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Differences between power and torque in a car?

  1. Sep 18, 2009 #1
    Hi guys, have been doing some thinking about cars recently, just wanted to find out what exactly is the difference between power and torque when it comes to motor vehicles?

    I think that more power allows you to have a higher max. speed, you can also carry heavier stuff in your vehicle if you have more power. On the other hand, i think that if you have more torque, your wheels should be able to spin faster as well, hence, also more speed?

    I have heard that torque also contributes to how fast your car can accelerate, so more torque = higher acceleration, but some people told me that petrol cars accelerate faster than diesel cars, which kinda shattered that theory...

    And when it comes to the torque and power curves of a vehicle, i think that it stays the same dispict whichever gears you are in (certain rpm will give you certain power & torque). So what is the significance of that, what characteristics can you tell by looking at the curves, are we always trying to achieve the max torque at the lowest rpm possible? Why don't we try to get both max power and max torque at the same rpm??

    Thank you all!!!
     
  2. jcsd
  3. Sep 18, 2009 #2

    Borek

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    Staff: Mentor

  4. Sep 18, 2009 #3
    First of all, I am no way getting into a discussion about this. The thread above nearly killed me but baching my head against the wall.


    Torque is a twisting motion around an axis. It has the units of force* distance. It can be views as the roatational equivilant of force.

    Power is the rate of work, so work/energy per unit time.


    None is more important then the opther as they are both mathematically interlinked.Power is Torque * angular velocity.

    So for a handy equation.
    Power (Bhp) = (Torque (Ft/Lbs.) x Rpm) / 5250
    Power (Watts) = Torque (Nm) * RPM * 2 * PI / 60

    You have to realise the gearbox playes a large role in a car as it acts as a torque multiplyer.
    Accelerration is determined by the thrust at the wheels, which is determined by the torque at the wheels.

    Torque provides the acceleration. Wheel torque is Engine torque * drive ratio. The the torque output of the engine provides the actualy 'push' to move.

    You cannot compare diesels and pertol directly as you arent takeing into account the gearbox. Diesels produce mroe torque, but they do it at a lower engine speed requiring different gearing.

    Diesels can go incredibly quickly and accelerate harder than a pertrol with tuning for speed and Just look at the audi R10 and Peugeot 908.

    Power is the amount of work done in time. So it's basically the potential of the engine. An gine with high power will hav ethe potential to do more.

    You can have a high torque engine have a high top speed with correct gearnig or a high power engine go slowly but lift great weights.

    So in short gearing is the key. You cant say diesel accelrates better then pertol as it makes more torque. or that petrol can go faster then diesel because it makes more power.

    GEARING (capped and bolded for emphasis.) is the key.


    The power curves is defined by the torque curve. Menging that they can NEVER! occur ar the same rev value. Notice on dyno graphs that the curves always cross at 5250 revs. Google them and look.

    The only way you can have max torqur and max power at the same rev is by having the engine limited to a certain revs.

    You want to produce max torque in the region where the engine will be used the most.




    ITs common to think that low down torque is what is designed for becuase we drive road cars. We commonly spend all out time at low rpm. A race engine will be turned to produce torque at high rpm.
    Turing torque to peak will make the powerband of the ngine very narrow and will require lots of gears and skill to drivel. An egnien with a wide high torque #peak' will be easy to drive and require less gear chagnes.

    Tuning torque for high rpm will make much more power, as power is torque * angualr vel. But will make it horrid to drive at low engine speeds.

    That is the key to engine design, getting the balance between practicality, drivaility and power.
     
  5. Sep 19, 2009 #4

    Ranger Mike

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    how did i miss the referenced post above????...anyway..well said, Chris, as usual
     
  6. Sep 22, 2009 #5
    Thanks a lot for the Information .. nice question posed.!!
     
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