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).
Engine induction, valve timing, and exhaust back pressure effects on power output.
| Identifier | ExFiles\Box 134\1\ scan0008 | |
| Date | 12th July 1937 | |
| -3- Rm{William Robotham - Chief Engineer}7/R.12.7.37. (2) On a main aero engine, with induction passages branching off a main passage, it is assumed that ramming will be non-existent. On a car engine, however, fitted with two carburettors and unequal lengths of induction passages, this assumption cannot be made, and we are, therefore, once more confronted with the same problem as on the single, viz., what is the extent of this ramming, and what is its effect on power output and valve timing ? It is suggested, therefore, that as a preliminary similar tests to those on the single be made, finding a value for the bare engine without pipes. This should then line up, making due allowance for mechanical efficiency with the single cylinder unit of the same type. Further tests should then be made to find out whether or not the main engine induction system does indulge in ramming, and if so, where and to what extent. The next job would then appear to be to try the effect of various camshafts on the main engine, and try to get some relationship between these figures and similar single cylinder figures. If these tests are satisfactory, a basis should be obtained from which it will be possible to correctly forecast a suitable timing, and also to forecast with some accuracy changes in performance which may be expected on a main engine, as deduced from results obtained on the single, In addition, as work on the job progresses, it may be found possible to considerably simplify it. II. Exhaust Back Pressure. Not a great deal of work has been done on exhaust systems, but great surprise was expressed that it was necessary to lose 20% of power through the silencing system. This power loss Ricardo put down to two things, viz., the actual back pressure, and the fact that the mass of residuals in the combustion space was greater with the greater pressure. With half an atmosphere back pressure, these items will account for about 5% and 10% respectively. This, however, still leaves a balance, which the Writer thinks may be due to resonance in the exhaust pipe. | ||