Considerations on the combustion of biofuels in [615845]

Considerations on the combustion of biofuels in
compression ignition engines
Um Min Allah Fazal1, Marin Bică*1, Alexandru Gruia1, Dragoș Tutunea1, Mădălina
Călbureanu1
1University of Craiova, Faculty of Mechanics, Romania.

Abstract. Biofuels are considered to be attractive alternative fuels due to
rising oil prices and the depletion of fossil sources. It is produced by a
process of transesterification of vegetable oils and animal fats with an
alcohol in the presence of a catalyst. At present, biodiesel and bioethanol
are considered solutions, and almost all EU countries have prepared a
policy on the production and use of biodiesel in the transport and heating
sectors. Biodiesel has certain advantages, such as a higher number of cetan,
biodegradability, low sulfur content and aromatic content, and lower
carbon monoxide emissions, unburned hydrocarbons and particulate
matter. One of the main issues related to biodiesel and biodiesel blends is
the high viscosity of the fuel. Among the biodiesel properties, density,
viscosity and heating value affect engine performance and emission
characteristics. The presented paper highlights the differences that occur in
comparison to the classical fuels and the performances obtained when
using biodiesel.
1 General consideration
The development of the transports required the improvement of Diesel engines, especially
since the steam engines that used the railway transport locomotives did not respond to the
needs (tasks and speeds) The emergence of oil, the rapid development of the oil industry,
led to the neglect of the use of biodiesel, due to the undeniable qualities the refined fuels
and the low price at that time. The increase in oil consumption during World War II has led
to the depletion of their production. The idea of using biofuels has become a solution. For
engines evolved at that time, the high viscosity of biodiesel did not allow it to be used. In
1937, G. Chavanne, a Belgian, invented transesterification of vegetable oil, a process that
enabled the efficient use of these fuels, which proved to be environmentally friendly. In the
1980s, environmental pollution increased, global temperature increased – global warming.
Since then, the European Union has become the largest biofuel producer. The first motors
were built to work with vegetable oils without being subjected to further processing.
Biodiesel burns clean, which means there will be a reduction of all kinds of pollutants that
add smog and global warming. Only alternative fuels as biodiesel and bioethanol were

* Corresponding author: Marin Bică, [anonimizat]

approved by the Environmental Protection Agency (EPA). It passed every test of clean air
health effects and meets the needs of the atmosphere. Biodiesel fuel is an alternative clean
combustion fuel from 100% renewable resources. Most people believe that biodiesel fuel
must be the fuel of the future. It does not contain oil, but it can be mixed with oil to produce
a mixture of biodiesel, which is then used in many different vehicles. Pure biodiesel can
only be used in diesel engines and is biodegradable and non-toxic, is completely safe. There
is an increase in nitrogen oxides and degradation of tropical rainforests replaced by palm
plantations [1].
2 Physical-chemical properties
The structure of the injection jet from combustion chamber determines the increases of
engine performance and reduces pollutant emissions. The geometry of the injector, size and
nozzle is selected function of the fuel properties, the combustion chamber architecture and
the engine's energy solution. The larger angle of the injection cone decreases the
penetration and interference between the jets. Hiroyasu and Arai proposed the relationship
for determining the angle of the injection jet cone:

𝜃= 83.5ቀଵ
ௗቁି଴.ଶଶ
ቀௗ
஽ೀቁ଴.ଵହ
ቀ⍴೒
⍴೗ቁ଴.ଶ଺
, [degrees] (1)

d- diameter of the spray jet, [m]
D0 – diameter of the cylinder, [m]
ρg – gas density in the combustion chamber, [kg / m3]
ρl – density of spray liquid, [kg / m3]

The spraying angle is based on:
– the geometry of the spray jet hole;
– kinematic viscosity of the fuel – affects only the limits of the jet;
– injection pressure – increases the angle of the jet;
– fuel density – decreases the angle of the jet;
– temperature – decreases the angle of the injection jet.
The characteristics of the spraying of biodiesel-diesel mixtures were determined by using
an atmospheric pressure spray gun using two types of injectors, figures 1, 2 and 3. The
density and viscosity of the fuel are important in determining the spray characteristics. The
kinematic density and viscosity of the fuel mixture has an increase as the biodiesel
concentration of the mixture increases in the blend.

Figure 1. Bosch common rail piezoelectric injector, BMK type engine

Figure 2. Fuel spraying

Figure 3. Injector Delphi.Tip VM Motori engine 2,5 l. Spray in the atmosphere

Table 1. Biofuel properties
The determined size Value Testing method EN 14214
Density at 15°C , Kg/m3 875 EN ISO 3675 860-900
Refractive index 1,4562 ASTM D 1298 ≥1,4900
Viscosity at 40°C, mm2/s 4,719 EN ISO 3104 3,5-5
Acidity, mg KOH/g 0,370 ≤0,5 PeEN 14104
Iodine index, gI 2/100g 128 PrEN14111 <120
Saponificat index, gKOH/g 187,4 AOAC (1999) 185-190
Flash point, °C 178 ASTM D 6751 Min 101
Freezing point, °C -7 EN 2315 –
Cloud point, °C -2 – –
Stability at oxidation at
110°C, h 8 EN 14112 Min 6
Distillation, °C 307 ASTM D 6751 Max 360

3 Influence of parameters determined on biodiesel quality
3.1 Density
It is defined as the mass per unit volume of a fluid. Fuel density is a key property that
affects engine performance and pollutant emissions. Since fuel injection pumps deliver fuel
by volume, not by mass, a larger or smaller amount of fuel is injected, depending on its
density. Thus, the air-fuel ratio and the energy content from the combustion chamber are
influenced by the fuel density. Normally, the biodiesel fuel densities are slightly higher
than those of diesel fuel. For all types of biodiesel, the tendency to change the density is the

same, over time it increases with storage time. This increase is the result of the presence of
oxidation products. Samples containing hydrocarbons with shorter chains and saturated
fatty acids tend to crystallize more quickly. This has as a result the decrease in volume and
thus the increase in density. The mass of the sample also increases as a result of insoluble
sediment formation, which also leads to increased density
3.2 Kinematic viscosity
It is a parameter that expresses the friction between the interstices of a fluid being a major
parameter in assessing the quality of liquid fuels. Viscosity affects the atomization of a fuel
at injection into the combustion chamber and, consequently, ultimately the formation of
deposits in the engine [2]. The higher the viscosity, the greater is the fuel tendency to cause
such problems. The viscosity of a transesterified oil (biodiesel) is about an order of
magnitude smaller than that of the base oil [2, 3]. High viscosity is the main reason why
some vegetable oils have been largely abandoned. Kinematic viscosity has been included in
most biodiesel-specific standards. The difference in viscosity between base oil and alkyl
ester derivatives can be used to monitor biodiesel composition. Viscosity increases with
chain length (number of carbon atoms) and degree of saturation. This also applies to the
alcoholic portion because the viscosity of the ethyl ester is slightly higher than that of the
methyl ester [2]. Viscosity of fuels is taken into account in engine design specifications, is
directly responsible for flow and lubrication properties; the increase in viscosity leads to a
tempering of the injector nozzles. A too viscous biodiesel leads to the formation of too
large droplets that will penetrate the wall opposite the injector. The surface of the cylinder
is cold, the combustion reaction will be interrupted and black smoke will be formed (the
intermediate product of combustion consisting of aldehydes and acidic caustic-odorous
acids). Incomplete combustion results in lower engine power.
3.3 Flash point
An increased flammability of biodiesel indicates that it has a low probability of accidental
ignition. Changing the flash point can be influenced by the residual methanol in the
biodiesel production process. The methanol residue may cause degradation of engine
elastomers and seals, as well as the corrosion of the aluminum and zinc present in the
injectors. The flash point gives indications of the presence of unreacted methyl alcohol in
the biodiesel process. It is also important in relation to the legal requirements and safety
measures involved in fuel handling and storage and are usually specified in the regulations
on fire safety measures [4].
3.4 Cloud point
It is the temperature at which the first crystals appear in the mass of biodiesel. They notice
visually and under these conditions biodiesel can only be used at a higher temperature. The
cloud point variations leads to problems related to the decrease of the flow through the
injection pump (decrease of the engine power), the clogging of the filters and the supply
pipes. The parameter is an indicator of the temperature at which saturates can flocculate
(coagulated as vials), which leads to increased viscosity; at the same time, sets the
temperature at which a fuel can no longer be filtered within a specified time limit; shows
the injection system blocking potential; shows the destination for use in cold weather or
cold season; allows analysis of the need to add additives; can be controlled by mixing raw
materials; the parameter also characterizes the determination of the fog point (precipitation
of crystals). The cloud point is relative to the degree of saturation and the number of carbon
atoms in the fatty acid chain. Saturated fat compounds have significantly higher melting
points than unsaturated fatty compounds and crystallize in a mixture at higher temperatures
than unsaturated compounds. Thus, biodiesel fuels derived from fats or oils with significant

amounts of saturated fatty compounds will have greater cloud points. The increase in the
flash point can be improved by the addition of additives [5].
3.5 Freezing point
It is the temperature at which biodiesel gels and does not flow. The mixture can no longer
be pumped to the engine and it is necessary to heat it. A high value can limit its use in cold
climates. An important role in modifying the freezing point is paraffinic substances which,
upon cooling, form a network of crystals, thus preventing the movement of the oil. Apart
from the presence of paraffin, the freezing point also depends on a number of factors
including: the content of resinous and asphaltic substances, the water content of the oils, the
preheating temperature, and the cooling temperature of the oils [6].
3.6 Acidity index
Indicates the amount of free acids in biodiesel. They come from two sources: from acids
used in the production of biodiesel and which have not been completely eliminated and
from oxidative degradation. Increased acidity can cause corrosion, deposits in the feed
system or clogging of filters due to increased viscosity at the same time may be a symptom
of the presence of water in the product.
3.7 Iodine index
The iodine value is an indicator of stability and a measure of total unsaturation; increased
iodine value is closely linked to polymerization to oligomers and leads to damage to the
injector; low-value iodine oils are less sensitive to oxidation processes.
3.8 Distillation temperature
In the case of unusually low distillation temperatures, these were effectively combined with
a low boiling point and flash point indicating high residual amounts of methanol and / or
glycerin content. Lower the 10% distillate temperature value is, easier will be the engine to
start in cold climate. The temperature at which it distills 50% of the volume of biodiesel
exerts an influence on the good running of the engine, that is, these average fractions
influence the quality of the fuel mixture in the case of a heated engine leading to a stable
engine operation and a smooth passage from the running regime empty at the medium
speed. The wear of the piston rings, of the bearings, of the cylinders liners is influenced by
the heavy fraction characterized by the temperature at which it distills 90% of the fuel,
since the heavy fractions in a high percentage remain unvaporized and dissolve the oil thus
creating a possibility of dry friction between the engine organs. Also, heavy fractions in
liquid form are not in contact with air and do not burns completely in the combustion
chamber, creating calamines that produce engine wear [7].
3.9 Saponification index
Indicates the amount of saponifiable units (acyl groups) per unit of biodiesel weight. A high
level indicates a higher proportion of low molecular weight fatty acids in oil or vice versa.
The saponification index is used to measure the average molecular weight of biodiesel and
is expressed in milligrams of potassium hydroxide (mg KOH / biodiesel). Saponification is
a chemical reaction involving the production of a metal salt or soap. The reaction involves
attack on a methyl ester, a free fatty acid, triglycerides or another glyceride with a
hydroxide ion, -OH. The hydroxide ion implies the presence of water in the system. If the
water could be removed, then there would be no soap formation. However, this is a
practical impossibility. There is always a little water and there is always a soap that is
formed when biodiesel is produced [8,10]. The exception to this is when using a solid
(heterogenous) catalyst that does not provide the free metal ions needed to form soap.
These catalysts have to provide biodiesel and glycerin that do not contain soap. In fact,

many of these allegedly heterogeneous catalysts eliminate metal ions in the liquid and thus
require a cleaning of the reaction products. Soap should be removed from the biodiesel
after the reaction, and this can be done either by washing with water using a liquid-mixed
solid adsorbent or by passing the liquid through an ion exchange resin bed.
3.10 Refractive index
It changes when the temperature of the biodiesel is near the point of disruption being a
significant parameter for assessing its state. An increased refractive index indicates the
presence of unsaturated substances.
3.11 Oxidation stability
Refers to the ability of a fuel to resist chemically to the changes occurring over time over
the storage period. It is a major factor in the commercialization of biodiesel. One of the
major issues limiting the use of biodiesel as a fuel in compression ignition engines is its
poor oxidative stability. Biodiesel may be affected by oxidation during storage (in contact
with air) and hydrolytic degradation (in contact with water). The two processes can be
characterized by oxidative stability and hydrolytic stability of biodiesel. Stability of
biodiesel can refer to two aspects: long-term storage stability or aging and stability at high
temperatures or pressures as fuel is recirculated through the engine fueling system. For
biodiesel, storage stability is very important. Storage stability refers to the ability of a fuel
to withstand chemical changes during long-term storage. These changes usually consist of
oxidation due to contact with oxygen in the air. The composition of biodiesel greatly affects
its stability in contact with air. Unsaturated fatty acids, especially polyunsaturated (e.g.,
C18: 2 and C18: 3), have a high oxidation tendency. After oxidation, hydroperoxides (one
hydrogen atom and two oxygen atoms) are attached to the fatty acid chain. Oxidation
reactions can be catalyzed by some of the materials present (the material from which the
tank is produced) and light. After chemical oxidation reactions, hydroperoxides are
produced, which in turn can produce short chain fatty acids, aldehydes and ketones.
Hydroperoxides can polymerize to form large molecules. Thus, oxidation increases the
viscosity of biodiesel. In addition, oxidation increases the acid value, the color changes
from yellow to brown and the appearance of a paint smell, solid deposits in the engine fuel
system (pipes and filters) [11,12] can form. For example, calculations were made for
burning diesel and biodiesel. Results are shown in tables and graphics.

Table 2. Elementary composition of diesel fuel, colza biodiesel and FAME (Biodiesel)
Elements Colza biodiesel Diesel Biodiesel
Carbon (%) 76 86.8 76.2
Hydrogen (%) 14.5 12.6 12.6
Oxygen (%) 6.3 0 11.2
Azote (%) 3.2 0 0

*Laboratory tests were performed with colza biodiesel

Table 3. Values V air, Vi and V O2 for full burning of colza biodiesel
𝜆 Vair[kmolair/kg.comb] V i [kmol/kg.comb] V O2 [kmol/kg.comb]
1.2 12.489 12.495 0.437
1.4 14.518 14.524 0.874
1.6 16.664 16.67 1.31
1.8 18.74 18.7468 1.75
2 20.83 20.8368 2.18

Table 4. Values for 𝑉஼ைమ,𝑉ுమை, 𝑉ேమ și 𝑉௚at the full burning of colza biodiesel
𝜆 𝑉஼ைమ 𝑉ுమை 𝑉ேమ 𝑉௚
1.2 1.42 1.62 9.987 15.11
1.4 1.42 1.62 11.63 18.836
1.6 1.42 1.62 13.16 22.449
1.8 1.42 1.62 14.924 26.296
2 1.42 1.62 16.569 30.024

Table 5. Values V air, Vi și VO2for complete burning of Diesel
𝜆 Vair[kmolair/kg.comb] V i [kmol/kg.comb] V O2 [kmol/kg.comb]
1.2 13.296 13.30 0.465
1.4 15.512 15.517 0.93
1.6 17.728 17.733 1.396
1.8 19.944 19.95 1.86
2 22.16 22.165 2.326

Table 6. Values 𝑉஼ைమ,𝑉ுమை, 𝑉ேమ și 𝑉௚for complete burning of Diesel
𝜆 VCO2 VH2O VN2 V g
1.2 1.62 1.41 10.5 15.746
1.4 1.62 1.41 12.25 19.702
1.6 1.62 1.41 14.005 23.673
1.8 1.62 1.41 15.75 27.634
2 1.62 1.41 17.506 31.606

Figure 4. The air required for burning (୫ొయ
୩୥.ୡ୭୫ୠ)
11131517192123
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2Vair[kmol/kg. fuel]
λBiodiesel of Colza
Diesel fuel

Figure 6. Comparative chart between the excess air λ and the number of kilograms of the substance
in the reaction

4 Conclusions
Based on the experimental results the biodiesel spray has a narrower angle and the
penetration is larger than diesel fuel. The biodiesel properties that affect the injection of
fuel are fuel vaporization, higher injection pressure and worse atomization. The atomization
of biodiesel is the consequence of higher values of surface tension and viscosity
comparative with diesel fuel. The results are in accordance with similar rapports from
various authors [6,9,12,14].Calculation of combustion indicates that less air is required for
biodiesel due to higher content of oxygen.
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