GPracer2500
01-05-2007, 03:11 AM
This is from a Kawaski 750 SXi stand-up; 750cc, two stroke, twin cylinder, liquid cooled, pistons are 80mmx74mm and handled about 45hp each. It had been running at 180psi on AZ 91 pump for many hours. A batch of suspect gas combined with little to no "extra" deto protection did it in.
Deto erroded the ring lands near the exhaust port until the top ring gave way. The broken piece(s) proceded to bounce around the chamber for a while. That's what all the chickenscratch on the top is from.
http://img300.imageshack.us/img300/2018/p7090021pe2.jpg
Here's the other piston. The rings are removed in this pic. They were badly worn but did not break. Same thing happened--detonation--except the damage wasn't as bad. Had the engine continued to run this one would have met the same fate.
http://img299.imageshack.us/img299/7249/p7090026hp3.jpg
This is actually a really good example of pure detonation damage. That pitting and errosion all over the top ring land is from the piston's protective boundry layer being blasted away by detonation. The boundry layer is a very thin layer of gas that is mostly stagnent. Once that thermal protection is compromised the combustion gasses can actually touch the piston--these gasses are more than hot enough to melt aluminum.
The damage concentrates on the faces of the ring lands because of the tiny gap between the cylinder wall and piston. Pressure waves that occur in the combustion chamber at the moment of ignition (by the plug) race ahead of the flame front. They can bounce around in that gap and create localized areas of increased pressure. This auto-ignites (detos) air/fuel mixture in those pressure spots before normal combustion has a chance to get there.
Here's an example of deto damage on an air cooled Honda XR engine. The circumstances were different with this piston but the end result is pretty much the same.
http://img80.imageshack.us/img80/5633/660pistontopsu6.jpg
Deto erroded the ring lands near the exhaust port until the top ring gave way. The broken piece(s) proceded to bounce around the chamber for a while. That's what all the chickenscratch on the top is from.
http://img300.imageshack.us/img300/2018/p7090021pe2.jpg
Here's the other piston. The rings are removed in this pic. They were badly worn but did not break. Same thing happened--detonation--except the damage wasn't as bad. Had the engine continued to run this one would have met the same fate.
http://img299.imageshack.us/img299/7249/p7090026hp3.jpg
This is actually a really good example of pure detonation damage. That pitting and errosion all over the top ring land is from the piston's protective boundry layer being blasted away by detonation. The boundry layer is a very thin layer of gas that is mostly stagnent. Once that thermal protection is compromised the combustion gasses can actually touch the piston--these gasses are more than hot enough to melt aluminum.
The damage concentrates on the faces of the ring lands because of the tiny gap between the cylinder wall and piston. Pressure waves that occur in the combustion chamber at the moment of ignition (by the plug) race ahead of the flame front. They can bounce around in that gap and create localized areas of increased pressure. This auto-ignites (detos) air/fuel mixture in those pressure spots before normal combustion has a chance to get there.
Here's an example of deto damage on an air cooled Honda XR engine. The circumstances were different with this piston but the end result is pretty much the same.
http://img80.imageshack.us/img80/5633/660pistontopsu6.jpg