Relation between torque and effect. Why not linear?

[RWebb]

OK, I'll have a shot at an analogy. Imagine you have a nice chariot that will be powered by YOU. The wheel has a shaft sticking out of it and you have to grab that shaft and twist it to make your chariot go. You obviously want a good grip and a LOT of arm muscle to twist it. That is torque.

[Zeke]

Originally Posted by RWebb

OK, I'll have a shot at an analogy. Imagine you have a nice chariot that will be powered by YOU. The wheel has a shaft sticking out of it and you have to grab that shaft and twist it to make your chariot go. You obviously want a good grip and a LOT of arm muscle to twist it. That is torque.

And it was always explained to me that keeping the chariot going was HP.

[Bill Verburg]

Originally Posted by milt

Let's try one more for the uneducated: What is the definition of the "power band?"
IIRC, when setting up a trans for a particular track, you want to keep the revs within the PB as you make your way thru the gears and around the track. So, if not going below max TQ when upshifting, what is the goal?

And why?

You want to keep the rpm in the range where peak hp is generated( not peak torque), the width of the power band depends on the shape of the engine torque curve and the gear ratios, higher numerical ratios make the power band narrower. You shift past the rpm peak because it takes time to shift and the revs fall more than is desired, ideally if you could shift instantaneously you would shift to stay in the rpm range symmetrically w/ peak hp, this is the beauty of the newer e shifters and pdk transmissions.

When setting up a trans for a particular track you want to
* not shift in a corner, or other awkward spot, the esses at WGI come to mind
* not run out of rpm too close to a corner, ideally you would hit redline in a gear at the brake point
* minimize the total # of shifts per lap

the fewer gears you have the harder it is to do 1 & 2, the more gears you have the harder it is to do 3

[Flieger]

Torque times the gear reduction times the final drive ratio divided by the rolling tire radius gives thrust. If you integrate thrust with respect to time or distance, it gives an impulse or a work, respectively. These will change the linear momentum or the linear kinetic energy of the car by causing a linear acceleration.

This is why integrating a torque curve over the "power band" is a measure of how well a car can accelerate. I define "power band" as the rpm range where no gear shifting is occuring.

Complicating factors on this are that there are many rotating components on the car with significant inertia. The linear force applied to the chassis is only a result of the angular acceleration of these components, like wheels. This is why lightweight wheels are so much more important than loosing a few pounds off the chassis (or driver).

[911st]

Milt,

I am not an expert but here is my best guess.

Starting with the longest straight decide on the top speed you should be able to obtain. This should be at close to or just past HP peak.

Look at your slowest corner and use that to set the first so you get your best acceleration out of that corner.

Start with setting the remaining gears to close to equal rpm drops between first and high gear.

Adjust splits up or down to fit exit speed's on other corners or to maximize speed on the important straights or sections.

Tighter gear spacings make for higher average HP.

On street cars gear spacings are wider at lower speeds and tighter at higher speeds. Doing that will help higher speed acceleration rates at the expense of lower speed acceleration rates.

There could be trade offs from there like sacrificing first gear out of the slowest corner for tighter gears on a more important section.

Better yet, check with a real expert or someone that is running a car under similar conditions.

[911st]

...
yes, there is a difference between flywheel torque and wheel torque, the difference is simply the amount of multiplication done by the gear sets, cwp & tires

This point has me thinking.

Again, the term Torque might be getting us in trouble.

The measurements at the crank or wheel are not really motor or wheel, HP or TQ.

They are more of a gross power measurement (load or acceleration rate) that has to be converted to something usable. They are not HP though that is probably a closer description.

HP is probably easyer to derive first (wheel or crank). Then from HP and by adding RPM into the equation, 'motor TQ' values are probably then derived.


Not an expert, just my amateur deduction.

[livi]

Oh dear. This tread virtually exploded over night. There is now so much insightful information that I did not know where to start reading when I just opened up the computer at work this morning. I was going threw the list of patients coming to see me today and I thought - man, my work is a simple one compared to the intricate knowledge these guys hold.

I will print out all your posts and dig in during the weekend.

You guys are truly beautiful! Thanks for sharing your knowledge!

[psalt]

Hello Markus,

See what you started ? There are now more false statements in this thread than true ones. The answer to your original question why a large truck engine has more torque than a small race engine is still displacement. Torque is proportional to displacement and no matter what the design of the engine or speed, peak torque per litre (NA) is limited to around 80 ft/lbs liter.

Like I said, the hp vs torque debate will continue, primarily because of the limits of words. I have read many threads, on many lists, on this topic and still there are disagreements. The only F1 Chief Engineer to soil his hands in one of these, put it this way. Peak Hp determines top speed and top gearing. Everything else, acceleration, gearing, when too shift, etc. is determined by the torque curve.

[Bill Verburg]

Originally Posted by 911st

This point has me thinking.

Again, the term Torque might be getting us in trouble.

The measurements at the crank or wheel are not really motor or wheel, HP or TQ.

They are more of a gross power measurement (load or acceleration rate) that has to be converted to something usable. They are not HP though that is probably a closer description.

HP is probably easyer to derive first (wheel or crank). Then from HP and by adding RPM into the equation, 'motor TQ' values are probably then derived.


Not an expert, just my amateur deduction.

what is measured n any dyno is torque and rpm

hp is then calculated from that info

if you want to go back to basics the most fundamental things that are measured are
M - mass
L - length
T - time

all else is derived from those measured quantities

Force =MLT^-1
Power = ML^2T^-3
torque = ML^2T^-2
work = ML^2T^-2
velocity = LT^-1

[docrodg]

To put some of this in a different context, that of the human body, I shall throw my hat into the ring.

Torque and HP have been described fairly well and are not too difficult, their relationship and why they drop off is the harder part to understand IMHO.

Think of the respiratory system as an engine - mainly because that is what it is. The lungs work off of a pressure differential caused by the diaphragm and intercostal muscles changing the volume of the lungs. When they increase volume a pressure differential exists and air comes in for the body to then burn in it's cells. When the musculature reverses direction the pressure goes the other way and the air is expelled. This is exactly as in an engine (Intake and Exhaust strokes). The musculature is like the throw and the combustion stroke added together - throw (or stroke of the crankshaft) is the diaphragm muscle changing the volume either a little or a lot, the combustion stroke is like the muscular energy of the diaphragm and the intercostals working together.

The curves that you see are not linear for the same reason that the arterial oxygen levels are different in a patient depending on their respiratory rate. This is called Volumetric Efficiency (VE) in an engine. Too slow a respiration and there is not enough oxygen, too fast and the lungs are not done filling before the air is exhaled. Bradypnea is akin to an engine at too slow an idle, not enough air is getting in the piston over time to cause a good burn. Hyperpnea is the opposite where the engine is moving the lungs too fast and there is not enough air coming in to burn well because the airway can only let a certain amount of air flow at one time - if we had larger brachia and trachea we could breathe at a higher rate and be fine. Same in the engine - the intake and exhaust passages can only move a certain amount of air in a second. This point of drop-off in air volume is where the curve drops off - the engine has developed hyperpnea and to get back to running well it must be slowed down. As you know, different people with different respiratory systems have different optimal respiratory rates when they are exercising, if they slow down breathing they have less strength, if they breathe too fast they have less strength. Plot that and it would be like an engine torque curve. Do this for a runner in a sprint race and you could plot air flow in and out, strength (force applied by muscles in the legs), and speed that the legs are moving (number of steps per second). You have plotted an engine Volumetric Efficiency, torque, and horsepower. Put longer legs on the runner or larger muscles and you get more torque for each step.

The reason a truck engine has more torque than a small race engine is that the lungs and musculature driving them are much larger (bigger cylinder displacement, bigger throw). So all the energy in the cylinder is acting on a larger lever (the throw of the crankshaft) to help produce more power in each stroke. When that piston goes down the crank throw produces the most power when it is 90 degrees ATDC. Get a longer stroke crankshaft you get more torque, get a larger cylinder you get more torque.

Another one that comes to mind, but leaves out the curve portion is the strength of extension and flexion. Two people have different length humerous and Ulna/Radius - but their muscles are the same strength (obviously different lengths). In addition, the long boned person is a weight lifter with asthma and the short boned person is an aerobics instructor - obviously the aerobics instructor can breathe better (at a higher respiratory rate) and has a faster muscle retraction time due to their exercise style and neural/muscular fitness. Have them do some weight lifting curls with a weight. The weight lifter has more torque than the aerobics instructor and can curl a heavier weight. But, if the aerobics instructor has faster muscle retraction then they can move their arm faster (with less torque than the weight lifter) over a period of time, and does more curls and that means they moved their arm a larger distance than the weight lifter - a greater HP. The aerobics instructor just takes longer to get the weight started than the weight lifter. The weight lifter can only go so fast though before they cannot breathe well enough to speed up more. The Torque is thus king at getting a car rolling, but HP is king when it comes to going at a high speed for a period of time. Thus the difference between a dragster and a race car (or a truck and a car). The weight lifter is a truck and does the curls slower but with more raw power and starts faster, the aerobics instructor is a race car and takes a little time to get up to speed but then can go faster before they are breathing too fast to support the oxygen demand on the muscles.

[RWebb]

you beat me to it - I'd use bird lungs for the analogy (they are superior, being flow-thru "devices")

livi - post any questions you have

[hcoles]

this is late but can't resist putting in my 2 cents.

You see engine torque values listed as e.g. 300 lb-ft

This is the energy the engine puts out with one revolution when it is held by the dyno. for the rpm being tested by the dyno.

HP is easily calculated from there. 1 HP = 550 ft-lbs/sec

[WERK I]

Originally Posted by psalt

...........................edit snippet...................................

Like I said, the hp vs torque debate will continue, primarily because of the limits of words. I have read many threads, on many lists, on this topic and still there are disagreements. The only F1 Chief Engineer to soil his hands in one of these, put it this way. Peak Hp determines top speed and top gearing. Everything else, acceleration, gearing, when too shift, etc. is determined by the torque curve.

Spot on.

[Matt Monson]

Great thread.

[stownsen914]

Originally Posted by Bill Verburg

You shift past the rpm peak because it takes time to shift and the revs fall more than is desired, ideally if you could shift instantaneously you would shift to stay in the rpm range symmetrically w/ peak hp

This must be a complex topic or there wouldn't be so much debate! Re: shifting beyond the hp peak, I thought the purpose was to maximize hp under the curve. Some will argue that it's torque that induces acceleration, but maximizing hp under the curve makes more sense to me.

Scott

[Bill Verburg]

Originally Posted by stownsen914

This must be a complex topic or there wouldn't be so much debate! Re: shifting beyond the hp peak, I thought the purpose was to maximize hp under the curve. Some will argue that it's torque that induces acceleration, but maximizing hp under the curve makes more sense to me.

Scott

yes it is torque that induces acceleration but hp is the rate at which the work is done, and there is a period beyond the torque peak where hp is increasing, i.e. the actual torque is decreasing but it is applied at a fast enough rate that net work is done faster, look at the dimensions of hp, see the extra T in the denominator. Ideally yes, the shift would be such that there is rpm symmetry on either side of the hp peak. For example w/ a hp peak @6800rpm and an rpm drop of 600 one would want to shift @6800+300 =7100. But it takes time to shift so you will have to take the engine to somewhat higher rpm. This is of course if the rpm limiter allows it.

[hcoles]

Bill,
no disagreement, I wonder if this way of describing the situation is worthwhile, for max. acceleration for the desired period of time, put the most energy into the driveshaft during the period of time. This allows shifting past the torque peak because more energy per unit time is available there compared to short shifting in the next higher gear. You use the HP curve, when the HP is higher than the rpm you would be shifting you don't shift, when lower you shift. Of course there is all sorts of other things happening that may cause shifting a bit earlier or later than that criteria. This subject seems to flair up every few months, I don't worry about this type of thing right now, I'm trying to get the rocker shafts installed and try my new G50 clutch shaft invention.

[Flieger]

All you need to know about power and torque is...

More Is Better!

as Mark Donohue put it, I will never have enough power until I can spin the tires from one end of the straightway to the other in every gear. (Or something like that.)

[Bill Verburg]

here's an overlay of hp and thrust for my car


and a close up of just 2 gears




what's going on in other cars will depend on the toque curve and gearing besides the issue of time I noted above here's another thing to consider, in general you need to shift beyond the hp peak to maximize the thrust in the next gear

[RWebb]

I wonder where the shift points are set on the sport programs for PDK?

recent Excl. article says the thing shifts so fast that you can change gears in the middle of a flat out corner & not upset the car's balance (!)