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).
The investigation of piston knocks.
| Identifier | WestWitteringFiles\N\2October1925-December1925\ Scan1 | |
| Date | 12th June 1925 | |
| R.R. 498A (50 H) (D.D. 31, 12-6-25) J.H.L. Expl. No. REF: Hs{Lord Ernest Hives - Chair}/ACL/LGS.10.25. EXPERIMENTAL REPORT. PISTON KNOCKS. We have been troubled with what appeared to be piston knocks. The symptoms however are not the same as ordinary piston slap caused by loose pistons inasmuch as it increases with the temperature of the water jackets and is less evident when cold than when hot. After various tests which eliminated other factors such as loose gudgeon pins etc. we were able to say that the seat of the trouble was the piston itself. We found the general characteristic of the trouble to be as follows :- The knock could not be immediately cut out by shorting the ignition of the particular cylinders where it occurred but would gradually decrease which rather seemed to indicate that the decrease was due to the cooling down of the piston head. Again, the knock would usually decrease as the jacket water was cooled down and was least evident of all when starting up from cold. From these observations it seemed to indicate that the trouble was due to tight pistons which was further supported by an experiment in which we drove an engine, with the cyl. head removed, by means of a belt. Of course for this test we had neither compression nor explosion to give any side thrust, the only side thrust being due to the inertia - which was very small at the low speed at which we run - and friction! During this test we found that a tight piston invariably would invariably knock. The cyl. block was heated to approx. 90°C by steam. contd :- | ||