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Is the energy output of an engine constant?

  1. Jun 24, 2015 #1
    I know that the power of an engine is equal to (rpm*torque)/ 9.54 (SI
    units). But does this mean that an engine when turned on is outputting the same amount of energy per unit time, every time ? . What happens when we press the gas pedal, does the power output increases? When i drive my car faster, the fuel consumption increases, is that due only to dissipative forces, or does the power output also increases.

    Another thing:

    When we run, for example 1 km , we do a certain amount of work. Say that instead of running we walk the same distance. In which occasion do we spend more energy? (regarded that the only force in play is gravity, and there no resistive forces )

    Thank you in advance :)
  2. jcsd
  3. Jun 24, 2015 #2


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    Hi there
    welcome to PF :)

    so what do you think the answer is ?

  4. Jun 24, 2015 #3
    In the case of the engine my intuition tells me that the energy output does change, but when i try to reconcile my intuition with my knowledge of physics i don't see how that can be, it must remain constant.

    The same case with running or walking. The only force we are subjected to is gravity, and gravity does no work on us since we don't move in the y direction, and therefor if there is no dissipative forces to overcome we wouldn't need to do any work. But in reality we do work, and i guess i do more if when i run than when i walk. Can someone help me to understand this?
  5. Jun 24, 2015 #4
    Use the first law of thermodynamics, more fuel is used, ie more energy is used, so more energy is transforming + a contribution lost to heat, so yes, hitting to pedal will make the car output more power,
    As for the second case, it a bit more complicated because human dynamics is different and the will allways be resistive forces, but if you replace it with the old fashioned point that if move it it will stay in motion, to get to 1km faster you need more velocity, and so more kinetic energy, the work needed is equal to that KE, good luck
  6. Jun 24, 2015 #5
    So if an engine has 120 kW of power. That doesn t mean the engine will be using 1000 j per second, every time?Does the relation between power torque and rpm still hold, when the engine is not operating at full power?

  7. Jun 24, 2015 #6


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    Note: 120 kW = 120,000 J / s, from the definition of the watt.

    Why wouldn't it?
  8. Jun 25, 2015 #7
    If an engine is rated it is for a particular power at a given constant rpm at a given torque.
    For.e.g. 12000kW at 127 rpm
    600kW at 900rpm.
    Or 120kW at ? rpm
    A car at 100kW can run with lets say 500rpm for x liters of fuel. Same car will get only 400rpm at an incline for same fuel and power.
    To get same speed of 500rpm you need more fuel per stroke of piston. Hence more power.
    Power rating in general is the highest safe rated power.
  9. Jun 25, 2015 #8
    OK, i got the ideia. The power advertised by the car brands is bound to a specific rpm.
    Imagine this:
    We have two blocks on a frictionless surface, both weight the same. The first covers 100 meters in 6,66 seconds the second does it in 20 seconds. Which block needs more work?

    The first would need more power than the second, but will run for less time.In the other hand, the second will need less power, but will run for a greater time. So the work should be the same, right?

    Ty very much for the help so far
  10. Jun 25, 2015 #9


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    Neither needs any work to move on a frictionless surface.
  11. Jun 25, 2015 #10
    If they start at rest, they need some work to move at given speed, am i wrong
  12. Jun 25, 2015 #11
    ASSUMING that these blocks are free floating in space.
    Power is the RATE of doing work. Since the first block moves faster it requires more power.
    Work done = force x distance
    Now distance is the same but did you give the same force to both blocks?
  13. Jun 25, 2015 #12
    Yes, but which one will need more work?
  14. Jun 25, 2015 #13


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    Depends how you accelerate them. If both have constant acceleration the quicker one will need more work.
  15. Jun 25, 2015 #14
    Distance and weight are the same. But one has a greater speed in the end of 100 meters. One does the 100 m in 6.66 seconds, the other needs 20 s. So the forces need to be different, i think...
  16. Jun 25, 2015 #15
    The one who cover the distance in less time has more speed, so more kinetic energy (assuming the value of M2 is around M1), so you need to do more work on it to get it to that speed, but once reached you need no more work ( so no power), driving a car is very different because resistive for cannot be neglected at all
  17. Jun 25, 2015 #16
    Yes I know that the example is an oversimplification, but i needed it to make sense. N
    I know this is an oversimplification but it is easier to understand the concepts. Noctisdark can you explain me why a engine with more power going at the same speed as other with half the power uses more fuel?
  18. Jun 25, 2015 #17


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    Power is defined as energy per time. So assuming the same efficiency, more power means more fuel per time.
  19. Jun 25, 2015 #18
    Your observation was wrong, how do cars accelerate then ? there is allways a friction force that tries to pull back the car, you hit to pedal we overcome that force (or just cancel it to maintain the same speed), so your observation is only valid if the friction force vary, ie more friction force will require you you hit the pedal harder but move at the same speed, mathematically, let f be the friction force, to be at isolated the car also has to force with magnitude f then the work is f*d, when the friction force is more intense then you'll need more work thus more power !
  20. Jun 25, 2015 #19


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    This is the actual power output at one particular time. If you change the torque or the rpm, then obviously the power changes.
    The car goes faster, so the rpm increases and the torque must have increased to cause that acceleration. So power increases on both counts.
    Why must power remain constant? When the car is sitting in the garage, it outputs no power. When you start and it ticks over it outputs just enough to overcome engine "friction". When you load up the family and luggage and attach a camper van, then accelerate hard, it must be putting out a lot of power. I just can't imagine what knowledge of physics is predicting otherwise? I think your intuition seems to be a better guide.
    I think this is the clue! If you have a 120kW engine (lucky you, mine is only 40kW) then the manufacturer is claiming that you can get it to output this much power - it is the maximum, you cannot get more. Most of the time, nearly all of the time, it will be producing much less than this.
    The relation between power, torque and speed is always true, but the torque and speed vary, so the power varies.
    Actually we do: we bob up and down a bit. In the case of running, when you may be out of touch with the ground between footfalls, this is more obvious than in walking.
    Even if we do no work against outside forces, we move our limbs causing internal "friction" using inefficient muscles, pumping blood and air, etc. As for the vertical bobbing, we do work lifting ourselves each step, but do not recover all of that energy when gravity pulls us back down - in fact we have to do yet more muscular movement to cushion the impact.
    I don't know if you do more work running than walking, but you do it in a shorter time, so the power is greater. It is conceivable that running could be more efficient than walking. I think I feel more tired after going for a walk with my partner, who is very slow, than if I walk a similar distance more quickly on my own, but there are many confounding issues there.
    Other posters comments are agreed.
    Assuming you apply a constant force, then you are doing work to accelerate the blocks and the one that gets there first must have greater acceleration.
    a = 2xdist/time2 so one block a= 200/ 6.62 = 4.6m/sec2 other a= 200/400 = 0.5 m/sec2
    So the faster one has 9.2x the acceleration, so needs 9.2x the force.
    Work = force x dist, so faster block needs 9.2x the work, done in shorter time 6.6/20.
    So average power for faster block is abot 27.8x the av. power for slower block.
    For constant force, accn is constant, so speed increases linearly and so does rate of doing work. So power is increasing linearly.

    When you stop pushing both blocks continue at constant speed, one being about 3x faster than the other and having 9.2x the KE than the other.

    I think that however you accelerate the blocks, even with a varying force, you will always need to do more work and apply greater average power on the block that gets there faster and that will be reflected in a faster speed and greater KE for that block.
    Last edited: Jun 25, 2015
  21. Jun 25, 2015 #20
    By Merlin's Beard!!! What an answer!!
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