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Old 11-02-2012, 04:42 AM   #3158
roger 04 rt
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Originally Posted by The Evil Twin View Post
I had the same thought until PS corrected me. I started thinking about it in the sense of thermodynamics. To put it simply, if the cylinder temperatures got hotter as the mixture was leaned out then the absence of any fuel would result in infinite heat Now we know that won't work. Unfortunately.
Thinking of small aircraft, as you ascend you would lean the mixture to account for less dense air. As you do so the exhaust gas temp increases, yes. Not because lean= hotter, but because the excess fuel acts as a coolant. It absorbs the heat. As the mix is leaned out there is less "coolant" (vaporizing fuel) and the EGT goes up. However, only so far. If the mix is leaned further then it starts dropping (less fuel, less work, less heat) until combustion can no longer be supported.
Originally Posted by Wallowa View Post
Two ways of stating same thing...lean mixture = less fuel = less "coolant"....therefore, lean mixture does result in increases of EGT or in my case the cylinder/engine temp on my boxer. More fuel in response to an ambient temp sensor that registers lower than actual ambient temps [hence air density is less than temp sensor would indicate] results in the fuel management putting more fuel [more coolant] into the cylinders resulting in a richer mixture. Sorry I know this not complicated, but I will be damned if I can see it...thanks for patience.

Kinda ...less fuel more temp [lean] : more fuel less temp [richer]
There are three different temperatures of interest: intake air, cylinderhead and exhaust temperatures.

Intake Air: For any desired air/fuel ratio (AFR) colder air, because it packs more oxygen, needs a little more fuel. Hotter air needs a little less fuel. For instance, if it takes a 2 millisecond injector pulse at 68 degrees Fahrenheit to reach an AFR of 14.7:1, then at 32F it takes a pulse of 2.12 milliseconds. At 104F it requires a pulse of only 1.88 milliseconds to achieve that same 14.7:1 AFR.

Cylinderhead: This reaches its peak temperature when the mixture is at its Best Power ratio. If normal cruising is at 14.7:1, the Best Power AFR is roughly 12.5:1. The cylinder head gets hottest because the combustion is making most power. Richer or leaner than Best Power and the CHT goes down.

Exhaust Gas: This reaches its peak at roughly the ideal combustion mixture (stoichiometric) of 14.7:1 for gasoline. (E10 reaches a peak at about 14.1:1 but you don't have to think about it because your O2 sensor makes this adjustment automatically.) richer or leaner than Stoic and the EGT goes down.

So as you can see CHT and EGT reach a peak at different mixtures. Richer isn't always cooler, and leaner isn't always hotter. But leaner than Stoic is always cooler for both CHT and EGT; the leaner you get above Stoic, the cooler that CYT and EGT get.


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