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).
Article describing the working principle, setting, and testing of the Carburvac, a new vacuum feed carburetter.
| Identifier | ExFiles\Box 122\4\ scan0033 | |
| Date | 3rd October 1921 | |
| Reprinted from Motor Transport. October 3rd, 1921. A New Vacuum Feed Carburetter. Known as the Carburvac, this Instrument has the Useful Property of being able to Draw Petrol from a Tank some feet below its own Level. UTILISING the reduction of pressure in the induction pipe to secure petrol feed from a tank below carburetter level is a principle which the Autovac and similar devices have made quite familiar; the idea has also been applied to the carburetter itself, in order to avoid the expense of a separate device. A new carburetter embodying this principle and possessing certain other advantages is now on the market: it is entitled the Carburvac, and is made by Carburvac, Ltd., 28, Grange Road, Willesden Green, London, N.W.10. Principle of Working. The general design of the carburetter shown by the accompanying sketch. Petrol is supplied by a top-feed float chamber of the usual type. The regulating jet is below the float chamber, and is fed from above; the jet is large, and pierced by a tapered needle. In contact with the lower end of this needle is a cap in the form of a hollow cylinder with its open end upwards. This cap fits over the metal surrounding the regulating jet, and its lower edge fits inside the coned air inlet. On its extreme lower edge are three narrow slits, which act as the spraying jet. While the engine is stationary, the jet needle is forced down on to its seating by the spiral spring above it; but as soon as there is a reduction of pressure in the induction pipe, the air pressure lifts the cap, and thus air is admitted to the carburetter body, while at the same time the cap lifts the jet needle, and the jet passes petrol. Those who have compared the action of various carburetters will realise at once that this carburetter is of the type known as the “constant vacuum,” for the effect of the variations of air inlet opening with the engine suction is to produce a practically constant vacuum within the carburetter body, whatever may be the depression in the inlet pipe. In this particular carburetter there is an ingenious extension of the constant vacuum idea. A passage is drilled through from the carburetter body up to the top of the float chamber, thus bringing the vacuum to bear on the petrol supply, and rendering the carburetter capable of lifting its own petrol from the tank without the assistance of a separate vacuum feed device. The actual lift obtainable will naturally vary with the adjustment of the spring above the jet needle; but the strength of this spring has to be made suitable to the engine itself, and therefore cannot be used to control the vacuum. However, there is an ample margin, as it is found that with the carburetter as issued (which is suitable for all ordinary engines) a lift of somewhere near 4ft. 6in. can be easily obtained. Part sectional sketch showing principal features of the Carburvac carburetter. Obtaining a Correct Setting. As has been just mentioned, the strength of the jet needle spring is adjusted permanently by the makers. The setting of the carburetter for various engines and different weathers is obtained by screwing in or out the bottom air cone. Obviously, if this is lowered, air will be admitted to the carburetter, while if this is screwed further down the jet, i.e., the proportion of air to petrol will be increased. This cone is fitted with a quick thread screw, so that one-quarter of a turn gives ample adjustment, and a lug is provided so that the control of this cone may be brought up to the dashboard. The control rod lug is held in place by a split ring, so that the carburetter may be brought into adjustment with the engine once and for all, after which the control rod gives a sufficient adjustment. There is usually a certain fear among practical users that carburetters with floating air valves of this type may give trouble owing to the valve chattering in use, but this trouble has been ingeniously avoided in the carburetter in question. It will be seen that the lower end of the jet needle spring is finished off with a piston, which is a fair fit in the cylindrical spring housing. This space is completely full of petrol when the carburetter is working, and the piston acts as a most efficient dashpot, holding the needle and the air cap quite steady. The designer of the Carburvac carburetter informed us that the first users had a certain amount of trouble, through failure to realise one important point in connection with carburetters of this type. This point is that the upper part of the carburetter must be kept suction-tight, and in this it differs from the ordinary carburetter, in which there is usually an air vent to the float chamber. In order to ensure the fulfilment of this condition, the float chamber cover of the Carburvac is fitted with a ground joint, and is held in place by two grub screws. The jet cover also is specially constructed, and, in fact, makes two tight joints. The cover itself is ground on to the upper edge of its housing, while a rod projects down from it carrying a cover which fits tightly on to the top of the dashpot, and also acts as a backing-plate for the spring. This latter cover is held down to its work by a strong spring washer, to allow for any minute alterations or inaccuracies of machining in the body. PASSAGE TO TOP OF FLOAT CHAMBER A Practical Test. We had an opportunity of examining the behaviour of the carburetter as applied to a Ford car. We were greatly pleased at the excellent slow running obtained, together with the instant answer to a sudden full opening of the throttle. A test was also made to see how long it would take to fill the carburetter. The carburetter and petrol pipe (from a tank at the rear of the car) having been completely emptied, it was found that, after pulling the engine over compression four times it started immediately. On a running test, it was found that the power was normal, while the sweetness of running and the slow running under load were a great improvement on the usual type of carburetter. The designer of the Carburvac also claims for it a considerable decrease in petrol consumption. A special Ford model is not being made, as the standard model is found to fit all engines equally well. The designers have however, standardised Ford fittings, consisting of inlet pipe and hot air pipe, controls, etc., at such an extremely low rate that the total cost of fitting to a Ford is very small. Morton, Burt & Sons, Ltd., Printers, Willesden Green, N.W.10. | ||