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 cause of an engine 'boom' in Phantom II cars at 2200 r.p.m.
| Identifier | ExFiles\Box 182\M19\ img152 | |
| Date | 18th April 1931 | |
| To R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} ORIGINAL Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls}1/KT.18.4.31. c.c. to Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager} c.c. to E.{Mr Elliott - Chief Engineer} Da.{Bernard Day - Chassis Design} c. to Hy.{Tom Haldenby - Plant Engineer} y684. P.II BOOMS. We do not yet know why Phantom II cars boom at 2200 r.p.m. whereas no boom existed in Phantom I's. We are able to say that neither alterations to the flywheel by stiffening or ball bearing, nor stiffening the crankshaft flange, nor the addition of balance weights eliminate or alter the frequency of the boom. The boom is always worst on the overrun and is usually undetectable under full gas torque. Removal of the slipper wheel although bringing up torsional periods did not affect the boom. We think, therefore, that we can say definitely that the boom is not due to a torsional period but more probably to inertia torque. When we towed a car in gear with the pistons removed, the boom had disappeared causing us to believe that the piston motion is the disturbing influence. To find the frequency of the boom we excited in the car, a loudspeaker direct from the primary of a magneto. The P.F. from this source is unfortunately not a pure sine wave, the 3rd. harmonic existing to a marked degree. Running the engine at 2200 r.p.m. we discovered that resonance observable by beat notes occurred when the loudspeaker frequency was 4400/minute and 6600/minute, the latter being more pronounced. This again points to the piston motion as being the cause of the boom. As the boom at 60 m.p.h. is always accompanied by a steering wheel vibration, we found the frequency of the vibration by holding a microphone against the steering wheel and connecting it to a loudspeaker, the note emitted being gauged against tuning forks. This vibration was found to be 4400/minute or twice engine speed. This points to two disturbing influences as suggested in R1/M12.4.31. EV.{Ivan Evernden - coachwork} has suggested that the engine speed of the boom varies with different types of body but we do not believe this to be the case. Our experience of many cars is that the boom is at 60 m.p.h. or 2200r.p.m. According to a memo. from R.{Sir Henry Royce} we examined engines running on a test bed for a period at 2200 r.p.m. but we were unable to find one. The resonance causing the boom is not therefore in the engine but in the chassis subframe or body. When travelling over 5" tram sets at 32 m.p.h. Phantom II cars boom, the boom being accompanied by considerable vibration. At 32 m.p.h. the rate of encountering tram | ||