Intake air amount measuring system for an internal combustion engine

An intake air amount measuring system for an internal combustion engine which has an intake air passage and a throttle valve positioned therein, comprising:

• an electric heater positioned upstream of the throttle valve, for heating a part of intake air flowing through said intake passage in accordance with a direct current voltage applied thereto through a voltage detecting resistor connected in series therewith;
• a first thermally-sensitive resistor positioned between said electric heater and said throttle valve to be responsive to temperature of said part of the intake air heated by said electric heater;
• a second thermally-sensitive resistor connected in series with said first thermally-sensitive resistor and positioned upstream of said throttle valve to be responsive to temperature of the remaining part of the intake air flowing through said intake passage;
• series-connected reference resistors connected in parallel with said series-connected first and second thermally-sensitive resistors to constitute a bridge circuit therewith;
• an oscillator circuit, connected to said bridge circuit, for supplying said bridge circuit with a pulsated voltage the amplitude of which is proportional to said direct current voltage supplied thereto;
• a heat control circuit, connected to said bridge circuit, for supplying said electric heater and said oscillator circuit with said direct current voltage which varies to maintain a voltage difference between a voltage at a junction point of said first and second thermally-sensitive resistors and a voltage at a junction point of said series-connected reference resistors to be constant;
• and
• computation means, connected to said electric heater, for computing an amount of the intake air in response to the direct current voltages across said electric heater and across said voltage detecting resistor.