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 design, comparing aluminium and steel, and discussing clearances and materials.
Identifier | ExFiles\Box 65\1\ scan0035 | |
Date | 23th December 1924 | |
R.R. 493A (50B) (D.B. 175 25-9-24) J.H.D. -2- Expl. No. REF Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}/LG23.12.24. EXPERIMENTAL REPORT. The main point is that with this clearance, we have not had the two pistons running up together. Initially, we suggested an aluminium piston for the following reasons :- (a) Owing to its light construction it allows a sharp advance at the base of the ignition curve. (b) It allows a greater clearance when cold. Now that we find we can work with such large clearances however, the value of the greater expansion of the aluminium is reduced, as this expansion is only .001" greater than that of the steel piston of similar dimensions over the range of oil temperature encountered. We believe that if we do not aspire to obtain a perfect ignition advance curve under the extreme of heat and low oil pressure previously mentioned, we can still further reduce the diameter of the small piston with consequent increased immunity from sticking. We are carrying out tests to determine the limits to which we can go in this direction. We do not see that it should be necessary to go to hardened and ground piston and cylinder for these parts. In the carburetter air valve, we have a larger diameter aluminium piston working in a brass cylinder with practically no clearance and directly exposed to the grit which is sucked in through the extra air ports. If this works satisfactorily, we cannot see why similar materials should not be successful where we have very much greater clearances and unlimited lubrication. contd :- | ||