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Old 11-02-2012, 05:45 AM   #3159
The Evil Twin
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Originally Posted by roger 04 rt View Post
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.


As a disclaimer, I (and probably "we") am talking about 10ths of AFR when talking about lean/ rich,
To add, as seen in the chart, depending on where you start with an engine, you could very well say that as you enrich the mixture the EGT temp goes up. So point of view has something to do with it Overly lean mix will not be any hotter than a overly rich mix.You can have too little or too much fuel to support strong combustion.Lean is typically fine at cruising/ low load situations. The AFR purpose is not necessarily to control temperatures on vehicles. Regardless, excess heat is wasted energy no matter if it is the exhaust or the cylinder head.
I will only agree with the above chart to an extent, though. It looks as if it is depicting temps for a moderately loaded engine. Under high loads, adding fuel (to an extent) will reduce temperatures in "best power" mode. IE, for gaining altitude in an aircraft. I whole heartedly agree with the relationship between cooler air being more dense and containing more O2 per unit volume. With piston engines, rich or lean/ CHT and EGT and what is an ideal ratio has much to do with loading.
Regardless, the vast majority of engines are lean running out of the gate.
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The Evil Twin screwed with this post 11-02-2012 at 05:55 AM Reason: change "lean" to "enrich" doh!
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