PURPOSE: To measure the air-fuel ratio quickly and highly accurately in a wide measuring range from rich to lean, by using an oxide semiconductor sensor only on the rich side and switching to a threshold current type oxygen sensor on the lean side.
CONSTITUTION: An output of a thermocouple of a TiO2 sensor 1 is introduced into a temperature regulator 23 through an automatic cold junction compensator 19, a measuring amplifier 20, an attenuator 21 and an automatic cold junction compensator 22. A characteristic of resistance of TiO2 to the air-fuel ratio is changed suddenly in about 104 times at a boundary of rich and lean. Accordingly, its middle value is set at the standard resistance in a standard resistance setting device 29 and on the other hand, dimensions of the resistance of the sensor are extracted by a temperature compensator 25 and then, both dimensions are compared by a rich and lean decision device 30. When the sensor resistance is small, it is decided as rich and when said resistance is large, it is decided as lean. A threshold current type oxygen sensor 6 is connected to a threshold current voltage and current detection part. The voltage is applied only in case of deciding as lean by the device 30.
TAKAHASHI HIDEAKI
SAJI KEIICHI
TAKEUCHI TAKASHI
