Running Lean and Hot

[HPetrino]

We pretty much all know that when the carb mixture is too lean the engine will run hotter. I know this is a fact because I've experienced it.

Frankly, I don't actually understand it. Simple Vulcan logic would suggest just the opposite. More fuel = more heat not less fuel = more heat. What's the reason for this well known dynamic? It must have something to do with the ratio of oxygen to fuel, but I've never been able to figure it out. Anybody out there have an "explanation for dummies"?

[CudaMan]

Here's my understanding of it, FWIW.

For gasoline, the chemically ideal ratio of air to fuel is around 14.7 to one. This assumes perfect mixing of the air and fuel.

Since the mixing is rarely perfect, most cars are set up to run a bit on the rich side of ideal, this helps ensure that all of the oxygen in the intake air gets used.

If you run a lot richer than the ideal ratio, some of the energy from combustion gets used heating the excess fuel, so the combustion temperature is reduced. Supercharged engines are often set up to run very rich under boost, specifically to cool the mixture.

If you run leaner than the ideal ratio, you're not burning all of the available oxygen. Peak combustion temperature is still reduced somewhat, but the excess oxygen can react with the materials in the combustion chamber, almost like a cutting torch. Think burning pistons, exhaust valves, and spark plug electrodes.

A leaner mixture also burns slower than a mixture at the ideal ratio, so if the spark advance isn't changed, the gases are still burning when they go out the exhaust, which can dump extra heat into the combustion chamber walls (and therefore into the coolant) and cause burned exhaust valves and a glowing red hot exhaust manifold.

[HPetrino]

Thanks Mark! That makes all sense.

[Burger in Spokane]

Nothing that duct tape can't fix, though ....