Theoretical aspect of knock in respect to piston speed.
Whether you like the way 94AWDcoupe put it, I'll say again that everything he has written in this thread I agree with, and that some of the other information is recycled misconceptions or outdated ideas.
I was considering replying in more detail yesterday, but when you believe something from a source you trust, I'm not going to change your mind unless I start referencing Heywood's Fundamentals of Internal Combusion or Bell's Maximum Boost, and I haven't got the time to get into a theoretical argument.
I was considering replying in more detail yesterday, but when you believe something from a source you trust, I'm not going to change your mind unless I start referencing Heywood's Fundamentals of Internal Combusion or Bell's Maximum Boost, and I haven't got the time to get into a theoretical argument.
that is all.
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Honestly I think 94AWDcoupe has given one, but here is a quote from Maximum Boost by Corky Bell:
"Detonation is the spontaneous combustion of the air/fuel mixture ahead of the flame front - combustion by explosion rather than controlled burning. It occurs after the combustion process has started and is usually located in the area last to burn. As the flame front advances across the chamber, the pressure - and thus the temperature - in the remaining unburned mixture rises. If the autoignition temperature is exceeded, this remaining mixture explodes. The audible ping is the explosion's shock wave.
Detonation is extremely destructive. This is a result of the temperatures that can reach 18000F in the center of the explosion. The pressure spikes caused by the explosion can reach several thousand psi, and the pressure rise is rapid enough to be considered an impact load. These temperatures and pressures are almost ten times higher than those accompanying controlled combustion.
No metals in existence today, no forged pistons, and no special head gaskets can withstand sustained detonation. Virtually nothing can withstand sustained detonation. Consider that at 6000 RPM, fifty explosions can occur in each combustion chamber in one section. Thus: if you ever hear a ping, you lift your foot."
"Detonation is the spontaneous combustion of the air/fuel mixture ahead of the flame front - combustion by explosion rather than controlled burning. It occurs after the combustion process has started and is usually located in the area last to burn. As the flame front advances across the chamber, the pressure - and thus the temperature - in the remaining unburned mixture rises. If the autoignition temperature is exceeded, this remaining mixture explodes. The audible ping is the explosion's shock wave.
Detonation is extremely destructive. This is a result of the temperatures that can reach 18000F in the center of the explosion. The pressure spikes caused by the explosion can reach several thousand psi, and the pressure rise is rapid enough to be considered an impact load. These temperatures and pressures are almost ten times higher than those accompanying controlled combustion.
No metals in existence today, no forged pistons, and no special head gaskets can withstand sustained detonation. Virtually nothing can withstand sustained detonation. Consider that at 6000 RPM, fifty explosions can occur in each combustion chamber in one section. Thus: if you ever hear a ping, you lift your foot."
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Tough crowd, but I'll stick my neck out and add some stuff:
The definition above is fundementally correct, detonation is an abnormal burn, though current research seems to suggest that it is more of a high speed shock wave than an explosion.
In a modern engine, some knock during accelleration from low torque/low RPM is pretty normal.
Detonation at high power is always a bad thing.
An earlier post mentioned heat and pressure. There is definately a connection, but the heat and pressure alone do not cause knock, the fuel is also involved. Put simply, the heat and pressure break down the hydrocarbons in the fuel into compounds which are sometimes referred to as 'auto igniters'. These compound then can be involved in a very fast chain reaction that ignites the whole mixture dramatically faster then desired.
This gives us some insight into why we hear so much knock in new cars accellerating from idle. Idle is held closed loop at stoichiometric, so we have quite a lot of heat. However, the strokes of the engine are somewhat proportionally fixed, so we are actually leaving the mixture under heat and pressure longer, causing more breakdown.
Last, if you read on detonation, check the age of the paper/book. A lot of experimentation has been done in the last few years.
-jjf
The definition above is fundementally correct, detonation is an abnormal burn, though current research seems to suggest that it is more of a high speed shock wave than an explosion.
In a modern engine, some knock during accelleration from low torque/low RPM is pretty normal.
Detonation at high power is always a bad thing.
An earlier post mentioned heat and pressure. There is definately a connection, but the heat and pressure alone do not cause knock, the fuel is also involved. Put simply, the heat and pressure break down the hydrocarbons in the fuel into compounds which are sometimes referred to as 'auto igniters'. These compound then can be involved in a very fast chain reaction that ignites the whole mixture dramatically faster then desired.
This gives us some insight into why we hear so much knock in new cars accellerating from idle. Idle is held closed loop at stoichiometric, so we have quite a lot of heat. However, the strokes of the engine are somewhat proportionally fixed, so we are actually leaving the mixture under heat and pressure longer, causing more breakdown.
Last, if you read on detonation, check the age of the paper/book. A lot of experimentation has been done in the last few years.
-jjf
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