Why can't I understand Torque?

[Guest]

I

SO: A crane requires a certain amount of torque from its motor, based on the mechanical advantage of the gears and rigging and the lifted load. That amount of torque is constant whether you are lifting that load at 5 feet per minute or 10 fpm. But, if the *5* fpm version uses an 1800 rpm motor, the 10 fpm one needs a 3600 rpm motor. The 3600 rpm motor will have the same torque as the 1800 but with twice the speed it will have twice the power.

Sounds right to me, with the noted correction.

 

[clusterm2]

Put a wrench on a tight bolt and try to get it loose. You're applying torque (rotational force), equal to the force you're applying to the wrench handle (force) times the distance your hand is from the bolt's axis (distance). Torque = force * distance.

You know about rotational speed, revolutions per minute (RPM). That's analogous to straight ahead velocity.

Horsepower = Torque * RPM / 5252, where torque is measured in lb-ft (one pound of force applied a foot away from the axis).

So, horespower can increase by either increasing the torque an engine can provide at a given speed, or increasing the speed that it can provide a given torque at. Small Formula One engines have VERY high horsepower ratings because they turn at insane RPMs and still provide moderate torque. As far as the car's concerned, with the right gearing it will all be the same: more acceleration.

So, torque is force and horsepower is torque times rotational speed. Simple?

If that was the case then the torque and horsepower curves, when graphically illustrated, would have the same shape...They don't

 

[garys_2k]

If that was the case then the torque and horsepower curves, when graphically illustrated, would have the same shape...They don't

The equation is correct but engines may not behave the way you may think they do.

Engines produce only torque and speed, horsepower is a calculated value showing how much energy they are producing per unit of time. Engine torque is multiplied with gears to create a lateral force on the contact patch of the tire where it meets the road. More torque means more force and more acceleration.

Horsepower is a measure of "how quickly" the engine can produce that torque (note to LucyR, please give me some latitude here). Producing a very high torque at a low speed, like a truck engine, is fine for applications where ultimate acceleration isn't needed because the total POWER, torque multiplied by RPM, doesn't have to be too high. But if you want plenty of acceleration for a sustained period of time, like a dragster, go for horsepower and the proper gearing.

The horsepower curve usually peaks a bit after the torque curve because it benefits from both torque and speed. Once the engine's friction (which increases as a quadratic function of engine speed, with constant, linear and squared terms) becomes too great, and induction and cylinder filling losses increase at higher speeds, the torque drops with increasing speed faster than the speed itself increases. This causes the horsepower to drop with increasing RPM after the peak: torque is going down too fast.

So, engines ONLY produce torque and speed. Horsepower is proportional to their product. Does this help?

 

[Iconoclast]

The horsepower curve usually peaks a bit after the torque curve because it benefits from both torque and speed.

Usually? It's mathematically impossible for the power peak to occur before the torque peak. Furthermore, only with a (theoretical) torque profile that drops immediately to zero after it's peak will the power and torque peaks occur at identical engine speeds. So, for real world engines, the power peak must ALWAYS occur after the torque peak.

 

[clusterm2]

The equation is correct but engines may not behave the way you may think they do.

Engines produce only torque and speed, horsepower is a calculated value showing how much energy they are producing per unit of time. Engine torque is multiplied with gears to create a lateral force on the contact patch of the tire where it meets the road. More torque means more force and more acceleration.

Horsepower is a measure of "how quickly" the engine can produce that torque (note to LucyR, please give me some latitude here). Producing a very high torque at a low speed, like a truck engine, is fine for applications where ultimate acceleration isn't needed because the total POWER, torque multiplied by RPM, doesn't have to be too high. But if you want plenty of acceleration for a sustained period of time, like a dragster, go for horsepower and the proper gearing.

The horsepower curve usually peaks a bit after the torque curve because it benefits from both torque and speed. Once the engine's friction (which increases as a quadratic function of engine speed, with constant, linear and squared terms) becomes too great, and induction and cylinder filling losses increase at higher speeds, the torque drops with increasing speed faster than the speed itself increases. This causes the horsepower to drop with increasing RPM after the peak: torque is going down too fast.

So, engines ONLY produce torque and speed. Horsepower is proportional to their product. Does this help?

Still doesn't explain a very flat torque curve along with a steep horsepower curve like say on a Buell. Here there appears to be no relationship between the two.

 

[scotth]

Still doesn't explain a very flat torque curve along with a steep horsepower curve like say on a Buell. Here there appears to be no relationship between the two.

I wide flat torque curve will always cause a steadily rising HP curve.

If torque stays constant across and RPM range, the product of torque * RPM (HP) will have to go up across that range.

 

[Iconoclast]

Still doesn't explain a very flat torque curve along with a steep horsepower curve like say on a Buell. Here there appears to be no relationship between the two.

Well sorry mate, but rotational power is defined as the product of torque and angular velocity. Either the curves you have are wrong or you haven't done some test calculations against them to check whether or not the power at each point on the curve is equal to the torque at the same point multiplied by the angular velocity at that point.

 

[garys_2k]

Usually? It's mathematically impossible for the power peak to occur before the torque peak.

True.

Furthermore, only with a (theoretical) torque profile that drops immediately to zero after it's peak will the power and torque peaks occur at identical engine speeds.

Not at all. All you need is for the torque curve to drop faster per RPM increase than the RPM increases. For example, if the torque drops off at a 3X downslope with a 2X increase in speed, the horsepower will drop after the torque peak. Some engines, usually real stump pullers, have torque and horspower curves with nearly identical peak points. There's no real reason they couldn't be at identical points.

 

[Iconoclast]

There's no real reason they couldn't be at identical points.

Yes there is. It's called calculus.

 

[garys_2k]

Yes there is. It's called calculus.

Really?

What physical impossibility is expressed in this hypothetical torque output?

RPM - - - - Tq - - - - Hp
500 - - - - 75 - - - - 7.14
1000 - - - 85 - - - - 16.2
1500 - - - 100 - - - 28.6
2000 - - - 65 - - - - 24.8
2500 - - - 50 - - - - 23.8
3000 - - - 25 - - - - 14.3

Using advanced engine control you can pretty much make a torque and horsepower curve anything you want, within the physical limits of the hardware, cam, induction. I know, see my home state for a hint at how I know.

 

[scotth]

Really?

What physical impossibility is expressed in this hypothetical torque output?

RPM - - - - Tq - - - - Hp
500 - - - - 75 - - - - 7.14
1000 - - - 85 - - - - 16.2
1500 - - - 100 - - - 28.6
2000 - - - 65 - - - - 24.8
2500 - - - 50 - - - - 23.8
3000 - - - 25 - - - - 14.3

Using advanced engine control you can pretty much make a torque and horsepower curve anything you want, within the physical limits of the hardware, cam, induction. I know, see my home state for a hint at how I know.

Try doing this with a little finer RPM resolution and you will almost certainly find that it fails.

In real life, torque curves do not drop rapidly enough after peak for peak HP and peak torque to coincide.

 

[garys_2k]

Try doing this with a little finer RPM resolution and you will almost certainly find that it fails.

In real life, torque curves do not drop rapidly enough after peak for peak HP and peak torque to coincide.

Oh, but we CAN now make torque curves drop as fast as we want. "Traction control" schemes can allow us to manipulate an engine's torque curve dynamically, setting the peak or slopes to almost arbitrary values.

I was only pointing out that there is no inherent, mathematically-based reason that a torque and power peak couldn't happen at the same engine speed. As long as the torque falls off past that speed faster than the speed itself increases, the power will also decrease.

 

[scotth]

Oh, but we CAN now make torque curves drop as fast as we want. "Traction control" schemes can allow us to manipulate an engine's torque curve dynamically, setting the peak or slopes to almost arbitrary values.

I was only pointing out that there is no inherent, mathematically-based reason that a torque and power peak couldn't happen at the same engine speed. As long as the torque falls off past that speed faster than the speed itself increases, the power will also decrease.

I would agree that if a torque curve has a very sharp break downward starting at its peak, HP and torque would be at a peak at the same RPM.

Generally, without some out of the ordinary contrivance this doesn't happen.