Greg, the cut-off changes afforded by the reverser (whether powered by air or the lever called the Johnson Bar) do nothing for adhesion or mechanical advantage. The cut-off is merely the limitation of the admission of steam while the piston is moving ahead of the pressure imparted by the steam. With full cut-off, often down to 15% of the duration of movement of the piston, or its length of stroke, take your pick, the valve leaves the port open for a very short duration. This is used at speed when piston motion is rapid and the cycling rate is near or above 4 cycles per second. This conserves fuel because the boiler doesn't have to supply all the steam that can occupy the full volume of the cylinder once the piston slows, stops, and then returns in the other direction, now accelerating in front of another wavefront of admitted steam. With 15% cut-off, the valve admits very little steam that must then expand and fill the increasing volume behind the advancing piston.
When 'lifting' a train, a steam locomotive is typically going to need a more constant pressure applied to the advancing cylinder so as to fill the expanding volume, so the reverser is positioned to allow the valve to admit steam for a much longer time. In many cases, up to 85% of the piston's stroke. This keeps the pressure high, as opposed to what happens when the cut-off is only 20% and the limited steam must expand to fill the void. Understandably, at 20% cut-off the steam's pressure drops quickly in front of the advancing piston. This is just sufficient to impart a continuance of momentum if the track conditions and rolling conditions don't change for the train.
What would change the gearing for the locomotive would be the length of the main crank, or the diameter of the drivers, neither which change.
Steamers produce their highest horsepower between 3-4 cycles per second. A 6000 hp Niagara isn't going to be providing that to the pistons when it is running at 80 mph. But it will be providing much more HP to motive effort at 70 mph than a diesel will, the latter having fallen off near the 40-50 mph mark, depending on gearing and other factors. At least, that is what respected authors on the subject have related to their readers over the years.
Fun fact:
The Reading Co. tested using 1000 hp NW-2's on commuter trains and while they could reach speed, they couldn't accelerate fast enough to maintain schedule, the RDG went with FP7's instead.
Dave H. Painted side goes up. My website : wnbranch.com
dehusmanIf you have an ISL, then it matters even less because below 20 mph the acceleration rate is more determined by the electrical gear and turbocharger than the hp. For example GE's accelerate really slow.
And if you use older GP7/9s you have the EMD lag in transistion even when switching whereas Alco RS-1 ,RS-3 and RS-11 has instant throttle response.
IMHO Geep 7/9s on a ISL demands monmentum with speed step for closer to prototype operation.
Larry
Conductor.
Summerset Ry.
"Stay Alert, Don't get hurt Safety First!"
ATLANTIC CENTRALI have never cared for using any throttle with momentum, DC or DCC.
Sheldon,Momentum has always been a favorite mode for me since I like realistic switching.
Another favorite thing was pulse power on MRC's Golden Throttle pack of the early 60s since it slowed those old open frame motors down.Today I use momentum and speed step on my GP7/9s and just speed steps on my Alco RS-1,RS-3 and RS-11.
i was grossly wrong when I said "that the leverage between the piston and wheel can vary (Johnson bar?)". (yet another learning experience)
i've read about cutoff. But I found the Tractive Effort Calculator confusing when changing the cutoff also changed the Weight on Drivers and there was no setting for speed.
unlike a diesel locomotive where you dial up the power to the motor, it seems that a steam engine is more like a fuel injection on a car. The Bosch system i'm familiar uses a constant pressure fuel system and controls the fuel amount by controlling the time the injectors are open.
it seems that cutoff controls the power to the cyclinders by controlling how much steam is allowed to enter.
selector... the piston slows, stops, and then returns in the other direction, now accelerating in front of another wavefront of admitted steam.
it also makes sense to me that cutoff is necessary as speed increases to limit steam entering the piston to the beginning of the stroke where its effect is maximized and to limit the pressure in the cyclinder at the end of the stroke to make it easier to be forced out of the cyclinder when it reverses direction.
An engineer obviously has to get a feel for this and I assume that in order to maximize power/acceleration the cutoff has to be reduced as speed increases until it is reduced further to simply maintain speed. Presumably the fireman had stopped adding fuel to the firebox before reaching the desired speed anticipating the reduced need for steam.
acceleration and speed depend on drawbar force (== tractive effort). I believe the plot of diesel vs steam horsepower provides a comparison for determining acceleration for a steam locomotive. I assume the steam plots represent what a well operated steam locomotive is capable of doing. This obviously depends on the proper use of cutoff.
greg - Philadelphia & Reading / Reading
dehusmanThe Reading Co. tested using 1000 hp NW-2's on commuter trains and while they could reach speed, they couldn't accelerate fast enough to maintain schedule, the RDG went with FP7's instead.
When practical railroad men hear the size of cylinders, the diameter of driving-wheels, and the boiler dimensions of a locomotive, mentioned, they understand what kind of service the engine is adapted for, and about the weight of train it can haul.
i don't have the experience of many of you. This is why i generated the plots which i feel show the effect of varying amounts of horsepower on train speed and acceleration. I believe they account for the maximum TE due to adhesion, the decreased TE as speed increases with constant horsepower and the effect of train resistance as speed increases resulting in non-constant acceleration.
Randy pointed out why a 1000 HP switcher is adequate to move entire trains at slow speed in a yard. And the plots showed the need for a 3000 HP locomotive to get a train up to speed quickly when a 2000 or 1500 HP engine could also achieve the same speed.
gregcRandy pointed out why a 1000 HP switcher is adequate to move entire trains at slow speed in a yard. And the plots showed the need for a 3000 HP locomotive to get a train up to speed quickly when a 2000 or 1500 HP engine could also achieve the same speed.
Its not that you need a 3000 hp locomotive, its that you need 3000 hp. Two 1500 hp or one 3000 hp engine would have similar acceleration rates for a light to medium sized train. For a very heavy train, 2 1500 hp engines would have better acceleration because they have twice the axles so can apply a higher % of their power to each axle without slipping.