Is the energy output of an engine constant?

[Thom_Silva]

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 :)

 

[davenn]

Hi there
welcome to PF :)

so what do you think the answer is ?Dave

 

[Thom_Silva]

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?

 

[Noctisdark]

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

 

[Thom_Silva]

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?

Ty

 

[SteamKing]

So if an engine has 120 kW of power. That doesn t mean the engine will be using 1000 j per second, every time?

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

https://en.wikipedia.org/?title=Watt

Does the relation between power torque and rpm still hold, when the engine is not operating at full power?

Why wouldn't it?

 

[Rippetherocker]

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 let's 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.

 

[Thom_Silva]

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

 

[A.T.]

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?

Neither needs any work to move on a frictionless surface.

 

[Thom_Silva]

Neither needs any work to move on a frictionless surface.

If they start at rest, they need some work to move at given speed, am i wrong

 

[Rippetherocker]

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?

 

[Thom_Silva]

Yes, but which one will need more work?

 

[A.T.]

Yes, but which one will need more work?

Depends how you accelerate them. If both have constant acceleration the quicker one will need more work.

 

[Thom_Silva]

ASSUMING that thes

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?

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...

 

[Noctisdark]

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

 

[Thom_Silva]

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

Yes I know that the example is an oversimplification, but i needed it to make sense. N

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

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?

 

[A.T.]

can you explain me why a engine with more power going at the same speed as other with half the power uses more fuel?

Power is defined as energy per time. So assuming the same efficiency, more power means more fuel per time.

 

[Noctisdark]

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 !

 

[Merlin3189]

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

This is the actual power output at one particular time. If you change the torque or the rpm, then obviously the power changes.

What happens when we press the gas pedal, does the power output increases?

The car goes faster, so the rpm increases and the torque must have increased to cause that acceleration. So power increases on both counts.

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.

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.

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?

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.
==================

... 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,

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.

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?

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.
======================

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?

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.

 

[Rippetherocker]

By Merlin's Beard! What an answer!

 

[Thom_Silva]

require you you hit the pedal harder but move at the same speed, mathematically, let f be the

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/sec²First of all, thank you very much for your extended answer i really appreciated your kindness and effort. Just to clarify i thought the output energy remained constant because i was not aware the engines are defined in power vs rpm curves. So i thought that if you wanted to increase rmp the torque will have to go down. But i know now that is a caractheristic of an engine.

 

[Thom_Silva]

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 !

"To put two cars with the same mass going at a given speed, we will need the same amount of energy" i think this assumption is right. I'm trying to reason why one car being more powerful than other seems to use more energy, even if they go at same speeds. That i can't understand. Assuming that there is no slipping, the energy of the fuel is being used to produce a torque and a RPM, and of course there will be losses to internal friction in the engine components, air darg an so on. So, to put a mass going at a given speed will need a certain amount of energy, and that amount should be equal to every car for that same speed, regardless if an egine is more powerful than the other.That will only influence time needed to acquire the given speed.

Sorry if I'm being annoying :)

 

[Merlin3189]

... thank you very much for your extended answer i really appreciated your kindness and effort.

It's nice to be appreciated. Thanks. I guess like many people here, I find it interesting to think about and discuss these things, so it's not totally altruistic.

i thought the output energy remained constant because i was not aware the engines are defined in power vs rpm curves. So i thought that if you wanted to increase rmp the torque will have to go down. But i know now that is a characteristic of an engine.

Now there's an interesting thing. I'm aware of these curves and I can get some info from them, but I've never found a clear explanation of what they actually show. I assume that they show the MAXIMUM torque or power available from an engine at a particular rpm, but I would have thought they should really be a family of curves showing the torque or power output vs rpm at different throttle settings. If they really are maximum graphs at full throttle (or full gas or whatever it is called now we have fuel injection) then 99% of the time we operate our engines somewhere under these curves, but we have little idea where.

 

[A.T.]

So, to put a mass going at a given speed will need a certain amount of energy, and that amount should be equal to every car for that same speed, regardless if an egine is more powerful than the other.That will only influence time needed to acquire the given speed.

Yes, ignoring losses, that is true. If by "put a mass going at a given speed" you mean accelerate a mass from zero to a certain speed.

 

[Thom_Silva]

Yes, ignoring losses, that is true.

So, ignoring losses, a more powerfull car should use the same amount of fuel... If in reality this hardly happens, then more powerful engines must be associated with less efficiency, is this assumption right?

 

[Merlin3189]

Re. 2 cars of equal mass.
To get both cars up to the same speed does require the same amount of energy, because they then have the same kinetic energy.
But if that were the whole, or even then main part, of the story, we could just slip into neutral and coast to our destination. In fact a lot of the power during a journey is put into overcoming "frictional" forces like air resistance and rolling resistance. These will depend on the car's shape, wind, wheel and surface properties.
Also your speed does not stay constant, particularly in urban driving. Every time you have to increase your speed, you have to put in more energy again, but you don't get it back as you slow down. If you go up hill you also need to use energy to work against gravity, though you do get some of that back when you go down again.

And that is thinking about the useful energy output by the engine. A lot of the fuel energy put into the engine gets wasted in the conversion to mechanical energy, in moving the engine parts and ancillary equipment (eg. alternator, pumps, air conditioner) and even in inspiring and exhausting gases.

 

[A.T.]

So, ignoring losses, a more powerfull car should use the same amount of fuel...

... to accelerate from zero to a certain speed assuming the same mass.

 

[Thom_Silva]

Re. 2 cars of equal mass.
To get both cars up to the same speed does require the same amount of energy, because they then have the same kinetic energy.
But if that were the whole, or even then main part, of the story, we could just slip into neutral and coast to our destination. In fact a lot of the power during a journey is put into overcoming "frictional" forces like air resistance and rolling resistance. These will depend on the car's shape, wind, wheel and surface properties.
Also your speed does not stay constant, particularly in urban driving. Every time you have to increase your speed, you have to put in more energy again, but you don't get it back as you slow down. If you go up hill you also need to use energy to work against gravity, though you do get some of that back when you go down again.

And that is thinking about the useful energy output by the engine. A lot of the fuel energy put into the engine gets wasted in the conversion to mechanical energy, in moving the engine parts and ancillary equipment (eg. alternator, pumps, air conditioner) and even in inspiring and exhausting gases.

So is theoretically possible to have a very powerfull car (a super car for example), that can use less fuel to mantain a given speed than a less powerful car. If that is true, that's an interesting realization. I wonder why doesn't that happen more frequently, if it happens at all.

 

[Noctisdark]

So the most important thing you get out of this is that when you hit the pedal you accelerate the car in some direction, and you feel that acceleration, like a force you pushes you toward you seat, hitting the pedal make the car use more energy hence accelerate (in some direction), two cars with the same mass use the same amount of energy to accelerate but reach different speed mean that one car is loosing energy more than the other, a car will never stop losing energy by contact with the floor, the more massive the car will more will be the energy loss, the car won't stay at motion so we need a force to let it be at motion, that's why the car output some power, to overcome the force of friction and keep moving at the same speed

 

[russ_watters]

So, ignoring losses, a more powerfull car should use the same amount of fuel... If in reality this hardly happens, then more powerful engines must be associated with less efficiency, is this assumption right?

You are exactly right. That's why for otherwise identical cars, the upgraded engine version almost always gets poorer fuel economy.

The lower efficiency comes largely from the fact that the larger engine has more/larger moving parts, so it loses more energy at a given output than a smaller engine.

 

[russ_watters]

... to accelerate from zero to a certain speed assuming the same mass.

I think the qualifier also implied only losses internal to the engine, so it is true at constant speed as well -- the real world application being what I described above.

 

[russ_watters]

So is theoretically possible to have a very powerfull car (a super car for example), that can use less fuel to mantain a given speed than a less powerful car. If that is true, that's an interesting realization. I wonder why doesn't that happen more frequently, if it happens at all.

Some of the losses are a fixed function of size, not a fraction of output. So for identical cars and driving, the smaller engine will essentially always produce better fuel efficiency.

Say, for example, 2 engines are each 30% efficient at peak power, 25% efficient at half power. If one engine can produce 100 hp while the other can produce 200, then at 100 hp, the smaller engine is 30% efficient and the larger engine 25% efficient.