Gas sensor

A gas sensor of a limiting current type, the gas sensor including a sensor element formed of an oxygen ion conductive solid electrolyte, the gas sensor being capable of specifying a concentration of NOx in a measurement gas,

• said sensor element includes:
• a gas inlet into which a measurement gas is introduced from an outer space;
• a first inner space communicated with said gas inlet under a predetermined diffusion resistance;
• a second inner space communicated with said first inner space under a predetermined diffusion resistance;
• a main pump cell which is an electrochemical pump cell constituted by an inner pump electrode including one or two unit electrode parts and provided to face said first inner space, an external pump electrode provided on a surface of said sensor element, and said solid electrolyte located between said inner pump electrode and said external pump electrode;
• a measurement electrode provided to face said second inner space and covered by a porous protection film providing a predetermined diffusion resistance, the measurement electrode functioning as a reduction catalyst for NOx;
• an atmospheric air introduction layer into which atmospheric air is introduced from outside of said sensor element as a reference gas;
• a reference electrode covered with said atmospheric air introduction layer;
• and
• a measurement pump cell which is an electrochemical pump cell constituted by said measurement electrode, said external pump electrode, and said solid electrolyte located between said measurement electrode and said external pump electrode,
• wherein
• when said inner pump electrode includes said two unit electrode parts, said two unit electrode parts are disposed to face each other,
• said main pump cell pumps out oxygen in said measurement gas introduced into said first inner space through an application of a predetermined main pump voltage between said inner pump electrode and said external pump electrode, so that oxygen partial pressure of said measurement gas in said first inner space is lowered,
• said measurement pump cell pumps out oxygen generated by a reduction of NOx in said measurement gas reaching near said measurement electrode in said measurement electrode, through an application of a predetermined pump voltage between said inner pump electrode and said external pump electrode,
• said gas sensor further includes:
• a controller which receives signals corresponding to a magnitude of an NOx current flowing between said measurement electrode and said external pump electrode in said measurement pump cell and specifies a concentration of said NOx based on the magnitude of the NOx current,
• a diffusion resistance from said gas inlet to said first inner space is 200 cm−1 or larger and 1000 cm−1 or smaller, and
• when said first inner space has a space length L1 as a size in a longitudinal direction of said sensor element, a space thickness t1 as a size in a thickness direction of said sensor element, and a space width w1 as a size in a width direction perpendicular to both of said longitudinal direction and said thickness direction, and said unit electrode part has an electrode length L2 as a size in said longitudinal direction and an electrode width w2 as a size in said width direction,
• said space length L1 is 2.5 mm or larger and 10 mm or smaller,
• said space thickness t1 is 50 μm or larger and 300 μm or smaller,
• a ratio of said electrode length relative to said space length is 0.5 or more and 1.0 or less, and
• a ratio of said electrode width relative to said space width is 0.5 or more 1.0 or less.