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Auto specifications: Power & Torque [and other questions]

  1. Apr 5, 2008 #1
    On all the auto specifications card that I read, they mention 'bhp', which i guess is the power provided by the engine, when none of the transmission losses are counted. But, is the power of a engine constant?

    Secondly, the also provide a specification 'Torque @ a certain rpm'. But, isn't Torque x Angular Velocity = Power? Then why are there 2 separate specifications given?

    Also, what exactly happens when I press the accelerator of my car?

    Also, lower gears have a low gear ratio. Hence, for every single rotation of the wheel, more torque is provided on lower gears than for higher gears [am i right?]. Why is a higher torque needed at lower speeds?

    Thanks,
    rohan
     
  2. jcsd
  3. Apr 5, 2008 #2

    brewnog

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    Gold Member

    No, the power of a given engine at full throttle depends entirely upon engine speed.

    The peak power and peak torque occur at different engine speeds. For example, a particular engine develops 150bhp at 4,000rpm. This is the power peak. However, the torque peak for the same engine is 300Nm at 2,000rpm.

    It depends on the type of car. The all-encompassing answer is that more fuel and air are put into the engine, allowing more engine speed/load to raise. The accelerator of a carburetted spark ignition engine simply opens a valve (the throttle) which lets more air into the engine; the carburettor then adds more fuel. A spark ignition engine with electronic fuel injection will also open a throttle, but also electronically squirt more fuel into the inlet tract, and also modify ignition timing.

    A simple diesel engine will typically just squirt more fuel into the cylinders, but a more advanced one will also modify the injection timing profile and (if fitted) the turbo boost characteristics.

    More torque is provided to the wheels in a lower gear. This is to facilitate moving off from a standing start, and to give high acceleration from low speeds.
     
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