# Why would steam trains puff smoke out rhythmically (puff, puff, puff instead of smoothly) **URL:** \ **Category:** Factual Questions\ **Created:** [July 23, 2013, 2:36am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279 "2013-07-23T02:36:48Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 23, 2013, 2:36am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/1 "2013-07-23T02:36:48Z")
Title kinda says it all. Here is an example: [Steam Train struggles on 1 in 48 grade. SRV Robinvale Weekender: Australian Trains - YouTube](https://www.youtube.com/watch?v=fpzkAZmJ77I) I would think the coal burning would be steady and produce a constant stream of smoke out the smoke stack. I get the chug chug noise is the actuators(?) moving the wheels compressing and decompressing. But why would we see the huff-puff in the smoke stack? Wouldn’t the coal be burning at a constant rate and boiling water? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 23, 2013, 2:56am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/2 "2013-07-23T02:56:28Z")
Exhaust steam is routed back through the chimneys. From the [wiki](http://en.wikipedia.org/wiki/Steam_locomotive#Steam_circuit): > [@](#): > > Exhaust steam is directed upwards to the atmosphere through the chimney, by way of a nozzle called a blastpipe that gives rise to the familiar “chuffing” sound of the steam locomotive. The blastpipe is placed at a strategic point inside the smokebox that is at the same time traversed by the combustion gases drawn through the boiler and grate by the action of the steam blast. The combining of the two streams, steam and exhaust gases, is crucial to the efficiency of any steam locomotive, and the internal profiles of the chimney (or, more strictly speaking, the ejector) require careful design and adjustment. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [July 23, 2013, 2:57am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/3 "2013-07-23T02:57:19Z")
I believe that exhausted steam from the cylinders (the ‘chug chug’) is jetted up the smokestack to draw more air through the fire and make it burn hotter, especially when starting. I could be wrong though. ---
**Author:** ![Glazer](https://avatars.discourse-cdn.com/v4/letter/g/a88e57/32.png) [@Glazer](https://boards.straightdope.com/u/Glazer)\ **Post date:** [July 23, 2013, 3:14am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/4 "2013-07-23T03:14:37Z")
The pistons are pulling air into the fire box to stoke the fire. Once a head of steam has been built up the engine runs to the rhythm of the pistons. Cut off steam to the pistons and your fire will return to normal convection. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 23, 2013, 3:18am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/5 "2013-07-23T03:18:53Z")
Does it matter if the wheels are run synchronously or asynchronously? Essentially is it important for the wheels on both sides to be in unison or do you want them staggered so one can always get more power? Or does it not matter? (hope that makes sense) ---
**Author:** ![tellyworth](https://avatars.discourse-cdn.com/v4/letter/t/977dab/32.png) [@tellyworth](https://boards.straightdope.com/u/tellyworth)\ **Post date:** [July 23, 2013, 3:24am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/6 "2013-07-23T03:24:55Z")
I don’t know if it applies to the engine as well, but train carriages have a fixed axle (wheels turning in unison) in order to [stay on the track](http://www.youtube.com/watch?v=6-a3itxDiiQ). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 23, 2013, 3:36am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/7 "2013-07-23T03:36:02Z")
> [@tellyworth](#): > > I don’t know if it applies to the engine as well, but train carriages have a fixed axle (wheels turning in unison) in order to [stay on the track](http://www.youtube.com/watch?v=6-a3itxDiiQ). Good point although the pusher rods could be out of synch even with a common axle. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [July 23, 2013, 3:40am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/8 "2013-07-23T03:40:52Z")
> [@Whack-a-Mole](#): > > Does it matter if the wheels are run synchronously or asynchronously? > > Essentially is it important for the wheels on both sides to be in unison or do you want them staggered so one can always get more power? Or does it not matter? > > (hope that makes sense) One solid axel with wheels connected to each side. The wheels have to run at the same speed because they are one body. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [July 23, 2013, 3:42am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/9 "2013-07-23T03:42:00Z")
> [@Whack-a-Mole](#): > > Good point although the pusher rods could be out of synch even with a common axle. The rods are a little less than 180 degrees from each other. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 23, 2013, 3:42am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/10 "2013-07-23T03:42:24Z")
> [@Whack-a-Mole](#): > > Good point although the pusher rods could be out of synch even with a common axle. They woudn’t be out of sync so much as running on opposing phases, if there is a common axle. I don’t know if the cylinders have to run at the same speed though. Read the whole wiki article, it’s pretty detailed. I’m sure there’s a ton more information out there, and there are some train dopers who may drop by with more specific information. ---
**Author:** ![Glazer](https://avatars.discourse-cdn.com/v4/letter/g/a88e57/32.png) [@Glazer](https://boards.straightdope.com/u/Glazer)\ **Post date:** [July 23, 2013, 4:13am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/11 "2013-07-23T04:13:18Z")
> [@Snnipe\_70E](#): > > The rods are a little less than 180 degrees from each other. Since there are only 2 of them wouldn’t that make them a little more than 180[sup]o[/sup] as well?🆒 ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [July 23, 2013, 5:07am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/12 "2013-07-23T05:07:01Z")
Locomotive cylinders are double acting (power on both directions of stroke) so the phase angle on a conventional two cylinder loco is 90 degrees. Compounds, with 3 cylinders (the 3rd is in the middle between the wheels) can be 120 degrees or 135 degrees (if the cylinder is angled. Obviously with single acting cylinders, if you have two, the phase angle needs to be a bit under 180 to always be able to start rotation. ---
**Author:** ![BlakeTyner](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@BlakeTyner](https://boards.straightdope.com/u/BlakeTyner)\ **Post date:** [July 23, 2013, 5:20am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/13 "2013-07-23T05:20:11Z")
(6 years as a steam engineer) Each distinct “chuff” you hear is one side of one piston exhausting. As the valve gear moves and admits high-pressure steam from the boiler into the cylinder, the used, expanded, low-pressure steam that has just pushed the piston now must be exhausted out the stack. The valve gear opens the port on that side of the piston as it also opens the port to allow live steam to the other side. The high-pressure steam expands, pushes the piston the other way, and as it travels it forces the low-pressure steam out in front of it. Then the valve gear do the same thing the other way. With a 2 cylinder engine, you don’t want the pistons on both sides to be in sync. I’m sure there’s math for efficiency when at speed, but the main reason is that there is a certain point where a piston is “high centered” in the cylinder, and admitting steam via the throttle will have no effect on it. In that case, you want the other piston to be able to get power and start the train moving. Ideally, the “chuffs” would be timed equally apart, but the valve gear timing has a tendency to get off, and so you might end up hearing something like "chuff…chuffchuffchuff…chuff…chuffchuffchuff. A pipe wrench and 15 minutes of running back and forth fix that. There is usually a steady exhaust out of the stack when the train is moving slowly or stopped. This is the blower, which is just a pipe that exhausts live steam from the boiler into the stack, which creates the draft for the firebox so that the fire gets enough air. When moving, the blower is reduced or shut off, because the natural exhaust from the cylinders will create that draft. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [July 23, 2013, 6:57pm UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/14 "2013-07-23T18:57:23Z")
> [@Sunspace](#): > > I believe that exhausted steam from the cylinders (the ‘chug chug’) is jetted up the smokestack to draw more air through the fire and make it burn hotter, especially when starting. I could be wrong though. Another reason is that routing damp steam through the stack prevents built-up carbon from catching fire (like a chimney fire in a house). This first came to my attention when reading about the ironclad gunboat USS _Carondelet_ running past the gun batteries of Island No. 10 in the US Civil War. Planning to sneak past under cover of darkness, the _Carondelet’s_ engineers re-routed her steam into the paddle box to muffle the chuff-chuff sound. But that led to a problem, as Wikipedia notes (bolding mine): > [@](#): > > On April 4, conditions for running past the remaining batteries were satisfactory. The night was moonless, and after dark a thunderstorm came up. _Carondelet_ made her way downstream, and was not discovered until she was abreast of the Confederate Battery No. 2. **She might have escaped detection completely had not her smokestacks blazed up; the buildup of soot, no longer dampened by escaping steam, caught fire and revealed her position**. The batteries opened, but their fire was inaccurate, and _Carondelet_ completed the run unscathed. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 24, 2013, 12:01am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/15 "2013-07-24T00:01:27Z")
[This youtube video](http://www.youtube.com/watch?v=WueHGOCP8b8) (starting at 25s) shows an interesting exception: the [South African Class-25 condenser locomotive](http://en.wikipedia.org/wiki/South_African_Class_25_4-8-4). It was built for long desert routes where water is scarce, so instead of venting the steam through the chimney, it pipes the steam into a huge condenser and reclaims it. But because there is no smoke to vent the smoke and move the hot air/fire through the boiler, they fitted a steam-driven fan under the chimney to do this job. So this locomotive makes a steady whine instead of a chuff-chuff noise. You can hear it best around 2:40 into the video. (There is a faint chuff-chuff noise since the steam is still vented out of the cyliders, but it’s muffled because it doesn’t go up the chimney.) ---
**Author:** ![VunderBob](https://avatars.discourse-cdn.com/v4/letter/v/839c29/32.png) [@VunderBob](https://boards.straightdope.com/u/VunderBob)\ **Post date:** [July 24, 2013, 11:42am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/16 "2013-07-24T11:42:46Z")
> [@Glazer](#): > > Since there are only 2 of them wouldn’t that make them a little more than 180[sup]o[/sup] as well?🆒 No. The rods are 90[sup]o[/sup] apart, so one cylinder is taking steam at any one time. FWIW, steam cylinders are double sided, vs internal combustion that are single sided. ---
**Author:** ![VunderBob](https://avatars.discourse-cdn.com/v4/letter/v/839c29/32.png) [@VunderBob](https://boards.straightdope.com/u/VunderBob)\ **Post date:** [July 24, 2013, 11:58am UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/17 "2013-07-24T11:58:39Z")
> [@Sailboat](#): > > Another reason is that routing damp steam through the stack prevents built-up carbon from catching fire (like a chimney fire in a house). That’s a side benefit, not a design feature in a locomotive. I’ve shoveled enough carbon black from a smokebox to know. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [July 24, 2013, 12:11pm UTC](https://boards.straightdope.com/t/why-would-steam-trains-puff-smoke-out-rhythmically-puff-puff-puff-instead-of-smoothly/664279/18 "2013-07-24T12:11:16Z")
When I was a kid we used to love to stand on a bridge when a train went under. Of course the ‘smoke’ would totally blind us, but it did not sting our eyes and make us cough as you might expect. [![](https://img.youtube.com/vi/WwaRc8t7Wos/hqdefault.jpg "Steam train going under bridge") ](https://www.youtube.com/watch?v=WwaRc8t7Wos)