Exhaust gas sensor operating temperature detection for fuel mixture control system

Claim 1

In a mixture control system for an internal combustion engine having an exhaust gas sensor for supplying a control signal to the mixture control system, a detection system for detecting when the temperature of the gas sensor is at or above its operating temperature to allow said mixture control system to operate in a closed control mode and generating a disabling signal to operate said control system in an open mode when the temperature of the gas sensor is below its operating temperature, wherein said gas sensor is operable to generate low and high voltage signals in response to the presence and absence, respectively, of a predetermined constituent gas in the exhaust gases when operating at or above its normal operating temperature, said gas sensor having an internal impedance varying inversely with the temperature of said gas sensor from a very high impedance at its low, nonoperable temperature to a very low impedance at its high operating temperature, wherein said high voltage signal decreases with said internal impedance and said low voltage signal increases to approach said decreasing high voltage signal and then decreases with said internal impedance, said detection system comprising: a differential amplifier having a first input terminal receptive of the output from sad exhaust gas sensor and a second input terminal receptive of a variable reference voltage for generating a differential signal; a source of voltage of a magnitude intermediate the magnitudes of said high and low voltage signals which will be generated when said exhaust gas sensor is operating at or above its normal operating temperature; means for detecting said low voltage signal; means for adding a predetermined constant DC voltage to said detected low voltage signal; said variable reference voltage being the one of the output from said adding means and said intermediate voltage which is greater than the other; means for detecting said high voltage signal; and a comparator for generating a comparator output when said variable reference voltage is greater than the detected high voltage signal, said comparator output being said disabling signal.