Calculating Instantaneous Net Power
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).
:?
PWHP calculation - comments
I used to do this in ATAP with just acquiring VSS anf the time stamps
and the throttle position (as a check what my foot was really doing).
I converted to an excel sheet and did the calculation afterwards.
I found that you also need to take into consideration in the
mass (vehicle weight) the effect of accelerating rotating mass
(tires/wheels, drivetrain etc...) . This is different for each gear selected.
The factors for multiplying your actual mass (17"wheels, 3.42 gears)
1st gear: 1.2
2nd 1.12
3rd 1.085
4th 1.06
I also plotted a power friction loss curve based on an assumed cd
value. The power at the rear wheels you calculate does not include
the loss due to wind-resistance and wind (head wind or back wind etc.).
This needs to be added to the EFILive measurement to get a comparison
to the dyno reading because there is no wind-resistance to overcome on a dyno. Combined you can get pretty good results, assuming you numbers are close to actual (for example, at around 95 mph in 3rd
near 5700rpm, power peak area, the loss due to windresistance is about
70 or so Hp or more with no wind).
Gert