Barometric correction

[botasjr]

hi

im reading about barometric correction, and im more and more confused,

im my mind if the atmosferic pressure rise, i need more fuel, i have more oxigen, i need more fuel, is like a boosted engine, when the boost rise, more fuel i need.

is that correct?

the best way is to travel the car all around the world, or put it on a controleed chamber.

i tune at a value of 955 - 960

what is the correct picture 1st or 2nd?

do i need to tune barrometrico correction on a turbo engine?

regards


http://fotos.sapo.pt/xpower/fotos/corre ... OzNttGpIIZ

http://fotos.sapo.pt/xpower/fotos/corre ... zBOzr30tZc

[dancrev]

im my mind if the atmosferic pressure rise, i need more fuel, i have more oxigen, i need more fuel, is like a boosted engine, when the boost rise, more fuel i need.

I have a similar question.

The larger barometric pressure means you need more fuel, but if you are using speed density then the manifold pressure is being measured. If the manifold pressure is measured, then why is barometric correction necessary? The density is being calculated from measured quantities. So if MAP is 50kPa at my elevation (7000 feet) or at sea level, it's still 50kPa... just at a different throttle position. I don't think the percentage of oxygen per volume of air is changing much over drivable elevations, so no correction is needed for that.

From reading "The Internal Combustion Engine in Theory and Practice" by Charles F Taylor, one thing that does change is the volumetric efficiency of the engine based on the difference between MAP and exhaust pressure. Basically, the residual exhaust gas in the cylinder is roughly barometric pressure. Then when the intake valve opens, there is more pressure in the cylinder than in the manifold and exhaust gas first flows backwards before turning around and flowing into the cylinder with the new air. Those dynamics are tuned into your VE table for the particular barometric pressure (elevation) that your table was tuned at. As you change elevation, the VE values need corrected as a function of MAP and Barometric Pressure. If anything, it seems to me that barometric correction should be a table (MAP vs Baro) instead of just a curve depending on Baro alone. From the equation in that book, the largest difference is at small MAP/Baro, and becomes negligible as you approach wide open throttle.

Edit:
Actually, since the correction is a function of MAP/Baro, it probably could just be a curve based on that quantity (MAP/Baro). Doesn't need to be a table.

-Dan

[billr]

Dancrev, "baro" is a highly-opinionated subject (as well as IAT comp!), but I agree with you. With SD the engine doesn't care if MAP is 50 because of the throttle or because of the altitude; at least as far as the mass inflow is concerned. There are other factors, like reduced exhaust back-pressure (better scavenging and reduced shaft work) when baro is lower and less shaft work and inlet-air cooling caused by the throttle when baro is lower, but my perception is that those are very minor effects and can be ignored for most "non-racing" purposes. Perhaps the VE will be slightly different too, but that isn't clear to me, and I don't think it would be much if it is true.

[pit_celica]

So if MAP is 50kPa at my elevation (7000 feet) or at sea level, it's still 50kPa... just at a different throttle position.

I think that this sentence is where the problem is in your understanding.

If all the conditions are the same (MAP, RPM, IAT, CLT, steady-state (no accel/decel)), then the calculated fueling PW is the same, no matter the elevation/baro press. So, why do you need different fueling at different elevation/baro press : TPS will be different. The problem is that a speed density system won't know the different because it's not using the TPS except for accel/decel.

Different TP will mean different air flow, which require a fueling variable to be added to the equation. In our case, this is the Gbaro term. The main problem with the Gbaro term is that it needs to account for 2 differents physical changes : intake air density and variation in exhaust pressure at the exhaust valve. Both of these need to be taken in account. The default table only applic the first one (change in air density) and not the second one (exhaust pressure at the exhaust valve).

It's hard to come with a default table that can satisfy both of these conditions, because the second one is different for every engine (exhaust system, forced induction or not, exhaust valve specs, etc). This is why you should "tune" the barometric correction curve using real data :
-tune the VE table at a specific elevation/baro press
-get the car at another elevation/baro press and check the difference in fueling
-adjust the barometric curve for this baro press
-repeat at other elevations/baro press

Sam

[racingmini_mtl]

Different TP will mean different air flow, which require a fueling variable to be added to the equation. In our case, this is the Gbaro term. The main problem with the Gbaro term is that it needs to account for 2 differents physical changes : intake air density and variation in exhaust pressure at the exhaust valve. Both of these need to be taken in account. The default table only applic the first one (change in air density) and not the second one (exhaust pressure at the exhaust valve).

I agree with your process to get the correct correction but what you wrote above is not correct. Under the conditions you mention you do know the intake air density with MAP and temperature. What you don't have is the variation in VE due to the barometric pressure. This is what the GBaro term corrects and it includes (or should if correctly set) the exhaust pressure but also any other factors that affect VE such as throttle position, crankcase pressure, ...

So the correction is for the different physical changes that affect VE due to a change in barometric pressure. You already have the information on the pressure and temperature directly measured so the only remaining one to compute the mass of the air going to the cylinders is the volume. This is where VE and Gbaro come in.

Jean