Gas sensor

1. A gas sensor including:

a sensor element capable of detecting the concentration of predetermined gas in measured gas,

an inner protection cover having a bottomed cylindrical shape and covering a tip end of the sensor element, where the inner protection cover has at least three first inner gas holes and at least one second inner gas hole formed at a position further away in a tip direction of the sensor element than the at least three first inner gas holes,

an outer protection cover having a bottomed cylindrical shape and covering the inner protection cover, where the outer protection cover includes a cylindrical body portion having a plurality of first outer gas holes formed therein and a bottomed cylindrical tip end portion having at least one second outer gas hole formed therein and having an inner diameter that is smaller than an inner diameter of the body portion,

a first gas chamber formed as a space between the body portion of the outer protection cover and the inner protection cover, where the first gas chamber communicates with the inside of the inner protection cover through the at least three first inner gas holes, and

a second gas chamber formed as a space between the tip end portion of the outer protection cover and the inner protection cover, where the second gas chamber does not directly communicate with the first gas chamber and communicates with the inside of the inner protection cover through the at least one second inner gas hole,

wherein the at least three first inner gas holes and the plurality of first outer gas holes are formed so that the following conditions are satisfied: 0<an inner/outer hole count ratio Nr≦0.5 and 0<an inner/outer hole-area ratio Ar≦0.25, and

wherein a first-inner-hole count Nin represents a total number of the at least three first inner gas holes, a first-inner-hole average area Ain [mm 2 ] represents (a total opening area of the at least three first inner gas holes)/(the first-inner-hole count Nin), a first-outer-hole count Nout represents a total number of the plurality of first outer gas holes, a first-outer-hole average area Aout [mm 2 ] represents (a total opening area of the plurality of first outer gas holes)/(the first-outer-hole count Nout), the inner/outer hole count ratio Nr represents the first-inner-hole count Nin/the first-outer-hole count Nout, and the inner/outer hole-area ratio Ar represents (the first-inner-hole average area Ain/the first-outer-hole average area Aout).

2. The gas sensor according to claim 1 ,

wherein the body portion of the outer protection cover includes a side portion having a side surface extending in the direction of the central axis of the outer protection cover and a stepped portion that connects the side portion to the tip end portion,

the plurality of first outer gas holes includes at least three horizontal holes formed in the side portion of the outer protection cover and at least three vertical holes formed in the stepped portion of the outer protection cover,

the horizontal holes are formed such that 0≦a horizontal hole non-existence maximum angle θhmax≦180°, wherein when the horizontal holes and the central axis of the outer protection cover are projected onto a plane that is perpendicular to the central axis to form a projected central axis and projected horizontal holes and the plane is viewed from the projected central axis in a radial direction of the outer protection cover, an area of the plane in which the projected horizontal holes do not exist is referred to as a horizontal hole non-existence area, and a largest angle among central angles of the horizontal hole non-existence area is referred to as the horizontal hole non-existence maximum angle θhmax, and;

the vertical holes are formed so that 0 a vertical hole non-existence maximum angle θvmax≦180°, wherein when the vertical holes and the central axis of the outer protection cover are projected onto a plane that is perpendicular to the central axis to form a second projected central axis and projected vertical holes and the plane is viewed from the second projected central axis in a radial direction of the outer protection cover, an area of the plane in which the projected vertical holes do not exist is referred to as a vertical hole non-existence area, and a largest angle among central angles of the vertical hole non-existence areas is referred to as the vertical hole non-existence maximum angle θvmax.

3. The gas sensor according to claim 2 ,

wherein the horizontal holes and the vertical holes are formed so as to be alternately disposed in a circumferential direction of the outer protection cover when the horizontal holes and the vertical holes are projected onto the plane that is perpendicular to the central axis of the outer protection cover.

4. The gas sensor according to claim 1 ,

wherein the at least three first inner gas holes are formed such that the first-inner-hole count Nin≧3 and 0≦a first-inner-hole non-existence maximum angle θinmax≦180°, wherein when the at least three first inner gas holes and the central axis of the inner protection cover are projected onto a plane that is perpendicular to the central axis to form a third projected central axis and projected first inner gas holes and the plane is viewed from the third projected central axis in a radial direction of the inner protection cover, an area of the plane in which the projected first inner gas holes do not exist is referred to as a first-inner-hole non-existence area, and a largest angle among central angles of the first-inner-hole non-existence area is referred to as the first-inner-hole non-existence maximum angle θinmax, and

the plurality of first outer gas holes is formed such that the first-outer-hole count Nout≧6 and 0≦a first-outer-hole non-existence maximum angle θoutmax≦90°, wherein when the plurality of first outer gas holes and the central axis of the outer protection cover are projected onto a plane that is perpendicular to the central axis to form a fourth projected central axis and projected first outer gas holes and the plane is viewed from the fourth projected central axis in a radial direction of the outer protection cover, an area of the plane in which the projected first outer gas holes do not exist is referred to as a first-outer-hole non-existence area, and a largest angle among central angles of the first-outer-hole non-existence area is referred to as the first-outer-hole non-existence maximum angle θoutmax.

5. The gas sensor according to claim 4 ,

wherein the at least three first inner gas holes are formed such that the first-inner-hole count Nin≧3 and 0≦the first-inner-hole non-existence maximum angle θinmax≦120°, and

the plurality of first outer gas holes is formed such that the first-outer-hole count Nout≧6 and 0≦the first-outer-hole non-existence maximum angle θoutmax≦60°.

6. The gas sensor according to claim 1 ,

wherein the plurality of first outer gas holes is formed such that an opening area of each of the first outer gas holes ranges from 0.196 mm 2 to 3.14 mm 2 .

7. The gas sensor according to claim 1 ,

wherein each of the at least three first inner gas holes has an opening area of 0.2 mm 2 or greater.

8. The gas sensor according to claim 1 ,

wherein a central point of an opening of each of the at least three first inner gas holes is located 5 mm or more away from the tip end of the sensor element towards a rear end of the sensor element.

9. The gas sensor according to claim 1 ,

wherein the at least one second outer gas hole includes at least three horizontal holes formed in a side surface of the tip end portion and at least three vertical holes formed in a bottom surface of the tip end portion.

10. The gas sensor according to claim 1 ,

wherein the plurality of first outer gas holes is formed such that an opening area of each of the plurality of first outer gas holes ranges from 0.785 mm 2 to 3.14 mm 2 .