Liquid Fuels
Example

What is the air/fuel ratio and the exhaust
products when ethanol is used as an
engine fuel?
Solution
C2 H 6O  3O2  11.28 N 2  11.28 N 2  2CO2  3H 2O
1(46) 3(32) 11.28(28) 11.28(28) 2(44) 3(18)
1 2.087 6.866 6.866 1.913 1.174
A / F  (2.087  6.866) / 1  8.95
Calculate the Stoichiometric Air ?
Calculate the theoretical CO2 content
in flue gases ?
Heating Value Estimates for
Petroleum Fuels

Heating values are estimated from the API
gravity,
H g  42,860  93   API  10 (kJ / kg)
H n  0.7190  H g  10,000

(kJ / kg)
where Hg is the gross (high) heating value and
Hn is the net (low) heating value.
Properties of Liquid Fuels
 Density
• Ratio of the fuel’s mass to its volume at 15 oC,
• kg/m3
• Useful for determining fuel quantity and quality
6
Liquid Fuels
 Specific gravity
• Ratio of weight of oil volume to weight of same
water volume at a given temperature
• Specific gravity of water is 1
• Hydrometer used to measure
Table Specific gravity of various fuel oils
Fuel oil
type
LDO
(Light Diesel Oil)
Furnace oil
LSHS (Low Sulphur
Heavy Stock)
Specific
Gravity
0.85-0.87
0.89-0.95
0.88-0.98
7
Liquid Fuels
 Viscosity
• Measure of fuel’s internal resistance to flow
• Most important characteristic for storage and use
• Decreases as temperature increases
 Flash point
• Lowest temperature at which a fuel can be heated
so that the vapour gives off flashes when an open
flame is passes over it
• Flash point of furnace oil: 66oC
8
Typical Units

Centipoise (cP) was the popular unit of dynamic
viscosity.
1 cP  1 mPa  s

Centistoke (cSt) was the popular unit of
kinematic viscosity.
1 cSt  1 mm2 / s
Reporting of Viscosity

Kinematic viscosity (n) is reported as,
m

r

where m is absolute (or dynamic) viscosity, and
r is the fluid mass density.
Table SAE Motor Oil
Classification
Cloud and Pour Points





Cloud point is the temperature at which crystals
begin to form in the fuel.
Pour point is the temperature at which the fuel
ceases to flow. Indication of temperature at
which fuel can be pumped
Cloud point are typically 5 to 8 C higher than
pour point,
Not an issue for gasoline.
Values are important for diesel.
Fundamental Definitions


Calorific value
Amount of heat librated by the combustion of unit
quantity of fuel. kcal/ kg , kcal / m3
Gross Calorific Value (G.C.V) or HCV

heating value measurement in which the product
water vapour is allowed to condense
Net Calorific Value (N.C.V) or LCV
heating value in which the water remains a vapor
and does not yield its heat of vaporization

HHV = LHV + (mwater /mfuel)ʎwater
Liquid Fuels
 Calorific value
• Heat or energy produced
•
Gross calorific value (GCV): vapour is fully
condensed
• Net calorific value (NCV): water is not fully
condensed
Fuel Oil
Kerosene
Diesel Oil
Furnace Oil
Gross Calorific Value (kCal/kg)
11,100
10,800
10,500
14
Liquid Fuels
 Sulphur content
• Depends on source of crude oil and less on the
refining process
• Furnace oil: 2-4 % sulphur
• Sulphuric acid causes corrosion
 Ash content
•
Inorganic material in fuel
• Typically 0.03 - 0.07%
• Corrosion of burner tips and damage to materials
/equipments at high temperatures
15
Liquid Fuels
 Carbon residue
• Tendency of oil to deposit a carbonaceous solid
residue on a hot surface
• Residual oil: >1% carbon residue
 Water content
•
Normally low in furnace oil supplied (<1% at
refinery)
• Free or emulsified
form
• Can damage furnace surface and impact flame
16
Four stroke cycle theory
Intake stroke
Piston moving down
Intake valve open
Exhaust valve closed
Four stroke cycle theory
Compression stroke
Piston moving up
Intake valve closed
Exhaust valve closed
Four stroke cycle theory
Power stroke
Piston moving down
Intake valve closed
Exhaust valve closed
Four stroke cycle theory
Exhaust stroke
Piston moving up
Intake valve closed
Exhaust valve open
Engine measurements
Bore
• Diameter of cylinder
Stroke
• Distance between TDC & BDC
Engine measurements
Displacement per cylinder
•  r² S
Displacement for the engine
• Disp per cylinder times the
Number of cylinders
Engine measurements
Compression ratio
D + CV
CV
To calculate clearance volume
D .
CR-1
Abnormal Combustion in SI Engine
Knock is the term used to describe a pinging noise emitted from a SI engine
undergoing abnormal combustion.
The noise is generated by shock waves produced in the cylinder when
unburned gas autoignites.
Knock in SI engines.
Octane Ratings



Octane is a measure of gasoline’s
resistance to “knock.”
“Knock” is the uncontrolled release of
energy when combustion initiates
somewhere other than the spark plug.
Symptoms of engine “knock” include an
audible “knocking” or “pining” sound
under acceleration.
How to Reduce Engine Knock
Use gasoline with higher octane ratings –
these ratings are associated with gasoline
that has few straight chain carbons have
longer ignition delay times.
Octane Rating Measurement



Procedure developed by the Cooperative
Fuels Research Committee (CFR).
The committee proposed a single cylinder
SI engine to measure octane – the CFR
engine has an adjustable compression
ratio.
Engine is driven at a constant speed with
an electric motor.
Octane Rating Measurement




Octane ratings are obtained by
comparing fuel in question to iso-octane
(Octane Rating of 100) and heptane
(Octane Rating of 0).
CR is adjusted until “knocking” is
detected with fuel being tested.
Blends of iso-octane and heptane are
tested until the same level of knock is
obtained.
Octane rating is % of iso-octane in test
blend.
Fuel Knock Scale
To provide a standard measure of a fuel’s ability to resist knock, a scale has
been devised by which fuels are assigned an octane number ON.
The octane number determines whether or not a fuel will knock in a given
engine under given operating conditions.
By definition, normal heptane (n-C7H16) has an octane value of zero and
isooctane (C8H18) has a value of 100.
The higher the octane number, the higher the resistance to knock.
Blends of these two hydrocarbons define the knock resistance of intermediate
octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an
octane number of 90.
A fuel’s octane number is determined by measuring what blend of these two
hydrocarbons matches the test fuel’s knock resistance.
Octane Number Measurement
Two methods have been developed to measure ON using a standardized
single-cylinder engine developed under the auspices of the Cooperative Fuel
Research (CFR) Committee in 1931.
The CFR engine is 4-stroke with 3.25” bore and 4.5” stroke, compression
ratio can be varied from 3 to 30.
Research
Inlet temperature (oC)
Speed (rpm)
Spark advance (oBTC)
Coolant temperature (oC)
Inlet pressure (atm)
Humidity (kg water/kg dry air)
52
600
13
Motor
149
900
19-26 (varies with r)
100
1.0
0.0036 - 0.0072
Note: In 1931 iso-octane was the most knock resistant HC, now there are
fuels that are more knock resistant than isooctane.
Octane Number Measurement
Testing procedure:
• Run the CFR engine on the test fuel at both research and motor conditions.
• Slowly increase the compression ratio until a standard amount of knock
occurs as measured by a magnetostriction knock detector.
• At that compression ratio run the engines on blends of n-heptane and
isooctane.
• ON is the % by volume of octane in the blend that produces the stand. knock
The antiknock index which is displayed at the fuel pump is the average of
the research and motor octane numbers:
Antiknock index 
RON  MON
2
Note the motor octane number is always lower because it uses more severe
operating conditions: higher inlet temperature and more spark advance.
The automobile manufacturer will specify the minimum fuel ON that will resist
knock throughout the engine’s operating speed and load range.
Fig. CFR Engine
Octane Ratings





CFR developed initial method (Motor Octane
Number – MON).
ASTM developed a new method (Research
Octane Number – RON).
RON octane ratings are 8 points low than MON
for most gasoline.
Most retailers report the Anti-Knock Index which
is an average of MON and RON.
Octane ratings of fuel are adjusted for elevation
– lower atmospheric pressure reduces the
tendency for engine knock to occur.
Cetane Ratings



Cetane rating are an indication of the
fuel’s anti-knock resistance for CI engines.
Fuels with high cetane ratings are created
by increasing the proportion of long chain
molecules, thereby reducing the ignition
delay.
Fuels with high Octane Rating have low
cetane ratings!
Cetane Ratings

CFR cetane rating process is similar to the
Octane process with a couple of
differences:



Cetane and Alpha methyl naphthalene are the
reference fuels.
Cetane is given a cetane number of 100.
Alpha methyl naphthalene has cetane rating
of zero
Hyptamethylnonane has a cetane rating of 15.
Effect of Cetane Rating






If cetane rating is too low, the ignition
delay results in hard starting (combustion
after piston is moving downward) and
characteristic ”white smoke.”
High cetane ratings start the combustion
process to soon, and some of the fuel is
not volatized and does not burn.
“Black smoke” in heavily loaded engines is
a symptom of high cetane ratings.
Minimum cetane rating for CI engines is 40
according to SAE.
Commercial fuels seldom exceed 50.
Cetane rating should never exceed 60.
Cetane Ratings and CI Engines




Octane rating is not a good way to predict
“knock” in CI engines.
Combustion in diesel engines consists of a two
part delay – physical and chemical.
Physical - the fuel is injected and atomized.
Chemical - process proceeds with a pre-flame
chemical reaction, similar to that of SI engines.
Altering Knock in CI Engines


Ignition delay controls the relative release
of energy between the two phases of
combustion – a longer delay results in
more energy produced in the pre-mix
phase.
Since “knock” occurs when more energy is
released at the start of combustion, it
follows that “knock” is reduced with short
delay periods.
Fuel Viscosity



Viscosity is a measure of the flow
resistance of liquid.
Fuel viscosity must be high enough to
insure good lubrication of injection pump
mechanisms in CI engines.
Fuel viscosity must be low enough to
insure proper atomization at the time of
injection.
Fuel Impurities - Sulfur


Sulfur oxides – can convert to acids which
corrode engine parts and cause increased
wear.
Assessed by immersing copper strip in fuel
for three hours, then comparing corrosion
to standard strips.
Fuel Impurities - Ash



Ash – small solid particles or water-soluble
metals found fuels.
Defined as un-burned fuel residue left
behind.
Can cause accelerated wear of close-fitting
injection system parts.
Fuel Impurities – Water and
Sediment


Moisture can condense in fuel storage
tanks, or seep in from underground leaks.
Fuel should be bright and clear, and visibly
free of water and sediment.
Fuel Impurities - Gum


Gum can form in gasoline, leaves behind
deposits on carburetors. Gum is dissolved
by gasoline – more prevalent in gasoline
that is made by cracking.
Antioxidants are now added to both diesel
and gasoline to extend storage life without
gum formation.
Fuel Additives


Until 1970, gasoline contained TEL
(tetraethyl lead). TEL was used as an
octane booster.
MTBE (methyl tertiary butyl ether) is often
substituted as an octane booster – could
be phased out/banned soon.
Table 5.5: Gasoline additives
Fuel Storage


Fuels classified according to
flammability – gasoline is more
dangerous with a flash point of -40
C.
Major concern with regard to
environmental contamination
Lubricating Oil Additives