James Wrote:
-------------------------------------------------------
>
> You'll note at starting--meaning .5 or 1 MPH--the
> horsepower generated is indeed very very low.
> This is because at such a low speed the cylinders
> cannot use much of the steam the boiler can
> provide, and all that happens is the safety valves
> pop and the extra potential power goes to waste.
> As speed increases, the cylinders can use more of
> the potential power, and hence horsepower also
> increases and keeps increasing until you reach a
> speed where the boiler can no longer keep up with
> cylinder demand and cutoff has to be shortened or
> throttle reduced. That's when you've reached
> maximum horsepower
I’m definitely NOT a technical person, which is why I remember an article in TRAINS many moons ago that was exceptionally well written and quite fascinating. The author was the engineer I think on SP 4449 (the details are fuzzy!). They were unassisted on a fan trip going up Tehachapi or somewhere steep, and he commented to the railroad’s pilot that he hoped the signals would at least stay yellow and they wouldn’t get a red. As they slowed for a signal they finally got a green. The engineer widened the throttle and the pilot was absolutely astounded that they actually accelerated up the grade.
The pilot was used to diesels, which are of course more specifically diesel-electric locomotives, which are the exact opposite. Their highest pulling power (tech term please!) is actually when they’re standing still, and it decreases as they gain speed. That’s why the first diesels were used as switchers - they could start a mile long cut of cars but only get them up to 4 or 5 mph, ideal for yard service. Adding more units was required not to start a train, but to make it go faster once it was rolling.
Being the opposite, the power of steam locomotives is more on a bell curve. They may need assistance to get a heavy train started, but once rolling they’re off and running until their peak power is achieved, at a sweet spot speed determined in large part by their driver diameter. A low drivered freight loco would maybe be 30 mph, while a high drivered passenger loco would be 50-60 mph, etc.
Anyway, it was a super explanation, written in layman’s terms, that explained exactly why the SP pilot was amazed that the 4449 could accelerate up the hill when a diesel would be on its knees.
Cheers,
Ralph
Edited 4 time(s). Last edit at 09/17/2020 05:29AM by tgbcvr.
