Fuel Properties Fuel Properties

 Why do we study about Fuel properties? Fuel Properties

 There are some international organization who engaged in measurement and control of fuel properties

 SAE (Society of Automotive Engineers)

 decides on need for new or modified standards

 ASTM (American Society for Testing Materials)

 develops the testing procedures to measure properties

 API (American Petroleum Institute)

 works with fuel suppliers to produce fuels with appropriate properties Fuel Properties

 What are the important properties

 Specific gravity  Heating value  Volatility  Flashpoint  Viscosity  Pour points  Impurities  Octane & Cetane numbers Specific Gravity

 Measure of density of liquid fuel  It can be measured by hydrometers  Density of fuel usually less than that of water, i.e. fuel floats on water  Specific gravity (SG) is a dimensionless number

 Density of Fuel @15.60C  Fuel Density (kg/l) =   SpecificGravity(SG)  0   Density of Water @15.6 C  Gasoline 0.72-0.78  kg    Density of Fuel    Diesel 0.82-0.86  L  =   Methanol 0.79 1kg    L  Ethanol 0.79 Heating value

 Heating or calorific value

• Heat or energy produced during combustion per unit mass of fuel

• Gross calorific value (GCV) or Higher heating value: Assume all vapor produced during combustion is fully condensed

• Net calorific value (NCV) or Lower heating value: Assume water vapor is not fully condensed

Fuel Heating Value (MJ/kg)

Gasoline 44 Diesel 42.5 Methanol 19.7 Ethanol 26.8 Heating Value of Fuels

 For petroleum fuels, heating value can be estimated from fuel specific gravity

 Procedure:  Use hydrometer to measure specific gravity at 15.60C  Calculate API0 = (141.5/SG)-131.5  Some hydrometers calibrated directly in API0  Estimate heating values from equations  HHV = 42860+93*(API0 -10) kJ/kg  LHV = 0.7190*HHV+10000 kJ/kg Volatility

8  Volatility is A fuel’s ability to vaporize or change from liquid to vapor  Fuels won’t burn till they vaporize  The volatility characteristics of fuel in a spark ignition (SI) engine are of prime importance.  The main parameters to establish volatility limits are

 Vapor/Liquid Ratio (V/L)

 Reid Vapor Pressure (RVP)

 Distillation curves Volatility

Volatility too low Volatility too high

• Poor cold start • High evaporative emissions, • Poor warm up performance • Hot drivability problems, vapor lock • Poor cold weather drivability • Fuel economy may deteriorate • Unequal fuel distribution in carbureted vehicles • Increased deposits: crankcase, spark plugs, combustion chamber Vapor/liquid ratio (volume)

 If mixture of hydrocarbons is enclosed in a variable volume container such that the pressure is always atmospheric, the vapor fraction will increase with increasing temperature.

 The temperature at which V/L = 20 is used as a key indicator of “vapor lock” tendency; the malfunctioning of a vehicle because there is too much vapor in the fuel delivery system.

 More volatile fuels require lower temperatures to achieve this ratio while less volatile fuels require higher temperatures to create the same ratio. Volatility

 Reid Vapor Pressure (RVP)

 Test used on gasoline fuel

 Measured @ 37 0C in a vapor pressure bomb

 Higher vapor pressure means more volatile fuel

 RVP expresses volatility with a single number

 Adjusted seasonally and geographically at the refinery by relative abundance of C4 compounds (butane and isobutane)

 Winter gasoline R V P, 60-80 kPa

 For Summer RVP = 56 kPa Volatility

 Distillation curves

 Upon heating a mixture of hydrocarbons, lighter (more volatile) compounds are driven off first; remaining mixture has higher boiling point. Volatility

 Two ways of expressing key points along the distillation curve  T10, T50, T90  The temperature at which the indicated % (by volume) has evaporated  E70, E 100, E150 (C)  The percentage (volume) evaporated at the indicated temperature Volatility 14 T10 Temp

 Associated with Engine Cold Starting  The 10% evaporated temperature must be low enough to provide easy cold starting but high enough to minimize vapor lock as well as hot driveability problems.  In winter, this temp lowered to allow enough fuel to evaporate to form a combustible mixture. Put in more light molecules (butane)  In summer, raise T10 point by adding less light molecules to prevent vapor lock Volatility 15

 T50 Temp  Associated with engine warm up  The 50% evaporated temperature must be low enough to provide good warm up and cold weather driveabilty.  Low temp allows engine to warm up and gain power quickly without stalling Volatility 16  T90 Temp  Associated with crankcase dilution and fuel economy  The 90% and end point evaporation temperatures must be low enough to minimize crankcase and combustion chamber deposits as well as spark plug fouling and dilution of engine oil.  If too high, large fuel molecule will condense on cylinder liners & pass down into crankcase without burning Volatility 17

 Interpretation of Curves: Diesel

 Volatility is important but not as critical as for gasoline, because chamber is hot when fuel is injected

 If too volatile, fuel droplets evaporate too quickly to permit adequate spray penetration in combustion chamber

 Low T10 point aids starting

 Low T50 point reduces smoke and odor during warm up

 Low T90 reduces crankcase dilution and improves fuel economy Starting and Warm up

 A certain part of fuel should be vaporize at the room temperature for easy staring of the engine

 The portion of the distillation curve between about 0 and 10 % boiled off have relatively low boiling temperature

 As the engine warms up, the temperature will gradually increases to the operating temperature.

 Low distillation temperature are desirable through out the range of the distillation curve for best Drivability

 Drivability of a vehicle is the extent to which a car starts and operates smoothly and reliably during the first few kilometers of operation.

 Drivability malfunctions, which occur during this period, include stall, stumble, surge, backfire, and idle roughness or quality, and stretchiness.

 Drivability can be improved by increasing the fuel volatility

 A Drivability Index (DI) has been developed using the temperatures for the evaporated percentages of 10 percent (T10), 50 percent (T50) and 90 percent (T90): Drivability

 An attempt to quantify cold start and warm-up performance

 DI = 1.5(T10) + 3.0(T50) + T90, (for conventional gasoline)

 The lower this value, the more volatile the fuel, and the better the drivability, particularly in cold weather after a cold start.

 Typical values 850 - 1300

 Varies with gasoline grade (regular, premium) and season Operating Range Performance

 Low distillation temperatures are preferable in the engine operating range.

 Better vaporization tends to produce both uniform distribution of fuel to the cylinders as well as better acceleration characteristics by reducing the quantity of liquid droplets in intake manifold. Crankcase dilution

 Liquid fuel in the cylinder causes loss of lubricating oil (by washing away oil from cylinder walls) which deteriorates the quality of lubrication and tends to cause damage to the engine through increased friction.

 The liquid fuel may also dilute the lubricating oil and weaken the oil film between the rubbing surface

 To prevent these possibilities, the upper portion of the distillation curve should exhibit sufficiently low distillation temperatures to insure that all fuel in the cylinder is vaporized by the time the combustion start Vapor Lock

 High rate of vaporization of gasoline can upset the carburetor metering or even stop the fuel flow to the engine by setting up a vapor lock in the fuel passages

 This characteristics, demands the presence of relative high boiling temperature hydrocarbons throughout the distillation range

 Since this requirement is inconsistence with other requirements ( cold start and crank case dilution) so that it needs a compromise Carburetor Icing

 This is due to condensation of the water vapor in the air on to the throttle plate.

 Can be reduced by using manifold heating or by use of additives.

 Volatility also plays an important role here. Flash Point of Fuels 25

 Flash point of a flammable liquid  is the lowest temperature at which it can form an ignitable mixture in air

 Varies with fuel volatility but is not related to engine performance

 Relates to safety precautions that must be taken when handling a fuel

 Gasoline necessarily has low flash point & thus requires more stringent handling procedures than diesel fuel

 Flash point of diesel is 52oC or higher, therefore, at ordinary ambient temperatures, it does not form enough vapor for combustible mixture Viscosity

 Measure of resistance to flow

 Important for diesel fuel

 1) To lube the injection equipment

 2) Get proper spray pattern from injectors

 Limits established by SAE

 Kinematic viscosity @ 40 oC

 #1 diesel (min 1.3 mm2/s; max 2.4 mm2/s)

 #2 diesel ( min 1.9 mm2/s; max 4.1 mm2/s)

 Measured by viscometer Cloud & Pour Points

 Cloud Point

 is the temperature at which large molecules start to form crystals

 Pour Point

• Lowest temperature at which fuel will flow

• Indication of temperature at which fuel can be pumped Fuel Impurities: Sulfur

 Sulfur compounds naturally present in crude oil but most of sulfur removed during refining

 Must be limited to prevent corrosion in engine and exhaust system

 Sulfur compounds react with combustion water to produce H2SO4- rust out exhaust system and affect exhaust after-treatment systems

 In USA, sulfur content of gasoline averages less than 0.03% by weight

 SAE limit for sulfur in diesel is 0.05% (500ppm)

 Low sulfur grades of diesel have been developed recently to meet more stringent emission requirements Other Fuel Impurities

 Gum-

 Viscous liquid formed in gasoline during storage, limits storage time for fuel

 Ash-

 Small solid particles found in fuels- particularly harmful for diesel engines because of abrasion in fuel injection system

 Water & sediment

 can enter during handling and storage

 Water

 can promote the formation of slime/algae

 can undermine lubricity of diesel fuel Gum deposits

 Factors which Increase Gum Formation

1. Increased concentration of oxygen in fuel.

2. Rise in temperature.

3. Increased exposure to sunlight (this influence is considerable).

4. Contact with metals which act as catalysts; copper is particularly reactive.

5. Deactivators must be added to neutralize the catalytic action of the metal. Knock in SI Engines 31

 Knock in SI Engines (What is engine knock?)

 Occurs when end gases (gases ahead of the flame front) self-ignite and generate a rapid, uncontrolled release of energy

 Quick energy release causes sharp rise in pressure and pressure oscillations Effect of Knocking

32 Rating of SI Engine Fuels

 Hydrocarbon fuels used in SI engines under severe operating conditions cause engine knock.

 A given fuel will have an increasing tendency to knock with increasing compression ratio.

 Fuels differ widely in their ability to resist knock.

 Fuel knock rating for a particular fuels is accomplished by comparing its performance with that of a standard reference fuel which is usually a combination of Iso-Octane and normal heptane or iso-ocatane plus tetraethyl lead. Rating of SI Engine Fuels

 Iso-Octane, being a very good anti-knock fuel is assigned a rating of 100 octane number,

 Normal heptane, has a very poor anti-knock qualities and is given a rating of 0 octane number

 Octane Number

 Measure of the knock resistance of gasoline (resistance to Auto-ignition)

 The percentage by volume, of iso-ocatne in a mixture of iso-octane and normal heptane, which exactly matches the knocking intensity of a given fuel, in a standard engine under given standard operating conditions is termed as the octane rating of the fuel. Octane Number

 The higher the octane number of a fuel, the less likely it will self- ignite.

 Engines with low compression ratios can use fuels with lower octane numbers, but high-compression engines must use high-octane fuel to avoid self-ignition and knock.

 Things that affect ON are combustion chamber geometry, compression ratio, turbulence, swirl, temperature, inert gases, etc.

 Fuel components with long chain molecules generally have lower octane numbers, components with more side chains have higher octane numbers. How is Octane Number Measured? 36 1. In special CFR (Cooperative Fuel Research) engine with adjustable CR and made for a certain octane number

 The two standard reference fuels used for octane rating are

 iso-octane: ON =100 Sample Iso- n-heptanes  normal-heptane : ON= 0 Fuel Octane

Carburetor

Knock Meter Engine Cylinder How is Octane Number Measured?

 Procedure

 Run CFR engine, adjust CR for mid scale knock on test fuel (sample fuel) as shown with knock meter

 Varying the CR until the knock meter records some value let 55

 Open the valve of Iso-Octane of 85 % by volume and 15% n-heptane by volume to the engine in this condition the sample fuel cut off at the same CR

 If we get a reading of knock meter

 More than 55 %  Shows more n-heptance , less iso-octane and less ON 75 %  Less than 55 %  More Iso-Octane, less n- heaptane and more octane number 55 %  With same CR, try blends of isooctane and n-heptane 35 % to get same knock;

 Octane Rating = % iso-octane in blend ON Octane Rating

38  Two ASTM methods for octane rating use same engine but different operating conditions

 Research Method

 Research Octane Number (RON)

 Motor Method

 Motor Octane Number (MON) 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 Octane Rating

 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.

 MON

 Octane Sensitivity (OS) =RON-MON

 Anti-Knock Index (AKI) or Control Octane Number (CON) posted on some service station pumps = (RON+MON)/2

 Regular gasoline, RON =91, MON=83, (R+M)/2=87

 Minimum AKI In Canada: Regular (87), Mid-grade (89), Premium (91) Knock Characteristics of Various Fuels 40

 Formula Name Critical r RON MON

 CH4 Methane 12.6 120 120

 C3H8 Propane 12.2 112 97

 CH4O Methanol - 106 92

 C2H6O Ethanol - 107 89

 C8H18 Isooctane 7.3 100 100

 Blend of HCs Regular gasoline 91 83

 n-C7H16 n-heptane 0 0 Causes of engine knock

 Low octane number of gasoline fuel

 Due to engine ages, deposits might build up on the combustion chamber walls. This increases knock problems in two ways.

1. First, it makes the clearance volume smaller and consequently increases the compression ratio.

2. Second, the deposits act as a thermal barrier and increase the temperatures throughout the engine cycle, including peak temperature. Causes of engine knock

 Octane requirements can go up as an engine ages, with an average increase needed of about three or four for older engines.

 Engine knock can also be caused by surface ignition.

 If any local hot spot exists on the combustion chamber wall, this can ignite the air-fuel mixture and cause the same kind of loss of cycle combustion control. Diesel fuel

 Diesel fuel (diesel oil, fuel oil) is obtainable over a large range of molecular weights and physical properties

 Generally speaking, the greater the refining done on a sample of fuel, the lower is its molecular weight, the lower is its viscosity, and the greater is its cost

 Light diesel fuel has a molecular weight of about 170 and

can be approximated by the chemical formula C12.3H22.2 Diesel fuel

 Heavy diesel fuel has a molecular weight of about 200 and

can be approximated as C14.6H24.8

 Heavy diesel fuel can generally be used in larger engines with higher injection pressures and heated intake systems.

 Often an automobile or light truck can use a less costly heavier fuel in the summer, but must change to a lighter, less viscous fuel in cold weather because of cold starting and fuel line pumping problems Diesel fuel

 In a compression ignition engine, self-ignition of the air-fuel mixture is a necessity.

 The correct fuel must be chosen which will self-ignite at the proper time in the engine cycle. The property that quantifies this is called the cetane number.

 The larger the cetane number, the shorter is the ignition delay (ID) and the quicker the fuel will self-ignite in the combustion chamber environment.

 A low cetane number means the fuel will have a long ID. Diesel fuel

 Like octane number rating, cetane numbers are established by comparing the test fuel to two standard reference fuels

 The fuel component n-cetane (hexadecane), C16H34, is given the cetane number value of 100, while heptamethylnonane

(HMN), C12H34, is given the value of 15.

 The cetane number (CN) of other fuels is then obtained by comparing the ID of that fuel to the ID of a mixture blend of the two reference fuels

 CN of fuel = (percent of n-cetane) + (0.15)(percent of HMN) Effect of cetane number

 Normal cetane number range is about 40 to 60. For a given engine injection timing and rate, if the cetane number of the fuel is low the ID will be too long.

 When this occurs, more fuel than, desirable will be injected into the cylinder before the first fuel particles ignite, causing a very large, fast pressure rise at the start of combustion

 EUROPE: 43 - 57, average 50

 U.S. lower, minimum 40, average 43

 SAE minimum cetane rating set at 40 Effect of Cetane Rating

48  Higher cetane correlates with  improved combustion  improved cold starting  reduced noise, white smoke, HC, CO and particulate emissions, especially during early warm-up phase  Low Cetane benefits  More premixed combustion due to longer ignition delay  Higher efficiency  Less smoke  Low cetane disadvantage  Higher engine stress  More noise Additives used for gasoline

1. Anti-knock Additive:- Required to eliminate knock or increase the octane number the usual additive was tetraethyllead

2. Deposit-modifiers:- Used to modify the chemical character of combustion chamber deposits and so reduce surface ignition and spark plug fouling. They are usually a phosphorus or boron compound.

3. Anti-oxidants:- Used to reduce gum formation and decomposition of the lead compounds. They are usually an amine. Additives used for gasoline

50 4. Detergents:- Used to prevent deposits in the carburetor and manifold. They are usually an alkyl amine phosphate.

4. Lubricants:- Used to lubricate valve guides and upper cylinder regions. They are usually light mineral oils.

5. Metal de-activators:- Used to destroy the catalytic activity of traces of copper. They are usually amine derivatives.

6. Anti-rust Agents:- Used to prevent rust and corrosion due to moisture in the air. They are usually fatty acid amines, sulfonates, or alkyl phosphates.