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Car torque

  1. May 2, 2007 #1

    I'm trying to program a sort of small car simulator, but I have a problem.
    I managed to calculate the force on the car, acceleration, speed, air ressistance etc...

    My theory:
    We've got a car at a certain speed (v) and I calculate the RPM:
    Code (Text):
    rpm = ( v * gear-ratio * current gear ratio ) / ( 2pi * tyre_radius) * 60
    Now I calculate the torque:
    Also I calculate the engine power (Watt) from horsepower.
    Code (Text):
    torque =  ( horsepower * 745.69987158227022 ) / ( 2pi * (rpm / 60 ) )
    Then I am able to calculate the force on the car:
    Code (Text):
    F = ( torque * final_gear_ratio * current_gear_ratio) / tyre_radius

    1. it doesn't work when speed = 0
    2. it's better to work with a torque and power function because they aren't the same at different engine speeds (max power at certain RPM), but how do you create/calculate such a function. Or can this only be made by testing the engine to get the values?

    example of one:

    I'm from Belgium so please use the SI units!
  2. jcsd
  3. May 6, 2007 #2


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    Staff Emeritus
    Science Advisor

    There are two effects here resisting the torque of the engine or force applied by the tyres. One effect, which one mentions is air resistance, but the other effect is acceleration or change in speed of the car.

    The air resistance is proportional to the speed of the car (or = kvn, where n >1), and one could use a coefficient of drag.

    Yes torque and power vary with engine speed, much in the same way that pump performance varies with speed. One normally determines the relationship (function) by testing the engine. There is internal resistance in a rotating system, which increases with speed. In addition, in a pump, the fluid can only flow in only so fast, and similarly in some engines, the fuel and air supply can only get in so fast. Fuel injection and turbocharging (as compared to normal aspiration) are way to improve fuel and air flow for improved engineer performance.
  4. May 7, 2007 #3
    Ok, I understand, but is there a way to get or make such a relationship if it's determined by testing.
    Can I somewhere get these values/curves?
    On every sites, If they provide some info about torque, I only find max torque or power @ xxxx rpm.
  5. May 7, 2007 #4


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    Science Advisor

    The device used to measure these things is called a dyno (dynamometer). I think it's also referred to as a rolling road in the UK. If you search for these terms, the web is filled with the types of plots you're looking for.

    Also, your little program would work much better by assuming constant torque (at the engine) rather than constant power. In many modern engines, maximum torque doesn't change by more than 30% or so over most of the operating range.
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