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).
Inertia torque and local out-of-balance as causes of engine roughness.
| Identifier | WestWitteringFiles\V\December1930-February1931\ Scan187 | |
| Date | 27th January 1931 | |
| To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/ c. to SG.{Arthur F. Sidgreaves - MD} c. to Rg.{Mr Rowledge} c. to RY. /Tsm. Gry.{Shadwell Grylls}1/MJ.27.1.31. INERTIA TORQUE. In previous reports we have stated that roughness in a six cylinder engine must be due to inertia torque and that the lack of inertia torque in eight cylinder engines accounted for their remarkable smoothness on the over-run. At 3000 R.P.M. in the 25 HP. engine the third harmonic of inertia torque amounts to about 200 lbs. ft. Taking the moment of inertia of the engine as 800 lbs.ft.2 we should get a total movement of the engine at the engine feet of .0033". The speed of oscillation being 9000 per minute. From these figures we do not think that inertia torque is an important factor conducive to roughness. If we increase the China connecting rods 1" we should get a decrease in engine feet movement of .00045". Coupled with our 'Diamond' mounting of fore and aft suspension we do not think that the increase in smoothness due to 1" longer connecting rods would be noticed in a car. We have found published confirmation of EVC's statement that the magnitude of the inertia torque depends directly upon the ratio r/l and not on (r/l)^3. Local out of Balance. This we consider the main cause of engine roughness. Local out of balance is very large in six cylinder engines. The centre and end main bearings of C.7 unit after the last 10,000 miles test shewed very considerable wear. Balance weights would of course greatly reduce this. To get a smooth engine we must reduce local distortion. In view of the fact that crankshaft stiffness is almost negligible compared to crankcase stiffness we expect the 25 HP. engine to run more smoothly with an increase in crankcase rigidity. It appears necessary to rely to a great extent on the main bearings holding the crankshaft to get a smooth engine; and Nitralloy balanced crankshafts should help us to reduce bearing wear enormously. All the eight cylinder engines we know of have cast iron crankcases, which may be the cause of their smoothness. On the other hand, many six cylinder engines have only four bearings. These must rely on crankshaft stiffness to prevent local distortion. Hs.{Lord Ernest Hives - Chair} | ||