Article: Torque / Work / Power

[stradlater]

This article is written as a further follow up to my article on the ins and outs of power ratings.

The core aspects of an engine 'rating' are it's Power Output and it's Torque Output. Most people only rate engines in their Power output, however it's typically infrequently known how much torque is being put out by an engine which is funny considering the fact that the power output is infact derived from the torque output.

Torque

Torque is the quantification of a force exerted about an axis of rotation. In metric terms it is measured in Newton Metres (Nm) and in imperial form it is measured in Foot Pounds (ftLb). A newton metre is a measurement of the force exerted by the distance from the center of rotation.

Take for example a bolt you are tightening. If you have a socket wrench attached to the bolt that is 1 metre long, and you apply 40 newtons of force to the wrench, you are exerting 40 NewtonMetres of Torque. If you then used a 2 metre long wrench with the same force, you would be applying 80 NewtonMetres of Torque.

Work

Work is also a measurement of force distance, but it varies a little bit from Torque. It is still a force exerted multiplied by the distance which the force was exerted. For example, if you had to lift a 30kg box 1 metre up, you would have exerted 30 kg/m's of work.

Power

Power is the amount of work done over given period of time. IE: The rate of work done over time. The more power that is generated by a device, the more work that can be done in a given space of time. Consider an example.

Two people, a mid 20's man, and a young boy. Both are faced with the task of moving 50 bricks from one table to another some distance away. The man may take anywhere up 10 bricks per trip, thus only needing to make 5 trips. The boy on the other hand may only be able to carry 2 at a time, therefore requiring 25 trips to do the same work. Therefore the man would carry out the work in much less time than the boy would take. Therefore it can be said that the man is much more 'powerful' as he has been able to perform the work in a much smaller space of time.

HorsePower

Ironically, the concept of Horse Power was invented by the man name James Watt. He had just recently refined the design of a steam engine to make it a viable and efficient power generator. He decided that he needed a means on comparing his engine to other peoples engines. Given that he was familar with the concept of foot pounds, so he decided to see how many foot pounds of work a horse could exert. Obviously not every horse is the same, so he used a number of horses and averaged them all out.

He eventually measured an output of 22,000 foot pounds per minute. Then, to allow for people who may have had more powerful horses than he did, he added 50% to his figure, and resulted in 33,000 foot pounds per minute, or 550 foot pounds per second.

Hence the measure, 1 HP = 550 ftlb/sec.



Next up: How horsepower and Torque are related..

[JetspeedCamry]

Awesome man! Thanks for sharing that, very imformative.

[crowncustom]

Very well explained.This reminds me back when I did Mechanical Engineering back in the dark ages with all the different theories and equations we had to work with.
Keep up the good work.+rep.
Cheers Brett.

[stradlater]

Cheers guys, hopefully this stuff is of interest to people.

[BeRad]

ahh never fully understood how they measured torque makes it a little easier now but.....

haha this may sound stupid but im not one to get into this side of the automotive fun... it may make 40nm of torque with 1 metre of wrench, but how does this compare to a cars wheels spinning around rollers?

[YelloRolla]

calculating the torque of an engine from it's given power output is simple.

HP = torque x rpm / 5252

So if your engine is making 300hp at 2500rpm then torque (at 2500rpm) may be calculated as follows; Torque = hp/rpm x 5252 = 630ft/lbs

[YelloRolla]

haha this may sound stupid but im not one to get into this side of the automotive fun... it may make 40nm of torque with 1 metre of wrench, but how does this compare to a cars wheels spinning around rollers?

If you could keep that 40nM of value and spin the wrench very fast (like 2000RPM and 300RPM and 7000RPM) - then the power curve that that you would be making would look something like (I will quote 40ft/lbs for the sake of not converting power values)
2000RPM: 40 x 2000 / 5252 = 15.23hp
3000RPM: 40 x 3000 / 5252 = 22.85hp (which is 3/2 x 15.23)
7000RPM: 40 x 7000 / 5252 = 53.31hp
If you then nosed over (stopped making 40ft/lbs) in the top end and were only capable of say 30ft/lbs at 8500RPM (because you and your spanner are freaking quick) then 48.55hp would be the result.

Here we have created a power curve showing how engines can build power and how they would taper off. Fair to say that peak torque is created at maximum combustion efficiency (or peak average cylinder pressure in normal operating conditions).

Hope that helps

edit/hmmm just got around to reading the other post on this subject - maybe I have wasted my time?

[stradlater]

Which other post?

[YelloRolla]

Which other post?

Sorry, I meant thread where this information had already been posted.

[stradlater]

yeah, which thread was that?

[YelloRolla]

the one that you started about power conversion, the replies cover just about everything that I wrote ^^^there

[V-Na]

For example, if you had to lift a 30kg box 1 metre up, you would have exerted 30 kg/m's of work.

Actually you would have exerted 30 kilogram-force-metres. The "kilogram-force" refers to the force due to gravity (weight), on the surface of the Earth, exerted on one kilogram, a unit of mass, by the Earth. Also, the force is multiplied by the distance over which the force is exerted not divided (ie 30kg box 2 metres up = 60 kg-force-metres not 15).

EDIT: In Système International d'Unités this would be equal to rougly 294 joules.