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).
Effects of piston inertia torque on engine roughness for various cylinder configurations.
| Identifier | Morton\M19\ img022 | |
| Date | 17th February 1932 | |
| ORIGINAL. To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} c. to Sg.{Arthur F. Sidgreaves - MD} c. to Br.{T. E. Bellringer - Repair Manager} c. to Ev.{Ivan Evernden - coachwork} Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls}1/MJ.17.2.32. x634 PISTON INERTIA TORQUE. EFFECT OF ENGINE TYPE. We have found graphically the magnitude and frequency of the principal harmonics of piston inertia torque for engines of 4, 6, 8 and 12 cylinders. We see that we have been right in assuming the 3rd. harmonic for a 6 cylinder engine to be the only one of importance. The magnitude of the torque is directly proportional to the mass of the reciprocating parts. We have previously suggested that, if inertia torque is a cause of engine roughness, we should not be able to detect the improvement (12%) due to connecting rods 1" longer, but we ought to be able to detect the gain in smoothness by using magnesium pistons after a trial of cast iron ones. This should effect an improvement on the 25 HP. in the ratio of 1:4. If we use a balanced crankshaft lighter pistons will give us a gain in smoothness due (1) to less inertia torque and (2) less inertia forces causing local distortion. If we use an unbalanced crankshaft the gain from less local distortion will be too small to detect, and any reduction in roughness of the nature of 1:4 will be due to less inertia torque. From these two experiments we hope to separate two possible causes of engine roughness. The engines of least inertia torque are the straight 8 (either 4-4 or 2-4-2) and the 12 cylinder 60°. From graphical methods we cannot say that these have no inertia torque as higher harmonics of very small amplitude are present in the 8 cyl. and may exist in the 60° 12 cyl. The other two 12 cyl. engines namely with banks at 45° and 90° have an inertia torque equal to and greater than the 6 cyl. engine respectively. In each case the 3rd. harmonic is the dominating one. Inertia torque, if a reason for engine roughness, may cause this roughness in two ways. (1) The oscillation of the engine as a whole due to the total of inertia torque. (2) The crankshaft is no longer balanced due to local torques in individual journals being of large amplitude and torsional distortion taking place. Crankshafts stiffer torsionally are the only method of preventing this. | ||