Electrochemical oxygen sensing element, particularly for determination of oxygen content in the exhaust gases of automotive internal combustion engines

Electrochemical oxygen sensing element to determine the oxygen content in the exhaust gases from internal combustion engines having a housing (16, 17, 18, 19);

• a solid electrolyte tube (11) having a first end closed at the bottom (10) and having an opposite end mounted in the housing and projecting therefrom, the solid electrolyte tube forming a solid ion conductive electrolyte oxygen concentration measuring element;
• first electrode means (13) at the inside of the tube (11) extending into the region of the closed bottom thereof;
• means (20) formed in the housing providing access of ambient air to the inside of the solid electrolyte tube (11) to establish an oxygen reference potential;
• second electrode means (12) at the outside of the tube (11) and forming a catalyzing layer, connected to a terminal of said element;
• and a protective tube (22) surrounding the solid electrolyte tube, with clearance, and secured in the housing formed with opening therein to expose the outside of the tube (11) with said catalyzing layer (12) to the exhaust gases;
• wherein, in accordance with the invention,
• means are provided formed on the protective tube to establish a substantially uniform temperature gradient of the solid electrolyte tube, when said solid electrolyte tube is exposed to hot exhaust gases from the internal combustion engine including
• said openings (27) formed in the protective tube (22) having an increase in cross-sectional free area
• in the direction from the housing towards the bottom of the solid electrolyte tube, the largest open area of the openings (27) being approximately in the region of the closed region of the closed bottom (10) of the solid electrolyte tube (11) so that the area of said openings in the vicinity of the end of the tube in the housing has a smaller cross-section than the openings in the vicinity of the closed end.