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 design and operation of the 'Carburvac', a combined carburetter and vacuum fuel feed system.
| Identifier | ExFiles\Box 122\4\ scan0027 | |
| Date | 1st October 1921 | |
| Reprinted from The Autocar, October 1st, 1921. CARBURETTER AND VACUUM FEED IN ONE. How Fuel is Fed to the Float Chamber by the Suction of the Inlet Pipe without the Aid of a Special Tank. VACUUM tanks are being used for a great number of cars, but these tanks all depend on the suction or depression in the inlet pipe. Accordingly, many designers have attempted to supersede the more complicated mechanism by something which could be embodied in the actual carburetter. The Carburvac, as its name at least implies, is one solution of the problem. It is a carburetter having a float chamber which is air-tight and well above the level of an orifice forming the jet, so that the fuel feeds by gravity from the float chamber to this orifice. Immediately above the orifice is a chamber having in it a spring-loaded plunger terminating in a needle valve, of which the needle portion is in the jet orifice. On the needle stem is a cone, and when the engine is stopped this makes contact with a seating under the pressure of a coil spring, and so prevents any petrol from leaking through the jet. A part of the needle chamber forms a guide for the interior walls of a piston which covers the jet orifice completely, the piston being placed head downwards and serving to close entirely the choke tube by making contact with a special adjustable seating. When the suction in the inlet pipe causes this piston to rise, air is allowed to pass the adjustable seating, and, simultaneously, the inside of the piston head touches the lower end of the needle and raises it from the jet orifice. As the needle moves the cone clears its seat, and so fuel can flow from the float chamber. By this means fuel is fed to the inside of the piston in quantities increasing as the piston, and therefore the needle, is sucked higher. In the walls of the piston and close to the head, long slots are cut, and through these slots petrol flows to mix with air passing to the inlet pipe, the whole being governed by the position of the throttle. Feeding the Float Chamber. Variation of air to petrol in due proportion can be obtained by partly rotating the adjustable seating by means of a rod control and a lever, since the seat is secured to the body of the carburetter by a very thick thread. The more the seating is raised the nearer it approaches to the piston, so decreasing the amount of air available; consequently, the driver has always a very fine adjustment, and can run on a weak setting for economy, while he may increase the richness of the mixture for starting or when more power is preferred. So far we have dealt only with the operation of the carburetter as a carburetter; the great feature of the instrument lies rather in the method adopted to feed fuel from the rear tank to the float chamber, which, it will be remembered, is airtight. From the tank a single pipe is taken to the top of the float chamber, the latter being furnished with the usual needle valve controlled by the action of the float. At one side of the body of the carburetter is a passage communicating with the air space above the surface of the fuel in the float chamber, the lower end of this passage being affected by the suction caused by air passing through to the inlet pipe; in consequence, the air in the float chamber is always below atmospheric pressure when the engine is running. As the tank has an air vent, and the fuel therefore is under atmospheric pressure, petrol finds its way at once to the float chamber in quantities governed by the action of the float. Conditions of Working. It will be observed that this renders the action of the carburetter a little unusual, since in normal circumstances the orifice of the jet is surrounded by air considerably below atmospheric pressure, but the float chamber is subject to atmospheric pressure. In the Carburvac instrument there is a depression in both places, fuel therefore flowing through the jet merely by gravity, because the float chamber is considerably higher than the jet orifice. This makes it absolutely necessary to ensure that the cone seating of the needle valve in the jet orifice forms a petrol-tight joint when the throttle is closed and the instrument not working, but it introduces also other variations, since it has been found that a quite plain, regular taper will suffice for the needle valve, and it is not essential to make special provision to increase the amount of fuel to obtain acceleration or to decrease it at high speeds. Involving as it does a sliding piston, care also must be exercised so to design the latter that it does not stick on its guide and interfere with the operation of the jet. The instrument is made in two sizes; one to suit engines up to 20 h.p., the other of larger dimensions, the price of a Carburvac for a Ford car, for example, being £6 15s., with an addition of 3s. 6d. for sundry parts required in fitting. We were able to make a short test run on a Ford fitted with this carburetter, and there was no doubt whatever that the fuel feed was operating perfectly and that the engine would respond instantaneously to any throttle movement, gradual or abrupt, while pulling evenly at low speed. Messrs. Carburvac, Ltd., 28, Grange Road, Willesden Green, London, N.W. 2, are placing the Carburvac on the market, Mr. F.{Mr Friese} H.{Arthur M. Hanbury - Head Complaints} Arnott being responsible for the design. Image Caption: With the aid of this part sectional sketch of the Carburvac, the accompanying description will be easily followed. The device is peculiar in that it is not only a carburetter, but also an instrument which can supply itself with fuel from a tank either above or below its own level. Image Labels: PASSAGE TO TOP OF FLOAT CHAMBER M.{Mr Moon / Mr Moore} W. BOURDON | ||