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).
Engine valve particulars and their effect on performance across different models.
| Identifier | ExFiles\Box 31\2\ Scan091 | |
| Date | 18th May 1933 | |
| Y1097. To E.{Mr Elliott - Chief Engineer} from Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} c. to Rg.{Mr Rowledge} Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} 2/KT.18.5.33. VALVE PARTICULARS. We have compiled the attached table with a view to comparing the speeds and outputs we should expect from a knowledge of valve area and camshaft durations with the actual test bed results. It will be seen for instance that the optimum speed of the car engines is proportionate to the area past the valve per litre. By comparing geometrically similar engines at the same M.E.P. and the same area past the valve per litre and therefore the same piston speed, we should expect the power output to vary as the square of the linear scaling and the optimum speed inversely as the linear scaling. The table therefore also contains values of BHP/Litres², etc. which should be constant for all engines, were they geometrically similar. The advantage of a short stroke is immediately apparent, P.II. and the Kestrel being above the average, and J.2. well below, although J.2. is better valved than J.3. In the car engines the speed of the engine varies almost as the valve area per litre, the figure being smaller the larger the engine. The Kestrel, however, has the same valve area per litre as J.1. Although it does not run at the expected 4200 r.p.m. judging by the area past the valve or the gas velocity, the output per litre instead of being low, because of the cylinder size, is surprisingly high, due we think (1) to the large valves and (2) the small stroke/bore ratio. In the matter of valve size car engines are restricted by considerations of slow running and low speed M.E.P. The figure of 2.7 valve area per litre for Peregrine has necessitated a very short duration camshaft to get good slow running. The small valve Peregrine was able to run with a longer duration camshaft. The valving and camshaft durations of car engines cannot therefore be chosen for maximum power. The larger the cylinder size, (and hence smaller valve area per litre), the lower duration camshaft can be used without sacrificing slow running. Possibly therefore the solution of the difficulty of a good power output is the use of two inlet valves per | ||