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).
Investigation into piston knock, theorizing it is caused by resonance rather than oil film breakdown or piston deflection.
| Identifier | WestWitteringFiles\O\2April1926-June1926\ Scan007 | |
| Date | 1st October 1925 guessed | |
| contd :- -7- knock and in the light of our recent experiments it still seems a reasonable explanation. The reason stated was as follows :- "The piston ----- preserves a film of oil between it and the bore when in motion but when the direction is changed the film at any particular high spot may momentarily break down but is immediately restored when motion in the other direction starts. This breaking down of the film it seems probable could give rise to a knock and actually the increased resistance when reversing the direction is very noticeable". It therefore seems that the benefit derived from thicker section skirts is not due to the prevention of measurable deflection, but rather to the reduction of resonance. This theory is supported to some degree by the Silver Ghost in which precisely similar pistons are used without any indication of piston knock. The base of these cylinders in addition to being thicker than the Phantom - .250 compared with .175 - have the stiff holding down flange both of which tend to reduce the resonance factor. It is realised that curing the knock trouble by means of thicker skirts on the particular engine used for the experiments, as shewn in diagram 2, is not sufficient evidence to indicate that it will effect a cure in all cases. Therefore it has been deemed advisable, in order to fully test this method, to produce several sets for trial on cars suffering in a marked degree from this fault. contd :- | ||