Pre-detonation

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I see a number of members using the term "pre-detonation" when referring to pre-igniton or detonation.

Pre-ignition is what happens in an internal combustion engine when fuel is ignited by a hot spot prior to the desired ignition point, resulting in higher than normal combustion pressures.

Detonation in an internal combustion engine is when the fuel explodes rather than burns. The technical definition of an an explosion is when the burn front exceeds the speed of sound. The sound you hear when an engine detonates is the "sonic boom" generated by the supersonic flame front. This also results in extremely high combustion pressures.

Pre-detonation is what happens in an atomic weapon if the sub-critical fuel masses are exposed to stray neutrons and undergo spontaneous fission, destroying the fuel before it can be taken super-critical. Needles to say, but I'm going to say it anyway, this is not what is happening in your engine!

Ed
 
LOL!! Thank you Ed
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Thanks, Ed. Those really are important distinctions.

By the way, could anything resembling pre-detonation have happened in a certain DeLorean that was alternatively fuelled?
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Originally Posted By: Garak

By the way, could anything resembling pre-detonation have happened in a certain DeLorean that was alternatively fuelled?
wink.gif


I came >< this close to mentioning that as a possible exception.
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Ed
 
Thanks Ed...

need to add in "end gas autoignition", where the flammable gasses remote from the ignition source are "dieseled" by being compressed by the burning gasses.
 
Originally Posted By: Shannow
"end gas autoignition",


Detonation is not pre ignition, it's post ignition.
 
Originally Posted By: edhackett
Detonation in an internal combustion engine is when the fuel explodes rather than burns. The technical definition of an an explosion is when the burn front exceeds the speed of sound. The sound you hear when an engine detonates is the "sonic boom" generated by the supersonic flame front. This also results in extremely high combustion pressures.


But gasoline can't actually detonate, can it? It is still "just" rapid combustion since there are no oxygen atoms on the octane molecule. I always thought that detonation was a reaction propagated by a shock wave and required the reactants to be on the same molecule?
 
Originally Posted By: edhackett
I see a number of members using the term "pre-detonation" when referring to pre-igniton or detonation.

Pre-ignition is what happens in an internal combustion engine when fuel is ignited by a hot spot prior to the desired ignition point, resulting in higher than normal combustion pressures.

Detonation in an internal combustion engine is when the fuel explodes rather than burns. The technical definition of an an explosion is when the burn front exceeds the speed of sound. The sound you hear when an engine detonates is the "sonic boom" generated by the supersonic flame front. This also results in extremely high combustion pressures.

Pre-detonation is what happens in an atomic weapon if the sub-critical fuel masses are exposed to stray neutrons and undergo spontaneous fission, destroying the fuel before it can be taken super-critical. Needles to say, but I'm going to say it anyway, this is not what is happening in your engine! About 80 doublings and 3 percent yield?

Ed
 
Originally Posted By: kschachn
Originally Posted By: edhackett
Detonation in an internal combustion engine is when the fuel explodes rather than burns. The technical definition of an an explosion is when the burn front exceeds the speed of sound. The sound you hear when an engine detonates is the "sonic boom" generated by the supersonic flame front. This also results in extremely high combustion pressures.


But gasoline can't actually detonate, can it? It is still "just" rapid combustion since there are no oxygen atoms on the octane molecule. I always thought that detonation was a reaction propagated by a shock wave and required the reactants to be on the same molecule?


Yes, detonations can occur with any mix of flammable gas and air. A molecule containing its own oxidizer is not required. It happens with finely divided solids too. You may be aware of flour and "dust" explosions that happen in factories and farm barns and silos. Here's a Wiki article that talks about flammable gasses. It's not specific to engines but gives examples of deflagration to detonation transition.
https://en.wikipedia.org/wiki/Deflagration_to_detonation_transition

Ed
 
Originally Posted By: edhackett
Yes, detonations can occur with any mix of flammable gas and air. A molecule containing its own oxidizer is not required. It happens with finely divided solids too. You may be aware of flour and "dust" explosions that happen in factories and farm barns and silos. Here's a Wiki article that talks about flammable gasses. It's not specific to engines but gives examples of deflagration to detonation transition.
https://en.wikipedia.org/wiki/Deflagration_to_detonation_transition


OK, thanks.

Yeah, growing up in the corn belt there were a few such explosions that I remember. In fact, in high school we did a science experiment using (IIRC) a coffee can and bicycle pump where we introduced a spark into an air/powder mixture. I remember using several different powders, all of them made quite a bang.
 
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