Discussion on some tuning theory

[pauly24]

Ive been looking at the the tute posted by swing tan " E38 VVE Setup Guide"

And Ive got some questions about the methods used.

First, the VE table is volumetric efficiency, so it lets the ecu know the amount of volume of air that is going into the engine at each RPM and MAP point. But what the car really needs to know is how much mass of air is entering the engine. So it can match it with the right amount of fuel.

Because of the laws of physics (PV=nRT), you can have say 80% efficiency at 2000rpm and 60kpa, but this volume of air can bring in say 200grams of air or if the temp is hotter it can bring in only 180 grams of air (numbers are completely made up) but you get my point.

So I understand this is where the intake temp multiplier comes into play.

But when you dial in the VE table, you are setting the intake temp multipliers all to 1.

So if you went out and tuned your VE table on a cool night, your numbers will be different to if you tuned it on a hot night.

Effectively saying your Volumetric Efficiency of the engine changes depending on intake temps, but this is untrue.

The volumetric efficiency stays the same (Car takes in same amount of air regardless of intake temps), it's the density of the air that changes which gives the different AFR results.

And therefor the intake temp scalars are what needs to be worked on.

When the ECU works out the mass of air from the volume, it obviously needs to know the density of the air. This density of air will only hold true for one temp. And whatever temp this is that the density is worked out from is the temp that ones intake temp should be at, AND THEN you will have a TRUE VE table.

Now someone reply and tell me you understand what I just said :Eyecrazy:

[redhardsupra]

oh i completely understand everything you said ;) this is why i think the IAT multiplier is a completely wrong way of accounting for temp changes. the ideal gas law you've mentioned will account for that just fine. when calibrating you do have to attribute all the changes properly. some airmass changes due to temps, some to pressure, and some to efficiency. to calculate that efficiency, you gotta make sure that the pressures and temps are accounted for. the same thing goes for the source of airmass calibration--the fuel model. if your nonlinear fuel model isnt perfect, if your wideband doesn't read precisely, all the errors are gonna end up getting (wrongly!) attributed to the VE.

i hope you understand what i just said ;)

[joecar]

The VE table has units [g.K/kPa] meaning each cell is the cylinder airmass "normalized" for temerature and pressure.

The Ideal Gas Law says: P.V = n.R.T = (m/M).R.T

Rearranging: V.(M/R) = m.T/P

In this you see that (M/R) is constant, and so V is directly proportional to m.T/P which has units g.K/Pa (or g.K/kPa if you scale for Pa->kPa conversion).

The point is that temperature and pressure are already taken into account (i.e. VE[g.K/kPa] is divided by DAT[K] and multiplied by MAP[kPa] to arrive at cylinder airmass)...

That is why the IAT scaler is disabled.

[ of course, I am not sure what units the E38/E67 virtual VE tables are displayed in, but it seems to be related to g.K/kPa ]

[redhardsupra]

parametric VE is just a different representation of the GMVE table we know from E40s. once you crunch the numbers, GMVE comes out on the other end.

[joecar]

Marcin, ok thanks, that's what I thought.

[pauly24]

Arh ok, I believe this is where I got confused.

As you stated, when the ECM looks up a VE table point, the values are in g.K/kPa. It then looks at the MAP reading and multiplies this VE value by the map, then it looks at the IAT and then it multiplies this value by the IAT.
Then it has a value left in grams, which it can use to match against the fuel.

But after all that is done, the IAT scalar is something else that is multiplied through correct? I thought by removing the IAT scalar you are removing the part of the fueling equation stated above that multiplies the VE number by Temp to arrive at the fuel mass.

Its two completely different things, one is the ECM working out the mass of air by multiplying through the temp, the other is a scalar used to change the desired AFR against temp (ie like a choke)

Light bulb moment, thanks guys.

[joecar]

...
... the other is a scalar used to change the desired AFR against temp (ie like a choke)
...Does the scaler modify VE/airmass or desired/commanded AFR...? There's a difference.

[pauly24]

I don't have a table in mind, I was just talking in theory terms.

But it definitely is crystal clear now.