What is the relationship between puffs of smoke and steam cylinder strokes

[Brian Rupnow]

Yeah, I know, a really weird sounding question. On old movies of steam trains that have a sound track, it seems that every stroke of the steam cylinder is accompanied by a puff of smoke from the smokestack. I am assuming that there is some tie in there, but I don't know what it is. Can anyone explain it to me, please???

 

[b.lindsey]

Taking a stab at this...as memory serves, looking at one cylinder only...since it is double acting, each exhaust stroke (that would be twice per wheel revolution) a portion (of not most or all) of the exhaust is diverted up the stack to assist in the boiler draft. Given that, since the opposite side cylinder is offset by 90 degrees (so the engine will be self starting), that would be two more exhaust strokes for that cylinder in between those of the first cylinder. Thus looking at one driven wheel on one side of the locomotive, there would be four "puffs" for each wheel revolution. I am sure others will chime in if this is incorrect.

Bill

 

[GailInNM]

The exhaust from the cylinders is run up the smokestack to increase the draft for the fire. In general, with a locomotive type boiler, the fire is at the rear of the boiler and the smoke stack at the front and they are connected by firetubes that the hot combustion products pass through to heat the boiler. Smoke and other hot gases like to go up and not sideways so some form of draft is needed to make them go through the tubes to heat the water. When running, the exhaust steam is used to do this. When standing, the fireman has a valve that he can open to pass boiler steam up the smoke stack to increase the draft to build the fire up to get underway, or close to reduce the fire if they are going to be standing for a while.

Starting from cold was normally done in the engine shed hours before a run started. There a stationary boiler was used to supply steam to the blower. Some times duct work was tied to the stationary boiler smoke stack and that draft was used to get steam up on a cold boiler. Other methods were also used.

A tall smokestack also induces draft, but it is not practical on locomotives. This is the reason for the very tall smoke stacks at factories with their own power plant. Most people think that it was to get the smoke away from the people, but that was just a side benefit.

Gail in NM,USA

 

[tel]

Yep, Gail has pretty much covered it. Each 'puff' of exhaust up the chimney creates a partial vacuum behind it, which draws the fire, and combustion products thru the tubes and into the smokebox, where it then exits the stack.

 

[sparky961]

Wow, the things you never knew you never knew...

I had always wondered that too, Brian, but never really thought about it. Thanks for asking, and thanks for the detailed explanations.

-Brian

 

[Brian Rupnow]

Thank you to everyone for the explanations.---Brian

 

[GailInNM]

Brian,
Not that you probably want to know, but the second video posted by Florian at

http://www.homemodelenginemachinist.com/index.php?topic=2619.msg23048;topicseen#new

illustrates one other fine point about the train exhausts and it applies to other double acting twin cylinder steam engines with 90 degree crank settings. Taking a standard side rod locomotive as an example, there are four power strokes per revolution. Two of them are with the pistons moving from the front to the rear, and two are with the pistons moving from the rear to the front. First one piston moves to the rear, then the second piston moves to the rear. Then the first piston moves forward followed by the second piston moving forward. The movement of the piston from the front to the rear takes more steam than the movement from the rear to the front because the area of the piston rod has to be subtracted from the piston area on the forward movement. On a high power, high pressure engine this area difference is considerable at the piston rod is large to transmit the power. The result of this is that there are two power strokes followed by two slightly weaker power strokes. This is not noticed when the engine is running light, but when it has considerable load it is easily noticed. The video is of an engine working hard on a grade and it is easy to hear the PUFF-PUFF-puff-puff beat of the engine.

The link to the same video is here also.
[youtube=425,350]95adgcPoJXE[/youtube]

Another neat thing in that video is the blowing down of the sediment in the boiler as the engine crosses the trestle. It was normal to do it on trestles as the hot water/steam comes out at body height and would damage any wild life, people or vegetation that might be along the track. There is nothing on the sides of a trestle that might be damaged.

Gail in NM,USA

 

[Brian Rupnow]

Thanks Gail---I can remember when I was a boy, and the steam trains would pull into Detlor station. They would load up with cordwood for fuel in the cities, and then head down to Toronto, 150 miles south. There was always lots for a boy to see at the old station and yards.---Brian

 

[chuck foster]

excellent video..............but one question i have is what pressure would a locomotive typically be run at?

chuck

 

[GailInNM]

Chuck,
It mostly depends on the era of the locomotive. The operating pressure kept increasing as metallurgy developed. In the civil war era, 50 to 75 PSI was typical with many of the boilers still being built with wrought iron plates. As steels developed the pressure kept increasing and by the early 20th century pressures were about 200 PSI max. At the end of the steam locomotive era, 250 PSI was a rather common upper end. As those pressures were reached, lubrication was becoming a problem with piston locomotives as the high temperature steam made the common lubricants ineffective as they were washed away by the steam.

There were a few turbine locomotives built and some hybrid turbine-piston locomotives, where the turbine ran at high pressure and the exhaust was used as part of the steam supply for the lower pressure pistons. In one system, both a high pressure and low pressure boiler were operated by the same heat source. The high pressure portion of these locomotives and the pure turbine locomotives was 1200 to 1500 PSI.

The one in the video was probably in the 225 to 250 psi range as it is a fairly modern locomotive. They are still building and using conventional piston locomotives in China as they can use low grade fuel which is easily available. Higher grades of fuel, such as Diesel, are in short supply.

Gail in NM,USA

 

[Mcgyver]

Gail, nice video and commentary. thanks

 

[rake60]

I love that video!

The power of steam is incredible and that's a great display of it
with its work clothes on.

Rick

 

[bigal2749]

Thanks for posting the video.
You learn something everyday