Detonation - What and why?

[SwampMatt]

Can someone offer up a good definition of exactly what "Detonation" is? I have some ideas - but based on how I see the word used in reference to pistons burning up. I've never been able to get a solid definition of what it is - relating to internal combustion engines.

Looking for some description of it in everyday language, How it happens, the basic physics of it, and what to do to stop or avoid it. And why it is bad for a motor.

How can I tell if I've got it, or if I've HAD it in the past. Spark plugs tell a story?

Thanks.

 

[Anonymous]

Hey Mood....I started to type one of my lenghty deals, and realized I was probably better off givin ya some 411 I agree with. If the terms in it aren't what the doctor ordered, or answerin the particular question you're lookin for, I'll be more than happy to throw my spin on it.

http://www.contactmagazine.com/Issue54/EngineBasics.html

Felber

 

[spooledup50]

ITS WHEN A GAS MOTOR DIESELS WHEN RUNING UNDER LOAD ITS A TIMMED EVENT AND WHEN HAPPENS OUT OF TIME ITS LIKE A BOMB GOING OFF ON TOP OF THE PISTON AND IT STOPS THE PISTON FROM GOING UP AND AROUND AND FORCES IT BACK DOWN REVERSING THE ENGINES ROTATION AND BANG THATS WHEN THINGS BREAK THE REMEDY IS ONE OF TWO THINGS HIGHER OCTANE GAS OR LESS TIMMING THEY GO HAND IN HAND MORE OCTANE HIGHER TIMMING MORE POWER AND VICE VERSA

 

[SwampMatt]

So it's deiseling (run-on) that is detonation? But only when it turns the motor backwards?

Felber - that is a great writeup - but it was just a tad over my head. I did bookmark it and I'll try it comprehend it again sometime. Thanks for the link!

I was under the impression that detonation could be something that happened under normal operating conditions. I guess I had the wrong idea of what it was - Glad I asked!

Thanks all.

 

[Anonymous]

Mood,

Yur not gonna let me get off easy are ya? LOL I'll try to break it down in simple terms, so No google searchin from the friggin peanut galley nitpickin me aight?

OK....Deiseling perse is more along the lines of preignition. It happens when pressure rise/heat within the cylinder cause the mixture to ignite prematurely. It cause a loss of power for obvious reasons, and damage as well.

But detonation when speakin of gas engines is something that happens much closer to TDC and usually happens when an angine is "on the edge". On wedge chamber heads especially it starts over on the quench side of the head and two flame fronts collide. Unlike pre ignition it builds a lot more heat and the explosion is at a point where the mixture is that much more volatile.

It's caused by a lot of things, just like pre ignition. Chamber temp, static compression, load, fuel octane, etc. and ignition lead too high for the combination of the formentioned.

In airboats, it's especially a concern due to HEAT. When I say heat, I mean in the chamber, not water temps. Props load the engine so hard and the people who own them tend to load them even more. In other words...An engine that may live fine with 11:1 compression, 180deg WT on pump gas and 40 degrees of timing in a car would DIE in an airboat because the temps in the chamber. pistons, rings etc would put it on the edge due to sustained cylinder pressure/load.

It is easily read in early stages as little black balls on your spark plug porcelain. In later stages, you can find pieces of piston and rings when you drain the oil. How was that?

Felber

 

[SwampMatt]

I understand it much better now! Spontaneous Combustion could be another word for Detonation it seems. I can see how it could happen with extra high compression. In my camping gear, I have a fire starter gizmo that uses fast and high compression to create an ember. Same principle I guess. Very similar to dieseling.

So it can occur when running at normal speeds ( ? ) (Not just when the motor is shut down)

I also get it about how the two flame fronts collide. The other flame is from the spark plug, right?

Now the next question to follow up.....
I was told in my 500 cad motor to run 93 octane gas. I asked what would happen if I ran a lower octane gas.
The answer: Because it is a big piston motor, I'd get detonation. And normally, you can hear the motor ticking/clicking in a car running cheap gas, but in an airboat, the prop sound would hide that subtle engine sound.

I just don't understand how a lower octane fuel - less flammable? can become more explosive when compressed.

Still something I'm not understanding. Why would a lower octane fuel cause detonation? You'd think it'd be the other way around.

 

[Waterthunder]

The lower the octane of a fuel the faster it burns, in simple terms cheap gas explodes or burns quicker and good gas burns slower and more consistent! I know this is not scientific enough for some but it's a good way to get the idea quickly! That is why higher compression motors need slower burning gas because the cylinder pressures are higher which will cause a more volatile burn so they require a slower burning fuel. Detonation will occur when fuel explodes instead of burning. So the cheaper the gas or the lower the octane the more prone it is to explode or I should say technically burn to fast!