Alternative Fuels For Diesel Engines

70
Advances in Science and Technology
Research Journal
Volume 7, No. 20, Dec. 2013, pp. 70-74
DOI: 10.5604/20804075.1073063
Review Article
Received: 2013.09.19
Accepted: 2013.10.14
Published: 2013.12.06
ALTERNATIVE FUELS FOR DIESEL ENGINES
Jacek Caban1
, Agata Gniecka2
, Lukáš Holeša3
1
Institute ofTransport, Combustion Engines and Ecology, Faculty of Mechanical Engineering, Lublin University
of Technology, 36 Nadbystrzycka Str., 20-618 Lublin, Poland, e-mail: j.caban@pollub.pl
2
Maria Curie-Skłodowska University, Plac Marii Skłodowskiej-Curie 5. 20-031 Lublin, Poland
3
Department of Road and Urban Transport, Faculty of Operation and Economics of Transport and Communi-
cations, University of Žilina, Univerzitná 1, SK-010 26 Žilina, Slovakia, e-mail: Lukas.Holesa@fpedas.uniza.sk
ABSTRACT
This paper presents the development and genesis of the use of alternative fuels in in-
ternal combustion ignition engines. Based on the analysis of the literature, this article
shows various alternative fuels used in Poland and all over the world. Furthermore,
this article describes the research directions for alternative fuels use in road transport
powered by diesel engines.
Keywords: biofuel, diesel engine, eco-fuels.
INTRODUCTION
The increase in the emission of pollutants
into the environment coming from road transport,
depletion of natural resources, and economic
considerations are the main reasons for the de-
velopment of alternative fuels to power internal
combustion engines. Fuel consumption by mod-
ern road vehicles is an interesting parameter for
all users and producers. [18]. In order to meet the
energy needs, there has been increased interest
in alternative fuels, such as biodiesel, methanol,
ethanol, biogas, hydrogen and gas to ensure ade-
quate replacement of diesel fuel for internal com-
bustion engines [2]. Vegetable oils were applied
for the first time as power by Rudolf Diesel – in
the engine he constructed [6]. Vegetable oils are a
very promising alternative to diesel fuel because
they are renewable and have similar properties [2].
Vegetable oils offer almost the same power with a
slightly lower thermal efficiency when used in SI
engines [21]. The interest in alternative fuels were
noted again during the great oil crisis in the 1970’s
all over the world and this interest lasts until now,
for economic and environmental reasons.
This paper presents the development of the
use of alternative fuels in internal combustion en-
gines. A careful analysis of the literature allowed
us to present the main trends in the development
of alternative fuels in the country and all over the
world. This article describes the research direc-
tions in the use of alternative fuels in transport
vehicles powered by diesel engines.
PHYSICO-CHEMICAL CHARACTERIZATION
OF ALTERNATIVE FUELS
Research of alternative fuels based on bio-
components to power diesel engines focus main-
ly on identifying their impact on the environment
[11]. Alternative fuels can be divided into the fol-
lowing categories:
• alcohols: methyl and ethyl,
• bioethanol,
• biogas,
• alkyl esters of fatty acids (FAME, FAEE),
• CNG gas (Compressed Natural Gas),
• vegetable oils (biodiesel),
• hydrogen.
In Europe and Poland, because of the climate
and agronomic reasons, rapeseed oil methyl es-
ters and their mixtures with diesel fuel [5] are

71
Advances in Science and Technology Research Journal vol. 7 (20) 2013
most commonly used as liquid biofuels. Bio-
fuel or motor fuel biocomponents will constitute
7.55% of the total fuel market in 2014 [8, 9].
Despite significant advantages, in the papers [1,
4] the drawbacks of biofuels and their negative
impact on the sustainability and viability of the
internal combustion engine, and the environment,
are presented.
In Europe, the basic raw material for the pro-
duction of biofuels is rapeseed. Basic biofuel
used to power diesel engines are esters of fatty
acids (FAME, FAEE). Esters of pure vegetable
oils have good solubility in diesel fuel. This fea-
ture allows to create commercial mixtures of
biofuels such as B10, B20. Due to high viscos-
ity, low freezing point, water content and organic
acids using pure rapeseed oil is not desirable. It
may lead to seizure or damage of injection equip-
ment and engine. The fuel in the supply system
is also the only lubricating medium. The amount
of hydrocarbons containing carboxyl group deter-
mines the lubricity of the fuel, what makes them
chemisorbed on a clean metal surface [5]. Ethyl
and methyl esters differ in products of incomplete
combustion. In the process of incomplete combus-
tion of the methyl esters, toxic substances, such as
formaldehyde and radicals, are exuded, which do
not occur in the case of incomplete combustion
of the ethyl esters [23]. Exhaust gasses of diesel
engines powered by RME contain three times
more free methyl radicals than exhaust gasses of
engines powered by REE [17]. The comparison
of some properties of ON diesel oil, vegetable oil
OR, and rapeseed oil methyl ester RME is pre-
sented in Table 1.
Subjecting the oil to esterification allows to
break heavy molecular structure of tri-glycerides.
The reduction of particle size is accompanied by
the reduction in viscosity – which substantially
simplifies and improves the fuel injection spray
[8]. In this process, the reaction of esters neutral-
ization occurs. The unquestionable advantage of
esters of rapeseed oil is very low sulphur content
translated into a reduction of particulate emis-
sions [8]. The disadvantage is hygroscopy, which
favors the proliferation of bacteria and fungi
which translates into low chemical stability of the
fuel. In Polish climatic conditions, the use of pure
ester as a fuel without the additive depressant de-
creasing the pour point causes a lot of problems,
especially in the conditions of autumn – winter
[24]. Another problem associated with the esters
are the adverse effects on the structure of the plu-
rality of polymers (for example, rubber, some of
the seals, coatings, etc.). Important from the op-
eration point of view is the property of sediment
dissolution resulting from diesel engines power.
Diluted sludge can cause clogging of filters, and
damage to the injectors. For this reason a num-
ber of engine manufacturers make restrictions on
the use of biofuels and in case of their use they
demand more frequent servicing. These prob-
Table 1. Comparison of selected physical and chemical properties of fuels ON, OR and RME [23]
No. Parameter Unit
ON
Diesel
OR
Rapeseed oil
RME
Rapeseed oil methyl ester
1 Viscosity T0
= 40 °C mm2
/s 2–4.50 20 4.32
2 Mass calorific value MJ/kg 42–43 36–38 36–38
3 Cetane number – Min 51 40–50 50–55
4 Theoretical air consumption kg/kg 14.4–14.6 12.2–13 13.4–13.8
5 Density T0
= 15 °C kg/dm3
0.8–0.845 0.90 0.88
6 Surface tension T0
= 20 °C N/m 24×10-3
36×10-3
–
7 The cloud point °C -12 18 -9
8 Cold filter plugging point °C -31 (IZ 40) 9 (-9) – (-11)
9 Freezing point °C -40 4 -15
10 The sulfur content % 0.001 0.0002 0.0002
11 Carbon residue % 0.01 0.17–0.5 0.5
12 Ignition point °C >55 200–300 130
13 Initial boiling point °C 175 300 300
14
Average elemental composition
C
H
% m/m 86.4 77.6 76.8
% m/m 13.4 11.7 12.1
% m/m – 10.5 11.0

72
lems, however, did not stop the development of
biofuels. The authors in [8] indicate that the use
of B100 fuel and CE (camelina esters) is accom-
panied by higher specific fuel consumption than
in case of diesel. However, the authors of [9] con-
firm that the emission of nitrogen oxides is higher
when the engine is fueled with biofuels than with
diesel engine. Emission of hydrocarbons for fuel
B100, CE and HE is on the same level, particu-
late emissions are highest for CE [9]. Reduction
of CO and THC value during powering by esters
of vegetable oils is related to the oxygen content
chemically bonded in the molecules of esters.
Motor power supply with B100 bioester very pos-
itively influenced the smoke opacity. According
to the authors [11], at low and medium engine ro-
tational speed of the engine crankshaft, the smoke
opacity was more than three times lower on aver-
age, and at the high rotational speeds was lower
by tens of percent, compared to smoke opacity of
diesel-powered engine. Less clear is the effect of
the application of bioester concerning the emis-
sions of hydrocarbons and nitrogen oxides [11].
However, taking into account the fact that in real
conditions the test engine works more time in the
field of small and medium speeds than large ones
[16], the impact of bioester on the emission of hy-
drocarbons is favorable and unfavorable in case
of nitrogen oxides.
In [10], the authors draw attention to the
mechanisms responsible for blocking by a mix-
ture of biodiesel the fuel distributor or fuel filter
of the vehicle. These mechanisms are complex
and vary depending on the following factors [10]:
• FAME source and type of treatment used for
their production,
• characteristics of the solvency of the base oil,
• the level of impurities and contaminants in the
fuel,
• fuel storage and working conditions.
The authors [10] state that the increase in the
tendency to block the filter of FAME blended
mixtures is likely due to complex interactions
of trace constituents present in various types of
FAME and is also strongly dependent on the stor-
age temperature, storage time, and the properties
of the base oil. Filter blocking issues have been
seen in a field trial using experimental B20 and
B13 blends HVO7 fuel, where the FAME por-
tions of the blends were produced using a 50:50
RME: SME mix. Residues from the blocked
filters were confirmed to be a good match with
sterile glucosides using IR analysis, and saturated
monoglycerides were also found in some filter
samples using GC analysis.
A further study attempting to assess the indi-
vidual contribution of various trace components
in FAME towards an increase in FBT, showed
that there was no effect on FBT due to the ad-
dition of 0.3% by mass monoglyceride to B20,
made from distilled RME. The presence of sterile
glucosides and water in combination had the most
significant detrimental effect to FBT in B20 RME
blends. This effect was identified at a constant
MG level of 0.3% by mass.
Ethanol has been considered an important
alternative fuel for internal combustion engines
for a long time, because it solves a number of
problems of traditional crude based fuels such as
emission of greenhouse gases and particulates. As
a fuel for diesel engines, it suffers from a very
low cetane number (around 5…8, while diesel is
typically higher than 51), and consequently a high
auto-ignition temperature (around 640 K while
diesel is around 500 K). Ethanol has a heating
value of about 60% of diesel. Igniting ethanol in
a 4-stroke compression engine either requires use
of a higher compression ratio, use of glow plugs
or a catalyst [15]. Despite this, ethanol has suc-
cessfully been used for example in Sweden as an
alternative to diesel in bus fleets [20].
Diesel fuel-alcohol blends (known by a num-
ber of names – including E-diesel, M-diesel,
Oxy-diesel and diesohol) also can be used as fuel
in diesel engines [13, 19]. Generally, ethanol can
be blended with diesel without engine modifica-
tions [12]. Because ethanol is highly hygroscopic,
additives must be used in order to ensure solu-
bility of anhydrous ethanol in diesel fuel under a
wide range of conditions. Miscibility is limited,
especially at lower temperatures. The addition
of ethanol to diesel fuel can reduce lubricity and
create potential wear problems in sensitive fuel
pump and injectors designs. Ethanol possesses
also a lower viscosity and calorific value, with
the latter imposing minor changes to the fuel de-
livery system in order to allow injection of larger
quantities of fuel, what is likely to be required for
engine performance and for fuel injector/pump
durability. Because ethanol has a very low cetane
number it reduces the cetane level of the diesel-
ethanol blend.
However, a major drawback in ethanol-diesel
fuel blends is that ethanol is immiscible in diesel
over a wide range of temperatures and water con-

73
tent due to their difference in chemical structure
and characteristics [12]. These can result in fuel
instability due to phase separation. Prevention of
separation can be accomplished in two ways: by
adding an emulsifier, which acts in order to sus-
pend small droplets of ethanol within the diesel
fuel, or by adding a co-solvent, which acts as a
bridging agent through molecular compatibil-
ity and bonding to produce a homogenous blend
[12]. Emulsification usually requires heating and
blending steps to generate the final blend, where-
as co-solvents allow fuels to be “splash blended”,
thus simplifying the blending process [7]. Blends
of diesel and bioethanol can be used as fuel for
compression ignition engines, but with limited
scope, primarily due to the reduced mingle capa-
bility of ethanol with diesel, as well to the worse
ignition properties of the mixture.
Promising results have been obtained for util-
itarian bi-fuel (with initiating dose ON) diesel en-
gines fueled, especially CNG [22]. Biogas is a re-
newable alternative fuel currently used to produce
electricity and heat. Using it as fuel should strive
to bring these properties, which , is unfortunately
complicated by the widespread use of biogas. To
meet the expectations associated with the ecologi-
cal and economical operation of vehicle a number
of modifications of standard engine is required.
These include the change of shape of the piston
crown and the engine head as well as a change of
the fuel supply system. Additionally, the catalyst
choice and special fuel tanks are required. Both
manufacturers of new engines as well as users of
many older vehicles will be able to take some ad-
vantage of natural gas application which in turn
may lead to an increase in the number of new and
adopted vehicles fuelled with this fuel.
Despite the many drawbacks, the technology
of CNG fueling is becoming increasingly popular
amongst the crowd of consumers, individual and
fleet users, and public transport, as an economic
and ecological fuel. The low price of CNG fuel
and available technology of vehicle conversion
play a dominant role.
CONCLUSIONS
Modern pro-ecological attitude associated
with environmental protection against increased
toxic exhaust emissions and over-exploitation of
natural resources have forced to look for inno-
vative liquid fuels. For efficient use of different
fuels in existing engines, fuel or power systems
of internal combustion engines modifications are
applied. As demonstrated in the paper, the prob-
lem is to produce fuel similar to a classic one,
whose properties can be used as best as possible,
withdrawing from the available raw material.
Experience from the test stand and the opera-
tion show that the power of internal combustion
engines with alternative fuels is a complex and
long-term process and possible use of renew-
able natural resources are among the most envi-
ronmentally friendly. It is stated that you cannot
clearly determine which fuel is the best. In case
of small motors it can be quite different fuels than
for tractors’ engines or trucks and buses. Climatic
conditions of exploited vehicles are of great im-
portance when using biofuel. Advantageous ef-
fects of RME on exhaust emissions from older
diesel engines have been confirmed in many pre-
vious studies. However, in case of modern diesel
engines, the influence of RME seems to be less
recognized.
The main scientific problems relating to al-
ternative fuels include: accurate knowledge of
physical, chemical, thermodynamic, supplies and
logistics processes. Issues mentioned in the arti-
cle, the problems associated with the use and pro-
duction of alternative fuels show that research on
the development of alternative fuels is a complex
and constantly current research field.
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