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Torque question.

  1. Feb 20, 2010 #1
    I am working on a vehicle simulation and I have calculated the torque to the wheel using engine RPM and the corresponding amount of HP. Then I multiplied that torque by the gear ratios but I do not understand how at a higher gear the ratio is smaller but it is suppose to go faster than the torque provided at a lower gear. I think it has something to do with MOI and the AA but I do not get it please help. Also how does cruse control work if you set your engine to 3000 RPM's and the same amount of torque is being applied to the wheel wouldn't you continue to accelerate?
  2. jcsd
  3. Feb 21, 2010 #2
    The power in the drive train at any given moment is the same. The power leaving the engine and going into the gearbox will be the same power that leaves the gearbox (ignore losses in the gearbox), and goes into the drive shaft, but because the speed is changed the torque must change,(power is proportional to torque times speed) same thing happens through the diff.
    I think that you are slightly confused about power speed curves for a engine the power quoted at a certain speed is the maximum power, you can run at that speed at a lower power without any problem.
  4. Feb 21, 2010 #3


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    Staff: Mentor

    You've made one error and applied it twice. The power output of an engine is not always the same at a given RPM. It varies with throttle position. So applying that back into your first question, torque at the wheel need not be lower at higher rpm. It is if your foot is on the floor and the throttle wide-open, though. So what does that tell you? It tells you your acceleration is better at low speed partly because you are in a lower gear.
  5. Feb 22, 2010 #4
    Ok so i can be at 3000 RPM's with the throttle fully opened and the engine's power would be ~100 HP but if im at 3000 RPM's and the throttle only halfway open then the engine's power would be ~50 HP?
  6. Feb 22, 2010 #5
    I'm sure the relationship between throttle position and power isn't that linear, but yes, you can be at 3000 rpm and practically anywhere between 0-100 hp.
  7. Feb 22, 2010 #6
    Power (watts) = torque (Newton-meters) x 2Ï€ RPM/60

    HP = power (watts)/746

    An engine could generate 100 HP at 3,000 RPM, but to drive at say 35 MPH (15 meters per second), usually less than 10 HP (7,500 watts) is required.

    Bob S
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