Chapter 4 fuels.pptIts talking about fuels we used

Its talking about fuels we used

1. 4.3. Comparison between diesel and gasoline fuels

2. Gasoline It is a hydrocarbon compound produced by the refining of crude oil. the most predominating type is octane (C 8 H 18 ) Gasoline contains octane improvers and other additives Antiknock additives slow down the burning of gasoline. This helps prevent engine knock.

3. Main Properties of gasoline It is highly volatile; that is, it evaporates easily at normal temperatures. It is colourless, transparent, and aromatic. Has low flash point (-10 to -15 o C). Has low specific gravity (0.6 to 0.78) It can dissolve oil and rubber It leaves little residual hydrocarbon (ash) after combustion. It generates large amounts of heat (9,500 to 10,500Kcal).

4. Normal & Abnormal Combustion

5. Normal combustion Normal combustion occurs smoothly and progresses across the combustion chamber from the point of ignition.

6. Normal combustion propagation Between 45 and 90 MPH (72 and 145 km/h)

7. Abnormal Combustion Abnormal combustion occurs when the flame does not spread evenly and smoothly through the combustion chamber.

8. Abnormal Combustion • Instead of continuing to burn smoothly and evenly, the unburned portion of the air/fuel mixture self-ignites towards the end of combustion there by burning in an uncontrolled or violent state and explodes violently from spontaneous combustion.

9. Speed increases by upto 10 times to near the speed of sound. Abnormal combustion can quickly destroy an engine.

10. Abnormal combustion can quickly destroy an engine

11. What is engine knock? high frequency vibrations caused by the premature burning of fuel inside the engine's cylinders. Knocking is created by an abnormal combustion of gasoline in the cylinder.

12. Engine knock is a metallic noise an engine makes, usually during acceleration, resulting from abnormal or uncontrolled combustion inside the cylinder.

13. This causes: sharp rise in pressure and pressure oscillations. Depending on its intensity the piston and crank components may be overloaded, or the engine may overheat.

14. Effects of Engine Knock Audibly annoying Can cause burning of piston crown and other engine damage

15. Factors of knocking Too low a grade of fuel Too advanced ignition timing Compression ratio

16. Gasoline octane rating The octane rating of gasoline is a measurement of the fuels ability to resist knock. A high octane rating indicates the fuel will not knock or ping easily.

17. High-octane gasoline should be used in high comression engines and turbocharged engines. Low octane gasoline is suitable for low-compression engines.

18. Octane number Octane number gives the antiknock value of gasoline. Gasoline with high octane number will resist knock better than gasoline with a low octane number. Example: A 93-octane gasoline is more knock resistant than an 89-octane gasoline.

19. Gasoline Grades And Octane Number

20. Diesel fuel Diesel fuel (sometimes called "light oil") is a mixture of hydrocarbons. It is made from the same refining process that produces gasoline. It must have the proper viscosity, Volatility, and cetane number.

21. Diesel fuel A gallon of diesel fuel contains more heat energy than a galon of gasoline. It is a thicker part of crude oil.

22. Main properties of diesel A colourless or light yellow aromatic liquid. Slightly viscous (thick) and normally does not evaporate under normal temperature. Has a high flash point ranging from 40 to 100 o C.

23. Main properties of diesel Ignites spontaneously at about 350 o C a fairly low temperature compared to gasoline, which ignites at around 500 o C. Has a specific gravity of 0.82 to 0.86. Generates a large amount of heat (10,500Kcal). Has a higher sulfur content than gasoline

24. Since diesel fuel is thicker and has a different burning characterstics than gasoline, a high-pressure injection system must be used to spray the fuel directly into the combustion chamber. A low-pressure injection system or carburetor would not meter the thick diesel fuel properly.

25. NOTICE: • Unlike gasoline, diesel fuel also serves as a lubricant. Do not interchange the fuels because if gasoline is poured into a diesel engine by mistake, it could damage the injection pump and the nozzles.

26. Diesel fuel cetane rating A cetane rating indicates the cold starting ability of diesel fuel.

27. Cetane number The cetane number indicates the ignitability of the diesel fuel. The higher the rating, the better the fuel's ignitability and the lesser the amount of knocking

28. A low cetane number means the fuel is slower burning and requires a high temperature to ignite. The acceptable minimum cetane rating for a fuel to be used in high-speed diesel automotive engines is generally 40 to 50.

29. A cetane number, in some ways, is the opposite of a gasoline octane number. A higher cetane number means the fuel will ignite easily from heat and pressure.

30. Diesel Knocking Diesel fuel with a low cetane number takes a little longer to ignite. This may cause excessive ignition lag. In this delay, the fuel collects in the cylinder. When ignition does occur, all the fuel ignites at once. The pressure goes up quickly and combustion knock results.

31. In short, Diesel knocking is caused when the time before ignition (retarded ignition timing) becomes long, such as when using low rating cetane fuel, at low temperatures or low rpm. When the interval prior to ignition becomes longer, the fuel remaining in the cylinder will combust or explode all at once, causing the pressure to rise abruptly. This creates a rattling sound.