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Automotive Another hp v tq thread this ones optimistic

  1. Jul 4, 2012 #1
    OK, I'm seriously struggling with this horsepower torque conflict, mostly because of all the god damn bias going around in these forums. It seems that the facts only go so far as to saying hp is a measure of power, and torque is a measure of turning force, then when people try to bring about examples, they start going off on a tangent, ignoring the whole controversy of WHERE, WHEN, and IN WHAT BALANCE, (numerically) you would want torque and horsepower; all in order to propose theoretically optimal lap-times or 1/4mile times or any other physical situation.

    Sometimes I wonder if the understanding of this could be exemplified in the theoretical situation of a continuously variable transmission. Now, obviously racers don't use these, because the belt (in theory), is far to weak to handle high hp applications. But what if for example you hooked one up to a sports car with typical track inclined characteristics:

    Boss 302 mustang, plenty of torque, but even more power.
    hp: 440 at 7400rpm
    tq: 380 at 4500rpm

    what if the car wasn't using a sequentially geared transmission to compicate matters. What if a continuously variable trans. was used to simplify things: once the car took off so launch was taken out of the equation, at what rpm would be optimum for setting the CVT, so that the maximum acceleration would take place? When would the acceleration rate decrease/increase and WHY? What if a car with virtually no torque was used instead. Could it be JUST as fast with a CVT, because all you would have to do is continuously adjust the CVT depending on the vehicle speed, to keep the engine speed locked at a specific RPM.

    See what I'm asking here? I guess I may be proposing that a car with little to no torque could be just as a fast, as long as gearing was theoretically optimal, AND IM SAYING IN THEORY. Its obvious we dont have the technology to adjust the the CVT with minor loads such as bumps in the road and turning forces when the car is being turned.

    Couldn't this cvt theory allow a complete top end racecar to act as a heavy duty truck? by adjusting its rpm to the max level so max output is being used to tug a heavy object?

    Thankyou
     
  2. jcsd
  3. Jul 4, 2012 #2
    If the unobtainable infinite gearing were obtainable and traction were infinite, power is all that matters, as the gearbox makes it usable. For a real gearbox, you need a good compromise.

    There is no point in considering power and torque as separate entities. P = Tw.
    Just consider them as two parts of a single 'engine output'. Once you do this, the conceptual problem goes away. However you are still left with the design challenge of finding the best compromise.

    And you also need to consider engine duty cycle. For heavy lifting (high loads) a large high torque, low rpm engine will cause less damage to bearings (ie do the same work but run longer) than a low torque high revving engine that is then highly geared.
     
  4. Jul 6, 2012 #3

    jack action

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    First, we need to understand that only power is important in comparing performance. The rpm is dictated by the wheel's rpm and we matched it to the engine's rpm by the use of a gearbox. The power at the wheel is the same as the one at the engine crank (assuming 100% efficiency) and the engine and wheel torque are found by dividing the power by their respective rpm.

    If engine «A» produces the same power that engine «B», but at a higher rpm (assuming same technology), it can only be because it has a smaller displacement.

    Now the real question is: If we can build a small, high-revving engine that produces the same power as a large, slow-revving engine, why don't we used them in all vehicles (including large heavy duty trucks)?

    I'll attempt at an answer.

    We must first agree on some terms: Both engines should be similar in design, the only difference being their displacements. But what are the right characteristics that must stay the same? Here is my list of what should be the same to compare both engines:

    If all of these are the same for both engines, then the total bore area must also be the same.

    If you want to have a similar combustion chamber design (that will produce the same BMEP), you will also need to keep the bore-to-stroke ratio constant.

    So, let's assume we want an engine twice as small as your 302, producing the same power at the same BMEP and mean piston speed. Since the total bore area will also be the same, the only way to achieve our goal will be to reduce the engine stroke by half. This will give us a 151 ci that will produce its 440 hp at 14 800 rpm.

    But the bore-to-stroke ratio need to stay the same. The only way to achieve that - while keeping the same total bore area - is to increase the number of cylinders. So, comparing engine «A» with engine «B»:

    NAA = N BB
    NA(D/S)² S²A = N B (D/S)² S² B
    NAA = N BB

    NA / NB = S² B / S²A


    Where N is the # of cylinders, D is the bore and S is the stroke. So, if the stroke of engine «B» is half the stroke of engine «A», then engine «B» must have 4 times the # of cylinders to keep the same bore-to-stroke ratio. Since our 302 has 8 cylinders, the 151 would need to have 32 cylinders to produce similar performance!

    So the answer to the question why don't we use more powerful small engines? is:

    They have to be more complex (and more expensive) to do the same job as the engines with bigger displacement.

    In theory, you could use an engine of zero displacement to produce any amount of power ... as long as it has an infinite number of cylinders!

    Just an opinion; open to discussion.
     
  5. Jul 7, 2012 #4
    I've just re read the op, and I think he has a deeper conceptual problem than I first thought.

    The two phrases that worry me are: balance of torque and power. And the 308 has plenty of torque but more power.
    As though they were mutually exclusive.

    Im on my phone atm, but it may be worth talking through a torque and power curve.
     
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