Calc.rwhp

[TFZ_Z06]

Are there updates to this CALC PID? Has anybody compared the output to a real dyno?

[joecar]

You can add this pid after having logged... when you view the log, select this pid, replot, and save the log file (can do this with any calc pid).

This pid computes the power that is left over to accelerate the vehicle mass (including payload) after all resistance (road, air, mechanical) has been overcome driving on the street (it is based on the rate of change of VSS)... it probably won't match what the chassis dyno reports.

[vzsv8]

I have just ordered an accelerometer development board, based on the chip used in laptops and Nintendo Wii's to sense movement. It can output acceleration in 3 planes from 0 to 2 g plus or minus, as a 0 to 5 Volt linear signal. My thoughts are that rear wheel Hp (or Kw for the PC) depends on
-speed
-acceleration
-slope of the road
With the Vss signal and long axis acceleration, all three can be calculated and aerodynamic drag can be approximated and added in. When I get a circuit up and some data I will post it up.
All I need now is a rear wheel dyno run to compare it to:), for scientific purposes only.
Cheers, Steve

[joecar]

Hi Steve :cheers:

Also depends on mass of vehicle.

What is left over after overcoming all "friction" losses is what propells the vehicle mass... this is P = m.v.dv/dt where v is VSS and dv/dt is the rate of change of v.

When v = constant, dv/dt is zero, so P is zero... i.e. vehicle is not accelerating, and is at steady speed, even though power is being consumed to overcome friction losses.

Friction (mechanical, aerodynamic, etc) needs to be accounted for only if you're calculating FWHP (gross power)... otherwise you end up with FWHP rather than RWHP... :D ...or do you want to calculate FWHP...?


If you first log P accelerating up to some speed V you get a positive graph of net power;
If you then log P coasting down from the same speed V you get a graph of net collective friction losses (negative values);
you then sort those two data sets by speed, and subtract the losses (negative) from the power (postive) for each sorted speed... this gives an approximate indication of gross power vs speed (which can be mapped to rpm by gear ratio).


You can run on the chassis dyno and compare to the chassis dyno...
but this will be very different compared to running on the street...
there is no air friction and no road slope when running on the chassis dyno.


The accuracy depends on the sampling rate of VSS... 10 samples/second is too slow... 40 samples/second might be just barely ok... the iPhone app "Dynolicious" and other windscreen mounted g-meters sample much faster than that (they sample at 100 samples/second).

[vzsv8]

The purpose of the dedicated accelerometer, when acc can easilily be calculated by the rate of change of vss, is that if the road is not perfectly flat then acc reported from EFILIVE does not reliably translate to power.
By comparing the two readings I should be able to correctly calculate both slope and power, even on a bit of a slope (which represents where I live).
As for frictional losses, air resistance would need to be added to acc derived power to get a meaningful value. Consider top speed, no acc giving no power other than overcoming wind resistance.
Anyhow, as I said, its a work in progress and the minister of war keeps adding things to the priority list above it.:doh2:
PS the original reason for getting the accelerometer was to try to fix the fuel guage, readings in Australia only work when the car is on level ground.:confused:
Cheers, Steve