Keywords: biodiesel, fuel drop size, FAME, HVO, diesel engine
- Introduction
The use of biofuels is growing in the world. The EU directive prescribes
that by the year 2020 10% of the energy used in the transport sector
must be constituted by biofuels \cite{Kuut}. The Paris Agreement aims to increase
further the share of biofuels in the transport sector. Several studies
have been performed on the use of biofuels in internal combustion
engines. The main focus has been on the effects of biofuels on engine ‘s
exhaust gases, work surfaces, fuel preservation, blending with fossil
fuels etc. The results show that when, for example, biodiesel (for
example, FAME or RME) is used as engine fuel, then the level of soot
decreases in exhaust gases. At this point, the decrease of the level of
soot in exhaust gases is explained by more efficient combustion as
biodiesel contains oxygen \cite{Xue11,Haz09,Ozs09,Utl08,ozg07,Kap06,Rey06,Rah04,Ulu04,Cho06,Lin09,QiD09,Lap08,Kim10,Men08,Hul06,Gum10,Pal10,Ram05,Sha09,Gur10,Zhu10,Ryu10,Luj09,Che09,Des09,Tso07,Rah07,Aga06,Puh05,Can05,Can03,Sen00,Haa01,Sah07,Bai09,WuF09,Ulu09,Lin091,Tzi09,Zhe08,Tat07,Kal03,Lap02,Jun06,Mon01,Gra03,Wan00,Car02,Kad03,Lap00,Arm06,Yam01,Lap081,Lap05,Din08,Fon17}.
At the same time, when HVO is used, the level of soot in engine exhaust
gas is also reduced \cite{Boh18,VoC18}. Therefore, the oxygen content in the fuel cannot be
used as the actual reason for explaining the reduction of the level of
soot.
In order to provide a better overview, a theoretical analysis of the
injection of biofuels into engine must be performed. Nowadays there is
no summary available on the injection mechanisms of the biofuels, the
behavior of fuel drops in fuel sprays and the distinctive features of
the behavior of biofuels compared to regular fuels. As the drop size is
an important factor in determining fuel evaporation and combustion in
engine cylinder, then this analysis may provide some explanations about
the formation of fuel sprays of biofuels and about the characteristics
of the combustion of biofuels.
Therefore, the aim of the article is to provide an overview of the
behavior of fuel drops and their size in fuel sprays when various
biodiesels (hereinafter biofuels) are used. The reason for determining
of drop size and behavior is the possible assessment of the quality of
air-fuel mixture in order to explain the reduction of soot emission when
biofuels are used. The theoretical part is based on the fuel drops’
formation models, which are used to perform the calculations to describe
the behavior of various biofuels in the fuel spray. The article
describes the formation of fuel drops, points out their impact
parameters and analyses the behavior of the drops of biofuels in the
fuel spray.
The main theoretical assumptions on which this paper is based:
- Sprayed fuel drops are being considered as (symmetrical) physical
bodies, which have the ability to bounce, coalesce and separate from
each other \cite{Bre11,Nik09,Nob95,LiJ16,Liu161}.
- The ability to bounce, coalesce and separate from each other is
dependent of the intrinsic and the environmental physical properties
(pressure, temperature, etc.) \cite{Qia97,Vol15,Ant16}.
- The spraying process is considered as a two-phase event: primary
breakout of the fluid and the formation of droplets. Several theories
describe this event: WAVE-RT, WAVE-TAB, WAVE-KH, etc., each with a
respective mathematical interpretation \cite{Rei86,Rei87,ORo87}.
The detailed mathematical background will be discussed in Sections 3
(Parameters describing fuel drop formation and collision), 4 (Fuel drop
size after leaving the injector), 5 (Hybrid breakout model) and 6
(Mathematical representation of reflexive and stretching separation).
The topic of the article is related to the scope of the Journal of the
Power and Technologies by the theme of renewable energy. The article
provides an overview of the behaviour of biofuels’ drops in the spray,
what is more, it supplements the database of the journal with
explanations of the problems of biofuels’ spray.
- Problem description
When biofuels, for example, FAME, is used as fuel in a diesel engine,
then generally the soot level decreases in the exhaust gas and the
number of soot particles, emission of carbon dioxide and nitrogen
compounds increases in the exhaust gas. The increased level of nitrogen
compounds and CO2 and the reduction of soot level is
caused by the more efficient combustion of biofuels (HVO, FAME) in the
engine. The more efficient combustion is justified by the biofuel’s
oxygen content, which improves the combustion of the fuel. In addition,
sources discuss thoroughly the carbon-hydrogen ratio in the fuel \cite{Ram05,Puh05,Can05,Ulu09,Fon17,Boh18,VoC18}.
Unfortunately, the reasons given in these scientific sources are not in
conformity with generally known theories, because, for example, the
diesel engine always works with lean mixture, where the value of the
air-fuel equivalent ratio is usually greater than 1.25. For turbo
engines, this value is greater than ~4 \cite{Die06}. Therefore, the
cylinder of a diesel engine contains theoretically sufficient amount of
oxygen for the complete combustion of fuel. In addition, the engine
tests of HVO fuel are in contradiction with the FAME results. The HVO
fuel does not contain oxygen, but the soot level in the emission gas is
reduced. It is also questionable how the carbon-hydrogen ratio affects
the emission gas. If we presume that for the engine to work on same
load, the same amount of energy must be added and this is derived from
the fuel carbon-hydrogen ratio, then the fuel added to the engine has
always the same magnitude of carbon-hydrogen atoms. Further, the test
results show a contradiction in fuel properties and fuel behavior during
injection.
Table 1 compares the physical properties of diesel fuel (DF), HVO and
FAME obtained by testing according to the standard EN-590. The
properties of gasoline are obtained from source \cite{Bud01}. In order to avoid the
fuel’s possible different properties listed in sources, the data listed
in the table has been obtained by testing. In the table, gasoline has
been given as reference fuel for comparing low viscosity fuels with high
viscosity fuel. Table 1 shows, for example, that the viscosity of HVO
and FAME is greater than that of diesel fuel. According to general
knowledge, when the viscosity of the fuel increases, the fuel drop size
in the fuel spray should increase, which also increases the combustion
time. The longer combustion time prevents large fuel drops from
combusting completely, which increases the level of soot in the emission
gas. In our case, this is in contradiction with the results given in
previous studies. When comparing fuel weight fractions, then HVO fuel
contains lighter fractions compared to diesel fuel. It can be said about
the FAME fuel that this fuel contains significantly more heavy fractions
compared to diesel fuel (when the temperatures of the evaporated parts
(10%-90%) of fuel are compared). Likewise, the heavy fractions of fuel
need more time for combustion. Therefore, the soot level of emission gas
of the FAME fuel must be at least in the same magnitude as diesel fuel.
The following chapters provide an overview the behavior of fuel drops in
the fuel spray and describe the effect of the properties of biofuels on
the fuel drop size.
Table 1. Properties of diesel fuel and biofuels used in diesel
engines.