The basics. What AFR makes best power?

[Brew]

Its fairly simple, really. An engine will make the most power when it is using up all the available air. This tends to be at about 12.5:1, as you need a richer-than-stoichiometric mixture to be able to use up all the available oxygen.

[Insanity]

Its fairly simple, really. An engine will make the most power when it is using up all the available air. This tends to be at about 12.5:1, as you need a richer-than-stoichiometric mixture to be able to use up all the available oxygen.

This makes absolutely no sense. Stoichiometry means that your reactants combine perfectly with no waste. By your logic, the best power would be produced running as rich as possible, because you'd obviously use all the oxygen.

The problem with your already hideously flawed logic is that in order to completely vaporize the remaining fuel, it absorbs heat, which takes away power.

A lean burn makes your combustion temps higher due to how oxygen interacts with combusting things (fanning the flames effect). However, obviously running lean can introduce serious drawbacks.

Turbos run on hot gases as well, so running more heat per psi of air in the turbine is better as more heat will turn the turbine more.

Where did you hear that? Heat has no capability to turn anything by itself. If you could invent a system that converted heat directly to mechanical energy, you'd have the holy grail of automotive technology.

It's true that the heat of a gas and the kinetic energy of that gas are inexorably linked, however, the loss in efficiencies at higher temperatures outweigh the benefeit of the higher kinetic energy pretty quickly, hence why smaller turbos run out of steam at high exhaust gas volumes.

[mspiegle]

This makes absolutely no sense. Stoichiometry means that your reactants combine perfectly with no waste. By your logic, the best power would be produced running as rich as possible, because you'd obviously use all the oxygen.

I think he should have rephrased his statement to be "within certain limits". Obviously, trying to run a 5.0 AFR isn't going to help you. For one, you don't have enough oxygen to combine with the fuel, the sheer volume of fuel is probably displacing some necessary O2, and on an unrelated note - you're clensing the cylinder walls of their much needed film of oil.

Turbos run on hot gases as well, so running more heat per psi of air in the turbine is better as more heat will turn the turbine more.

I believe what he was getting at is that the additional heat excites the gas molecules to a point where they have a higher velocity and can perform "more" work. True, heat on its own won't make a turbo work (how about 300hp by sticking your turbo in an oven? lol), but the heat becomes a potentiator to the force moving the turbo (exhaust gas).

I don't think I agree with your statement that higher temperatures create a loss in efficiency. If anything, it is our inability to properly harness the energy from heat and do something useful with it. The most common example of heat being beneficial is with ceramic coatings on headers. You're essentially keeping the exhaust gasses hotter which increases velocity which increases scavenging. Another example is with fueling - If you heat fuel up, it is better able to vaporize and properly atomize with the oxygen in the combustion chamber. When your car is running cold, it runs significantly richer because the fuel can't atomize properly and you need more of it to get the same effects.

Now, obviously there is a limit to heat with regards to materials since our current technology only allows us to dissipate a certain amount of it. If you were able to make a graph of automotive cooling technology from 25 years ago to now, you would see a steady increase in the combustion chamber and coolant temperatures. The most modern cars often run 10-20 degrees celcius hotter because modern technology allows us to use this heat to our advantage AND our refined materials can cope with the heat.

Check out this cool link. It is an example of someone making better use of heat for increase efficiency: http://www.damninteresting.com/?p=467

[Insanity]

I think he should have rephrased his statement to be "within certain limits". Obviously, trying to run a 5.0 AFR isn't going to help you. For one, you don't have enough oxygen to combine with the fuel, the sheer volume of fuel is probably displacing some necessary O2, and on an unrelated note - you're clensing the cylinder walls of their much needed film of oil.

But the problem is, neither of you are understanding why a rich burn creates poor running conditions. Heat is the source of your power in an internal combustion engine, it creates pressure which drives the piston. Less heat = less power. Gasoline (as most liquids) have a very high heat of vaporization (the heat required to turn the liquid into a gas). Therefore, any gas that isn't combusted will absorb heat, and quite a lot of it.

I don't think I agree with your statement that higher temperatures create a loss in efficiency. If anything, it is our inability to properly harness the energy from heat and do something useful with it.

You're misunderstanding what I said... more heat is less efficient for a turbocharger. The actual device. It performs substantially worse when hot. Not only is significantly more friction created (which is a big deal when the shaft is spinning at 200,000RPM), but it also superheats the intake charge on the compressor side. That's why turbo cars will generally be tuned on the rich side, because you actually make more power with the lower EGTs by improving the efficiency of the turbo, even though the exhaust energy is lessened.

http://www.damninteresting.com/?p=467

Yeah, like I said, actually harnessing that heat energy instead of just bleeding it off in the cooling system would be an automotive holy grail. The cooling system ends up dissipating more heat from the combustion than your engine actually uses on the combustion.

[mspiegle]

But the problem is, neither of you are understanding why a rich burn creates poor running conditions. Heat is the source of your power in an internal combustion engine, it creates pressure which drives the piston. Less heat = less power. Gasoline (as most liquids) have a very high heat of vaporization (the heat required to turn the liquid into a gas). Therefore, any gas that isn't combusted will absorb heat, and quite a lot of it.

I think we actually agree on all these points, just not quite so much on the AFR range which makes a good amount of safe power.

[Insanity]

Well, as I said, the MOST power is definitely lean, due to the "fanning the flames" effect (I don't know the actual scientific terminology, because I don't think it's a catalyst, but something else), but it's certainly not the most reliable power. Definitely the best for emissions purposes (unless your state measures NOx, which will skyrocket).

14.0 is usually pretty safe, and leaves some room for timing screw ups without popping the engine.