Torque and RPM relation

[xpensive]

There you go grunts, I was beginning to suspect that we were talking different languages here, the assignment?

[bhall II]

Ohhh THAT work!

[...]

Something like that. Though, I'm not exactly sure how it wound up here.

Oh, well. I guess this is what happen when one touches the third rail.

[gruntguru]

I guess this is what happen when one touches the third rail.

That's when you really start to feel the "power".

[GitanesBlondes]

For some reason I struggle so much with torque.

I don't know if it is because the more that people chime in, it takes what I thought was an understanding, and makes it feel uncertain.

Where I am currently:

Torque without a gearbox means nothing right?

What everyone considers "torque", it's just a measure of the turning force on the flywheel, no?

However that turning force measured (torque is a measurement of force) at the flywheel is irrelevant because what really matters is how the gearbox multiplies that turning force created by the power (the amount of energy being unleashed by the engine and defined generally as horsepower?) of the engine at various RPM?

Over the course of the way from the flywheel, through the gearbox, down the driveshaft (assuming RWD) through the rear end to the wheels there is a loss of power that varies from vehicle to vehicle?

[dren]

Think of it like you're kicking something. How hard you kick once is torque ... force. Now start kicking more and more, faster and faster. Assuming all kicks are the same strength, the more you apply over a given amount of time, the more power you apply.

A transmission takes power and adjusts the speed and torque numbers from input to output. One goes up, the other down.

[Jersey Tom]

Torque without a gearbox means nothing right?

I would rephrase that as, "torque without a relevant measure of speed" means nothing... which ultimately gets us back to power.

What everyone considers "torque", it's just a measure of the turning force on the flywheel, no?

I'd say so, yes.

However that turning force measured (torque is a measurement of force) at the flywheel is irrelevant because what really matters is how the gearbox multiplies that turning force created by the power (the amount of energy being unleashed by the engine and defined generally as horsepower?) of the engine at various RPM?

You're on the right path here, yes. You've got how fast the engine is turning (or can turn fastest), and then you've got how fast you need to wheels and tires to turn to go a certain speed. So with that you're forced to be in a certain ballpark range for overall reduction ratio between the two.

So if your engine makes lots of torque but at pitifully low RPM - who cares? You'd have to gear the thing in such a way that the torque at the wheels would be tiny and your car would be very slow. If you have an engine that makes the same torque at higher RPM you don't have to have as tall of gear ratios and you can deliver much more torque to the driving wheels.

That's why I think power is really a much better overall view of what an engine is capable of, all other things being equal (you can still make poor choices in gear ratios and be slow even with a high HP engine).

Over the course of the way from the flywheel, through the gearbox, down the driveshaft (assuming RWD) through the rear end to the wheels there is a loss of power that varies from vehicle to vehicle?

Yes

[Just_a_fan]

So if your engine makes lots of torque but at pitifully low RPM - who cares? You'd have to gear the thing in such a way that the torque at the wheels would be tiny and your car would be very slow. If you have an engine that makes the same torque at higher RPM you don't have to have as tall of gear ratios and you can deliver much more torque to the driving wheels.

I guess it depends what your intended use is. In a racing engine, yes you want power at high revs because with a limited set of gears that gives you best top speed. In a lugging engine, you want power at lower revs (and hence what is sometimes referred to as a "torquey engine") coupled to a gearbox that allows you to lug but still have useful speed when required.

My vehicle has the same peak torque as one of the great supercars - the McLaren F1. The F1 does speed and acceleration like few others. Mine doesn't but it can pull itself and a 3.5 tonne trailer up a steep incline at walking pace with ease. The difference is that mine makes peak torque at about 1800rpm, the F1 at about 4800rpm. The F1 is making much more power at its peak torque than mine does at its peak torque because, as we know, power is proportional to torque and revs. And likewise, mine is making more power at 1800rpm than the F1 is because mine is making more torque at that speed. Couple my low speed torque/power to a 2-speed transfer box and 6-speed gearbox and you get its lugging ability. It tops out 130mph though and it takes a while to get there once you get over about 110mph because drag is eating the power available (270bhp at 3800rpm).

The problem in this discussion is that people want to try to separate power and torque. You can't. They are interdependent - all you can do is determine what you want your engine to do and then tune it to give the power and torque characteristics necessary for that use. Gearing is then used to allow you to get to your performance envelope from a stand still and then stay in that envelope as road speed changes.

[langwadt]

For some reason I struggle so much with torque.

I don't know if it is because the more that people chime in, it takes what I thought was an understanding, and makes it feel uncertain.

Where I am currently:

Torque without a gearbox means nothing right?

What everyone considers "torque", it's just a measure of the turning force on the flywheel, no?

However that turning force measured (torque is a measurement of force) at the flywheel is irrelevant because what really matters is how the gearbox multiplies that turning force created by the power (the amount of energy being unleashed by the engine and defined generally as horsepower?) of the engine at various RPM?

Over the course of the way from the flywheel, through the gearbox, down the driveshaft (assuming RWD) through the rear end to the wheels there is a loss of power that varies from vehicle to vehicle?

the electrical equivalent to speed and torque would be voltage and current.

a 12Volt heater drawing 10 amp = 120Watt
a 120Volt heater drawing 1 Amp = 120Watt

would both heat a liter of water at the same speed because they have the same power

the equivalent to a gear box would be a transformer

120V through a 10:1 transformer would power the 12V, 10A heater, while drawing 1A at the 120V side

[jz11]

you can't measure torque of a running engine directly

What do you think an engine dyno does? ..darn sure is running. Ever scatter an engine on a dyno? big bang fer sure.

maybe you didn't understand what I tried to say - for an object that is rotating continuously - there is no way to measure its torque directly, so on a dyno you let this rotating object do some work, generate electricity, move water, spin up heavy object, whatever - do things that you can more or less precisely calculate the amount of energy required to do this work in some period of time, so you have work done and you have time it took to do this work, and from this you determine how much energy it took to change the state of the thing you were looking at (rotational speed of a steel drum, electric current flowing through a load circuit etc.), and from that you calculate the equivalent momentum of the rotating object at some point in time (rpm) - this torque by itself does not do any work at all - this is fundamental to understand - it is simply a potential of the object to do work, then you give it some time and get work done

same way you don't measure speed directly, you measure change of things over time and can calculate the speed

so power is not more fundamental than torque, it is simply different expression of the same thing

understanding how things are measured, what are the sources of possible errors, and how it can affect the final quantification of something is very important to avoid such mistakes as was the initial rule of the fuel flow this year

[gruntguru]

https://www.google.com.au/webhp?hl=en&t ... d=0CBEQ1S4

[langwadt]

you can't measure torque of a running engine directly

What do you think an engine dyno does? ..darn sure is running. Ever scatter an engine on a dyno? big bang fer sure.

maybe you didn't understand what I tried to say - for an object that is rotating continuously - there is no way to measure its torque directly, so on a dyno you let this rotating object do some work, generate electricity, move water, spin up heavy object, whatever - do things that you can more or less precisely calculate the amount of energy required to do this work in some period of time, so you have work done and you have time it took to do this work, and from this you determine how much energy it took to change the state of the thing you were looking at (rotational speed of a steel drum, electric current flowing through a load circuit etc.), and from that you calculate the equivalent momentum of the rotating object at some point in time (rpm) - this torque by itself does not do any work at all - this is fundamental to understand - it is simply a potential of the object to do work, then you give it some time and get work done

same way you don't measure speed directly, you measure change of things over time and can calculate the speed

so power is not more fundamental than torque, it is simply different expression of the same thing

understanding how things are measured, what are the sources of possible errors, and how it can affect the final quantification of something is very important to avoid such mistakes as was the initial rule of the fuel flow this year

measuring torque directly isn't a problem it is done all the time, measure the twist of a calibrated shaft or measure the force it takes to prevent the motor from rotating

[dren]

I guess it depends what your intended use is. In a racing engine, yes you want power at high revs because with a limited set of gears that gives you best top speed. In a lugging engine, you want power at lower revs (and hence what is sometimes referred to as a "torquey engine") coupled to a gearbox that allows you to lug but still have useful speed when required.

The power at high revs in racing engines is largely due to a limit on engine displacement. You want power, to get it with a limited displacement you have to rev higher and higher. You then use gearing to get what you want out of that power. You can tow a boat with an F1 engine, you just have to rev the piss out of it and gear it accordingly.

[strad]

jz11: ok understood

[machin]

For some reason I struggle so much with torque.

It is probably just easier to look at how torque and engine speed are used to calculate a vehicle's performance, rather than thinking about analogies. The equation below is basically a slightly rearranged and expanded version of Newton's F=mA:-

Vehicle Acceleration = ((Flywheel Torque x (Engine RPM / Road Wheel RPM) x Driven Wheel Radius) - (Resistance Forces)) / (Mass+Inertia effects)

The same equation can be used to calculate a vehicle's top speed, i.e. when the driving forces = the resistive forces, the acceleration = 0.

So Flywheel torque IS important to vehicle performance.... but only if you also know the flywheel rpm at which that torque is being generated.

Now since engine Power = flywheel Torque x Engine RPM we can simplify the above equation to:-

Vehicle Acceleration = (((POWER / Road Wheel RPM) x Driven Wheel Radius) - (Resistance Forces)) / (Mass+Inertia effects).

So now if we have two cars, otherwise identical except the engine power output, it is clear to see that the one with the higher power should accelerate faster and have the higher top speed (assuming suitable gear ratios are available, and there is enough grip to transmit the motive force), and we didn't once need to look at Torque and engine RPM (although you could, if you wanted to: the answer would be the same).

[Greg Locock]

you can't measure torque of a running engine directly

What do you think an engine dyno does? ..darn sure is running. Ever scatter an engine on a dyno? big bang fer sure.

maybe you didn't understand what I tried to say - for an object that is rotating continuously - there is no way to measure its torque directly, so on a dyno you let this rotating object do some work, generate electricity, move water, spin up heavy object, whatever - do things that you can more or less precisely calculate the amount of energy required to do this work in some period of time, so you have work done and you have time it took to do this work, and from this you determine how much energy it took to change the state of the thing you were looking at (rotational speed of a steel drum, electric current flowing through a load circuit etc.)....

understanding how things are measured, what are the sources of possible errors, and how it can affect the final quantification of something is very important to avoid such mistakes as was the initial rule of the fuel flow this year

Actually you do measure the torque directly, it is reacted through the engine mounts. Apparently you know bugger all about engine dynos. Old ones used to have a little arm stuck out sideways with a weight on them that you used to balance the torque. I left the rest of your pompous gibberish in because it is so funny in context.