cylair formula's units

[redhardsupra]

This is what I found in the little explanation/help window on VE table:

The air mass per cylinder can be determined from the VE table using the following formula:
g/cyl = VE*MAP/charge temperature
Ve is in g*K/kPa,
MAP is in kPa,
and then in the temp blend explanations i found:
273.15+IAT+((ECT-IAT)*factor) with the factor of 0.042969 for any significant airflow.
so with VE of 100 g*K/kPa,
MAP of 100kPa, and
IAT of 82F and ECT of 190F

so i tried to see if it works so i did:
airmass= 100*100/303=32.9 g
that ain't right, so i thought maybe the VE should be as % so:
airmass= 1 * 100/303=0.329g which is too low for a WOT condition on a typical engine.

what am i doing wrong?

thanks,
Marcin

[SSpdDmon]

Did you convert to *C before the *K calculation?

[redhardsupra]

yes

[SSpdDmon]

**Edit** I just noticed that your VE listed in your first post is 100. That means it's 100% of the theoretical maximum (based on cylinder volumes I believe). You need to go to Edit>Properties and change the VE drop-down menu to Grams*K/kPa and reload your file. See below for mathematical explanation.

Double check your math and make sure your VE is in Grams*Kelvin/kPa units under edit>properties. I just checked mine where I usually record ~.80(grams/cyl) in my logs. Using your temps converted to Celcius, I get a charge temperature of 303.50591(K). At 100kPa on my VE table, I have 2.4365(Grams*Kelvin/kPa) at 4400(RPM), which is where I see most of my airflow with the stock cam (peak torque).

If I take 2.4365(Grams*Kelvin/kPa)*100(kPa) and divide that by 303.50591(K), I get 0.80278 (grams). :)

[SSpdDmon]

TTT for the edit above...

[joecar]

Yes, I agree, VE ranges somewhere in the vicinity of 2.0 g.K/kPa (+/- some amount)...

100 g.K/kPa seems way too high...

[redhardsupra]

oh yea, good catch...thanks guys.
so how do we arrive at the % theoretical? is that what the unit HPT displays too?

[SSpdDmon]

I'm pretty sure that's what HPT shows. PM Blacky...he might be able to hook you up with the equation for VE% based on theoretical maximum.

http://forum.efilive.com/member.php?u=4

[redhardsupra]

eh, no need, i figured it out, thanks guys

[joecar]

eh, no need, i figured it out, thanks guysRHS,

Did you calculate the mass of a cylinder volume's worth of air and compare with VE table cell(s), is that what you did to get VE% of theoretical max...?

Joe

[redhardsupra]

i tried to go from airflow numbers (which i already got) and theoretical airflow, but frankly it's kicking my ass. well, i know how to calculate it, but it wants air density which i've been able so far to eliminate from all other equations..grr..
if you want more details, email me, i'll send you all 11 pages of math for it...

[blandmiller]

I don't think you will ever calculate cylinder air mass or VE without first calculating the air density. How have you eliminated air density from other equations?

You have an email coming, I would like to see the math..

[blandmiller]

Marcin, what do you have that compares airflow numbers to the theoretical maximum airflow? Your 'cylair' spreadsheet calc can be corrected to the top table if the air density (rho) is taken into effect. I have verified several points and get accurate numbers until I get into the low MAP regions. I think there is some IAT/ECT temperature blending affecting the low massflow numbers.

Keep in mind that "CylAir" is the absolute 'mass' of air entering the cylinder and the airmass is directly related to the temperature and pressure of the air. IE, rho cannot be a constant.

The table is referenced to 75 degrees F

[redhardsupra]

i got a formula for that, working on it today, expect an update soon ;)

[joecar]

Can rho be calculated from PV=nRT where m=nM...?

[blandmiller]

Since the tables are MAP specific, Rho can be calculated based on the linearity of air density to absolute pressure. I calculated Rho (based on the tablulated info provided) to be 1.173 Kg/M3 @ 100 Kpa. The metric system makes the rest easy; use the MAP values as a % multiplied by the 1.173 air density (ie, MAP=50, 1.173*0.5=0.5865 air density {rho} at vacuum).

[redhardsupra]

check your mail, i dropped a big one in there for you ;)

[joecar]

I got it thanks, I'm a little rushed at work, but I'll read it over dinner. :)

[joecar]

Marcin, email sent. :)