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).
Comparison of pedestal piston efficiency against a standard piston and an analysis of piston knock characteristics.
| Identifier | ExFiles\Box 48\1\ Scan177 | |
| Date | 4th April 1926 guessed | |
| contd :- -3- the average pedestal piston would give a slightly better mechan- ical efficiency than standard. At present we have only one comparison of the mechanical efficiency obtained with the standard and new pistons, and apart from the fact that too much importance must not be attached to one set of readings, the results were obtained on two different engines where we cannot guarantee that the various other losses were identical. The tests shewed the friction losses on the standard engine amounted to 26.75 HP. at 2000 r.p.m. and with pedestal pistons they were 23.5 HP. at the same speed. The resulting mechan- anical efficiencies were 80 and 81.5% respectively. Naturally, a slight reduction in the friction losses, although amounting to little gain at full load, is more evident with regard to efficiency on small throttle openings, such as when the car is coasting along at about 20 to 30 m.p.h. PISTON KNOCKS. The characteristics of the particular knock with which we are troubled and experiments conducted, are as follows :- The knock does not sound the same as that given by a 'slack' piston and is not evident when starting from cold until the engine has 'warmed up'. It will not disappear when the ignition of the particular cylinder giving trouble, is 'shorted'. The noise seems to emanate from the skirt of the cyl. and is nearly always more evident on No.5. cyl. than any other. By listening to each skirt with a sounding rod - where holes have been cut in the crankcase to permit this - the noise appears contd :- | ||