Why do high performance engines run rich?

[autogyro]

If you are setting up a race engine using a dyno and tuning has little to do with it, AFR is of secondary interest.
The things you set up to are the torque and bhp maximums.
AFR, lambda and other tailpipe gas measurements define other requirements.

[xxChrisxx]

If you are setting up a race engine using a dyno and tuning has little to do with it, AFR is of secondary interest.
The things you set up to are the torque and bhp maximums.
AFR, lambda and other tailpipe gas measurements define other requirements.

Say whaaaa? Auto, you describe things in such confusing ways. I'm unsure of the point you are making, could you clarify a little please?

AFR and lambda (not the measured exhuast value - they really should use another greek letter for that) or phi (equivilance ratio which is 1/lambda) are just ways of describing the fuel to air ratio, nothing more.

[strad]

Sorry..been gone..health issues,,scared myself only to find out it was severe vertigo..Which I'm sure you guys would say just figures, since ya don't think I know my ass from a hole in the ground.
Look this just plain has to be miscommunication..13:1?? 14:1?? to me these are just numbers. When I first put them out it was just as an example and really something learned 40 years ago....What I think..what I believe and what I have used successfully would if displayed as a line with real close to fouling plugs rich on one side and burn up lean on the other it would be about 3/4s of the way to the lean side.
Now as I said before..numbers mean nothing,,quite frankly jet sizes etc mean nothing. What really counts is how well the intake tract keeps the air/fuel mixed well and a piston shape that allows good flame propagation and complete burn. I don't care what the numbers say...I care what those plugs look like,,how the pistons are colored when I pull it back apart...stuff like that...so can we drop it please?
No performance engine runs RICH..quite frankly rich is a term of being out of adjustment and we don't want our competition engines out of adjustment..they run just right.
BTW...Thank you for your input and please know that I rate your opinions high enough that I did go out and talk with two of the top drag racers I could get in touch with and they agreed I was right..thus the reason I think I am just saying (maybe) what you're saying,,,but different.

[xxChrisxx]

Look this just plain has to be miscommunication..13:1?? 14:1?? to me these are just numbers.

You've worked with engines for 40 years, and don't know about air to fuel ratios? Really? Then how do you even try to meter fuel flow on a freshly designed engine, just blind trial and error? Science > serendipity.

Now as I said before..numbers mean nothing,,quite frankly jet sizes etc mean nothing. What really counts is how well the intake tract keeps the air/fuel mixed well and a piston shape that allows good flame propagation and complete burn.

So we have a huge engine displacement and tiny jets, whats going to happen?

I don't care what the numbers say...I care what those plugs look like,,how the pistons are colored when I pull it back apart...stuff like that...so can we drop it please?

You know, we used to gauge the temperature of steel by it's colour, now we use digital infrared thermometers. It's called progress.

No performance engine runs RICH..quite frankly rich is a term of being out of adjustment and we don't want our competition engines out of adjustment..they run just right.

Which is a statement, if you are using correct terminology, is just plain wrong no matter how you spin it. As stated before if you are just going to make up terminology then all form of meaningful communication breaks down.

BTW...Thank you for your input and please know that I rate your opinions high enough that I did go out and talk with two of the top drag racers I could get in touch with and they agreed I was right..thus the reason I think I am just saying (maybe) what you're saying,,,but different.

This is a technical forum, therefore we have technical posts. Your two mates may think you are right, but every single engine design book in the land disagees with you.

[xxChrisxx]

Chris
Ya know, even if you were right, you're one hell of a jerk.

EDIT: I've got rid of a long winded responce to this comment, as it's not really relevment. Over the next few days i'll make a technical post just explaining Why it is that maximum power is obtained when running slightly rich.

[riff_raff]

As edis noted, a gasoline (SI) engine will make best power at or near stoichiometric mixtures, all other things being equal. This becomes apparent when one considers the basic process of combustion chemistry. With a homogeneous charge like that in an SI engine, the maximum heat release (and thus available power) of the fuel mass combustion will occur at the stoichiometrically correct fuel/oxygen ratio. A leaner mixture means less oxygen atom pairs are available for combining with each fuel carbon atom (creating CO2). It's the breaking of these atomic carbon-carbon bonds during combustion that produce the thermal energy, thus an insufficient mass of fuel for a given intake mass flow of oxygen (ie. lean conditions) means that less than maximum thermal energy is being produced by the engine.

SI ignition timing is always mapped for MBT (Max Brake Torque), within knock limits. Knock hurts engine performance because it increases the amount of negative work the combustion gas produces on the piston ahead of TDC.

As for the original post asking why racing engines run rich, the reason is because they can. SFC and emissions are not a big concern with race engines. The reason you see exhaust flames from race engines during closed-throttle overrun is mostly due to how their ECU's work. Race engines typically use a more open loop fuel injection control strategy, which means they don't have the feedback from an intake mass airflow sensor that your typical production car has.

With the refueling ban and engine durability rules in F1, fuel consumption will become very important.

Regards,
riff_raff


Regards,
riff_raff

[machin]

Riff Raff, your post is a good one, although I would just want to clarify a couple of statements:-

A leaner mixture means less oxygen atom pairs are available for combining with each fuel carbon atom (creating CO2).

A leaner mixture is the opposite: one where there are less carbon atoms than there are oxygen pairs (i.e. more oxygen than fuel)

As for the original post asking why racing engines run rich, the reason is because they can.

..I'd say its because they can and because no real engine has a completely truely homogenous charge... it is impossible to achieve an exact mix in the cylinder, and therefore to ensure that all the oxygen is burnt (oxygen intake quantity being the limiting factor to an engine's power output at wide open throttle) a "little bit"* more fuel must be injected to make sure that all the oxygen is burnt, therefore the mixture must be slightly richer (than stoichiometric!) to achieve maximum possible power.

* The definition of a "little bit" depends on how homogenous the charge is, and this depends on a lot of things.. injector/carb design, inlet tract, injector timing, cam timing, etc. Good engine design probably arrives at a point where the charge is near homogenous, so the extra amount of fuel will be only slightly over stoichiometric, but it won't ever be under stoichiometric (what engineers call "lean") -because that means there's oxygen in your cylinder which you've spent a lot of effort to get in there (bigger valves, longer exhaust duration, intake and exhaust harmonics etc) which you're not using -and that's means there's untapped power from your engine.

..hence why a performance engine runs rich!

[autogyro]

Right on the point Machin.
Getting as much air in is what 'tuning is about'.
Setting up on a dyno is making the most use of it.
Adjusting in practice is making changes to suit conditions.
Getting fuel into the engine is never much of a problem.
Used to start one of my drag cars by pouring fuel into the intake with an oil can.

[strad]

Last post on the subject from me:
ANyone tuning a competition engine starts out rich to be safe and goes leaner and leaner until by checkin plug color etc decides he's gone too lean then backs it up a bit.
I guess it comes down to this definition of a "little bit"
I guess to you it's a little rich fine,,I say it's as lean as you want to go.
you will never in the combustion chamber,,or in the intake tract achieve this "stoichiometric" mixture.

[madtown77]

Anyone tuning a competition engine starts out rich to be safe and then leans it out UNTIL THEY START TO LOOSE POWER!!! Once you get close to stoich this will happen because of what machin said. You will always be running slightly rich, how close is dependent on how well you mix the charge and the flame propogation in your combustion chambers.

The only time a race engine runs lean is when they need to tune it down because of excessive fuel consumption.

[xxChrisxx]

I guess it comes down to this definition of a "little bit"
I guess to you it's a little rich fine,,I say it's as lean as you want to go.

Now that I agree with. It's rich but, learner than you can possibly go. If you check back I tried to clarify this in.

Do you see why that saying 'lean as you want to go' is totally different to saying 'the leaner you go, the more power you make'?

Relating this back to the topic at hand, do you also see why that you saying that all race engines run lean isn't correct?

you will never in the combustion chamber,,or in the intake tract achieve this "stoichiometric" mixture.

It's interesting you put the word stiochiometric into speech marks. As though it's some made up term. It's what defines if something is lean or rich. And you get a 'stoichiometric mixture' when you mix air with fuel in the ratio of 14:7:1 by mass. As that is the very definition of stoichiometric. It's where the chemical reaction that is the combustion of fuel is balanced.

[strad]

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

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 somehow missed this post...Answer...YES to the latter

Look Chris..I really feel it's like we are standing on this line from rich to lean and I'm saying you should be left of point x and you are saying you should be to the right of point y,,but the two spots we're pointing to are the same..I know I said I wouldn't post more but I just had to cause I really think this has been a huge misunderstanding...Did you really think I meant lean like LEAN? I mean within narrow confines of already close or however you might term it.
Now I'll shut up..much to your eternal happiness PAX

[Caito]

To sum up:

In an ideal world each 14.7g of air would combine with 1g of fuel. Since we can't achieve an ideally homogeneous mixture, to make sure all of our air reacts with fuel, we put more fuel than what would be ideally needed.

We will define rich as a mixture with more fuel than needed to achieve stoichometric figure. Lean is defined as a mixture with less fuel than needed to achieve stoichometric figure. In order to achieve maximum power our engine should run rich, but only rich enough to make sure all of the air has combusted. We could call this the least rich than it can run.


Being this point clarified, let me add a new question. If we are in the optimum point, where max power is achieved. How can it be than adding more fuel actually decreases this figure. I would have thought that it just adds fuel consumption but doesn't alter the power figures.



Thank you very much for all your answers.


Bye bye

Caito.-

[747heavy]

in part because the temperature will decrease.

in your sum-up you need to keep the fuel type in mind as well.
the ~14.7 figure is for gasoline, if you use other types of fuel
e.g. methanol (as in your RC plane engines) the figure will be different.

as a guideline:

Diesel 14.6:1 (diesel gets more complicated so i won't go there)
Gasoline 14.7:1
Methanol 6.4:1
Ethanol 9.0:1
Propane (LPG) 15.5:1
Methane (CNG) 17.2:1
Hydrogen 32.4:1

[riff_raff]

machin,

Thanks for the correction to my post. Sometimes I'm a bit dyslexic, but that's what I meant to say.

To make the whole rich/lean thing more understandable, consider it like this: An IC engine is a device for converting combustion heat energy into work. The amount of heat energy available from combustion to be converted into work is always based on the heat value (LHV) of the fuel mass. Lean combustion means less fuel mass per cycle, and thus less total heat energy released. And also by definition less power produced, all other things being equal.

747heavy,

Stoichiometric combustion temps are higher than lean combustion temps. That's why turbine engines cannot run anywhere near stoichiometric combustion. The TIT's would melt the turbine blades. SI recip engines tend to overheat with lean mixtures due to the excessive pressure rise that occurs with lean mixture detonation.

riff_raff