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).
Tests conducted on the Phantom II expanding carburetter to improve opening up from idling speeds.
| Identifier | ExFiles\Box 13\3\ 03-page392 | |
| Date | 3rd March 1932 | |
| R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Swdl.{Len H. Swindell} c. to SS.{S. Smith} WDF. c. to S. RV. c. to Ds. PHANTOM II EXPANDING CARBURETTER. The recent tests on the P.II expanding carb. have been directed towards producing good opening up from idling speeds. Difficulty was experienced with the initial opening of the air piston being too sudden upon "snap" opening of the throttle from idling speeds with the result that the additional air admitted was in advance of the increasing of the jet size accompanying the lifting of the taper metering needle. Damping down the rate of lift of the air piston was the next test indicated. This was done by substituting one .025 dia suction hole in place of the two .150 dia holes. The result was a definite improvement but 'popping back' could still be produced by vigorously opening the throttle and still remained short of what was desired. Attention was then paid to the diffuser and the bore area was increased by 37% giving a greater 'well' capacity to supply a momentary rich mixture for acceleration. With this quite good 'opening up' was obtainable no popping back was experienced but the actual acceleration was not as rapid as desired due to slight weakness which could be observed by the pale colour of the exhaust flame when rapidly opening the throttle. A defect which has been encountered with the highly 'damped' piston valve is its slow return to closed position when the throttle is shut down from full open roughly about 6 seconds. If the engine is again opened up before the air piston is well down cutting out of popping back occurs. The S.U. air piston falls quite quickly. It would appear that a larger fixed choke would give an improvement. The present size choke will supply about 30 HP i.e. at full throttle 650 R.P.M. before the air piston admits extra air. This represents about 30% of the max. full power of the engine. We know from previous tests that if we employ a simple fixed choke care of the H.C.M. aero type - which is small enough to limit the possible power output of the engine to about 75% we can obtain quite good opening up. with fixed choke - or static carburetter | ||