Just wonder can our knock sensor pick up pre ignition? or do pre-ignition always caused knock?

as have uses lower octane fuel for the past few days with regular hard driving but my PFC ecu commander just shown moderate knock figures.

 

any pre-igniion is knock. might not always come us as true knock when logging but it could cause probs down the road id stick to 91 fuel at least, i dont use anything less than 93 just for the safety factor. ya might be able to run like u are and be safe for a while but it only takes one random case of knock to really screw yourself lol

 

I can imagine a hot spot, maybe a bit of carbon, lighting off the mixture and it burning normally. The would be preignition but not be detonation.

 

So its mean when pre ignites occur, it wont always pick up from the knock sensor?
And if it does not pick up high knock figure from my knock sensor will it still be safe?

Thanks Guys

 

If I'm not mistaken...

By definition, "knock" is when PCP (peak cylinder pressure) is reached while the piston is still travelling upwards on it's compression stroke. So whether the pre-ignition is caused by too much timing advance, a hot spot, or otherwise, the way I see it, the sensor should still detect it.

 

Thanks Raptor.
as ive monitor closely on my knock level and lastly will advancing timing bring up a leaner afr?

 

timing will generally not change AFR.

If you have knock, add fuel or reduce timing.

knock = pre-ignition.
knock is uncontrolled ignition of fuel/air that you do not want. Pre-ignition is the same thing.

 

Preignition, by definition, takes place on the compression stroke before the spark plug is timed to fire, does it not?

 

Thanks Guys, so in conclusion i am still on the safe side if my commander register only moderate knock even after using lower octane fuel?

Just worry that my knock sensor will not detect the pre-ignition

 

James, if you reduce your timing by 2degree's, does the knock level go down?
reduce over the entire ignition map.

 

Hi RSMike,
im still in the mid of exploring my self tune and im mostly advancing timing at the moment and so far my knock register moderate figures (below 30 on my pcf ecu commander after few hard driving). Mainly adjusting timing on the lower load & rpm areas.

The reason i do my own exploration is bcos im still hope to get my car responsive whenever i full throttle but unfortunately the afr still deeps and ive to partial lift off and throttle again in order for the car to catapult.

Any tips to improve my Full Throttle response?

Thanks

 

When I was a kid I lived near a naval ammunition depot. Workers there were busy disposing of WWII ammunition. There were 16 inch shells for battleships, 40mm antiaircraft shells, rockets, depth charges - you name it. As these things were disassembled they would dump the powder on the ground and burn it. There would be a huge cloud of black smoke but no explosion.

Gasoline in an engine cylinder is supposed to burn like this powder did. Knock occurs when gasoline explodes. Preignition could be either one, it only means it occurred before the spark but only an explosion will be heard by the knock sensor.

 

Hahahahaha...+1

As long as there is no high knock detected via the knock sensor, i am pretty much safe ya...

 

Wrong!

Detonation and preignition are somewhat different animals. Detonation is the spontaneous combustion of end gases in the combustion chamber AFTER the spark plug event. Preignition is the combustion of the air-fuel mixture way BEFORE the spark plug fires. Preignition can occur anytime after BDC as the piston is ascending on the combustion stroke.

Knock sensors are tuned to 6400 Herz which is the frequency range in which detonation(spark-knock) occurs. So, knock sensors are tuned to detect the ping, or "knock" that occurs during detonation.

The following passage was taken from the article, "Detonation and Preignition", written by, Allen W. Cline. Herein the author makes several noteworthy distinctions between the two distinct, abnormal combustion events:


"....Remember, the spark plug ignites the mixture and a sharp pressure spike occurs AFTER that, when detonation occurs. That is what you hear. (By contrast, during)... preignition, the ignition of the charge happens far ahead of the spark plug firing....(With preignition) there is no sharp pressure spike as there is with detonation. Instead it is a tremendous amount of pressure which is present for a very long dwell time, i.e. the entire compression stroke (starting at BDC).. That is what puts such large loads on engine parts.

Therefore, with preignition there is no sharp pressure spike to resonate the head and the block to cause any (audible) sound. So, you never hear it. The engine just blows up. That is why preignition is so insidious. It is hardly detectable before it occurs......"


So, for the benefit of the OP's understanding....knock sensors per se, are designed and tuned to pick up the sharp pressure spikes known as knock or ping that are caused by detonation. Preignition on the other hand, is for the most part manifested as an inaudible event that may in fact, pass undetected by the knock sensor. Preignition is a mostly silent killer.

Note: I have taken some liberty in paraphrasing Cline's article because it was originally structured as one single, undivided, lengthy paragraph. I separated the author's original paragraph into two, shorter ones. Furthermore, I took the liberty of straightening out a few other compositional kinks. The words within parentheses and the italics employed are of my doing as well. Otherwise, this short extract quoted from Cline's complete article appears pretty much as the author penned it. Maybe some day before I pass on I can figure out how to post a link on here and so avoid having to go through the task of manually copying writings verbatim. But that is another story. LOL

 

Great read, thanks for contributing Sparky