Understanding Speed Density

[Gelf VXR]

After spending quite some time trying to understand speed density, looking at equations for the ideal gas law, density, molar mass of air and the gas constant R:book::online2long:

I came up with a working spread sheet to demonstrate how the function works to myself, and now i understand i thought i would share.

To use enter your cylinder volume, a charge temperature value and an air-mass value. Look up and down the table for pressure values.

[joecar]

Hi Gelf,

Cool spreadsheet :cheers:

questions:
- how do I know what g/cyl to enter...?
- in the spreadsheet VE goes up as pressure goes down, is this for a given g/cyl..?

[Gelf VXR]

Hi Gelf,

Cool spreadsheet :cheers:

questions:
- how do I know what g/cyl to enter...?
- in the spreadsheet VE goes up as pressure goes down, is this for a given g/cyl..?

Hi Joe

Its a theoretical calculation to demonstrate how speed density is calculated, the table computes theoretical cylinder air-mass, density, GMVE and %VE from the virtual data input values.

RPM is independent.

I enter values taken from a log to confirm the result, ie cylinder volume, charge temperature, MAP and grams per cylinder.

Indeed %VE does increase as MAP pressure kPa decreases for the virtual data inputted, you would normally see max VE at WOT at 101kPa normally aspirated, if you could achieve the same air-mass cylinder fill at lower pressures where density is decreasing, reducing the theoretical air-mass cylinder fill, %VE will be proportionally higher.

[joecar]

Ah, ok, understood, VE would indeed be higher if same cylinder fill airmass was achieved at lower pressure;

thanks.

[Gelf VXR]

What this does show is that %VE and GMVE are variable from day to day because density is affected by charge temperature. So it must be impossible to dial in VE calibration table 100%, any one with a SD only tune must at least need to change their calibration as the seasons change to account for the change in ambient temperature?

[Wheelz]

^^ except I thought that was why we had an IAT sensor and the tables to correct how the ECM calculates charge temp from ect and iat

[joecar]

GMVE = VE[g*K/kPa]

this is cylinder airmass normalized for pressure and temperature, it does not rely on density of air like VE[%] does...

i.e. density of air is used only for converting from volume (displacement) to mass.

[Gelf VXR]

:music_whistling_1: I see my error

[joecar]

Have a look at posts #103 and #104 here: New-Tuning-Tutorial-WeathermanShawn (http://forum.efilive.com/showthread.php?13152-New-Tuning-Tutorial-WeathermanShawn)

post #103 derives VE[g*K/kPa] from MAF, it does not use air density (IGL),

post #104 derives VE[%] from MAF and air density (IGL),

( noting that air density is the Ideal Gas Law (IGL) in re-arranged form )

:)

[joecar]

:music_whistling_1: I see my errorVE is not an easy topic.

[Gelf VXR]

I was confused because i had entered fixed values for temp and air-mass across the range of MAP to calculate GMVE, in the VE table GMVE and MAP values are fixed, where as Temp and air-mass values are variable

I added a new tab on the spread sheet to demonstrate air-mass and temperature variable for GMVE and MAP

[joecar]

Correct, VE[%] has constant temperature and pressure, whereas VE[g*K/kPa] allows variable temperature and pressure.