Rotax Piston Meltdown

[AndrewR]

Note it always mentions melting of the piston, and the picture of this Rotax clearly shows the melted top land.

The reason I was looking at this as a sign of detonation was this section:

Some engines ... have a very common detonation failure mode. What typically happens is that when detonation occurs the piston expands excessively, scuffs in the bore ... and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits.

In the shop someone looks at the melted result and says, "pre-ignition damage". No, it's detonation damage. Detonation caused the piston to scuff and this snowballed into loss of compression and hot gas escaping by the rings that caused the melting. Once again, detonation is a source of confusion and it is very difficult, sometimes, to pin down what happened, but in terms of damage caused by detonation, this is another typical sign.

The description of pre-ignition suggests the heat is felt through the whole combustion chamber, and the areas that melt are the areas least able to absorb the heat e.g. the centre of the piston. I don't see how pre-ignition melting would be confined to such a localized spot.

On the other hand, if the melting is due to hot gases blowing past a section where the rings lost their seal, that is more like what I would expect.

But pre-ignition or detonation is a bit of an academic distinction at this point (albeit interesting!).

[rv6ejguy]

The 1st ring land is sometimes described as part of the crown. This is the portion above the top ring. I've never seen this break from detonation. The 2nd and 3rd commonly break from detonation as below:





There is usually no further damage to the crown from this failure because parts can't migrate upwards into the chamber.

[rv6ejguy]

Originally Posted by AndrewR

The reason I was looking at this as a sign of detonation was this section:

Some engines ... have a very common detonation failure mode. What typically happens is that when detonation occurs the piston expands excessively, scuffs in the bore ... and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits.

In the shop someone looks at the melted result and says, "pre-ignition damage". No, it's detonation damage. Detonation caused the piston to scuff and this snowballed into loss of compression and hot gas escaping by the rings that caused the melting. Once again, detonation is a source of confusion and it is very difficult, sometimes, to pin down what happened, but in terms of damage caused by detonation, this is another typical sign.

The description of pre-ignition suggests the heat is felt through the whole combustion chamber, and the areas that melt are the areas least able to absorb the heat e.g. the centre of the piston. I don't see how pre-ignition melting would be confined to such a localized spot.

On the other hand, if the melting is due to hot gases blowing past a section where the rings lost their seal, that is more like what I would expect.

But pre-ignition or detonation is a bit of an academic distinction at this point (albeit interesting!).

From my experience, this is totally incorrect. You can see that detonation from the 2 latest photos I posted, simply snapped the ring lands with no other skirt or heat damage anywhere.

I've driven engines for hours after the rings lands broke from detonation. You just have to add lots of oil as it's woofing out the breathers. Never had one stop running yet and I've broken my fair share.

[AndrewR]

Originally Posted by rv6ejguy

You can see that detonation from the 2 latest photos I posted, simply snapped the ring lands with no other skirt or heat damage anywhere.

How long was that engine detonating at WOT? This engine might have been 1-2 minutes in detonation at WOT.

[rv6ejguy]

Originally Posted by AndrewR

How long was that engine detonating at WOT? This engine might have been 1-2 minutes in detonation at WOT.

The first set of 4 photos I posted, photo 1, the engine detonated moderately to heavily for about 1 second, 5 or 6 times over a couple days. I got out of the throttle/ boost immediately each time. Then this happened the 7th or so time. Immediately down on 3.5 of 4 cylinders. Looked at the breathers and knew the rings lands were broken.

Several nearly identical incidents driving turbo street cars.

I lightly detonated this same engine dozens of times over the next 15+ years and never broke any pistons.

On another turbo engine, the water injection tank ran dry at full boost. Instant, severe detonation broke ring lands on all 4 pistons and the head gasket in less than 1 second.

[Art_N412SB]

Originally Posted by Piper J3

Rotax 912 % Power Graph...
-

When I look at this chart I see a cruise chart with limited excursions into the grey area. What I take away from it is this. Take off with WOT and convert to cruise climb when able. Set your cruise power to the values in the white area.
Best regards,
Art

[tomkk]

Originally Posted by Art_N412SB

When I look at this chart I see a cruise chart with limited excursions into the grey area. What I take away from it is this. Take off with WOT and convert to cruise climb when able. Set your cruise power to the values in the white area.
Best regards,
Art

The note that just precedes this chart is "NOTE: Only applicable on pressure altitude below 3500 ft." (sl-912-016-r1). Not what the situation is above that ...

[bret]

Did I read that there was an ignition failure on one plug? if not, skip the rest of this but here is a theory from a HS grad with no formal training, so here is my uneducated guess, So everything is hot and ready for a detonation event, if one plug fires and as the flame front travels across the CC from the one good plug but with the flame front propagation not completely consuming all the gasses and then you have the (end) gasses spontaneously ignite and detonate with abnormal combustion that blows away the piston boundary layer you have heat transfer to the piston top that is no longer protected by the boundary layer and now exposed to that 1800 degree flame, then the explosive force and pressure of detonation to break stuff and blow holes through weakened soft hot aluminum? all happening within seconds depending on piston thermal inertia rejection capabilities. And this also might snowball by the head boundary layer absent and superheating the coolant in the head and vaporizing it making the coolant ineffective. May be jiberish, just kick me out of the sand box if I'm off base here

[waterboy2110]

Originally Posted by bret

Did I read that there was an ignition failure on one plug? if not, skip the rest of this but here is a theory from a HS grad with no formal training, so here is my uneducated guess, So everything is hot and ready for a detonation event, if one plug fires and as the flame front travels across the CC from the one good plug but with the flame front propagation not completely consuming all the gasses and then you have the (end) gasses spontaneously ignite and detonate with abnormal combustion that blows away the piston boundary layer you have heat transfer to the piston top that is no longer protected by the boundary layer and now exposed to that 1800 degree flame, then the explosive force and pressure of detonation to break stuff and blow holes through weakened soft hot aluminum? all happening within seconds depending on piston thermal inertia rejection capabilities. And this also might snowball by the head boundary layer absent and superheating the coolant in the head and vaporizing it making the coolant ineffective. May be jiberish, just kick me out of the sand box if I'm off base here

The ignition check failed after the engine melted down. Perhaps one of the #4 plugs was taken out - I'll check. Once I found the #2 melted I stopped looking at the ignition. I can't reason a dead cylinder causing a difference in RPM during an ignition check (500 RPM).

[rv6ejguy]

A melted ground electrode is usually a sure sign of pre-ignition which follows with the type of piston damage in the photos too. In all my years detonating turbo engines, never saw it melt off a ground electrode.

Detonation results in pressure spikes many times what you would get during normal combustion but it is of extremely short duration so it does not transfer much heat to anything which is why you see pressure damage but no heat damage.

Detonation also always occurs ATDC where cylinder volume is increasing and crank angle is more optimal where pre-ignition is of long duration (normal combustion started before spark initiation) and always occurs BTDC so pre-ignition adds a ton of heat while effective cylinder volume is decreasing, adding even more heat.