Quote:
Originally Posted by ALS
The fuel being injected in to the cylinder actually cools the piston and cylinder wall from the last power stroke. That is why people who run too lean will get a damaged piston, burnt valves or say their exhaust manifold is glowing cheery red. Many manufacturers are now install piston oil squirters. These oil squirters spray oil on to the underside of the piston to cool it down.
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Partly true. The fuel indeed helps with cooling the walls, but temperature peaks at a bit leaner than stoich, and tapers on both sides. Running very lean or very rich lowers temperature, running a bit lean leads highest temps.
See
the graph in this document.
The nasty stuff happens simply because you're running too hot, not as a direct cause of running too lean. You might say you're running too lean for for the temperature/load. As you said a solution to run cooler is to run richer. But if the engine could run very lean and without detonation, temps would be down. Since most of the time when you need to run cooler you also want to make big power, you go rich as going very lean leads to a reduction in power (and detonation).
Diesel engines run very lean and are cool. As you richer a diesel you'll see the EGT go up. If you would richen it past stoich (similar to a gasser stoich), the EGT would go down again, but it would be hurling out thick black smoke.