From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical memorandum discussing engine design considerations for cylinder grouping, camshafts, and valve tappets.
| Identifier | ExFiles\Box 134\2\ scan0008 | |
| Date | 11th May 1935 | |
| -2- Hs{Lord Ernest Hives - Chair}/Smth.14/KW.11.5.35. The shortest engine is obtained by grouping the cylinders in pairs with a common combustion chamber wall. This arrangement is also the best for the provision of camshaft bearings, although by no means ideal in this respect. The necessarily big distance between the exhaust and inlet cams for one cylinder means that between two pairs of cylinders there is no gap in the camshaft big enough to take a good bearing. We should be able to get good bearings between the exhaust cams of paired cylinders, but these would be about 8" apart, with the shaft in between carrying inlet tappets of two cylinders. We should probably have to put a small bearing between each exhaust cam and its nearest inlet cam. It does not appear to be very practicable to provide a separate hydraulic adjuster to each inlet valve, so that if we are to use automatic tappets for the inlet valves, we must resort either to Halford tappets or to a Wilcox-Rich bottom tappet with a screw adjustment on one of each pair of valves. This latter system will give full adjustment to one valve only, but has the advantage that the hydraulic unit would have a lower leakage factor than the Halford type and the inertia of the valve gear would be less. We think that the bottom tappet, which would have to be of the roller type, including the hydraulic unit, would weigh 30-40% more than the Bentley tappet. For an engine speed of 5,500 r.p.m.; exhaust valve 1.500 dia. x .400 lift; inlet valves 1.150 dia. x .325 lift; rocker arm ratio 1.33, and using Wilcox-Rich type tappets, we estimate the following spring loadings would be required when the valves were fully opened. Exhaust Valve 64 lbs. Inlet Valve 54 lbs. Tappet 60 lbs. Tappet 37 lbs. This would give a tappet loading some 70% greater than the Bentley figure, and which might prove excessive. In this case the engine would have to be lengthened by 2 or 3 inches in order to accommodate a tappet and push rod for each inlet valve, and becomes considerably less attractive in consequence. If we do use 3 tappets per cylinder, the narrow combustion chamber which we have asked Wd.{Mr Wood/Mr Whitehead} to test will be indispensable. | ||