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).
Piston failure due to high engine speeds and potential design improvements.
| Identifier | ExFiles\Box 66\1\ scan0018 | |
| Date | 16th March 1925 | |
| X. 8040 To E.{Mr Elliott - Chief Engineer} HS.{Lord Ernest Hives - Chair} } FROM R.{Sir Henry Royce} BY.{R.W. Bailey - Chief Engineer} S E C R E T A RL/M16.3.25 E.{Mr Elliott - Chief Engineer} A.{Mr Adams} C. PISTON FAILURE AND SCRAPER RINGS. This is quite a reasonable failure due to high engine speeds and explosions causing concentrated alternating stresses at the end of the horizontal saw cut. How can these stresses be reduced? My first idea was to drill a hole to finish the saw cut, then reinforce round the hole. Another was to use two ¼" fine thread bolts to reinforce the aluminium. These might bite into the piston pin at one or both ends. They could be used as a means of making the piston pin tight, but I should not recommend slotting (splitting) the boss. SEE ORIGINAL FOR SKETCH. Do we still use the diagonal webs? One thinks that we ought. My impression is that the aluminium is O.K. for compression due to the stresses caused by the explosions, but following these by the tension at high speeds at the end of the exhaust stroke is too much for the metal, and either the section or the metal is different in the die castings. It would seem that some definite improvement should be made and the engine should be fitted with suitable valve springs and extra high compression, and run at a speed of say 3,300 r.p.m. for a long time. This would prove or otherwise the theory of the failure, and shew how much difference it is necessary to make so that the pistons shall be free from such a failure. Higher speeds that are likely to occur on the car or additional weight of crown could be used to prove that we had a margin of safety, but we ought to reduce the stress at the beginning of the fracture at least 50%. E.{Mr Elliott - Chief Engineer} will let us know what is being tried, but certainly do something more than materials. In passing I would like to point out that my idea of all scraper rings has the defect of making the piston less gas-tight because the gas can run round the grooves. Perhaps the grooves should be milled in and stop short of the joint - SEE ORIGINAL FOR SKETCH. contd :- | ||