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 the effect of connecting rod length on Bentley engine performance, efficiency, and vibration.
| Identifier | ExFiles\Box 85\3\ scan0075 | |
| Date | 16th March 1934 | |
| x214 E.{Mr Elliott - Chief Engineer} Hs.{Lord Ernest Hives - Chair} Lm. Rg{Mr Rowledge}/Tsn.5/DM.{D. Munro}16.3.34. Length of Connecting Rods - Bentley. Attached are the results of the investigation you asked for into the effect of connecting rod length. The effect of the reduced side-thrust on the mechanical efficiency at high speeds should be quite appreciable. Taking this at 69.5% at 4250 R.P.M. as given in Hs{Lord Ernest Hives - Chair}/Mths.{Reg Matthews}1/KW.13.2.34, then the friction HP is 51.4, and of this 36 HP is accounted for by the pistons. I estimate that with 9" rods we might expect to gain 5½ HP and with 10" ones 9 HP at this speed. Ten inch rods give a 25% reduction in piston side thrust, and it is possible that with the reduced tendency to piston knocks we might be able to use larger clearances and get a little more power this way as well. The next most important effect seems to be in reducing torsional vibration periods. The figures given are for the inertia part of these only. Unfortunately, the low periods that trouble us on the 6-cylinder and the top period of a straight-eight are caused by harmonics of the gas torque curve for each cylinder not of the inertia torque curve. It is probable that rod length would affect these also but it is not possible to say how much. The whipping stress in rods only varies as the length. Although the bending moment varies as the square, this is offset by the angular acceleration which varies inversely as the length. The tendency to whipping vibrations may be important however. I suggest we should build up a Bentley unit with 10" rods, using Phantom forgings, in order to get information for future designs. Rg{Mr Rowledge}/Tsn. | ||