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 'Convac' carburettor.
| Identifier | ExFiles\Box 122\4\ scan0020 | |
| Date | 7th March 1914 | |
| MARCH 7, 1914. THE “CONVAC” CARBURETTOR. X4586 The AUTO MOTOR JOURNAL THE “Convac” Carburettor, which has recently been placed on the market by Messrs. W. J.{Mr Johnson W.M.} Bithell, Ltd., of 46, Cannon Street, London, E.C., is interesting alike to motor engineers and private motor users. Not merely because it is of British design, but because it embodies a new application of a well-known principle. As its name implies, the “Convac” carburettor is of the type known as “suction-controlled”; that is to say, the quality of the mixture is automatically controlled by the suction of the engine. The principle, of course, is not new, having been utilised in some of the most successful carburettors. Where, however, the “Convac” carburettor differs is in the application of the principle. A glance at the sectional drawing reproduced shows that the jet is arranged in the centre of a rather small choke tube in the main body of the carburettor. A number of holes are seen in the side of the choke tube; the air sucked in through these holes meets the current past the jet at an acute angle, with the result that the air and the petrol from the jet are well mixed, the liquid fuel being thoroughly broken up and atomised. On the left of the jet is the regulating valve. It consists mainly of two parts, the moving piston valve and the stationary cylinder dash-pot. The latter is screwed into the main body from above; it is in the form of a cylinder open at the bottom and having a small air hole near the top facing the suction side of the carburettor. The area of this opening can be regulated by means of a setscrew. The piston, or air valve proper, is the most important part in the working of the carburettor; it is the only moving part in the device. The movement of this valve is entirely controlled by gravity and by the suction of the engine—regardless of the position of the throttle—and serves the double purpose of regulating the amount of air admitted to the mixing chamber and the quantity of fuel passing through the jet. At its lower end the valve piston carries an extension in the shape of an internally coned tube, which latter surrounds a tube communicating with the float-chamber above the level of the fuel. A small passage is drilled in the lower half of the piston valve which leads from the conical interior of the extension to the suction side of the carburettor. By means of this air valve the suction or velocity of air past the jet is kept constant, while the flow of petrol is regulated according to the demands of the engine by varying the negative pressure in the float chamber. This apparently complicated process is effected automatically in a simple and ingenious manner. When the engine is at rest, the piston-valve in the carburettor is down on its seat, thus entirely closing all air passages into the carburettor except through the very narrow choke tube past the jet. The suction created through the engine being turned by the starting handle is hardly sufficient to lift the comparatively heavy piston valve, so that a strong mixture, which is so essential for easy starting, is produced. Immediately the throttle is opened, and the suction of the engine increased, the piston valve is lifted from its seat through the suction in the mixing chamber of the carburettor and the negative pressure in the interior of the dash-pot or cylinder. As the piston valve lifts, air is admitted to the mixing chamber through the opening; with the lifting piston valve its lower conical tube extension rises also, while the “vacuum tube” surrounded by it remains stationary. The rising of this conical tube opens the air space around [Image 1: Plan view of the carburettor] LABELS: “CONVAC” CARBURETTER, FLOAT CHAMBER, VACUUM TUBE, AIR INLET, JET, THROTTLE CAPTION: “Auto.” (Yellow Cover) Copyright. Plan view of the “Convac” carburettor showing that all parts are accessible from above. [Image 2: Sectional drawings of the carburettor] LABELS: HOT AIR, VACUUM TUBE, TO ENGINE, FLOAT CHAMBER, VENT HOLE, VACUUM TUBE, FROM TANK CAPTION: Part section of the new “Convac” carburettor. On the right, section through float chamber. “Auto.” (Yellow Cover) Copyright. [Handwritten in margin] X1481 | ||