GWR four-cylinder arrangement?

Industrial and marine steam engines were almost universally compounds (or strictly speaking triple expansion engines), although of course they generally had condensers. The SS Titanic and her sisters had triple expansion engines driving the outer screws, with the exhaust from these combining to feed a turbine which powered the middle screw. These were said to have advantages over other contemporary vessels.
Going back to locomotives, I've always been intrigued by the idea of having a four cylinder compound locomotive with a Lord Nelson 45 degree crank angle arrangement. Assuming early exhaust release has advantages, and the LP cut-off is less than 50%, the "chuff" from the HP engine should hit the LP piston when it is more or less at the dead centre position. I'm thinking that this would give more power, but I don't know whether anyone ever built one.

 

Mallets are best suited to running at 15 to 25mph, i.e. for slogging up steep grades. At higher speeds power declines because of the unpredictable filling of the receiver. As such, Mallets would have been unsuitable for UP.

 

The higher level of training in France was linked to compounding; most French compounds had two reversers and two regulators! Even Chapelon's compounds were built this way until he worked out the optimum arrangement for each design. Compounding was usually reserved for tender passenger locomotives and some mixed traffic classes. Most French locomotives were not compounds!

 

But it wasn't a fair comparison. Churchward's locomotives had large, long travel valves; the French Atlantics had narrow steam passages and slide valves for the LP cylinders.

 

No, British engineers weren't stupid but at the end of the 19th century, not surprisingly, they lacked sufficient knowledge of thermodynamics. Even Gresley made the mistake of placing the steam inlet in the P2 cylinder block too close to the exhaust, with the result that the former heated the latter! Chapelon's work on the 160A1 with steam jacketing of the cylinders (the steam being used to pre-heat the cylinder before use in the cylinder) showed that superheating for the HP stage was virtually superfluous. This would have greatly simplified the construction of compounds and their reliability, since the HP cylinders would not have been subjected to the high temperatures that broke down cylinder lubricants.

Compared with other locomotives, where fuel consumption increases once speed falls below about 20mph, the consumption per horsepower at the tender drawbar of 160A1 continued to fall with decrease in speed.

 

Yes, the Virginian 2-10-10-2 Mallets had 48" diameter low pressure cylinders, and it is practically impossible to fit large enough valves; even in America the loading gauge was an issue. In addition for any large Mallet the front low pressure unit had little intrinsic mass and was reported to ride badly at any sort of speed because of the heavy pistons thrashing around.

 

LP cylinder diameter could be reduced by increasing boiler pressure. The Y6 had 39" dia. cylinders and 300psi boilers. They could run at 50mph downhill (not uphill though!) without any problem - blistering speed for a Mallet.

 

Chapelon proposed a three cylinder low pressure unit which would have been helpful in a number of ways. I don't know if it was ever drawn up. The Y6s had very little weight on the two carrying axles unlike some other designs which had too much weight on undriven axles. For a heavy freight locomotive adhesion and tractive effort are pretty well everything.

 

Chapelon's alternative solution was to provide six cylinders for 160A1, 2 x HP and 4 x LP, all of the same size.

 

Chapelon moved on from the six cylinder designs. In his paper read at the Sorbonne in 1933 he looked at the technological developments which had allowed a Pacific built in 1909 to be rebuilt some twenty years later with its power increased from 2,000 ihp to in excess of 3,000 ihp and at the subsequent work from 1932 which allowed the creation of a 4-8-0 type conversion of the Pacifics which could deliver 4,000ihp. He then looked to the future and what further developments might be possible. The de Glehn system located the low pressure cylinders between the frames (so did du Bousquet but he also experimented with the low pressure cylinders being so large that the piston rod of the front cylinder ran down the outside of the cylinder set behind it - see his Baltics). To achieve larger low pressure cylinders Chapelon looked at the PLM and their Mountain and Mikado types which had the low pressure cylinder outside the frames and th H.P. inside. He was predicting maximum power outputs of between 4,000 and 5,000ihp however there was the issue of the crank axle which had limitations with regard to crank web thickness and axlebox journal width. Chapelon proposed 7 locomotive types in 1936/7:

A Heavy Express 4-8-4 or 4-6-4 rated at 160 km/hr.

A High Speed 4-6-0 rated at 200 km/hr

Heavy Freight would be either a 2-10-4 or a 2-12-0 with maximum speeds of 110 and 90 km/hr respectively.

A Heavy Suburban 2-10-2 tank locomotive designed for 130 km/hr running.

A Passenger 4-6-2 tank for secondary line services again designes for 130 km/hr running.

The high power types were designed to sustain 6,000 ihp and used six cylinder drive.


The creation of 242A1 proved the concept of the high power three cylinder compound and 6,000 ihp locomotives of this type were to meet future traffic requirements. The two throw crank flexed too much, it limited power output. 242A1 could deliver up to 2,500 hp from its inside cylinder. It was a proof of concept.

 

Not quite true. 160A1 was built in 1940 and, as far as I am aware, was Chapelon's only six-cylinder locomotive actually built.

 

160A1 was an experimental locomotive. It was designed to address the problems of efficiency and power output at low speeds. It gave good results but was built in 1940. Some 3 years after the proposed series of 7 types. The crank axle issue was known about at the time, hence the use of 6 cylinders, but the advantage of the single throw crank was recognised. The question was how well would it work in practice with locomotives capable of 6,000 ihp. 242A1 came into service in 1946, proved the system and the need for the double throw crank vanished. And we came very close to having the 152P.

 

How do you propose to fire a 6,000hp steam loco? That’s well beyond the capability of a single fireman, so you fall back presumably on a mechanical stoker - which rather throws away the enhanced cylinder efficiency by creating worse boiler efficiency.

Tom

 

Probably one of the reasons that Chapelon's work was abandoned by the senior management and the French railways adopted electrification, presumably paid for by the US of A.

 

I think you would have to go to oil firing, but that would have financial issues for France in the forties. On the other hand just double head!

 

It has a negligable effect, see the 2-4-0P. Rather like abandoning the first stage superheater and relying on the inter stage. Which makes sense when you consider how you are supposed to drive and work a compound.

 

Sorry, I don’t understand that? Are you saying that fitting a mechanical stoker has neglible impact on efficiency? Because my understanding was that typically you’d burn about 25% more coal for the same output. Which rather screws your overall efficiency.

Tom

 

SAR Class 26 was stoker fitted

The real issue with boilers is the unburnt fuel loss so sort the drafting and you are well on the way to an efficient loco

 

6000 ihp is power of 3 LNER V2 working flat out.

The Chapelon compound 152 goods engine was planned to pull fitted freigth at max 105 km/h.

700 american built 141 R,ca 300 141P and 25 240 in France had stokers.

Stokers lowering boiler effiency ,is even more reason to seek highest possible cylinder efficiency?

 

I would say that the unburnt fuel loss was more due the amount of fines that were generated by the screw feed system, which simply passed straight through the tubes and out of the exhaust without ever properly combusting.

Take a look at the exhaust on any stoker fired loco working at its full output, invariably very dark.

For this system to work efficiently I think you’d need to design a totally different firebox more suited to a pulverised coal feed, which in turn would be easier to fire mechanically than lump coal.