The idiot's guide to power & torque | Auto Expert John Cadogan | Australia

23 Nov 2016 17:03 323
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Can you explain power and torque? Like, explain it in three concise sentences. I bet you can’t.

Simplest way to look at it. Engine power makes shit happen, and by ‘shit’ I mean ‘acceleration’, and torque is one of power’s two fundamental building blocks. It’s power that throws a car into motion when the lights go green. Fuel and air goes into an engine - lots of stored energy. Magic happens inside. A lot of sucking, squeezing, banging and blowing - pornography on amphetamines - and much less energy comes out the tailpipe. The difference comes out the crankshaft, as power. Torque and revs.

Power causes acceleration. Twice the power, twice the acceleration - as long as a bunch of other things are equal, and you’ve still got grip. You literally do power out of a corner. So, this is like cooking. Jamie-friggin’-Oliver. If you cook a roast dinner, you need the ingredients, right? Dead chook, potatoes - whatever. If you want to cook power the ingredients are torque and revs. Simple.

This is the simplest equation on Earth: Power equals torque times revs. But there’s a catch for the mathematically illiterate: You need to convert revs (commonly expressed in revs per minute) to a maths thing called radians per second.

Ohhh - scary. But stay with me: it’s not quite Stephen Hawking o’clock just yet.

Radians are very cool, mathematically, because they’re dimensionless. Just accept it, and move on. Mathematically, in the physics beer garden, you need to take very good care of the units. Otherwise you break your equation. Then it betrays you. And you might end up being a journalist. And nobody who was ever an engineer would want that.

If you’re in Australia you’re working metric - watts (or kilowatts) for power and Newton-metres for torque. So, kilowatts equals Newton-metres x rpm x pi, divided by 30,000. That’s just how this works. Trust me. If you’re in North America (or anywhere else imperial) power in horsepower equals torque in foot-pounds times revs times pi divided by 16,500. It just is.

If you’re really illiterate mathematically - please don’t breed; the next generation does not need your genes - but if that’s you, put the condom on now, pre-emptively, and relalise pi is a big, long irrational number - but 3.1 will do for most jobs. Probably not a moonshot, but many other jobs. Three is almost good enough sometimes.

Just to put pie on the face of those two conversions: Kilowatts equals Newton-metres times revs divided by 9549. Horsepower = foot-pounds times revs divided by 5252. Every dynamometer on earth knows this. Dynamometers don’t actually measure power. They measure torque and revs, and they calculate power from those measurements.

Because that’s how this works.

Because: Science rocks, and being a dumb shit doesn’t.

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