Four Cylinder Locomotives
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;
- 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.
- 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% larger 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 .
- High boiler pressure - Churchward pushed boiler pressure on new GWR engines up from 200psi to 225psi, having made an extensive study of the improvements in hauling power and coal consumption he could obtain this way.
- Two engine frames, as opposed to the four frames prevalent on GWR express engines such as 'City of Truro'.
- 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.
- 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 200psi 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...
The King class was the final and most powerful development of the GWR’s 4 cylinder designs. GWR CME (Chief Mechanical Engineer) Charles Collett’s Castles were arguably the GWR’s finest passenger engine design, and on a tractive effort basis, had proved themselves more powerful than LNER’s Flying Scotsman. Nevertheless, the advent of the Southern Railway’s King Lord Nelson class as the new leader in British express steam tractive effort (33,500lbs) pushed the GWR’s publicity office to push for one greater engine class, with a nominal tractive effort of 40,000lbs.
The apparent need for such a class existed on the heavily graded routes in South Devon, en route to Plymouth. However the ‘hammer blow’ of the weight of each axle plus the effect of reciprocating parts was expected to be in the region of 22½ tons, 3 tons in excess of the 19½ ton static design limit of most GWR bridges. Luckily, across the huge and unwieldy GWR organisation, the civil engineering division had been independently raising the bridge loadings to 22 tons for the previous 22 years, and a concession of 22½ tons was made for 4-cylinder engines. For 22½ ton axle loading, routes from London to Plymouth and Cardiff, on a handful of bridges would need strengthening. These routes would become the ‘double red’ routes of the GWR, and the Kings would rarely stray from them.
Collett opted for smaller wheels on the King than his Castles, after casting aside the conventional wisdom that large wheel diameter was needed for the greatest speed. He had observed an express train being overhauled by a mineral train hauled by a close-coupled GWR 4-8-0 with small wheels. When compared with the Castle class, the slightly smaller wheels adopted for the King Class allowed more space above them for a fatter boiler to be built, as little extra engine height was available for expansion.
Nevertheless, such a reduction in wheel size was not without its corresponding problems. A long standing Swindon design constraint imposed by former CME George Jackson Churchward, was that the pistons would be level and not slanted. This meant the centre height of the pistons would be the same as the driving wheel axles. The smaller wheels thereby lowered the height of the pistons. At the same time, to achieve the 40,000lb tractive effort, the pistons were enlarged to 16¼” bore and 28” stroke. The outer pair of pistons, acting on the centre wheels, needed to hang either side of the rear wheels of the front bogie. The hidden inner pair of pistons, acting on the front set of driving wheels, hung low between the front wheels of the bogie. Both factors meant a traditional springing arrangement for the bogie was impossible. A compromise design with the front wheels sprung on the outside and the back wheels of the bogie sprung on the inside, produced the distinctive and decidedly odd-looking long front bogie of the Kings.
Like on the Stars and Castles before them, the Walschaerts valve gear was concealed between the frames, and driven off the front set of driving wheels. The two inner valves, directly above the inner pistons, were driven directly from these sets of valve gear. The outer valves, directly above the outer pistons, took a reflected drive from the inner valve gear via a pair of rocker arms emerging from the frames above the front bogie. Like the Stars and Castles, no external valve gear was visible, but this aesthetically pleasing arrangement made for a maintenance nightmare, with over 120 oiling points, many hard to reach, before the engine could take to the road.
Draughting arrangements included a 'jumper' blastpipe ring (the blastpipe is inside the smokebox and directly under the chimney - it directs the steam exhaust straight up into the chimney and in doing so creates a strong sucking action on the fire in the firebox, drawing the hot gases through the tubes of the boiler.) A jumper ring was designed to allow the blastpipe to lift and expand under heavy working.
The first member of the class, 6000 King George V, left Swindon in 192-, and after running in was sent to represent the GWR at the 100th Anniversary of the Baltimore & Ohio Railroad. It returned with a memorial bell that it wears to this day. Subsequent class members were named after Kings of England in reverse order from King George V.
Trials proved Kings to be only marginally better than Castles on gentle gradients, but the Devon banks proved their mettle.
The principles of the King design did evolve further, but not on the GWR. Collett’s assistant William Stanier became CME of the London Midland and Scottish Railway, and improved the design by having four full sets of valve gear, and a longer boiler with a wide firebox over lowered rear frames supported by a rear bogie. These giant 4-6-2 Pacifics were his Queen Elizabeth Class, further improved in the shape of the magnificent Duchess class.
The Kings played an important part in the evolution of British Express steam locomotives, but their further development within the GWR was unnecessary as the need for a more powerful express engine was not justified. The long runs from London to Scotland provided the need for bigger engines with more speed and more stamina, but these routes were under the LNER & LMS.
All GWR engines were designed for Welsh Steam Coal, with its high carbon content and low proportion of volatile gases. Poor coal quality during World War 2 forced changes in the draughting arrangements soon after the war, and the removal of the jumper ring and replacement with a larger plain blastpipe was found to improve performance on the poorer coal available, which had a high amount of volatile matter in it.
After 1956 the Kings were fitted with a double blastpipe and narrower double chimney draughting arrangement. Four row superheater elements were fitted to give a greater level of superheat. Performance shot up, with power rising by 20% at speed, and an 8% improvement in coal and water consumption.
6023 is to be returned to 'Single chimney' condition, unlike its sister 6024, which retains the double chimney and blastpipe..