BIOFUELS IN THE FUEL INJECTION SYSTEM OF A SINGLECYLINDER MEDIUM-SPEED DIESEL ENGINE
AKI TILLI
MARTTI LARMI
OSSI KAARIO
aki.tilli@tkk.fi, tel. 09 451 3463
Internal Combustion Engine Research group
TKK Helsinki University of Technology,
Helsinki, Finland
ABSTRACT
As the awareness of the climate change has become widespread and the forthcoming
fossil fuel production decline gets closer and closer, the biofuels and their behavior have
become one of the most important fields of research in the energy technology sector and
as well in the internal combustion engine studies. The diesel-type biofuels are naturally
more or less CO2 –neutral. Moreover, in renewable diesel studies also other emissions
have been promisingly low.
The focus of this research was to evaluate the compatibility of modern renewable diesel
fuels with the fuel injection system of the “Extreme Value Engine” (EVE), a singlecylinder medium-speed research engine of the Internal Combustion Engine research
group in Otaniemi. In the study, the objective was to determine whether biofuels and
especially their physical properties require changes in the fuel injection system. The
research consisted of a literature study of biofuels and their comparison, a report on the
simulation model designed and the simulations, and of the results and summary
presentation.
In the literature part, conventional diesel and different renewable diesel-type fuels are
introduced. The fuels introduced are traditional biodiesel (fatty acid methyl ester,
FAME), hydrotreated vegetable oil (HVO), Fischer-Tropsch (FT) diesel fuels and
dimethyl ether (DME). The raw materials, production and characteristics of the fuel types
are briefly introduced and their qualities and usability compared.
In the simulations part, first the software used (GT-FUEL) is introduced briefly. The
simulation model made as a part of the study and the simulations to fit the model to
measurements are presented. The simulations to actually compare different fuels in the
fuel injection system, the results and the alleged behaviour of the fuels in the EVE fuel
injection system are then introduced.
According to the simulations, the behavior of different renewable diesel fuels in the fuel
system is very similar. Of the physical properties, mainly the differences in fuel densities
and viscosities have some predictable effect on the fuel masses injected. The chemical
properties of the fuels (which the simulation model does not take into account) are
probably more important for the fuel injection system. Fuel spray behavior and
combustion of renewable diesels are important research issues and should be the next
step in the study of renewable diesel fuels in the EVE research engine.
Keywords: Biofuel, Renewable diesel, Biodiesel, Fischer –Tropsch, Dimethyl ether,
Hydrotreated vegetable oil FAME, HVO, BTL, GTL, DME, NExBTL, Internal
combustion engines, Extreme value engine, Fuel injection, Simulation, GT-FUEL
2
1. INTRODUCTION
The goal of this study was to prepare future research of fuel injection and alternative
fuels in the Internal Combustion Engine research group by studying different biofuels
that have been used in diesel engines, and evaluate their usability in the “Extreme Value
Engine” (EVE). The fuels deemed most suitable were traditional biodiesel (fatty acid
methyl ester, FAME), hydrotreated vegetable oil (HVO), Fischer-Tropsch (FT) diesel
fuels and dimethyl ether (DME). The data required was collected and a simulation model
of the fuel injection system of the EVE engine was developed. EVE is a single-cylinder
medium-speed 4-stroke diesel engine with a 200 mm bore. The current maximum
cylinder pressure of EVE is ~ 200 bar, rail pressure 1400 bar and engine speed 900 rpm.
It has a charge air compressor (1-8 bars) and floating engine bed on air springs. Special
characteristics include electro-hydraulic control of gas exchange valves, electro-hydraulic
control of injection valve, operation in motored mode, easy access for measurements and
optical access. The fuel injection system of EVE is based on Wärtsilä W32 common rail
accumulator and W20 injector. Especially important in EVE is that it is very flexible and
accessible test bed for research purposes with powerful and versatile control of engine
parameters. (Lehto K, 2007), (Olenius T, 2006)
The simulation model in this study was made with GT-FUEL, an object-oriented fuel
injection system simulation program. GT-FUEL is part of the GT-SUITE software
family. The simulation model created was first adjusted to measured boundary
conditions. Simulations using fuel models based on properties of different biofuels and a
standard fossil fuel as a reference were run with different loads. The results were used to
evaluate possible changes needed in the fuel injection system.
From the fuel injection system simulations, boundary conditions for the CFD calculations
concerning fuel spray behaviour and combustion were obtained. Biofuel compatibility
with EVE was evaluated and new parameters for the biofuel injection in EVE research
engine are being searched for. Eventually, the goal is to find optimum combustion
conditions and reach high engine efficiency with significant emission reductions.
2. THE FUELS STUDIED
2.1 Standard diesel fuel
Conventional diesel fuel quality requirements are based on international standards. EN
590, partly introduced in table 1., is a standard that determines the physical properties all
the diesel fuels sold in EU must fulfill.
Table 1. EN 590: standard for diesel fuel in EU.
In addition to the EN 590 requirements,
the demands of local climate and the
Criteria
EN 590
users of the fuel set a standard of their
Density @ 15°C (g/cm³)
0.82-0.845
Viscosity @ 40°C (mm²/s)
2.0-4.5
own. In this study the focus was on
Flashpoint (°C)
>55
physical properties, especially on density
Cetane number
>51
and viscosity. (Mikkonen et al., 2007)
1
2.2 Conventional biodiesel - FAME
According to many standards, directives and laws, only the traditional first generation
renewable diesels can be called “biodiesel”. These traditional biodiesels consist of fatty
acid methyl esters, usually referred to as “FAME”. These fuels comprise of moderately
long (C16-C18) fatty acid chains, mainly mono-alkyl fatty acid esters. (Mikkonen et al.,
2007), (Mikkonen, 2007)
Traditional biodiesel are usually produced by transesterification method from fatty acids
by using base as a catalyst. In transesterification processes the triglycerides of the fatty
acids react with an alcohol and thus mono alkyl esters and raw glycerol are produced.
This reaction only occurs in the presence of a strong base as a catalyst. The most
common catalysts used are water-free sodium- or potassium hydroxide. Before they are
added to the reactor, the bases are dissolved into the alcohol used in the reaction. The
most common alcohol used is
_______________________________________
methanol (ca. 10% of the reacting
substances). Other alcohols can also
be used, if not (yet) as efficiently.
(Yamane et al., 2001), (Van Gerpen
et al., 2004), (Knothe G, 2005)
Thermodynamically, FAME is quite
similar to traditional diesel fuels.
However, there are differences in
chemical behaviour and physical
Figure 1. Tr ansesterification reaction. (Van Gerpen et al.,
properties, especially in density,
2004)
viscosity and compressibility.
Because of its higher compression
modulus, with FAME, the pressure pulsations are faster, the injection starts earlier and
the maximum pressure is higher than with traditional diesel at low mean pressures and
temperatures (293 – 313 K). With higher injection pressures and temperatures the effect
disappears. (Yamane et al., 2001), (Knothe G, 2005), (Mikkonen S, 2007)
Traditional biodiesels are non-toxic and biodegradable, their production process is simple
and easy, and they are practically free of sulphur and ring-shaped aromatic carcinogens.
FAME is mainly produced from renewable or recycled components, usually vegetable
oils. Therefore, the impact on greenhouse effect should be smaller when FAME is used
rather than pure fossil fuels. Emissions of particles, hydrocarbons, and carbon monoxide
also seem to lessen when FAME is used. FAME also improves the lubricity of the fuel
mixtures even in small proportions, especially with sulphur -free fuels. However,
emissions of NOx, cold smoking, engine oil deterioration, corrosion, and carbon build-up
in the fuel injection system seem to increase if 100% FAME is used. Usually the cold
operation characteristics of FAME are worse than with a normal diesel fuel and the high
boiling range (340-360 °C) has caused problems. Because of these drawbacks, engine
manufacturers recommend, that the amo unt of FAME in the fuel should be 5 % (weight)
at maximum in the usual diesel engines. Still, even in relatively low quantities, the fatty
2
acids of FAME may harden and embrittle rubber parts (such as seals and hoses) that
endure acids badly. FAME-based fuels are difficult to store because of the
biodegradability and tendency to gather up water during transportation. Moreover, the
best quality-FAME oil (RME, rapeseed methyl ester) is derived from rapeseed oil that is
also used in food industry. Thus, its production competes with food production and its
availability is limited. (Yamane et al., 2001), (Van Gerpen et al., 2004), (Knothe G,
2005), (Laurikko J, 2007), (Mikkonen S, 2007), (Reinhardt et al., 2006), (Mäkinen et al.,
2005)
2.3 Hydrotreated vegetable oil – HVO
FAME fuel quality differences can be quite large. Therefore, there has been demand for
processes that use bio-based raw materials to produce high quality diesel fuels in the
scope of traditional oil refineries. New kinds of renewable diesel refi nement processes
that produce large quantities of hydrocarbon-based diesel fuel have been developed.
Instead of the traditional esterification, in the process any vegetable oils and animal fats
are hydrotreated. The process is comparable to usual diesel fuel refinement. (Mikkonen
S, 2007)
The first stage of the production is
the pretreatment, where impurities
are removed. The actual NExBTLprocess has two parts:
hydrogenation and isomerisation.
The pretreated compound is
continuously fed into the
hydrotreatment unit. The fatty
acids are hydrogenated into
paraffinic hydrocarbons. The
hydrogenation reaction of the fatty
Figure 2. The simplified chemical production process of
acid molecules and the products
NExBTL. (Oja et al., 2005)
can be seen in figure 2. During the
hydrogenation temperature is 330-450 °C and pressure 5 MPa. Catalysts, such as
NiMo/Al 2 O 3 or CoMo/ Al 2 O 3 are used. (Reinhardt et al., 2006), (Oja et al., 2005)
After the hydrotreatment, the mixture of hydrocarbons is isomerised. The molecular
structures of the paraffinic hydrocarbons are treated with the help of a catalyst so that the
number of carbon atoms remains high and methyl branches are created in the carbon
chain. The end results of the reactions are also water, propane and small amounts of
carbon oxides. The isomerisation should not go too far, as the end-products cetane
number would decrease. During the isomerisation phase, the temperature is about the
same as in the hydtrotreatment. To stabilize the catalyst, a pressure of 3,5 - 4 MPa is
used. Side products of the NExBTL production process are fuel gas, which is burned for
energy, and a small amo unt of biogasoline. (Reinhardt et al., 2006), (Oja et al., 2005),
(Rantanen et al., 2005), (Juva A, 2007)
3
The life cycle emissions of HVO
are very small, only about 50% of
traditional biodiesel production
emissions. The renewability of
Regulated emissions
Unregulated emissions
NExBTL can be verified by
• NOx
- 0 ... - 20 %
• Aldehydes
- 40 ... - 45 %
radioactive C14 analysis. The
• Particles - 17 ... - 30 % • Benzene
- 40 ... - 45 %
engines and vehicles –group at
• CO
- 45 ... - 55 % • PAH
decrease
• HC
- 45 ... -55 % • Mutagens
decrease
VTT has made exhaust gas
measurements with mixtures of
NExBTL and ordinary diesel in passenger cars and buses. The measurements were made
with fuels consisting of 5, 15, 20 or 85 % of biocomponent. The base fuel used fulfilled
basic European requirements. The results showed emission reductions, especially with
carbon monoxide and hydrocarbons. With the highest NExBTL proportion –fuels, the
particle and even NOx emissions were also diminished, unlike with FAME –biodiesels.
According to the measurements, the emission reductions were as can be seen in table 2.
(Mäkinen et al., 2005), (Juva A, 2007), (Rantanen et al., 2005)
Table 2. Emission changes when using blends of traditional
diesel and NExBTL. (Mäkinen et al., 2005)
The characteristics of HVO are as good as or better than with usual good quality diesel
fuel, although the density of HVO is lower. With HVO, flammability is good, boiling
ranges lower than with normal diesel fuel and the cold operation characteristics are
adjustable. Moreover, HVO contains no aromatics or oxygen. Although the lubricities of
the fuels are poor, they can be easily improved with additives. (Mäkinen et al., 2005)
HVO can be used with more flexibility and without some restrictions that concern usual
FAME –biodiesel. The process also makes a wider range of raw materials possible.
Additionally, it enables better product optimization. NExBTL has a low cloud point,
which can be adjusted from - 5 to - 28°C by severity of process conditions in order to
produce either summer or winter type diesel fuel. The end product is very similar to
conventional synthetic diesel fuel and can thus be blended directly with it or be sold as
such to be further processed. No upper limits need to be set for HVO blends, as long as
the final blend fulfills EN 590 requirements. In practice, the lower density of HVO means
that the limits of EN 590 are reached with a 30% NExBTL blend. (Reinhardt et al.,
2006), (Mäkinen et al., 2005), (Rantanen et al., 2005)
2.4 Fischer-Tropsch –diesels
One of the most promising routes to produce liquid green fuels is the combination of
biomass gasification (BG) and Fischer-Tropsch (FT) synthesis. In FT synthesis, solid or
liquid raw materials are first gasified. If the synthesis gas feed is from biological masses,
the end product is called BTL (“Biomass to Liquids”). Current fuel research focuses on
both BTL and “gas-to-liquids” (GTL), that uses natural gas and refinery gas (“flared
gas”) from petroleum production. Both concepts use synthetic gas (“syngas”, mixture of
carbon monoxide and water) to produce a liquid fuel. After cleaning, the syngas is used
for FT synthesis to produce long-chain hydrocarbons that are converted into “green
diesel”. The syngas obtained from biomasses is termed “bio-syngas”. The process of FT
synthesis using biomass feedstock is under development. (Brown D, 2007), (Boerrigter et
al., 2002), (Srinivas et al., 2006)
4
The production process for BTL is started by grinding and drying of biomass, which is
then formed into pellets. The biomass-pellets are diverted into a gas (smoldering gas) and
solid fraction (charcoal) in a low temperature gasification process and transformed into a
synthetic gas in a second step. In syngas production, the coal of the raw material is
gasified by heating. The syngas mixture is formed in the presence of oxygen and water.
Since the reactions are mainly exothermic and dependent on temperature, good heat
transfer in the reactor is essential. An important and useful reaction is water-gas shift
(WGS): (Boerrigter et al., 2002), (Srinivas et al., 2006), (Hepola et al., 2002)
The syngas produced in the gasifier consists mainly of carbon monoxide (CO) and
hydrogen (H2). Also small amounts of carbon dioxide and methane are in the mixture.
The sulphur of the raw material is reduced to hydrogen sulfide and nitrogen to ammonia.
The raw syngas mixture has to be cleaned. The most important contaminants are
aromatics, ammonia and other nitrogen compounds, hydrogen sulfide and chloride and
solids. The major gas cleaning issue is the tars in the gas, so a tar cracker in the system is
required. Organic contaminants can be cracked or washed away. As the syngas needs to
be compressed to 25-60 bar for FT synthesis, the concentration of the organic compounds
must be below the dew point at FT pressure to prevent condensation and fouling in the
system. Sulphuric contaminant removal is especially important, as they are harmful for
the FT –synthesis catalysts. NH3 and H2 S can be removed via wet scrubbing followed by
active-carbon and ZnO filters. Solids must be removed essentially completely, as they
foul the system and may obstruct the fixed-bed. With respect to the other constituents of
biosyngas, i.e. H2, CO, CO 2, CH 4, N2, paraffins (ethane and propane), and olefins, there
are no specifications. The gas is suitable as feed for FT synthesis assuming the syngas is
free of tars and other impurities. (Boerrigter et al., 2002), (Srinivas et al., 2006)
After purification the gas is liquefied in the FT reaction, in which carbon monoxide (CO)
and hydrogen (H) react and form carbon-hydrogen chains. In the catalytic FischerTropsch synthesis one mole of CO reacts with two moles of H 2 to form mainly aliphatic
straight-chain hydrocarbons (CxHy ). Typical FT catalysts are based on iron or cobalt.
About 20% of the chemical energy is released as heat in exothermic reactions that form
paraffins (2), olefins (3) and alcohols (4):
As follows from equations (2) – (4), the FT reaction consumes hydrogen and carbon
monoxide in a ratio of H 2/CO = 2. When the ratio in the feed gas is lower, it can be
adjusted with the water-gas shift (WGS) reaction (1). (Boerrigter et al., 2002), (Vessia Ø,
2005), (Cao et al., 2003)
Typical operation conditions for FT synthesis, when aiming for long-chain products, are
temperatures of 200-250 °C and pressures of 25-60 bar. The resulting paraffin-like liquid
5
is isomerized to achieve better cold operation and then distilled or cracked. In this step,
the specifications of the fuel can be fine-tuned to match the requirements of the engines
by altering the form or length of the fuel molecules. The higher the temperatures, the
more short-chained hydrocarbons are formed. This fine-tuning is not possible in the
currently used standard refining process for diesel or gasoline. With respect to the
production of FT diesel, process conditions can be selected to produce maximum
amounts of products in the diesel-range or towards production of long-chained
hydrocarbons (wax). The wax can be cracked to yield predominantly diesel fuel. This
hydrocracking requires additional hydrogen, for instance from a syngas side-stream that
is completely shifted to hydrogen via the WGS reaction (1). Other fractions can be used
in the chemical industry or be further processed. (Brown D, 2007), (Boerrigter et al.,
2002), (Srinivas et al., 2006), (Lieberz S, 2004), (Hepola et al., 2002)
Today, it is not yet economic to convert biomass to fuels using conventional FT
technologies. Production costs are about three times as high as with traditional diesel.
Commercial plant capacity, a secure feedstock supply and existing pipelines to move
syngas to large central facilities are yet to be established for the BTL route. The high
capital cost is a significant barrier. Energy efficiency should still be increased, it being
about 45% from tree-to-barrel at the moment. (Brown D, 2007), (Srinivas et al., 2006),
(Lieberz S, 2004), (Cao et al., 2003)
Fischer-Tropsch diesel fuel can be produced so that it is similar to fossil diesel with
regard to almost all properties, the exception being density which tends to be lower. If the
relatively small density differences are not regarded, FT-diesel can be blended with fossil
diesel in any proportion without the need for engine or infrastructure modifications. As
FT-fuels and HVO consist of same kinds of hydrocarbons, also their characteristics are
the same. FT fuel lubricity is low, and the cold operation properties depend on the
production process conditions. Cetane numbers of FT-diesels are very high and aromatic
contents low, which results in lower NOx and particle emissions. BTL is also CO2 –
neutral as the comb ustion only releases the cola in the biomass. (Berlowitz et al., 2003)
2.5 Dimethyl ether – DME
Table 3. DME compared with propane and butane.
(Haikonen T, 2004)
Dimethyl ether (DME, CH 3OCH 3)
is structurally the simplest ether,
chemically stable and colorless. Its
Property
DME Propane Butane
qualities
are most comparable with
Boiling Point, °C
-24,9 -42,1
-0,5
propane
or
butane (table 3). In
Vapor Pressure @ 20 °C, bar
5,1
8,4
2,1
normal temperature and pressure
Liquid Density, @ 20 °C ,
668
501
610
kg/m3
DME is gaseous, having a boiling
Lower Heating Value, kJ/kg
28,43 46,36
45,74
temperature of –25 °C in
Auto Ignition Temperature @
235atmospheric pressure. DME is easy
470
365
1 atm, °C
350
to liquefy by increasing pressure,
Explosion/Flammability Limit 3,4since its steam pressure is 5,1 bar at
2,1-9,4 1,9-8,4
in air, vol %
17
20 °C. DME disintegrates quickly
in atmosphere and isn’t carcinogenic or dangerous to the ozone layer.
6
DME can be produced by dissociating methanol into DME and water or from syngas by
direct synthesis. In dissociating DME from methanol, following reactions take place:
These reactions require a catalyst that is stable in high temperatures. Such are for instance
nickel and zinc oxide. This so-called Topsoe process can be seen in figure 3. The process
consists of
desulphurization, Auto
Thermal Reforming (ATR),
removal of carbon dioxide,
oxidation synthesis and
final purification.
DME is suitable for the
diesel process, but it has
very low viscosity (1/10 of
conventional diesel fuel
Figure 3. Schematic of DME production process. (Haikonen T, 2004)
viscosity), lubricity, density
and heating value. The
compressibility of DME is three times the conventional diesel compressibility. DME
requires e.g. a high-pressure injection system. Because of the low viscosity, traditional
seals will leak when using DME. The viscosity can be increased with additives, but the
additives deteriorate properties of the fuel. DME may also erode conventional rubber and
plastic parts. The Young’s modulus of DME is very dependent on temperature, which
causes problems with regular pumps, as the amount of pumped fuel decreases as
temperature rises. Because of the low heating value, DME has to be injected 1,8 times as
much as regular diesel for the same power. The low lubricity can be increased with
additives quite easily. The high compressibility of DME causes problems with injection
system as high-pressure peaks are formed in the injector after injection. Also cavitation
becomes a problem.
No soot is formed in DME combustion. Compared with conventional diesel, the DME
spray injected into the combustion chamber mixes better with air. Therefore good
combustion will take place with less air. DME combustion is quicker, late combustion
phase longer and thermal efficiency better than with diesel. The emissions are lower than
with diesel, if the injection system is optimized. (Berlowitz et al., 2003), (Haikonen T,
2004), (Yu et al., 2002)
DME is an interesting fuel in many ways, and it has to be taken into account when
assessing possibilities for future fuels. However, in this case it was estimated that the
differences in the physical properties of DME and conventional diesel are very
significant. At this stage it was not deemed sensible to make the needed changes to the
7
research engine EVE, especially as at the same time it should be run with other diesel
fuels. Therefore, DME wasn’t part of the simulation program.
2.6 Fuel Comparison
Renewable diesels have both advantages and disadvantages when compared with regular
diesel. A comparison of renewable diesel raw materials, processes, and end products
can be seen in table 4. FAME is a mature technology with low capital costs, but limited
quality, feedstocks and views for the future, BTL technologies are very young, hindered
with high costs but have much potential with large feedstocks and high quality.
Hydrotreated oils fall in
Table 4. Renewable diesel fuels: basic information. (Bown D. 2007)
between, as the feedstocks
are same with FAME,
FAME
HVO
BTL
production commercialized
Process
Gasification,
Transesterification Hydrotreatment
and
the quality is as with
route
FT
Feed
BTL. The main advantage of
Vegetable oils
Oils, fats
Biomass
Product
all different renewable diesel
Isomerized
Isomerized
fuels is their reduced
Product
Fatty acid methyl
paraffinic
paraffinic
(type)
esters
emissions. The CO2 emissions
hydrocarbons
hydrocarbons
of FAME depend heavily on
Product
Consistency and
High
High
the raw materials and their
quality
stability issues
cultivation, and the emission
0.3-1.5 kg
0.5-1.5 kg
CO2
1.6-2.3 kg CO2 /kg
CO2/kg oil
CO2/kg oil
emissions
reduction can be nonexistent.
oil equivalent
equivalent
equivalent
(LCA)
BTL has at its best as high as
Note: Fossil diesel fuel value reported as 3.8 kg CO2 / kg oil
90% CO2 life cycle reduction.
equivalent
(Vessia Ø, 2005)
Although thermodynamically similar or even better than conventional diesel, renewable
diesels exhibit some differences in chemistry and in such physical properties like density,
viscosity and isothermal compressibility, that strongly affect injection pressure, injection
rate, fuel spray characteristics, combustion characteristics and emissions. The different
diesel fuel characteristics can be seen in table 5. (Yamane et al., 2001) The properties
mainly affecting the functioning of the fuel injection system (not the combustion) are
compressibility, density and viscosity. The boiling points don’t seem to be a problem, as
they are either about the same or higher than with ordinary diesel. (Juva A, 2007),
(Nylund et al., 2007)
The main renewable diesel divergences from EN 590 requirements for a typical diesel
fuel are lower (HVO and BTL) or higher (FAME) densities. The different densities mean
that there will be differences in fuel spray. The higher the density, the higher the mass
flow will be. In order to achieve the same power with low-density fuel, a larger volume
of fuel per cycle should be injected. The volumetric flow should decrease with viscosity.
8
Table 5. Properties of different diesel fuels. (Juva A, 2007)
Fuel
NExBTL
Diesel
GTL FT
Typical
FAME
Typical
Standard EN 590
diesel
Density at +15°C (kg/m3)
Viscosity at +40°C (mm2 /s)
Cetane number
10 % distillation (°C)
90 % distillation (°C)
Cloud point (°C)
Heating value (MJ/kg)
Heating value (MJ/l)
Polyaromatics (wt-%)
Oxygen content (wt-%)
Sulfur content (mg/kg)
780 - 785
3.0 - 3.5
98 - 99
260- 270
295 - 300
- 15
44
34,5
0
0
< 10
770- 785
3.2- 4.5
73 - 81
260
325 - 330
0 ... +3
43
33,8
0
0
< 10
885
4.5
51
340
355
0 ... - 5
38
34
0
11
< 10
835
3.5
53
200
350
-5
43
36
4
0
< 10
820-845
2.0-4.5
>51
<11
<50
Compressibility (bulk modulus) has its effect on fuel injection system functioning. For
instance, because of its very compressibility, DME was left out of the simulation program
and near-future plans for EVE. FAME has at low liquid pressure and temperature a bulk
modulus higher than conventional diesel. When using FAME, the rate of liquid pressure
rise increases and the injection timing advances with decrease in fuel temperature.
Therefore, also the peak injection pressure is higher at low mean injection pressure
conditions. The reason for the high biodiesel NOx emissions largely may be attributable
to the difference in bulk modulus between conventional diesel fuel and biodiesel
(FAME). At higher temperatures and pressures these differences in compressibility
disappear. An opposite trend is observed with paraffinic fuels. Their use leads to a
retarding of injection timing because they have a lower bulk modulus of compressibility.
This supports the observation that paraffinic (HVO and FT-diesel) fuels yield lower NOx
emissions. The conclusion is that in a high-pressure common rail system, the bulk
modulus values of renewable diesels are not considered critical to the functioning of the
fuel injection system. (Yamane et al., 2001), (Boehman et al., 2003)
Diesel engines are not as sensitive to vapour pressure variations as SI-engines. The
vapour pressures of renewable diesels are quite similar to traditional diesel. Thus, vapour
pressure has not been considered an issue when using different renewable diesels. (Beer
et al., 2001)
The physical properties of the renewable diesel fuels don’t have many differences with
conventional diesel and those deviations that do exist are relatively small. In order to
make EVE research engine compatible with renewable diesels, it shouldn’t be necessary
to make large-scale changes to the engine. Chemistry must be taken into account, and if
FAME is to be used in large quantities, rubber parts should be changed. The density,
compressibility and viscosity differences mean that the injection parameters must be
optimized for each fuel to reach the best possible performance.
9
3. THE SIMULATION MODEL
The one-dimensional compressible
flow solution of GT-FUEL is based
on a flow solution developed for 3D CFD applications (Gosman A,
1985), reduced to one dimension.
An explicitly conservative finite
volume solution of mass and
energy is applied. (Gamma
Technologies, 2003), (Seenikannan
et al., 2008), (Gosman A, 1985)
The GT-FUEL model (figure 4) of
EVE engine was created by using
30 different templates to describe
the physical parts, properties and
boundary conditions of the system.
The injector dimensions of were in
Figure 4. Model of the injector of EVE fuel injection system.
most cases gotten from Wärtsilä.
The liquid diesel fuel and their
gaseous forms were referenced in their own simulation objects. The simulated fuels were
a typical EN 590 specifications fulfilling summer-grade diesel fuel (here “EN 590”),
FAME, NExBTL, a typical Fischer-Tropsch fuel (“GTL”) and “Tempera-15”. “Tempera15” is a previously used fuel model in EVE-simulations, so it was used as a reference fuel
in the test runs to fit the simulation model to real EVE measurements.
To describe the properties of compressible fuels, two different equations of state for
density (polynomial and “GTI”), were used. The polynomial form is often used to model
fuel behavior and “GTI” is an improved power form for evaluating properties at high
pressures. The “GTI” equation has been shown to yield more accurate density predictions
when extrapolated to high pressures. (Kolade et al., 2003)
In the “Tempera-15” –model, the fuel density was modeled with the polynomial form (8)
of the density (kg/m3). In other modeled fuels the density model used was the “GTI”
power form (9):
where Tref = 298 K and p ref = 1 bar and T and p must be expressed in K and bar. (Kolade
et al., 2003)
10
Table 5 properties were used and the density curve of the GT-FUEL reference fuel was
fitted into the data. Coefficient a0
was changed so that table 5 values of
fuel density at 288 K were on the
curve. The fuel density curves (p = 1
bar) obtained thus can be seen in
figure 5. Surprisingly, the fuel model
used previously (Tempera-15) didn’t
seem to fulfill EN 590 requirements,
as it had a density of the same order
as FAME. The density seems to fit
very well with some checkpoints
obtained: FAME density of 841
kg/m3 ± 2.5% at 80 °C and normal
diesel density 788 kg/m3 ± 0.5% at
80 °C. (Ejima et al., 2006)
The viscosities of the fuels were also
obtained from table 5. In GT-FUEL
the dynamic viscosity values are defined as an array, where a certain viscosity value
corresponds with given temperature and pressure. Many points of reference can be given.
The program interprets the viscosity to change linearly between the reference points and
outside the range of the points uses
the value of the nearest point. The
viscosities in the different fuel
models were defined based on the
original GT-FUEL diesel fuel
behavior.
Figure 5. The simulated fuel density curves at p = 1 bar.
As an example, the viscosity
behavior of the fuel models at 1 bar
is illustrated in figure 6. The aim of
the procedure was to set the
extreme condition viscosity values
at a level “same or worse than in
reality”.
From test runs during April 2007,
injection durations (in seconds),
the start of injection (SOI) timings
(in crank angle degrees) and corresponding injection doses were gathered for different
rail pressures (1400, 1300, 1200 and 1100 bars). The needle valve was then adjusted to
open accordingly at each scenario. Corresponding cylinder pressure curves were set as
boundary conditions for each scenario.
Figure 6. The fuel viscosity curves at 1 bar.
11
At the test simulations, after the
needle valve opening arrays were
set to be such that the injection
timings and lengths corresponded
quite well to the measurements
made, the total injected masses
matched relatively well the reality
(figure 7). Also temperatures and
pressures had to be taken into
account when the simulation model
accuracy and the choices made
Figure 7. Simulated and measured injection doses.
were evaluated. The temperatures
had been evaluated to be below
400 K, remained at maximum around 360 K. The pressure peak reached as high as 1700
bar, which was judged to be normal high-pressure injector behavior.
Figure 7. Simulated and measured injection doses
4. THE SIMULATION RESULTS
Figure 8. The simulated mass flow rates as a function of
crank angle.
In the actual simulations the goal
was to evaluate the compatibility of
the research engine EVE and usual
renewable diesels. The simulations
were run with the rail pressures used
in the test runs (1100, 1200, 1300
and 1400 bars), but here we
concentrate on the simulations run
with 1400 bar rail pressure. As the
fuel behavior differences were
similar, so the emphasis on the
highest pressure with the most
extreme values does not leave out
any essential information.
From the figure 8, it can be seen that
the injection mass flow rates were as
expected. The mass flow rate curves
are shaped very typically for a
common rail engine and they are
very similar with the different fuel
models. The magnitude of the fuel
injection rates are in the order of
increasing density. The flow rates
naturally follow the pressure in the
fuel gallery (figure 9). Right after the
injection starts, the pressure drops
Figure 9. Fuel gallery pressure curves in the simulations.
12
rapidly to recover again. The injection flow rates follow (the rapid decline at about 358
degrees). The injection stops as the needle valve is closed and the needle settles on its
seat again blocking the injection flow. This causes a peak in the pressure at about crank
angle 390 degrees. The fluctuating pressures settle to the 1400 bars of the high-pressure
pump by the end of the cycle. The pressure curves of the different fuels are very similar
to one another.
The pressure drop between the upper
fuel gallery and the volume in the
upper end of the injector is essential
to the functioning of the injector,
since the pressure difference causes
the force difference that moves the
needle from its seat as the injection
starts. This pressure difference
caused by the opening of the needle
valve between the high-pressure and
the low-pressure sections of the
injector can be seen in figure 10. No
great differences can be seen
Figure 10. The pressure drop between the fuel gallery and
between the different fuels and the
the volume behind the needle.
injector works as expected. The
needle valve acts as the actuator of the operation of the injector, as with it the pressure
behind the needle is controlled (that is why it is also called “control valve”).
In the rail accumulator volume, the conditions change very little, as is predictable in such
a large volume (over 10:1 in relation to other parts of the fuel injection system). The rail
pressure behavior can be seen in figure 11. The pressure amplitude in the rail volume is
quite small, about 15 bars at maximum.
Figure 11. The simulated fuel pressure curves in the rail accumulator volume.
13
5. CONCLUSIONS
The examination of the simulated values and curves shows quite convincingly that the
behaviors of the different renewable diesels in the fuel injection system differ only little
from the behavior of conventional diesel. The greatest differences in the physical
properties of the renewable diesels, when compared with conventional fuel, were the
densities. However, this had little effect on the functioning of the fuel injection system in
the simulations. The observation is in line with previous studies. (Ejima et al., 2006)
Lower density fuels contain less energy per unit volume. This, consequently, results in a
loss of engine performance in conventional high pressure injector fuel injection system
engines. However, high pressure common rail fuel injection system compression ignition
engines can be operated with no performance debit and with a significant reduction in
emissions by using a low density diesel fuel. The fuel density should be about 0.83 g/cc
or less, and the viscosity should be about 3 cSt or less at 40 o C. Viscosity seems to be
more important for the fuel behavior in a diesel engine. However, it has mostly effect on
the fuel spray and droplet size. (Ejima et al., 2006), (Schilowitz et al., 2002)
Combined with the observation about the effect of fuel density on the performance in
power, the study on the effect of the physical properties should have its emphasis on the
fuel spray and the combustion in the cylinder. In the fuel injection system itself, more
important factors would be the chemical properties. If FAME is used, the possible fuel
injection system corrosion, and hardening or embrittlement of rubber parts must be
avoided by changing seals etc. to ones made of materials that can withstand the slightly
acidic fuel and/or by using only smaller fractions of FAME blended with other fuels. The
low lubricity of HVO and Fischer-Tropsch fuels must be corrected with additives.
An interesting possibility is to blend low -density paraffinic fuels (FT-fuels and NExBTL)
with FAME and thus get an all-biofuel blend with the weaker properties of both fuels
enhanced. The high density of FAME would be improved with the low density of the
paraffinic diesel and vice versa, the low lubricity of the paraffinic fuels improved with
FAME (that improves lubricity even in low portions), the high viscosity and boiling
ranges of FAME improved with the lower ones of the paraffinic fuels and the lower
heating value and cetane number of FAME improved by the higher ones of the paraffinic
fuels. Paraffinic fuels could also improve FAME cold flow properties. All the fuels
involved are readily biodegradable and non-toxic if spilt. Neither FAME nor the
paraffinic fuels contain aromatics or sulphur, which would normally limit blending ratios
with conventional diesel. More FAME (50%) could be mixed with paraffinic fuels than
with conventional diesel (20%). This method has been tried and patented with GTL and
conventional FAME - biodiesel. (De Boer et al., 2004)
All in all, the possibilities of using biofuels in EVE research engine seem to be very
good. No extensive modifications, but a possible change of seals in contact with the fuel,
to the fuel injection system seem to be needed. Only the injection itself should be
optimized for each fuel used, as the differences in the physical properties mostly seem to
affect the fuel spray and combustion properties.
14
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