Is it a lean A/F Ratio?
Is it a Rish A/F Ratio?
Is it timing? Too Advance or retarded?
Is it when you ahve a bad plug and the combustion chamber ignites incorrectly (too early or late)
I know it has something to do with the spark and fuel mixture, just not sure what.

I know it is bad and what it sounds like, but I don't know what causes it.

 

Thats the million dollar question. Or maybe just the 5000.00 question (the amount for a new motor.)


-It is my opinon that the biggest culprit would be poor gas (not enough octane) for a given tune.

But there are a lot of other factors that I'm not sure I understand. But yes the mixture is detonating on its own instead of when the spark plug wants it to. And the pressure on the piston going up on its way to TDC is tremendous

 

Premature ignition:
A hot spot in the combustion chamber causes mixture to fire at an inappropriate time. High temp, pressure, poor gas can all can cause flash over, at the wrong time, in the combustion chamber.

Something like that anyway.

Fred

 

Geez... now thats an open question... there's entire books on this. Why don't we try something of smaller scope like, "Whats the meaning of life?" j/k

The short answer is: heat. Heat in the wrong place at the wrong time.

A/F, intake charge temp, sharp points in the combustion chamber, exhaust restriction, excessive timing can all cause heat at the wrong place.

Lets take timing since its probably the hardest to understand. The piston is coming up during the compression stroke. As it approaches the top the spark plug fires. This ignites the mixture and flame front/combustion kernal expands. As the kernal expands its compressing the mixture around it and heating it and it ignites. As the pressure grows it pushes the piston down, which relieves the pressure. In good combustion processes the pressure generated from the combustion is equalized by the piston moving down and a nice equalibrium is hit the entire time until the exhaust valve opens.
When you increase the timing too much the cumbustion kernal expands and raises the pressure in the cumbustion chamber too much before the compression stroke is done. The piston isn't going to move down, its only choice is to keep squeezing. This causes the mixture to ignite, that raises the pressure more, which causes it to ignite in other places, etc. The cylinder pressure spikes and spikes at the worst possible time, when the most force is already on the piston and rods, at the top (or bottom) of the stroke.

 

a Quick and simple soultion to combat detonation is to lower the timing or lower the operating temp of the motor.

 

Detonation= spontaneous uncontrolled spark trying to reverse the direction of the piston while its still climbing up the bore.

 

Detonation can happen on the way down too.

 

Your getting sound answers so far.

In a gen 2 S/C motor the major area is the relationship between A/F ratio and timing.


Rich

 

http://www.hastingsmfg.com/Service%2...reignition.htm

Link to a brief primer on detonation and preignition.

 

so lets say the electrode from your spark plug comes off and welds itself to the top of your piston, being a different material than the piston, and perhaps retaining heat longer, that little chunk of metal could cause a hot spot and pre-ignition (detonation)>?


I've seen a ton of plugs that are missing electrodes that people assume go out the exhaust valve, but what if it is welded to the top of the piston?

 

WOW...that is an awesome question and you will likely get a million different answers.

I work in Fuels in the Air Force and know a bit about how fuel reacts, make up of it, and flash points (temperature at which fuel will ingnite).

Everyone here has a piece to the puzzle...because according to who you ask, it is a puzzle, wrapped in a mystery, surrounded by an egnima!

The bottom line is this....it is caused by heat and the ability of your plug to despense/get rid of the heat. (Increased pressure helps in raising the heat levels (i.e. adding boost)).

What temperature will the fuel ignite? Well, that depends on the rating of the fuel. Fuel is basically made of two compounds...Iso-octane and N-Hetane. Higher octane aides in the resistant to knock and ping....and detonation. But...nothing beats a good tune, good plugs and some a great tuner.

 

I've been avoiding these questions for a while, but if it's going to be discussed, then I'd like to know as well in this topic . . .

Why does a lean A/F cause detonation (I assume that it's detonation that causes the motor to go boom on a lean condition)?

I understand how detonation ruins the engine, and that it occurs because of preignition, which is caused when the temperature in the chamber exceeds the flash point (I may be using flash point incorrectly here - def? - temp at which a given substance will combust on its own?) and the gas ignites at the wrong time. This happens thanks to the PV=nRT thermodynamic equation of state (again, I think this is the rule at work here, pressure times volume equals amount of substance times temperature and a constant).

If I'm understanding this correctly, a lean cylinder condition is creating imbalances in the equation, which is increasing temperature to a point beyond the flash point of the combustion mixture and causing preignition. Right?

To help understand this better though, can anyone explain why the presence of excess o2 causes the imbalance? Excess fuel is just expelled through the exhaust, so why wouldn't excess air be expelled in the same way?

Or am I totally wrong is associating detonation with lean condition failures?

 

"Lean" in terms of an engine tune is still rich in respect to stochiometric (sp?) mix. I believe around 14:1 is "perfect" in terms of chemistry. As you get close to this however the burn happens too fast relative to the piston's movement so that equalibrium of kernal expansion to combustion chamber expansion isn't kept for a nice controlled burn. Instead cylinder pressure increases too much, igniting fuel beyond the flame front, which causes a viscous cycle that puts a bend in your rod or a hole in your piston.

 

ILLINI-SVT: no you are right. lean air/fuel can cause detonation.

without all the mumbo jumbo...

it's simple, the fuel cools the combustion chamber, air does not.

so if you've got more air in there(lean), the combustion chamber temps can climb higher increasing the chance of detonation. more fuel(rich) helps keep temps down.

later,
chris


BTW, Sublime, how exactly does detonation happen when the piston is on the way down? and if indeed this does occur, how could it be harmful to the engine? another commonly used term for detonation is "pre-ignition". in order for detonation to take place, the air/fuel mix ignites BEFORE the desired spark firing point (or timing). be it 13 or 30 degrees BEFORE top dead center. if the intended spark timing is any degree before top dead center and when the cylinder is on it's way back down it's obviously AFTER top dead center, then how does detonation occur when the piston is on the downstroke?

just curious.

 

BAD GAS, or a lean A/F ration, will get you some denotation.