LastspikemikeWere these fast steam locomotives steam flow limited or driver rpm limited?
In the days before Woodard and Eksergian, the idea of 'diameter speed' as a general rule of thumb was thought to apply: the driver diameter correlated with the top service speed of the locomotive. In practice that number increased dramatically within the first few years of lightweight rod 'understanding', to the point that Alfred Bruce (who was in a position to know if anyone was) thought the Milwaukee A design was good for 128mph or better (on 84" drivers without Timken rods) and Kiefer after WWII spun a J3a up to the equivalent of 161mph without damage doing augment testing.
On the other hand, relatively large drivers were not always indications of high speed; Golsdorf's 2-6-4s (which had Krauss-Helmholtz bogies and therefore better guiding than Bissel engines) used the large drivers to reduce machinery speed and lubrication requirements, not obtain higher balanceable rotational speed. Both Mallard and the T1 used drivers no larger than 80" for demonstrable speed well above that. And large drivers combined with short stroke can hurt high-speed acceleration capability... without which you can't reach high speeds you might otherwise be able to sustain. Meanwhile there are critical speeds and resonances which have a very practical speed limiting effect, too.
CPR fastest locomotives as tested were the Jubilee class 4-4-4 (come on Rapido, these just have to be icons of Canadian Steam)...[/quote]These are the moral equivalent of the Milwaukee 'heavy Atlantics' -- we're talking of course about the F2s, not the little wind-up toy F1s which are light Hudsons with a driver pair left out -- which had 80" drivers -- perfectly adequate for speed. I suspect an A could run away from one, but that's more an artifact of being able to oil-fire rather than having greater heating surface, etc.
They looked to be large locomotives also.
Throttle restrictions and steam flow issues matter not if the locomotive can't increase driver rpm anyway.
ICE have rpm limits that can be reached before the otherwise maximum vehicle speed can be reached, for example.
But IC engine limits are at far higher rpm and hence have much higher inertial peak forces than a properly-high-speed-balanced long-wheelbase high-polar-moment American locomotive involves. By analogy with the balancing method Riddles used on the 9F class in Britain (and the Australians tried on some surprisingly unexpected wheel arrangements!) a reciprocating locomotive can receive zero overbalance. The only hammer-blow tendency in such a locomotive is driven by the couple between counterweight and rod, which are within an inch or two of being in a common plane; the absence of overbalance means that the rotating balance of the side rods (and rotating part of the mains) can be 'perfect'. While the resulting wheels have enormous rotational inertia compared to parts of a vehicle IC engine, they also rotate at comparatively low rpm -- AAR recommended highest speed was about 510rpm but this could be practically exceeded in service at times; IIRC Glaze's J balance design (which was NOT even remotely zero-overbalance, although I was taught years ago that it was and that the lateral component of unbalance had to be absorbed in stiffer lateral in the lead and trailing trucks) was good for about 540rpm, and modern practice could well do better ... on smooth level track without the usual and perhaps frequent sort of perturbing effects.
There are people who will affirm that very high drivers are necessary for high steam speed. One justification is keeping machinery speed "low enough" (but they will often quote machinery speed, e.g. peak piston speed, that is conservative by late observed practice, or by what advanced design and tribology can support). A problem with higher drivers is that they have difficulties with starting torque at even relatively high peak piston thrust, and especially when combined with 'lowest possible stroke' as in the T1 (which only got to its stroke by absolutely minimizing the same metal web thickness between axle and main pin, and even then ground the main pin to an eccentric profile so the main rod-eye circle was less than that for the side rod). Doing this can produce the somewhat counterintuitive model of a 6400hp locomotive that cannot start its own train on a grade without assistance ... and was intentionally built without a proper reversible booster to assist with that issue.
OvermodBest short-term answer: see the N&W 610 testing on PRR. Verified speed in excess of 110mph...
is this for a passenger train? what about a 100 car freight?
greg - Philadelphia & Reading / Reading
It's not exactly 'inertia' but for many years I sat through explanations where people used the gas mass in momentum calculations -- complicated by the fact that the gas DOES have inertia and momentum when in motion and WOULD exert force when 'decelerated'. The point I'd nake is that the elastic effect of molecular motion at these densities and temperatures far outweigh the "momentum" contribution directly.
When the valve closes faster than a certain rate, steam will compress slightly (and probably reflect a shock back up the tract or 'ring' around in the chest a bit) but I don't expect inertial force to hold pressure for the length of time back to same-side opposite port admission. One of the things to be tested at high speed on 5550 is the actual gas kinetics at high speed with the eight-valve setup; at least some of those data will be highly applicable to the situation with large-port long-travel piston valves.
(If you have not seen a video explanation of long-lap long-travel valves, find and watch one. Some people think piston-valve travel is very short, as with older slide valve/riding cutoff practice where low travel and minimized mass was advantageous (and easily driven by eccentrics on an uncranked axle)).
The first place I recall seeing momentum effects at a macro level during admission involved comments in the test records of the PRR Q2, a design "infamous" for having been designed with colossal dead space by any published design criterion. It is difficult to imagine an actual mistake of that magnitude, especially on a carefully-considered engine with reduced steam-mass-flow rates into the smaller cylinders of the divided drive. Look critically at the series of surviving test results and think. (Keep in mind that the Q2 developed horsepower and the slightly higher V1 power were about the practical maximum sustainable horsepower for a single-unit locomotive, set by the water rate and practical cistern capability -- about 8000hp at large-locomotive efficiency...)
One of the points of piston valves is that the port volume is arranged circumferentially so any increased pressure is balanced on the rings and any momentary starvation is made up by very high and, ideally, reasonably flow-streamlined acceleration of mass into the passage through the port. What you're looking for would be some recovery of indicated cylinder pressure rather than continuous fall across the period of admission, to be 'as high as possible' at the moment admission stops.
Any pulse effect induced after admission ends will likely not affect potential admission elsewhere while it still has the effect to enhance flow -- which for steam is likely to be slight before the pressure effects move away from the immediate neighborhood of the port by the time subsequent admission starts. Compare this with either pulse ram induction or the principle of the pulsejet.
Poppets if a bit overenthusiastically closed may lead to worse flow issues, especially if the admission tract is as weirdly convoluted as in, say, most of the Franklin type B seems to be (the reasoning as far as I can see is to have all these bent volumes act as a 'steam chest' that recovers smooth pressure by the time the admission valves pop open -- not necessarily achieved in practice according to some accounts I'be seen go by. I was taught very early to use nothing but modified-trapezoid cam profiles to drive steam poppet-valve gear (no jerk!) and the form of the cam can determine admission opening asymmetrically to establish best early mass flow vs. pressure drop. Closing, though, in many Franklin applications is not desmodromic and excessively accelerated to get positive and debounced poppet closure... look for weird pressure effects there and then.
Balance chambers in slide valves are often fed via a different path than the main steam, and this may determine what the effect on the valves is. Anything that adds to the (differential) pressure on the chest side of the valve, even for a brief time, may upset the tribology or increase the resistance of valve motion -- even fretting motion may start to ruin the lapped surface