Two Cylinders, Three Cylinders or Four Cylinders - Design Considerations

[D854_Tiger]

Some of the most powerful of British steam locomotives seem to come in all flavours of cylinder configuration.

 

Suggesting that clearly no one way was the right way.

 

What were the design considerations in choosing one over the other and what were the compromises that resulted with each choice.

 

Then what was the preferred phasing of the power strokes as I believe not all were the same.

[Killian keane]

Two cylinder engines are invariably offset from one anither by 90 degrees, heft leading or right leading is fairly arbitrary and it differs from railway to railway, a four cylinder would also be offset by 90 degrees, really the tractive effort of an engine is not only defined by the number of cylinders, but also the size of the cylinders, number of driving wheels and boiler pressure

[New Haven Neil]

In the UK, the issue is the restricted loading gauge.  Two cylinders would be too big in diameter in more powerful locos (class 8+) to fit in the platforms etc.  That's why the Duke had 3, and Brits 2 cylinders, and the other class 8P locos 3 or 4.

 

It was thought at the time of steam that a smooth turning torque from multiple cylinders gave better traction - this isn't true, but it took many years to realise about hysteresis curves etc.  Modern traction systems work by allowing 'micro slips' and a recovery period between them, to simplify it.

[Bernard Lamb]

It was thought at the time of steam that a smooth turning torque from multiple cylinders gave better traction - this isn't true, but it took many years to realise about hysteresis curves etc.  Modern traction systems work by allowing 'micro slips' and a recovery period between them, to simplify it.

I don't think that the full implications were discovered until the introduction of electronic testing equipment in the early 1980s. It came as a shock to me when a sample of steel I was testing displayed such a loop. I believe it caused a few problems for the people working on Concorde at that time, but we better not go there.

Bernard

[LMS2968]

There was also a weight issue: multiple cylinders weighed more than a comparative two cylinder layout. On the other hand, multiple cylinders largely balanced the reciprocating masses, reducing - often substantially - hammerblow. If the Chief Civil Engineer appreciated this, it might be possible to provide a higher static axle loading, usually allowing a bigger boiler. Multiple cylinders engines were generally more expensive to build and to maintain - more moving parts to make and wear. But they also (usually) gave a smoother pull since the power strokes from three or four small cylinders were more evenly distributed than the heavy strokes from two big ones. This is one reason why GWR men, Canton excepted, never took to the Britannias. Comparing the ride of the four-cylinder Castle and the two-cylinder Brits showed this up. But two cylinders, provided they were mounted outside the frames, along with the valve gear, made preparation much easier and quicker. Hence in the USA multiple cylinders were largely confined to the Malletts; they were rarely used on rigid framed locos.

[Guy Rixon]

More cylinders allow better balancing of the motion and less hammer-blow to the track. Hammer-blow increases significantly the load that a locomotive puts on the track and, from c. 1900 if not earlier, designers were keen to reduce it. Hence, Churchward designed the 4-series "Star" class for the fastest trains, even though most of his standard designs had two cylinders.

[melmerby]

About the largest cylinders you were ever going to get on a UK loco are 20" x 30".

With a boiler pressure of 250psi and 6' 2" wheels the TE is about 34460lbs (a good class 7MT) if you up the wheels to 6' 8" you are down to a TE of 31680lbs (about 7P)

If you could increase the cylinders to 21" diameter and use 280psi you could have an 8P but the CE might complain about the hammer blow!

 

Keith

[LMS2968]

About the largest cylinders you were ever going to get on a UK loco are 20" x 30".

With a boiler pressure of 250psi and 6' 2" wheels the TE is about 34460lbs (a good class 7MT) if you up the wheels to 6' 8" you are down to a TE of 31680lbs (about 7P)

If you could increase the cylinders to 21" diameter and use 280psi you could have an 8P but the CE might complain about the hammer blow!

 

Keith

...and the works complain about boiler maintenance costs. This was largely why Bulleid (MNs and light pacifics) and Hawksworth (Counties) reduced from 280 p.s.i to 250. Drivers generally ran with only part regulator anyway so the extra pressure wasn't being used, but the costs were still there.

[peach james]

...and the works complain about boiler maintenance costs. This was largely why Bulleid (MNs and light pacifics) and Hawksworth (Counties) reduced from 280 p.s.i to 250. Drivers generally ran with only part regulator anyway so the extra pressure wasn't being used, but the costs were still there.

 

& the drivers who didn't run regulator wide open, driving on the reverser, were wasting coal...

David Wardale had the notches ground out of The Red Devil in the rebuild process- leaving 2- fully open and fully closed.  Anywhere else, you had to hold the throttle, which is a good incentive to wind the reverser back and the throttle wide open.

 

On the ship. we had 7 nozzle throttles- 

HP Turbine Throttle Box 2 by Peach James, on Flickr

 

for similar reasons.

 

Now, the 2 vs 3 vs 4 debate:  a lot comes down to hammer blow, the other portion in the UK comes down to loading gauge.  As has been posted above, it is hard to get above 20" diameter on outside cylinders, and the same limit applies to inside cylinders.  So, in order to get 8P power, you are basically limited to 3+ cylinder designs based on starting TE.  (or smaller wheels...but the extreme with that is a 9F being used at 90+ MPH...)  So, as is usual, the outermost examples of power dense engines are not all that efficient, not of necessity all that effective from a $ prospective, but boy do they look good ! (thinking of a Coronation here...)

 

James

[jim.snowdon]

... But they also (usually) gave a smoother pull since the power strokes from three or four small cylinders were more evenly distributed than the heavy strokes from two big ones. This is one reason why GWR men, Canton excepted, never took to the Britannias. Comparing the ride of the four-cylinder Castle and the two-cylinder Brits showed this up. But two cylinders, provided they were mounted outside the frames, along with the valve gear, made preparation much easier and quicker. Hence in the USA multiple cylinders were largely confined to the Malletts; they were rarely used on rigid framed locos.

With rare exceptions, the cranks on four-cylinder engines were arranged at 90 degrees, thus the tractive effort has the same number of peaks for revolution as does a two cylinder engine. What is different is the net reciprocating mass, as in a four-cylinder engine each pair is set 180 degrees apart, so that the effects of the reciprocating masses balance each other out. A few engines, memorably the SR Lord Nelsons were set with the cranks at 135 degrees, which will produce a tractive effort profile that has lower peaks but eight instead of four per revolution. Like almost everything in engineering, any design solution is a compromise between all sorts of factors - two notable ones in this instance are the complications of crank axles vs the ease of access provided by putting everything on the outside. Both have costs associated with them, it is does not take much inagination to realise that a 2-cylinder engine will be cheaper.

 

Boiler maintenance costs are something of an old chestnut and it is intersting to look back at how many and how long some engineers clung to 180psi as the maximum pressure before it evntually became accepted that going to 250psi didn't really come with a proportionate increase in maintenance costs. The boilers cost more to start with, obvioulsy, and were heavier, but what can do just as much harm to a boiler is the thermal and mechanical cycling that it is subjected to in its working cycle. Working the locomotive with only partial regulator is not efficient, and once engineers had cottoned on to the use of generously proportioned long travel long lap valves, the preferred driving technique was to regulate the power output by using the valve gear and getting the best out of the expansion of the steam, not by throttling it at the regulator. Different with the older pre-Churchward generation of locomotives, where any attempt to notch up simply lead to wholly inadequate valve openings, leaving the only option open to the driver that of driving on the regulator and keeping the valve travel long.

 

True, in US practice it was rare to find either inside cylinders or 3-cylinder locomotives, although there were some rather well known 3-cylinder 4-12-2s by way of the 88-strong Union Pacific 9000 class. On the whole, the use of 4-cylinder locomotives in the US was the result of compounding, either as Mallets or the non-articulated Vauclain compounds; 4-cylinder simples grew out of the problems with the problems with constricted steam distribution and excessive reciprocating masses on the LP units of the Mallets, apart from the Pennsylvania's Duplex locomotives, where the primary object was to keep the crankpin forces within tolerable limits.

 

Jim

[t-b-g]

Power and "tractive effort" are two quite different things and are measured in different ways.

 

I once read a letter in a magazine as a response to the claim by the GWR that their King Class had the highest tractive effort of any express passenger loco and was therefore the most powerful.

 

The letter writer pointed out that if you took the coupling rods off the loco and replaced the Swindon boiler with one for a Sentinel shunter (higher pressure than the Swindon boiler fitted), you would increase the theoretical tractive effort but the resulting loco probably wouldn't be able to move itself.

[LMS2968]

The 'full regulator' is certainly the theoretically most efficient way of driving a steam loco, but unless the power output is allowed to rise beyond what's needed, the reverser has to be pulled back a long way. This, depending on the loco, can give very unwanted results, especially with an outside two-cylinder loco, such as a County. These include severe axlebox knock and surge. Generally, the driver, if he understood what was happening, which was not always the case, would find a happy medium between the regulator and reverser.

 

I don't have figures for boiler maintenance cost between similar boilers of different pressures, but presume the CMEs did, and made their judgements accordingly.

[melmerby]

Power and "tractive effort" are two quite different things and are measured in different ways.

 

I once read a letter in a magazine as a response to the claim by the GWR that their King Class had the highest tractive effort of any express passenger loco and was therefore the most powerful.

 

The letter writer pointed out that if you took the coupling rods off the loco and replaced the Swindon boiler with one for a Sentinel shunter (higher pressure than the Swindon boiler fitted), you would increase the theoretical tractive effort but the resulting loco probably wouldn't be able to move itself.

This is the same old chestnut rolled everytime TE is mentioned.

 

TE is a good indicator of what sort of power the loco could be expected to produce.

It's a bit like quoting the engine size in a car. You wouldn't put a 4.7litre V8 in a car and then feed it with a single 1¼" carburetor!

In the same way you wouldn't put 4 large cylinders on a loco motive and try to provide it with steam from a 2 pint kettle.

The CME's weren't such dunces that they would create such a ridiculous combination.

 

All things are taken into account and providing a boiler that can produce the steam output for the wanted performance is one of the parts of the equation.

Occasionally they got it wrong with a poor steam producing boiler throttling the loco's performance but generally they got it right.

 

Keith

[Poor Old Bruce]

Most four cylinder designs got away with only two sets of valve gear with the adjacent valves operated through rocking levers (the adjacent cranks being set at 180 degrees meaning that the pistons were going in opposite directions). Most three cylinder locos used separate valve gears for each cylinder although some (notably Gresley designs - there were others) used conjugated gear to drive the valves for the middle cylinder.

[LMS2968]

This is the same old chestnut rolled everytime TE is mentioned.

 

TE is a good indicator of what sort of power the loco could be expected to produce.

It's a bit like quoting the engine size in a car. You wouldn't put a 4.7litre V8 in a car and then feed it with a single 1¼" carburetor!

In the same way you wouldn't put 4 large cylinders on a loco motive and try to provide it with steam from a 2 pint kettle.

The CME's weren't such dunces that they would create such a ridiculous combination.

 

All things are taken into account and providing a boiler that can produce the steam output for the wanted performance is one of the parts of the equation.

Occasionally they got it wrong with a poor steam producing boiler throttling the loco's performance but generally they got it right.

 

Keith

No. In simple terms, power is the Tractive Effort being produced by the locomotive multiplied by the speed at which it is being produced. The TE produced is not constant. It is at its maximum when the loco is stationary and then declines exponentially as speed rises. From the above, TE x Power, it will be realised that at the maximum Tractive Effort, Power Output is zero. As speed rises, the power at first increases as the decline in TE is gradual, but as that decline increases a point is reached where the rise in speed cannot increase the power output, which then plateaus, and then begins an also exponential decline.

 

At best, Nominal Tractive Effort is an indication what a loco could get moving from rest. Hence many a shunter had a high TE but did not have, to use your words, a boiler that can produce the steam output for the wanted performance, so power output was low.

 

By your standards, the most powerful loco in Britain was Gresley's Garratt, and it has often been described as such. Now imagine the shedmaster at Kings Cross allocating it to the Down 'Elizabthan' in place of the regular A4...

[Titan]

 

The CME's weren't such dunces that they would create such a ridiculous combination.

 

 

 

At least one did:

 

 

Ok it had three (huge) cylinders rather than four, but guess what, it was not very powerful...

[D854_Tiger]

Thank you for all the replies.

 

I raised this question after watching the BBC4 program (shown again this week) on the last days of BR steam when they stated that the Standard designs were all two cylinder designs a decision made for ease and speed of maintenance but I suspected there must have been a down side to such a compromise.

[melmerby]

At least one did:

 

 

Ok it had three (huge) cylinders rather than four, but guess what, it was not very powerful...

Really?

 

It did precisely what the designer wanted, A quick accelerating loco to compete with electric traction.

Once it proved it could be done it was quickly forgotten & rebuilt into a 0-8-0 tender engine.

 

Keith

[Grovenor]

At least one did:

 

 

Ok it had three (huge) cylinders rather than four, but guess what, it was not very powerful...

I always understood that this loco did exactly what it was designed to do.

 

In the early 1900s, the Great Eastern Railway (GER) was threatened by the City & North Eastern Surburban Electric Railway - a scheme for an electric railway which would compete with the GER's lucrative commuter traffic. In order stop this proposal in Parliament, the GER's Management ordered James Holden to produce a locomotive that could compete with an electric railway. The locomotive had to accelerate a 300 ton train to 30mph in 30 seconds. The 0-10-0T No. 20 'Decapod' was Holden's response to this challenge. In tests, the Decapod appears to have met the acceleration goal, and the electric scheme was defeated.

Regards

[The Johnster]

There was also a weight issue: multiple cylinders weighed more than a comparative two cylinder layout. On the other hand, multiple cylinders largely balanced the reciprocating masses, reducing - often substantially - hammerblow. If the Chief Civil Engineer appreciated this, it might be possible to provide a higher static axle loading, usually allowing a bigger boiler. Multiple cylinders engines were generally more expensive to build and to maintain - more moving parts to make and wear. But they also (usually) gave a smoother pull since the power strokes from three or four small cylinders were more evenly distributed than the heavy strokes from two big ones. This is one reason why GWR men, Canton excepted, never took to the Britannias. Comparing the ride of the four-cylinder Castle and the two-cylinder Brits showed this up. But two cylinders, provided they were mounted outside the frames, along with the valve gear, made preparation much easier and quicker. Hence in the USA multiple cylinders were largely confined to the Malletts; they were rarely used on rigid framed locos.

 

A significant feature of the Canton liking for the Britannias, which even there were considered as good as, and not preferred to Castles, is that much of their work was on heavy 14 coach unassisted Paddington trains over a route restricted to 75mph until the other side of Stoke Gifford, and involved a continuous uphill stretch from the bottom of the Severn Tunnel to Badminton, about 20 miles.  An engine with 6'2" driving wheels and two big cylinders is a very good tool for this work, as it is on the North to West route to Shrewsbury.  The 4 cylinder Castles were considered superior for high speed running, but this was not considered to be the defining part of the performance on up South Wales-London trains; uphill slogging at about 40-50mph when the hammerblow and poor ride didn't matter too mucn was!  A Brit lurching about on a Bristol-Paddinton with 10 on at 90 odd would have been very unpleasant to ride on; no wonder the men preferred their Castles and Kings!

 

What Canton really wanted was Kings, the ideal combination of both abilities, and they eventually got them briefly in the early 60s, at which time they were happy to lose the Brits to the LMR .  It is not insignificant that Kings have the same diameter driving wheels as Britannias.  

 

This is slightly OT as a matter of the performance of the locos concerned rather than their design considerations.

[asmay2002]

It is not insignificant that Kings have the same diameter driving wheels as Britannias.

Britannia 6'-2"

King 6'-6"

[New Haven Neil]

Thank you for all the replies.

 

I raised this question after watching the BBC4 program (shown again this week) on the last days of BR steam when they stated that the Standard designs were all two cylinder designs a decision made for ease and speed of maintenance but I suspected there must have been a down side to such a compromise.

 

Except 71000 of course....had three last time I looked!

[Poor Old Bruce]

Except 71000 of course....had three last time I looked!

 

One out of 999. A true 'one off'.

[Titan]

Really?

 

It did precisely what the designer wanted, A quick accelerating loco to compete with electric traction.

Once it proved it could be done it was quickly forgotten & rebuilt into a 0-8-0 tender engine.

 

Keith

 

Yes really, The statement was not 'did it do what the designer wanted', It was 'The CME's weren't such dunces that they would create such a ridiculous combination', to which the answer 'really' is 'At least one did'.

 

Whilst it could do what was intended, i.e to accelerate a train to 30mph as fast as an electric, that was the only thing it could do! it had insufficient power to haul a train any distance at a reasonable speed, and when converted to an 0-8-0 goods engine was equally useless at that too, being withdrawn not long after. It was your 'ridiculous combination' statement I was responding to and no other issue.

 

It certainly proved that tractive effort  does not correlate to power!

[peach james]

The 'full regulator' is certainly the theoretically most efficient way of driving a steam loco, but unless the power output is allowed to rise beyond what's needed, the reverser has to be pulled back a long way. This, depending on the loco, can give very unwanted results, especially with an outside two-cylinder loco, such as a County. These include severe axlebox knock and surge. Generally, the driver, if he understood what was happening, which was not always the case, would find a happy medium between the regulator and reverser.

 

I don't have figures for boiler maintenance cost between similar boilers of different pressures, but presume the CMEs did, and made their judgements accordingly.

 

If the valve gear isn't designed right, then yes, it can give interesting effects.  If you take a look at Dockstader's valve gear program, then you can see what properly designed valve gear does, and how little wear limits there really are in it.

 

( http://jf2.com/bcwrr/Dockstader-Valve-Gear.html )

 

I have the info for LBSC's Britannia on the computer here- I'd have thought it would have been a .txt file, but it isn't.  

 

My brit has >1000 km of running, and I'm not known for hanging around the station platform.  Lots of wide open throttle, run on the reverser type running.  It will quite happily run at 7 turns back (of 13 forward-reverse), so the valve gear events must be fairly well right.

 

BritVime98 by Peach James, on Flickr

 

 

( note, the LBSC version in 3.5" most likely had unofficial help from BR in the design, as Riddles was a friend of LBSC)

 

Suffice to say, that there are many ways to make valve gear that is not quite right, and does something a bit interesting when you start changing it from it's setpoint.  On Traction Talk right now, there are comments about Sentinel waggons as to valve setting to .002" or better accuracy.  Dad said that (Sentinel tractor) 7529 had a badly worn camshaft when they owned it (59-62), and that it went much better in reverse than forward, which isn't normally the case.  That is with machine ground cams that are effectively non adjustable, like any other poppet valved engine of the time.  Now, I suppose, some wag would make it with VVIT.  Its not a simple nut to crack,as you need quite an amount of force to open & close the valves.  When you introduce more pins (*), which every valve gear has, then you are introducing ever more probability of there being creeping errors which affect valve timing.  That's not to say that variable valve timing isn't important, because it is far more important to efficient fast running than most other things which are designed into a steam engine.

 

(*) pins in this case meaning as per mechanics, a free to rotate joint)

 

I'd reccomend Ing. Porta's papers that Camden republished as a good jumping off point, along with "The Red Devil", as being a really good look at the (re) design process for steam.  The book is incredibly dense in information, as to how David Wardale came up with the plans for "The Red Devil" and the test data to back it up.  Just consider that it was a 3'6" gauge engine with a higher HP than any UK mainline engine...(mind, the cape loading gauge is about equal, it's the track gauge that is narrow...)

 

The trick with all of this is to design a valve gear that gives variable cut-offs that remain consistent, from at least 70% to under 7% cut off.  The majority of the running should be done at less than 15% cut off, or elsewise, you should be thinking seriously about compounding.  The mechanical complexities of the system need to be looked at as well, with fuel cost being one portion of the cost of ownership.  I think the best review of this was published very late in the day, and is referenced in "The Red Devil" very briefly, but it had to do with costing diesel's (first and 2nd generation) vs steam, and how fraudulent the whole process of dieselization was.

 

James Powell