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The mkiv Supra Owners Club

Can some help me understand detonation ?


Clarkey

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We all know detonation is unhealthy. That if an engine is running lean or insufficient level of octane - it could lead to detonation. But I've never actually understood the physics of it.

 

This is a bit from this website:

 

Detonation, or engine knock, occurs simply when fuel pre-ignites before the piston reaches scheduled spark ignition. This means that a powerful explosion is trying to expand a cylinder chamber that is shrinking in size, attempting to reverse the direction of the piston and the engine

 

Does this mean that if there is high pressure in the cylinder(s) (I would guess that the piston would be upwards - compressing the gas) - the mixture is ignited from the heat in the chamber causing the piston to go down .. when in actual fact it should be up.

 

The piston going up/down at irregular intervals with alot of force causes the pinging noise and the erroding of the piston itself.

 

No doubt my interpretation/understanding of that quote is wrong .. could someone correct me ?

 

Ta!

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im afraid I'm no mechanic (or artist at that) but my understanding is that knocking is heard whenever the fuel ignites on its own. If the fuel is ignited before or after the spark plug lights it up then the burn pattern of the fuel changes and you hear the knocking sound.

Worst case is when the piston is compressing the mixture (or whatever gas is in the chamber) and the gas is ignited before the piston reaches TDC, therefor trying to push the cylinder down when its moving up putting a massive force on the cylinder.

I'm sure I'll be corrected if wrong:read:

knockithink.JPG

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Worst case is when the piston is compressing the mixture (or whatever gas is in the chamber) and the gas is ignited before the piston reaches TDC, therefor trying to push the cylinder down when its moving up putting a massive force on the cylinder.

 

In this scenario - if the spark-plug fired after this event .. been trying to think what would happen then - if the plug fires attempting to ignite a mixture that's already been ignited.

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Det is not necessarily the mixture igniting before the spark ignites it - that is pre-ignition. Normally it takes time for the mixture to burn from the bit right by the spark plug to the rest. Det occurs when this happens uncontrollably, much faster than normal - the sudden increase in pressure causes a pressure wave.

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Normally it takes time for the mixture to burn from the bit right by the spark plug to the rest. Det occurs when this happens uncontrollably, much faster than normal - the sudden increase in pressure causes a pressure wave.

 

I'm not sure I understood that ... can you give an example ?

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Well if you think of the fuel/air mixture in your cylinder like a match, normally the spark is like striking the match at one end. It takes time to burn right to the other end and use all the fuel up. Energy is released throughout that time. Pre-ignition is like the match just spontaneously catching fire. But det could also happen by you striking the match and the heat from it bursting into flames making the far end catch fire at the same time. The match would burn much faster than normal and release all that energy super fast.

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The match would burn much faster than normal and release all that energy super fast.

 

Ah ok. And it's this additional ignition in the chamber which causes the piston to travel in the opposite direction that it should be, at a greater force causing the knocking sound ? Right ?

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as far as I'm aware no. When detonation occurs the fuel burn pattern changes (like stated above, the match may light at both ends or all at once). Because of this difference in how the fuel ignites, it changes where force is exerted inside the combustion chamber. As the force is created in the wrong place inside the combustion chamber it creates a pressure wave inside the chamber making the noise that can sometimes be heard.

In (crap) fig a the fuel is being burnt right to left as normal.

In (crap) fig b the detonation of the fuel has happened on the left and changed how the fuel is burnt. This creates abnormal pressures inside the cylinder making the noise.

Does this make any sense?

det1.JPG

det2.JPG

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Not necessarily. The whole piston moving in the wrong direction thing is the absolute worst case. Basically when the fuel burns it wants to expand. That's what normally pushes the piston down. When it burns you get a wave of pressure in the cylinder from the spark outwards. When it burns in more than one place you get another wave of pressure going the other way. They crash into each other, and the pressure is much higher than normal too because the fuel has burned faster than normal so the piston is not as far down as it would be normally, so they resonate which is what makes the knock noise. The resonating plus high pressure can cause big damage, even if the piston is on its way down as it should be.

 

EDIT: beaten to it!

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