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).
Technical analysis of creating a 3-cylinder engine by removing half the pistons and rods from a six-cylinder engine.
| Identifier | ExFiles\Box 159\5\ scan0039 | |
| Date | 14th August 1940 | |
| COPY. To Rm.{William Robotham - Chief Engineer} c. to Jnr.{Charles L. Jenner} 3-CYLINDER ENGINES. ----------- The following rough calculations have been made in view of your proposal to get the geel of a 3-cylinder engine by removing half the pistons and rods from a six. The front three pistons should be removed, for otherwise the length of crankshaft between the working cylinders and the flywheel will have a bad effect on torsionals. The longitudinal couple on a B.60 @ 4500 r.p.m. when running on the rear three cylinders only is about 1570 lbs. feet. The longitudinal couple on a 3-cylinder 2 1/2" bore (7.5 HP) engine running at 6000 r.p.m. is 360 lbs/ft. (It is estimated that the max. r.p.m. on the small engine would be around 6000, hence it would not be fair to compare the engines at the same speeds). It is further estimated that the moment of inertia about the "rocking" axis is approximately 10 times greater for B.60 than for the 3-cyl. We suggest that when an engine is subject to a rocking couple its angular acceleration is a measure of the roughness which will be felt in the car. Angular acceleration is equal to couple / moment of inertia. Substituting the above values it is found that the angular acceleration of the 3 cylinder would be about 2.3 times greater than that of B.60 with the engines running at the respective speeds. Logically therefore one would expect that the removal of three pistons from a B.60 would give less than half the roughness to be expected on the baby 3 cylinder engine. This is assuming the 3 cylinder crankshaft to have the same amount of balance as B.60, namely 70% of the rotating masses. In point of fact, however, considerably more balance could be used with advantage on a 3-cylinder engine. If all rotating masses and 50% of the reciprocating masses were balanced the longitudinal rocking couple would be replaced by a longitudinal couple rotating in the opposite direction to the crankshaft, and of half the magnitude of the rocking couple which it replaces. Therefore a B.60 with 3 pistons removed should have a couple of about the same magnitude but of a different type to that which would be expected on the 3-cylinder engine having this added balance on the crankshaft. It seems doubtful whether the removal of half the pistons from a B.60 engine would have any effect noticeable to the driver other than loss of power. Rm{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden} | ||