At the instant of starting a cold engine, atomization of the fuel is usually insufficient and ignition is therefore difficult. To correct this, it is necessary to supply a relatively large quantity of fuel to the cold engine by reducing the effective area of the valve at the inlet of the carburetor. As the engine warms up, the choke valve is moved progressively to its fully opened position to yield an optimum air-fuel ratio.
It has long been known in this art to operate the choke valve automatically by means of a thermostat responsive to the temperature of the engine cooling water or of the engine exhaust gases. In general, arrangements for heating the thermostat with coolant engine are complicated, while arrangements for heating the thermostat with air warmed by the exhaust gases are susceptible to improper operation of the choke valve in the presence of foreign materials which are often introduced into the heating air.
More recently, simpler choke valve control arrangements have been developed wherein the thermostat is heated by an electric heater. Unfortunately, the response of presently known designs of this type to changes in the engine water temperature is slow; this is particularly true in the case where the vehicle is equipped with a separate room heater which, when made operative, absorbs heat from the engine cooling water to warm the passenger compartment. Because the room heater and the thermostat heater operate independently in such designs, the heat absorbed by the room heater when the latter is operative is not taken into account in the operation of the thermostat heater so that the latter functions as if the engine were colder than it actually is. Accordingly, the choke valve dictates an unduly rich air-fuel mixture to the engine before actual engine conditions require it. As a result, operating conditions for the engine will not be optimum, and the latter will tend to exhibit incomplete combustion with the accompanying danger of producing poisonous exhaust gases.