The physics behind air/fuel ratios in 4-stroke engines.


by Holocene
Tags: 4stroke, air or fuel, engines, physics, ratios
Holocene
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#1
Jul27-08, 09:32 PM
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Correct me if I'm wrong, but the hottest combustion temperatures will result when the air/fuel ratio is ideal, or stoichiometric, no?

I ask, because there seems to be this mentality that a lean mixture will result in even hotter temperatures, and could cause engine damage.

This has not been my experience. In aviation, Cessna recommends leaning the mixture until peak exhaust gas temperature is reached. Further leaning only reduced the EGT reading, it does not increase it.

What is the truth?
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pixel01
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Jul27-08, 09:58 PM
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Quote Quote by Holocene View Post
Correct me if I'm wrong, but the hottest combustion temperatures will result when the air/fuel ratio is ideal, or stoichiometric, no?

I ask, because there seems to be this mentality that a lean mixture will result in even hotter temperatures, and could cause engine damage.

This has not been my experience. In aviation, Cessna recommends leaning the mixture until peak exhaust gas temperature is reached. Further leaning only reduced the EGT reading, it does not increase it.

What is the truth?
As I understand, the stoichiometric ratio will give off highest heat, but in case all the fuel molecules combust. In fact, the molecules do not burn completely and some side reactions occur which reduce the temp and thus reduce the efficiency. So adding some more air will increase the chance of complete combustion, but we can not dilute the fuel forever. There should be an optimum mixture.
mgb_phys
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Jul27-08, 10:04 PM
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Quote Quote by Holocene View Post
Correct me if I'm wrong, but the hottest combustion temperatures will result when the air/fuel ratio is ideal, or stoichiometric, no?
Not necessarily

I ask, because there seems to be this mentality that a lean mixture will result in even hotter temperatures, and could cause engine damage.
Burning in excess air could ensure that al the fuel burns most completley to CO2 / H2O and so generate most energy.

Holocene
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Jul27-08, 10:09 PM
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The physics behind air/fuel ratios in 4-stroke engines.


Quote Quote by mgb_phys View Post
Burning in excess air could ensure that al the fuel burns most completley to CO2 / H2O and so generate most energy.
But isn't a "most complete" combustion accomplished via a stoichiometric ratio as well?

I know what you're saying, but if the fuel hasn't completely burned, that is indicative of mixture that is too rich.
mgb_phys
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Jul27-08, 10:22 PM
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Quote Quote by Holocene View Post
But isn't a "most complete" combustion accomplished via a stoichiometric ratio as well?
In a perfect world, but it depends on the reaction vessel and conditions - it's usually best to have one component in excess to guarantee the reaction goes to one end point. BUT on the other hand this will dilute the reaction products and so reduce the average temperature even if it generates most energy.

It's likely to need a very complicated model of the fluid dynamics in the cylinder, unfortunately.
gmax137
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Jul28-08, 11:05 AM
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Many engine tuners can attest to the fact that leaning out the mixture too far results in holes melted in the piston crown. I had a badly pinking engine once, turned out to be an air leak between carb & inlet... I can't say whether this is consistent with what you say about exhaust temperature reaching a peak. Keep in mind that the piston/valve/seat/head temperatures are not the same as the exhaust temp.
brewnog
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Jul28-08, 02:38 PM
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Quote Quote by Holocene View Post
Correct me if I'm wrong, but the hottest combustion temperatures will result when the air/fuel ratio is ideal, or stoichiometric, no?
Not normally. For liquid fuels, evaporative effects help to cool the charge and end gas. Leaning the AFR off beyond stoichiometry can cause an increase in combustion temperatures because this cooling effect reduces.

Quote Quote by Holocene View Post
But isn't a "most complete" combustion accomplished via a stoichiometric ratio as well?
In a quasistatic environment, yes. But in an internal combustion engine, breathing effects mean that it's impossible to completely burn all the fuel given stoichiometric conditions. Providing excess oxygen gives a greater possibility of complete combustion, but even this is not easily achieved.
nucleus
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Aug1-08, 12:36 AM
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Holocene
Yes the EGT is max at stoichiometric, or close to it, but it is the worse place to operate an aero engine. It gives you neither best power nor best economy. Auto engines operate close to it because it gives the lowest overall emissions.

If you look at the first page at the site below you can see how the EGT peaks. To the right of peak it is called “rich of peak” (ROP) and engine is fuel cooled. To the left of peak it is called “lean of peak” (LOP) and engine is air-cooled. People often make statements like: “A lean mixture runs hotter”. This statement may or may not be true. If you start off ROP and then lean – then yes the EGT will rise, but if you start off LOP and then lean – the temperature will drop and rise when mixture is richened.

http://www.lycoming.textron.com/supp...es/SSP700A.pdf
jefswat
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Aug1-08, 12:57 AM
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I do not know the science behind it, but as someone mentioned before, running a car too lean has a tendency to melt/blow up/destroy car engines. I think it is important to keep in mind that there are a lot of other variables involved. I personally have seen 3 engines blown up because of improper air to fuel ratios(thankfully they weren't mine)
gmax137
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#10
Aug1-08, 06:26 AM
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Thanks to nucleus for that link in post #8. A very interesting figure, worthy of a good look.


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