air flow modeling PV=nRT
05-05-2009, 08:17 PM
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air flow modeling PV=nRT Any system (e.f.i.) that models air flow uses the basis of the ideal gas law PV=nRT, to model airflow. I have calibrated/tuned e.f.i. vehicles for some time not really having to deal with this formula as long as the air mass was given to me on data. On mass air system the air mass is given directly granting that the maf is scaled correctly. Now on a speed density system i realize it is given to the user as well but knowing that the computer is using this equation there are some things I would like to know.
First I have been given information from several good sources that the constants used in the equation are easier to use SI units (pressure in kPa, Volume in Liters, Temp in Kelvin), then the value of R is 8.314.
That is not where I get hung up...
For the majority of the cars I tune, being factory ecu, hptuners is used. In the spark tables airmass in grams per cylinder was very acurate to tune with as long as I had a airmass output reading while data logging. But if noticed, g/cyl is ranged from .08 to 1.20 most of the time on the table. while I do like this way of mapping a table rather than manifold absolute pressure (kpa) vs. speed. I do not know why the g/cyl numbers are the way they are.
For instance, to my understanding V is the cylinder volume, the "airmass per cylinder". = pi x ((Bore/2)^2) x Stroke which gives me completely different numbers than that of on airmass tables in hptuners. where this ties into the paragraph above is using this what do I insert the the "V" variable. If anyone can help me with this it would be much appreciated as I am just aiming to better understand a few things.
First I have been given information from several good sources that the constants used in the equation are easier to use SI units (pressure in kPa, Volume in Liters, Temp in Kelvin), then the value of R is 8.314.
That is not where I get hung up...
For the majority of the cars I tune, being factory ecu, hptuners is used. In the spark tables airmass in grams per cylinder was very acurate to tune with as long as I had a airmass output reading while data logging. But if noticed, g/cyl is ranged from .08 to 1.20 most of the time on the table. while I do like this way of mapping a table rather than manifold absolute pressure (kpa) vs. speed. I do not know why the g/cyl numbers are the way they are.
For instance, to my understanding V is the cylinder volume, the "airmass per cylinder". = pi x ((Bore/2)^2) x Stroke which gives me completely different numbers than that of on airmass tables in hptuners. where this ties into the paragraph above is using this what do I insert the the "V" variable. If anyone can help me with this it would be much appreciated as I am just aiming to better understand a few things.
Last edited by avb0119; 07-21-2010 at 12:08 PM.
05-05-2009, 11:11 PM
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Thank you for the advice. Do you have any information relating to the reason the airmass values in tables are different than they would be when using the equation for airmas g/cyl?
05-07-2009, 03:25 PM
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Maybe you are not distinguishing between mass air
flow (g/sec) and cylinder air mass (g/cyl) properly.
To get from airflow to cylinder air mass you have to
get from RPM to cylinders per second. 4 cylinders
per revolution filled, makes it
4 cyl/rev * rev/min * min/60sec for a 1/15 RPM ->
cyl/sec factor.
So if you took a (say) 300g/sec airflow at 5000RPM,
your 5000RPM equates to 333 cyl/sec and your
300g/sec * sec/333cyl = 0.9 g/cyl.
Of course the real airflow and cylinder air mass are
going to differ from ideal by the volumetric efficiency.
It's also worth noting that the PV=nRT is an
-equilibrium- gas law, and the goings-on in a pulsed
flow system like your motor are a fair bit non-
equilibrated. You've got a bunch of gas expansion,
resonant gas columns and such all going on in-the-
moment. So if you get a really, really ideal result in
anything, be suspicious.
flow (g/sec) and cylinder air mass (g/cyl) properly.
To get from airflow to cylinder air mass you have to
get from RPM to cylinders per second. 4 cylinders
per revolution filled, makes it
4 cyl/rev * rev/min * min/60sec for a 1/15 RPM ->
cyl/sec factor.
So if you took a (say) 300g/sec airflow at 5000RPM,
your 5000RPM equates to 333 cyl/sec and your
300g/sec * sec/333cyl = 0.9 g/cyl.
Of course the real airflow and cylinder air mass are
going to differ from ideal by the volumetric efficiency.
It's also worth noting that the PV=nRT is an
-equilibrium- gas law, and the goings-on in a pulsed
flow system like your motor are a fair bit non-
equilibrated. You've got a bunch of gas expansion,
resonant gas columns and such all going on in-the-
moment. So if you get a really, really ideal result in
anything, be suspicious.
05-07-2009, 07:52 PM
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Maybe you are not distinguishing between mass air
flow (g/sec) and cylinder air mass (g/cyl) properly.
To get from airflow to cylinder air mass you have to
get from RPM to cylinders per second. 4 cylinders
per revolution filled, makes it
4 cyl/rev * rev/min * min/60sec for a 1/15 RPM ->
cyl/sec factor.
So if you took a (say) 300g/sec airflow at 5000RPM,
your 5000RPM equates to 333 cyl/sec and your
300g/sec * sec/333cyl = 0.9 g/cyl.
Of course the real airflow and cylinder air mass are
going to differ from ideal by the volumetric efficiency.
It's also worth noting that the PV=nRT is an
-equilibrium- gas law, and the goings-on in a pulsed
flow system like your motor are a fair bit non-
equilibrated. You've got a bunch of gas expansion,
resonant gas columns and such all going on in-the-
moment. So if you get a really, really ideal result in
anything, be suspicious.
flow (g/sec) and cylinder air mass (g/cyl) properly.
To get from airflow to cylinder air mass you have to
get from RPM to cylinders per second. 4 cylinders
per revolution filled, makes it
4 cyl/rev * rev/min * min/60sec for a 1/15 RPM ->
cyl/sec factor.
So if you took a (say) 300g/sec airflow at 5000RPM,
your 5000RPM equates to 333 cyl/sec and your
300g/sec * sec/333cyl = 0.9 g/cyl.
Of course the real airflow and cylinder air mass are
going to differ from ideal by the volumetric efficiency.
It's also worth noting that the PV=nRT is an
-equilibrium- gas law, and the goings-on in a pulsed
flow system like your motor are a fair bit non-
equilibrated. You've got a bunch of gas expansion,
resonant gas columns and such all going on in-the-
moment. So if you get a really, really ideal result in
anything, be suspicious.
I am more than familiar. This is not anything that is going to change the way I do things necessarily. Just doing some research on airflow modeling. Thank you for the reply, it is much appreciated.
Last edited by avb0119; 07-21-2010 at 12:11 PM.
