Really good ping vs knock explination

[nipper]

http://www.streetrodstuff.com/Articles/Engine/Detonation/

 

thought i would share this, this is a very good explination.

 

 

nipper

[wiwrx]

Good read!

[NorthWet]

It is a good explanation, but it seems to me that it is biased towards largish domestic V8s. Not bad in and of itself, but it leads to errors and omissions regarding other engine designs.

[nipper]

It is a good explanation, but it seems to me that it is biased towards largish domestic V8s. Not bad in and of itself, but it leads to errors and omissions regarding other engine designs.

 

The theory is the same weather its one cylinder or multiple cylinders. I can steer you towards REALLY heavy engineering books if you want.

The combustion porcess doesnt care if the engine is 10cc or has a displacement measured in cubic feet.

 

Sorry (waving automotive engineering degree) the article is dead on. I posted it as I got tired of having to re hash it every time it came up.

 

nipper

[jamal]

great article, thanks.

 

And considering it's for streetrodstuff.com I would expect a bit of V8 bias. What kind of omissions and errors are you talking about?

[OB99W]

Thanks nipper, interesting read.

 

[...]And considering it's for streetrodstuff.com I would expect a bit of V8 bias. What kind of omissions and errors are you talking about?

Here's one generalization I picked up on:

"Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock."

 

Knock frequencies are dependent on engine design, in particular things such as cylinder bore (diameter). The frequency given in the article (6400 Hertz) is only an average one, and may be at what the NorthStar engines the writer is apparently familiar with resonate. Also, many knock sensors are of the non-resonant (untuned) type, and respond to a wide range of frequencies (2kHz-20kHz or even beyond is common). Often the output of those untuned sensors is filtered, or "listening" time is limited, so that the ECU can obtain more useful data. Even so, untuned sensors can pick up sounds other than knock, leading to unintentional retarded timing.

[BigMattyD]

Thanks nipper, interesting read.

 

Here's one generalization I picked up on:

"Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock."

 

Knock frequencies are dependent on engine design, in particular things such as cylinder bore (diameter). The frequency given in the article (6400 Hertz) is only an average one, and may be at what the NorthStar engines the writer is apparently familiar with resonate. Also, many knock sensors are of the non-resonant (untuned) type, and respond to a wide range of frequencies (2kHz-20kHz or even beyond is common). Often the output of those untuned sensors is filtered, or "listening" time is limited, so that the ECU can obtain more useful data. Even so, untuned sensors can pick up sounds other than knock, leading to unintentional retarded timing.

 

Sometimes, if I don't have my coffee in the morning, I have "unintentional retarded timing." Just ask my wife...

 

matt

[OB99W]

Sometimes, if I don't have my coffee in the morning, I have "unintentional retarded timing." Just ask my wife...

I just thought of a whole bunch of puns and wisecracks, none of which I'm going to post.

[NorthWet]

The issues that I have are with what appear to be simplifications that can lead to erroneous conclusions. Things such as detonation only occurring if the sparkplug ignites the mixture. Detonation is the abnormal combustion of the endgas, and it does not care how the combustion process initiated. (You can get detonation in a compression ignition engine running supplimental fuel... no spark event involved.)

 

The assertion that the frequency of detonation noise is such and such because of the resonance of the engine's metal sounds falatious: Resonance of a material is directly related to its density, stiffness, and mass. I CAN however believe that the frequency is set based on the pressure propogation speed through a gas of fairly consistant density striking a material of density within a certain range.

 

Discussion of causes of detonation leave out fuel/air mixture and turbulence (though the latter could be covered by CC geometry, turbulence is more complex than that).

 

Discussion of pre-ignition states that BDC is the most likely point that pre-ignition will occur. "Since the spark voltage requirements to light the charge increase in proportion with the amount of charge compression; almost anything can ignite the proper fuel/air mixture at BDC!!" Falatious conclusion based on impoper premise. He states the pre-ignition is caused by a non-spark event, then equates the ignition source to the physics that apply to an electrical-discharge spark event. The fact may be that pre-ignition is more likely at BDC, but this conclusion can NOT be drawn by when an electrical spark can bridge the spark gap. (Given the rest of his reasoning, the ACTUAL most likely moment is closer to when the exhaust valve closes and there is a less-than-atmospeheric pressure in the combustion environment.)

 

The theory is the same weather its one cylinder or multiple cylinders...

It is not the one or many cylinders to which I referred. It is issues of bore stroke ratios and dimensions, along with conrod angularities.

 

...The combustion porcess doesnt care if the engine is 10cc or has a displacement measured in cubic feet...

The process doesn't vary from its physics, but it will vary from its environment.

 

Geometry matters. Mercedes Benz made its 5-cylinder engine (the 300D) because they needed a larger engine than the 240, but they had "perfected" the geometry of the 240's combustion environment and didn't want to stray from it. They did 5 cylinders because they felt that a 3.6 liter 6-cyl was too big. Due deference to the Northstar team, but I would listen to engineers from companies that have been designing "high-performance" (and ultra-HP by GM standards) engines for much longer than I have been alive.

 

I also feel (just in my gut, though ), that LPP is affected by rod angularity. It used to be considered so, though perhaps recent information makes that obsolete.

 

I know that theories are just that, and beliefs are at at even lower level. Much VERY GOOD information is presented in this article, but for me much of it is tainted by the author's may of expressing himself.

 

 

We should agree to disagree. I will agree that the article is definitely worth the read, and will now leave it at that. Thank you for bringing it to us.