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Physics and chemistry of why lean-running air/fuel mixes produce more heat?


fuzzybabybunny
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So the general explanation for why a lean fuel mixture produces more heat and can lead to an engine seizure is that it's the same as if you took a fire in a fireplace and fanned air into it. With the added air the temperature of the fire goes up.

To me this doesn't quite make sense around the subject of engines.

If you have a general combustion formula:

CH4 + 2O2 -> combustion -> 2H2O + CO2 + 100 kJ heat (or whatever)

If you simply add more O2 without increasing fuel (CH4) the amount of energy and heat released should still be the same:

CH4 + 48O2 -> combustion -> 2H2O + CO2 + 46O2 + 100 kJ heat

The only way for it to increase heat is if more fuel was burned. 

And this is what I don't understand about lean-running engines producing more heat. When you tune the carb to run lean, you're restricting the amount of fuel that gets into the combustion chamber, right? So less fuel in the combustion chamber should mean less heat is produced, right?

The reason the fire in the fireplace gets hotter when you fan more air is because more fuel is being burned in, say, 1 second than if there wasn't air being fanned onto it. All that extra fuel (wood) is just laying there in the fire itself. 

But in the case of engines the combustion chamber only has a set amount of fuel, whatever amount the carb is set to deliver.

Edited by fuzzybabybunny
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With 2 stroke motors the operating conditions aren't quite that simple. The added oil in the fuel is to keep the engine lubricated, as there is no sump, but the amount of fuel does have a cooling effect on the inlet phase in my understanding.

Pete do you have further thoughts?

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Yeah, I understand that in rich conditions the excess fuel can evaporate, cooling the engine down. To me this is a little weird because I wouldn't think that such a little bit of evaporation would actually effect that much...

But it's the lean-running condition that confuses me. Like I said in the OP, the chemical formula is pretty straightforward.

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The term 'lean burn' is relative to the 'normal' condition which is slightly rich. In the rich condition, the reaction does not complete to CO2 throughout the charge. Some of the reaction results in CO, with an attendant reduction in heat output.

The effect on the engine components is the result of both the additional heat of the lean burn, and the additional oxygen which remains after combustion. This combination promotes damage to parts, notably the piston crowns of 2 stroke engines. It is easy to see how this is perceived as simply more heat.

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My understanding was that 2 strokes run on the 'rich side' of a perfect stoichiometric ratio (where the fuel/oxygen mix is perfectly balanced). If you run an engine either side of this ratio it runs cooler as the fuel is not all used up and acts to cool the combustion chamber.

I'm very happy to be wrong as that always seemed far too simple an explanation.

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I guess the part that I get stuck on is this:

When an engine runs lean, it means that there is less fuel, *not* that there is more oxygen. When I go to adjust a carb for leanness/richness, I adjust the screws that control the flow of fuel. I don't adjust anything that has to do with the flow of air.

So it would be something like this for a "normal" mixture:

50 mol CH4 + 100 mol O2 -> combustion -> 100 mol H2O + 50 mol CO2 + 5000 kJ heat (or whatever)

And for a lean mixture:

40 mol CH4 + 100 mol O2 -> combustion -> 80 mol H2O + 40 mol CO2 + 40 mol O2 + (something less than 5000) kJ heat

So I would think that the "something less than 5000 kj" would mean that it should run cooler since there is simply less moles of fuel being burned. The amount of O2 in the combustion chamber is the same because the carb adjustments only restrict the moles of fuel getting into it. In a lean mixture there is more complete combustion of the fuel, but there is also less fuel. So is it fair to say that despite there being more fuel in a rich mixture, MORE fuel is being burned in a lean mixture anyway? So running an engine slightly lean will produce more power than an engine running slighting rich? And because a rich mixture burns less fuel and has more evaporative effects than the lean mixture, the engine runs cooler?

I get that a stoichiometric mixture in real life won't have complete combustion and that lots of other things will be made from incomplete combustion and all the other elements in air besides O2 when running rich. And there's more excess fuel being evaporated so the heat of evaporation factor is more.

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Your equations are correct, but based on stoichiometric mixture being the norm and lean burn being an excess of oxygen. The reality is that a rich mixture is the norm and 'lean' refers to a mixture which is probably almost stoichiometric.

The rate at which air and fuel flow through the carb is not so simple. What you adjust is the ratio of fuel to air, whether by adjusting a fuel restrictor or an air restrictor screw. The total flow rate is influenced by what happens in the combustion chamber, especially so in 2 stroke engines. The exhaust system is designed to use gas momentum to draw the next charge into the cylinder. A hotter burn and higher cylinder pressure will create higher exhaust gas velocity and draw more air/fuel mixture into the cylinder. This is only one of the factors at work, there are others

Thus the fuel mixture cannot be considered in isolation.

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Two stroke motors run rich, I could go into the equations, but simply they rely on forcing fuel into the exhaust and blowing/compressing it back into the cylinder.  That is why tuned pipes give more power.  Essentially if you burn all the fuel it will run hotter as the unburned fuel aids cooling.  Run it too lean and you will melt a hole in the piston, I did it on my RD350 when I was younger..

Edited by G-UTSI
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