Torque and Horsepower - A Primer

[machin]

I think Belatti is proving a point N_S;

With a single power figure, and knowing gearing is free we can work out the maximum possible acceleration of the car at one speed, since F=mA and power=force.speed. (assuming of course that we ignore drag, inertia and grip -to keep things simple)

The same can't be said of torque since u need to know the wheel diameter, the road speed and the engine speed at which the engine makes the torque.

Both methods can be used to work out the acceleration, but u need to know a lot more data to work it out using the torque method. And that's the reason why power is the "easier" to work with. However as everyone says; a single point on the engine output curve doesn't tell you how fast the car will accelerate over a range of speeds, so assessing real-world performance using a single data point is plagued with pitfalls!

[xxChrisxx]

I didn't realy read all of that, because I know what I am talking about 100%

It was mostly equation manipulation (ie not long) and was rather simple to follow.

In any case I wouldn't dream of wasting my time, answering questions to someone that already knows all the answers.


EDIT: Because I can't help my self. I'm not explaining, I'll just give the equations. As they can't be argued with.

1. Where does the torque come from in the piston engine? (trace it right back to the intake and fuel)

Leading question, as I know what you want me to say.

And it’s a subtle difference because energy and forces are different concepts. Which is why you can consider them as simultaneous products or inputs, not consequences of each other.

Just as we consider power output and work output as simultaneous and non consequential. (as P = Tw).
We consider power in and heat in as simultaneous and non consequential. (deltaE/T = deltaQ)

So we can still quite rightly talk about:
Power in = work ouput.
Heat input = power output.
Power input = Power output
Heat input = work output.

All the above 4 are equally correct. And each has their specific use.

2. How efficient is the typical IC Petrol engine?)

η = 1 – (1/Rc^(γ-1))
Where gamma is 1.4 for air cycles. And typically 1.3 for stoichiometric AFR.

3. Where does the.... REST OF the UNUSED POWER.... go to??)

δE = δQ – δW
1st law - standard convention of signs.

As torque clearly relates to mechanical work, any arugemnt that Q is not Tw is obvious and meaningingless.


For example if we only ever used power for designing engines, how could we design the components? As power tells you nothing about physical loads due to combustion. You’d need to infer it by trying to work out how much fuel was burnt and estimate gas quantities and cylinder pressures. In these cases, measuring forces and torques is far more important than knowing your power output.

Just as calculating acceleration from torques only is more difficult that power only.

Right tool for the right job.

[xpensive]

This is a case of rather simple dynamics, just forget about gearing, grip and aerodynamic resistance for a moment;

Accelleration equals Force over Mass; a = F /m (1)

Power is Force times Speed; P = F * v (2)

(1) + (2);

Accelleration is Power over Speed times Mass; a = P / (v * m)

This makes of course Accelleration infinite at zero speed, which is why you spin the tyres, but if you plot the curve for a 1000 kg and 500 000 Watt (680 Hp) object, as a function of speed from 10 to 50 m/second, and you will get the point.

There is simply no need to involve Torque at all in this, it's all about Power.

[machin]

There is simply no need to involve Torque {in the calculation of vehicle acceleration}

I like to look at it as two different languages.. one is the langauge of "Power" -its very simple, the other is the language of "Torque" -its very long winded.

You can use either language to say "hello", but the Torque language takes you much longer, so its easier to use the power language since both give you the same results!

[n smikle]

It was mostly equation manipulation (ie not long) and was rather simple to follow.

In any case I wouldn't dream of wasting my time, answering questions to someone that already knows all the answers.


EDIT: Because I can't help my self. I'm not explaining, I'll just give the equations. As they can't be argued with.



Leading question, as I know what you want me to say.

And it’s a subtle difference because energy and forces are different concepts. Which is why you can consider them as simultaneous products or inputs, not consequences of each other.

Just as we consider power output and work output as simultaneous and non consequential. (as P = Tw).
We consider power in and heat in as simultaneous and non consequential. (deltaE/T = deltaQ)

So we can still quite rightly talk about:
Power in = work ouput.
Heat input = power output.
Power input = Power output
Heat input = work output.

All the above 4 are equally correct. And each has their specific use.

2. How efficient is the typical IC Petrol engine?)

η = 1 – (1/Rc^(γ-1))
Where gamma is 1.4 for air cycles. And typically 1.3 for stoichiometric AFR.

Just say a percentage. (yes this one is a loaded question too, you are falling right into my trap)

3. Where does the.... REST OF the UNUSED POWER.... go to??)

δE = δQ – δW
1st law - standard convention of signs.

I was expecting you to say waste energy goes to sound, coolant and exhaust. I was priming you up so you realise that all those forms of energy/power came from the combustion and that they really have nothing to do with torque.. It is only the geometry of the engine that torque is developed.

As torque clearly relates to mechanical work, any arugemnt that Q is not Tw is obvious and meaningingless.

This is where you get caught! Because you and I know that that is not the argument. The argument, well my argument is that Power is developed first before Torque in any heat engine.. It is 100% Fact. No real argument there.
You use an equation that has the same units of Joules/second, but no physical relation to waste energy lost to the coolant, exhaust or sound - which CANNOT be expressed as a Torque and angular velocity. There is simply no physical reason to express waste heat using Tw.

For example if we only ever used power for designing engines, how could we design the components? As power tells you nothing about physical loads due to combustion. You’d need to infer it by trying to work out how much fuel was burnt and estimate gas quantities and cylinder pressures. In these cases, measuring forces and torques is far more important than knowing your power output.

Yes it does actually. Again it all starts with the combustion in the cylinders.. where there is no torque. In fact, disregarding the friction of the lubricant, and the minor rotational accelerations of the connecting rod (energy of which is wasted anyway), there is no torque until you reach the crankshaft. (torque cannot be transmitted through ideal radial bearings).

[xxChrisxx]

You use an equation that has the same units of Joules/second, but no physical relation to waste energy lost to the coolant, exhaust or sound - which CANNOT be expressed as a Torque and angular velocity. There is simply no physical reason to express waste heat using Tw.

I've just said that Tw is the mechanical work. I suspected were going to say that "Q is not Tw (which you have). Therefore power is not Tw, Therefore power is more important."

Tw not being heat is obvious. Yet somehow you took it to mean that heat is expressed as Tw?


EDIT:

Any arguement that "Q is not Tw" is obvious.

Was it a genuine misunderstanding as to my meaning? Reading the original statement back it doesn't read well. I hope this post clarifies my meaning.

I also said it was meaningless, because although torque is dimensionally equivilant to energy, they aren't the same thing as one is a vector and the other is scalar. It's just a curiosity of the unit system.

[strad]

xpensive wrote:There is simply no need to involve Torque {in the calculation of vehicle acceleration}

Dare you to try to tell John Force that.
.
Can I just add that when you put an engine on a dyno it does not give you a horsepower figure.
It gives you the torque, and from that you deduce the horsepower using the formula
torque X RPM ÷ 5252
edit

[volarchico]

I don't think anyone is arguing that torque and power in a car are inextricably related by a simple equation. Nor has anyone said that MEASURING power is the better way (because it is actually quite difficult to measure power). A dyno measures torque and power is calculated from that measurement. Simple enough.

What Machin and xpensive seem to be saying is that once you have these values determined, it is easier to use power for future calculations and comparisons.

[DumHed]

power to weight ratio is an important figure in comparing a car's performance to others.

torque to weight ratio is utterly meaningless unless you know the other values needed to calculate the power, or have all the gear ratios, tyre diameters, etc.

[strad]

Then they should create a topic on torque multiplication through gearing or the like.
I sometimes think the logo for the site should be

[Belatti]

xpensive wrote:There is simply no need to involve Torque {in the calculation of vehicle acceleration}

Dare you to try to tell John Force that.
.
Can I just add that when you put an engine on a dyno it does not give you a horsepower figure.
It gives you the torque, and from that you deduce the horsepower using the formula
torque X RPM ÷ 5252
edit

so?

[strad]

Torque is what overcomes that silly principle about an object at rest and all that.

[Raptor22]

somebody gets it.

[victorsaver]

Gears allows you to change the speed of the various components of power, torque and angular. So gear is very important, but critical, gears can be changed any power you want, because the physical size of the mass, and, above all, the inertia of the rotating gears themselves.

[machin]

Raptor22/Strad... are you suggesting it doesn't take energy to move something from rest?

If it takes energy (I can assure you; any movement requires energy) and it takes a certain time (again; it does) that means you can describe the motion in terms of power just as well as you can in terms of torque.

I repeat; power and torque are simply two different ways of describing engine output. Using either power or torque to work out acceleration will get you the same answer; as long as you do the maths correctly!