Why do high performance engines run rich?

[747heavy]

thanks riff-raff
for the explaination, but I´m not quite sure to which statement it is relatet.

Me saying "temperature has something to do with it" was in response to Caito´s question:
>>
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.
<<

as I think, that additional fuel will lead to lower temperatures and therefor limit the termal efficency.

But if this is wrong, I´m happy to be corrected, and to learn something new.


The second part, was just to point out that the stoichiometric ratio -> theoretical optimum combustion, is depending from the fuel type used and that 14.7 is not a universal ratio for all fuel types.

Not more not less

[strad]

Heavy, I'm not sure, but I think we have two types here and one is the guy that thinks he can go faster by bolting dual quads on his stock small block.
We may have argued over my terms but....Any more fuel than necessary will cool things off and by doing so make less power. Right???

[machin]

Any more fuel than necessary will cool things off and by doing so make less power. Right???

Yep, -a little bit richer than stoichiometric gives you more power, any more or less than this and you will see you move away from the peak and power will drop off. -It is safer to go slightly slightly richer tho to save the internals (i.e. the cooling effect from the extra fuel) than it is go slightly leaner.

Oh, that's for petrol....

Diesel is the opposite.... more air (leaner) makes the engine run cooler (and cleaner).

[xxChrisxx]

Any more fuel than necessary will cool things off and by doing so make less power. Right???

Now we get to the inevitable phrase 'it depends'.

On face value you make max power with an equivilance ratio of 0.9 which corresponds to an AFR of about 13.1:1. This is slightly rich, but in reality, due to imperfect mixing is the condition where you get all the fuel to combust.

As you say, all things being equal either side of this value will give you less power. The leaner you go, not all the fuel will be combust or you aren't burning all the fuel you could do, much richer and you don't get as good a combustion.

However, if running more rich allows you to do something else to increase power, gains may be found. So for example if you go to 12.9:1, and that allows you to increase compression, power gains may be found. This is why it's common in highly turbocharged cars to run very rich, the extra fuel actively hinders combustion but the cooling effect allows higher boost pressues to be run so there is a net gain.

As a general rule i've found that anything richer than 12.5:1 (ish) will yield no power gain no matter what you do, but will provide cooling.


Like all things motorsport, this stuff is all on a knife edge and is only valid for very specific cases. There really isn't an all sweeping rule of thumb that works.

[riff_raff]

The reduction in peak cycle temperatures due to the increased latent heat effect of a slightly rich intake charge would be minimal. If what you want to achieve is reduced peak combustion temps, then cooled EGR is a more reliable approach. Exhaust gas is composed of mostly water, carbon dioxide and nitrogen. Thus it is chemically inert and would not contribute to the combustion process or change the effective equivalence ratio. It would however reduce peak combustion temps by absorbing heat along the flame front.

Excessively rich mixtures do not necessarily equate to lower temperatures within all parts of an engine. If the exhaust gas contains enough unburned hydrocarbons, these can quickly reignite within the exhaust ports or exhaust pipes when encountering sufficient oxygen from the header reversion flows. The exhaust burning can overheat exhaust primaries and turbocharger turbine wheels.

Enjoying the discussion.
riff_raff

[machin]

you make max power with an equivilance ratio of 0.9 which corresponds to an AFR of about 13.1:1. This is slightly rich, but in reality, due to imperfect mixing is the condition where you get all the fuel to combust.

Just to clarify, I think you meant "air" here... you need to put a bit more fuel in to make sure that you combust all the air, as its air which is the limiting factor to an engine at wide open throttle (its easy to inject more fuel).

[xxChrisxx]

you make max power with an equivilance ratio of 0.9 which corresponds to an AFR of about 13.1:1. This is slightly rich, but in reality, due to imperfect mixing is the condition where you get all the fuel to combust.

Just to clarify, I think you meant "air" here... you need to put a bit more fuel in to make sure that you combust all the air, as its air which is the limiting factor to an engine at wide open throttle (its easy to inject more fuel).

Yeah I did mean air.

[alelanza]

Very nice topic

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.

In addition to what´s been already said, i´d imagine that very rich in a non direct injection engine, would mean that your fuel starts taking up volume that would have been otherwise occupied by oxygen, so now you end up with fuel molecules having other fuel molecules adjacent instead of oxygen, the more this happens the less power you make i´d imagine. Instead of having unburnt oxygen as in the lean case scenario, now you have unburnt fuel that has displaced oxygen. I may be wrong so anyone feel free to correct.

[riff_raff]

alelanza,

The power output of a N/A piston engine is affected by the volumetric efficiency of that engine. Or more specifically, the mass of oxygen trapped within the cylinder at each engine cycle. Since a cooler intake charge is denser, it also naturally has a greater mass of oxygen. The fuel injected into the intake airflow absorbs energy as it evaporates (ie. its latent heat), thus lowering the airflow temperature and increasing its density and oxygen content.

This is why you typically see the fuel injectors of F1 engines way out at the end of the intake manifold runners. This maximizes the fuel latent heat effect and results in the highest density intake charge flow past the intake valve/seat restriction.

A DI engine has higher intake air mass flow due to the fact that the fuel mass flow, which amounts to about 7% of the intake mass flow, does not have to pass by the intake valve restriction. This 7% increase in intake mass flow and engine power would amount to about 50hp on a typical F1 engine.

Regards,
riff_raff

[alelanza]

Thanks for that Riff Raff. Now, I imagine that at some point, despite the higher density allowed by the cooler charge, the fuel itself displaces enough air that the resulting mixture sucked by the cilinder (or pushed by the atmosphere some would say ) is oxygen starved for the lack of a better term?
I guess the colder the ambient tempearature is, and/or the warmer the fuel is, the sooner this situation is encountered?

[747heavy]

how much, does the mixture cooling effect in a port injected engine increases the air mass Riff_Raff?
What is the delta T of the mixture? Any guess?
I understand, what you say ( Ithink ), so a DI injected engine will "suffer" in hot conditions, because it´s advantage will become less.
As I understand it (which is may wrong) a engine sucks a given volume of air, the temperature of the air will account for the oxigene content, in terms of mass.
Cooler air -> higher density -> higher oxigene content in terms of oxigene mass pe cylinder charge.
So if this is correct, a DI engine will "loose" part of it´s advantage as higher the ambient temperature - No?

[ringo]

Yes it will , but i think what was said is that even though the fuel is absent in the charge coming pass the valves, taking away the advantage of cooling the air, the absence of fuel molecules in that 7% of the volume is replaced by air molecules.
This replacement of air, an increase of 7% of the total volume, is greater than the increase that a temperature change would garner.

We can look at the latent heat of vaporization of the fuel for a certain pressure.
This heat value is taken from the air. Knowing this we can find the temperature drop of the air from the specific heat value of air.
What's interesting is the difference between the cylinder pressures where the DI will inject the fuel, and the intake manifold or ahead of the throtle plates pressure. This should affect the vapour pressure of the fuel.

[riff_raff]

As ringo points out, the trade-off between the denser intake charge produced by the latent heat effects of port injection versus the accompanying reduction in air mass flow displaced by the port-injected fuel mass, is very complex.

The critical parameter for engine performance is mass flow, not volume flows. Injecting fuel ahead of the intake valve creates a cooler, and thus denser intake charge, due to the latent heat effect of the fuel. This also improves mass flow past the intake valve. But that fuel mass also displaces about 7% of the oxygen mass flow fraction that must pass by the intake valve, so there is a trade off.

With GDI, the fuel mass does not have to flow past the intake valve. So the intake air mass flow should be about 7% higher, all other things being equal. And engine power should be about 7% higher.

Regards,
riff_raff

[747heavy]

Yes, I understand this.
But as the air would be hotter, less dense in the case of the DI engine (for a given ambient).
I don´t think, we have the full 7% increase - hence the question.

[riff_raff]

747heavy,

"I don´t think, we have the full 7% increase - hence the question."

I would agree, thus the caveat "all other things being equal". Which of course they never are, especially with engines, right? All recip engines are heat engines, who's efficiency is a product of how effectively they convert the thermal energy of combustion into useful work at the flywheel. The GDI engine recovers some useful amount of heat energy through the latent heat effect, more so than a port injected engine since the intake air charge is at a higher temperature when the fuel is injected.

As for the cooling effect of the fuel latent heat, that depends upon the fuel chemistry, among other things.

Regards.
riff_raff