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).
Carburettor operation, focusing on the idle system, reverse air bleeds, and performance comparisons.
| Identifier | ExFiles\Box 123\1\ scan0049 | |
| Date | 24th April 1940 | |
| - 4 - When the engine is idling, the maximum depression in the carburettor is at the throttle edge. As the throttle is opened this maximum depression is gradually transferred to the inner venturi, until at full throttle there is maximum depression at the throat of the inner venturi. Now the idle system continues to function as a "mixture supplier" whilst ever the depression at the throttle edge exceeds that at the inner venturi. As soon as the inner venturi is in ascendance the idle system acts as a reverse air bleed for the main jet system, air being drawn from the throttle bores through the idle discharge holes. Since with smaller throttles the butterflies are further open at any given speed, this reversal of flow must take place earlier. Hence although the main jets come in at the same time, the idle system is cut out earlier, and the period of overlap during which both main jet and idle systems are running is reduced. Thus the economy is brought about by a reduction in the quantity rather than the strength of the mixture, and the flat spots which you might expect do not materialise. Curve C shows a gain of about 10% by using overdrive and curves D and E show comparative consumption figures for the Buick and Phantom III cars. Since the Buick has a slightly smaller engine and a more favourable overall gear ratio a correction factor has been applied and the result is shown in curve F.{Mr Friese} This correction factor may be a little unfair where engine capacity is concerned, but the results do show that on a comparative basis we can hold our own. IDLE SYSTEMS. Attached is a sketch of the idle system in use on our Stromberg dual carburettors. It differs from that used on earlier types, e.g. the A.A.25, in that it has a plain idle tube and a secondary air bleed to replace the ball idle tube. The latter is an idle tube, containing a non-return ball valve designed to prevent the idle system from acting as a reverse air bleed, thus preventing a flat spot which is inherent at the change over period. Unfortunately, the balls have a habit of sticking so the scheme is not all that it might be. Alternatively, secondary air bleeds are used. When the engine is idling these act as ordinary air bleeds. As the throttle is opened the increased air flow past the projecting secondary bleed causes a reversal of the flow through it. Flow through the idle discharge holes ceases, but an air flow through the system is maintained. Thus the system does not become a reverse air bleed until later in the main jet period, and the danger of a flat spot is removed. | ||