Mark Huber Wrote:
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> Poundforce, lbf, #, pounds, etc. - all the same -
> are a force.
>
> Horsepower - is a unit of power. Power is energy
> per second; energy (many different ways to arrive
> here, but for our case) is force times a distance.
> Force at the piston, times crank radius gives you
> your energy. The faster you can do that, the more
> power you get.
>
> If that doesn't make sense - think of it this way:
> More energy per second, more horsepower. This is
> why car engines can put out lots of horsepower -
> lots of RPM's. In the case of a steam locomotive,
> the engine is directly coupled to the speed
> without shifting - hard to get that force per
> second going on. Hence why horsepower is not a
> great comparison for steam locomotives vs. other
> machinery.
Good way of putting it. Steam engines essentially generate large force slowly. Force times speed: Large force at low speed or low force at high speed, either way gets you the same number and the equation doesn't care. Out in the real world your load definitely cares though, so horsepower is a rather limited metric for comparing different kinds of engines. For comparing the same type of engines doing the same type of work it's quite useful (which was James Watt's purpose), but unfortunately it's overused as a general catchall in much modern advertising.
On top of that a lot of folks forget the distinction between maximum and sustainable power. I run into this a lot when some dude scoffs that his pickup truck makes as much horsepower as some old steam engine. Sure it might, but let's see that truck try to make that maximum all day long--can't, it'll wind up broken down on the roadside in short order. This is why railroads aren't falling over themselves to put ~10,000 horsepower top fuel drag racer engines in their locomotives, they need engines that'll make maximum power for more than ten seconds without blowing up.
We can finally toss in horrible misleading terms like "boiler horsepower" which has nothing at all to do with any of the above and it illustrates why over the years I gradually resent the word "horsepower" more and more.
