Knocking in si & ci engine

Knocking in SI and CI engine

1. SAMBODHI Enlightment of Knowledge Knocking in SI & CI engines 28-07-2020 1

2. SAMBODHI Enlightment of Knowledge Definition Abnormal combustion occurring in the combustion chamber give sudden pressure rise accompanied by a metallic, hammer like sound leading to loss of performance and possible engine damages . This abnormal combustion is commonly known as “Knocking Combustion” 28-07-2020 2

3. SAMBODHI Enlightment of Knowledge Comparison 28-07-2020 3

4. SAMBODHI Enlightment of Knowledge Classification of IC Engine SI engine ( Spark Ignition)or Gasoline or Petrol Engine CI engine (Compression Ignition) or Diesel Engine 28-07-2020 4

5. SAMBODHI Enlightment of Knowledge

6. SAMBODHI Enlightment of Knowledge Engine Cylinder

7. SAMBODHI Enlightment of Knowledge What is Engine Knock? F uel burns unevenly in your engine’s cylinders. Incorrect balance of air and fuel , fuel will burn in small, regulated pockets instead of all at once. S hocks go off at the wrong time . An annoying noise and potential damage to your engine’s cylinder walls and pistons. 28-07-2020 7

8. SAMBODHI Enlightment of Knowledge SI ENGINE 28-07-2020 8

9. SAMBODHI Enlightment of Knowledge What Could Cause Engine Knocking in SI engine? Faulty Spark Plugs Low-Octane Fuel Carbon Deposits High compression ratio Advanced Ignition timing . Knocking takes place at end of combustion. 28-07-2020 9

10. SAMBODHI Enlightment of Knowledge What Could Cause Engine Knocking in SI engine? Decrease in Turbulence Low Engine Speed Increase in flame travel distance 28-07-2020 10

11. SAMBODHI Enlightment of Knowledge CI ENGINE 28-07-2020 11

12. SAMBODHI Enlightment of Knowledge What Could Cause Engine Detonation or Pinking in CI engine? Increase in Ignition lag Increase in Engine speed Reducing the flame travel distance Increasing in delay period of the fuel. Reduce in cetane Number. Detonation takes place at the beginning of the combustion. 28-07-2020 12

13. SAMBODHI Enlightment of Knowledge 28-07-2020 13

14. SAMBODHI Enlightment of Knowledge Factors Tending to reduce knocking in IC engine S.No Factors S.I Engine C.I Engine 1. Self Ignition temp of fuel High Low 2. Time Lag or Delay period of fuel Long Short 3. Compression ratio Low High 4. Inlet Temperature Low High 5. Inlet Pressure Low High 6. Combustion chamber of wall temperature Low High 7. Speed High Low 8. Cylinder size Small High 28-07-2020 14

15. SAMBODHI Enlightment of Knowledge OCTANE NUMBER Its the % of iso -octane in a mixture of iso -octane and n-heptane . It is a standard measure of the performance of an engine fuel . It is the measure of resistance of gasoline against knocking or pre-ignition of fuel . Octane number is related to petrol. 28-07-2020 15

16. SAMBODHI Enlightment of Knowledge Higher the octane number , better the quality of petrol, and hence the vehicle gives higher efficiency. The octane no. of iso -octane is 100 , while that of n heptante is zero(0) . Increase in octane number reduces the knocking. 28-07-2020 16

17. SAMBODHI Enlightment of Knowledge CETANE NUMBER Its the % of n-hexadecane in a mixture of n-hexadecane and 1-methyl naphthalene. It is an indicator of combustion speed of Diesel Engine . It is the measure of ignition quality of fuel. Alkyl nitrates and Di- tert - butyal peroxide are used as additives to raise the Cetane number. 28-07-2020 17

18. SAMBODHI Enlightment of Knowledge High cetane number fuels have low octane numbers , and vice versa. This is one reason why we can't use petrol in diesel engine and vice versa . High cetane number reduces the pinking or detonation in CI engine. In India Current BIS specification of Anti-Knocking Index of petrol is 84 (unleaded regular) and 88 (unleaded premium), And cetane index of diesel is 45-50 . 28-07-2020 18

19. SAMBODHI Enlightment of Knowledge COMPRESSION RATIO: The ratio of the maximum to minimum volume in the cylinder of an internal combustion engine. For SI Engine : 10 to 14 For CI Engine : 18 to 23 28-07-2020 19

20. SAMBODHI Enlightment of Knowledge Air-Fuel ratio (AFR) for various fuel: S.NO FUEL AFR 1. Methanol 6.47:1 2. Ethanol 9:1 3. Butanol 11.2:1 4. Diesel 14.5:1 5. Gasoline 14.7:1 6. Propane 15.67:1 7. Methane 17.19:1 8. Hydrogen 34.3:1 28-07-2020 20

21. SAMBODHI Enlightment of Knowledge Air fuel ratio It is the ratio between mass of the air to the mass of fuel. AFR = m a / m f Inverse ratio = 1/AFR is called as Fuel – air ratio. The theoretical air – fuel ratio for a complete combustion is called as stoichiometric air fuel ratio . 28-07-2020 21

22. SAMBODHI Enlightment of Knowledge For petrol engine fuel Stoichiometric ratio is 14.7:1 i.e In order to burn completely 1 kg of fuel , we need 14.7 kg of air . AFR > Stoichiometric ratio = Lean Mixture AFR < Stoichiometric ratio = Rich Mixture 28-07-2020 22

23. SAMBODHI Enlightment of Knowledge AFR for S.I Engine - 12: 1 (Rich Mixture) 20: 1 ( Lean Mixture) AFR for C.I Engine – 18:1 to 70:1 (Lean Mixture) C.I Engine – Air and fuel are mixed randomly in CC, so it is a Non –Homogenous mixture S.I Engine – Air and Fuel is mixed well as a single mixture so it is a Homogenous mixture 28-07-2020 23

24. SAMBODHI Enlightment of Knowledge AFR GRAPH 28-07-2020 24

25. SAMBODHI Enlightment of Knowledge S.I Engine AFR For Engine starting , AFR = 9: 1 ( Rich Mixture) For Warming up ( Idling ) AFR = 12: 1(Rich Mixture) For Accelerating, AFR =11:1 – 13:1 ( Rich Mixture) 28-07-2020 25

26. SAMBODHI Enlightment of Knowledge C.I Engine AFR For Heavy load, AFR = 12: 1 ( Rich Mixture) For Constant speed,( Cruising ) AFR = 14.7: 1 (Stoichiometric Mixture) For Decelerating AFR = 17:1 ( Lean Mixture) Air-fuel ratio of 12:1 is considered as maximum output ratio, where as the air-fuel ratio of 16:1 is considered as maximum fuel economy ratio. 28-07-2020 26

27. SAMBODHI Enlightment of Knowledge Physical Delay : Time b/w beginning of the injection and attainment of chemical reaction. Chemical Delay: Time b/w beginning of the combustion reaction and then accelerating . C.I Engine – Physical + Chemical delay ( Paraffins ) S.I Engine – Chemical delay ( Aromatics ) 28-07-2020 27

28. SAMBODHI Enlightment of Knowledge Air–fuel equivalence ratio ( λ ) The ratio of actual AFR to the Stoichiometric AFR is given by symbol Lambda If < 1 - Rich Mixture . = 1 - Stoichiometric > 1 - Lean Mixture . Fuel–air equivalence ratio ( ϕ ) It’s a Vice versa of 28-07-2020 28

29. SAMBODHI Enlightment of Knowledge GATE QNS A diesel engine is usually more efficient than a spark ignition engine because (Gate ME-03) Diesel being a heavier hydrocarbon, releases more heat per Kg than gasoline. The air std.efficiency of Diesel cycle is higher than the Otto cycle, at a fixed compression ratio . The compression ratio of a diesel engine is higher than of an SI engine. Self Ignition temperature of diesel is higher than that of gasoline . 28-07-2020 29

30. SAMBODHI Enlightment of Knowledge At the time of starting, idling and low speed operation, the carburetor supplies a mixture which can be termed as (Gate ME 04) Lean Slightly leaner than stoichiometric Stoichiometric Rich 28-07-2020 30

31. SAMBODHI Enlightment of Knowledge Knocking tendency in a SI engine reduces with increasing (GATE ME 93) Compression ratio Wall temperature Supercharging Engine speed Presentation prepared by C.PRASATH 28-07-2020 31

32. SAMBODHI Enlightment of Knowledge 28-07-2020 32