Alan Parkin has now produced several manufacturing drawings for the approximately 140 individual components which are required to fabricate the cylinder block. At the same time David Elliott is building up the scale 3D printed model to determine the best construction sequence to ensure that all seams can be fully welded. The intention was to issue a “request for expressions of interest” to a number of fabricators during December. The target is for an almost full set of manufacturing drawings at the end of January to enable shortlisted companies to quote for the whole job.
The latest cylinder CADs and the assembled scale model of the cylinder block. – David Elliott
In parallel with discussions are at an advanced stage with a specialist company in CFD (computational fluid dynamics) to model the steam flow through the cylinder block to ensure that we do not have avoidable pressure reducing features to enable us to optimize the detail of the design – there is little scope for significant redesign of the block as the steam and exhaust passages are already as large as practical within the tight limits of the envelope of the block, however some gains may be possible through increasing radii on the inner corner of bends etc.
It is expected that this work will lead on to a more extensive model to analyse steam flow from the regulator to the blast pipe which will assist in fine tuning the profiles of the cams and the diameter of the blast pipe tops.
A further use of CFD will be look at smoke lifting. The original P2 No. 2001 Cock o’ the North did not have smoke lifting issues as even at low powers, as students of the history of the P2s will be aware, but the second engine, No. 2002 Earl Marischal (as first built with Walschaerts valve gear and the same smoke box and smoke lifting plates as No. 2001), immediately suffered from drifting smoke affecting the driver’s view resulting in early fitment of large and rather ugly smoke deflectors. The main difference is that piston valves open and close the exhaust valves gradually, with very little cross section for gas flow at the start and end of each event resulting in the exhaust appearing to “leak” from the chimney encouraging it to stick to the boiler top, which with the cross winds and the Coanda effect draws the exhaust down on the leeward side of the boiler. By contrast, poppet valves open and close to full area quickly resulting in sharper edged exhaust beats. Whilst not a problem on No. 2001, with all the computer modelling to optimize efficiency and power, there is a risk that the “squareness” of 2007’s exhaust beats may end up less sharp. If a smoke lifting problem is identified it is anticipated that with 80+ years of aerodynamic development since No. 2002 appeared, some relatively unobtrusive aerodynamic fences or strakes could be applied to solve the problem.
A short-term lack of Solidworks drafting capability has been identified and steps are in hand to recruit addition resources to cover this to expedite production of the detailed drawings for the valve gear. Low risk items such as reversing gear, return crank gear boxes and cardan shafts, and casting and machining valve covers can then be put into manufacture. In parallel it is intended to prototype a cam/follower/tappet/valve assembly and subject it to some fatigue testing before committing to a full set.
3D CAD view of poppet exhaust valve. – David Elliott
Cylinder drain cocks – Considerable progress has been made (mainly by Ian Matthews) in installing Alan Parkin’s cylinder drain cock design onto the frames. The design is closely based on that on Tornado to eliminate the use of Bowden cables (as used on the original P2s).
Cylinder drain cock actuating linkages. – Ian Matthews