pre- ingnition

[ricks450]

what ignition sources cause pre-ignition?

[ghott]

Here's a helluva read:

https://home.pacbell.net/sfnelson/fuel__detonation__preignition__etc_.html

[GPracer2500]

Any abnormally hot spot in the combustion chamber has the potential to cause pre-ignition. It could be an overheated spark plug, valve, piston or a speck of super-hot carbon.

[GPracer2500]

Originally posted by ghott
Here's a helluva read:

https://home.pacbell.net/sfnelson/fuel__detonation__preignition__etc_.html

Yes, a helluva read. That link doesn't identify the source of that info. But FYI, it is part 6, 7, 8, and 9 of Bruce Hamilton's excellent Gasoline FAQ. It's getting a bit old being written 10 years ago, but is still chocked full of solid, extensively referenced information. I would love to see Mr. Hamilton update it with an addtion that focused on racing gasoline.

[ricks450]

damn, im tired now (thanks guys) you mean like the temp rating on spark plug being to hot or just getting hot do to fuel mixture?

[GPracer2500]

A plug of an incorrect temp range can be the culprit. Usually in order to get some spot hot enough to actually ignite the mixuture there's got to be something going pretty wrong inside the combustion chamber. Pre-ignition isn't especially common. And thankfully so because a pre-ignition condition can destroy an engine in seconds. The other and far more common abnormal combustion phenomenon--detonation--is gentle by comparison.

[250X_project]

Now what are the key differences between the two- pre-ignition and detonation?

[GPracer2500]

Originally posted by 250X_project
Now what are the key differences between the two- pre-ignition and detonation?

Well, they are two completely different things. What can make things confusing is that one can lead to another and both can occur at the same time. But one doesn't always lead to another and they don't always occur at the same time.

Deto always occurs after the air fuel mixture is ignited and has begun to burn. This ignition source is usually/normally the spark plug firing. The plug fires, the mixture begins to burn, but before the mixture at the edges of the combustion chamber has a chance to burn (if conditions are just right) it explodes violently instead of burning. That is detonation--when some of the a/f mixture explodes rather than burns. Burning (deflagration) and exploding (detonation) are not the same thing.

Pre-ignition is simply when the a/f mixture is ignited and begins to burn (deflagrate) well before the spark plug fires. Deflagration is usually a normal occurance. But what makes it abnormal in the case of pre-ignition is that it's happening at the wrong time.

Deto is the wrong kind of combustion (deflagration is the good kind, detonation is the bad kind). With pre-ignition it is the right kind of combustion but it happens at the wrong time.

Most engines can withstand a certain amount of mild detonation without sudden, catostrophic damage. Deto also usually provides an audible warning that it is happening. Pre-ignition can destroy an engine in a second or two. I'm not aware of an engine that can withstand a continuous pre-ignition condition for any period of time. Deto is somewhat common while pre-ignition is comparatively rare.

Hope that helps some....

[ricks450]

lets say your not geting pre-ingnition so your fine on that part and your spark plug is firing when its supose to. but your getting detno. what causes that? how would you cure that problem? it would come from end gases right or wrong? thanks man!!

[250X_project]

Detonation is timing and pre-ignition is low grade fuel?

[TWILES]

Basically if your piston is 12.1 or better you need race gas and 92+ with the stock piston and the stock plug unless otherwise noted.

[ricks450]

i understand that you need race gas to stop pre-igintion. but does race gas stop detonation. detonation happens after the spark plug fires. i have built alot of race motors i have always called pre-ignition (pre-detonition). which is caused by compressing air so much it gets hot (with high compression piston)causing the fuel to ignite before the spark plug has a chance. but pre-ignition is proper term. dont really understand how you get detonation after curing pre-ignition with higher octane fuel and how to cure it, if you got detonation.:confused:

[GPracer2500]

Originally posted by 250X_project
Detonation is timing and pre-ignition is low grade fuel?

Nope. Detonation is caused by some combination of too low a grade fuel, too much dynamic compression, too much ignition advance, and too much heat. Lean air/fuel mixtures can also be a contributing factor and there could be other factors I'm not thinking of at the moment.

Pre-ignition is caused by the creation of an ignition source in the combustion chamber other than the normal firing of the spark plug. High octane fuel only indirectly helps prevent pre-ignition. High octane fuel can help stop deto and deto can cause the hot spot that allows a pre-ign condition to start.

[ricks450]

glad to see you! so if you stop pre-ingnition with race gas it will stop detonation to? im cumfused as hell:confused:

[ricks450]

Originally posted by ricks450
glad to see you! so if you stop pre-ingnition with race gas it will stop detonation to? im cumfused as hell:confused: pre-ignition happens before spark plug fires and detnotion happens after right or wrong?detno is the end gases exploding right or wrong?

[GPracer2500]

Originally posted by ricks450
i understand that you need race gas to stop pre-igintion....
Race gas only helps prevent pre-ign by stopping detonation. Detonation can lead to a hot spot and the hot spot causes pre-ignition. It's in this way that deto and pre-ign become intertwined. If a hot spot somehow develops in the combustion chamber, the mixture could pre-ignite regardless of octane rating.


...but does race gas stop detonation...
It can, if the primary factor causing the deto is insufficient octane. This is usually the case. But in some cases--like having way to much ignition advance, a lean mixture, really hot temps, and really high compression--the engine's octane requirement may be higher than any fuel available. If an engine deto's no matter how high the octane rating of the fuel, the engine is badly mis-tuned.


detonation happens after the spark plug fires. i have built alot of race motors i have always called pre-ignition (pre-detonition). which is caused by compressing air so much it gets hot (with high compression piston)causing the fuel to ignite before the spark plug has a chance. but pre-ignition is proper term. dont really understand how you get detonation after curing pre-ignition with higher octane fuel and how to cure it, if you got detonation.:confused:

Yes, you're confused. ;) Pre-ignition has little to do with compression. It is simply when there is some glowing hot spot that ignites the a/f mixture before the spark plug fires. Compression has nothing to do with that. Again, deto is the wrong kind of combustion that happens at the right time. Pre-ignition is the right kind of combustion that happens at the wrong time.

Detonation happens when the fuel is subjected to such high heat and pressure that it spontaniously ignites without any ignition source (auto-ignites). The piston rising and compressing the unburned a/f mixture (this is prior to the plug firing) is not nearly enough pressure to cause detonation. Diesel fuel will auto-ignite like this, but gasoline won't. The only situation any of the air fuel mixture sees enough heat and pressure to detonate is after some of the air fuel mixture has already started to burn normally. As the normally burning mixture expands and adds heat and pressure to the combustion chamber, the a/f mixture at the edges of the combustion chamber (that the normal flame front hasn't reached yet) might reach a state where it will detonate. This is why detonation can only happen after some of the air/fuel mixture has already started to burn.

[GPracer2500]

Originally posted by ricks450
pre-ignition happens before spark plug fires and detnotion happens after right or wrong?detno is the end gases exploding right or wrong?

Correct!

Pre-ignition is when the a/f is ignited before the spark plug fires. Detonation happens after the spark plug fires. And yes, it is the end gases (i.e. the a/f mixture at the very edges of the combustion chamber) that can detonate. The whole mixture doesn't detonate--some/most of it deflagrates normally. The end gases, being the last part of the mixture to burn, is the only part of the mixture that could ever see the heat and pressure needed to cause detonation.

The only exception to the rule that deto can only happen after the spark plug fires is if the mixture pre-ignites first. When that happens it's possible for the end gasses to deto prior to the plug firing. But in either case, the mixture must begin to burn first from some ignition source before any of the mixture will see the heat/pressure needed to detonate the fuel.

[ricks450]

thanks CPRACER! i have built alot of motors and made them run wright but i never understould how the fuel burned in the cumbustion camber. it is starting to make alot of sence now. never understould how pre-ignition and detonation occured evidently. you da man!!!! :blah: