Detonation

[hightower99]

I was also thinking that like a bomb the most destructive part is the point of ignition. So if detonation burns the spark plug the point of origin is the spark plug. Similarly, if the piston is damaged, I assume that the point of ignition is carbon on the piston at that point.

Anyone have any ideas on this?

I have seen several explosions that would disagree with your idea. Normally the point of ignition suffers much less damage compared to what the wave front hits at max speed. I remember reading somewhere that detonation normally has an ignition point that is somewhere in the volume of the cylinder (ie away from the walls or the piston of the spark plug). This would explain why holes are punched into the piston and somtimes break the valves as these are the weakest points. I don't know if there is an example of detonation damaged engine the suffered both a hole in the piston and damaged valves?

[Old Sage]

Pre-ignition occurs before the plug fires, detonation occurs afterward. Pre-ignition causes mostly heat related damage where detonation tends to break parts. Detonation can send a cylinder into pre-ignition.

[hightower99]

Pre-ignition occurs before the plug fires, detonation occurs afterward. Pre-ignition causes mostly heat related damage where detonation tends to break parts. Detonation can send a cylinder into pre-ignition.

Detonation just means that the mixture exploded there is no set timing as to when detonation occurs.


Pre-ignition is when the mixture begins to burn before the spark plug fires, obviously there is a logical time constraint on this as it has to happen before the spark plug fires.

The only difference between them is the difference between a burning mixture and one that explodes.

[Alastor]

The only difference between them is the difference between a burning mixture and one that explodes.

But this is a fairly important difference, is it not?

[hightower99]

But this is a fairly important difference, is it not?

Yes and it is the only difference...

Therefore if you want to discuss detonation keep it to exploding mixtures, stop bringing unneccessary time constraints into the discussion.

[Old Sage]

Yes and it is the only difference...

Therefore if you want to discuss detonation keep it to exploding mixtures, stop bringing unneccessary time constraints into the discussion.

The previous response was edited out. Since you believe detonation is some type of army corp of engineers exploding mixtures phenomenon that can strike at any time, further discussion is somewhat pointless.

[hightower99]

The previous response was edited out. Since you believe detonation is some type of army corp of engineers exploding mixtures phenomenon that can strike at any time, further discussion is somewhat pointless.

WTF!?!?!

Detonation is just when the fuel air mixture explodes instead of burns like it does during normal operation. Detonation can occur BTDC and ATDC.

[BradG]

Very nice, revetec! I agree that detonation is so damaging because it is a violent explosion causing a large increase in cylinder pressure, thus causing more detonation in different parts of the cylinder, thus more pressure, this is exacerbated by the fact that the piston may still be trying to compress the fuel-air mixture. Regular ignition does occur before TDC but since the burn of the fuel-air mixture is more controlled (flame front being the progression of the burn). As to why the damage occurs to different components in different instances, I really don't know.

You gave a great summary.

[Old Sage]

WTF!?!?!

... Detonation can occur BTDC and ATDC.

Post #17 said nothing about tdc.

[revetec]

Pre-ignition occurs before the plug fires, detonation occurs afterward. Pre-ignition causes mostly heat related damage where detonation tends to break parts. Detonation can send a cylinder into pre-ignition.

I think your first sentence was right but your last sentence was wrong. I think it would be: Pre-ignition can cause a detonation to occur.

[Old Sage]

In this case, severe temperatures caused by detonation can form an alternate ignition source resulting in pre-ignition and a sudden meltdown of the cylinder.

[revetec]

Pre-Ignition can be caused from detonation, but it will still occur before the correct ignition firing. This is rare because to get the cylinder temperatures up so high that it explodes before an ignition point is created, is unlikely, but not impossible. It is usually the other way around.

Quote from Wikipedia:

"Pre-ignition is a different phenomenon from detonation, explained above, and occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition is caused by an ignition source other than the spark. Heat or hot spots can buildup in engine intake or cylinder components due to improper design, for example, spark plugs with heat range too hot for the conditions, or due to carbon deposits in the combustion chamber. Spark plugs with a high heat range will run hot enough to burn off deposits that lead to plug fouling in a worn engine, but the electrode of the plug itself can occasionally heat soak, and begin glowing hot enough to become an uncontrolled ignition source on its own. Bits of carbon that build up in a combustion chamber can also heat soak to the point where they also are glowing hot and ignite the air-fuel mixture before the proper time.

Pre-ignition and "dieseling" or "run on" are the same phenomenon, except in the latter case the engine continues to run after the ignition is shut off with a hot spot as an ignition source. Pre-ignition might cause rough running due to the advanced and erratic effective igniton timing and may cause noise if it leads to detonation. It may also cause "rumble" which is fast and premature but not detonating combustion.

This heat buildup can only be prevented by eliminating the overheating (through redesign or cleaning) or the compression effects (by reducing the load on the engine or temperature of intake air). As such, if pre-ignition is allowed to continue for any length of time, power output and fuel economy is reduced and engine damage may result. The engine might be slightly harder to get running at once after pre-ignition.

Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may exist separately."

[Old Sage]

I know you meant well and this is not targeted at you so please don't take it that way. That article is full of inaccuracies.

Instead of debate, let me suggest an alternative. There is a whole series of tech articles authored by the Chief Engineer at UEM/KB (Keith Black) Pistons. Several cover this subject in varying degrees. One of them is titled Combustion Science and Theory. For those really interested in the finer details of this subject, I would highly recommend this reading.

[revetec]

Do you have any links to post?