AFR's - Talk to me

[Sean1933]

Very eye opening, thanks John!

[JohnA]

I'll elaborate further:

 

In an *ideal* world max power would be available not far from stoich (14.7 for petrol) even at full boost. No excess oxygen, no excess fuel.

 

Of course we don't live in an ideal world, so there are inefficiencies everywhere and we try to patch them up. The most prominent of these 'botches' is thermal-related:

Chargecooling: it goes over ambient and we have detonation? No problemo, we'll squirt a bit more fuel and it will absorb some of this heat (but it won't burn, because there not enough oxygen molecules available)

Also fuel distribution*: not trusting all injectors to be delivering the exact same quantity per cylinder? No problemo, we'll squirt a bit more fuel in general, so we'll make sure no cylinder runs less than stoich. (but it won't burn most of the time because it is excessive by design!)

Fuel atomisation: dodgy patches and turns? Does it fall out of suspension? No problemo, we'll throw in some extra in order to make up for this during cold days or whatever

 

You get the idea: for all sorts of potential deficiencies, we add fuel to mask the problem (real or imaginary). So we feel 'safe' and all is well.

It makes economic sense since the fuel system is already there, has extra delivery capacity and we simply use some of that.

 

But let's not forget whythe extra (over and above stoich) fuel is there;) The excess fuel is not free or totally inert either, so it costs in ultimate power and emissions.

 

With this in mind, we can better regulate the need for excess fuel in the combustion chambers. There is no need to run pig-rich if intercooling is really good, fuel atomisation is also really good and delivery is uniform.

Even more so if we can substitute the in-cylinder cooling function of the excess fuel (it is not very good at that, water is much better at it)

 

That's why I don't agree with the one-size-fits-all AFR theories.

The only AFR that I see as a 'standard' is stoich (where every oxygen molecule finds a fuel partner and vice versa)

How much excess fuel is wise to have under boost is dependent on variables that need to be clearly listed.

 

 

* Also let's not forget that the wideband only shows the average of all cylinders. If one is running 16:1 (blocked injector) and the others at 10:1 we'd be looking at a 'safe' 11:1, wouldn't we

[Ryan.G]

Interesting read John i must say while training at motec i found very different results to what you have found with regards to A/f and power output but as you say every car is different and the Water/meth on yours will certainly be helping keep the cylinder temps down at the Leaner mixtures.

[JohnA]

...while training at motec i found very different results to what you have found with regards to A/f and power output

This is not surprising, as training at Motec would focus around specific fuelling needs for specific conditions.

The fact that there can be safe (and cleaner) power at 'lean' AFRs (close to stoich more accurately) is not new. There are several SAE papers on the subject and it is the main reason behind the manufacturers' trend towards direct injection. The closer to stoich under load, the easier it is for them to meet the new emissions regime. They avoid excess fuel that won't burn at all or won't burn completely.

 

... the Water/meth on yours will certainly be helping keep the cylinder temps down at the Leaner mixtures.

Yes, by design. Water is four times more efficient in absorbing excess heat during the compression stroke, so it's a good start.