Spark ignition internal combustion engines

A spark ignition internal combustion engine installation which includes a carburetor and an exhaust system;

• the carburetor having a body in which an induction passage is formed, the induction passage providing a flow path for air which is drawn into the engine by operation of the engine, a driver operable throttle valve for controlling the mass flow of air through the induction passage, and a fuel supply system comprising a fuel metering section, a fuel discharge nozzle and a source of liquid fuel, the fuel supply system being arranged so that metered quantities of liquid fuel are drawn from said source through the fuel metering section and the fuel discharge nozzle into the induction passage by air flow through said throat whereby fuel drawn through the discharge nozzle is atomized and dispersed within that part of the induction passage between the throat and the throttle valve so as to mix with air that flows through said throat and to form an air/fuel mixture which flows to the engine;
• the engine installation also including sensing means which are operable to monitor the ratio between the quantity of one of the constituents of gas flow at a location downstream of the throttle valve and the remainder of that gas flow and to emit a signal when that ratio differs from a predetermined optimum ratio by a predetermined amount, the signal being indicative of the sense of the difference between the ratio that is sensed at any one instant and the predetermined optimum ratio;
• an air flow passage which is connected to the fuel supply system between the fuel metering section and the fuel discharge nozzle, variable air flow metering means for metering air flow through the air flow passage so that metered quantities of air are drawn into the fuel supply system upstream of the fuel discharge nozzle and are mixed therein with fuel that is drawn through the fuel metering section whereby a mixture of metered quantities of fuel and air is drawn through the fuel discharge nozzle into the induction passage by air flow through the throat, the quantities of air drawn from the air flow passage into the fuel supply system upstream of the fuel discharge nozzle being metered by the variable air flow metering means, varying means which are operable to vary the performance of the air flow metering means, and signal transmitting means connected to the varying means and the sensing means so as to transmit to the varying means the signal that is emitted by the sensing means, the varying means being operable in response to a signal received from the sensing means to vary the performance of the air flow metering means so that the pressure drop across the fuel metering section is varied by the consequent change in the air flow through said air flow passage with the result that the quantity of fuel drawn from the source through the fuel metering section and the fuel discharge nozzle into the induction passage by a given air flow through the throat is changed, the variation in the pressure drop across the fuel metering section being the form of variation that will lead to a reduction in the difference between the said ratio and the predetermined optimum ratio;
• a supplementary air supply passage which is connected to the induction passage downstream of the throttle valve, a variable flow restricting device in said supplementary air supply passage, control means for controlling the variable flow restricting device, and further signal transmitting means connected to the control means for transmitting to said control means the signal that is emitted by the sensing means, the control means being operable in response to a signal received via the further signal transmitting means from the sensing means to vary the restriction to air flow through said supplementary air supply passage afforded by said variable flow restricting device, the variation in the restriction to air flow through said supplementary air supply passage being the form of variation that will lead to a reduction in the difference between the said ratio and the predetermined optimum ratio when the engine is idling.