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 memo discussing engine compression ratios, piston materials, and detonations for the 40/50 chassis.
| Identifier | ExFiles\Box 35\2\ scan 053 | |
| Date | 18th February 1919 | |
| To R.{Sir Henry Royce} from EH. c. to CJ. c. to E.{Mr Elliott - Chief Engineer} c. to Ey. c. to Da.{Bernard Day - Chassis Design} EH1/LG18.2.19. X.2748 RE 40/50 CHASSIS X.790. X.2748. X.2768 X.682 Previous to the war we used to make cars with high and low compression. The capacity of the combustion chamber in the case of the high compression was 24½ cu.ins. in the case of the low compression it was 25½ cu.ins. The compression ratios were 4.06 to 1 and 3.89 to 1 respectively. It will be seen that the difference in compression was very slight. The point which governed the amount of compression to a large extent on the old cars fitted with cast iron pistons, was detonations. We suggest that now we are to use aluminium pistons we may have a wider range between the two compressions. 7 CA is now running with 22½ cu.ins combustion chamber or a compression ratio of 4.33 to 1. We consider that 7-CA is quite good for absence of detonations. We realise that the engine feels more vigorous but the idea of the higher compression was to use it on open touring cars where customers desired speed and power rather than soft running. Previously we made the difference in compression by means of a liner under the cylinder. This was satisfactory as long as the variation was only 1 cu.in. but if we think of using the same compression as on 7-CA for open cars, we shall have to do it by means of a piston. At present the new type pistons which we are running give a compression ratio of 4 to 1 or a capacity of 25 cu. ins. R.R. 235A (500) 9.17 R.M.(L'T'D.{John DeLooze - Company Secretary}) 2156/1S. | ||