Fuel Properties Fuel Properties
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1 Fuel Properties Fuel Properties 2 Why do we study about Fuel properties? Fuel Properties 3 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 4 What are the important properties Specific gravity Heating value Volatility Flashpoint Viscosity Pour points Impurities Octane & Cetane numbers Specific Gravity 5 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 6 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 7 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 9 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) 10 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 11 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 12 Distillation curves Upon heating a mixture of hydrocarbons, lighter (more volatile) compounds are driven off first; remaining mixture has higher boiling point. Volatility 13 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 18 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 19 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 20 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 21 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 22 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 23 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 24 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 26 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 27 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 28 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 29 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 30 Factors which Increase Gum Formation 1.