Liquid Fuels

1. Liquid Fuels
Liquid Fuels

2. Example
Example
• What is the air/fuel ratio and the
exhaust products when ethanol is
used as an engine fuel?

3. Solution
Solution
95
.
8
1
/
)
866
.
6
087
.
2
(
/
174
.
1
913
.
1
866
.
6
866
.
6
087
.
2
1
)
18
(
3
)
44
(
2
)
28
(
28
.
11
)
28
(
28
.
11
)
32
(
3
)
46
(
1
3
2
28
.
11
28
.
11
3 2
2
2
2
2
6
2
=
+
=
+
+
→
+
+
F
A
O
H
CO
N
N
O
O
H
C

4. Calculate the Stoichiometric Air ?
Calculate the theoretical CO2
content in flue gases ?
?

5. Heating Value Estimates for
Heating Value Estimates for
Petroleum Fuels
Petroleum Fuels
• Heating values are estimated from the
API gravity,
• where Hg is the gross (high) heating value
and Hn is the net (low) heating value.
( )
)
/
(
000
,
10
7190
.
0
)
/
(
10
93
860
,
42
kg
kJ
H
H
kg
kJ
API
H
g
n
g
+
⋅
=
−
⋅
+
=

6. 6
Density
• Ratio of the fuel’s mass to its volume at 15 oC,
• kg/m3
• Useful for determining fuel quantity and quality
Properties of Liquid Fuels

7. 7
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
0.88-0.98
0.89-0.95
0.85-0.87
Specific
Gravity
LSHS (Low Sulphur
Heavy Stock)
Furnace oil
LDO
(Light Diesel Oil)
Fuel oil
type
Table Specific gravity of various fuel oils

8. 8
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

9. Typical Units
Typical Units
• Centipoise (cP) was the popular unit of
dynamic viscosity.
• Centistoke (cSt) was the popular unit of
kinematic viscosity.
s
mPa
cP ⋅
= 1
1
s
mm
cSt /
1
1 2
=

10. Reporting of Viscosity
Reporting of Viscosity
• Kinematic viscosity (ν) is reported as,
• where µ is absolute (or dynamic)
viscosity, and ρ is the fluid mass density.
ρ
µ
υ =

11. Table SAE Motor Oil
Table SAE Motor Oil
Classification
Classification

12. Cloud and Pour Points
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.

13. Fundamental Definitions
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)Ȟwater

14. 14
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 Gross Calorific Value (kCal/kg)
Kerosene 11,100
Diesel Oil 10,800
Furnace Oil 10,500

15. 15
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

16. 16
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

17. Four stroke cycle theory
Intake stroke
Piston moving down
Intake valve open
Exhaust valve closed

18. Four stroke cycle theory
Compression stroke
Piston moving up
Intake valve closed
Exhaust valve closed

19. Four stroke cycle theory
Power stroke
Piston moving down
Intake valve closed
Exhaust valve closed

20. Four stroke cycle theory
Exhaust stroke
Piston moving up
Intake valve closed
Exhaust valve open

21. Engine measurements
Bore
• Diameter of cylinder
Stroke
• Distance between TDC & BDC

22. Engine measurements
Displacement per cylinder
• π r² S
Displacement for the engine
• Disp per cylinder times the
Number of cylinders

23. Engine measurements
Compression ratio
D + CV
CV
To calculate clearance volume
D .
CR-1

24. 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.

25. Knock in SI engines.
Knock in SI engines.

26. Octane Ratings
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.

27. How to Reduce Engine
How to Reduce Engine
Knock
Knock
Use gasoline with higher octane
ratings – these ratings are
associated with gasoline that has
few straight chain carbons have
longer ignition delay times.

28. Octane Rating Measurement
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.

29. Octane Rating Measurement
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.

30. 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.

31. 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 Motor
Inlet temperature (oC) 52 149
Speed (rpm) 600 900
Spark advance (oBTC) 13 19-26 (varies with r)
Coolant temperature (oC) 100
Inlet pressure (atm) 1.0
Humidity (kg water/kg dry air) 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.

32. 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:
Octane Number Measurement
2
MON
RON
index
Antiknock
+
=
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.

33. Fig. CFR Engine
Fig. CFR Engine

34. Octane Ratings
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.

35. Cetane
Cetane Ratings
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!

36. Cetane
Cetane Ratings
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.

37. Effect of
Effect of Cetane
Cetane Rating
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.

38. Cetane
Cetane Ratings and CI
Ratings and CI
Engines
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.

39. Altering Knock in CI 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.

40. Fuel Viscosity
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.

41. Fuel Impurities
Fuel Impurities -
- Sulfur
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.

42. Fuel Impurities
Fuel Impurities -
- Ash
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.

43. Fuel Impurities
Fuel Impurities –
– Water and
Water and
Sediment
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.

44. Fuel Impurities
Fuel Impurities -
- Gum
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.

45. Fuel Additives
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.

46. Table 5.5: Gasoline
Table 5.5: Gasoline
additives
additives

47. Fuel Storage
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

48. Lubricating Oil Additives
Lubricating Oil Additives