General Leaning for power

[Zufelt]

What's going on guys. I'm in class (for Aeronautics) and we started talking about AFR's. My teacher stated that MAX power will be when your burning a little rich.. I've always thought running a little lean will make more power, but can't explain my reasoning aside from that's what I have read.. Does anyone have am explanation? Any help would be greatly appreciated. This is at any throttle position.

 

[2gmitsueclipse]

Zufelt,

I will side with your professor on this one. From my understanding, (using gasoline) the optimal mix (stioch) would be 14.7g of air to every 1g of fuel. This can be a hard figure to meet under all conditions. It is easy to add fuel, but you can only cram so much air into the engine. The goal is to burn as much of the oxygen as possible to extract the energy out of the fuel. Optimally, you only need a stoich mixture for optimal burn. However, things aren't always perfect, so going slightly rich (or adding a little more fuel) will ensure that you burn as much oxygen as possible. A lean condition will not allow for a complete burn of the oxygen. The other benefit of running slightly rich happens to also help reduce temperatures as well.

I am going under these definitions of lean and rich:

• Rich: fuel mixture greater than needed to obtain stoich.

• Lean: Fuel mixture less than needed to obtain stoich.

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[Chumpaumpalumpa]

Zufelt,

I will side with your professor on this one. From my understanding, (using gasoline) the optimal mix (stioch) would be 14.7g of air to every 1g of fuel. This can be a hard figure to meet under all conditions. It is easy to add fuel, but you can only cram so much air into the engine. The goal is to burn as much of the oxygen as possible to extract the energy out of the fuel. Optimally, you only need a stoich mixture for optimal burn. However, things aren't always perfect, so going slightly rich (or adding a little more fuel) will ensure that you burn as much oxygen as possible. A lean condition will not allow for a complete burn of the oxygen. The other benefit of running slightly rich happens to also help reduce temperatures as well.

I am going under these definitions of lean and rich:

• Rich: fuel mixture greater than needed to obtain stoich.

• Lean: Fuel mixture less than needed to obtain stoich.

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I think you mean burning fuel. Oxygen doesn't burn. You need to burn all the FUEL off and that requires an oxidizer (O2 in the air). The way I understand is that if you have more oxygen per the same amount of fuel, you will make more power (Think Nitrous) because more of the fuel is able to be burned ergo more energy is being extracted, but it will be hotter (more energy extracted) which can lead to bad things including loss of power. There's a balance to it. Too rich or too lean will make less power. Also you have to consider knock. If you could fully burn all the fuel off at 14.7 without knocking, you will make the most power. This is because that is where the fuels energy is extracted most efficiently. Since we can't do this, we must richen it up to provide knock suppression and cooling to the motor. The interesting thing is that it depends on the atomization of the fuel as well. If the fuel is not atomized well, you will need to run richer to get the same amount of fuel to combust as with a well atomized fuel. It also has to do with how well the fuel is mixed in the cylinder. Honda was achieving some 20:1 AFRs safely around town because the fuel was really really finely atomized and because 1 of the 2 intake valves would open creating a swirling action which mixed the fuel and air better. Both of these things allowed much higher efficiency of extracting the energy out of the fuel without knocking or creating too high of temperatures.

Where did you pull that chart from?

Now OP when you say leaner mixture do you mean above 14.7 or above what we normally tune at due to knock?

Most NA cars and newer turbo cars on gasoline make the most power at 12.1-12.8. This a lot of the time is due to needing knock suppression and cooling.

If you've ever tuned nitro RC engines its really easy to see this principal work on a basic level. You could feel the difference in heat and see and hear the car accelerate faster with a leaner mixture. Of course that's not very scientific but is something more relatable to more people.

 

[Zufelt]

Hey guys, thanks for the reply. We did talk about stoich ratio being 14.7, and that it was the most efficient. My thoughts are this.. Taking our cars and running a 10.2 AFR at WOT will yield you LESS power than running 11.5 at WOT. Why is that? By what was said I would have thought the richer mixture would yield more power. Also if 14.7 is the optimum mixture, why don't we aim for that under WOT?

 

[Chumpaumpalumpa]

Hey guys, thanks for the reply. We did talk about stoich ratio being 14.7, and that it was the most efficient. My thoughts are this.. Taking our cars and running a 10.2 AFR at WOT will yield you LESS power than running 11.5 at WOT. Why is that? By what was said I would have thought the richer mixture would yield more power. Also if 14.7 is the optimum mixture, why don't we aim for that under WOT?

I explained this in my post. Read it again and here is a summary of your question:

11.5 Has more air comparatively to 10.2 so more fuel is able to be burned off and the energy extracted from. We can't hit 14.7 under WOT because of excessive heat and knock. Adding more fuel draws heat out of the engine because it is a medium to transfer some heat to and goes out the exhaust taking the heat with it. Tuning for 11.0-11.5 is pretty common in this platform. Keep in mind we are discussing gasoline and that E85 will have much different needed ratios.