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I.C. ENGINES

I.C. ENGINES. LECTURE NO: 06 (10 Mar, 2014). QUESTIONS. What are the types of fuels? Define Calorific Value? What are major characteristic of fuel? What are the qualities of S.I. fuels? What are the qualities of C.I . fuels ?

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I.C. ENGINES

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  1. I.C. ENGINES LECTURE NO: 06 (10 Mar, 2014)

  2. QUESTIONS • What are the types of fuels? • Define Calorific Value? • What are major characteristic of fuel? • What are the qualities of S.I. fuels? • What are the qualities of C.I. fuels? • Explain volatility, crankcase dilution, vapour lock, viscosity? • How common Hydrocarbon fuels are grouped?

  3. QUESTIONS • What are the types of alternate fuels? • Explain overall chemical equation for the complete combustion of one mole of propane (C3H8) with oxygen?

  4. The overall chemical equation for the complete combustion of one mole of propane (C3H8) with oxygen is: species # of moles Thus the stoichiometric reaction is: Combustion Stoichiometry If sufficient oxygen is available, a hydrocarbon fuel can be completely oxidized, the carbon is converted to carbon dioxide (CO2) and the hydrogen is converted to water (H2O). Elements cannot be created or destroyed, so C balance: 3=b→b= 3 H balance: 8=2c→c=4 O balance: 2a =2b + c → a=5

  5. The complete reaction of a general hydrocarbon CaHb with air is: Example: The stoichiometric reaction of octane (C8H18) a= 8 and b= 18 Combustion Stoichiometry Air contains molecular nitrogen N2, if products are at a “low” temperature the nitrogen is not significantly affected by the reaction, it is considered inert. C balance: a=b → b =a H balance: b =2c→c =b/2 O balance: 2a = 2b + c →a = b + c/2→ a = a + b/4 N balance:2(3.76)a = 2d → d = 3.76a/2 →d = 3.76(a + b/4)

  6. Substituting the respective molecular weights and dividing top and bottom by a one gets the following expression that only depends on the ratio of the number of hydrogen atoms to hydrogen atoms (b/a) in the fuel. Combustion Stoichiometry The stoichiometric mass based air/fuel ratio for CaHb fuel is: Note above equation only applies to stoichiometric mixture Example: For octane (C8H18), b/a = 2.25 →(A/F)s = 15.1 (gasoline (A/F)s≈ 14.6

  7. At low combustion temperatures, the extra air appears in the products as O2 and N2: Fuel Lean Mixture • Fuel-air mixtures with more than stoichiometric air (excess air) can burn • With excess air you have fuel lean combustion a fuel lean mixture has excess air, so g > 1 Above reaction equation has two unknowns (d, e) and we have two atom balance equations (O, N) so can solve for the unknowns

  8. Fuel Rich Mixture Fuel-air mixtures with less than stoichiometric air (excess fuel) can burn. With less than stoichiometric air you have fuel rich combustion, there is insufficient oxygen to oxidize all the C and H in the fuel to CO2 and H2O. Get incomplete combustion where carbon monoxide (CO) and molecular hydrogen (H2) also appear in the products. a fuel rich mixture has insufficient air → g < 1 Above reaction equation has three unknowns (d, e, f) and we only have two atom balance equations (O, N) so cannot solve for the unknowns unless additional information about the products is given.

  9. Stoichiometric mixture: Off-stoichiometric mixture: Off-Stoichiometric Mixtures The equivalence ratio, f, is commonly used to indicate if a mixture is stoichiometric, fuel lean, or fuel rich. stoichiometric f = 1 fuel lean f < 1 fuel rich f > 1

  10. Example: Consider a reaction of octane with 10% excess air, what is f? Stoichiometric : 10% excess air is: Off-stoichiometric Conditions Other terminology used to describe how much air is used in combustion: 110% stoichiometric air = 110% theoretical air = 10% excess air → mixture is fuel lean O balance: 1.1(12.5)(2) = 16 + 9 + 2a→ a = 1.25, N balance: 1.1(12.5)(3.76)(2) = 2b → b = 51.7

  11. FUEL PROPERTIES COMPARISON

  12. CETANE NUMBER • A measure of the ignition quality of a diesel fuel, as determined in a standard single cylinder test engine, which measures ignition delay compared to primary reference fuels. The higher the CETANE NUMBER, the easier a high speed , direct injection will start and the less white smoking and diesel knock after start.

  13. CETANE NUMBER • Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of the quality of a diesel fuel. A number of other measurements determine overall diesel fuel quality - these other measures of diesel fuel quality include density, lubricity, cold-flow properties, and sulfur content.

  14. CETANE NUMBER • Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of the quality of a diesel fuel. A number of other measurements determine overall diesel fuel quality - these other measures of diesel fuel quality include density, lubricity, cold-flow properties, and sulfur content. Generally, diesel engines operate well with a CN from 40 to 55. Fuels with higher cetane number have shorter ignition delays, providing more time for the fuel combustion process to be completed. Hence, higher speed diesel engines operate more effectively with higher cetane number fuels.

  15. OCTANE NUMBER • A measure of a fuel’s ability to prevent detonation in a S.I. engine. Measured in a standard single cylinder test engine, variable compression ratio engine by comparison with primary reference fuel. Under mild conditions, the engine measures research octane number (R.O.N.), under severe conditions Motor Octane number (M.O.N). The arithmetic average of (R+M)/2. It approximates the Road Octane Number, which is the measure of how an average car responds to the fuel.

  16. OCTANE NUMBER • Measure of the ignition quality ofgas(gasoline or petrol). Higher this number, the less susceptible is the gas to 'knocking' (explosion caused by its premature burning in the combustion chamber) when burnt in a standard (spark-ignition internal combustion) engine.Octane number denotes the percentage (by volume) of iso-octane in a combustible mixture whose 'anti-knocking' characteristic match those of the gas being tested. In the older vehicles,high octanenumber were achieved by adding lead tetraethyl to the gas (the 'leaded gas'), a pollutant that contributes to lead poisoning. In the newer vehicles, the same results is achieved by the engine design that increases turbulence in the combustion chamber, and/or by adding aromatic hydrocarbons andoxygenates (oxygen-containing compounds such as alcohols) to the gas (the 'unleaded gas. Also called Octane rating.

  17. OCTANE NUMBER • Measure of the ignition quality ofgas(gasoline or petrol). Higher this number, the less susceptible is the gas to 'knocking' (explosion caused by its premature burning in the combustion chamber) when burnt in a standard (spark-ignition internal combustion) engine.Octane number denotes the percentage (by volume) of iso-octane in a combustible mixture whose 'anti-knocking' characteristic match those of the gas being tested. In the older vehicles,high octanenumber were achieved by adding lead tetraethyl to the gas (the 'leaded gas'), a pollutant that contributes to lead poisoning. In the newer vehicles, the same results is achieved by the engine design that increases turbulence in the combustion chamber, and/or by adding aromatic hydrocarbons andoxygenates (oxygen-containing compounds such as alcohols) to the gas (the 'unleaded gas. Also called Octane rating.

  18. PRINCIPLES OF CARBURETION

  19. COMPOSITION OF AIR • Air is composed of various gases, mostly nitrogen and oxygen (78 percent nitrogen and 21 percent oxygen by volume). These gases, as are all substances, are made of tiny particles called molecules. In the air surrounding the earth, as In all gases, the molecules are able to move quite freely in relation to each other. The molecules of air are attracted to the earth by gravity, creating the atmosphere . The weight of the air molecules creates atmospheric pressure

  20. EVAPORATION • Evaporation is the changing of a liquid to a vapor. The molecules of the liquid, not being closely tied together, are constantly moving about among themselves. Any molecule that moves upward with sufficient speed will jump out of the liquid and into the air. This process will cause the liquid to evaporate over a period of time.

  21. RATE OF EVAPORATION • Evaporation is the changing of a liquid to a vapor. The molecules of the liquid, not being closely tied together, are constantly moving about among themselves. Any molecule that moves upward with sufficient speed will jump out of the liquid and into the air. This process will cause the liquid to evaporate over a period of time.

  22. EVAPORATION DEPENDS UPON • Temperature • The rate of movement of the molecules Increases with temperature. Because of this, the amount of molecules leaving the liquid for a given time will increase as the temperature Increases. • Atmospheric Pressure • As atmospheric pressure Increases, the amount of air molecules present over the liquid also increases. The Increased presence of air molecules will slow the rate of evaporation. This is because the molecules of liquid will have more air molecules to collide with. In many cases, they will fall back into the liquid after collision.

  23. EVAPORATION DEPENDS UPON • Closed Chamber • As evaporation takes place in a closed container, the space above the liquid will reach a point of saturation. When this happens, every molecule of liquid that enters the air will cause another airborne molecule of liquid to fall back. • Volatility • The term volatility refers to how fast a liquid vaporizes. Some liquids vaporize easily at room temperature. Alcohol, for instance, vaporizes more easily than water. A highly volatile liquid is one that is considered to evaporate easily. • Atomization • Atomization is the process of breaking up a liquid into tiny globules or droplets. When a liquid is atomized, the droplets are all exposed individually to the air. For this reason, atomization greatly increases evaporation by increasing the exposed surface area of the liquid.

  24. ATOMIZATION

  25. VENTURI EFFECT • Venturieffect is used by the carburetor to mix gasoline with air. The basic carburetor has an hourglass-shaped tube called a throat. The most constricted part of the throat is called the venturi. A tube called a discharge nozzle Is positioned in the venturi. The discharge nozzle is connected to a reservoir of gasoline called the float bowl. The negative pressure that exists in the combustion chamber because of the downward intake stroke of the piston causes atmospheric pressure to create an airflow through the carburetor throat. This airflow must Increase temporarily in speed as it passes through the venturi, due to its decreased size.

  26. VENTURI

  27. VENTURI

  28. THE BASIC CARBURETOR

  29. BASIC CARBURETOR PARTS • Carburetor Body • Air Horn • Venture • Throttle Valve • Float Circuit • Venting

  30. CARBURETOR BODY

  31. AIR HORN • The air horn is also called the throat or barrel. The parts which often fasten to the air horn body are as follows: the choke, the hot idle compensator, the fast idle linkage rod, the choke vacuum break, and sometimes the float and pump mechanisms.

  32. VENTURE • The venture produces sufficient suction to pull fuel out of the main discharge tube

  33. THROTTLE VALVE • The throttle valve is used to regulate the speed and power output of the engine. • It is controlled by the accelerator pedal, and usually consists of a flat, round plate that tilts with the throttle shaft. As the accelerator pedal is fully depressed, the throttle valve is moved from a position of completely restricting the throat to being completely open. • The idle stop screw is used to keep the throttle valve open slightly so that the engine may run at a regulated idle speed with no foot pressure on the accelerator. This screw may be turned in or out to regulate engine idle speed.

  34. THROTTLE VALVE

  35. FLOAT CIRCUIT • Purpose • The float circuit maintains a steady working supply of gasoline at a constant level in the carburetor. • This is very critical to proper engine performance. • An excessively high float level will cause fuel to flow too freely from the discharge tube, causing an overly rich mixture. • Whereas an excessively low float level will cause an overly lean mixture.

  36. FLOAT CIRCUIT • Operation • Fuel pump delivers gasoline to the fuel pump under pressure to the carburetor. The following events occur as the gasoline enters the carburetor through the fuel inlet: • The gasoline begins to fill the float bowl. • The float rises with the level of the gasoline. • The needle valve is closed by the rising float as the fuel reaches the desired level in the float bowl. • As the engine uses the gasoline from the float bowl, the level will drop. This will cause the float to drop, which will open the needle valve to let in more

  37. VENTING • The pressure in the float bowl must be regulated to assure the proper delivery of fuel and purging of vapors. The following systems and devices are added to the float circuit system to provide for these needs. • Balance Tube • Due to the restriction imposed by the air filter and changing air velocities because of varying engine speeds, the air pressure in the air horn is usually lower than atmospheric pressure. The pressure in the float bowl must equal that of the air horn in order for the carburetor to provide fuel delivery. A tube called a balance tube is run between the air horn and the float bowl to accomplish this task.

  38. VENTING • Idle Vent • Because gasoline Is highly volatile, it can create overly rich mixtures during long periods of engine idle. This is because the fuel begins to evaporate in the float bowl and the vapors get into the air horn through the balance tube. The solution to this problem is to have an outside vent for the float bowl that is opened whenever the engine is idling. The idle vent is activated by linkage from the throttle valve. The idle vent system on later vehicles may be part of the emission control system

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