6023 King Edward II Project

(Go directly to the GWR King Class if you wish to miss these technical notes on their evolution)

The use of 4 cylinder engines on the GWR came about after the CME (Chief Mechanical Engineer) George Jackson Churchward took over from William Dean in 1902. Churchward was a man of great vision and forward planning, and looked ahead to meeting the GWR's needs beyond his retirement in 1921. Churchward's mind was following several possible lines of improvement in steam locomotive performance;

1. Free steaming boilers - notably based on Belgian Inventor Alfred Belpaire's boiler with rectangular firebox. This was less prone to foaming and carry over of water to the cylinders, due to having a greater surface area for evaporation when compared with the narrowing upper space of a classic cylindrical boiler. Churchward improved the design with tapering of the boiler away from the point of highest steam production, applying mathematical principles to the flow of boiler water. The Churchward design of boiler also dispensed with the need for a large dome to collect steam, and Churchward's engines are notable for the brass 'bonnet' hiding the safety valves, and the top-feed of water supply from the injectors, in the approximate position where other boilers are weakened by a dome.
2. Long travel large piston valves - giving the minimum loss of pressure as steam passes to the cylinders. The travel and lap of Churchward's valves was 50% large than anything seen in the UK to that time, and Churchward engines can give the impression of the valves being a second slightly smaller set of pistons .
3. High boiler pressure - Churchward pushed boiler pressure on new GWR engines up from 200lb/psi to 225il/psi, having made an extensive study of the improvements in hauling power and coal consumption he could obtain this way.
4. Two engine frames, as opposed to the four frames prevalent on GWR express engines such as 'City of Truro'.
5. Concealed valve gear between the engine frames - reducing the need for a strip-down to get access to the connecting rods linking the wheels and pistons.
6. Compounding. A number of steam engine designs had applied the marine principle of re-using exhaust steam that still had considerable energy, in a second, lower-pressure set of cylinders. Churchward bought a French-built De Glehn compound engine No 102 'La France' in 1903 for comparative tests. This engine had two high pressure cylinders between the frames, with pistons linked to the front driving wheels, and two lower pressure cylinders visible on the outside of the frames and wheels, set further back and acting on the second set of wheels; 4 cylinders in all. The French compound had a high boiler pressure when compared with its test rivals, and only when tested alongside 200lb/psi two cylinder engines without compounding (simple engines) was a valid comparison made. Churchward found no efficiency advantage - but what he did find was that the smoother riding of the 4-cylinder engine encouraged economical use of steam by crews (who had tended to use long cut-offs to dampen rocking in two cylinder engines), and also gave huge scope for more power. The problems of loads on rods and axleboxes was also reduced by the French division of the drive across two sets of wheels. The concept of the GWR 4-cylinder engine was born.

The first of these new engine was No. 40, later named North Star, which emerged from Swindon works in 1906, in Atlantic 4-4-2 wheel configuration. It was as revolutionary to steam express locomotive building as Admiral Fisher's HMS Dreadnought was to battleship building in the same decade, and influenced all express steam engine designs in the UK after it. No. 40 had an obscure 'scissors' valve gear, hidden between the engine frames, designed by W.H.Pearce. The scissors gear had a technical drawback of hamstringing the whole engine if it failed, whereas other designs could work with one side out of action, in order to let the engine limp home. The scissors gear was not repeated, and when further engines were ordered, Pearce designed inside valve gear based on the invention of Belgian Egide Walschaerts (pronounced val-shirts). Walschaerts valve gear, which was a novelty in the UK, became as common in the steam age as the jet engine is today. Its main benefit is a derived motion from the piston crosshead via a combination lever that causes it to dwell at the end of its valve travel, giving greater ingress of steam.

The new engines of the 'Star' class, were all built with a 4-6-0 wheel arrangement, and No. 40, North Star was converted to this layout in 1909. 73 engines of this successful class were built, many lasting into the 1950s. One, Lode Star, remains as a static exhibit at the National railway Museum in York

In 1919, the need for greater performance, constrained by the loads the GWR express routes were capable of taking, led Churchward and his deputy Charles Collett (who took over as CME in 1922) to examine larger boilers for the 'Star' class - resulting in Collett's 'Castle Class' of 1923. The larger, fatter boiler, improved crew accommodation and outside steam pipes to the outer cylinders disguised what was basically a 'Super-Star'. It also allowed the GWR's publicity department to claim the most powerful express passenger locomotive in the UK on the basis of tractive effort. In 1924 and 1925 No 4073 Caerphilly Castle was positioned alongside LNER 'A1' 4472 Flying Scotsman at the Wembley Empire Exhibition. Dwarfed by 4472, the Castle's claim looked ridiculous. Nevertheless, the two railway companies put the engines to the test on the same routes, and the Castle out-performed the mighty Scotsman. New A1s were redesigned to incorporate GWR features such as longer travel valves and better boilers, but when they returned in 'A3' guise with these modifications, only narrowly improved on the Castles.

The efforts to maintain the lead in tractive effort were to take further turns - and the Star /Castle design would develop great one step further on the GWR......