Poppet Valves

Trains Magazine General Discussion
1

I seem to recall that towards the end of steam production there was a small movement towards poppet valves. I think the Pennsy used them on some of their stuff (the S1, T1 and Q2 duplexes maybe?)

I’m curious about their operation, and their advantages/disadvantages. Anybody know much about these creatures?

2

You may try posting this in the “Steam and Preservation” forum, now that everything is broken down into several catagories.

Edit: BTW, are you gettin’ spun up for the state fair run yet? That should be a great run along the joint line with some great phot ops. I haven’t quite figured out what we are going to do yet.

3

Smitty,

Can’t wait for the State Fair train. I need to get out on the joint line and figure out some places to shoot the train. It’s been awhile since I’ve been down that way, and I definately want to get some nice shots.

4

…Chris:

I just ran a “search” and found good info regarding railroad poppet valves. You might give it a try if your interested.

5

Chris

Here is what I found for the Franklin poppet valve that was used on the C&O’s streamlined engines.

http://findarticles.com/p/articles/mi_qa3943/is_200605/ai_n17184256?lstpn=search_sampler&lstpc=search&lstpr=external&lstprs=other&lstwid=1&lstwn=search_results&lstwp=body_middle

Unfortunately, this reprint doesn’t include the diagrams that were in the original article.

6

I think a Capprotti was a maker of poppet valve systems.

The B&O experimented with them.

Carl T.

7

Jay,

Thanks for the link. It’s a shame that they didn’t reproduce the drawings, but that’s OK. I still have a better understanding of the how and why of it all.

How about that bit about T1’s being run at up to 140mph to make up time?!?!?! [:O] I had heard rumors that they saw service above 120mph, but I didn’t know it was by that much. I can’t even fathom what it would be like to watch a T1 and a string of coaches fly by at 140 mph!

8

I rather suspect that the T-1 MIGHT have been run to 140mph on the test stand at Altoona - but I wouldn’t put money on it.

The N&W J was alleged to have been counterbalanced for 140mph, but it never came within 25mph of that speed on road tests (Maximum has been variously reported as 114mph and 110mph.) I’m sure that a 140mph T-1 run would be in the Guinness Book of Records if anyone could plausibly prove that it happened.

The big advantage of poppet valves was the separation of the inlet and exhaust events. That allowed maximum use of the expansive power of steam, since extremely short cutoff of the inlet would not result in any back pressure buildup from steam that couldn’t get out of the other end of the cylinder.

Chuck

9

Chuck,

Thanks for the note. I was up at the GLRR today, and I got the same response about poppet valving (i.e. that it’s main benefit was the seperation of inlet and exhaust timing to allow short cut-offs on the inlet side without the backpressure problems associated with really short cut-offs).

What’s also interesting (and makes sense to me) is that the goal is not to completely exhaust all steam from the cylinder. Rather, it’s good to leave a little in there to act as a “cushion” for the piston to compress against near the end of it’s stroke. Apparently, it greatly reduces wear and tear on the rods and such.

I’m really curious about the balancing of the J’s for 140mph…that must have been an incredible accomplishment. Didn’t the J’s have really small drivers (well…for a Northern anyways)? I imagine that balancing small drivers like that for 140mph must have been really, really tough. That’s really interesting.

10

Indeed it would be tough. Don’t hold your breath waiting for somebody to explain it.

11

Chris –

If I’m not mistaken that position on the Johnson Bar (slightly forward of dead center) was known as the “company notch” because in that position the locomotive ran at peak efficiency (fuel use). As you say, it left a little residual steam pressure in each cylinder so that when compressed by the piston near the end of each exhaust stroke it would help push (spring) the piston in the reverse direction – like air shocks. The position was slightly different on different types of locomotives – a top engineer could set the position by the sound of the engine.

PZ