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Engine horse-power

  1. Sep 13, 2006 #1
    hello everbody,
    in fact i was thinking about the horse-power which u can get from an internal combustion engine,i found there are many horse powers u can get-such as(brake hp,indicated hp,rated hp)and may others-
    anyway,i thought about the hp which is declared for the customer when he buy a car with an engine produce for ex:150hp
    my ques:how can we calculate the rated hp?the 150hp?
    the indicated hp=torque*speed(t*w),and the brake hp is the indicated substracted with the mechanical losses occured,and the brake can be measured by a dynamomter or whatever,i ask about the rated power which describes the engine in the markets
    thanks in advance
  2. jcsd
  3. Sep 13, 2006 #2
    dyno messured at flywheel before drivetrain loss
    drive wheel to ground ruffly 20% less

    SAE , DIN are slightly different standerds use in auto HP ratings
  4. Sep 14, 2006 #3


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    The bhp figure a car manufacturer will quote is taken at the engine dynamometer brake, ie at the flywheel.

    Brake power is a practical unit in the sense that this power can actually be used to do meaningful work. It takes into account factors such as bearing friction, pumping, and driving of things like alternator for ignition, water and oil pumps etc.

    Indicated power is the power which the engine would develop neglecting frictional, pumping and ancilliary losses in the system. It is the power the engine would produce if all the work done by the gases expanding in the cylinder went into doing meaningful work. For a four-stroke cycle, two terms are used. Gross indicated power is that delivered over compression and expansion strokes only, net indicated power is that delivered over the entire 720 degree cycle.


    brake power = (net indicated power) - (power lost to friction and ancilliary systems)


    brake power = (gross indicated power) - (power lost to friction, ancilliaries, and pumping)

    When dealing with indicated quantities, ALWAYS state whether you're dealing with net or gross units. This can make a big difference, particularly when you're dealing with forced induction engines where the positive volumetric efficiency can turn the pumping losses into pumping gains!

    Rated power is the power an engine is physically allowed to develop. This can either be maximum rated power, for short durations, or normal rated power for continuous operation.

    Road-load power is a useful measurement for testing car engines and drivetrains. It's a part-load power level (so not at rated power), and is the power required to drive the car along a level road at a steady speed. This power obviously overcomes rolling and air resistances. These resistances are accounted for by empirical formulae, and the power can be found.

    Incidentally, if you ever take your car to a rolling road, always insist on seeing the 'at the wheels' power. Since this is where it's measured, many establishments will divide this by an arbitrary efficiency to account for the drivetrain. Unfortunately, this estimation is often vastly inflated, usually to make the tuner look better than he actually is. Beware!
  5. Sep 14, 2006 #4
    thanks ray_b&brewnog for the answer,
    but i still wanna know how we can get the rated hp of an engine at a certain rpm,we could measure the bhp by the dyno-and then we could know the gross or the net indicated hp.

    my ques:how can we calculate the rated hp?(maximum power of engine at a certain speed)
    thanks again
  6. Sep 14, 2006 #5


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    The rated power is ultimately an arbitrary limit set by the manufacturer.

    You could calculate from a power figure at the wheels gained from a dyno if you knew the exact drivetrain and rolling inefficiencies. However, you don't. Estimations can be made by 'motoring' the drivetrain over and seeing what power is required to do this, but it's never entirely accurate.

    If you could, you'd divide the power developed at the road wheels at rated speed by the entire drivetrain efficiency. 0.8 is a conservative estimate, but it's dependent on so many figures you'll struggle to get a meaningful value for it. Just take what the manufacturer tells you the rated power is as the truth. It won't be right but it'll be near enough.

    At the end of the day, power at the wheels is all you're really interested in. You could have all the power at the flywheel in the world but if you can't get it down onto the road then there's no point at all.
    Last edited: Sep 14, 2006
  7. Sep 14, 2006 #6
    thank alot brewnog for the fast reply-:)
    yes,thats right we couldnt know the exact drivetrain and rolling losses,and it differs from engine to the other,but im agree with u that if we knew then we could know how much power we get at the wheels,at any rpm,then as u said only the manufacturer who hase the figure where we can get this rated power.
    thanks alot for ur help.
  8. Sep 17, 2006 #7


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    You're welcome.
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