GB1396289A - Carburettors - Google Patents

Abstract

1396289 Spray carburetters W R A ALLEN 13 Oct 1972 [23 July 1971] 34659/71 Heading F1B A carburetter comprises an air duct having an inlet 12 and an outlet, a liquid fuel duct 44 having an outlet 16 and a mixture zone 14 communicating with the outlet end of the air duct and with the outlet end of the fuel duct, the air duct being adapted to produce an air-flow condition in which there is a pressure loss which is directly proportional to the air-flow velocity therein and the fuel duct being adapted to produce a liquid flow condition in which there is a pressure loss directly proportional to the fuel flow velocity, whereby the velocity with which fuel is drawn through the fuel duct, by the depression caused in the mixture zone as a result of air-flow through the air duct, is directly proportional to the airflow velocity, and the air/fuel ratio remains substantially constant over a range of air-flow velocity. The carburetter intake 12 is divided by a number of parallel plates 20 into passages having a crosssectional area such that the Reynolds number for a given air-flow velocity is not more than 2000 and the pressure drop along the length of each passage is proportional to the flow velocity. Similarly, the cross-sectional area of a restriction 16 through which fuel from a float chamber 27 emerges, is such that the amount of fuel is also proportional to the amount of air flowing i.e. the air/ fuel ratio remains constant even under the pulsating conditions induced by the intermittent combustion cycles of an I.C. engine. Additional fuel for acceleration is provided through a restrictor 17 by a pump 41 mechanically actuated by the throttle linkage. Additional fuel for operation at full power is provided through a restrictor 15, flow of fuel from which is controlled by a valve 21 also mechanically connected to the throttle linkage. To compensate for variation of the fuel temperature there is a movable shutter 19 which covers the entry 12 and which is mounted on a piston 36 which controls the flow of fuel from a pump to the float chamber 27. Fuel enters by port 26 and acts on the upper surface of the piston to move it downwardly against a spring 30 until an aperture 35 is clear of the lower edge of a sleeve 29 which is supported by a bi-metal strip 28. If the fuel temperature increases, the sleeve 29 moves downwardly and the piston 36 takes up a new lower position, so that more of the passages between the plates 20 are open, the flow of air increases and the depression in the mixing chamber 14 is reduced, thus compensating for the reduction in density of the fuel. In a modification, Fig. 2, the position of the shutter 19 is controlled by a piston, one side of which is open to the depression in the mixing chamber 14. The piston-rod 38 bears against a hinged arm 33 which in turn bears against the rod of a fuel flow restrictor 31. Variations in fuel temperature cause the piston 36 to raise or lower the unit 32 bodily and thus alter the point of contact of the restrictor rod with the lever 33. In a further modification, Fig. 3 (not shown) the air/fuel ratio is controlled by a needle valve in the fuel line, the valve being moved axially by an electromagnet, the current supply to which is controlled by a bridge circuit. The arms of the bridge include sensors responsive to the pressure in the mixing chamber, the temperature of the fuel, the pressure difference across the fuel outlet to the mixing chamber and the difference in pressure at two points downstream of the throttle valve 18. A circuit diagram, with component valves, is given of a D.C. amplifier which amplifies the bridge output.