This post talks a bit about a concept called "Knocking" which is very common in Spark Ignition Engines having very high compression ratio.
The question we need to answer first is what is Knocking?
"Knocking" is a metallic pinging sound that is caused inside the engine cylinder & leads to high intensity vibrations in the Engine block ( usually causing metal fatigue in the cylinder walls). If the engine knocks for very long duration then this might have serious impact on the usability of the engine and on the engine life.
What causes "Knocking"?
Flame front:
The power inside the cylinder which is generated when a spark ignites the air-fuel mixture leading to a flame front. The flame front is a high velocity pressure wave that travels, after originating near the spark plug, downwards to push the piston head and ensure that our engine keeps running.
Now, what happens is that based on the fuel quality among many other factors there are certain spots inside the cylinder with some carbon deposits.If the cylinder temperature keeps rising due to ignitions then at some point of time these hot spots reach a temperature where they are capable to ignite the air fuel mixture or the yet unburnt gases (also called End gas). Simply put there is sufficient heat accumulation at certain points that these act as potential spark plugs.
The Problem:
This causes a problem because we do not have any control over these spontaneously created spark plugs ( so to say). This means that they can ignite the end gas at any point of time. In homogeneous operation of engine ( more about this mode in future posts), the mixture is fairly rich and these hot spots have energy sufficient to create a ignition of the air-fuel mixture. This phenomenon is often known as "Detonation".This ignition causes a pressure wave to travel from the spot towards the periphery. This Flame-Front if collides with the flame front travelling from the spark plug will result in wo very high pressure waves to bang into each other. This leads to very high pressure peaks which put extreme amounts of stress on the cylinder walls. Continuous Knocking might lead to permanent stresses getting formed in the cylinder walls & the piston head which eventually will lead to their failure.
How to Avoid it ?
A few years back when "leaded Petrol" used to rule the market the folks added something known as Anti-Knock in the petrol to ensure that it did not knock. This however, lead to higher amount of particulate matter in the exhaust and also was not good for the cylinder walls.The anti knock was a Lead compound which i cannot remember at this point of time.
These days we use more of "Unleaded Petrol" and there has to be different mechanism to control the "Knocking". This is done these days by ensuring that the engine temperature doesn't reach very high ( then there is lesser chance of hot spots getting created). One of the methods employed to do this is EGR. Exhaust Gas Recirculation, ensures that he temperature of the engine come down apart from having benefits like enhanced fuel efficiency and reduced NOx in the exhaust. However, that is a different topic and will be dealt a little later. The other method used in conjunction with the EGR is spark retard. Spark retard will ensure that you ignite the air-fuel mixture late enough that there will be no pressure peaks. When we do not have pressure peaks the chances of knocking are lesser.
Finally, use of high octane fuel will result in better combustion of the fuel and lesser hot spot contribution because of poor fuel quality. The reduction of hot-spots will result in reduced Knocking even at higher engine temperatures.
Some links that give more info on how it is detected etc
[1]
Please leave your comment. You can subscribe to this blog by using the links under "Subscribe" section.The question we need to answer first is what is Knocking?
"Knocking" is a metallic pinging sound that is caused inside the engine cylinder & leads to high intensity vibrations in the Engine block ( usually causing metal fatigue in the cylinder walls). If the engine knocks for very long duration then this might have serious impact on the usability of the engine and on the engine life.
What causes "Knocking"?
Flame front:
The power inside the cylinder which is generated when a spark ignites the air-fuel mixture leading to a flame front. The flame front is a high velocity pressure wave that travels, after originating near the spark plug, downwards to push the piston head and ensure that our engine keeps running.
Now, what happens is that based on the fuel quality among many other factors there are certain spots inside the cylinder with some carbon deposits.If the cylinder temperature keeps rising due to ignitions then at some point of time these hot spots reach a temperature where they are capable to ignite the air fuel mixture or the yet unburnt gases (also called End gas). Simply put there is sufficient heat accumulation at certain points that these act as potential spark plugs.
The Problem:
This causes a problem because we do not have any control over these spontaneously created spark plugs ( so to say). This means that they can ignite the end gas at any point of time. In homogeneous operation of engine ( more about this mode in future posts), the mixture is fairly rich and these hot spots have energy sufficient to create a ignition of the air-fuel mixture. This phenomenon is often known as "Detonation".This ignition causes a pressure wave to travel from the spot towards the periphery. This Flame-Front if collides with the flame front travelling from the spark plug will result in wo very high pressure waves to bang into each other. This leads to very high pressure peaks which put extreme amounts of stress on the cylinder walls. Continuous Knocking might lead to permanent stresses getting formed in the cylinder walls & the piston head which eventually will lead to their failure.
How to Avoid it ?
A few years back when "leaded Petrol" used to rule the market the folks added something known as Anti-Knock in the petrol to ensure that it did not knock. This however, lead to higher amount of particulate matter in the exhaust and also was not good for the cylinder walls.The anti knock was a Lead compound which i cannot remember at this point of time.
These days we use more of "Unleaded Petrol" and there has to be different mechanism to control the "Knocking". This is done these days by ensuring that the engine temperature doesn't reach very high ( then there is lesser chance of hot spots getting created). One of the methods employed to do this is EGR. Exhaust Gas Recirculation, ensures that he temperature of the engine come down apart from having benefits like enhanced fuel efficiency and reduced NOx in the exhaust. However, that is a different topic and will be dealt a little later. The other method used in conjunction with the EGR is spark retard. Spark retard will ensure that you ignite the air-fuel mixture late enough that there will be no pressure peaks. When we do not have pressure peaks the chances of knocking are lesser.
Finally, use of high octane fuel will result in better combustion of the fuel and lesser hot spot contribution because of poor fuel quality. The reduction of hot-spots will result in reduced Knocking even at higher engine temperatures.
Some links that give more info on how it is detected etc
[1]
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