Ok, so two flame fronts coliding in the combustion chamber make a sound that the knock sensor picks up.

Now my question is wouldnt lower rpm knock be more dangerous then higher rpm knock, when taking piston speed in to account,

Compression in the cylinder is at its highest point when the piston is at its highest point obviously, and after that the piston starts its down stroke,

what if multiple flame fronts dont start until right before TDC right at TDC or a lil after, if that is the case, isnt that technically harmless knock? ( because the flame front from the spark is already started its path to the piston and it just happens to meet up with other flame fronts on the way down that formed a little bit later but still early enough to colide with the intended spark or possibly each other.

wouldnt harmful knock be only if the unintended flame front started WAY before spark.? because technically then we shouldnt get harmful knock ( Taking the resonable and obvious in to account, half way decent timing and AFR with no ridiculous boost) Unintended flame fronts shouldnt start way early because the compression isnt that high in the cylinder ( again taking reasonable boost in to account) as it starts its up stroke it compresses it more and more as it travels, (so in theory the most dangerous time in the combustion process would be at TDC, because its compressed to its highest potential.

Finally what im trying to get at is, is it possible that all this knock we are picking up even counts as high as 6, 7 or 8 possibly harmless knock? flame fronts that start a little after intended spark, and the flame front from the spark just happens to run across them on the way down?

 

Harmful knock versus harmless knock, what an interesting concept. Knock at or near peak torque when cylinder pressures are already at their highest is always bad. Knock, detonation, preignition all can cause severe damage, why? Because the fuel is exploding (pressure spikes as high as 2000psi have been observed) rather than burning. Not all knock is going to be flame front interaction. Some knock might be less damaging (potentially) than others. For instance 2 stockish motors are both run at 25psi all the way to redline. One knocks at 3500 rpm and the other at 7500. Both times its 6 counts, so maybe its flame fronts maybe not, which would you rather have?

I still think its better to think of all knock as over aggressive for the fuel and only tune for the occasional 1-2 counts with zero at max BMEP. If nothing else its at least safer in the long run.

 

My understanding is that the "two flame fronts colliding" theory is now considered to be false, although it is still widely cited.

Some low compression normally aspirated engines may be quite tolerant of detonation, especially if the cylinder pressures are not too high, but our engines making around 200 BHP/litre (liter in American?) with basic modifications are not ones to test this on as more damage occurs more quickly.

Having said that, I think leaving our engines with minor knock sums is safe because the factory do it on some models, and also the ECU can retard for the next cylinder event thus keeping the detonation event very mild. By the time a human had reacted and lifted off you'd have had much more exposure to damaging effects. Some people say the factory knock control is cautious, but I'd rather it was that way for an engine I expect to last some time, especially since detonation tends to avalanche promptly.

 

two flame fronts colliding is not knock. if that where true cylinders with dual plugs would be a problem.

normal combustion is 2000-2600psi at peak. it is a slow controlled burn. peak pressurre occurs at 12-16 after TDC for best power. piston in good position to do work by spinning crank.

detonation is instataneous explosion caused by too much heat and compression. cylinder pressures skyrocket to 4000psi+. it happens at or near TDC where piston cant rotate crank. head lifting and blown gaskets occur as a result. pistons take a beating from the shock. rod bearings take a beating as well often leading to spun rod bearing.

Another problem with detonation is the extreme heat. normal combustion is around 2000degress. pistons and cylinder heads are protected from this heat because of natural vapor barrier. during detonation the barrier is broken down and heat enters the head and piston at greatly accelerated rates often resulting in melted pistons.

 

^ agree entirely with your excellent post

 

if flame fronts coliding is false, what makes the sound that the knock sensor picks up.?

 

detonation is an explosion. the shock wave makes the piston "ping"

knock sensors are microphones that are tuned to hear the frequency of the ping.

 

the fuel in the cylinders does not explode. The burning process is much slower.

As it was explained to me (by a engine calibrator who has been working in the industry for over a decade for GM) I will probably miss a couple of key points, but the cliff notes are:

the pinging you refer too is the collision of the shock waves of the leading edge of the burn hitting the trailing edge. When you start the combustion cycle, the spark plug creates a flame called "the acorn".

This acorn of flame is supposed to burn evenly through out the chamber to produce power. Now here are some interesting facts about the burning fuel.

* the leading edge of the burning fuel is typically hotter and traveling faster than the fuel closer to the plug.
* the shape of the cumbustion chamber, piston head and cylinder walls (I think) are all designed to shape the burn to prevent this collision of shock waves.

*if the leading edge of the burn travels too fast (I.E running too lean), it will collide with the edge of the piston and cylinder walls and cause knock. When you hear about people cracking the edges of the pistons, this is the primary cause.

I forget what happens when it runs too rich. Basically, this cools down the burn, and also causes the fuel to form dropplets of un-atomized fuel which also adversely affects your tune, but I don't recall if it can produce knock.
[edit] I think when it runs too rich, knock can occur when the leading edge reaches the cylinder wall which produces a shock wave. Th shock wave heads inward and collides with the trailing edge of the burn and can cause knock as well.


I don't think piston speed has much to do with knock, presure has a much bigger effect than the speed of the piston.

 

Spark is the one flame front we want

Ping is the flame front caused when one single problematic aromatic hydrocarbon desides to drop it's hydrogen bonds under pressure without spark and starts a chain reaction that my cost you a crappy engine

Pure bred fuel has no problem children, mixed or contaminated fuel has many

 

wait so detonation is not multiple flame fronts?

So it still is 2 edges hitting? and by shockwaves you mean flamefronts?
or is the shockwave created after the burn passes through a section?

 

there are so many things wrong with your explanation I am not even gonna correct you. suffice to say you should just stay out of these discussions.

 

wait 94AWDcoupe

let me get this straight, intended spark is a slow burn process, and detonation, is a spontaneous quick explosion that causes the piston to ping? when it hits it?

 

whatever... I clearly stated this was 3rd party info...

also, asking politely, what was said that was "wrong"?

you can either sound like an ****, or provide me and the rest of us a better explaination please? If you can't provide an explaination then please stay out of these discussions.

 

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.

 

Oh go on do! But can you wait untill I get back from the pub to make a post like that? It always reads better after a couple of pints of wife beater

MB