I asked a train fan friend, who had this answer that seems to explain it. — Mike
Begin forwarded message:
From: Claudio Topolcic <claudio@sunsetharrier.com> Subject: Re: [math-fun] Stupid steam train question Date: November 8, 2017 at 10:29:30 PM EST To: Mike Beeler <mikebeeler2@gmail.com> Cc: Claudio Topolcic <claudio@sunsetharrier.com>
Yes, you guys are figuring it out. This was worked out in the early days of steam locomotives. The first design was more like a tea pot, where the hot gasses went around the boiler. This is very inefficient since most of the hot gasses don't come anywhere near the boiler. So they ran the hot gasses through tubes in the boiler, like your house furnace. But you want to force the cold air through the burning coal to burn faster. I think this is what tall smoke stacks are for in power stations. So you could use a fan, and I think this design was used. But there is a simpler design. The spent steam from the cylinders goes up in the "blast pipe" in the smoke box at the front of the boiler, and that pipe ends somewhat below the bottom of the stack. So the exhaust steam passing out of that pipe and to the stack causes a Venturi effect that sucks the hot gasses through the pipes in the boiler through the smoke box and out the stack. So yes indeed the puffing of the smoke and steam exhausted out the stack are in synch with the cylinders.
I assume that below, Henry was seeing steam coming out of the cylinders, and this is what he was taking about. I assume he figured that was the spent steam being exhausted from the cylinders. That is not the case. There are various reasons why some steam would be exhausted directly from the cylinders. I don't know all, but an example that I do know is that there is a valve at the bottom of each cylinder that the engineer can open remotely. They open the valves when the cylinders are still cold because some of the steam condenses in the cold cylinders and if they left liquid water in the cylinders then that could break the pistons when they move to their extreme. But that is not the bulk of the exhausted spent steam, that goes up the stack.
I don't know about steam ships, but I would expect that they do the same thing with the spent steam. But there is a lot of plumbing between the boiler and the funnel, and I don't know what happens to puffs. Also big steam ships have been using turbines for a while, with no puffs. But I don't know.
Check out the Wikipedia page https://en.wikipedia.org/wiki/Steam_locomotive <https://en.wikipedia.org/wiki/Steam_locomotive>. It has this drawing, check out #12:
[ I think math-fun is only text, so I removed the illustration. It shows the outside and a cut-away of a steam loco. Part # 12 is the “blast pipe”. ]