Whilst Alan Parkin has been developing manufacturing drawings for the cylinder block, David Elliott has resolved one of the more tricky design issues with the poppet valve gear. One of the unsatisfactory features of the original No. 2001 cylinder block was having the inlet and exhaust valves on opposite sides of the middle cylinder. As a result, separate ports were required from the cylinder to the valve chests which gave rise to excessive clearance volume. Clearance volume is the space in the cylinder and its associated ports that is left when the piston is at the end of its stroke and is usually expressed as a percentage of the total volume for the cylinder when the piston is at the other end of its stroke. This space is not good for efficiency as for each power stroke of the piston, the clearance volume has to be filled with steam which does little work on the piston. The best figures achieved with poppet valves are around 10%. The original P2 design middle cylinder had clearance volumes of around 14% at one end and 16% at the other – the difference being due to the slope of the cylinder in the block to enable the connecting rod to clear the leading coupled axle whilst the valves were almost horizontal to be in line with those on the outside cylinders.
No. 2001 cylinder section drawing showing the clearance volume – LNER/David Elliott
To overcome this problem, the layout of the valves has been changed to mimic the outside cylinders with the inlet and exhaust valves together and in line with the centerline of the cylinder. This does create a challenge, as whilst the exhaust valves can be driven directly from the Driver’s side cambox (as on No. 2001), the inlet valves are on the other side of the engine compared with the cam driving them. After several iterations a pair of rocking shafts have been designed to transfer the movement of the inlet valve cam tappets on the Fireman’s side of the engine across to the inside cylinder inlet valves.
No. 2007 cylinder section drawing showing the clearance volume – David Elliott
Cylinder block section showing rocking shaft drive to inside cylinder inlet valve – David Elliott
3D printing – David Elliott has recently acquired a 3D printer. The first work undertaken is to reproduce all the individual parts of the cylinder block at 1:8 scale to ensure that the proposed weld sequence will actually work with sufficient access to achieve all the welds. It will also help prospective fabricators to understand the design when quotes are sought.
1:8 scale 3D printed cylinder parts – David Elliott
Shortly after the printer was first set to work, we were sorting out patterns for the regulator stuffing box that is needed to test the new boilers in Meiningen and were reminded that the cover pattern was missing. The 3D model was rapidly modified to include machining allowances and add “draft” to the edges and a pattern printed. This took the printer about 56 hours altogether – fortunately, provided it does not run out of plastic filament or suffer loss of power, it happily proceeds on its own until finished!
3D printing of regulator stuffing box cover – David Elliott
The printed patterns for the stuffing box – David Elliott