How Torque, HP, and max RPM relate to speed/cruise? How do Gas/Diesel compare?

Discussion in 'Powerboats' started by leaky, Oct 20, 2015.

  1. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    Quote: Originally Posted by powerabout View Post "Electric motor the only motor that can produce torque without rpm"

    Then one reply:

    "Rubber power also. And the coiled spring/flywheel energy storage on toy cars."

    And my 2cents worth: Steam. Steam engines can give full torque, or more at zero RPM, and steam turbines, they can produce about 200% of rated full power torque at zero RPM.

    Not that any of that is relevant to this discussion, except to clarify that torque can be either a static value, or a dynamic value. If we want to have any power, then the torque is a dynamic value, not static.
  2. powerabout
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    powerabout Senior Member

    Wow a steam turbine can produce more torque stalled than rotating?
    I didnt know that
  3. NavalSArtichoke
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    NavalSArtichoke Senior Member

    The speed of the engine where minimum fuel consumption occurs and the speed where max. HP is produced are pretty much two different things.

    Most gasoline or diesel engines have relatively flat torque curves between idle speed and max. RPM. Since HP is proportional to RPM, while torque is relatively constant, then the faster the engine turns, the more HP the engine will produce.

    Most fuel consumption figures are calculated as specific fuel consumption, i.e. lbs of fuel consumed per horsepower-hour

    This chart shows the performance curves for a Cummins 5.9L ISB diesel:


    The torque curve is dead flat at about 460 lb-ft; consequently, the HP curve is a straight line with a constant slope. The specific fuel consumption curve starts out and declines until a minimum consumption figure is reached, and then maxes out where the power output does.​
  4. gonzo
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    gonzo Senior Member

    Torque is always dynamic. The static similar dimension is moment.
  5. Barry
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    Barry Senior Member

    Torque is not always dynamic. Moment and torque can be used interchangeably. A twisting force applied at a distance.

    Below a description, not mine, that clarifies that though the shaft is rotating, the torque is a static load, as it pertains to the discussion on a propeller, because there is no angular acceleration.

    In a discussion of static vs. dynamic
    torque, it is often easiest to start with
    an understanding of the difference
    between a static and dynamic force. To
    put it simply, a dynamic force involves
    acceleration, were a static force does
    not. The relationship between dynamic
    force and acceleration is described by
    Newton’s second law; F=ma (force
    equals mass times acceleration). acceleration. The torque transmitted
    through a cars drive axle as it cruises
    down the highway (at a constant speed)
    would be an example of a rotating static
    torque, because even though there is
    rotation, at a constant speed there is
    no acceleration.
  6. Joakim
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    Joakim Senior Member

    Static vs. dynamic is not very well suited for this discussion about torque and power. Static vs. dynamic can mean different things. It can be not time dependent vs. time dependent or at rest vs. moving.

    A well know example of the latter is static pressuse vs. dynamic pressure.

    Static or dynamic doesn't fit well to torque in the sence we are discussing it.
  7. DCockey
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    DCockey Senior Member

    Barry, where did you find the definition of torque as static which you quoted, and what was the context?

    I usually think of torque as the rotational equivalent of force with a direction (axis of "twist") and a magnitude. Torque, just like force, can exist without anything moving, with objects moving at constant speed, or with objects accelerating.

    What is the relevance of whether torque is labeled "static" or "dynamic"?

  8. Barry
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    Barry Senior Member

    My original comment into this thread (#38) was when a contributor said that "propellers turn torque into thrust" which my summarized response was "propellers turn horsepower into thrust". And later, said that the correct terminology would be " propellers turn torque at an rpm into thrust, which is of course then power.

    I then discussed a horsepower curve of an engine and the relevance of the Torque curve being imposed upon the horsepower graph can cause misconceptions and some were being promoted in the thread.

    So as a quick overview, the torque when measure by a dyno is a static load and from the torque static load you multiply (with a constant) the rpm to get a horsepower value.

    As you stated torque can exist without motion or with constant motion, static torque and torque can exist with acceleration, dynamic torque.
    This is correct.

    Regarding your comment about where I got this definition.It was from working on a dyno with various power sources from a steam turbine, steam engine, a kerosene powered turbine, gas and diesel engines. All hooked to dynos and other torque measuring equipment, measuring torque, rpm, thermal efficiencies, the chemistry in the exhaust gasses under various loadings to determine the percentage of fuel burn, volumetric efficiencies and other things that I have forgotten over the years.
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