Power vs Torque Questionnaire -RESULTS

[Manoah2u]

thumbs up.

[Just_a_fan]

Since eternity, Power has been Force times Speed, while Force is Mass times Accelleration and that's all there is to it.

In an engine context, power is torque x speed. No torque, no power. No speed, no power.

Of course one can have torque without power but it's not really very useful in an engine...

[Charlatan]

Obviously, but when it comes to the performance of a vehicle, the below relations is all you need, no reason to involve torque.

A) Power equals Force times Speed.

B) Force equals Mass times Accelleration.

This torque-talk is a bit embarrassing, something you here in "Top Gear" and forums like that, but Jeremy Clarkson is no engineer.

[Cold Fussion]

Since eternity, Power has been Force times Speed, while Force is Mass times Accelleration and that's all there is to it.

In an engine context, power is torque x speed. No torque, no power. No speed, no power.

Of course one can have torque without power but it's not really very useful in an engine...

The point is that in vehicle context, torque measured at the flywheel is not a useful unit of measure without angular velocity, which gets you back to power. Talk of torque is fairly useless because vehicle performance is governed by power and as you say, you can have torque without rotation and not get anywhere at all (if you have an equal reactionary torque).

[Charlatan]

Indeed, also wheel-torque is useless, all that matters is the Force that can be generated at the tire's contact patch.

Said force can be derived from A) above.

[Just_a_fan]

Torque is, in effect, the "force" bit of your power equation. It's the simplest way to quantify the force being applied to the piston by the combustion of the fuel unless you're going to measure pressure in the cylinder.

If I tell you that an engine is running at 5k rpm you can't tell me how much power it's making. If I tell you it's making 500Nm at 5k rpm you can tell me it's power output.

To ignore torque is akin to ignoring mass when discussing gravity.

[Charlatan]

No, it's not when it comes to vehicle dynamics, but perhaps you still have a book from your Univeristy where you can look it up?

[FoxHound]

Can we all agree torque is invisible?

I must say there are some very insightful posts but Machin's epic post really does nail it to the wall.
It should be a reference.

[Charlatan]

I joined here as I thought it was a forum about engineering, but I now realize it's just another beauty-contest.

Mea culpa and have fun.

[Cold Fussion]

Torque is, in effect, the "force" bit of your power equation. It's the simplest way to quantify the force being applied to the piston by the combustion of the fuel unless you're going to measure pressure in the cylinder.

If I tell you that an engine is running at 5k rpm you can't tell me how much power it's making. If I tell you it's making 500Nm at 5k rpm you can tell me it's power output.

To ignore torque is akin to ignoring mass when discussing gravity.

Engine torque does not quantify vehicle performance in any useful way, if I gave an engine torque figure there is no way one could know anything about its performance. With engine power alone you quantify vehicle performance pretty well.

[Stradivarius]

Torque is, in effect, the "force" bit of your power equation. It's the simplest way to quantify the force being applied to the piston by the combustion of the fuel unless you're going to measure pressure in the cylinder.

If I tell you that an engine is running at 5k rpm you can't tell me how much power it's making. If I tell you it's making 500Nm at 5k rpm you can tell me it's power output.

To ignore torque is akin to ignoring mass when discussing gravity.

The only way this would be true, is if it wasn't possible to use gears in order to trade torque for rotatational velocity. But since this isn't only possible, but is done successfully on all motorized vehicles, there is simply no reason to worry about the torque when evaluating engine performance. The only ones who need to worry about the engine torque are the engineers who decide the gear exchange ratio. Yet, there is a lot of people who get confused and I admire machin for trying to clarify this, and I don't think it is fair accusing him of being arrogant for trying to explain something which is misunderstood as often as this.

Maybe this graph (which is just used as an example) can help illustrate that the engine torque is not relevant to engine performance (beyond the fact that the torque influences the power).


The special thing about this graph is that it contains all the relevant information necessary to evaluate the engine performance, but it does not contain any information about the torque and it is also impossible to determine the torque from the available information, since the engine speed is just given as a percentage of the maximum speed.

If it is possible to describe all relevant performance characteristics without even knowing the torque, there is no way the torque can contain any relevant information about enginge performance at all. That is pure logic, based on the very reasonable assumtion that the appropriate gear exchange ratios are chosen for the vehicle using the engine in question, which is always the case.

[machin]

The problem for us is that we don't have a nice short and sharp (but accurate!) statement to explain the two quantities, whereas the (INCORRECT!) statement "Torque gets you moving, Power keeps you moving" is short, sharp and easy to remember... even if it is wrong!

Earlier in this thread cycling was mentioned.. and this is an area closer to my heart than cars these days (Did I just say that on an F1 forum?!)... so I thought you guys might like to see these charts.

My bike has a "Power Meter", which means that for every pedal revolution I get a log of Torque (at the pedals), Cadence (RPM) and Power. A wheel RPM sensor gives me a wheel RPM log. This is converted to bike speed using tyre circumference and verified using GPS. Every ride I do is logged in this way and you can set up GPS "marker points" to allow you to easily compare your performance over the same stretch of road.

Below is a series of charts showing the average Pedal Torque, Cadence and Power for 95 separate occasions climbing up the hill outside my house. It is a short but steep hill, which normally takes me about a minute to climb at around 9 to 10mph.


It is interesting to see that whilst there is a general trend: more pedal torque or more pedal RPM tends to give a higher road speed, there is considerable scatter, such that making an accurate prediction of performance from either of these metrics is not possible.
When we look at the average power outputs there is a very clear and distinct correlation between power output and road speed, such that if we only knew my average power figure we would be able to quite accurately predict my climbing speed to within one or two mph.

It is quite interesting to see that the relationship between power and speed is almost linear at these low speeds... as speeds increase we would expect the relationship to become more and more "cubed" in nature, due to the impact of aerodynamic drag (rolling resistance and gradient resistance dominate at low speeds whilst climbing, hence the almost linear graph).

[Phil]

I think the problem with torque is that many people look at the number attached to it - either in NM or LB-FT and compare it to different cars/engines. Torque [at the engine] is not something that can be easily compared to with other cars. It's only real value is the maximum figure - the RPM at which it produces the most and the curve. What is also relevant, is how much torque you have at the wheels (after the gearbox).

Some people say power is everything. That is true, but a power figure as it is found in most car magazines isn't much better. It only shows the maximum power rating and at which point - the figure alone doesn't tell us how much power the engine produces at low rpm or mid rpm. Which is why a power curve is always the best for an accurate analysis in how an engine performs in and outside the entire package.

[machin]

A power figure at say 40% or 50% of maximum revs (in addition to the peak power figure) would be good replacement for the Torque figure in my opinion, and takes up no more room in a car magazine or brochure.....

[Phil]

EDIT: Deleted (errors!)