torque.....what the...?

[mutt]

Okay. So- torque is a measurement of how much "work" a spinning shaft can do.
HP is how quick that work can be done.
if torque is a measurement of how much energy is in a rotating mass.....why, or HOW, does torque DECREASE above some certain RPM?
Why would, iow, a rotating mass produce (say) 120 ft lbs of torque at (say) 2800 rpm, but only posess 98 ft lbs of torque at 3500 rpm?

 

[ludwig]

So- torque is a measurement of how much "work" a spinning shaft can do.

Torque is a measure of TWISTING FORCE. As the engine speeds up, the twisting force rises too. At a certain point, where the two curves of RPM and Torque cross, the twisting force falls off due to ineffeciency, maximum total force generated by the motor and is more affected by the resistance of the load.
If you pedal a bicycle on the big ring only, you mash the pedal very hard. This gives a lot of twisting force to the gear wheel. A a certain point you achieve a lot of speed but the amount of force you can apply on each rotation diminishes. You can only stomp the fast moving pedals so hard at a high speed and the total twisting force falls off from its peak.

 

[StrangeRanger]

Torque is twisting force but it is produced by combustion of the fuel-air mixture. The amount of mixture an engine consumes per rev varies as a function of RPM. If you were to plot torque vs. RPM and Volumetric Efficiency vs. RPM on the same graph, the curves would be amazingly similar.

 

[Lazy JW]

These terms can be a bit confusing.

As mentioned above, Torque is a twisting force. Torque can be applied with zero work being accomplished. Think about what happens when you torque the headbolts on your Shovelhead. When you are turning the bolt, work is being accompsished (tightening the bolt). As you get up to torque and hold it there without moving, there is no work actually being done (the bolt is stationary) even though torque is being applied, but you will get mighty tired holding it.

Some steam engines and certain electric motors are capable of applying torque at zero rpm; therefore no work is being done and no horsepower is being developed.

Horsepower is a measure of work done in a given amount of time. In order for horsepower to be developed, something must be moved; also the amount of time must be measured. James Watt defined the Horsepower as 550 lbs/feet of work per second, or 33,000 lbs/feet per minute. He conducted experiments and detirmined that an average work horse could produce this amount of work for relatively long periods of time.

Horsepower does not need to be rotary motion. A hydraulic cylinder can develop measureable horsepower in linear motion without applying any torque at all! A rifle develops horsepower when firing a bullet, although it is not customarily described as such. Still, work is being done in a measureable time frame, so the definition of horsepower still fits.

So we see that torque does not necessarily generate horsepower, and horsepower can also be exclusive of torque.

A typical car engine does require rotary motion to generate both torque and horsepower. This is where the confusion starts.

Torque will be maximum whenever there is the most "push" on the piston. This is going to take place pretty close to the time when the most air/fuel mixture is drawn into the cylinder which is also known as maximum volumetric efficiency. When we rev our low speed 300's too fast, this volumetric efficiency drops off because the cams, etc. aren't right for high speed useage. Less air/fuel enters the cylinder and torque also decreases, even though the engine spinning very fast.

I like to think of torque as the "push", and horsepower is how many "pushes" per minute. More "pushes" equals more horsepower. Stronger "pushes" equals more torque.

Clear as mud, eh?
Joe

 

[ludwig]

Very nicely explained.

Torque will be maximum whenever there is the most "push" on the piston. This is going to take place pretty close to the time when the most air/fuel mixture is drawn into the cylinder which is also known as maximum volumetric efficiency.

Past this point the effeciency drops off and the torque curve - twisting force - diminishes even as the engine goes faster.

I like to think of torque as the "push", and horsepower is how many "pushes" per minute. ... Stronger "pushes" equals more torque.

My point with the bike in high gear. High speed but pushes of lower power at each pedal stroke as the RPM rises. Conversely, as the grade rises more torque is produced when the RPM slows and the rider stomps harder - in the same gear, yet the vehicle slows.

 

[XPC66]

At one time torque wrenches were correctly called tension wrenches.

Torque measuerment in a combustion engine is related to BMEP, so just below peak BMEP is peak VE and correspondingly peak torque.

Torque is a cross product of two vectors = power and annular displacement. As has rightly been said power is not derived from torque, but torque is derived from the power vector, whether that be gravity acting over time or the liberation of heat via the combustion process. A car sitting in the driveway will not move until the joules/sec liberated by the burning fuel are used to drive the pistons, no matter how many times you sit there changing gears and playing vroom vrooms.

The piston generating the power from the combustion process is linear and the car travelling along the road is linear. The rotating stuff in between is of little consequence and therefore torque has taken on an undeserved cult like status. The g force that people attribute to torque is in reality the power delivery from a linear process being delivered to another linear process. A rocket strapped to the car will slam you back in the seat just as easily and it isn't a rotating machine

Acceleration, by virtue of it being a rate of change will have the same curve characteristic as the torque. That is simple physics and could just as easily be written with power and annular acceleration nomenclature..

The misconception that the torque curve crosses the horsepower curve at a magical number, is purely because that is the scale that has been adopted. Dyno curves with the tractive effort plot from which the torque and power curves are derived have no constraints of the artistic flair of the classic hp/ftlb curves.

The true calculation for torque in SI units is Nm = watts/(2Pi x RPS)

As torque is tracking VE, it stands to reason that as VE falls past the peak BMEP range so will the torque value. It simply reflects the reduction of specific power per stroke, which occurs on the other rpm side too.

 

[mutt]

AHA! I thought it had something to do with mass, now I see low mass can have high torque if it has enough push behind it.
That was my stumbling block, assuming it was tied to rotating mass.
Thank you, fellows.....When my Capn Midnite Ovaltine Decoder Ring arrives, Ill parse out brother xpc66's response, and draw together all the threads....appreciate it, youve explained a lot.......

 

[mikeyo]

I hope I remember the numbers correctly but as I recall:
Torque will always be higher than HP below 5250 RPM and HP higher
above this. 5250 I think is the crossover point.
This would be why a very large diesel engine might seem to have lower
HP numbers for it's displacement. It may produce max torque at 1500 RPM. A large cummins in a semi for example may be rated at 400hp. Doesn't sound like a lot until you read the rest of the specs, that 400hp may be at say 1800 RPM and the torque required to make that horsepower number at that low RPM may be 1900 lb/ft.
A worked over 302 or 350cheby might also dyno at 400 hp but have to spin up to 5500RPM to make that number.

 

[mikeyo]

I hope I remember the numbers correctly but as I recall:
Torque will always be higher than HP below 5250 RPM and HP higher
above this. 5250 I think is the crossover point.
This would be why a very large diesel engine might seem to have lower
HP numbers for it's displacement. It may produce max torque at 1500 RPM. A large cummins in a semi for example may be rated at 400hp. Doesn't sound like a lot until you read the rest of the specs, that 400hp may be at say 1800 RPM and the torque required to make that horsepower number at that low RPM may be 1900 lb/ft.
A worked over 302 or 350cheby might also dyno at 400 hp but have to spin up to 5500RPM to make that number.
Effectively torque continues to climb with RPM, but above 5250 RPM hp climbs faster (provided the engine VE peaks above this RPM). Torque and hp begin to decline once the engine VE peaks, due to design and restrictions, simply the engines in-ability to move any more air.

 

[XPC66]

Torque units and horsepower units are different, so one can't be higher than the other. The 5252 is just a constant : if you used the metric equivalent you'd see the crossover at ~9549 rpm using the same y axis units for each measurement.

Torque is power divided by radians per second. From this relationship you can easily see that by gear reduction you reduce the radians per second and thus increase the torque with the same power available from the engine.

 

[69.5Mav]

To complecated

Think of it this way

You have two different engines, One with high torque and low horsepower the other with low torque but high horsepower.

The high torque engine won't go very fast because of the low horsepower but it will reach that speed very quickly because of the high torque.

The low torque engine will go much fatser because of the high horsepower but it will take a long time to reach that speed because of the low torque.

So generally speaking a fast accelerating car has a lot of torque and a high speed car has a lot of horsepower. At the drag races you want both.

Now the two engines I proposed can not in fact be built because the production of horsepower and torque are interlinked but it help to show the effects of each on performance.

 

[80broncoman]

As mentioned above, Torque is a twisting force. Torque can be applied with zero work being accomplished. Think about what happens when you torque the headbolts on your Shovelhead. When you are turning the bolt, work is being accompsished (tightening the bolt). As you get up to torque and hold it there without moving, there is no work actually being done (the bolt is stationary) even though torque is being applied, but you will get mighty tired holding it.

Some steam engines and certain electric motors are capable of applying torque at zero rpm; therefore no work is being done and no horsepower is being developed.

funny you should mention electric motors, Yesterdy at work I was looking at westinghouse 2,000HP motor (We have lots of motors this sizie in the plant) but I am alway wondering at just how much torque these things make. Well this one had it on the info plate :!: 700 volt DC

It said torque 481,000 foot lbs. RPM 0-56

 

[mutt]

IS THERE A CAM FOR THAT!!!!??????MUST POSSESS.........hmmmm....i prolly havta reinforce the econos chassis, fer starters.....

"It said torque 481,000 foot lbs. RPM 0-56"

 

[80broncoman]

Ya I always look for the rpm and torque but this is the first motor I have seen it on. And this is ONLY a 2000 hp motor I know where there are some 4000 and 6000 hp motors in the plant.
Just to let you know the overall diameter of the 2k hp motor is about 10 feet.
I'd say it weighs 15-20 TONS a little too much for your van.

 

[mikeyo]

I was doing a job at a nuculear power plant awhile back and was amazed at the size of the electric motors that ran the cooling water pumps.
I asked one of the plant engineers and he told me they were 10,000hp motors.
Wonder if the equation is linear on electric motors? That would equate to....hmmm..... ........a whole bunch of torque.
I also asked the engineer how much water was being pumped per each unit....I got a technical nuclear engineer answer-not-.....after a moment of deep thought he anwered "a butload" and grinned.

 

[XPC66]

To complecated

Think of it this way

You have two different engines, One with high torque and low horsepower the other with low torque but high horsepower.

The high torque engine won't go very fast because of the low horsepower but it will reach that speed very quickly because of the high torque.

The low torque engine will go much fatser because of the high horsepower but it will take a long time to reach that speed because of the low torque.

So generally speaking a fast accelerating car has a lot of torque and a high speed car has a lot of horsepower. At the drag races you want both.

Now the two engines I proposed can not in fact be built because the production of horsepower and torque are interlinked but it help to show the effects of each on performance.

You lost me on that post.

When you say high torque, what is the benchmark?

 

[Lazy JW]

...

The high torque engine won't go very fast ...but it will reach that speed very quickly because of the high torque.

You obviously have never driven a Poppin' Johnny tractor. Rapid acceleration is not one of their strong points despite the very high torque rating.

The low torque engine..... will take a long time to reach that speed because of the low torque.

The late Gene Berg would smile at THAT statement while his Volkswagen left you in the dust. Many misconceptions in this business, eh?
Joe

 

[Lazy JW]

...
Wonder if the equation is linear on electric motors?....

Not linear but some designs are potentially much closer to linear than internal combustion engines. Electric motors suffer from inherent inefficiencies such as eddy current losses, slippage, etc. There are many different configurations also depending on ultimate useage that give different torque characteristics. AC vs. DC, shunt wound, series wound, squirrel cage, slip ring, synchronous, etc. etc. There are folks who have earned their PhD studying this very topic so obviously a lot more is involved than can be detailed here.
Joe

 

[XPC66]

Many misconceptions in this business, eh?
Joe

Too right. It seems that peaks are king when it comes to internet chatter. To get that "high torque" motor the power has to be availalble at that rpm.

Your tractor starts off with a fairly high power level and presumably tails off after a fairly narrow rpm range, while a short stroke touring car would have relatively lower power at the same rpm tractor range, but have a much wider overall range, so that that both have the same potential peak power, but the tractor has possibly higher average torque.

So 69.5Mav, say Joe's tractor engine had an effective operating rpm range of 2000rpm with average of 250 ftlbs and my equal weight tractor had a 4 pot, same peak power, Jap engine with an effective rpm range of 6000 rpm and an average of 150ftlbs, do you think if I changed my tractor final reduction ratio to 6000/2000 = 3 times that of Joe's I would stand a chance over the quarter?

 

[69.5Mav]

OK

Remember these are not real engines!

This is a thought experiment only; you could never build an engine with, for the sake of argument, 1000 f/lbs of torque and only say 50 HP. But if you could its Acceleration would be very high because Torque is directly proportional to Acc., and its top speed would be low, say maybe 80 mph or less. So it would go slow but get to that speed very fast.

The same works in reverse with a high HP engine with low Torque.

Once again these are not real engines and any attempt to compare them to real engines will only result in confusion and discussions over semantics.