What is the factors effecting knock?

Answer 1

Factors Affecting KnockFrom the above discussion on knock, it may be seen that four major factors are involved in either producing or preventing knock. These are the temperature, pressure, density of the unburned charge and the time factors.

Temperature factor includes inlet temperature of the mixture and temperature of the combustion chamber walls. Increase in inlet temperature of the mixture makes the charge more vaporized than the required at the end of compression. This increases the tendency of knocking. Temperature of the combustion chamber walls plays a predominant role in knocking. In order to prevent knocking the hot spot in the combustion chamber should be avoided. Since, the spark plug and exhaust valve are two hottest parts in the combustion chamber; they mostly converted in to sources of autoignition after a long run. Furthermore, most of the two wheelers use air cooling which is not at all efficient to cool these two hottest parts. This is the main reason because of which a single cylinder engine could also have knocking problem.

Pressure factor includes the final pressure of fresh charge that it can reach after completion of compression stroke. Obviously, high pressure again converted in to high temperature factor and boosts the knocking process. No doubt, pressure factor is not at possible to control externally. It depends only on design parameters of engine.

Density factor comprises mass of inducted charge and fuel-air ratio. A reduction in the mass of the inducted charge into the cylinder of an engine by throttling reduces both temperature and density of the charge at the time of ignition. This decreases the tendency of knocking.

The flame speeds are affected by fuel-air ratio. Also the flame temperature and reaction time are different for different fuel-air ratios. So tendency of fresh charge to reach up to its self ignition temperature directly depends on the composition of that charge. As the carburettor or injection systems are the only responsible components for fuel-air ratios, it is advisable to keep them cleaned and tuned to avoid knocking.

Time factors consist of engine speed and spark timing. An increase in engine speed increases the turbulence of the mixture considerably resulting in increased flame speed, and reduces the time available for preflame reactions. Hence knocking tendency is reduced at higher speed. Adversely, prolonged high speed increases working temperature of engine and increases amount of hot spots, so prolonged and constant high speed should be avoided to minimize the knocking. Wrong spark timing always leads to incomplete combustion. This incomplete combustion can be possible either as pre-ignition or post-ignition. Both these incomplete combustion increases knocking tendency.

Miscellaneous factors include location of spark plug and octane value of the fuel. In order to have a minimum flame travel, the spark plug is located at proper place in the combustion chamber. DTSI, DTS-SI and now three spark plugs (Pulsar 200) are few of the examples where efforts have been made to reduce flame travel by igniting the fresh charge from different locations, of course in proper manner. High octane fuel has the higher self ignition temperature and low preflame reactivity which reduces the tendency of knocking.

How to Recognize KnockFollowing are some symptoms which can help to identify the knock.

• Metallic sound in harmony with engine firing
• Sound comes out after long running
• Sound disappears at idle speed
• Drastic drop in fuel efficiency
• Drop in power output
• Engine run with high temperature and noise

It is very true that one has to develop some skills to identify the particular sound of knock. Many videos are available on YouTube where you can have idea about knock sound and you can save your engine by identifying the knock in preliminary stage. I hope this will be useful to you all in future. Waiting for your valuable comments.