Originally Posted by

**redhardsupra**
CALC.VE is horsepoop. ignore at will.

The CALC.VE pid that comes predefined with the V7 scantool is defined as:

Code:

VE% = {SAE.MAF.gps}*({SAE.IAT.C}+273.15)/((displacement()*61.024)*{SAE.RPM}*{SAE.MAP.kPa})*212544
VEpcm = {SAE.MAF.gps}*({SAE.IAT.C}+273.15)/((displacement()*61.024)*{SAE.RPM}*{SAE.MAP.kPa})*6155274.24

I saw several problems with those definitions:

- they use IAT instead of DAT;

- the constants don't reconcile (VE% is close, VEpcm is nowhere near).

Here's my analysis/derivation of calculated VE (I keep track of the constants):

Code:

In one cycle (4 strokes, 2 revs) all N cylinders are filled/emptied once.
Time for one cycle (2 revs):
t[s] = 2[rev] * 60[s/min] / RPM[rev/min]
= 120[s*rev/min] / RPM[rev/min]
Mass of air thru engine in one cycle (N cylinders):
VE[g] = MAF[g/s] * t[s]
= MAF[g/s] * 120[s*rev/min] / RPM[rev/min]
Mass of air normalized for temperature and pressure in one cycle (N cylinders):
VE[g*K/kPa] = VE[g] * DAT[K] / MAP[kPa]
= MAF[g/s] * DAT[K] * 120[s*rev/min] / RPM[rev/min] / MAP[kPa]
Normalized mass of air thru one cylinder:
VE[g*K/kPa] = MAF[g/s] * DAT[K] * 120[s*rev/min] / RPM[rev/min] / MAP[kPa] / N
= MAF[g/s] * DAT[K] * 15[s*rev/min] / RPM[rev/min] / MAP[kPa] when N=8
Ideal Gas Law equation (PV=nRT):
P[Pa] * V[m^3] = n[mol] * R[J/K/mol] * T[K] = (m[g] / M[g/mol]) * R[J/K/mol] * T[K]
where:
M = 28.96[g/mol] = molar mass of air (average molar mass of air composition)
R = 8.31447[J/K/mol] = Universal gas Constant
T = DAT[K]
P = MAP[Pa]
V = displacement[m^3] of engine (all N cylinders)
Theoretical air mass contained in engine displacement V (N cylinders), rearrange IGL:
m[g] = V[m^3] * MAP[Pa] / DAT[K] * M[g/mol] / R[J/K/mol]
Note conversions:
1[Pa] = 1[N/m^2] = 1[J/m^3] since 1[J] = 1[Nm]
1[kPa] = 1000[Pa]
1[m^3] = 1000[L]
So, converting to [L] and [kPa], theoretical air mass becomes:
m[g] = V[L] * MAP[kPa] / DAT[K] * M[g/mol] / R[J/K/mol] * 1000[Pa/kPa] / 1000[L/m^3]
= V[L] * MAP[kPa] / DAT[K] * 28.96[g/mol] / 8.31447[J/K/mol] * 1[Pa/kPa*m^3/L]
= V[L] * MAP[kPa] / DAT[K] * 3.4831[g*K*/kPa/L]
So, mass volumetric efficiency of engine (N cylinders):
VE[%] = VE[g] / m[g] * 100[%]
= MAF[g/s] * 120[s*rev/min] / RPM[rev/min] / V[L] / MAP[kPa] * DAT[K] / 3.4831[g*K*/kPa/L] * 100[%]
= MAF[g/s] * DAT[K] * 3445.2[s/g*kPa/K*rev/min*L*%] / RPM[rev/min] / MAP[kPa] / V[L]
Mass volumetric efficiency of 1 cylinder would be:
VE[%] = (VE[g] / N) / (m[g] / N) * 100[%] = VE[g] / m[g] * 100[%]
= same as for N cylinders.
i.e. VE[%] is the same regardless of number of cylinders.
Writing VE[g*K/kPa] and VE[%] using pids (assuming engine 5.669L V8):
VE[g*K/kPa] = {SAE.MAF.gps}*{CALC.DAT.K}/{SAE.RPM}/{SAE.MAP.kPa}*15 for 1 of 8 cylinders
VE[%] = {SAE.MAF.gps}*{CALC.DAT.K}/(SAE.RPM}/{SAE.MAP.kPa}*3445.2/5.669

So I would ignore the CALC.VE pid (like you said) and instead use the VE derived above.