A question about horse power

[Mac]

Horse power is the measure of the efficiency of torque over time.


My question is, why is an engine developing 100 ft.lbs. of torque @ 1000 rpm less efficient/powerfull than a similar engine putting out a similar 100 ft.lbs. of torque @ 10,000 rpm?

Is it due to the higher reving engine's quicker power pulses? Less loss due to friction? Inertia of rotating mass?

The question has been bothering me for a few days now.  

[Mookie]

Why are you adding efficiency into the equation?  HP is just:

                                      Torque * RPM
        Horsepower      =       ------------
                                         5252


since you're always dividing by 5252, the higher the RPMs the bigger the HP is going to be.  It's not really saying anything about efficiency, just how fast it can do the work.


/i think

[baldheadeddork]

Horse power is the measure of the efficiency of torque over time.


My question is, why is an engine developing 100 ft.lbs. of torque @ 1000 rpm less efficient/powerfull than a similar engine putting out a similar 100 ft.lbs. of torque @ 10,000 rpm?

Is it due to the higher reving engine's quicker power pulses? Less loss due to friction? Inertia of rotating mass?

The question has been bothering me for a few days now.  

I don't know if engine speed alone can be used to evaluate efficiency. Friction losses increase with speed, but that doesn't mean that slower speed engines are inherently more or less efficient.

[Mac]

Then what is the benefit of a higher hp engine that puts out the same amount of torque as a lower hp one?

[Green01SprintST]

I believe the benefit is the higher horsepower potential of the high revving engine.  

[Mookie]

Think about a motor that only made 1 ft/lb of tourque but revved to 500,000 rpm

It's still make almost a hundred HP but it'd be worthless to drive.

[garry]

Torque provides acceleration. Horsepower provides top speed. It's hard to push a motorcycle thru the air once you get into triple digits.

The other thing is that torque curves aren't flat due to the efficiencies of moving air/fuel in and out of the engine at different RPMs.

[kevin_stevens]

Horse power is the measure of the efficiency of torque over time.


My question is, why is an engine developing 100 ft.lbs. of torque @ 1000 rpm less efficient/powerfull than a similar engine putting out a similar 100 ft.lbs. of torque @ 10,000 rpm?

Is it due to the higher reving engine's quicker power pulses? Less loss due to friction? Inertia of rotating mass?

The question has been bothering me for a few days now.  

It may or not be more efficient.  It's less powerful because it's capable of applying that amount of FORCE through only one-tenth the DISTANCE in the same TIME as the the same amount of FORCE spinning/turning ten times the DISTANCE in the same TIME.

Torque is force.  Power is force applied over a distance for a period of time.

To look at it another way, if it takes 100ft-lbs to turn a printing press, your first engine will be able to produce 1000 pages per minute.  Your second engine will be able to produce 10,000 pages per minute.  If you had an engine that revved to 10K but only produced 50 ft-lbs of force, you could gear it down 2:1 (with a transmission) and produce 100 ft-lbs (so you could get the press to turn) and 5,000 pages per minute.  Without a transmission it couldn't turn the press at all, it would just stall.

KeS

[mtrider16]

I'm thinking out loud, and may be incorrect.

Higher rpm, allows more hp with a smaller engine.  

Torque requires longer moment arm and/or larger diameter pistons.  Torque = Force * Moment Arm

The efficiency of pumping air is set by valve timing as well as piston speed.  Thus most engines have a torque curve that drops off at the maximum rpm.

The 5252 equation relates HP, RPM, and T.  However the definition of HP is work per unit of time.  Work is force x distance.

HP then is (F X d)/t
Also F=ma
and T=F*L

Hmmm.....

Now I'm stuck.  I will need to think about this more.

David

[Tritium]

My question is, why is an engine developing 100 ft.lbs. of torque @ 1000 rpm less efficient/powerfull than a similar engine putting out a similar 100 ft.lbs. of torque @ 10,000 rpm?

Here's how I explain it to myself, I have no idea if it's completely correct or not.

Imagine if you have that engine connected straight to a set of wheels with a locked differential, no transmission (one rotation of the crankshaft = 1 rotation of the tires)

In one minute, the 1000rpm car has moved 1000 wheel circumferences. The 10,000rpm engine has gone 10,000 wheel circumferences. The    wheels from both engines were exerting the same amount of instantaneous force, but the 10,000rpm engine did it over a longer distance.

[jschmidt]

Horsepower is simply a made up number invented by James Watt in order to sell steam engines. The multiplier 5252 is simply a random number that compares well with a horse.

Its fine for comparison and a great sales tool but as an actual thing, its completely contrived.

[Tritium]

Horsepower is simply a made up number invented by James Watt in order to sell steam engines. The multiplier 5252 is simply a random number that compares well with a horse.

Its fine for comparison and a great sales tool but as an actual thing, its completely contrived.

Every unit is based on something, whether it's random or not. Mac's question was more along the lines of "what's the difference between force and power?"

[jschmidt]

Well, in his example, the opposite is actually true. The engine making the torque at fewer revs is more efficient.

[kevin_stevens]

Well, in his example, the opposite is actually true. The engine making the torque at fewer revs is more efficient.

No, you can't tell anything about the efficiency of either one because there's no information given about what the energy consumed by either engine is.

KeS

[highside]

Horsepower is simply a made up number invented by James Watt in order to sell steam engines. The multiplier 5252 is simply a random number that compares well with a horse.

Its fine for comparison and a great sales tool but as an actual thing, its completely contrived.

It's really no more arbitrary than any other unit of measure.

[Mac]

Maybe I should have asked

why is it apparently more desirable to have your max torque at a high rpm vs. a low rpm?


Max torque at hi rpm yields hi HP, max @ low yields low hp. Same torque, bu is there the same amount of work being done?


I'm still not quite getting it.


Maybe it is that for a given unit of time, the higher rpm engine will perform more work than the lower rpm engine.


I think thats it.  5252 is just a constant to derive the amount of work done in a easily managable number.

it could just as easily be,

100*10,000=1,000,000

100*1,000=100,000


I guess it turns out Hp is just a math problem.  

[mtrider16]

I still vote that part of it is the size of the power plant.  There are reciprocating industrial and marine engines that will make as much power as a funny car and do it at 1000 RPM or less.  They won't be moving 300mph at the end of a 1/4 mile though.

Metric units can also be used to compare torque, rpm, and power.  They have a different constant than 5252.

That rpm of the crank vs rpm of the wheel sounds right.  By spinning faster, you spread the power aplication out vs the rpm of the wheel.

Interesting way of thinking about it.

David

[jschmidt]

As I understand it, 5252 wasn't the original modifier. Watt adopted it when his original formula didn't prove to be attractive enough to farmers.

[cbsnbiker]

Maybe I should have asked

why is it apparently more desirable to have your max torque at a high rpm vs. a low rpm?


Max torque at hi rpm yields hi HP, max @ low yields low hp. Same torque, bu is there the same amount of work being done?


I'm still not quite getting it.


Maybe it is that for a given unit of time, the higher rpm engine will perform more work than the lower rpm engine.


I think thats it.  5252 is just a constant to derive the amount of work done in a easily managable number.

it could just as easily be,

100*10,000=1,000,000

100*1,000=100,000


I guess it turns out Hp is just a math problem.  

I think you're still not grokking the difference between power and torque.

Torque is force applied in rotation. Power is force (in this case, torque) times distance over time.

What does this mean in real life terms?

Picture using a wrench to try to unstick a stuck bolt. You can apply a pretty impressive torque (force) on the bolt. If you use a torque wrench, you can measure how much torque you are applying to the bolt to try to unstick it. But how much power are you producing? None. Why? Because the bolt hasn't moved. With no movement there is no power. So, you may be creating a lot of torque, but zero horsepower until and unless that bolt moves.

Now let's think about an engine and a crankshaft. An engine could be producing a huge amount of torque, but if the crankshaft isn't moving, no horsepower is being produced. Of course, internal combustion engines don't produce torque if they're not spinning, but electric motors do. In fact, many electric motors produce maximum torque at 0 RPM. (I don't know why.)

Now let's picture two engines. They are identical, except that one produces x amount of torque spinning at, say, 5000 RPM, and the other produces x torque spinning at 10,000 RPM. The second engine produces twice the horsepower. Why? (I leave that to you to answer.)

So, is maximum torque at a high RPM always best? No. It depends on the job you want to accomplish. Do you want to get going from a stop easily, like cruisers? Then you want high torque at low RPM. Do you want to go fast, like a sportbike? Then you want high torque at higher RPM. Do you want easy drivability on the street, with usable ummph from the engine at low, medium, and high RPMs, like a sport-tourer? Then you want a broad torque curve.

Does this help?

[Bjorn Toulouse]

Torque gets the load rolling.
Horsepower keeps the load rolling.

My 11 liter truck engine is rated 400hp @ 1800rpm, peak torque is 1450lb.ft. @ 1500rpm.


Rex