Fuel And Combustion PDF

Fuel and Combustion Dr. Suad Alhaj Mustafa  Definition It is a term given to any substance that produces heat when burns with air. The chemical reaction that permits the release of this heat energy is quite fast. The preparation of the fresh mixture is time consuming a...

Fuel and Combustion Dr. Suad Alhaj Mustafa  Definition It is a term given to any substance that produces heat when burns with air. The chemical reaction that permits the release of this heat energy is quite fast. The preparation of the fresh mixture is time consuming and depends on 1) The nature of the fuel, 2) The method used for the preparation of the mixture and 3) The method used for the introduction of the mixture to the combustion chamber.  Types Of Fuels Generally, fuels can be divided into three basic categories as show below : 1) Solid (coal, wood, oil shale..etc) 2) Liquid (Gasoline, Alcohol, Diesel..etc) 3) Gas (Hydrogen, Natural gas, LPG …etc) 4) Dual (Liquid-Liquid, Liquid-Gas, Gas-Gas)  Remarks Solid fuels are little used nowadays because of the problems of handling, storage and feeding to the engine as well as disposing off the products of combustion like ash. Gaseous fuels looks the most promising among all alternatives. 1) They burn lean and clean. 2) They eliminate the problems of starting and distribution and 3) They make good homogeneous mixture with air. 4) However, their usage is restricted because of handling and storage problems.  General Requirements 1. It should have sufficient energy content. 2. It should be easy to handle. 3. It should be economically available in large quantities. 4. It should be free from hazards as far as possible. 5. It should not have any chemical reaction with any of the engine components. 6. It should be easy to store. 7. It should easily mix with the air for combustion. 8. It should produce lesser pollutants when burned with air. 9. It should have least deposit formation tendency. 10. It should have god combustion qualities.  Specific Requirements : Since the time consumed by the engine to run for 1 degree = 1/6*RPM seconds during which the fuel has to be brought from the tank, supplied to the carburetor/injector, gets evaporated then mixed with air then distributed to the required cylinder in correct proportion …etc, this process is time consuming and puts heavy design load on the key parts of system i.e. fuel, fuel supply system and ignition system. Hence, further restrictions on the requirements of the engine fuel may be : 1. The combustion has to take as little time as possible and during which it has to release as maximum amount of its heat energy content as possible. 2. Those properties mentioned must be fulfilled as long as the fuel is in use. 3. The products of combustion should not be harmful neither ecologically nor biologically, and, 4. The fuel used should help in reliable and easy starting under ambient conditions.  Structure Of Petroleum Petroleum is a mixture of many different hydrocarbons, with some sulfur and other impurities. These hydrocarbons are grouped into four general categories: --------------------------------------------------------------------------------------- Name Structure Staurated Stable --------------------------------------------------------------------------------------- Paraffin (CnH2n+2) Chain Yes Yes Olefins (CnH2n) Chain NO NO Naphthenes (CnH2n) Ring Yes Yes Aromatics (CnH2n-6) Ring NO Yes  Paraffin Series of HCs begins with CH4 (methane). This has a straight chain molecular structure, and Their names end up with “ane”. Examples are:  Olefins These are also straight or open chain hydrocarbons but have one or more double-bonded carbon atoms in a straight chain. The names of the hydrocarbons having a single double-bond ends with “ene”, and those with two double-bonded ends with “adine”. These compounds, however, has to be kept as low as possible because the tend to form gummy deposits when they oxidize in storage.  Naphthene Has the formula as those for olefins, but with different properties. They have cyclic or ring structure. They are saturated.  Aromatics Ring structure compounds having the benzene molecules (C6H6) as their central structure.  General Remarks Normal Paraffin exhibits the poorest antiknock characteristics when used in SI Engines while Aromatics exhibits the best. Antiknock characteristics of paraffins can be improved by increasing the No of carbon atoms as well as its molecular compactness. For CI Engines Paraffins offer the best choice while Aromatics the worst. The boiling point of the fuel increases as the No of atoms in its structure increases. Hence fuels with lesser atoms tends to evaporate easier.  REFINING PROCESS Based on the simplest fact that the constituents of petroleum have different boiling points varying roughly with the molecular weight. The general processes followed in the plants are as below :  Fuels For Spark Ignition Engines Gasoline.  Used in more than 99% of the cars.  The lightest liquid petroleum fraction.  All material boiling up to 200oC is generally considered as gasoline. This is a mixture of a number of hydrocarbons (more than 40).  The composition depends upon the crude oil and refining process.  The requirements of an ideal gasoline : 1. Readily mix with air. 2. Knock resistant. 3. Clean and produce no corrosion. 4. Should not form gum. All these properties are connected to the following properties: volatility, knock resistivity, sulfur content, gum content and contamination.  Fuel Volatility Volatility is the tendency to evaporate under a given set of conditions or the readiness with which the liquid changes to vapor. If the fuel is heated at steadily increasing temperature and the percentage of fuel vaporized is plotted against the temperature rise we obtain a curve called “Distillation Curve” as shown below.  Measurement Process There are various methods by which the volatility of the fuel could measured, however, the volatility of Gasoline is generally characterized by two basic laboratories tests: 1. ASTM distillation test, and 2. Reid vapor test.  Effect on Engine Operation This is the most important property as it affects a wide range of engine characteristics e.g. startability (hot and cold), vapor lock, short trip economy, smooth running of the engine, worm up period, hot stalling, carburetor icing, acceleration and power and deposit formation.  Front end volatility affects the following engine characteristics : 1. Cold starting, 2. Hot starting, and 3. Vapor lock. Mid range volatility affects the following : 1. Engine warm up, 2. Acceleration 3. Short and long trip economy, and 4. Carburetor icing. Tail end volatility affects the following : 1. Crankcase dilution 2. Spark plug fouling 3. Evaporative losses, and 4. Varnish and sludge deposits.  Cold starting.  Occurs specially in winter when starting the vehicle becomes difficult.  Caused primarily due to the fact that the engine is not getting enough fuel vapor due to lower atmospheric temperature.  To solve this problem the fuel has to have higher volatility. Hot starting.  This phenomenon called “Percolation” is the difficulty to restart a vehicle, specially in hot days, which has been stopped for a short duration.  This occurs due to the excess evaporation of the fuel caused by the hot engine and weather which leads to the supply of very rich mixture to the engine.  To remedy this problem, the fuel has to have low volatility.  Vapor lock. Serious restriction of the fuel supply occurring due to excessive and rapid formation of vapor in the fuel system. This causes the fuel supplied to the engine to be very lean. This requires proper designing and low volatility fuels.  Warm-up period.  Warm-up can be defined as the interval between the starting of the engine and the time at which flexible operation is possible.  This requires that the mid-range volatility should be high enough for smooth engine operation. Engine acceleration, smoothness and fuel economy.  This part of the engine operation is very sensitive and the air-fuel ratio has to be controlled.  A low mid-range and tail-end volatility is required so that the fuel supply can response well with the engine operation.  Short and long trip economy.  For short trip, since the warm-up period consumes the maximum time, the fuel should have good mid- range volatility.  For long trip, the engine should have lower volatility fuel. Carburetor icing.  At higher humidity and lower atmospheric temperature, water condenses out of air and freezes on the carburetor surface. This is aggravated by the fact that fuel vaporization process causes significant temperature drop inside the inlet manifold if it has high volatility.  This is remedied by having lower volatile fuels.  Crankcase dilution.  Because of the fact that when starting the engine specially at low temperatures, a rich mixture has to be supplied to the engine. This causes some of the fuel to pass through the piston walls unevaporated. This causes the dilution of the crankcase lubricating oil by the unevaporated fuel which reduces the crankcase oil viscosity, and it also washes away the lubricant at the cylinder surface.  This can be solved either by proper venting of the crankcase, or having high volatility fuels. Evaporation losses.  Loss of vapor from the venting holes of the storage tanks.  The remedy requires that the fuel volatility to be low.  Varnish and sludge deposits and spark plug fouling. Certain constituents of the fuel, specially the high boiling point HCs have the tendency to produce varnish and sludge when vaporised which causes troubles to the piston rings plugging or sticking of the valves, on the tip of the spark plug..etc. Hence, lower the tail-end volatility, lesser is the amount of such high boiling point hydrocarbons.  Gasoline Additives 1. Anti-knocking agents. 2. Deposit modifiers. 3. Antioxidants. Reduce gum formation and decomposition of TEL. 4. Detergents. Prevent deposits in carburetor and manifold. 5. Lubricants. 6. Metal deactivators. Destroy the catalytic activity of traces of copper. 7. Antirust agent. Prevent rust and corrosion due to water presence. 8. Anti-icing agent. To prevent gasoline freezing because of water. 9. Dyes. To identify TEL in the fuel.  Knock Rating Of S.I. Engine Fuels There are several methods by which the knock rating of S.I. fuel could be rated. 1. Highest Useful Compression Ratio. 2. Octane Number. 3. Sensitivity. 4. Performance Number.  Octane Number HUCR is only applicable to a particular engine and at a particular engine set of conditions. This method is the determination of the fuel Octane Number (ON). It consists of testing the performance of an unknown gasoline fuel (Test Fuel) and comparing it with a series of reference fuels consisting of a mixture of Iso-octane and N-heptane. Hence ON indicates the percentage of Iso-octane in a mixture of Iso-octane + N-heptane.  Research and Motor Octane Number.  Since the tendency to knock varies from engine to engine, and for the same engine, it varies with the operating conditions, so, ON is made to be determined under more than one set of test conditions to give a reasonable agreement between the laboratory and actual service conditions.  Thus, tests under the CFR (Cooperative Fuel Research) engine are done under rather severe (Motor ON) and a rather moderate (Research ON) duty conditions as described below :  Sensitivity  Since both RON and MON reflects the difference between results obtained in laboratories and actual road conditions, the difference between RON and MON must be a factor of load rating.  This difference is called Sensitivity.  Performance Number The scale defined by the ON fails to serve the purpose when testing or rating fuels harder knocking than the N-heptane or others more knock resistant than Iso-octane. Hence in order to extend the octane scale, the knock resistance behavior of the fuel is measured in terms of Army-Navy Performance Number (PN). It is defined as the ratio of the knock-limited IMEP (KLIMEP) of the fuel to the knock-limited MEP of Iso- octane  C. I. Engine Fuels Diesel fuels Petroleum fractions that lie between kerosene and the lubricating oils. Diesel fuel properties are influenced by the crude source and method of refining. The basic important properties that influence the characteristics of diesel fuel are:  Viscosity A measure of the fluid’s internal friction or resistance to flow. It is expressed as the time required for a given volume of fluid at some standard temperature to flow out through an orifice or hole of certain small diameter. The instrument used for this purpose is called Saybolt Viscometer. The more the time taken, the higher the viscosity is. In diesel fuels, it indicates the ability of the fuel to flow in the fuel system and also determines the lubricating value needed to lubricate the fuel pup and injector. Now, since the lubrication of both the injection nozzle and fuel pump is entirely dependant on the fuel oil, its viscosity must not be too low. Viscosity of the fuel affects the spray characteristics of the fuel.  Volatility It is indicated by the 90% distillation temperature. It affects the HC, NOX emissions, smoke density and odor of the engine’s exhaust. Higher volatility reduces the HC emission and increases the NOX emissions. Carbon monoxide, fuel consumption and power are little or not affected by fuel volatility.  Ignition Quality Is the ability of the diesel fuel to ignite by itself or self ignite, under the conditions existing in an engine cylinder. A fuel with good ignition quality is the one which will self-ignite at low temperature. If the ignition delay (which is the time period between the start of injection and start of combustion) is too long, then the combustion and pressure release would become so rapid that knocking occurs, and if it is too short, then there would be not enough time for the combustion to be completed.  API gravity and specific gravity Specific gravity is the weight of the fuel compared with the weight of an equal volume of water. It indicates the grade of the fuel (heavy and light), hence, a measurement of its flow property in the fuel system. API is a more accurate representation of the fuel’s specific gravity. It is mainly used in the US. It is calculated as  Heat of Combustion. Contamination. Sulfur content. Carbon Residue. It is the remains after all the amount of fuel gets evaporated off by heating in the absence of air. It indicates the tendency of the fuel to form carbon deposits when burned. Water and sediments content. Ash content.  Cetane Rating of Fuel This is the measure of the fuel’s ability to autoignite quickly when injected into the compressed and heated air inside the cylinder. It is the prime factor affecting the engine’s emission, noise and startability. Lower cetane number produces more pollutants and more noise. Higher CN gives better startability. Cetane number is the percentage by volume of cetane (C16H34) in a mixture of cetane + α- methylnapthalene (C10H7CH3) that has the same performance as that of the fuel under test.  Cetane is arbitrarily given the number 100 i.e. it ignites quickly, and α-methylnapthalene is given the number 0, i.e. it ignites very slowly. For gasoline fuels, spontaneous combustion is the main concern while in diesel fuels it is the main criteria. Hence, both cetane and octane numbers are inversely measures to the same property. Further, it became clearer that a good C.I.E. fuel is bad S.I.E. fuel. Further, at the time diesel fuels are having CN = 40-60, gasoline fuels are having CN = 10-20.Cetane number can be calculated by the following formula :