Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Desired Accelaration to Torque

  1. Nov 22, 2016 #1

    I would like to know the basic eqns in a simple Powertrain model. Where i have the Desired accelaration and the other quantities to calculate the tractive force.

    But i need to calculate the torque to the wheels. can anyone help me ?
  2. jcsd
  3. Nov 22, 2016 #2


    User Avatar
    Gold Member

    Have a look at the units:

    [ Nm ] = [ N ] * [ m ]

    Torque = Force * radius
  4. Nov 22, 2016 #3
    Yeah, perhaps my question was too short.

    I want to calculate the drive torque to the wheels, taking into consideration of the tractive forces.

    Torque [wheels] = F tractive * dynamic wheel radius

    F tractive = F air + D drag + F accel

    But will this be dynamic enough ?
  5. Nov 22, 2016 #4


    User Avatar
    Gold Member

    How will you calculate Fair? Read the weather forecast, wind direction, will it rain? Will you calculate the cw-value for each wagon with its load?

    Drag is not constant. It goes uphill/downhill.

    You are the one to know.

    If an exact time of arrival is important, I would install a speed/acceleration control in the train, increasing the turbo pressure if needed.
  6. Nov 23, 2016 #5
    Nice pun !! But you are mistaken - I am not looking to calculate for a power TRAIN, but i am talking about vehicle powertrain (automobile)...

    Sorry for the typo...
    But what i meant is air resistance (drag). Let me rephrase it..
    F tractive = F air resistance + F rolling resistance + F accelaration
    =0.5*(air density*drag co-eff*cross sec area*(velocity)^2) + m*g*coeff-roll resistace + (m*desired accel)

    In this case i take air density constant - 1.2 kg/m3
    Cross sec area - i have example data for the vehicle
  7. Nov 23, 2016 #6
    What do you mean by dynamic?

    If you want to calculate Force/Power at a given instant in time your equation is ok.
    If you wish to plot Force/power over a given time period for a constant acceleration value, you need to either use 'dt' or do the calculation numerically (iteration per iteration).
  8. Nov 23, 2016 #7
    Yes , i am doing a simulation of my mathematical model therefore numerical calculation by iteration is dynamic .

    My question of dynamics was that, i try to make a simple physical equation of powertrain using the formula i mentioned above. When i do that, without considering driveline dynamic is that a right approach.

    Or is my question too vague to comprehend ? please let me know
  9. Nov 23, 2016 #8
    Well, i am missing some data in your explanation, but if you don't like sharing the details, that's allright. We'll find a way around it.

    What do you mean with 'driveline dynamic'?

    If you are worried about using your non-dt formula, don't worry, just use a sufficiently small timestep in your iteration. 0.1s should do, but if you are using a computer to churn out the results you might as well go with 0.01s. Difference will be minimal though.

    What order of accuracy are you looking for?
  10. Nov 23, 2016 #9

    jack action

    User Avatar
    Science Advisor
    Gold Member

    I use a mass factor to model the rotational inertia like shown here (I think that is what you are talking about). Learn more about the 'equivalent mass' theory.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted