1. Liquid Fuels
Physical Properties

2. Liquid Fuels Heating Value Volatility Density/Gravity Viscosity
Flash point
Fire Point
Cloud & Pour Point
Conradson Carbon Residue
Reid Vapour Pressure
Corrosion Test
Sulfur
Octane Number/ Cetane Number
The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends on the temperature and decreases as the temperature increases. Any numerical value for viscosity has no meaning unless the temperature is also specified. Viscosity is measured in Stokes / Centistokes. Sometimes viscosity is also quoted in Engler, Saybolt or Redwood. Each type of oil has its own temperature - viscosity relationship. The measurement of viscosity is made with an instrument called a Viscometer.
Viscosity is the most important characteristic in the storage and use of fuel oil. It influences the degree of pre-heating required for handling, storage and satisfactory atomization. If the oil is too viscous, it may become difficult to pump, hard to light the burner, and difficult to handle. Poor atomization may result in the formation of carbon deposits on the burner tips or on the walls. Therefore pre-heating is necessary for proper atomization.
(Click once) The flash point of a fuel is the lowest temperature at which the fuel can be heated so that the vapour gives off flashes momentarily when an open flame is passed over it. The flash point for furnace oil is 66 0C.

3. Heating Value Estimates for 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.

4. Density Properties of Liquid Fuels
Ratio of the fuel’s mass to its volume at 15 oC,
kg/m3
Useful for determining fuel quantity and quality
Density is defined as the ratio of the mass of the fuel to the volume of the fuel at a reference temperature of 15°C. Density is measured by an instrument called a hydrometer. The knowledge of density is useful for quantitative calculations and assessing ignition qualities. The unit of density is kg/m3.

5. 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
This is defined as the ratio of the weight of a given volume of oil to the weight of the same volume of water at a given temperature. The density of fuel, relative to water, is called specific gravity. The specific gravity of water is defined as 1. Since specific gravity is a ratio, it has no units. The measurement of specific gravity is generally made by a hydrometer. Specific gravity is used in calculations involving weights and volumes.
Table Specific gravity of various fuel oils
Specific Gravity
LSHS (Low Sulphur
Heavy Stock)‏
Furnace oil
LDO
(Light Diesel Oil)‏
Fuel oil type

6. Liquid Fuels Viscosity Flash point
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
The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends on the temperature and decreases as the temperature increases. Any numerical value for viscosity has no meaning unless the temperature is also specified. Viscosity is measured in Stokes / Centistokes. Sometimes viscosity is also quoted in Engler, Saybolt or Redwood. Each type of oil has its own temperature - viscosity relationship. The measurement of viscosity is made with an instrument called a Viscometer.
Viscosity is the most important characteristic in the storage and use of fuel oil. It influences the degree of pre-heating required for handling, storage and satisfactory atomization. If the oil is too viscous, it may become difficult to pump, hard to light the burner, and difficult to handle. Poor atomization may result in the formation of carbon deposits on the burner tips or on the walls. Therefore pre-heating is necessary for proper atomization.
(Click once) The flash point of a fuel is the lowest temperature at which the fuel can be heated so that the vapour gives off flashes momentarily when an open flame is passed over it. The flash point for furnace oil is 66 0C.

7. Typical Units
Centipoise (cP) was the popular unit of dynamic viscosity.
Centistoke (cSt) was the popular unit of kinematic viscosity.

8. Reporting of Viscosity
Kinematic viscosity (n) is reported as,
where m is absolute (or dynamic) viscosity, and r is the fluid mass density.

9. Table SAE Motor Oil Classification

10. 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.

11. 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

12. 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
The calorific value is the measurement of heat or energy produced, and is measured either as gross calorific value or net calorific value. The difference is determined by the latent heat of condensation of the water vapour produced during the combustion process.
Gross calorific value (GCV) assumes all vapour produced during the combustion process is fully condensed. Net calorific value (NCV) assumes the water leaves with the combustion products without fully being condensed.
Fuels should be compared based on the net calorific value. Ask the audience why. Answer: the GCV includes the heat content of the water vapour, but many appliances cannot use heat content of the water vapour. The NCV therefore allows you to compare fuels, especially when gaseous fuels are compared. For liquid and solid fuels this is less an issue so these are often compared on GCV
The typical Gross Calorific Values of some of the commonly used liquid fuels are given in this table.
Fuel Oil Gross Calorific Value (kCal/kg)‏
Kerosene 11,100
Diesel Oil 10,800
Furnace Oil 10,500

13. Liquid Fuels Sulphur content Ash 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 %
Corrosion of burner tips and damage to materials /equipments at high temperatures
The amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to a lesser extent on the refining process. The normal sulfur content for the residual fuel oil (furnace oil) is in the order of %. The amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to a lesser extent on the refining process.
The main disadvantage of sulphur is the risk of corrosion by sulphuric acid formed during and after combustion, and condensing in cool parts of the chimney or stack, air pre heater and economiser.
(Click once)
The ash value is related to the inorganic material or salts in the fuel oil. Residual fuels have higher ash levels. These salts may be compounds of sodium, vanadium, calcium, magnesium, silicon, iron, aluminum, nickel, etc.
Typically, the ash value is in the range %.
Excessive ash in liquid fuels can cause fouling deposits in the combustion equipment. Ash has an erosive effect on the burner tips, causes damage to the refractories at high temperatures and gives rise to high temperature corrosion and fouling of equipments.

14. Liquid Fuels Carbon residue Water content
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
Carbon residue indicates the tendency of oil to deposit a carbonaceous solid residue on a hot surface, such as a burner or injection nozzle, when its vaporizable constituents evaporate. Residual oil contains carbon residue of 1 percent or more.
(Click once) The water content of furnace oil when it is supplied is normally very low because the product at refinery site is handled hot. An upper limit of 1% is specified as a standard.
Water may be present in free or emulsified form and can cause damage to the inside surfaces of the furnace during combustion especially if it contains dissolved salts. It can also cause spluttering of the flame at the burner tip, possibly extinguishing the flame, reducing the flame temperature or lengthening the flame.

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

16. Solution

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

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

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

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

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

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

23. Engine measurements
Displacement per cylinder
 r² S
Displacement for the engine
Disp per cylinder times the
Number of cylinders

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

25. 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.

26. Knock in SI engines.

27. 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.

28. Causes of Engine Knock
Knock is caused when the temperature in the cylinder reaches the self ignition temperature (SIT) of the end gases.
The end gases do not readily ignite, rather there is an ignition delay caused by pre-flame reactions.
Engine knock is more prevalent under conditions that include:
Lean air/fuel ratios
High compression ratios

29. 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.

30. 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.

31. 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 which gives same level of knock as produced by the actual fuel sample.

32. 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.

33. 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)
Speed (rpm)
Spark advance (oBTC) (varies with r)
Coolant temperature (oC) 100
Inlet pressure (atm) 1.0
Humidity (kg water/kg dry air)
Note: In 1931 iso-octane was the most knock resistant HC, now there are
fuels that are more knock resistant than isooctane.

34. 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:
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.

35. Fig. CFR Engine

36. 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.

37. 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!

38. 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
Heptamethylnonane has a cetane rating of 15.

39. 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.

40. Table : limiting values for diesel fuels.

41. 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.

42. 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 produces 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.

43. Distillation Tests 100 ml sample is distilled.
Fuel temperature is recorded for first condensed drop (boiling point), and then at 10 ml intervals during the distillation process.
T10, T50 and T90 temperatures are important to engine characteristics which include ease of starting, warm-up, and crankcase dilution and fuel economy, respectively.

44. Fig. Fuel distillation apparatus.

45. Adjusting Distillation Temperatures
Gasoline sold during the winter must be more volatile for easy starting in the winter.
Gasoline sold for use in high elevations must be less volatile to avoid “vapor lock” in the summer.
Volatility is adjusted by adding butane and lighter hydrocarbons.

46. Adjusting Distillation Temperatures
For diesel engines:
Low T10 values aids cold weather starting.
Low T50 values minimize smoke and odor.
Low T90 values reduce crankcase dilution and improve fuel economy.

47. Fig. 5.11: Distillation curves.

48. 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.

49. 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.

50. 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.

51. 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.

52. 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.

53. 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 by EPA soon.

54. Table 5.5: Gasoline additives

55. 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

56. Lubricating Oil Additives

57. Distillation Temperatures
30 to 230 C for Gasoline
230 to 370 C for Diesel
Most refineries utilize “cracking units” where catalysts at high temperatures and pressures crack the larger hydrocarbon molecules into smaller ones shifting production towards gasoline.
Fractionating towers allow smaller molecules to condense out at cooler temperatures in the upper portion of the tower.