Difference between SI & CI Engine ppt.pptx

Si & ci engine, applied thermodynamics

1. Comparison of S. I. Vs C. I. Engine No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 1. Preparation of A: F mixture Outside the cylinder in Carburetor Inside the cylinder (Fuel Injection) 2. During suction stroke Air + Fuel inducted in cylinder Only air inducted in cylinder 3. Nature of mixture Homogenous Heterogeneous

2. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 4. Method of Ignition Spark plug ignition (S. I). Self ignites due to compression pressure (C. I.) 5. Ignition occurs at Single point (Air gap of Spark plug) and single flame travels in combustion chamber Multiple points (Wherever mixture reaches SIT it ignites) 6. Spark plug Present (Due to high SIT – 246 C) Absent (uses cold plug or glow plug heater for cold starting) (Low SIT – 210 C)

3. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) Cost of fuel High Low Sp. Gravity Low High Fuel sold Volume basis (in litres ) Mass basis (kg) Performance For the same compression ratio efficiency of Otto cycle is more than dual and diesel Cycle For all other condition efficiency of diesel cycle is greater than dual and Otto cycle. Dual cycle efficiency is always in between Otto and Diesel cycle.

4. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 7. Carburetor Present Absent (Uses fuel injector) 8. Thermal Efficiency Low High 9. Air standard cycle Otto Diesel 10. Fuel Economy (km/ litre ) Low (12 km/ litr ) 40 % Higher than petrol engines (16 to 17 km/ litre )

5. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 11. Compression ratio 6-10 16-20 12. Limit on Compression ratio Yes due to Knocking (HUCR) No limit 13. Noise & vibrations of engine Low High 14 Size of the engine Small Bulky due to high CR

6. No Parameter Spark Ignition (S. I .) Compressed ignition (C. I) 15. Application Light duty automobiles (Two-wheeler and low duty car) Heavy duty (Trucks, buses power plants, diesel genets etc.) 16. Bore size / Cylinder diameter Limited due to knocking (Since flame will need more time to reach end mixture for large size resulting in knocking) max. 100 mm No limit (50 – 900 mm) 17. Pre-ignition of mixture (before spark) Possible Not possible (No sparking)

7. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 18. Number of Valves 2 (Suction and discharge valve) 3 (Suction, discharge and Fuel injector) 19. With accelerator increase Throttle valve opening increases Fuel injection amount increases 20. Quantity of air Varies with load /Throttle valve opening /Acceleration Deceleration Constant amount of air inducted during suction stroke

8. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 21. Type of governing Quantity governed Quality governed 22. As the load changes Quantity of A: F mixture changes Air supply is constant, quantity of fuel sprayed changes and hence quality of mixture or A:F ratio changes 23. Low load condition Rich mixture required Lean mixture required

9. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 24. Operating range of A:F ratio 12-17 Cold starting: 7:1 Idling condition: 12.5 Normal power: 16 Maximum power: 13.5 20-100, Idling or low load very lean 100:1 Maximum power 20: 1 (Below 20 A:F ratio problem of black smoke and odour from the exhaust even though max. power possible) 25. Air supply preference Turbulence of air (disorderly random motion with no direction) to increase flame velocity Air Swirl motion (Orderly motion in circular direction

10. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 26. Stages in Combustion Three (Ignition lag, Flame propagation and after burning) Four (Ignition delay, Rapid or Uncontrolled combustion, Controlled Combustion, After burning) 27. Number of flames Single definite flame Multiple flames 28. Peak pressures 25-50 bar 150-200 bar

11. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 29. Injection pressures 6 – 10 bar 150-200 bar 30. Delay period No physical delay as mixture prepared outside cylinder. Only chemical delay (Ignition lag) Physical + Chemical delay (Ignition delay) Physical delay – Mixture preparation phase Chemical delay – Pre-flame processes 31. Physical delay period (Mixture preparation) Absent (Mixture prepared outside cylinder) Mixture prepared inside cylinder (After fuel sprayed)

12. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 32. Ignition Delay period (Time after SIT is reached for flame to appear) High to avoid end mixture to self ignite Low – to avoid large amount of diesel getting accumulated during delay period

13. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) Combustion           Ideal Combustion Curve - Ignition and Combustion occurs at TDC - Pressure rise instantaneous - Temp. rise instantaneous. - No un burnt HC. - No late burning.

14. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) Combustion           Combustion Curve

15. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 33. Suction Air + Fuel Only Air 34. Stages of combustion 3 4 35. Point A Spark is supplied Fuel injection starts 36. Point B Flame appears Mixture reaches SIT 37 Point C Maximum Pressure Flame appears (B-C: Pre-flame processes)

16. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 37. Stage I A-B: Ignition lag A-C: A-B Physical delay (Mixture preparation )+B-C Chemical delay (Pre flame processes) 38. Stage II B-C: Flame propagation C-D: Rapid or Uncontrolled combustion of diesel accumulated during delay period 39. Stage III C-D: After burning D-E: Controlled combustion after accumulated diesel is burnt 40. Stage IV Absent After burning

17. No . Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) Abnormal combustion / Knocking and factors affecting knocking   Abnormal Combustion    

18. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) Abnormal combustion / Knocking and factors affecting knocking 41. Definition of Knocking or abnormal combustion Happens at the end of combustion due to self ignition of end mixture (Hence requires long delay period to avoid knocking) Happens at the start of combustion due to long delay period large amount of diesel getting accumulated inside the cylinder which burns and creates large pressure during uncontrolled combustion stage. 42. Knocking occurs at End of combustion Start of combustion

19. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 43. Delay period required to avoid knocking Long Short 44. SIT to avoid knocking High (To avoid self ignition of end mixture) Low (Avoid large amount of diesel does not get accumulated)

20. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 45. Octane number to avoid knocking (ON is directly proportional to SIT) Should be high. High SIT desirable Should be low. Low SIT desirable   46 Cetane number to avoid knocking (CN is inversely proportional to SIT) Should be low. High SIT desirable Should be high. Low SIT desirable

21. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 46. Flame speed High Low 47. Engine size Low (Flame reaches faster to end of chamber) High 48. Density Low High 49. Supercharging preferred No Yes

22. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 50. Compression ratio Limited by HUCR ( 6-10) High (16-20) 51. Initial pressure and temperature Low ( To avoid burning of end mixture) High ( To help self ignition and decrease delay period) 52. Cylinder Wall temperature Low ( To avoid burning of end mixture) High to assist self ignition

23. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 53. Engine speed High (To increase flame velocity) Low (To reduce delay period) 54. Turbo charging Not desirable Desirable 55. Cooling water temperature Low High 56. Exhaust No smoke or odour from exhaust Black Smoky and odour (heavy Soot in exhaust)

24. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 57. Detection of engine knocking Possible by human ear (Petrol engines are less noisy) Difficult to detect by human ear (Diesel engine are more noisy) 58. Intake fuel temperature Low High 59. Engine load Low (high load increases operating temp of cylinder cooling water and hence knocking) High (Increase in operating temperatures helps in Self ignition and reduces detonation)

25. No. Parameter Spark Ignition (S. I.) engines Compressed ignition (C. I) 60 Exhaust gas temperature Higher (Due to lower heat extraction) Lower (Due to high heat extraction)