surely promote the complete combustion and increase
the BTE. Both NOx and soot emissions of DME/diesel blend
fuels are lower than those of diesel (Fig. 10).
Youn et al. (2011) investigated the combustions and
emissions characteristics of DME compared to conventional
diesel fuels. In combustion characteristics, the peak
combustion pressure and the ignition delay of DME fuel is
higher and faster than those of ultra low sulfur diesel
(ULSD), respectively. The NOx emission of DME fuel shows
slightly higher than that of diesel at the same engine load
condition, while HC and CO emissions were lower. Also, the
soot emission of DME fuel is nearly zero level (Su and
Chang, 2014; Park and Lee, 2013). Hou et al. (2014)
investigated combustion and emissions characteristics of a
turbocharged compression ignition engine fueled with DME
and biodiesel blends. The result shows that with the
increase of DME proportion, ignition delay, the peak in-
cylinder pressure, peak heat-release rate, peak in-cylinder
temperature decrease, and their phases retard. Compared to
biodiesel, NOx emissions of blends significantly decrease; HC
emissions and CO emissions increase slightly (Su et al.,
2014). Zhao et al. (2014) investigated the effects of DME
(dimethyl ether) premixing ratio and cooled external EGR
(exhaust gas recirculation) rate on combustion, performance
and emission characteristics of a DME-diesel dual fuel
premixed charge compression ignition (PCCI) engine. The
Fig. 9 e Comparison of mutual solubility and BSEC between DME and diesel (Wang et al., 2008b). (a) Mutual solubility. (b)
BSEC.
Fig. 10 e Comparison of NOx and soot emissions between DME and diesel (Wang et al., 2008b). (a) NOx emission. (b) Soot
emission.
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334 327
result showed that HCCI combustion of the premixed gas
promoted the in-cylinder pressure and temperature,
resulting in an earlier SOC and a shortened diesel ignition
delay. The decrease in the diesel fuel during the diffusion
combustion improved the mixing uniformity between the
fuel and air. Thus, the combustion became more complete
and the brake thermal efficiency improved. A higher DME
premixing ratio caused lower smoke and NOx emissions but
higher HC and CO emissions. PCCI engine with EGR
exhibited an obvious postponed SOC and prolonged
combustion duration. Thus, the maximum values of in-
cylinder pressure, mean charge temperature, heat release
rate and pressure rise rate all decreased. As the EGR rate
increased, NOx emission decreased, but smoke, CO and HC
emission increased. Wang et al. (2013, 2014) found that both
port DME quantity and injection timing remarkably
influenced the combustion process and exhaust emission of
engine. They had little impact on the peak position of HRR
during low temperature reaction (LTR) phase. However, the
peak value of HRR increased and the crank-angle
corresponding to the HRR peak advanced with an
incremental DME quantity or an early injection during high-
temperature reaction (HTR) phase. The peak value of HRR
dropped with an incremental DME quantity or a late
injection during the diffusion combustion phase. Peak
values of in-cylinder pressure and temperature increased
with an incremental DME quantity or an early injection. For
the fixed injection timing, NOx emissions presented a
decreasing trend with a rise of DME quantity but this
decreasing trend ceased at a higher DME quantity. Smoke
emission reduced, but CO and HC emissions increased with
a rise of DME quantity. Su et al. (2016) also found that the
ethanol fraction have a more obvious effect on the indicated
mean effective pressure (IMEP) for advanced in-cylinder
injection timings than around the top dead center (TDC)
conditions. The application of the DME-ethanol dual-fuel
combustion strategy caused a significant reduction of
indicated specific NOx without deterioration of indicated
specific soot. In addition, a high ethanol fraction led to a low
NOx for the same premixed combustion duration, while HC
and CO emissions increased slightly.
Although DME is usually thought to be an alternative fuel
for CI engines, the SI DME engine could be started successfully
and realize the stabile running. Shi et al. (2018) investigated
the combustion and emissions characteristics of a SI engine
fueled with gasoline-DME blends. Test results showed that
the addition of dimethyl ether resulted in the raised
indicated mean effective pressure for the gasoline engine.
Over increased and decreased spark timing tended to cause
the dropped indicated mean effective pressure. The
coefficient of variation in indicated mean effective pressure
was diminished with the spark timing advances and
dimethyl ether addition. NOx and HC emissions were
dropped with the spark timing decrease. NOx emissions
from the dimethyl ether-mixed gasoline engine are
decreased with the decrease of spark angle. Ji et al. (2011)
indicated that thermal efficiency, NOx and HC emissions are
improved with the increase of DME addition level. The
combustion performance was improved when DME addition
fraction was less than 10%. CO emission first decreased and
then increased with the increase of DME enrichment level at
stoichiometric condition.
PODEn, with the structure CH3eOe(CH2O)neCH3 and with
no CeC bond, are promising blend fuels for diesel due to low
viscosities and pour points, high oxygen contents and high
CNs. The “n” of CH2O group is from 1 to 8, and the main
composition is from 2 to 6. Properties of PODEn components
are listed in Table 2. PODEn can also be soluble with diesel at
any proportion. Added into the diesel blending with 10%e
20%, PODEn can significantly reduce the diesel cold filter
point, can improve the diesel combustion in the engine
quality, and improve thermal efficiency (Shi et al., 2012).
Feng et al. (2013) found that the ignition delay of
PODE3e8ediesel mixed fuel is shortened, the fuel
consumption is increased, but the effective thermal
efficiency is improved, compared with diesel fuel.
Yang et al. (2015), investigated the performances of engine
fueled with PODE2e4 blend fuel. The result showed that with
the increase of the mixing ratio of PODE2e4, the diesel
engine power and torque drop, but thermal efficiency
increase. Xie et al. (2017) made a study about PODEn and its
high proportion of diesel blended fuel on the combustion
and emission of the engine. It was found that the engine
was pulsed with pure PODEn, and its effective thermal
efficiency was improved and discharged relative to diesel oil.
So PODEn can be used as an alternative fuel for diesel alone
(Burger et al., 2010; Feng et al., 2013). The NOx-soot trade off
relationship can be dramatically improved (Burger et al.,
2010). The NOx and soot emissions can meet Euro Ⅴ
standards at high load and Euro Ⅵ standards at medium
load. Oxygenated fuel is an important method to inhibit the
formation of soot emissions and improving air entrainment
by blending high volatility fuel is another approach. PODEnhas lower distillation temperature than diesel and thereby
higher volatility (Liu et al., 2016a, 2017b). Furthermore,
PODEn has lower viscosity than diesel. As a result, blending
PODEn in diesel is helpful to improve the forming quality of
air-fuel mixture and the spray quality.
Furthermore, Liu et al. (2017) found the combustion
efficiency can be dramatically improved which leads to
lower HC and CO emissions. In order to reduce emissions
and improve thermal efficiency of diesel engines, blends of
GDP (gasoline/diesel/PODEn) were proposed and studied. GDP
blends have shorter ignition delay, lower max pressure rise
rate and COVIMEP (coefficient of variation of indicated mean
effective pressure) than GD blends. GDP blends also have
higher combustion efficiency and thermal efficiency than GD
blends, even slightly higher than diesel fuel. Pellegrini et al.
Table 2 e Properties of PODEn components (Chen et al.,2017a).
Component Density(g/cm3)
Boilingpoint(�C)
Cetanenumber
Oxygencontent
(%)
PODE2: CH3O(CH2O)2CH3 0.96 105 63 45.3
PODE3: CH3O(CH2O)3CH3 1.02 156 78 47.1
PODE4: CH3O(CH2O)4CH3 1.06 202 90 48.2
PODE5: CH3O(CH2O)5CH3 1.10 242 100 49.0
PODE6: CH3O(CH2O)6CH3 1.13 280 104 49.6
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334328
(2013) studied PODE3e5 on a diesel engine and the results
showed that the use of 12.5% PODE3e5 mixture reduced PM
emissions; high mixing ratio PODE3e5/diesel could
simultaneously optimize NOx, PM and noise, but may cause
problems with the engine hardware. Chen et al. (2017a)
found that both soot and ultrafine particles (UFP) emissions
obviously decreased with the increase of PODEn ratio. At low
engine loads, the reduction effect for UFP is especially
significant, as shown in Fig. 11.
BTEs of PODEn/diesel blend fuels firstly increase compared
to diesel with the blending ratio of PODEn and then decrease,
which is similar with DME. This condition is attributed to the
lower LHV of PODEn. The above reviews concluded that with
the blending of PODEn, HC, CO and smoke emissions decrease,
while NOx emission increases. Compared to P20, BTE of P30
decreases and thus reduce the NOx emission. High oxygen
content still has obvious effect in reducing smoke emission.
Fig. 12 indicates the soot formation process of diesel engine
(Dale and Kenth, 2007). Alkynes are polymerized into aromatic
hydrocarbons (PAHs) through fuel pyrolysis. The growth of
PAHs leads to the formation of soot (core formation). On the
whole, blending oxygenated fuel in diesel may provide
oxygen in the fuel-rich area of the diesel jet, which can
inhibit the soot formation. Common ethers used as diesel
additive were DME with the molecular formula of CH3OCH3
and PODEn of CH3O(CH2O)nCH3. There are no CeC bonds in
both DME and PODEn and these ethers have the best effects
in soot reduction. Zhu et al. confirmed that the Re(C]O)
OeR0 group in biodiesel was less efficient in suppressing the
soot precursor's formation than the R�OH group in n-
pentanol (Zhu et al., 2016). Further, it was confirmed that
soot precursor generated in the biodiesel pyrolysis was
proportional to the concentration of unsaturated fatty acid
methyl ester (the number of C]C double bonds) (Wang
et al., 2016b).
6. Greenhouse gas emissions of alternativefuels based on life cycle assessment
Greenhouse gas (GHG) emissions derived from vehicles are
the significant contributors to the global warming and the
climate change as shown in Fig. 13. Life cycle assessment
(LCA) methodology is commonly used to evaluate the well-
to-wheel greenhouse gas emissions of alternative fuels.
Ou and Zhang (2013) found that CHG-powered and LNG-
powered vehicles emit 10%e20% and 5%e10% less GHGs
than gasoline- and diesel-fueled vehicles respectively, which
has the similar results with Rose et al. (2013) that a 24%
reduction of GHG emissions (CO2-equivalent) realized by
switching from diesel to CNG. Since NG has a lower carbon
content than petroleum, gas to liquid (GTL)-powered
vehicles emit approximately 30% more GHGs than
conventional fuel vehicles. The carbon emission intensity of
the LNG energy chain is highly sensitive to the efficiency of
NG liquefaction and the form of energy used in that process
(Ou and Zhang, 2013).
It was confirmed that biodiesel appears attractive since its
use results in significant reductions of GHG emissions in com-
parison to gasoline and diesel (Nanaki and Koroneos, 2012).
Nocker and Torfs (1998) made a comparison of LCA and
external-cost analysis for biodiesel and diesel, which found
that both approaches confirm that although biodiesel offers
advantages in terms of greenhouse gas emissions, and it has
similar or higher impacts on public health and the
environment. However, from a LCA perspective, it is not an
Fig. 11 e Comparison of UFPs between diesel and its blends with PODEn (Chen et al., 2017a). (a) Number concentration. (b)
Volume concentration.
Fig. 12 e Schematic diagram of soot formation process (Dale and Kenth, 2007).
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334 329
accurate way for only focusing on the use phase of the fuels.
Carneiro et al. (2017) found that some biodiesel production
pathways perform satisfactorily in terms of GHG emissions
compared to other biofuels, but some others can be even
worst than fossil diesel, but energy and GWP performances
still can be improved if production pathways are carefully
chosen and optimized. Further, Collet et al. (2014) found that
a large fraction of environmental impacts and especially GHG
emissions stem from the production of the electricity
required for producing, harvesting and transforming algae, in
that case the source of electricity as well as algae production
technology may also play an important role in GHG reduction.
As for ethanol, Blottnitz and Curran (2007) reported that
bio-ethanol results in reductions in resource use and global
warming. A life cycle environmental impacts of selected U.S.
ethanol production and use pathways in 2022 was
conducted and indicates that one kilometer traveled on E85
from the feedstock-to-ethanol pathways evaluated has 43%e
57% lower GHG emissions than a car operated on
conventional U.S. gasoline (base year 2005) (Hsu et al., 2010).
Even though bio-ethanol production from sugarcane is
considered to be a beneficial and cost-effective greenhouse
gas (GHG) mitigation strategy, it is still a matter of
controversy due to insufficient information on the total GHG
balance of this system (Lisboa et al., 2011). In sum, the
utilization of alternative fuels including CNG, biodiesel, and
ethanol is helpful to control the GHG emissions.
7. Conclusions and suggestions
Diversification of fuels for automobiles is an inevitable strat-
egy and trend of social and economic sustainable develop-
ment. Environmental pollution effects of automobiles fueled
with alternative fuels are extremely complicated, determined
by fuel properties, engine technology and application modes.
In sum, improvement of BTE generally accompanies with the
increase of NOx emission. High oxygen content will surely
prohibit the formation of polycyclic aromatic hydrocarbons
and soot.
(1) Natural gas is the most important and successful
alternative fuel for automobiles. NG/gasoline bi-fuel
automobiles have higher BTE than gasoline ones,
producing less HC, CO, and PM emissions, while more
NOx emission. Pure NG automobiles have similar regu-
lations with bi-fuel mode compared to gasoline auto-
mobiles. NG/diesel dual fuel automobiles are commonly
compared with diesel ones, emitting higher HC and CO
emissions and lower NOx and PM emissions with BTE
decreasing. HCNG improves the BTE of NG engine and
as a result the NOx emission increases significantly.
(2) Methanol and ethanol are generally applied on SI au-
tomobiles, through mixing with gasoline in certain
volume proportion together with the additive. High
intramolecular oxygen contents accelerate the com-
bustion speed, shorten the combustion duration and
thus improve the BTE, promoting the complete com-
bustion. Accordingly, HC and CO decrease obviously.
Methanol and ethanol can also be used on diesel en-
gines, although their ignitability is poor due to the low
cetane number. However, high oxygen contents are
helpful for inhibiting the formation of PM and their ef-
fects on the combustion process are similar. The solu-
bility of alcohols in diesel is very poor and many
additives including cosolvents and surfactants must be
used, leading to the poor application.
(3) Biodiesel is an ideal and renewable alternative fuel for
diesel and its physical and chemical properties are close
to those of diesel. The most important advantage is that
biodiesel can be mixed with diesel in any volume ratio.
Although the feedstocks for the production are
extremely abundant, most biodiesel has five major
compositions and as a result its intramolecular oxygen
content is 10% or so byweight, accounting formore than
70% by weight. To balance the trade-off relationship
between properties including oxidation stability (induc-
tion period) and ignitability (cetane number) and low
temperature fluidity (freezing point), the mass fraction
of U-FAMEs is controlled no less than 70%. High viscosity
and low volatility of biodiesel result in poor homogene-
ity of mixture gas and spray quality. Biodiesel has lower
BTE than diesel at low loads, and higher at medium and
high loads. Biodiesel produces higher NOx emission than
diesel, while less HC, CO, and PM emissions.
(4) DME and PODEn do not contain CeC bonds and have
high oxygen contents. They are considered as the most
promising blend fuel in diesel. Both of themhave higher
Fig. 13 e The transport sector as a major contributor to global energy-related CO2 emissions (Ashnani et al., 2015).
J. Traffic Transp. Eng. (Engl. Ed.) 2018; 5 (4): 318e334330
BTE than diesel, due to the more concentrated heat
release andmore completed combustion. DME blending
in diesel can reduce both NOx and soot emissions.
PODEn blending can also decrease the soot or PM
emission of diesel engine significantly, while increase
the NOx emission in general. In general, increasing the
blending ratio of DME and PODEn reduce the soot or PM
emissions, mainly due to the increasing of intra-
molecular oxygen content. However, BTE does not in-
crease throughout with the blending ratio and HC and
CO will increase when BTE decreases.
(5) In most cases, high BTE means high NOx emission and
low soot (or PM) emission for diesel automobiles,
exhibiting a trade-off relationship between NOx and
soot (or PM). Automobiles equipped with dual fuel en-
gines, including both NG/diesel and methanol/diesel
dual fuel, simultaneously reduce the NOx and PM (or
soot) emissions compared to diesel. The decline of BTE
leads to the higher HC and CO emissions.
Conflicts of interest
The authors do not have any conflict of interest with other
entities or researchers.
Acknowledgments
This study was supported by National Engineering Laboratory
for Mobile Source Emission Control Technology
(NELMS2017B02), and the Special Fund for Basic Scientific
Research of Central Colleges, Chang'an University
(310822172203).
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Dr. Yisong Chen is lecturer in Chang'anUniversity, School of Automotive. Hereceived his PhD degree in vehicle engi-neering from Hunan University in China at2014, done postdoctoral research at Tsing-hua University form 2014 to 2015. Hisresearch interests include alternative fuelsof automobiles, life cycle assessment ofautomotive and strategic research into theautomotive industry in China.
Dr. Hao Chen is an associate professor inChang'an University, School of Automotive.He obtained his PhD degree and Master de-gree in vehicle engineering from Chang'anUniversity. He is interested in the fields ofalternative fuels of automobiles, diesel en-gine, engine and fault diagnosis, life cycleassessment of automotive, etc.
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