Abnormal combustion in si engines ppt ic engine

Abnormal Combustion Types, pre_ignition And Knocking

1. Abnormal combustion in si engines Presented by Anurag ratan shakya b.Tech mechanical Roll no- 1700850 Final year

2. What is mean by Combustion? As per Oxford Dictionary : The process of burning something (from Latin: comburere) Chemistry : Rapid chemical combination of a substance with oxygen, involving the production of heat and light. Thermo Dynamics : Combustion is a chemical reaction in which certain elements of the fuel like hydrogen and carbon combine with oxygen liberating heat energy and causing an increase in temperature of the gases

3. Combustion in SI Engines In any conventional spark-ignition (SI) engine, the fuel and air are homogeneously mixed together in the intake system, inducted through the intake valve into the cylinder where it mixes with residual gases and is then compressed. Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge. A turbulent flame develops following the ignition and propagates through this Combustion in SI Engines

4. Combustion in SI Engine Normal Combustion Abnormal Combustion When the flame travels evenly or uniformly across the combustion chamber. When the combustion gets deviated from the normal behaviour and resulting in loss of performance or damaging the engine

5. Abnormal Combustion in SI Engine Pre-Ignition Knocking Pre-ignition (self- ignition) occurs when the fuel mixture in the cylinder burns before the spark-ignition event at the spark plug. Knocking is due to auto-ignition of end portion of unburned charge in com b ustion chamber.

6. PRE-IGNITION Pre-ignition is the ignition of the homogeneous mixture of charge as it comes in contact with hot surfaces, in the absence of spark. Pre-ignition is initiated by some overheated projecting part such as the sparking plug electrodes, exhaust valve head, metal corners in the combustion chamber, carbon deposits or protruding cylinder head gasket rim etc. Auto ignition may overheat the spark plug and exhaust valve and it remains so hot that Engine efficiency will decrease due to pre- ignition Pre-ignition causes holes melted in pistons, spark plugs melted away, and engine failure happens pretty much immediately.

7. Knocking or Detonation Normal flame front travels at 15 to 30 m/s speed Temperature increases due to flame front advance, radiation and reactions of unburnt mixture During abnormal combustion end charge auto-ignites before the flame front reaches Velocities reached during detonation are 300 to 1000 m/s Press rise almost 3 to 4 times i.e. 50 bar to 150-200 bar

8. Knocking or Detonation This pressure frequency or vibration frequency in SI engine can be up to 5000 Cycles per second. Denotation is undesirable as it affects the engine performance and life, as it abruptly increases sudden large amount of heat energy. It also put a limit on compression ratio at which engine can be operated which directly affects the engine efficiency and output.

9. Effect of Knocking The harmful effects of detonation are as follows: Noise and roughness Mechanical damage Carbon deposits Increase in heat transfer Decrease in power output and efficiency

10. Cause of Abnormal combustion 1.Fuel condition including quality , quantity and other attributes. 2.Engine scantling (Condition of piston, Piston rings, Turbocharger functioning (if fitted),Bearing condition, nuts and bolts, Valves, Lubrication, Spark plug etc.) 3.Suction air quality and quantity. 4. Engine colling efficiency 5. Vibration.

11. 6. Atmosphere in which engine is operating . 7. Orientation of engine. 8.mechanical efficiency of parts. 9. Mechanical properties of different scantling materials. 10.condition of timing gears and chains operating cams