Why do high performance engines run rich?

[WhiteBlue]

,,You make the most power when it's as lean as you can get away with....

And it obviously depends of the injection technology, the ignition technology and the fuel chemistry how lean you can go. To optimize this point modern engines use ion flow measurement. It gives an optimum signal curve for each cylinder at best combustion. Those engines are dynamically optimized from shot to shot on each cylinder for leanest combustion and best ignition point.

[strad]

No..somewhere in the 13-14:1 range..whether you use injection or carb.

[xxChrisxx]

No..somewhere in the 13-14:1 range..whether you use injection or carb.

That's running rich though?
When you say max power is as lean as you can get it, do you mean acutally running lean? Or running leaner that you could(but still rich)?

I was going to make a technical style post on this, but don't really have the time to.

[WhiteBlue]

No..somewhere in the 13-14:1 range..whether you use injection or carb.

I don't see many high performance engines using carb in the future. Probably only in historic formulae. The above obviously relates to port injection and spark plug ignition. Future HP engines will almost certainly be turbo charged. NA is largely a thing of the past if you look at specs of HP cars like the McLaren MP4-12C or the Ferrari 458 Italia. Things will look different with turbo, high performance direct injection, variable valve timing and lift, laser ignition and ion current measuring. The optimum should be slightly over stoichiometric in that case for full power homogeneous combustion. So 14.7-15.3 should be more likely. Future racing engines will follow the trend in commercial high performance cars.

In part load conditions there could be significant stratification taking lambda up to 18 - 20. This does not take into account tricks like retarding ignition for blown diffusors.

[machin]

A lot of us on here have first hand motorsport experience... and I simply don't want people going away with the wrong info: Slightly rich is better for full power.

The engine is nothing more than an air pump

...I'm sorry, but you've taken that quote to its absolute extreme and come up with the wrong answer... you can think of an engine as an air pump when determining certain characteristics of the engine... but it ISN'T AIR AIR PUMP... An air pump takes power in to move air. An engine puts out power by BURNING fuel and air (actually, just the oxygen)...

Its easier to put more fuel into the mix (using carbs or injection), but it gets increasingly hard to get as much air as you can into the engine. To get as much power as possible you need to burn as much of the oxygen as possible to release the energy in the fuel. In order to ensure that you burn all of the oxygen possible scientifically you need only a stoichiometric mix.. in reality all of the oxygen molecules won't "find" a fuel molecule to react with so you need to go slightly rich to make sure they do...


As a reference, Below is a fuel ratio meter made by Alpha for the motorsports industry. It measures free oxygen levels in the exhaust gas to determine what air:fuel ratio you are running at. Best power is achieved when the green lights are illuminated, i.e. a lambda reading of approx 0.9.



Spotted you're mistake I think:-

No..somewhere in the 13-14:1 range..whether you use injection or carb.

FYI... an Air:fuel ratio of 13-14:1 is RICH!!!! it tells you that you have less air than in a stoichiometric mix.... (i.e. relatively more fuel).

[strad]

I think we're talking real world vs theoretical world..
Doesn't matter.

[xxChrisxx]

I think we're talking real world vs theoretical world..
Doesn't matter.

Erm, no. We're talking about reality, you can't sell this off cheap as 'theoretical vs real'. Not to put the squeeze on you, but you've just realised you've dropped a clanger and are trying to skirt round the issue.

The 'theoretical world' as you put it defines the real world. They aren't two entirely seperate entities, one of which is on paper the other is real. There are subtleties that mean the two aren't identical, but the basic facts of the matter are the same. 13-14:1 is rich and slightly rich gives max power. A lambda of about 0.9 (as shown in that nice little picture) is ideal for max power.

I can gaurantee you that if you have two identical engines, fuel one at 15.5:1 AFR, and one at 13:1, the 13:1 will make more power.

[strad]

Not at all...
For one, I thought we were talking in the narrower confines of a racing engine .
Two I've already tried to explain there has to be some miscommunication here cause you're not dumb guys..Surely you realize all mixture settings are going to be affected by things like intake tact..length,,surface,,efficency..things like the shape of the piston top..all of which affects how well said mixture burns because of how well or not the fuel mixes with that oxygen you mention..
I stand by my statement..You keep making more and more power the leaner you go,,til you go too far and burn her up...parts are expensive and you can't win with burnt up internals so ya back off a tad toward the richer side, but as little as you can get away with,,Although I will say that in drag racing you are apt to push it a tiny bit further toward rich, because you will be less sensitive to changes in the air.
Now since I really don't like these Internet debates that only serve to up my blood pressure (and yours apparently)and won't change either of our minds let's drop it and you believe what you will...and I'll take the gold and

[strad]

I think we're talking real world vs theoretical world..
Doesn't matter.

I can gaurantee you that if you have two identical engines, fuel one at 15.5:1 AFR, and one at 13:1, the 13:1 will make more power.

Isn't that what I've been saying??????

[machin]

I think we're talking real world vs theoretical world..
Doesn't matter.

I can gaurantee you that if you have two identical engines, fuel one at 15.5:1 AFR, and one at 13:1, the 13:1 will make more power.

Isn't that what I've been saying??????

Strad, I'll say it again; An Air:Fuel ratio of 13:1 is RICH!!!!!!!

[xxChrisxx]

Isn't that what I've been saying??????

As machin has been saying, that's running rich, not lean.

You seem to have learnt all this the old school way, I've found that it causes problems becuase although you know what you mean, the 'non-standard' way of trying to convey it makes it sometimes difficult to understand.

That's why I tried to clarify a few posts ago, as what you were saying was contradictory. You were saying that max power was got when running as lean as possible (which suggests you mean acutally running lean), but then saying AFR of 13:1.

[machin]

Strad.... I've figured it out... probably what you/your team did was set your engines up initially really rich (say lambda 0.8, to make absolutely sure you don't burn through your exhaust valves etc), and then you progressively reduced the fuelling level (in effect you made it more lean than your starting point, but still richer than stoichiometric) until you found the high point in your BHP (at around lambda 0.9 -check out the gauge on alpha meter in my earlier post), and then you stopped. So you did "lean it out", but your engine would still have been running rich when it gave max power.

That to me that sounds like a plausible explanation for your observations. Its also the safest way to find the correct A:F ratio when running an engine for the first time.

[G37Sam]

Perhaps the definitions of rich & lean aren't the same to everyone

When I had my cars AF's read out before getting her tuned, it read out to be in low 14s to high 13's @ WOT. The tuner told me: "Dayum son, your cars running too lean, definitely needs a tune"

[xxChrisxx]

Perhaps the definitions of rich & lean aren't the same to everyone

When I had my cars AF's read out before getting her tuned, it read out to be in low 14s to high 13's @ WOT. The tuner told me: "Dayum son, your cars running too lean, definitely needs a tune"

Thats like saying a circle isn't the same to everyone, becuase I don't define the circumfrence of a circle as Pi*D.

The phrases lean and rich in this context have rigid definitions. Lean is any AFR greater than stoichiometric, and rich is any AFR less than stoichiometric.

There really is no play in this, otherwise you can just make up any old --- you want and pass it of as your own 'definition' when it doesn't tally with what everyone else uses.

It's for precisely this reason why people don't tend to talk in tems of lean/rich when being technical. They will either explicity state the AFR or give an equivilance ratio, or use lambda.

[G37Sam]

Totally agree with you Chris, perhaps we need to bring together all mechanics and tuners and explain this to them