James Wrote:
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>
> 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.
>
> 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.
Horsepower is a useful knowledge factor for comparing engines, but under two assumptions. You are talking only about drawbar horsepower--which is really the only number worth mentioning. That's what gets to the rail and the coupler. The other point is that with the steam power curve, horsepower varies with speed and maximum horsepower has to come with a quantified of speed. Really, a power curve is the key thing, and that requires sticking a dynamometer car behind the engine to get accurate ratings.
In simplest form, tractive effort is how much you can start. Horsepower is how fast you can pull the load.
For a real life example, you can use Rich Melvin's article in Railfan & Railroad comparing Milwaukee 261 and Nickel Plate 765 on the New River Train in West Virginia. Both engines had enough tractive effort to start the train and keep it moving. Different years, but same size and weight of train. The difference was really noticeable on the grades at Cotton Hill deep in the gorge. 261 was running wide open most of the eastbound (uphill) run and was not able to maintain track speed. 765 could move the train at maximum allowable speed with a measure of reserve on the throttle. 261 couldn't generate enough power with that tonnage to move at track speed on that route. 765 could. That is horsepower.
Another real life example from a few years ago. N&W 611 was on a southbound excursion at Danville, VA with about 20 cars including some heavy superdomes. They had to stop in Danville to exchange EMT crews, and just beyond the depot was the start of the grade to the North Carolina state line--something the Southern referred to as Cemetary Hill. With the rain pouring down and only a couple hundred feet before the grade began, 611 couldn't get a run for the hill. Just into the hill, there are some back to back reverse curves that put a lot of drag on the train. The engine got to slipping and was soon down to a walking pace, but it didn't stall. Once out of the curves, the track had some long straight stretches to the top. That engine took off like a rocket and was easily doing all of the 40mph limit NS placed on it by the top of the hill. That's horsepower in action. The faster they got, the more power 611 could put to the rail. The acceleration riding the train was incredible. Another engine with the same tractive effort but less horsepower would have been able to accelerate, but may have topped the hill at say 20mph. It didn't have the horsepower to be able to accelerate the train any faster. Power at speed. That's what late steam era superpower design was all about.
That's also why when comparing steam and early diesel that it would take multiple diesels to equal the performance of one steam locomotive. The individual units didn't have enough horsepower (say 1500hp for an F-unit). The steam engine they were replacing was making 3500hp at 40mph. So you had to put several together to be able to pull the consist at the same speed down the railroad.
Kevin
