Engine Knocking | Types | Causes

Hello readers, welcome to all of you, you have come here in this post to know what is engine knocking. How it happens and what are its reasons In this post, you will be able to know very well what is engine knocking and what causes it, so let’s know about engine knocking.

Engine Knocking

petrol engine knocking

As you all know that spark plug is used in a petrol engine to ignite the fuel. As soon as there is a spark in the cylinder, the ignition of the fuel is done by the spark from the spark plug.

And the fuel burns evenly from start to finish. Due to this, the piston gets a powerful thrust. And the pressure in the cylinder also increases uniformly. but sometimes there is a problem

In which the last part of the fuel burns with a sound even before it comes in contact with the spark. Due to this, there is sudden pressure on the piston. Due to this the pressure in the cylinder also increases. Due to this, a special sound is produced. Also called engine knocking or detonation




due to knocking or detonation

Knocking is a problem occurring in engines. In which a sound is generated from the engine due to which problems arise in the engine. There are a few reasons for knocking on an engine. Let’s discuss a little about them, and how they arise, some of the reasons are as follows


It is important to know that generally, the ignition of the fuel occurs at the end of the compression stroke. But if at some point the ignition is on time. So it is called pre-ignition or pre-ignition.

There can be many reasons for this situation. Like the ignition timing is bad. or the higher compression ratio. Or carbon has accumulated in the combustion chamber. Or spark plugs are too hot Engine knocking occurs even if the mixture is hot.

And also the combustion chamber should be such that when there is a spark, there should be proper ignition all around, lack of this also leads to knocking. This type of situation arises.

Read Also  Magneto Ignition | Parts | Working | Types 

Some ways to prevent knocking defect

When the mixture is compressed in the cylinder during the compression stroke. So its temperature increases a lot. And it is lit by the spark. But if the temperature of the mixture increases too much that it goes up to the auto-ignition temperature

So the fuel then ignites this mixture on its own before giving a spark. The auto-ignition temperature of any fuel is that temperature. On which he himself starts burning. causing engine knocking problem

During compression, it is necessary that there should be no auto-ignition of the mixture. For this, the quality of petrol should be such that there should be no auto-ignition even at extreme temperatures in compression so that there is no problem of knocking. For this some chemicals are added to the petrol which is

octane number

It is a special property of petrol which prevents auto-ignition of the mixture even at high temperatures. And prevents knocking by having controlled ignition. Also called octane number or octane rating.

Or in simple words, the condition showing antiknock properties of petrol is called octane number or octane rating. This same chemical is mixed in it.

This substance is tetraethyl lead (T E L). Due to this, ignition does not happen with a jerk. It is mixed up to 0.07%. At present, lead-free petrol is used.

Normal heptane (C7H16) has very little anti-knock property for the octane number of petrol. And iso-octane (C8H18) anti-knock property is very good. Various mixtures are made by mixing.

The octane number is the percentage of iso-octane in petrol. The higher the octane number of the petrol, the more it is a knock inhibitor.

If the octane number of petrol is 93. So it means it has 93% iso-octane and 7% normal heptane.

The octane rating of a Buell can exceed 100.

And the octane rating of C.N.G is around 125. ie it has 25% anti-knock properties than iso-octane

Presently 93 to 94 octane number is of regular grade petrol

The octane number of premium grade petrol is 90 to 100.

Whereas the octane number of super premium petrol is 101 to 102.

The normal petrol used in India has an octane rating of 91. it extra

Speed ​​93 petrol has an octane rating of 93 and Speed ​​97 has an octane rating of 97. And the octane rating of Power 99 is 99.

What is a diesel engine knocking?

Only clean air enters the diesel engine during the suction stroke. Then it is compressed. Due to this, the air pressure increases a lot. And diesel is sprayed on it.

Diesel burns when it comes in contact with hot air. Diesel is injected into the cylinder through injectors. and when the ignition is on. So the time between spray and ignition is called ignition lag. Due to this engine knocking happens. and the engine makes a screeching sound

The ignition lag is as low as in a diesel engine. The engine is that good. And the higher the ignition lag. So problems remain. The depression between fuel injection and ignition is very high.

So that more fuel enters the engine. When fuel burns. So the pressure and temperature increase more. And to some extent, the fuel enters the engine in liquid form. This is called engine lacing. Due to this engine knocking happens.

How to stop diesel engine from knocking

Some measures can be taken to prevent diesel knocking. like

by increasing the temperature of the combustion chamber
air turbulence

raising the senses
increasing the compression ratio
increasing injector pressure
or by increasing the inlet air pressure

Apart from this, knocking can also be stopped by increasing the quality of diesel. Which is called the cetane number. Let’s know about it in detail

cetane number

The quality of diesel fuel is indicated by the cetane number. In general terms, normal hexadecane (C16H34) is called the cetane number. The percentage of cetane number in any fuel. That is called its cetane number.

Suppose the cetane number of diesel is 55, it means that it has 55% cetane and the rest is methyl naphthalene.

The higher the cetane number of diesel, the shorter its ignition delay. Diesel burns easily.

The value of the cetane number usually lies between 35 and 60. The cetane number of diesel used in India is around 51.

Now you must be thinking that if the cetane number of diesel is high. So why is cetane mostly not kept in diesel. Which is the reason. that if the cetane number of diesel is higher than the required

Diesel will burn before mixing with air and some amount of diesel will remain unburned which will cause excessive smoke and engine knocking.

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