Torque and power quiz 4

[yoshimitsuspeed]

I spend a lot of time in the automotive community and a lot of time on car forums and groups.
Ever since I started to learn the actual relationship of torque and power it drove me crazy how few others in the automotive world actually understood this basic formula. People capable of building motors that make 500 hp per liter and who still think that torque is low end power.
I have gotten into enough arguments with people to learn that most would rather argue relentlessly cause that's what theys daddy taught them than sit down and think about the simplicity of the formula long enough to understand the relationship of the two.
I have decided to try a different tact and make a little quiz that gets people thinking about this from a different angle and maybe hoping they will get the point that torque and power can't be compared, and that torque does not mean low end power.
I just started on this tonight and it's 3AM. I want to do more to improve it but I also would love some input from others on ways I could improve it.
I would like it to be as detailed and informative as possible while still being interesting and keeping the person engaged and interested.

Tell me what you think of what I have so far.
What could I do better?
What are other questions I could ask or ways I could put things to get people thinking about the relationship without getting too bogged down in the math to loose too many people?
On that note should I focus more on the math or stay more with the basic relationship and principles?

http://matrixgarage.com/content/think-you-understand-torque-and-power

 

[Tmoose]

Land and Sea is a dyno manufacturer in Concord, NH. They offer a bunch of dynos and dyno upgrade options.

From Land and Sea's website -
" inertia-only "dynos" can not directly measure torque "

http://www.land-and-sea.com/dynamometer/dynamometer-comparison.htm

So, for dynos that track roller rpm vs time, and thus compare the change in kinetic energy, HP is the more direct measurement, and torque is "derived."

 

[gruntguru]

Yes. The fact that measuring power output of a rotating type engine is most easily and accurately achieved by measuring torque and rpm, does not make torque a more fundamental quantity.

What engines do is convert heat energy into mechanical energy. The rate at which they perform this conversion is the power output and is the most fundamental quantity to be measured.

There are engines coming with no rotating parts whatsoever, converting linear motion of free pistons, directly into electricity via induction coils. What will be the fundamental output from these engines - Volts?, Amps? or power?

je suis charlie

 

[NormPeterson]

Combustion pressure times the area of a cylinder bore = some force.

Some force operating at the crankpin = a torque about the crankshaft's axis.

Seems pretty fundamental to me, though I guess you have to be thinking in terms of a free-body diagram. The analogous linear motor's property would probably have to be the force required to push the free pistons through the magnetic field.


Norm

 

[inline6]

torque is fundamental but it still doesn't tell you how useful the "engine" is unless you also know rpm.

or in the terms of a linear frame of reference where force is fundamental you need linear velocity to know how useful the "engine" can be.

i like the comments about converting chemical/heat energy to mechanical energy being why power is more relevant. if only people looked at energy methods for thinking about things.

For example:

- Accelerating a car from rest to say 60mph requires none other than a change in kinetic energy, the time taken to achieve this governed by energy/time (i.e.POWER)

- Going up a hill with 200m change elevation at constant speed is simply change in potential energy, the time taken to achieve this i.e the SPEED at which you can do this is governed by energy/time (i.e.POWER)

 

[NormPeterson]

- Accelerating a car from rest to say 60mph requires none other than a change in kinetic energy, the time taken to achieve this governed by energy/time (i.e.POWER)

- Going up a hill with 200m change elevation at constant speed is simply change in potential energy, the time taken to achieve this i.e the SPEED at which you can do this is governed by energy/time (i.e.POWER)

Power may make for the simplest explanations in such cases, but it's not the only possible approach.

If, and let's say in the absence of discussions such as this one, you were to look at any number of dyno sheets with both torque and HP plotted, you'd probably conclude that an ICE is closer to being a constant torque device than a constant power machine. Much closer in most cases, I suspect.


Norm

 

[inline6]

the fact is that peak hp is a better indicator of performance potential than peak engine torque. the truth is the average hp in the operating band is more accurate again, yet the average crank torque is next to meaningless as a measure of performance potential.

if one was to use a CVT transmission with infinite possible gears and wanted to generate the most tractive effort at every possible speed assuming sufficient traction then they would run it at the rpm of peak hp as opposed to the rpm of peak torque.....

 

[inline6]

when you look at performance potential based on engine torque numbers you need rpm, whereas with power you do not as it comes out in the wash so to speak

 

[NormPeterson]

I'm glad that you included the word 'potential'. Because in virtually all daily driving, non-competitive track day sessions, and even for a large part of actual competition where there are corners to be negotiated you won't be operating at peak power very much of the time. Even conventional (non-CVT) automatic transmissions don't kick down such that the engine is driven to run all the way up there either.

Really, you should keep rpm in mind when using an estimate of actual acceleration based on power, if only in order to have some idea about how much of it you really have available for the situation at any given instant.

I think I mentioned something about CVTs a month or so ago . . .


Norm

 

[JackAction]

Really, you should keep rpm in mind when using an estimate of actual acceleration based on power, if only in order to have some idea about how much of it you really have available for the situation at any given instant.

I don't understand what you mean, could you give an example? Like I showed you in my simulator, I don't know anything about engine rpm (or transmission or wheel rpm for that matter) and can still give an estimate of acceleration. What if you have an engine that doesn't rotate, as previously mentioned by others? The fact is that once you know the velocity of the car, every other car components' velocity is a mere geometrical relationship for «matching» purposes.

In the end of it - to get the greatest acceleration - if you want to do the proper «matching» of the engine rpm to vehicle speed, do you need to match the maximum torque of the engine or the maximum power of the engine? The maximum power will always give the greatest traction force at the wheel.

And even if you don't want the greatest acceleration; Or better, you want zero acceleration and keep the speed constant, are you gonna match your engine rpm and load to obtain a defined torque or a defined power? The answer is again power. Yes, at the wheel you need a defined traction force and velocity, say 1200 lb @ 45.8 ft/s or 100 hp. But you could set your transmission in low gear, setting your engine at 6000 rpm and adjusting your throttle to get 87.5 lb.ft or you could be in high gear, setting your engine at 2000 rpm and adjusting your throttle to get 262.6 lb.ft. As long as you get 100 hp. That is why we say that torque or rpm doesn't matter, only the power, i.e. the combination of both torque and rpm.

 

[inline6]

once the rpm get to peak power at 40mph they stay there

https://www.youtube.com/watch?v=inzDzycg9n0

kicks down to 5500rpm which is very near peak hp

https://www.youtube.com/watch?v=-h57k1VWV34

naturally the goal isnt always maximum performance in a road car so the system can be "tuned" to give certain results at part throttle

of course with conventional manual you try and maximse power band but sometimes the gearing is non ideal for the corner and you do as best you can.

 

[NormPeterson]

Interesting videos, inline, and very much what I'd expect.

The way I understand it, there is some sort of torque management going on in Nissan's CVTs. You'll note that the revs initially flash to about 3000, well below the peak torque point (4400), and remain there until the car has moved through about 15 mph. My guess is that the torque capacity of the drive "pulley" (available friction at the small diameter for large "gear reductions"? ) represents some sort of limitation. Nissan's CVT did have a few "teething pains" as I recall.

That at least Nissan's V6 CVT-equipped cars offer soft initial acceleration is a fairly common complaint on the Maxima forum that I belong to. But it's not at all surprising that those who can get past the slightly sluggish zero-to-20 are quite happy with the performance otherwise.


Norm

 

[BrianPetersen]

The initial flash to 3000 rpm is due to the torque converter - these transmissions have a conventional torque converter. It locks shortly after starting off from a stop. Note the flashing orange light on initial start-up ... no doubt this is the traction control system limiting torque output.

Tests like this don't expose that style of CVT's weakness ... transient response when an abrupt ratio change is demanded. Yes, normal automatics can have that weakness, too, but it's dependent on programming rather than on how quickly the cones and belt can shift position. Yes, you can ask either transmission to hold a lower ratio ... thus throwing away the efficiency benefit.

 

[NormPeterson]

Examples are easy, Jack

Street driving - and your satisfaction with your car in that environment - is more about response than it is about drive-it-like-you-stole-it absolute maximum performance. Quite a lot of near-wide-open-throttle and WOT driving falls not just within the regions identified as "unavailable power", but also to the left of the intersection with the line representing availability of a still lower gear. In nonsteady speed driving averaging 40 to 45 mph I commonly drive around in 4th which is about 2000 rpm, which is neither lugging the engine nor forcing it to feel "too busy". Should I feel a need for more acceleration than 4th can provide, 3rd has enough to get the job done. I just don't encounter situations where grabbing 2nd for 10 or 12 mph would be necessary or make much sense.

There are turns on the road courses I have driven on where running in 2nd gear would just barely be possible. However, since cornering eats up a good amount of drive axle grip it is worse than pointless to use 2nd, especially given that an upshift to 3rd would be required barely past the apex for rpm reasons regardless of how much or how little of the "extra" acceleration potential you could actually use. Non-optimal gearing? Of course. But no single transmission gearset, axle ratio, and tire size combination can be optimum for all corners at all road courses. You work with what you've got, just like you do on the street, and it's almost never along the max power line.

Maybe it's because I am NOT oriented toward drag racing (or its illegal street cousin and the sort of bragging that goes with that) that it's always been easier for me to work with torque. Doesn't mean I don't understand it from a power point of view. The easiest way for me to illustrate this is that when I downshift I'm specifically seeking a larger mechanical advantage for the available torque rather than accessing a greater amount of power. Pretty much the same thing with a different mindset . . . notice that I didn't say anything about chasing a bigger torque number either.


Norm

 

[JackAction]

The easiest way for me to illustrate this is that when I downshift I'm specifically seeking a larger mechanical advantage for the available torque rather than accessing a greater amount of power.

But when you say this, combined with your previous statement «that an ICE is closer to being a constant torque device than a constant power machine» I would think that a downshift is more of a power increase than a torque increase.

When you downshift - assuming same throttle opening - the engine torque stays more or less the same and the engine rpm increases, such that the net effect is a power increase.

Of course, at the wheel, the rpm is the same and the gear ratio translates the power increase into a torque increase at the wheel, but still, wouldn't you agree that it is fundamentally an increase of power from both the engine and wheel points of view?

 

[NormPeterson]

Like I said above, I'm looking at a downshift as providing a more favorable mechanical advantage (it's certainly what you make happen inside the transmission, regardless of what's going on with the engine ahead of it).

Yes, I could think of it in terms of accessing more power, but that just isn't the way I visualize it.

As a structural guy, things like torque (aka "moment"), mechanical advantage(s), drive tire radius, vehicle mass, rotating masses, etc., are all easier to picture as pieces of the whole than power is as sort of a shortcut around needing to know some of them. It's certainly easier from a torque perspective to compare the results of changes made individually to, say, transmission or axle gearing or tire size, when you're actually working with those items as variables.

That does bring up a different point - why would people do these sorts of performance predictions or analyses? If it's for estimating maximum performances only, using power is appropriate. But if your intent is for help in deciding whether some given mechanical modification is going to give you what you hope for, or if the improvement it's predicted to give is going to be worth the time and expense, maybe the power approach isn't sufficiently detailed.

If I was looking to maximize acceleration around some specific speed or speeds, or to maximize top speed, of course I'd choose gearing that would have those speeds put the engine up around or right at peak power. If I was a drag racer and serious enough about it to keep tinkering, I'd probably gear my car to be running a little past peak power rpm as my car crossed the finish line, which would either require knowing the peak power rpm or involve a number of gear swaps. But once that gearing had been finalized, what I might want to know about acceleration at other speeds and/or in other gears would still be based on torque and all the rest of the variables.


Norm

 

[gruntguru]

Link
Scroll down for scatter plots illustrating the relationship between acceleration and power/wt and torque/wt for a number of actual vehicle tests. Shows a clear correlation of acceleration to power and none for installed torque.

je suis charlie

 

[dicer]

If there was an EDIT function I would have included RPM that is the time factor that determines the HP from the measured torque.

 

[LionelHutz]

So, for dynos that track roller rpm vs time, and thus compare the change in kinetic energy, HP is the more direct measurement, and torque is "derived."

I've seen that argument before for an inertia dyno, but the dyno very much uses the basic torque = inertia x delta speed / delta time formula for the torque around the roller shaft. But, it will typically show the HP curve first because the HP is simply the HP so it's the easiest curve to create. Heck, a meaningful HP curve can even be created without knowing engine rpm. But, for the torque to be meaningful it really needs to have engine rpm on the X axis. So, the dyno first has to take the engine rpm calculation from the ignition pickup and figure out the effective gear ratio between the engine shaft and the dyno roller shaft before calculating the engine torque vs rpm curve.

 

[NormPeterson]

Scroll down for scatter plots illustrating the relationship between acceleration and power/wt and torque/wt for a number of actual vehicle tests. Shows a clear correlation of acceleration to power and none for installed torque.

I made a few stops along the way, and the derivation specifically starts out with torque, power not appearing until later. Hmmmm.

SO WHAT IS POWER AND WHY IS IT USEFUL?

An Engine's power output is simply Flywheel Torque x Flywheel RPM. Essentially it is a figure which tells us how much potential the engine has for accelerating a car when it is provided with the correct gear ratios. We can use it to simplify the equation above:-

(((Power / Road Wheel RPM) x Driven Wheel Radius) - (Resistance Forces)) / Mass = Acceleration

He also uses the word 'simplify' . . . IOW, a convenience (without implying there's anything wrong with making life easier here).

Now we can see that more Power = more acceleration; we don't need to know if this power is generated by having more flywheel torque, or a higher RPM, we just need to know it is higher.

For theoretical comparisons, sure, that is all you need (I've never argued otherwise). For design/redesign purposes with at least some of the hardware "fixed" and only knowing what that power is, probably not. Using power at 50% of peak power revs might be more useful, but it's not inherently better than using something like 85% or 90% of peak torque given that the values of such variables as gearing and tire size are readily available.


Norm

 

[NormPeterson]

Heck, a meaningful HP curve can even be created without knowing engine rpm.

I'm afraid I'm not grasping what you'd be plotting HP against if not rpm.


Norm