Knocking in SI (spark ignition) and CI (compression ignition) engine is an adverse phenomenon that takes place due to abnormal combustion. Sometimes terms knocking and detonation are used interchangeably but generally the term knocking is used more popularly in SI engines and detonation in CI engines, the phenomena of knocking and detonation are exactly the same and the reason will be explained later after building basic concept.
You may also curious and searched for some of the most common Questions regarding engine knocking and Detonation like:
- What is Engine Knocking?
- Why does Engine Knocking occur?
- what does engine knocking sound like? or How to identify engine knocking?
- Do engine knocking rises when accelerating ?
- What are Various methods of improving anti knocking quality of an engine?
- How to reduce engine knocking?
- What are harmful effects of Knocking?
and many more. Just hold on we will find out answers of all related queries. But first lets build our strong foundation in combustion and look at basics of combustion.
Distinction between Normal and Abnormal combustion w.r.t SI (spark ignition) or petrol engine.
Normal Combustion
During the compression stroke, temperature of air fuel mixture rises and as the temperature of air fuel mixture is greater than or equal to the flash point temperature just an external source is required to initiate the combustion and rest will be done automatically. That external source in SI Engine is popularly known as spark plug.
As the very first spark hits the nearest fuel particle in the combustion chamber it will lead to formation of flame front i.e. flame front is appeared for the first time in the combustion chamber. Now the charge temperature is already greater than the flash point in the cylinder which leads to the further increase in the temperature of next layer particles through radiation heat exchange from flame front and hence the new flame front will appear and this process continues till the end charge evenly throughout chamber.
Hence it appears that flame front is travelling from the first charge to the end charge known as propagation of flame front.
Abnormal Combustion
As the name suggest itself Abnormal is that combustion which does not follow the pattern of normal combustion. In case of normal combustion there is only one flame front from the first charge to the end charge but in abnormal combustion there are more than one flame front and those will developed due to the hot-spots present within the combustion chamber zone which itself able to increase the fuel temperature greater than the fire point temperature.
Hot spots were developed due to the many reasons like, due to the increase in temperature because of the remaining's of last burnt fuel which left in chamber or because of fuel itself (depends on fuel anti knock properties).
Now in the system there are more than one flame fronts one is released from spark plug and others are released from hot spots. These flame fronts have huge amount of pressure and heat energy when meet and colloid against each other will generates high energy shock waves (huge amount of energy for very short period) this collision is known as knocking.
Such amount of energy is unable to handle by piston and cylinder assembly which results in the collision of piston and cylinder and hence will generate metallic sound termed as detonation.
We can easily identify Engine Knocking by this metallic sound.
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