PHENOMENON OF KNOCK IN SI ENGINE BY : MOHAMMAD JOMA’A Ala’a Z Allan

PRESENTATION GOALS PRESENTATION GOALS * what is the phenomenon of knocking * what is the phenomenon of knocking ? * How the phenomenon of knocking occur? * How the phenomenon of knocking occur? *The disadvantages faced by the company *Recommendations

PHENOMENON OF KNOCKING IN SI ENGINE * Knocking (also knock, detonation, spark knock, pinging or pinking) in spark- ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder does not start off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. * The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

HOW THE PHENOMENON OF KNOCKING OCCUR In a spark ignition engine combustion which is initiated between the spark plug electrodes spreads across the combustible mixture. A definite flame front which separates the fresh mixture from the products of combustion travels from the spark plug to the other end of combustion chamber In a spark ignition engine combustion which is initiated between the spark plug electrodes spreads across the combustible mixture. A definite flame front which separates the fresh mixture from the products of combustion travels from the spark plug to the other end of combustion chamber Heat release due to combustion increases the temperature and consequently the pressure, of the burned part of the mixture above those of the unburned mixture. In order to effect pressure equalization the burned part of the mixture will expand, and compress the unburned mixture adiabatically there by increasing its pressure and temperature. This process continues as the flame advances through the mixture and the temperature and pressure of the unburned mixture are increased farther. Heat release due to combustion increases the temperature and consequently the pressure, of the burned part of the mixture above those of the unburned mixture. In order to effect pressure equalization the burned part of the mixture will expand, and compress the unburned mixture adiabatically there by increasing its pressure and temperature. This process continues as the flame advances through the mixture and the temperature and pressure of the unburned mixture are increased farther. If the temperature of the unburned mixture exceeds the self ignition temperature of the fuel and remains at or above this temperature during the period of pre flame reactions, spontaneous ignition or auto ignition occur at various pin point reaction. The phenomenon is called knocking If the temperature of the unburned mixture exceeds the self ignition temperature of the fuel and remains at or above this temperature during the period of pre flame reactions, spontaneous ignition or auto ignition occur at various pin point reaction. The phenomenon is called knocking

This phenomenon of knock may be explaned by referring to Fig (a) which show the cross section of the combustion chamber with flame advancing from the spark plug location A without knock where as Fig(c) shows the combustion process with knock This phenomenon of knock may be explaned by referring to Fig (a) which show the cross section of the combustion chamber with flame advancing from the spark plug location A without knock where as Fig(c) shows the combustion process with knock Normal combustion in the normal combustion the flame travels across the combustion chamber from A towards D. The advancing flame front compressed the end charge BBD farthest from the spark plug thus raising its temperature. The temperature is also increased duo to heat transfer from the hot advancing flame front. Also some pre flame oxidation may take place in the end charge leading to further increase in temperature. In spite of these factors if the temperature of the end charge had not reached its self ignition temperature, the charge would not auto ignite and the flame advance further and consume to charge BBD. This is the normal combustion process which is illustrated by means of the pressure time diagram, Fig(b) Normal combustion in the normal combustion the flame travels across the combustion chamber from A towards D. The advancing flame front compressed the end charge BBD farthest from the spark plug thus raising its temperature. The temperature is also increased duo to heat transfer from the hot advancing flame front. Also some pre flame oxidation may take place in the end charge leading to further increase in temperature. In spite of these factors if the temperature of the end charge had not reached its self ignition temperature, the charge would not auto ignite and the flame advance further and consume to charge BBD. This is the normal combustion process which is illustrated by means of the pressure time diagram, Fig(b)

However, if the end charge BBD reaches auto ignition temperature and remains for some length time equal to the time of pre flame reactions the charge will auto ignite, leading to knocking combustion. In Fig(c), it is assumed that when the flame has reached the position BB, the charge a head of it has reached critical auto ignition temperature, during the pre flame reaction period if the flame front could move from BB to only CC then the charge ahead of CC would auto ignite

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