The P2s were designed to handle the express passenger traffic on the Edinburgh to Aberdeen main line, which is noted for its steep gradients and tight curves. Double-headed Pacifics were prohibited from the line, so the heaviest traffic was usually handled by double-heading two smaller engines. Gresley sought to overcome this need with a Mikado (2-8-2) design with four driving axles for greater adhesion compared to an A3 Pacific. Initial plans were drawn up in 1932, and consisted of a 2-8-2 Mikado design using an A3-type boiler and dual chimney. Whilst the design was still being finalised, two engines were authorised in February 1933. At the end of March, this initial order was reduced to one locomotive. The firebox was lengthened, and a double blastpipe was added. The Kylchap chimney/blastpipe arrangement was chosen, along with an ACFI feed water heater, and smoke deflectors reminiscent of the W1 No 10000 "Hush-Hush".
After the successful experiments with the Lentz rotary cam poppet valve gear on the D49s, Gresley decided to fit similar valve gear to the P2s and directed Associated Locomotive Equipment Ltd to design a set of Lentz valve gear for the P2s. The gear was powered by two camshafts above the cylinders and at right angles to the locomotive horizontal centre line. These camshafts were synchronised with the rotation of the driving axles. Admission and exhaust valves were controlled by six scroll cams mounted on these two camshafts.
Associated Locomotive Equipment Ltd also provided the Kylchap double blastpipe, which was designed to take different blastpipe tops, and taper blocks allowing different blastpipe cross-sectional areas to be tried.
Initial drawings were provided in July 1933. Further changes would be made through to March 1934. The smoke deflector plates were moved forward so that they extended far ahead of the smokebox. Wind tunnel tests led to the deflectors being made vertical. A V-shaped cab front was also added. During these changes, five more P2 locomotives were ordered, but with a choice of valve gear left open.
This first locomotive, No. 2001 Cock o' the North was completed in May 1934. Early indicator tests showed low final compression pressures. This was found to be due to excessive clearance spaces in the poppet valve layout, and was fixed by adding a new exhaust cam for the left-hand cylinder, and advancing the closure point of the exhaust valves from 65 percent to 50 percent of the stroke. The scroll cams and rollers experienced very heavy wear due to their contact occurring at a point. These were substituted with stepped cams with parallel surfaces which contacted along a line. In March 1935, a circulating system for the cambox oil was added. This was powered by small pumps and incorporated a small copper pipe radiator mounted on the frame.
In December 1934, No. 2001 accompanied by O.V.S. Bulleid travelled to the locomotive testing station at Vitry-sur-Seine near Paris for two months of testing. The ambitious test programme had mixed success due to problems maintaining boiler pressure, and overheating axleboxes and bearings. The latter appear to have been due to the relative lack of vibration on the test station, leading to occasional breaks in the lubrication films failing to seal immediately. Blastpipe trials at Vitry found the best combination to be a 6in diameter blastpipe top with No. 1 taper blocks fitted. The exhaust beats were found to be uneven, and unequal clearance volumes were suspected. These were reduced slightly by modifying the steam passages on its return to Doncaster, but this did not fix the problem. After further testing on its return, No. 2001 entered service in Scotland.
The ACFI feed water heater on No. 2001 gave a lot of problems, and was never fitted to the other P2s. The heater used exhaust steam to heat the water before it was injected into the boiler. To work properly, the locomotive had to run continuously with its regulator open. This was not possible on the Edinburgh to Aberdeen route which the P2 was designed for, and so the ACFI feed water heater was wasted.
The second locomotive, No. 2002 Earl Marischal was completed in October 1934. This was identical to No. 2001 Cock o' the North but with piston valves, Walschaerts/Gresley valve gear, and the ACFI feed water heater was removed. This also ran a series of trials before entering regular daily service in Scotland in June 1935. Due to the use of Walschaerts and Gresley valve gear rather than the poppet valve gear, No. 2002's draughting characteristics were different to those of No. 2001.
2001 Cock o' the North May 1934 September 1944
2002 Earl Marischal October 1934 June 1944
2003 Lord President June 1936 December 1944
2004 Mons Meg July 1936 November 1944
2005 Thane of Fife August 1936 January 1943
2006 Wolf of Badenoch September 1936 May 1944
29 авг 2015
