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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