By-pass valves for steam locos

[tulip bone]

They perform several important functions:

1. They prevent alternating vacuum and compression in the cylinders when the locomotive is drifting, thus insuring the free movement of the pistons.
2. By permitting the circulation of the free air drawn into the cylinders through the vacuum-relief valves, they prevent this air from being overheated by the churning of the pistons and thus destroying the lubrication, when the locomotive is drifting down a long hill.
3. By destroying the vacuum which, without them, would be formed by the large piston, they prevent the smoke and gases from the smoke box being sucked into the cylinder.
4. They prevent excessive fanning of the fire from the pumping action of the large pistons when drifting

[clever token]

Snifter valves?

The little thing under the brake pump

[tulip bone]

No, thats another thing. It is mostly used when coasting.

http://advanced-steam.org/5at/technical-terms/steam-loco-definitions/bypass-valves/

[tulip bone]

Here is the driver operates this valve, it is near the window. https://www.youtube.com/watch?v=J9DrwxFdg_s

[clever token]

Snifters open automatically when the regulator is shut…

The driver dosent open them

[tulip bone]

This one is not a snifter 🙂

[jagged glacier]

yes, snifter and bypass are a completely different thing

snifter opens the admission to the atmosphere, the bypass joins both ends of the cylinder together

[clever token]

I didn’t have time to watch the video, i thought u were saying the driver operated the snifter

I’m too tired lol, my brains not working today

[shy holly]

what would they accomplish that the cylinder cocks don't do?

I see that they operate in different ways, but I'm wondering if there's a reason for a locomotive with cylinder cocks to also have a bypass valve

[jagged glacier]

cylinder cocks are often not as large as bypass valves

so you still experience a slight compression braking and pull air from outside

while the bypass valve would allow the air to just go back and forth with a large section to make sure it doesn't resist

[shy holly]

I see

[shy holly]

In this article it says that bypass valves are automatically operated (presumably off of the steam chest pressure); are they manually operated on that japanese locomotive?

Seems reasonable to make them automatic so the engineer doesn't have to worry about it every time they close the throttle

[tulip bone]

Yes, on this locomotive they are manually operated along with the drain valve (cylinder cocks). Anyway this valve should affect loco drifting, it should drift more freely then without it. As there will be no vacuum in cylinders.

BTW I have noticed steam locos drift not as good as diesel ones. With maximized reverser and opened cylinders drain. Probably this is because steam loco has more moving parts and plain bearings instead of roller ones. Or this is because of vacuum in cylinders. I'm not sure.

[clever token]

Combination of both probably

[shy holly]

I'd guess that the plain bearings are the culprit there

[tulip bone]

Vacuum can be very powerful, nearly same effect in jake break on trucks. It looks like in DV this effect is just ignored, i.e. no vacuum is simulated in cylinders at all. So it is about the same as to have automatic by-pass valve. And my idea was to make it manual, as it was really manual on many steamers. It will definitely increase immersion, as without opening it in in the correct moment you should feel engine break effect and damage to the pistons.

[shy holly]

What I meant is that IRL if you're seeing drag with the bypass valves open, I would assume that's mostly the plain bearings. But yeah if no bypass valves or snifters or anything, and the throttle is closed, then yeah it would be both

I could probably easily implement the vacuum simulation in the steam sim tweaks mod I'm working on, so you'd have to crack the throttle open to get rid of the braking effect. But adding a drifting throttle or some other kind of manual control would be annoying

[tulip bone]

Without possibility to open valve using handle in the cab will be just pain and no drifting at all 😦

[shy holly]

You'd have to crack the throttle open, which seems to have been common practice on many railroads

[clever token]

Just add snifters, problem solved

[shy holly]

The game already simulates snifters

They're not shown, but the "no vacuum at 0 throttle" is the game's simplification of how snifters work

From looking at photos, the USATC S200 (the basis for the S282) doesn't seem to have had snifters or bypass valves, although I could be wrong

[tulip bone]

Have you found in-cab photo of it?

[shy holly]

no, I'm just going off of what the cylinders look like from the outside