Open loop electronic circuit for altitude compensation

An open loop electronic circuit for altitude compensation of a carburetor installed on an internal combustion engine, the carburetor having an associated solenoid controlling auxiliary air bled into the fuel circuits of the carburetor thereby to control the air-fuel ratio of the mixture produced by the carburetor, the circuit comprising a bridge circuit having a variable resistant element which includes a strain gage the resistance of which changes as a function of altitude, the bridge circuit producing an electrical signal the amplitude of which changes in response to changes in air density;

• means responsive to the electrical signal for generating a variable pulse width control signal which is supplied to the solenoid, the signal response means operating in the range of 10-100 Hz and varying the pulse width of the control signal in response to amplitude changes in the electrical signal, the signal responsive means being calibrated to generate a control signal having a zero percent duty cycle at sea level and an increasing duty cycle at increasing altitude above sea level;
• and, thermal responsive means connected to the output of the signal responsive means, the thermal responsive means changing the pulse width of the control signal produced by the signal responsive means if the engine is operating at colder temperature conditions than for which the circuit is otherwise calibrated, the thermal responsive means affecting the control signal for the carburetor to produce a richer air-fuel mixture than what otherwise be produced thereby making it easier to start and operate the engine.