Example • What is the air/fuel ratio and the exhaust products when ethanol is used as an engine fuel?
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, • 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
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 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
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
Typical Units • Centipoise (cP) was the popular unit of dynamic viscosity. • Centistoke (cSt) was the popular unit of kinematic viscosity.
Reporting of Viscosity • Kinematic viscosity (n) is reported as, • where m is absolute (or dynamic) viscosity, and r is the fluid mass density.
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
Fuel Oil Gross Calorific Value (kCal/kg) Kerosene 11,100 Diesel Oil 10,800 Furnace Oil 10,500 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
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 /equipmentsat high temperatures
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
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.
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 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.
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.
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.
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