Hot, hot, hot!

[openspaceman]

Emmm....not sure about that.

On a 4 stroke (what i know best) running lean definitely causes ext temp to go skyward. As you go lean, combustion slows down. Slow combustion means you get less energy converted into mechanical energy - imagine the extreme case where the bulk of combustion is taking place as the piston is way past tdc and travelling down the cylinder, you just dont get the cylinder pressure at the right time to do any good. And if you don't remove the energy from the gases they stay hot - so higher exhaust temps. Pistons and exhaust valves also get toasted.

It's got to be the same with a two stroke. Plus, and maybe more importantly the higher engine speed you get with a lean engine means 1) more combustion cycles so even if every thing else were equal that's more heat to get rid of and 2) combustion takes a finite time so at some point the engine is reving too fast for the engine to be efficient (just like the running lean logic above)... and that's more heat out the exhaust.

 

Conversely a bit of extra fuel - running rich - the extra mass of the fuel absorbs some of the combustion heat causing cooler combustion and a cooler engine.

 

I know which I'd rather have.

 

bmp01

 

Yes you are right and my explanation was bad, the maximum temperature is with the stoich combustion but it completes soon after tdc, so the hot gases expand after completion and in pushing the piston down to do work they cool as they expand. The heat of combustion goes three ways, about 25% is converted to motion and most of the remainder goes out the exhaust.

 

As you rightly say a weak mixture burns slower, in the extreme it is still burning as the exhaust port is open, this means that as piston is already travelling down heat is still being produced and it expands over a shorter period and so less of the energy goes into pushing the piston down. The heat still has to go somewhere and it is rejected both to the exhaust and the cylinder, I was wrong to say the exhaust gas would be no hotter but the main point is more heat goes to the cylinder which leads to break down of lubricant and seizure.

 

Conversely I don't think the heat absorption of the extra fuel is as significant as the fact that with a two stroke and simple carburation the over rich mixture 4 strokes (doesn’t fire on every stroke as the previous combustion hasn't burnt and scavenged well so the mixing of this and the fresh charge won't fire with the spark, the next cycle clears the way for a fresh charge which them burns on the 4th stroke) and this limits revs.

 

Not a lot of sense there.

 

No too much scotch, see above, the point being to the OP that its hot engine rather than hot exhaust that is the problem with a weak mixture and his hot exhaust is probably nothing to worry about if his spark plug colour looks good and the engine isn't over revving.