Biofuel : Limitation of Using Straight Vegetable Oil and Ethanol as Transportation Fuel
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Transcript of Biofuel : Limitation of Using Straight Vegetable Oil and Ethanol as Transportation Fuel
BIOFUEL
Limitation of Using Straight Vegetable Oil and Ethanol as
Transportation Fuel
IBUKUN OLUWOYE
NICOSIA 2012
i
Abstract
The ecological and economic use of Bio-fuels is increasing on daily bases. Basically due to
environmental hazard of the non-sustainability usage of fossil fuel, the European Union had set
a standard for the 2020 most especially in transportation sector.
Major form of bio-fuel remains biodiesel and bioethanol which are produced from vegetable oil
and starchy contents respectively.
All these are only possible if the right physical and chemical processes are being put into
consideration with possibility of blending with fossil fuels because there are limitations in using
vegetable oil and ethanol directly as transportation fuel. In the review paper, such limitations
will be described in detail with necessary engine modification.
ii
Table of Contents
Abstract ……………………………………………………………………………………………………………..i
Table of Content ………………………………………………………………………………………………..ii
1. Introduction …………………………………………………………………………………………..1
2. Limitations of Vegetable in Diesel Engine ………………………………………………2
2.1 limitations based on physical properties
2.2 limitations based on structural and chemical properties
2.3 other limitations
3. Limitations of Ethanol in Gasoline Engine ………………………………………………7
3.1 limitations based on physical and combustion properties
3.2 limitations based on structural and chemical properties
3.3 other limitations
4. Possibilities of Engine Modification ……………………………………………………….10
4.1 Modification of Gasoline engine
4.2 Modification of Diesel engine
5. Conclusions ……………………………………………………………………………………………16
6. References …………………………………………………………………………………………….18
1
1. Introduction
Generally, biofuel are referred to liquid, gas and solid fuels predominantly produced
from biomass. A variety of fuel can be produced from biomass such as ethanol,
methanol, biodiesel, etc [1].
In the transportation industries, the use of biofuel which are basically biodiesel and
bioethanol is increasing on daily bases. The following figure shows the use of biofuel in
different energy mode in transportation.
FIG 1. Global energy use in transport sector (left) and the use of biofuels in different
transport modes (right) in 2050 (BLUE map scenario) [2].
It must be noted that the scenario can only be made possible only and only if the right
physical and/or chemical transformation are made on the biomass; in the case of
biodiesel, we have vegetable oil and in the case of bioethanol, we need blend with 90 %
gasoline.
The question remains: instead of inputting effort into the transformation of the
biomass, is the any possibility of using direct vegetable oil and ethanol?
In the following chapters, the limitation of using direct vegetable oil and ethanol will be
described in full details with possibility of engine modification to suit the direct use if
possible.
2
2. Limitations of Vegetable Oil Compared to Biodiesel.
There are limitations in using straight vegetable oil (SOV) directly in the diesel engine
compared to convectional diesel or biodiesel fuel specifications.
The specification is simply diesel fuel specification ASTM D 975.
2.1 Limitation Based on Physical and Combustional Properties.
The use of straight vegetable oil directly in diesel engine has some limitations based
on the following physical properties.
o Cetane quality: Cetane number is a measure of the ignition quality of the
fuel. Cetane number affects combustion roughness. The minimum cetane
number for diesel fuel (Grades No. 1 and 2) is 40.
As for vegetable oil, the cetane minimum number is 35 which is considered
as poor cetane number [3][4].
Result of Inadequate Cetane Number
Poor Ignition Quality
Long Ignition Delay
Abnormal Combustion
Abnormally High Combustion Pressure
Potential Uneven Thrust on Piston/Cylinder
Louder Engine Knock
Excessive Engine Knock and Smoke at Cold Start
FIG 2. Limitations of vegetable oil due to inadequate cetane number [3].
o Volatility: This is the property of changing readily from liquid to vapor.
According to ASTM D 975, the volatility of normal diesel the range for No. 2
grades of diesel fuel is 282ºC to 338ºC. This limits the level of high boiling
point components that could lead to increased engine deposits.
Vegetable oil has a relatively low volatility which leads to more engine
deposit
3
o Viscosity: The viscosity of diesel fuel is an important property which impacts
the performance of fuel injection systems. ASTM D 975 requires a kinematic
viscosity range of 1.9 minimum to 4.1 maximum mm2/S at 40ºC, for No. 2
diesel fuels (note that the term mm2/S replaces the former term of
centistokes [cst]).
The viscosity of vegetable oil is around 35 cst which is too high for the
engine. It causes too much pump resistance, filter damage and adversely
affect fuel spray patterns.
FIG 3. Viscosity affecting spray pattern [3].
o Flash point: The flashpoint is the lowest fuel temperature at which the vapor
above a fuel sample will momentarily ignite under the prescribed test
conditions. For No. 2 diesel grades, the flashpoint is a minimum of 52ºC. For
vegetable oil, the flash point is 220ºC which is too high.
o Low temperature operability: the cloud point and pour point of a vegetable
oil is high compare to diesel. Therefore, vegetable oil cannot work at low
temperature weather condition.
2.2 Limitations based on chemical and structural properties.
According to chemical and structural composition properties of a vegetable oil, it has
some limitation in using it as alternative fuel for a diesel engine.
Examining Rapeseed and Canola:
4
Oil sample Contamination
Mg/Kg Acid
value mg
KOH/Kg
Oxidation
stability
(1100C) h
Phosphorous
content mg/kg
Ash
Mass % Water
Mass %
Rapeseed 25 2.0 5.0 15 0.01 0.075
TABLE 1: Chemical content of rapeseed [4].
Component Canola Rapeseed Soybean
Triglycerides(%) 94.4- 99.1 91.8 – 99.0 93.0 – 99.2
Phospholipids (%)
Crude oil Up to 2.5 Up to 3.5 Up to 4.0
Water-degummed Up to 0.6 Up to 0.8 Up to 0.4
Acid-degummed Up to 0.1 - Up to 0.2
Free fatty Acid(%) 0.4 -1.2 0.5 – 1.8 0.3 – 1.0
Unsaponifiables(%) 0.5 – 1.2 0.5 – 1.2 0.5 – 1.6
Tocopherols (ppm) 700 - 1200 700 - 1000 1700 - 2200
Chorophylls (ppm) 5 - 35 5 - 35 Trace
Sulfur (ppm) 3 - 15 5 - 25 Nil
TABLE 2: Constituents of Canola, Rapeseed and Soybean Oils [3].
Oil sample phosphorus Iron Calcium Sulfur Zinc Lead
Canola 1190 3.52 296.0 6.5 2.4 0.24
TABLE 3: Simple Mineral Element Content in Canola Oil [3].
It must be noted that all these chemical composition contributes to limitation of
using straight vegetable oil in diesel engine. Details are given in table 4.
5
summary of limitations based of chemical properties
Sulfur content Low, not enough to control emission equipment
Water and Sediment Content Filter plugging, injector wear, increased corrosion
Ash Content Injector and fuel pump wear, piston and ring wear
Engine deposit.
Carbon Residue Fuel system deposit and Combustion Chamber deposit
TABLE 4: Limitations due to chemical properties.
The structural property also evoke limitations
Vegetable oil are composes of Triglycerides which are the main constituents of
vegetable oils and animal fats. Triglycerides have lower densities than water (they
float on water), and at normal room temperatures may be solid or liquid. When
solid, they are called "fats" or "butters" and when liquid they are called "oils".
A triglyceride, also called triacylglycerol (TAG), is a chemical compound formed from
one molecule of glycerol and three fatty acids.
Each triglyceride is composed of fatty acid three long chain fatty acid of 8 -22
carbons attached to a glyceride backbone [1]. This makes the vegetable oil to be
very different form the conventional diesel fuel.
6
2.3 Other limitations
The following are some other limitations of straight vegetable oil.
o Carbon Deposits: Excessive carbon deposits and contamination of engine
lubricant both result from SVO's higher boiling point relative to diesel or
blended biodiesel fuels. U.S. Department of Energy studies show prolonged
use dramatically reduces engine life because of this property. The over-
wetting caused by high viscosity exacerbates carbon buildup and lubrication
contamination.
o Catalytic Converter Damage: Modern clean diesel engines use catalytic
converters and fuel traps. SVO's higher boiling point causes saturation of
these components and can poison the catalytic converter with long-term use.
o Clogged Fuel Pumps: Both the increased viscosity and higher boiling points of
SVO lead to fuel pump clogging. While short-term use was deemed effective
in terms of engine performance and emissions, long-term use was deemed a
hazard to the durability of the fuel pump and other fuel system components
[5].
7
3. Limitation of Ethanol in Gasoline Engine
Ethanol is an alcohol-based fuel made by fermenting and distilling starch crops, such as
corn. It can also be made from "cellulosic biomass" such as trees and grasses. The use of
ethanol can reduce our dependence upon foreign oil and reduce greenhouse gas
emissions. A pure ethanol is the one with 95% ethanol.
It must be noted that the 95% cannot be use directly in gasoline engine without been
blended with gasoline (between 10-85 percent value, E10-E85) or necessary engine
modification [6]. This is due to the following limitations.
3.1 Limitations Based on Physical and Combustion Properties.
The following table shows the physical and combustion properties of ethanol
Characteristics of chemically pure fuels.*
Chemical formula Chemical
weight
(lb/mole)
Specific
gravity
Boiling
point
(C)
Latent
heat
(Btu/lb)
Combustion
energy
(Btu/lb)
Vapor
pressure
@100F
(psi)
Solubility
part in
100
parts
H2O
Stoichio-
metric
air-fuel
ratio
Methyl
alcohol
CH3OH 32 0.79 65 503 10,260 4.6 infinite 6.5
Ethyl
alcohol
CH3CH2(OH) 46.1 0.79 78 396 13,160 2.2 infinite 9
Butyl
alcohol
C2H5CH2CH2(OH) 74.1 0.81 117 186 15,770 0.3 9 11.2
Octane C8H18 114 0.70 210 155 20,750 1.72 insoluble 15.2
Hexa-
decane
C16H34 240 0.79 287 -- 20,320 3.46 insoluble 15
*To convert to metrics, use the following conversion factors: 1 pound = 45 kilogram; 1 degree F = degrees C - 32 x
5/9.
TABLE 5: Properties of Ethanol [7].
Based on the above properties the following limitations can be deducted:
o Driving ability of ethanol is lower.
Lower per liter energy value (EV)
Takes more to drive the same distance
Consumers have to fill their cars more often and pay more for ethanol fuel.
8
o Ethanol can absorb water & if water enters the fuel tank
It dilutes ethanol, reducing its value as a fuel;
It causes problems with corrosion and phase separation in the gasoline mixture.
o Ethanol dissolves almost everything.
It absorbs and carries dirt inside the fuel lines and fuel tank, thus contaminating
the car engine system.
o Ethanol is rich in octane content.
It is highly flammable and explosive compared to gasoline.
It requires more attention to handle in daily life.
o Low flash point 13 – 140C
It burns easily
3.2 Limitation Based on Chemical and Structural Properties.
The following are the major chemical properties of ethanol
Molecular formula C2H6O
Molar mass 46.07 g mol−1
Exact mass 46.041864814 g mol−1
Acidity (pKa) 15.9[2]
Basicity (pKb) -1.9
Refractive index (nD) 1.36
Viscosity 0.0012 Pa s (at 20 °C)
Dipole moment 1.69 D
TABLE 6: Some chemical properties of ethanol.
o Ethanol is a reducing agent -- it can be oxidized by strong oxidants, to
acetaldehyde and to acetic acid Ethanol is also weak acid.
Therefore Alcohols may be corrosive to certain materials used in engines.
Generally, methyl alcohol is the most corrosive and butyl alcohol is least
corrosive. Alcohols also can cause injury or physical harm if not used properly.
9
People who use alcohol in motor fuels should observe warning labels and follow
precautions to avoid problems [7].
o Aldehyde, a function of ethanol volume, is a threat to nose, eyes, throat &
possibly causes cancer [8].
3.3 Other Limitations
Other limitation includes:
Instability of the micro-emulsion (separation of ethanol phase
o Another problem is that ethanol has a smaller energy density than gasoline. It
takes about 1.5 times more ethanol than gasoline to travel the same distance.
However, with new technologies and dedicated ethanol-engines, this is expected
to drop to 1.25 times.
o Another problem is that ethanol burning may increase emission of certain types
of pollutants. Like any combustion process, some of the ethanol fuel would come
out the tailpipe unburned. This is not a major problem since ethanol emissions
are relatively non-toxic. However, some of the ethanol will be only partially
oxidized and emitted as acetylaldehyde, which reacts in air to eventually
contribute to the formation of ozone. Current research is investigating means to
reduce acetylaldehyde emissions by decreasing the engine warm-up period [9].
10
4. Possibility of Engine Modification.
In case of using straight vegetable oil or ethanol, necessary engine modifications must
be considered. The following explains the possibilities of modifying an engine for their
usage.
4.1 Gasoline Engine Modification
o MAINJET CHANGES
The first thing to alter is the main metering jet in your carburetor. In most
carburetors, this is a threaded brass plug with a specific-sized hole drilled through
the center of it. This hole is called the main jet orifice, and its diameter dictates how
rich or lean the air/fuel mixture will be when the car is traveling at cruising speeds.
Naturally, the smaller the hole is, the less fuel will blend with the air and the leaner
the mixture will be. As the orifice is enlarged, the mixture gets richer.
Since alcohol requires a richer air/fuel ratio, it's necessary to bore out the main jet
orifice when using ethanol fuel. The standard jet size in MOTHER's alcohol-powered
truck was .056" ... in other words, this was the diameter of the jet orifice. In order to
operate the engine successfully on alcohol fuel, it's necessary to enlarge this opening
by anywhere from 20 to 40%.
o IDLE ORIFICE CHANGES
Most carburetors will require additional idle circuit enlargement in order for the
engine to run at slowest, or idle, speeds. This is because the circuit that's fed by the
main jet operates fully only when the throttle plate within the throat of the
carburetor is opened past the idle position. When the plate is in the idle position,
the air/fuel mixture is allowed to enter the manifold only through the idle orifice
itself ... which, if it isn't large enough, will not provide the needed amount of air/fuel
blend to keep the engine running.
On some engines, it may only be necessary to loosen the idle mixture screw at the
base of the carburetor in order to provide the correct amount of fuel, since this
11
threaded shaft has a tapered tip which allows more mixture to pass as the tip is
backed off. On other engines, it's possible that the seat itself, into which the tapered
screw extends, must be enlarged in order to accomplish the same thing.
In most cases, if the seat has to be bored out, it can be enlarged by 50%, using the
same method of measurement as was detailed in the main jet section. This will allow
a full range of adjustment with the idle mixture screw, even if you should want to go
back to gasoline fuel. (When drilling, be careful not to damage the threads in the
carburetor body.
As a precaution against the idle screw's vibrating loose from its threaded opening,
you can shim the idle mixture screw spring with a couple of small lock washers ...
this will prevent the screw from turning even if it's drawn out farther from the seat
than it normally would be.
o POWER VALVE CHANGES
Most modern auto carburetors have what is known as a power valve that allows
extra fuel to blend with the air/fuel mixture when the accelerator is depressed, in
order to enrich the mixture under load conditions. This vacuum-controlled valve is
spring loaded, and shuts off when it isn't needed in order to conserve fuel.
The power valve used in the carburetor illustrated is somewhat difficult to alter and,
besides, is sufficient for alcohol use in its normal configuration if it's working
properly. However, there are other carburetors - specifically the Holley and Ford
(Autolite or Motorcraft) brands - that have easily replaceable power valves which
are available from auto parts stores in various sizes. If you use a power valve with a
25% or so greater flow capacity than the one that originally came with the
carburetor, your air/alcohol mixture will be sufficiently enriched when your engine
needs more power.
o ACCELERATOR PUMP CHANGES
In addition to a power valve, almost all automotive carburetors utilize an accelerator
pump. This is a mechanically activated plunger or diaphragm that injects a stream of
raw fuel directly down the throat of the carburetor when the accelerator is suddenly
12
depressed. The fuel is injected through a small orifice located in the throat wall at
some point above the carburetor venturi (the point at which the throat narrows).
The reason the accelerator pump is incorporated into modern carburetors is that as
the accelerator is pressed and more air/fuel mixture is drawn into the cylinders,
some of the liquid particles in the blend tend to stick to the walls of the intake
manifold, effectively leaning out the mixture by the time it reaches the combustion
chambers. The extra squirt of fuel that's added by the accelerator pump makes up
for this initial lean condition.
In order to adapt your accelerator pump to use alcohol effectively, you'll probably
have to enlarge the size of the injection orifice slightly (anywhere from 10 to 25% is
fine ... if you go larger than that, you'll risk the possibility of altering the pump
pressure enough either to turn the fuel stream into a dribble or to empty the pump
reservoir before the pump has made a full stroke).
As an alternative to enlarging the hole, you may be able to simply adjust the stroke
length of the pump arm in order to feed more fuel. Most carburetors installed on
Ford products already have a provision for seasonal adjustment, so it's just a matter
of putting the pump on its richest setting. Other carburetors, too, have threaded
rods that can be adjusted to accomplish the same thing.
o CHOKE ALTERATION
Although it's not absolutely necessary to adapt your car's choke system to burn
alcohol fuel, it has been our experience that a manually operated choke is more
desirable on an alcohol-powered car. If your vehicle's engine is already so equipped,
fine. If not, you can purchase - for about $7.00 from any auto parts store - a manual
choke conversion kit that will allow virtually any automatic choke to be adapted for
manual control.
o IGNITION TIMING
In order to take advantage of the great antiknock qualities that alcohol fuel provides,
you'll have to advance the engine's ignition timing by turning the distributor housing
13
opposite to the direction in which the rotor spins (the housing is held in place by a
bolted clamp).
Normally, an engine using gasoline has its timing set so the spark occurs at anywhere
from 8 deg BTDC (Before Top Dead Center) to TDC (Top Dead Center). Since alcohol
has a higher "octane" rating, you can advance the timing considerably more than
this. (In the case of MOTHER's truck, we adjusted it to operate at approximately 22
deg BTDC without any sign of pre-ignition, even under load.) Of course, care should
be taken when you adjust the timing on your vehicle, since a 22 deg advance might
be excessive for your car. Remember, it's not safe to be just short of detonation,
since inaudible knocking can also damage the engine ... the best procedure is to set
the distributor timing at least two degrees retarded from the point of detonation.
o COMPRESSION RATIO CHANGES
Increasing the compression ratio of the engine will be impractical for most people,
because of the expense and work involved ... however, this modification will do a
great deal to improve engine performance and economy. Just like a timing advance,
a compression ratio hike will take advantage of the potential that alcohol has to
offer as a fuel. Optimally, the ratio can be increased to 14- or 15-to-1 ... but even a
nominal increase - to perhaps 12-to-1, a figure that some manufacturers have
already offered in the past for premium gasoline use - will result in a vast
improvement over the standard 8- or 8.5-to-1 that most manufacturers incorporate
into their engines today.
o FUEL PREHEATING
In extremely cold climates, it may be necessary to preheat your alcohol fuel before it
enters the carburetor float bowl. This can be accomplished easily by splicing into the
fuel feed line - near the point where it passes the upper radiator hose - and installing
a fuel heater at this location.
o AIR PREHEATING
Most trucks and autos have air filter housings which are designed to allow heated air
from around the exhaust manifold to channel through a duct and enter the
14
carburetor when the engine first starts from a cold state. As the engine warms up, a
flap within the air cleaner "snorkel" shuts off this supply of warm air and allows
ambient air from the engine compartment to enter in its stead.
This flap is usually either thermostatically or vacuum controlled ... but either way,
you may find it helpful during the winter months to leave this valve closed to the
cold outside air. This can be done either by disconnecting the bimetallic thermostat
spring that controls the flap and installing a small spring of your own that will hold
the valve in the required position, or - if the flap is vacuum activated - by connecting
an existing permanent vacuum line to its control fitting. (You can, of course, remove
the control line entirely, plug it up, and hold the flap closed with a spring if you
wish.)
o THERMOSTAT CHANGE
In order to get maximum efficiency from your engine, you may need to change the
thermostat within the engine block. Thermostats are available in various heat ranges
from 140 to 200 deg F, and these temperatures indicate how hot the engine coolant
will be allowed to get before the thermostat opens to initiate the cooling process. (A
thermostat is designed to hold the coolant within the cylinder head till it achieves
the desired temperature ... at which point the heated liquid is allowed to escape into
the radiator to be cooled, and is replaced by a fresh supply of cool fluid. Depending
on the engine's operating conditions, the thermostat may cycle open and shut
regularly over the span of a few minutes.)
By using a hotter thermostat, you'll be able to warm up the entire engine, including
the intake manifold.
o COLD WEATHER STARTING
Since alcohol doesn't vaporize as easily as does gasoline, cold weather starting can
be a problem ... especially if the engine itself is cold. To alleviate this undesirable
situation, MOTHER's research staff has designed a combination coldstart/dual-fuel
system that'll work with any car.
15
All it requires is a five-gallon fuel storage tank with a fuel filler neck brazed into its
top (we used an old propane bottle), an auxiliary electric fuel pump, some steel
brake or fuel line, neoprene hose, an elbow, a length of copper pipe, a small
metering jet, and several needle valves, tees, and hose barbs. (Details and
illustrations of the installation are shown in the article reprints from MOTHER NOS.
59 and 60, which are included in this workbook.)
The five-gallon tank is mounted in some safe place on the truck or automobile and
used to store gasoline. This cache of petroleum fuel serves a dual role: When it's
needed for cold starting purposes, the electric pump is activated momentarily from
inside the car and a fine stream of gasoline is injected down the throat of the
carburetor. And, in the event that your alcohol supply is unexpectedly depleted on
the highway, the gasoline stored in the small tank can be routed into the carburetor
normally for emergency use.
o FUEL INJECTION SYSTEMS
Since some vehicles are equipped with fuel injection rather than carburetors, we will
briefly touch on the use of alcohol with that system. There are two important factors
in a fuel injection setup: injection timing and control jet diameter. Fortunately - since
many systems now use an electronically controlled timing sequence - injection
timing is not critical in a fuel injected engine. Neither performance nor economy
improve substantially by either advancing or retarding the injection timing process.
Control jet diameter, on the other hand, is an important factor. If you increase the
size of the control jets (which are the equivalent of the metering jets in a
carburetor), the engine will operate well on alcohol fuel. An increase of 15-20% is all
that's necessary to accomplish the conversion. (Ignition timing should, of course, be
advanced as explained previously.)
An interesting feature of the fuel injection system is that it doesn't require any
gasoline during the cold weather starting process to fire the engine up. Since the
fuel is injected at a pressure of about 250 PSI, the alcohol fuel is sufficiently
vaporized to ignite easily within the combustion chamber.
16
[10][11].
4.2 Diesel Engine Modification
To run a diesel engine on straight vegetable oil (SOV) the following modifications
must be considered.
o Preheating the oil, to make it less viscous
o Avoiding injector coking by replacing it often
o Adding extra engine coolant
o Vegetable oil filter
o Fuel filter preheater
o Thermoswitch
o Necessary parts includes
3-port solenoid valves
The fuel return loop
Custom-made heated tank
The hose within hose
Detail information on installation can be found in [12].
17
5. Conclusions
The limitations of using straight vegetable oil or ethanol in diesel engine and gasoline in
respectively have been discoursed. It is clear that they cannot be use directly without an
engine modification which is important or a long term use.
18
References
[1]: ME541-Biofuel- Lecture notes-1, 13/04/2012
[2]: Technology roadmap, Biofuel for Transport, International Energy Agency, page 21.
[3]: http://www.biodiesel.org/docs/ffs-performace_usage/service-technician's-guide-to-diesel-
fuel.pdf?sfvrsn=4
[4]: http://journeytoforever.org/biodiesel_yield2.html
[5]: http://www.ehow.com/list_5783028_disadvantages-using-vegetable-oils-fuel.html
[6]: http://www.fueleconomy.gov/feg/ethanol.shtml
[7]: http://www.ext.colostate.edu/pubs/farmmgt/05010.html
[8]: Advantages and Disadvantages of Using Ethanol: The Consumer Viewpoint, Ngo Anh-Thu and Gale
West, AIEA 2nd International Conference and Workshop, Laval University , Quebec, 2004
[9]: http://science.jrank.org/pages/2576/Ethanol-Disadvantages-ethanol-an-alternative-fuel.html
[10]: http://running_on_alcohol.tripod.com/id32.html
[11]: http://running_on_alcohol.tripod.com/id32.html
[12]:http://www.autonopedia.org/renewable_energy/Biofuels/How_To_Run_Your_Diesel_On_Staight_
Veg_Oil.html