Calculating Instantaneous Net Power

[joecar]

Edit: This power calculation comes already provided by the EFILive V7 and V6 Scan Tools as the built-in pid "{CALC.POWER_RW}".
Edit: This was written a long time ago when EFILive V5 was current, which is why it contains references to "{SAE.J1979.VSS}".

Using EFILive's calculated PID feature, you can calculate the "instantaneous net road power" at all recorded sample points using the following equations:

(for sake of example, I'm using my vehicle's mass (3750 LB = 1701 kg))

Using Imperial units (mass in LB, VSS in MPH):
3750 * {SAE.J1979.VSS} * dx({SAE.J1979.VSS}, 6) / 8226.63

Using Metric units (mass in kg, VSS in km/h):
1701 * {SAE.J1979.VSS} * dx({SAE.J1979.VSS}, 6) / 12960.0

Notes:

a. Vehicle total mass must include driver and all payload mass (to be accurate, you must account for all the weight).

b. The constant number 6 is the dt factor in the derivative function; you may want to read up the EFILive help on this function and play around with some different dt values.

c. The constants 8226.63 and 12960.0 are due to the various conversion factors between Metric and Imperial units and between non-mks and mks units (m, kg, s: the default units of physics).

d. The above equations are arrived at from the standard physics definitions of velocity, acceleration, force, work, etc. (i.e. they do not include any empirical influence or corrections).

If the units used were the classic physics units (kg, m, s) then the
instantaneous net power at each sample point would simply be:

P = m . v . dv/dt

(m is in kg, v is in m/s, and dv/dt is the slope of the VSS "curve")

These equations calculate the instantaneous (at each sample point) net road power that's left over to accelerate the vehicle (i.e. after mechanical friction, road friction, air resistance, and anything else you can think of, has been overcome by engine power).

:?