what will cause piston to break or melt

[odd boy]

Gents,

What will melt the piston? lack of fuel or extra timing?


What will break the piston? lack of fuel or extra timing?

I heard some people say if the piston is broken at the edge that means.... and a lot of other says. If someone has ideas about this subject plz teach us.

[Gelf VXR]

I dont quite understand how, but lean mixture melts pistons, how ever if i'm not mistaken petrol burns with the highest temperature at stoich???

[joecar]

Lean mixture does 2 things:
- increases combustion chamber temperature (extra fuel has a cooling effect)(stoich is too lean at WOT),
- increases ability to knock/ping.

Both of those will damage pistons, and knock damages head gaskets, rod/main bearings, and rods.

Timing that is too advanced (even by a few degrees) also does both of those things (temp. and knock).

[Stealth97]

Gents,

What will melt the piston? lack of fuel or extra timing?


What will break the piston? lack of fuel or extra timing?

I heard some people say if the piston is broken at the edge that means.... and a lot of other says. If someone has ideas about this subject plz teach us.

So far I have learned, and I hope this helps:

A hole burned through the top of the piston in only one cylinder usually means that specific cylinder was running lean in comparison to the rest (usually a failed fuel injector).

Inspect your pistons, if the ring lands are cracked, you have detonation issues.

Inspect your rod bearings on the big end of the rod (crank attachment). If they have low mileage but are worn through in the center past the outside coating...then you may also have detonation issues.

When tuning, most people are only checking AFR. I wonder if any of these setups are running too hot EGT (exhaust gas temperature) that causes damage.

I wonder if some of these extreme setups need to have EGT sensors put in the headers while being tuned or at least check the EGT while doing an extended highway pull in 80-90F heat and see what REALLY happens. Too much heat in the motor and then jumping on it again=popped motor.

[Stealth97]

I dont quite understand how, but lean mixture melts pistons, how ever if i'm not mistaken petrol burns with the highest temperature at stoich???

But compress air/fuel 10x and the rules of the game change I believe. The properties are different when compressed, no?

[Gelf VXR]

But compress air/fuel 10x and the rules of the game change I believe. The properties are different when compressed, no?

Compression doesn't affect AFR

[Stealth97]

Compression doesn't affect AFR

I mean that petrol might burn it's highest temperature at stoich (which is 1bar, 14.7psi/atmospheric), but take compressed air/fuel, and it may have a different burn temperature. I'm not totally sure on this but just thinking out loud.

[joecar]

I'll have to do some more reading in my Heywood.

[gmh308]

Petrol/gasoline doesnt burn hottest at stoich. The leaner it is, the hotter it burns. It burns most "efficiently" at stoich.

Leaner = hotter = higher Nox / lower HC. Higher Nox because more nitrogen is oxidized. Lower HC as HC's are burned more effectively by the higher temperatures.

Knock will hammer a piston, lean mixtures weaken the piston, increase tendency to knock. Generally with GenIII/IV, to much knock breaks the top land (weakest area).

If it is too lean and knocking, likely the hottest part of the piston, the centre, will weaken and get a hole "knocked" through it.

They generally fall apart before they get hot enough to really melt. At least cast pistons do. Forged resist the extremes much better!

[joecar]

Yes, Heywood says exactly that... leaner than stoich burns hotter (provided there is sufficient fuel, otherwise it won't burn at all... but that's pretty lean)... and the N2 (nitrogen molecule) requires quite a high temperature to oxidize (...lol... otherwise our 79% N2 atmosphere would oxidize with each and every open flame...).