# Why do steam locomotives "chug" (the noise they make...chugga chugga chugga) **URL:** \ **Category:** Factual Questions\ **Created:** [April 19, 2020, 8:21am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114 "2020-04-19T08:21:38Z")\ **Posts on this page:** 20\ **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:** [April 19, 2020, 8:21am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/1 "2020-04-19T08:21:38Z")
Title kinda says it all. You can hear the noise [here.](https://youtu.be/WuM9CSCyNZQ?t=160) I thought the smoke stack was for wood/coal being burned to heat the water in the boiler. Steam was used to push the piston that drove the wheels. Excess steam is dumped out the sides (as seen in the video). So why the chugga chugga out of the smoke stack? Was steam released there too? If so seems really inefficient to be dumping that much energy. That said, I am no expert on steam engines. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [April 19, 2020, 8:52am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/2 "2020-04-19T08:52:47Z")
Steam is released from the cylinder during the piston’s exhaust stroke and is directed out of the stack. This interrupts the steady flow of dark smoke from the firebox and creates the puffing effect. ---
**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:** [April 19, 2020, 8:55am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/3 "2020-04-19T08:55:42Z")
> [@Bear\_Nenno](#): > > Steam is released from the cylinder during the piston’s exhaust stroke and is directed out of the stack. The link I provided was later in the video. Earlier you can see steam blasting out the sides and top (apart from the stack). Why push steam out the stack? It seems there is no shortage of ways to unload steam pressure from the engine when needed. Not to mention it seems a waste of energy making the overall machine less efficient. ---
**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:** [April 19, 2020, 9:05am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/4 "2020-04-19T09:05:44Z")
> [@Bear\_Nenno](#): > > Steam is released from the cylinder during the piston’s exhaust stroke and is directed out of the stack. This interrupts the steady flow of dark smoke from the firebox and creates the puffing effect. My bad…you said it was the exhaust stroke. Why pipe it through the smoke stack? Why not just dump it out the side? Again, as you see in the video (from [earlier than I linked](https://youtu.be/WuM9CSCyNZQ?t=32)) they can and do dump steam out the side. Adding an exhaust pipe to the stack seems unneeded. Not to mention there is no chugga chugga there. ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [April 19, 2020, 10:17am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/5 "2020-04-19T10:17:45Z")
It’s piped out the stack to assist in the draft, pulling air through the firebox in a venturi effect. There’s also some dilution of the combustion gases going on and the “wet” steam at atmospheric pressure would extinguish sparks. That’s a speculation on my part. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [April 19, 2020, 10:30am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/6 "2020-04-19T10:30:51Z")
> [@Bill\_Door](#): > > It’s piped out the stack to assist in the draft, pulling air through the firebox in a venturi effect. There’s also some dilution of the combustion gases going on and the “wet” steam at atmospheric pressure would extinguish sparks. That’s a speculation on my part. It is indeed piped out the stack to assist in the draft. The stack actively pumps air through the boiler this way. There should be at least four “chugs” per revolution of the drive wheels, as the cylinders are driven in both directions, and the two sides of the locomotive are 90 degrees out of phase. I’ve never heard about the spark extinguishing effect, though it’s plausible. I don’t think the dilution is intended at all. For one thing, the same amount of exhaust gases are being thrown into the environment, regardless of whether the steam is added or is dumped elsewhere. ---
**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:** [April 19, 2020, 11:15am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/7 "2020-04-19T11:15:09Z")
> [@Bill\_Door](#): > > It’s piped out the stack to assist in the draft, pulling air through the firebox in a venturi effect. There’s also some dilution of the combustion gases going on and the “wet” steam at atmospheric pressure would extinguish sparks. That’s a speculation on my part. I do not understand this. Mind you I am on high school physics here but what I remember is the venturi effect is a volume of liquid (or gas) speeds up and is at a lower pressure as it moves through a smaller diameter pipe. Not sure what pumping steam out the stack has to do with that. We can see earlier in the video that the boiler is running well with no chugga chugga (not pulling steam up the stack). Smoke is flying out the stack and steam is jetting out of the sides. Piping steam out the stack seems a waste of energy. That said, I totally get I am no engineer and the people who made these had good reason for what they did and knew their job waaay better than my dumbass guesses. Still [trying to understand it.](https://i.pinimg.com/originals/0d/65/e4/0d65e4dfda0afad7dadb228aaa637dc7.gif) ---
**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:** [April 19, 2020, 11:27am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/8 "2020-04-19T11:27:31Z")
The ‘boiler’ on a steam loco is actually a container full of pipes. This is a far more efficient method of converting water to steam than having a large tank of boiling water. Hot air and smoke from the firebox is sucked through the pipes which heat the water, ending in the smokebox at the front, under the chimney. Waste steam is utilised to force this mixture out of the chimney and to create a draught through the boiler. The effect is identical to a fireplace with a chimney in a house, nothing to do with venturi effects. [![](https://img.youtube.com/vi/nd2Ii4OGQ4k/maxresdefault.jpg "How Do Steam Locomotives Work - Steam Engines Explained") ](https://www.youtube.com/watch?v=nd2Ii4OGQ4k) ---
**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:** [April 19, 2020, 11:36am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/9 "2020-04-19T11:36:42Z")
[quote=“bob\_2, post:8, topic:852114”] The ‘boiler’ on a steam loco is actually a container full of pipes. This is a far more efficient method of converting water to steam than having a large tank of boiling water. Hot air and smoke from the firebox is sucked through the pipes which heat the water, ending in the smokebox at the front, under the chimney. Waste steam is utilised to force this mixture out of the chimney and to create a draught through the boiler. The effect is identical to a fireplace with a chimney in a house, nothing to do with venturi effects. [/QUOTE] Ahh… I see now. Thanks. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [April 19, 2020, 11:47am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/10 "2020-04-19T11:47:50Z")
Already answered, but there are many cites on the [Wiki page](https://en.wikipedia.org/wiki/Steam_locomotive#Steam_circuit) > [@Wiki](#): > > Exhaust steam is directed upwards out of the locomotive through the chimney, by way of a nozzle called a blastpipe, creating 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, strictly speaking, the ejector) require careful design and adjustment. This has been the object of intensive studies by a number of engineers… > [@Wiki](#): > > Steam locomotives consume vast quantities of water because they operate on an open cycle, expelling their steam immediately after a single use rather than recycling it in a closed loop as stationary and marine steam engines do. Water was a constant logistical problem, and condensing engines were devised for use in desert areas… The most notable engines employing condensers (Class 25, the “puffers which never puff”[39]) worked across the Karoo desert of South Africa from the 1950s until the 1980s. ---
**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:** [April 19, 2020, 11:55am UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/11 "2020-04-19T11:55:11Z")
nm ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [April 19, 2020, 12:43pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/12 "2020-04-19T12:43:31Z")
> [@Whack-a-Mole](#): > > The link I provided was later in the video. Earlier you can see steam blasting out the sides and top (apart from the stack). The steam you’re seeing blasted out the sides is not part of the power production. This is a driver controlled effect. The engineer opens the cylinder cocks and steam blows out the sides. This allows the cooler steam and condensation to blow out of the cylinders to make way for the good, hot steam and to get the piston warmed up. You’ll see this performed when locomotives are starting up after they’ve sat for a bit. The engineer might do this while slowing down as well, or anytime he/she feels steam might be cooling off in the piston. And, of course, the engineer can do it just to show off. ---
**Author:** ![Sangahyando](https://avatars.discourse-cdn.com/v4/letter/s/f14d63/32.png) [@Sangahyando](https://boards.straightdope.com/u/Sangahyando)\ **Post date:** [April 19, 2020, 12:51pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/13 "2020-04-19T12:51:18Z")
Railway enthusiast with too much time on my hands, gratuitously latching further on to the Wiki quote given above, “The most notable engines employing condensers (Class 25, the ‘puffers which never puff’) worked across the Karoo desert of South Africa from the 1950s until the 1980s.” Afrikaner railwaymen called locos of this type, “Kameels” [=camels]. Another country which made much use of locos which condensed exhaust steam on this principle, was the USSR: employing them in water-poor areas such as the Central Asian " 'Stans". The Soviet railways, however, went in on a large scale for modernising traction – with electrification, and adopting diesel locos – from a quite early date: sensibly, this was implemented particularly early-on in those regions where water was a problem. Soviet condensing steam locos thus ceased to be used, a fair while before their South African counterparts suffered that fate. ---
**Author:** ![Yakamaniac](https://avatars.discourse-cdn.com/v4/letter/y/a5b964/32.png) [@Yakamaniac](https://boards.straightdope.com/u/Yakamaniac)\ **Post date:** [April 19, 2020, 1:39pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/14 "2020-04-19T13:39:56Z")
On a somewhat related note, is it true that steam locomotives have no torque at a standing stop? I have read that this is the reason that steam locomotives will spin the drive wheels when first starting out, to get the train moving, and then slow the drivers down and take hold. I am not sure if I am describing the thing correctly, but apparently the pistons will burst if pressure is gradually applied when they are stopped, or something like that. I have always seen steam engines spinning their wheels when starting out from a dead stop, and wondered why. ---
**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:** [April 19, 2020, 1:50pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/15 "2020-04-19T13:50:10Z")
Just going to point out that ‘chugga chugga chugga’ is the steam train language phrase ‘I think I can’, or depending on which ‘chugga’ is emphasized, ‘I know I can’. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [April 19, 2020, 2:30pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/16 "2020-04-19T14:30:45Z")
> [@Yakamaniac](#): > > On a somewhat related note, is it true that steam locomotives have no torque at a standing stop? No. What is true is that each cylinder has two dead spots where they provide no torque, since the connecting rod is exactly in line with the axis of the cylinder (basically if the point where the connecting rod is attached to the wheel is at the 9 and 3 positions on a clock as the wheel rotates). If the cylinders operated in phase with each other, and the engine happened to stop at the point where the cylinders were on these dead spots, then the engine would be stuck since its cylinders would be unable to provide any torque to the wheels. As **Napier** said upthread, the two sides are 90 degrees out of phase, and this is why. This prevents both sides from being simultaneously stuck at the point in the wheel rotation when the cylinder can provide no torque. So, depending on where the wheels happen to be in their rotation when the engine stops, one side or the other might not be able to produce any torque, but not both at the same time. If only one side of the engine is producing torque, the wheels might slip until they spin around to the point where both cylinders are producing torque. Otherwise, wheelspin is caused by the engine producing too much power for the load being pulled, or there is something on the track that is reducing friction, like oil. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [April 19, 2020, 3:08pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/17 "2020-04-19T15:08:03Z")
> [@Bill\_Door](#): > > It’s piped out the stack to assist in the draft, pulling air through the firebox in a venturi effect. There’s also some dilution of the combustion gases going on and the “wet” steam at atmospheric pressure would extinguish sparks. That’s a speculation on my part. My grandfather patented a [spark extinguisher](https://books.google.com/books?id=rUc1AQAAMAAJ&pg=RA3-PA79&lpg=RA3-PA79&dq=glafke+spark&source=bl&ots=KNdYPSniYR&sig=ACfU3U0ZkDAi38vNcrh_N881g3K9GsSPeQ&hl=en&sa=X&ved=2ahUKEwjyvMes4fToAhVMnp4KHYQbAS0Q6AEwAXoECAcQLg#v=onepage&q=glafke%20spark&f=false) here in Portland. It was for logging, but the same idea, it seems. ---
**Author:** ![EdelweissPirate](https://avatars.discourse-cdn.com/v4/letter/e/8491ac/32.png) [@EdelweissPirate](https://boards.straightdope.com/u/EdelweissPirate)\ **Post date:** [April 19, 2020, 7:56pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/18 "2020-04-19T19:56:07Z")
It’s not clear to me how your grandfather’s invention functions, but it looks like it used liquid water to extinguish sparks. Am I mistaken about that? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [April 19, 2020, 8:02pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/19 "2020-04-19T20:02:25Z")
“Steam locomotives consume vast quantities of water because they operate on an open cycle, expelling their steam immediately after a single use rather than recycling it in a closed loop as stationary and marine steam engines do. Water was a constant logistical problem…” This is the origin of “jerkwater” to describe a town that was almost nothing. It’s just a place they can jerk water up to the boiler to replace what’s been boiled off. A terrible logistical problem indeed. ---
**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:** [April 19, 2020, 8:39pm UTC](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114/20 "2020-04-19T20:39:10Z")
The steam coming out of the sides is not exhaust steam. When the engine sits the cylilnders cool off and water can condense in the cylinders. If that water is not removed the pistons may try to compress the water at top dead center, a very bad thing. So the cylinder vents are opened before the throttle is opened. Depending on how cool the engine has become the engineer may crack the throttle to blow steam through the engine. He leaves the vents open until he believes there is no danger of water still in the engine. Done on starting train, stationary, and marine engines. I guess an engineer could use the vents to help with throttle control. Recips can start with max torque. The wheels will spin if the torque is greater than the friction of the wheels. If the engineer opens the throttle too fast the wheels are going to spin. And this is a valve pulled by hand, making it a little harder to control than a valve that is open by turning a handle. The steam exhaust and the steam exhaust are designed. the exhaust steam blowing up the stack will decrease the pressure in the flue. Sucking more air through the tubes and fire box. The chugs are the exhaust valve opening 4 times each reveloution. [Next page](https://boards.straightdope.com/t/why-do-steam-locomotives-chug-the-noise-they-make-chugga-chugga-chugga/852114.md?page=2)