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Thread: Calculating Instantaneous Net Power

  1. #21
    EFILive Developer Site Admin Blacky's Avatar
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    Yes, you did get up too early, I have no idea what you are tolking about :?

  2. #22
    Joe (Moderator) joecar's Avatar
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    I think he means having the HP calculation be included with V6
    (unlike V5 where you have to paste it in);
    and, it seems he is in luck, because it looks like it already is included .

    I like how you included the gForce pid
    (very cool for measuring braking mods).

    Paul, if you can, can you post the new HP charts...
    oh wait... it looks like you already updated them
    (very happy to help out).

    Regards,
    Joe

  3. #23
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    Default 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

  4. #24
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    IN that sense the TTS Powersystems dyno from datamaster takes into account all that it works pretty darn well I suppose...
    "All that is needed for the triumph of evil is that good men do nothing..."

  5. #25
    Joe (Moderator) joecar's Avatar
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    The formula P = m.v.dv/dt.k is measuring the power available to accelerate the vehicle after all the resistance/friction has been overcome; it does not include corrections for anything; it calculates the actual power to the road that is making the vehicle increase speed; it makes no other claims;

    however, if you know the various corrections, you modify the formula and post it, someone with a dyno can compare the formula to their dyno and see how close it is.

  6. #26
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    The P= m v dv/dt formula posted here assumes that m is constant.
    However, due to all the rotating parts which have to be accelerated
    the effective mass is actually higher than the static mass of the vehicle.
    I posted above the multiplication factor for the first four gears with an
    M6 and 17" tires, 3.42 rear. I derived these from the automotive tables
    for V8 equipped cars. One can debate the accuracy of these factors
    but they need to be taken into account for better accuracy.

    Also, because in a road test, air resistance must be overcome, the
    power to do this is also delivered at the rear wheels.
    So on the road at 6000 rpm in 4th,it will give much lower output power than at 6000rpm in 2nd gear because of the speed difference and resulting air load difference. This effect does not show up on a dyno because the car is not pushing air out of its way - so dyno numbers should be much higher than this actual test.
    The problem in road testing is that to get actual power readings there is little time because our engines run through 2nd gear in a hurry, yet
    the air load is much less important at the lower speed (goes with v cubed)
    One can get much better readings in 4th because because the engine spends more time there during a full throttle run,
    however the air resistaqnce will make the power look really low near the
    typical peak rpms of 5500....6000rpm. So, knowing the losses due to air and adding them can give the best results based on resolution etc.

    So once you calculate both parts and add them you can get roughly what
    a dyno would yield.

    However, in actual driving, a head wind can make a difference as well as a tailwind etc.,.

    Gert

  7. #27
    Joe (Moderator) joecar's Avatar
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    Gert (2000ssnb),

    I' not saying you're wrong and I can appreciate the fact that you took the time to look up tables of data; and you are correct that the formula does not consider air resistance (it measures what remains after air resistance has taken its bite);

    All I'm saying is the following (this sounds funny because the following is a bit long...)...

    The formula calculates the power left over after all the resistances, frictions and inertias have been overcome; the formula does not care what is propelling the vehicle; it uses the vehicle's measured rate of change of motion/velocity to calculate the net power that is remaining at the drive wheels after all the resistance and air friction has been overcome; if the vehicle is moving at a constant velocity, the formula gives zero power since the rate of change of velocity is zero;

    I agree with you that rotational inertia (angular mass) influences angular acceleration, but it does not influence the vehicle's mass; the angular mass of a given powertrain and the torque developed by that powertrain's engine both determine the rate of increase of angular velecity of the whole powertrain within the vehicle; internal mechanical friction throughout the driveline subtracts from the available engine torque and so reduces the rate of increase of driveline angular velocity;

    at the drive wheels (ignoring tire slip and deflection for a moment), angular force (torque) is converted to linear force; the whole vehicle's rate of change of velocity is determined by this force and by the vehicle's mass; air resistance subtracts from this force; so finally, after all the resistances/frictions have subtracted, the remaining force determines the rate of change of velocity of the vehicle (at that once particular instant in time);

    if the vehicle is accelerating, the remaining force is positive;
    if the vehicle is at steady speed, the remaining force is zero;
    if the vehicle is decelerating, the remaining force is negative;

    the formula does not take into account any losses between the VSS (which is where we're measuring the instantaneous speed, unlike a dyno) and the drive wheels; these losses are: tire slip, tire deflection, drive shaft twist, U-Joint flex, axle twist, differential gear friction, differential bearing losses, wheel bearing losses, transmission output shaft bearing losses; the formula comes from the physics definition of power and work; dynomometers use the same formula with correction factors and terms; the G-Force windshield mounted accelerometer uses the same formula (would anyone be willing to compare a G-Force power curve to an EFILive calculated power curve...? I am curious, I think I'll have to get one...)

    if you don't have dyno access, the formula can be used to compare before and after doing a mod, provided you do the runs on the same stretch of road under the same conditions;

    if you do have dyno access, there is no air resistance, so the formula (EFILive capture during dyno run) should come close to the uncorrected dyno results; someone on this thread (I think it was Highlander) found something like a 10% difference, which is not so bad considering the formula does not include losses between the VSS sensor and the drive wheels (a differential ring and pinion has a lot of mechanical friction due to the hypoid arrangement (non-intersecting centerlines between the ring and the pinion));

    yes, a headwind makes you look bad, and a tailwind makes you look real good, as does going down a big hill); and you do bring up a very good point: air resistance is proportional to the speed cubed, so the net power will be less at higher speeds on a road test; the hard part is knowing the losses due to air;

    if you capture a trace going up thru some speed (say 80 mph), and then on the same spot on the road you capture a trace going down thru the same speed (coasting, measures air resistance, road friction, and mechanical friction), and you subtract the two calculated power numbers at that speed (since the coast down number is negative), you will get an approximate flywheel power number (e.g. I got 260 - (-70) = 330); of course, this is approximate, and is based on being consistent, and picking a perfectly calm day, and doing both runs minutes apart;

    also, while the engine is running it is consuming fuel which means the vehicle mass is reducing; but over a a short distance (1 mile say) the change of mass is very small compared the the vehicle mass, so it can be ignored;

    good discussion, shows some people are thinking
    (you sound like you might be an engineer...)

    Joe
    :wink:

  8. #28
    Joe (Moderator) joecar's Avatar
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    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
    Gert,

    Would you please look up the factors for my car...?
    My car is:
    2001 TA WS6
    275/40R17
    3.23 axle,
    A4 (1st 3.059, 2nd 1.625, 3rd 1.000, 4th 0.696),
    Alum drive shaft

    What about the Cd for it (with Ram Air hood)...?

    Thanks, appreciate it.

  9. #29
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    Joecar,

    These factors are called the "rotational inertia coefficient."

    I dervied these from a handbook using some graphs etc.

    There you multiply the gear you are in with the rear end gear
    ratio. Since your transmission gear ratios
    are bit higher than my manual gear ratios but your rear end
    is a bit lower (3.23 vs. 3.42) the factors are probably
    pretty much the same. I did this work over a year ago
    at home on paper etc. and it will take me some time
    to go through the procedure. Your wheels seem to be the same
    etc.

    I used 0.3 for our cd value but this is just an educated guess
    based on "wedge shaped vehicles." I can't imagine that our
    cd would be a lot worse,

    Gert

  10. #30
    Joe (Moderator) joecar's Avatar
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    Gert,

    Yeah, our sleds are pretty aero.

    Thanks for the rotational inertia coefficients, appreciate it.

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