Driving a compound locomotive

Afternoon all,

Since driving a 5" gauge model of one I've become slightly fascinated by the Midland Compounds. Until now I've always assumed they were driven much the same as any other loco, but from O S Nock's logs, the drivers rarely notched them up beyond 55% in the high pressure and 48% in the low pressure. Nock has the line "Cut-offs of 63% in the high pressure cylinder and 57% in the low would be equivalent to about 25% in a simple engine" but doesn't elaborate further.

I'm assuming this is because you didn't want an early cut off in the high pressure cylinders as you need the steam to still have lots of expansive power for use in the low pressure cylinders?

Does anyone have anything to explain this, or could point me towards sources from the UK or abroad?

Thanks in advance, Andy

 

Yes, I think that's basically correct. If you assume that the compound uses a similar amount of steam as a simple engine for the same power output, 55% of the volume of a single HP cylinder will be roughly the same as 25% of the volume of the cylinders in a simple expansion machine.
Obviously you could also argue that a compound is more efficient, so it would use a little less steam for the same power output, but there are a lot of other variables involved.

 

Here I will reveal my ignorance - do I understand correctly that compounds have separate valve gear controls for the high and low pressure cylinders?

Steve B

 

They can do, but in some examples I think they are linked.

 

I am pretty sure the HP and LP valve gears on the majority of Midland and LMS compounds linked up together. French compounds generally had separate reversing levers or wheels



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I've never been really sure how you start one. Obviously if you just open the regulator when stationary, steam will just go to the HP cylinder(s) which might not be enough to start the train. I heard of bypass valves and so on, but never really sure what the arrangements were.

 

There was some confusion in the UK with respect to the best way to drive compound expansion locomotives. Professor W. E. Dalby was considered to be one of the country's experts on the subject of locomotive valve gears and his book Valves and Valve Gear Mechanisms was published in 1906 and reprinted in 1918 and is available to this day courtesy of the Book Depository. He was advising people at during an interesting period in the development of valves and their gear but I have read that his work was a little to theoretical, but I cannot recall where. This was a period when valve travel was exercising minds but his advice lagged behind the intuition of some that he advised.

Chapelon worked out that long h.p. admission periods were necessary and performance on the rails proved him right. Gresley consulted Dalby w.r.t. the original Pacifics and then you have the "Hush Hush" which was never driven in accordance with Chapelon's ideas. Gresley would have been better following his own developing ideas which were not too far behind A.X.C.

On the subject of the three cylinder compound, 242A1 developed 1,920hp in the h.p. cylinder and 1,790hp in each of the low pressure cylinders, This was achieved with a 60% h.p. cutoff, a 40% l.p. cutoff,, a boiler pressure of 285 psi and a reciever pressure of 135 psi. If you are going to work in this fashion and are wanting to achieve good results you need to be sure that your steam circuit is fit for purpose. So this is the question; how good is the circuit on the 5" compound?
There again steam doesn't scale too well - look at the IMLEC results.

You can carry out much better development and testing on 15" and look at the Bure Valley development and testing. So far as I know no one has a 15" compound to work with but that shouldn't stop you enjoying what you have

 

Of interest, how much difference to all the above is there when considering the 2-cyl Von Borries compounding system? As applied on the B&NCR (5'-3" & 3'-0" gauge) the system used a 'simpling' valve to admit hp steam to the lp cylinder on starting but know precious little beyond that. Some of the NG machines survived into 1954, so (unless there were any still lurking on industrial lines) must've been the last 2-cyl compounds in these islands.

 

What the bypass system did was supply boiler steam to both sides of the HP piston(s), so putting the HP cylinder(s) into equilibrium and effectively out of use, and boiler pressure directly to the LP cylinders as in a simple engine, so the engine started as a two cylinder simple. Once moving, the valve was shut off, boiler steam provided to the HP cylinder(s) which then exhausted to the receiver / LP cylinders to assume compound working. In some systems, such as the Midland Compounds, there was a gradual progression from full simple to full compound depending on the regulator opening.

 

A lot of traction engines appear to be 2-cylinder compounds. So perhaps traction engine enthusiasts may be able to help answer questions on how to drive compounds?

 

There were various starting mechanisms, with the Midland compounds, early types had separate valve gear control with a reducing valve which fed steam at a suitable pressure to the receiver for starting. Later the separate valve control was done away with and the reducing valve was replaced with an extra valve included in the main regulator valve which allowed steam to pass directly to the receiver via a small bore pipe and it was arranged that this auxiliary valve should close when the regulator was half open. So the engines could be driven like a simple by means of paying careful attention to the regulator opening when starting.

 

Generally no different to a single, but with greater range to a tank of water.

....except when starting off if it won't "go", then give a sniff on the LP to get things moving rather than swing the lever, and when you're just about to stall out on a steep hill then gently bring in the after burners and hang on 'til you're over the top!

 

On all the compound traction engines that I have driven there is a spring loaded valve which admits steam from the H.P. to the L.P valve chest. On some engines the spring loaded valve is activated by pulling a handle (e.g. Marshall) others by pushing a button. If the crank has stopped on 'dead centre' most times it is enough to pull the simpling valve and immediately let go so that it turns the crank a quarter turn as the regulator is cracked open. The engine will then run in compound mode. Although frowned on (because of the uneven loading on the crank) it has been known for drivers to hold the simpling valve open (i.e.working as a 'double high') for longer periods to get the engine over a steep bit or out of a hole. Foden steam wagons had a different system whereby, IIRC, you pulled the regulator one way for normal compound working and the other way for 'double high'.
On the Austrian Zillertalbahn loco No3 is a two cylinder compound with one large dia. outside cylinder and a smaller one on the other side. It has a very different beat to a simple.
Ray.

 

I have been on the controls of a marine triple expansion engine.

As described above there is a simpling or Impulse valve that allows HP steam to be admitted to the IP or LP cylinder, arrangements vary. On the Duchess of Fife there was a 2 crank 4cyl engine with two HP cylinders working in tandem with the HB & LP cylinders and the HP's could be bypassed so the engine ran as a compound when high power output was required (ie racing a NBR or GSWR steamer)

Until the thing is well warmed through most of the work will be done by the High Pressure cylinder, in my case as we were only pootling round the City Docks in Bristol most of the time restarting could be a bit amusing involving swinging the expansion links with the reversing engine and a whiff of steam on the impulse.

 

Has anyone on here had experience of driving K1 on the WHR or at Statfold? How does the compounding work out on that?

Steve B

 

It rather depends upon degree - if it's a case of tackling an entire hill on the button then it's probably more appropriate to change gear, but if momentum is being lost as you reach the conclusion of the steepest part then bringing in extra for a short time seems perfectly reasonable - part of the art is not suddenly wanging it into double high by leaning on the button or swinging the lever wide open in one sudden go. With the Burrell tractor I have most road miles on you can gently just ease it and gain a bit more chuff to pull you over the top without usually needing it wide open.

Of course the only person's opinion that matters is the that of the engine's owner, 'cos as with everything - they're the one that foots any bills.
If the owner is happy driving that way, then the view of anyone else is totally irrelevant.

 

Aerolite in the NRM is a 2 cylinder Von Borries Compound

 

Observing a Goelsdorf 2-cylinder compound shunting in Austria, it was driven just like a simple. There was an additional port in the low pressure steam chest that was uncovered when in full gear and allowed high pressure steam into the steam chest and thus into the low pressure cylinder. When linked up, the additional port was covered up and the locomotive worked as full compound. No additional mechanism necessary.

 

The RPSI has a lot of experience of driving a compound in the shape of No.85. Perhaps the OP may wish to ask the question of them.

 

The book Compound Locomotives was published in 1994 by Atlantic Transport Publishers. It was written by J. T. van Riemsdijk who was Keeper of Mechanical Engineering at the British Science Museum and very much behind the setting up of the NRM in 1975.

It might not be perfect but it is rated highly and copies are available second hand from the usual sources, Abe Books and so on and unlike One Man's Locomotive by Vernon Smith are not expensive. It is has a full title of Compound Locomotives an International Survey which lets you know what you are getting yourself into.

This work answers a lot of questions that might just arise when folk start expanding their traction interests beyond simple expansion and, by extension, largely beyond these shores. I found it to be very useful and continue to do so but I am not trying to sell you something, I am merely offering a source of information and would like to share it with those who are sufficiently interested.