In one of your 765 videos you mention cylinder "back pressure"(I guess this is residual pressure?) Can you explain the significance of this and how knowing it affects the way you handle the locomotive? Did most modern locomotives have a back pressure gauge?
Most modern locomotives have a back pressure gauge. It help in setting the cutoff. When a steam engine is operating, the valves which admit steam into the cylinder can be set to admit steam for varying lengths of the piston stroke. When starting from a stop, a long, steady push on the piston is needed to get the train moving. So the valve gear will be set "down in the corner" at maximum cutoff, which will admit steam to the cylinder for as much as 80% of its stroke.
However, if left in this position as the engine accelerates, the engine will quickly reach a point where things are occurring too fast for the steam to be exhausted from the cylinder before the next stroke begins! The Back Pressure will rise to 40+ pounds in this situation. The engine will be making a lot of noise, but fighting itself to run and not developing very much power.
In the 765, with the cutoff set correctly, the Back Pressure will be 5% to 7% of the cylinder pressure. If I'm working a light throttle and have 100 psi in the cylinders, the back pressure will be 5 to 7 pounds. With the throttle open further and 200 psi in the cylinders, back pressure will now be around 10 pounds and the engine will be making a LOT of noise! This Back Pressure is measured in the exhaust passage.
As the engine accelerates, the engineer has to "hook up" the valve gear, shortening the time that the steam admission port is open. When running fast, the cutoff may be set so that steam is admitted for only 20-25% of the stroke. The steam works expansively for the rest of the stroke. When hooked up properly, a steam loco at speed will have a sharp and distinct sound for every piston stroke. With the cutoff set too long, the exhausts begin to all run together and the steam loco will sound like a jet engine, with a steady roar at the stack. Not good.
Rich
You stated, "You cannot feel any sense of acceleration when making throttle changes. Steam locomotives are too big and heavy for that."
I remember back in the early 90's when the 765 and 1225 met in Lima, OH. I had the privilege of riding in the cab of the 765 south towards Cincinnati. One thing I really felt was the quartering of the drivers as the engine accelerated. That awesome feeling of torque and horsepower is something I will never forget...
Gene, I hate to burst your bubble, but what you were feeling when in the cab of the 765 when you rode out of Lima was not the quartering of the drivers as the engine accelerated. You were feeling the result of the drivers being out of round and out of balance! Today, with the drivers all re-balanced and round again, the 765 is as smooth as any diesel.
Hmm. That was one of the biggest differences I felt between the handling of steam and diesels. The instantaneous result when increasing the throttle, no delay.
Yes, Wyhog, the throttle response is instantaneous...no load regulator to wait for in a steamer! What I mean is that you don't feel any sense of acceleration. It doesn't push you back in your seat.
