I have been understanding the deeper physics of a motor and trying to comprehend everything but am still clueless on some things?
Can someon tell me what physically happens during detonation?
How does it chew up the spark plug and worse cases leave marks in the piston?
I understand that detonation can occur when the timing is set to high with boost or nitrous. But in the harder physics? Why does detonation occur more with more timing.If you are at lets say 40 degrees of timing under WOT, isn't that 40 degrees after top dead center?
Why would more timing hurt the engine since the piston is on its way down in the cylinder. I could understand if the piston was coming up in the cylinder and you tried to fire, it would create massive problems and the explosion would be trying to push the piston down and it was actually moving up.
But why would you hurt it firing the cylinder later?
Am I looking at this right? Or is advancing the timing making it fire earlier?
Please shed some light on this?
Thanks

 

With respect to timing, the 40 degrees is before TDC.

The ignition has begun the combustion process as the piston is ascending to
the top.

Since the mixture needs time to ignite and expand, it must be triggered early
as the piston is moving at high speeds.

If the spark lead was only 5 degrees BTDC, the force of combustion would happen
as the piston was too far down the bore during the power stroke.

This will result in less piston face pressure which equates to less torque at
the crank.

The trick is to start combustion so that the punch of the "explosion" happens
as the piston is rounding out TDC and about to swing down.

If you trigger the combustion too soon, the punch of the expanding charge occurs before the piston has reached TDC. Now you have a problem! The
mixture is expanding too quickly and forcing the piston down as the piston
wants to come up.

If you retard timing, you can reduce cylinder pressure/piston pressure as the
piston face has created more combustion volume as it descends down. This
will result in a lower effective compression.

 

Air/fuel mixtures do not explode (in the true sense of the word this is detonation) but they burn. Normally a flame front is initiated by the spark plug, the flame then propogates through the mixture. This takes time and is controlled. Detonation is when the mixture becomes unstable and the entire mixture is oxidized almost instantly, unleashing all the energy of the charge immediately into the cylinder (an explosion!).

Timing is BEFORE top dead center. The ignition is started before the piston reaches the top of the stroke. Proper ignition timing will lead to maximum cylinder pressure just after TDC. If the mixture is ignited too early, pressures will increase exponentially before the piston reaches TDC. When the mixture becomes too hot or under too much pressure it detonates.

Instead of a push on the piston it gets, quite literally, hammered.

 

What Buttercup said.

It's the hammering than overstresses and breaks parts.

 

An alternative explanation, and a reference for anyone who wants more info....

Detonation actually involves the formation of two flame fronts. The first is the spark ignited combustion front, accurately described above, which propegates from the spark plug. If pressure rises to quickly and too high - following the intial spark ignition - it can promote the formation of "end gasses", a chemical transformation of some of the fuel components into other compounds that will "auto-ignite". Rapid pressure rise can be attributed to a spark that is too early, excessive heat input in the form of pre-heated combustion air, excessive coolant temperatures that impede the removal of heat, etc.

When the end gasses form, and auto-ignite, it generally occurs at a point remote from the initial spark induce flame propegation. Now we have two combustion events, moving rapidly toward each other. When the two flame fronts collide, there is a huge pressure spike, followed by an oscillating decay of the pressure spike, which causes further damage with vibrations.

Good reading:

http://www.faqs.org/faqs/autos/gasoline-faq/part1/

 

yall know more about this stuff than me. but i dont know that quench play a big part in predetnation. a tighter quench is a better fuel mixture, leaving less pockets of leaner air that will ignite under pressure sooner than you want. and im assuming that this is the second flame that injuneer is speaking.

and injuneer dont post ip numbers act on it.

 

No, the second flame front Injuneer was talking about is from a source of ignition other than the spark plug. A hot spot in the cylinder can ignite the mixture.

Quench helps a lot. Good quench increases mixture quality and the rate at which the mixture is burned. The more rapidly it is combusted the less chance there is for irregular combustion.

This whole process is about time. It may seem like an instant to us but even a few milliseconds can be an eternity relative to the combustion events.

 

Carbon Build-up can cause the second flame front as well. Carbon holds heat.