2 CI Engine Normal and Abnormal Combustion-Knocking

knocking combustion(Abnormal combustion) in CI engines In CI engine, auto ignition of first part of droplet which is responsible to produce knocking and rough running of engine.  Delay period is more, more droplets are accumulated in the combustion chamber  That Accumulated fuels start burning with creates high rate of pressure rise.  Heavy vibration accompanied by a knocking sound .  Causing  Over heating of piston ,  Cylinder head  Drop in power and damage to bearings  Possible to piston seizure . . . • reducing peak cylinder pressure by decreasing the engine revolutions (e. • decreasing the manifold pressure by reducing the throttle opening. there is also evidence that knock occurs more easily at high rpm than low regardless of other factors). reduces the combustion temperature and increases the margin above detonation. shifting to a higher gear. boost pressure or • reducing the load on the engine. which increases the combustion temperature of the fuel and reduces the proclivity to detonate.knocking/detonation prevention • Detonation can be prevented by any or all of the following techniques: • the use of a fuel with high octane rating. .g. • enriching the air-fuel ratio which alters the chemical reactions during combustion. Comparison of Knock in SI and CI Engines . engine takes place in an homogeneous mixture.Comparison of Knocking in S.I. Engines  In S.  The knocking in S. engine. and C. the delay period should be as small as possible and fuel self –ignition temperature should be as low as possible. engines. In C. engines.I.  The knocking in S. the mixture is heterogeneous and hence rate of pressure is lower. engine. To avoid this.  The question of preignition does not arise in C. To avoid knock in C. this occurs at the beginning of combustion.I. the knocking occurs near the end of combustion whereas in C. . the rate of pressure rise is high. engines as the fuel is supplied only near the end of compression stroke. engines is because of auto-ignition of the last part of the charge.I.I.I.I. therefore.I.I. the fuel must have long delay period and high self ignition temperature. engines High Long Low C. 5.I. engines S. Inlet temperature Inlet Pressure Speed Cylinder size Combustion chamber wall Temp Low Low High Small Low High High Low Large High .I. Factors affecting Knock Self ignition temperature Delay period of fuel Compression Ratio S.I. 2. No.I. engines Low Short High 4. 8. 1. 3. and C. 7.Factors which reduce Knocking in S. 6. Injection angle . Cetane number . Compression ratio .