Dumb QOTD: Why does load induct pre-ignition / detonation?

[hoodeng]

Somewhere around 2010 [yes i know that is a century ago in engineering] Cosworth were up to 20,000rpm, Honda not far behind at 19,000rpm, i am going to have to dig but i read at the time, yes spark knock was of a concern at this rpm but also was spark timing scatter, their explanation was that at this rpm the engine had so much inertia that it still made power enough for it to be relevant as a performance factor.

I also mentioned 'chassis' in my dyno description,[ i know a lot of guys don't consider motorcycles in their performance vocabulary],, a good friend of mine dyno's bikes five days a week and he has told me of a number of occasions where mechanical issues have fallen into the parameters of the knock sensors, and only after changing fuel/ignition settings to a point they should not be contributory that the mechanical factor has been investigated and found to be wanting.

Keith Duckworth said ' if you can keep air up to an engine it will continue to make more power', but he also said 'the closer you are to destruction the better the performance will be'!

Amen to Keith.

[Truckedup]

A good reputation dyno shop near me does only bikes an sleds....Japanese manufacturer field engineers rented dyno time for a few days...In their bag of tricks was a length of rubber tubing with head phones on one end and a copper lug on the other. The lug was bolted or clamped to the cylinder head, the dyno rider could hear detonation through the head phones...Crude but effective on a single cylinder racer..
The vintage bikes I race and street ride are detonation prone by design. Pre ignition ,if it occurs, seems to come at at mid rpm with load. Instantly there's a hole in the center of the piston...Detonation generally occurs from the same factors , takes a bit more time and causes a piston seizure if severe enough...I'm sure there's a combination of the two at times...

[Helmantel]

Interesting discussion!

One of the reasons why knock is RPM sensitive (i.e. the engine is more knock prone at low speed/high load) is that the turbulence in the combustion chamber is lower at lower engine speeds. Turbulence is very effective in spreading the flame front, forming "islands" of end gas that are eaten up from all directions, rather than by a flame front sweeping from left to right. This allows the end gas less time to heat up to the point of auto ignition. That doesn't mean that knock at high engine speeds is not possible though.

For this same reason, an engine doesn't need much more ignition advance at maximum RPM than at mid RPM. The available time is reduced in half but the faster combustion compensates for that and the combustion duration in crank angle degrees remains constant. Well, it's not quite that simple, but you probably get my point.

[AUTOMAN]

Pre-ign. IS pre-ign. short of spark!

It can happen at just after BDC on comp. stroke.

If and when it does, bang without too much noise.

2 or 3 bangs... it's over hole in piston-right in middle.

Never detect it usually.

Avoid hot spots/glow plugs/carbon/hot seats or valves.

cliff

[David Redszus]

Pre-ign. IS pre-ign. short of spark!

Some have correctly used the term to indicate initiation of burn prior to spark.
But what of the condition whereby burn is initiated at a second point but
after spark. A second flame front may not be detonation.

It can happen at just after BDC on comp. stroke.

That seems highly unlikely considering the pressure and temperature conditions at BDC.
I would be very curious to see evidence of such an occurrence; perhaps Nitro
has seen combustion traces to support the issue.

[Tuner]

Pre-ign. IS pre-ign. short of spark!

Some have correctly used the term to indicate initiation of burn prior to spark.
But what of the condition whereby burn is initiated at a second point but
after spark. A second flame front may not be detonation.

It can happen at just after BDC on comp. stroke.

That seems highly unlikely considering the pressure and temperature conditions at BDC.
I would be very curious to see evidence of such an occurrence; perhaps Nitro
has seen combustion traces to support the issue.

Pre-ignition can occur any time prior to the spark, up to and including during the intake stroke. Things can get exciting when it happens with the intake valve open.

A hot spark plug side electrode is the enabler because it gets hotter with each earlier ignition event, igniting earlier each time on the compression stroke and finally ignites before the intake valve closes. Been there, done that, on an air cooled Honda generator converted to run on natural gas. Had to use a surface gap plug to prevent the pre-ignition. Standard plug for gas is #5 NGK, but even a #10 NGK would pre-ignite and it had to be run at 1.4 or 1.5 lambda and the spark timing suitably optimized (advanced from the gasoline timing) to keep it from overheating at full load.

[David Redszus]

Pre-ign. IS pre-ign. short of spark!

Some have correctly used the term to indicate initiation of burn prior to spark.
But what of the condition whereby burn is initiated at a second point but
after spark. A second flame front may not be detonation.

It can happen at just after BDC on comp. stroke.

That seems highly unlikely considering the pressure and temperature conditions at BDC.
I would be very curious to see evidence of such an occurrence; perhaps Nitro
has seen combustion traces to support the issue.

Pre-ignition can occur any time prior to the spark, up to and including during the intake stroke. Things can get exciting when it happens with the intake valve open.

A hot spark plug side electrode is the enabler because it gets hotter with each earlier ignition event, igniting earlier each time on the compression stroke and finally ignites before the intake valve closes. Been there, done that, on an air cooled Honda generator converted to run on natural gas. Had to use a surface gap plug to prevent the pre-ignition. Standard plug for gas is #5 NGK, but even a #10 NGK would pre-ignite and it had to be run at 1.4 or 1.5 lambda and the spark timing suitably optimized (advanced from the gasoline timing) to keep it from overheating at full load.

Your observation regarding spark plug heat range raises an interesting issue.
If we cross the NGK to Bosch we find
NGK 5 ==> Bosch W8CS
NGK 10 ==> Bosch W08CS

But these numbers are relative and tell us very little. So Bosch scaled the heat range of their
plugs to the number of seconds a test engine could be operated at wide open, full load.

W8CS ==> 150 secs
W08CS ==> 350 sec

which works for the center electrode, but not for the ground strap.
There have been engines that when the spark ignition was shut off, the engine continued to run.

As Tuner correctly observes, when making substantial changes to the design of an
engine, the timing, mixture, plug heat range and tip style, load and mode of
operation must all be considered and tuned accordingly.