Mechanical Engineering - Automobile Engineering - Discussion

Pre-Ignition is the ignition of air-fuel mixture before the spark plug is activated.

Knocking is the ignition of air fuel mixture in locations other than main flame front(occurs after spark plug activation).

What is difference between knocking and pre ignition?

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.

Knocking should not be confused with pre-ignition "They are two separate events". However, pre-ignition is usually followed by knocking.

What is knock property in CI & SI engine ?

Amyl nitrate is the name of a colourless synthetic liquid having the formula C5H11NO3, which is normally used as an additive in diesel fuel so as to improve its ignition properties. Medically, this liquid is used to treat cyanide poisoning.