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Automotive Car engine: how to calculate power and torque?

  1. Jun 10, 2004 #1
    Hello, everybody. I'm in trouble. I have to create program calculating the power and torque four stroke car engine. Unfortunatey, got no formulas. Could you help me, giving those formulas with any describition of them? And, have you got any other advice? Please, it's very urgent.
  2. jcsd
  3. Jun 10, 2004 #2


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    Staff: Mentor

  4. Mar 26, 2009 #3
    All of the websites say that you need a dynamometer to calculate torque, and then from the torque curve produced, you may calculate the hp curve. However, i dont have access to a dyno... is there any way possible to calculate the hp of an engine using mathematics? I know the max torque and the rpm at which it occurs as given by the car specs, i found the drag, and also the tire resistance.. then i used that data to come up with a hp formula, but this is only when the car is in 5th gear.. however, my formula is an x^3 graph, not a logarithmic looking graph like the ones professionally done... so is it possible to make a professional one, without using a dyno? or am i just dreaming?
  5. Mar 26, 2009 #4


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    An engine dynamometer measures torque and rpm, but a chassis dynamometer measures force and speed. Either can be used to calculate power:

    power / horsepower = torque (ft lb) x rpm x 2 x pi / 33000 = torque (ft lb) x rpm / 5252.113122...

    power / horsepower = force (lb) x speed (mph) / 375

    Drivetrain losses are usually 13% to 17%, depending on the car.

    Depending on where you live, you might be able to find a chassis dyno shop that can do a dyno run on your car.
    Last edited: Mar 26, 2009
  6. Mar 26, 2009 #5
    Here is an article by Marc Ross (U. Mich.) on the physics of automobile engines.
    Of particular importance is the BSFC (brake specific fuel consumption) plot showing engine efficiency plotted vs. RPM, measured on a flywheel dynamometer. (83 grams of fuel per kilowatt-hour is 100% efficiency, 330 grams is about 25%. Note that maximum efficiency is at about 35% of redline and 80% of maximum torque.
    Last edited: Mar 26, 2009
  7. Mar 22, 2010 #6
    electric engine calculations

    I have a 24 inches lever, an electric engine is at one end of it and it has to lift 30 pounds of weight on the other end. The engine end of the levere is fixed to the base of the machine, the other end is fixed on the base of the burden. I need to know the needed specs of the engine (rpm wattage and torque) so that I will not buy a wrong one.

    Attached Files:

  8. Mar 22, 2010 #7
    You can start a new thread you know :)

    Without using a dyno you will need to know the weight of the reciprocating components to build an inertia load vs crank angle.

    You will also need a map of the pressures in the cylinder over 2 crank rotations (ie all 4 cycles). From this you can work out the pressure load vs crank angle.

    This will give you the force acting down the cylinder axis for a given crank angle.

    From this you need to convert the force ating down the axis into components at the crank (there is a 'turning' component used to create the useful torque, and a 'bending' component that tried to bend the crank).

    You can do this for a range of rev values to find a very rough idea of power output.

    WARNING: This method assumes a constant BMEP and will only be valid at RPM close to the BMEP it was taken at. You would need pressure vs crank angle map at multiple points to build a decent curve.
  9. Mar 29, 2010 #8
    how can I get those datas from the ones I have provided ? can anyone give me the exact calculations needed ?

  10. Mar 30, 2010 #9
    I actually didn't answer your question, I answered the OP's. Please ignore the above, this is also why you should start a new thread for new questions it's avoids confusion like this.

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