- Tue Feb 09, 2021 10:16 pm
#48199
A topic started in the STM32 topic but i hope to continue here. Below the start of it all
The mass of air and therefore oxygen entering an engine cylinder is determined by the pressure of the cylinder before closing the intake valve before compression (in a perfect engine this is the pressure before the intake valve (that is the MAP)) and the "fixed" volume of the cylinder . The mass/density of air is determined by the pressure AND temperature (there is the IAT correction), because cold air is more dense. See ideal gas law ideal gas law (PV=nRT) . Now we have
P=Cylinder pressure when closing intake valve (ideally this is MAP)
V=Cylinder volume
T= temperature
R = gas constant (does not change)
so n = can be calculated (this the amount of oxygen)
n is only affected by T and P(MAP) NOT by environmental or baro P in an ideal engine. This is used by tuner studio with your engine constants, desired AFR and injector flow to calculate required fuel. (with 100% VE and 100kpa MAP i assume)
P and V and T are all effected by scavenging, intake restrictions, intake duration of cams, heat soak etc.) but i see not why it is changed by barometric pressure.
If there is no intake restrictions and the cylinder is fully emptied by the exhaust stroke (infinite large inlet/outlet valve and no restrictions, cam overlap, intake closes when cylinder is perfectly down etc) the whole Volumetric Efficiency table should be 100% and follows the ideal gas law or in tuner studio therms Pulse width = MAP/100 * required fuel. No need for a special VE table anymore all cells 100% and the amount of fuel needed is calculated by the ideal gas law only. The pulse width for each intake stroke would be exactly determined by MAP only in that case. (A Turbo or compressor increases MAP above ambient increasing oxygen = more hp)
There are a couple of things influencing this
More cylinder intakes per second (higher RPM). Influences the cylinder filling time and therefore end cylinder pressure before closing the intake valve. So that is why the VE table has MAP and RPM axis. Still no barometric component. Temperature component is in the ideal gas law.
My main possible idea i have why there is a barometric effect
The back pressure on the exhaust is less with lower baro, making it easier for the exhaust gas to flow out. leading to more air be able to enter the cylinder? So in that case you would want baro correction maybe? But i assume the effect is neglect-able? Also the effect is the reverse of what is now implemented, more fuel for lower baro (no source sorry).
Other possible reason is oxygen content over altitude
The air gets more CO2 and N2 in them as a percentage of molecules the higher you go. (yes altitude sickness is a thing because at higher altitude leading to lower pressure and less molecules of oxygen per volume. still following (PV=nRT). This is NOT because of the part of oxygen per CO2 and N2 |(and other gasses) is getting lower i assume so also no altitude component there.
Why a carburetor is affected?
The carburetor uses the venturi effect see: http://ecoursesonline.iasri.res.in/mod/ ... php?id=677. So the system behaves differently. The venturi is before the butterfly valve and so the venturi itself is in the ambient (barometric) environment.
https://www.simscale.com/blog/2018/04/w ... ri-effect/ shows the venturi effect is influenced by mass of the flow passing the venturi. (and that is influenced by outside pressure). So maybe that is the reason? (not sure)
I do not have the knowledge on those. I do know airplanes with carbs can lean or richer fuel depending on altitude but i was always under the impression that was because better fuel economy (lean burn).
OEM ecu
My volvo 740 (MAF system) has no pins for a baro sensor any where. So no compensation i assume
My G16B Suzuki engine (MAP) also did not have OEM baro sensor pins. So no compensation i assume
This is why i put my baro correction to fixed (100%) also not effected by the first reading. That is also why i think baro correction as MAP/baro is absolutely not correct!
For alpha N this is different and you need baro correction.
So my question remains WHY does a MAP operated engine is effected by baro (in theory) because i see no influence of the baro pressure on engine operation using MAP readings, other than the exhaust pressure. So in my view baro correction should be a curve (for unforeseen very small changes) or disabled when using speed density algorithms.
PSIG wrote: ↑Tue Feb 09, 2021 1:42 am I apologize if this response is too far off-topic for STM32 dev.My main assumptions/theoryTjeerd wrote: ↑Mon Feb 08, 2021 10:24 pmI do live in a flat area so no baro issue.There is still a barometric issue even if staying at one altitude. This is called density altitude, and the baro pressure is affected by temperature and humidity, as well as altitude. This is why we use an intake temperature sensor, as another part of Speed Density fueling calculations based on the ideal gas law (PV=nRT) .Tjeerd wrote: ↑Mon Feb 08, 2021 10:24 pmAlso i think most cars with EFI have no baro sensor only MAP.Almost all do, including all OEM cars I've worked on. Most MAF cars also use a baro sensor to modify the MAF readings, as MAF cannot sense the pressure effects by itself. But, less-dense air would affect the MAF similarly to part-throttle, right? Not quite, in a similar way carburetors are also affected.Tjeerd wrote: ↑Mon Feb 08, 2021 10:24 pmAnd i still not sure what the reason for a baro sensor is. The MAP already reads a lower value and reduces fuel if it reads less dense air. So i still do not understand the theory why baro is necessary at all for EFI cars using MAP. .Because air has two commonly interchanged values of density and pressure. However, their effects are different, and carburetor users have had the same enrichment as you go higher (or weather causes lower density) for over 100 years. The effect on EFI is no different, as the lower pressure affects cylinder filling and exhausting, while the density affect available oxygen.
To use a carburetor analogy, they use air flow to pull fuel, and more air flow pulls more fuel. Cool. But while the air is lower density, it is also lower pressure with lower oxygen content. In order to offset all of these effects, aircraft carbs and some others use altitude-compensation that also corrects due to pressure effects, usually a big diaphragm housing on the carb or a crude manual mixture adjustment.
So, Speeduino (and other EFI systems) reads the baro value for relative effects at power-up. It even offers baro correction for Alpha-N (TPS-RPM) with no MAP sensor. Then it's corrected, but only at that moment, at that baro value. Real-time baro sensing allow ongoing corrections as weather or altitude change, with obvious benefits to correct fueling for whatever you're doing. I hope that helps.
David
The mass of air and therefore oxygen entering an engine cylinder is determined by the pressure of the cylinder before closing the intake valve before compression (in a perfect engine this is the pressure before the intake valve (that is the MAP)) and the "fixed" volume of the cylinder . The mass/density of air is determined by the pressure AND temperature (there is the IAT correction), because cold air is more dense. See ideal gas law ideal gas law (PV=nRT) . Now we have
P=Cylinder pressure when closing intake valve (ideally this is MAP)
V=Cylinder volume
T= temperature
R = gas constant (does not change)
so n = can be calculated (this the amount of oxygen)
n is only affected by T and P(MAP) NOT by environmental or baro P in an ideal engine. This is used by tuner studio with your engine constants, desired AFR and injector flow to calculate required fuel. (with 100% VE and 100kpa MAP i assume)
P and V and T are all effected by scavenging, intake restrictions, intake duration of cams, heat soak etc.) but i see not why it is changed by barometric pressure.
If there is no intake restrictions and the cylinder is fully emptied by the exhaust stroke (infinite large inlet/outlet valve and no restrictions, cam overlap, intake closes when cylinder is perfectly down etc) the whole Volumetric Efficiency table should be 100% and follows the ideal gas law or in tuner studio therms Pulse width = MAP/100 * required fuel. No need for a special VE table anymore all cells 100% and the amount of fuel needed is calculated by the ideal gas law only. The pulse width for each intake stroke would be exactly determined by MAP only in that case. (A Turbo or compressor increases MAP above ambient increasing oxygen = more hp)
There are a couple of things influencing this
More cylinder intakes per second (higher RPM). Influences the cylinder filling time and therefore end cylinder pressure before closing the intake valve. So that is why the VE table has MAP and RPM axis. Still no barometric component. Temperature component is in the ideal gas law.
My main possible idea i have why there is a barometric effect
The back pressure on the exhaust is less with lower baro, making it easier for the exhaust gas to flow out. leading to more air be able to enter the cylinder? So in that case you would want baro correction maybe? But i assume the effect is neglect-able? Also the effect is the reverse of what is now implemented, more fuel for lower baro (no source sorry).
Other possible reason is oxygen content over altitude
The air gets more CO2 and N2 in them as a percentage of molecules the higher you go. (yes altitude sickness is a thing because at higher altitude leading to lower pressure and less molecules of oxygen per volume. still following (PV=nRT). This is NOT because of the part of oxygen per CO2 and N2 |(and other gasses) is getting lower i assume so also no altitude component there.
Why a carburetor is affected?
The carburetor uses the venturi effect see: http://ecoursesonline.iasri.res.in/mod/ ... php?id=677. So the system behaves differently. The venturi is before the butterfly valve and so the venturi itself is in the ambient (barometric) environment.
https://www.simscale.com/blog/2018/04/w ... ri-effect/ shows the venturi effect is influenced by mass of the flow passing the venturi. (and that is influenced by outside pressure). So maybe that is the reason? (not sure)
I do not have the knowledge on those. I do know airplanes with carbs can lean or richer fuel depending on altitude but i was always under the impression that was because better fuel economy (lean burn).
OEM ecu
My volvo 740 (MAF system) has no pins for a baro sensor any where. So no compensation i assume
My G16B Suzuki engine (MAP) also did not have OEM baro sensor pins. So no compensation i assume
This is why i put my baro correction to fixed (100%) also not effected by the first reading. That is also why i think baro correction as MAP/baro is absolutely not correct!
For alpha N this is different and you need baro correction.
So my question remains WHY does a MAP operated engine is effected by baro (in theory) because i see no influence of the baro pressure on engine operation using MAP readings, other than the exhaust pressure. So in my view baro correction should be a curve (for unforeseen very small changes) or disabled when using speed density algorithms.