Sound of Three-Cylinder Loco

[Andy Kirkham]

 

In this video of a three-cylinder locomotive, it is noticeable, especially from about 6:09, that the exhaust beat is audibly in "6/8" time. Is this always the case with three cylinder machines, and if so, why? I appreciate that there will be six beats per revolution of the wheels, but why should one of those beats be more emphatic than the rest?

 

 

[The Johnster]

'Cos it don' mean a thing/if it ain' got that swing.

 

In a perfect world, 3-cylinder steam locomotives would give out an even 6-to-the-bar, and some might actually achieve this fresh form workshops at low mileages if the valve gear has been set up perfectly, but valve setting is a dark art, practised by men in hooded cloaks in gothic cellars with pentagrams and black candles to eldritch chanting and the screams of tortured sacrificial victims.  In other words, nobody knows how to do it properly, so the beats are slighly off most of the time and on high mileage engines a good bit off all of the time, resulting in the 6/8 groove.  Gresley conjugated valve gear is prone to some interesting synchopation as well.

[DavidB-AU]

There is some complex physics and mathematics behind this. The short version is because the connecting rod is (usually) angled, the mid point of piston travel and the top and bottom positions of the crank don't exactly correspond. As a result, valve events are very slightly offset from the theoretical intervals. What you hear as a volume difference is because valve timing is not uniform even when "properly" timed. Even on a 2 cylinder loco with perfectly quartered cranks, one exhaust beat per rotation will always be louder. This is less noticeable at low speed when there is a longer time between exhaust beats. This gets more complicated if any of the cylinders are inclined. W.A. Tuplin explained this in one of his books but the mathematics went way over my head.

 

It gets even more complicated with some 4 cylinder locos. The cranks are usually set at 90 degrees with the inside and outside cranks on each side at 180 degrees to each other, giving 4 beats per revolution. There were oddities like the Lord Nelsons which had the inside cranks at 90 degrees and the outside cranks at +/- 135 degrees giving 8 beats per revolution. It looks odd when drawn but gives one stroke every 45 degrees.

 

There is another effect causing a difference in volume because the forward portion of most cylinders contains slightly more steam than the rear portion, as the rear portion contains a piston rod. When the forward portion is compressed and discharges. the exhaust beat will be slightly louder. You don't get this effect on double guided pistons with rods protruding from the front of the cylinder. On a 2 cylinder loco you quite often hear 2 slightly louder beats followed by 2 slightly quieter ones. On a 3 cylinder loco you often hear 3 slightly lounder beats then 3 slightly quieter ones. EDIT: I can hear this in parts of the above video.

 

EDIT2: If you isolate the audio track from the video (done here with Audacity) it is indeed 3 and 3. So 3 consecutive louder beats per revolution rather than one

 

 

Cheers

David

Edited November 9, 2021 by DavidB-AU

[LMS2968]

Another issue with the Gresley system was that the inside cylinder was much higher than the outers in order to clear the leading coupled axle. The cranks were angled to allow for this, but since the inside valve was driven by the outers, they were set at 120 degrees. In theory, it should all have worked out, but . . . 

[The Stationmaster]

I think at the end of your final sentence you really meant ... 'BUT'

[Miss Prism]

The last track (At Steele Road Soon After Dawn On A Morning In May 1961) of Peter Handford's 'Trains in the night' album is classic 3-cylinder.

 

Edit: ah-ha, found it on Youtube! (track starts at approx: 36:00)

 

Edited November 9, 2021 by Miss Prism

[jim.snowdon]

5 hours ago, LMS2968 said:

Another issue with the Gresley system was that the inside cylinder was much higher than the outers in order to clear the leading coupled axle. The cranks were angled to allow for this, but since the inside valve was driven by the outers, they were set at 120 degrees. In theory, it should all have worked out, but . . . 

The two outside cranks were at 120 degrees but the inside crank was adjusted to allow for the angle at which the inside cylinder was set. From memory, I think the offset was 8 degrees, so the cranks would have been set at 120 / 112 / 128 which, with the inside valve still being horizontal, still gives a regular 6 beats to the revolution. What is different are the lengths, and therefore impedances, of the exhaust passages of the inside and outside cylinders, which will affect the exhaust pressure at the blastpipe.

[LMS2968]

Yes, I meant to say that the cranks were out of phase to compensate, hence the 'In theory, it should all have worked,' but in conjunction with backlash in the two-to-one motion levers, especially at higher mileages, it left a bit to be desired in practice.

[Nearholmer]

This seems like a good time to ask about this musical stuff: I’ve never understood what musical people mean when giving a timing as Andy did at the outset, 6/8, or any of the other timings so expressed.

 

Can any one explain?

 

Another musical thing that mystifies me is octaves and pitch etc. I think I might understand it, in fact I’m pretty sure I would, if someone could kindly explain it in terms of frequency (or wavelength, but I really prefer frequency).

 

Apologies if this is a diversion.

 

 

Edited November 9, 2021 by Nearholmer

[PenrithBeacon]

7 hours ago, LMS2968 said:

Another issue with the Gresley system was that the inside cylinder was much higher than the outers in order to clear the leading coupled axle. The cranks were angled to allow for this, but since the inside valve was driven by the outers, they were set at 120 degrees. In theory, it should all have worked out, but . . . 

It did all work out on Halford's drawing board, but he, like a good many other draughtsmen, forgot about working clearances, tolerances and wear on all those pin joints.

There is an issue of the ProcInstLocoEng of about 1920 where all the possibilities of derived valve events are laid out. Ok on the paper but would have been weird in practice. 

[Wickham Green too]

Then of course there's the mysterious exhaust sounds of three-cylinder compound locos which can start as a simple with different-sized cylinders and then run exhausting from the low-pressure only !

[rodent279]

7 hours ago, Nearholmer said:

This seems like a good time to ask about this musical stuff: I’ve never understood what musical people mean when giving a timing as Andy did at the outset, 6/8, or any of the other timings so expressed.

 

Can any one explain?

 

Another musical thing that mystifies me is octaves and pitch etc. I think I might understand it, in fact I’m pretty sure I would, if someone could kindly explain it in terms of frequency (or wavelength, but I really prefer frequency).

 

Apologies if this is a diversion.

 

 

Dunno, not sure I can explain, but an octave is I think a bit like a decade-in a decade, the highest frequency is 10 times that of the lowest, in an octave it's 8 times.

Pitch, as I understand it, is another way of saying frequency.

One thing that does interest me is how the "chuff" of a steam loco exhaust beat is produced. Is it just a function of the velocity (& density?) of the gasses passing through the chimney, and the diameter and length of the chimney, or does the size, diameter and length of the blastpipe & smokebox come into it as well? If it is related to the smokebox dimensions, then is it also a function of the speed of the gasses rushing along the firetubes, and crashing into the smokebox walls and door?

It ought to be possible, given an understanding of these things, to model and reproduce electronically the "voice" of a steam loco under a range of conditions, in much the same way as a human voicebox can be modelled and reproduced/synthesized electronically.

Edited November 9, 2021 by rodent279

[PhilH]

Further to the post re Gresley Pacifics earlier, on Bulleid Pacifics the inside crank is set back 6 degrees giving relative crank angles of 120-114-126 degrees. This gives rise to the typical uneven exhaust beat when in full gear or at longer cut offs, when the gear is pulled up to shorter cut offs the beat evens out.

Edited November 9, 2021 by PhilH

[Compound2632]

52 minutes ago, rodent279 said:

Dunno, not sure I can explain, but an octave is I think a bit like a decade-in a decade, the highest frequency is 10 times that of the lowest, in an octave it's 8 times.

Pitch, as I understand it, is another way of saying frequency.

 

If a note is pitched an octave above another note, then it has twice the frequency of the other note.

[rodent279]

Long time since I looked at that sort of thing! I just thought 8 due to the first 3 letters of the word.

[rodent279]

17 hours ago, DavidB-AU said:

There is some complex physics and mathematics behind this. The short version is because the connecting rod is (usually) angled, the mid point of piston travel and the top and bottom positions of the crank don't exactly correspond. As a result, valve events are very slightly offset from the theoretical intervals. What you hear as a volume difference is because valve timing is not uniform even when "properly" timed. Even on a 2 cylinder loco with perfectly quartered cranks, one exhaust beat per rotation will always be louder. This is less noticeable at low speed when there is a longer time between exhaust beats. This gets more complicated if any of the cylinders are inclined. W.A. Tuplin explained this in one of his books but the mathematics went way over my head.

 

It gets even more complicated with some 4 cylinder locos. The cranks are usually set at 90 degrees with the inside and outside cranks on each side at 180 degrees to each other, giving 4 beats per revolution. There were oddities like the Lord Nelsons which had the inside cranks at 90 degrees and the outside cranks at +/- 135 degrees giving 8 beats per revolution. It looks odd when drawn but gives one stroke every 45 degrees.

 

There is another effect causing a difference in volume because the forward portion of most cylinders contains slightly more steam than the rear portion, as the rear portion contains a piston rod. When the forward portion is compressed and discharges. the exhaust beat will be slightly louder. You don't get this effect on double guided pistons with rods protruding from the front of the cylinder. On a 2 cylinder loco you quite often hear 2 slightly louder beats followed by 2 slightly quieter ones. On a 3 cylinder loco you often hear 3 slightly lounder beats then 3 slightly quieter ones. EDIT: I can hear this in parts of the above video.

 

EDIT2: If you isolate the audio track from the video (done here with Audacity) it is indeed 3 and 3. So 3 consecutive louder beats per revolution rather than one

 

 

Cheers

David

Given a knowledge of the mechanical particulars of the locomotive, it ought to be possible to work out how fast it was travelling, and whether it was accelerating or decelerating.

[Compound2632]

This discussion gets to the heart of the appeal of the steam railway: the creative tension between the Dionysian thrill of a locomotive at speed and the Apollonian rationality of the complex engineering and physics that go into producing that thrill; the analogy with the aesthetic appeal of music is very close.

Edited November 9, 2021 by Compound2632

[rodent279]

3 minutes ago, Compound2632 said:

This discussion gets to the heart of the appeal of the steam railway: the creative tension between the Dionysian thrill of a locomotive at speed and the Apollonian rationality of the complex engineering and physics that go into producing that thrill; the analogy with the aesthetic appeal of music is very close.

 

Wot 'e said ^^^^

 

[rodent279]

Symphony in Steam indeed.

 

Diesels have similar qualities, though not in quite the same way. Listening to 40106 the other week at the SVR,  there's definitely music in that EE sound. The tuneful whistle, the rhythmic percussion of the exhaust. It's the EE 16 piece orchestra.

[jim.snowdon]

8 hours ago, PenrithBeacon said:

It did all work out on Halford's drawing board, but he, like a good many other draughtsmen, forgot about working clearances, tolerances and wear on all those pin joints.

There is an issue of the ProcInstLocoEng of about 1920 where all the possibilities of derived valve events are laid out. Ok on the paper but would have been weird in practice. 

When Gresley's team designed the first examples of the Holcroft 2:1 conjugated gear for the Great Northern there was a certain degree of assumption as to how the various levers and their mountings would flex at higher speeds. What they then found out from experience was that they had underestimated these factors, with the result that the middle valve spindles were hitting the ends of the valve chest. Various parts of the gear had to be stiffened and, from what I can recall, limitations placed on the maximum cut off for the valve gear. Play in the various pin joints didn't help either, as the design is such that the play in each joint is cumulative as far as the middle valve movement is concerned, resulting in further valve overtravel. The ouzzle is why Gresley stuck, almost religiously, to three cylinder engines with conjugated gear for the middle cylinder instead of adopting, as Peppercorn finally did, independent gear for each cylinder. Was the saving by not having a middle valve gear really worth the effort?

 

[Compound2632]

18 minutes ago, jim.snowdon said:

The ouzzle is why Gresley stuck, almost religiously, to three cylinder engines with conjugated gear for the middle cylinder 

 

From the common root of the words "conjugated" and "conjugal", I'd say he was wedded to it. 

[Michael Hodgson]

10 hours ago, Nearholmer said:

This seems like a good time to ask about this musical stuff: I’ve never understood what musical people mean when giving a timing as Andy did at the outset, 6/8, or any of the other timings so expressed.

 

Can any one explain?

 

When I was at school 6/8 was a third of a quid!

[Steamport Southport]

11 hours ago, Nearholmer said:

This seems like a good time to ask about this musical stuff: I’ve never understood what musical people mean when giving a timing as Andy did at the outset, 6/8, or any of the other timings so expressed.

 

Can any one explain?

 

Another musical thing that mystifies me is octaves and pitch etc. I think I might understand it, in fact I’m pretty sure I would, if someone could kindly explain it in terms of frequency (or wavelength, but I really prefer frequency).

 

Apologies if this is a diversion.

 

 

 

 

If you take "normal" music to be 4/4 time. Such as 1,2,3,4  / 1,2,3,4, etc

 

That's four beats per bar.

 

3/4 is three beats per bar, so 1,2,3 / 1,2,3, etc

 

6/8 is 3/4 but quicker....

 

in regards to music it's a waltz or a polka. Play with the sounds on here.

 

https://en.wikipedia.org/wiki/Time_signature

 

Also used for loads of famous rock, pop and other songs. 

 

 

Jason

Edited November 10, 2021 by Steamport Southport

[peach james]

Why to stick with 2:1 vs a 3rd set of valve gear?  Not sure...

There are some interesting assumptions here- I'd always thought that you didn't want 180% apart con rods,because effectively at top and bottom DC's, you have no angle to start the wheels moving.  

Setting valve gear- the "best" way is to do it with indicators.  But, as railway engines are hard to indicate, it becomes an issue.  Not like on a ship, or stationary plant, where you can adjust the valve settings to allow for the difference in cylinder area to equalize the work done top and bottom of the stroke.

Because there is limited ability to adjust the valve gear on a railway engine, it becomes a set of compromise and as the engine wears out, it becomes more problematic.  That's why some engines of a class would be a dog, and some a racehorse- because the valve event changes would affect performance that much...

Basically, you take your best guess on the final setpoints of the engine.  It's the same sort of thing on a traction engine, where the boiler expansion of 1/16" or more can be quite problematic.  We're in an area where thou matter, and it all moves by more than 0.032"...

Oh, and they WILL run with horrible valve events.  This engine- 

 (it's 2 cylinder...) had at least .100" of valve error (the valves were too long, piston valves), but it did run to some way or another.  Scale it up, and the valve error would be 1.6" or more !

James

[LNER4479]

3 hours ago, Steamport Southport said:

 

3/4 is three beats per bar, so 1,2,3 / 1,2,3, etc

 

6/8 is 3/4 but quicker....

 

Twice as quick, in fact ...

 

From the dim n distant days of O level music*, I recall 6/8 time being described as 1,2,3 / 4,5,6. It has much more of a rhythmical lilt to it and better mimics that lovely 3-cyl sound than strict 1,2,3 waltz time.

 

The poor ol' V2 at Steele Road is so off-beat that it's more of a 1...2..3/456, 1...2...3/456. Glorious stuff ... but only if you're a railway enthusiast (as opposed to a running shed fitter)

 

(*nerd alert. The base unit of '4' time is a crotchet and four of them make a semibreve (whole note). In '8' time, it's a quaver - half a crochet - so eight of them make a semibreve

https://en.m.wikipedia.org/wiki/Whole_note)

Edited November 10, 2021 by LNER4479