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Sheikh Mohammed Kaif

Why the knocking is there in ci engines??

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In ci engines, as we know that when the mixing of fuel and air is not very homogeneous as compared to si engines, the pressure rise is also not uniform. Due to this, the combustion takes place non-uniformly at many places at the time of ignition, and when more than two flames are generated on two different sites in piston cylinder, they follow the knock phenomena.

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When fuel is injected into combustion chamber, the droplets of fuel takes heat from surrounding air and vaporizes. Then it's temperature goes on increasing till it reaches to auto ignition temperature and then combustion starts. This whole process requires some time (fraction of second) and it is known as IGNITION DELAY OR DELAY PERIOD.

Ideally, as injrction starts, ignition should take place simultaneously. That is , ignition delay should be as small as possible.

Suppose auto ignition temperature of fuel is more then there will be more ignition delay period. This means that after injection start, the fuel will droplets will accumulate in combustion chamber. And after auto ignition temperature reaches, this accumulated fuel burns promptly. In fact it explodes. This is cause of knocking in CI Engine.

There are several factors which affects knocking such as A/F ratio, compression ratio, auto ignition temperature, engine speed, inlet temperature, intake pressure etc.

It is very difficult to differentiate between normal engine running and knocking in case of CI Engine.

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Knocking (also knock, detonation, spark knock, pinging or pinking) in start interior burning motors happens when burning of a portion of the air/fuel blend in the chamber does not result from spread of the fire front touched off by the start plug, however at least one pockets of air/fuel blend detonate outside the envelope of the ordinary ignition front. The fuel-air charge is intended to be lighted by the start plug just, and at an exact point in the cylinder's stroke. Knock happens when the pinnacle of the burning procedure never again happens at the ideal minute for the four-stroke cycle. The stun wave makes the trademark metallic "pinging" sound, and barrel weight increments drastically. Impacts of motor knocking reach from immaterial to totally ruinous.

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In my experience, it is usually caused by the air to fuel ratio, too much compression or a spark plug problem. Copper core plugs are supposed to be less likely to cause detonation, but they don't usually last as long. 

 

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