Science Forums 2

[marcringuette]

This thread has focused on the ability of excess fuel to carry away heat. However, I found an alternative explanation for why lean engines run hot that sounds very convincing to me:

"The bit that causes melted pistons is that lean mixtures extend the combustion duration due to the slow flame speed, and therefore the mean cycle temperature rises. Peak flame temperatures don't cause any damage to pistons, but it takes far lower mean cycle temperatures to damage them, usually first from lubrication failure before outright melting." -- TDIMeister, eng-tips.com/viewthread.cfm?qid=212876

However, I admit that I don't fully understand why slow combustion would raise the mean cycle temperature. It's not like the combustion products stop being hot once combustion is finished! My best guess is that if combustion is rapid (before the piston has moved far) then the expansion ratio is maximized and the gases can most quickly expand and cool down.


Marc