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.