Gas sensor and protection member for gas sensor

1. A gas sensor comprising:

a sensor element extending in an axial direction;

a metallic shell which surrounds a circumference of the sensor element and holds the sensor element;

a tubular outer surface member which is attached directly or indirectly to the metallic shell, constitutes an outer surface of the gas sensor, and extends toward a rear end side in relation to the metallic shell; and

a tubular protection member which is detachably fixed to a fixation region of an outer circumference of the outer surface member,

the gas sensor being characterized in that

the tubular protection member includes a first arc portion, a second arc portion, and a fixation portion which is brought into contact with and is fixed to the fixation region; and

when the tubular protection member fixed to the outer circumference of the outer surface member by fixing the fixation portion to the fixation region is viewed in the axial direction, the first arc portion and the second arc portion cover in total at least 90% of the entire circumference of the outer surface of the outer surface member,

wherein the tubular protection member is composed of two members which have the first arc portion and the second arc portion, respectively, and individual fixation portions connected to the first arc portion and the second arc portion, respectively, and the first arc portion and the second arc portion can be separated from each other.

2. The gas sensor as claimed in claim 1 , wherein the first arc portion and the second arc portion of the tubular protection member are connected by means of a connection portion, and the spacing between the first arc portion and the second arc portion in a radial direction can be expanded and contracted by rotation of opposite halves of the connection portion in the radial direction.

3. The gas sensor as claimed in claim 1 , wherein, when the tubular protection member fixed to the outer circumference of the outer surface member is viewed in the axial direction, the first arc portion and the second arc portion cover the entire circumference of the outer surface of the outer surface member.

4. The gas sensor as claimed in claim 1 , wherein

the outer surface of the outer surface member has a diameter-reduced portion recessed inward in the radial direction and serving as the fixation region; and

the fixation portion is fitted onto the diameter-reduced portion, thereby being fixed to the diameter-reduced portion.

5. The gas sensor as claimed in claim 1 , wherein

the outer surface member has a ventilation hole for introducing outside air into a space inside the outer surface member;

an air-permeable filter is disposed on an inner surface of the outer surface member to close the ventilation hole; and

the first arc portion and the second arc portion cover the ventilation hole from a radially outer side and form gaps for establishing communication between the ventilation hole and an outside atmosphere, the gaps being located at positions different from that of the ventilation hole in the axial direction.

6. The gas sensor as claimed in claim 1 , wherein the total sum of a center angle of the first arc portion and a center angle of the second arc portion is greater than 360 degrees.

7. The gas sensor as claimed in claim 6 , wherein

a diameter of the first arc portion and a diameter of the second arc portion are the same;

one end portion of the first arc portion in a circumferential direction covers the outer surface of the outer surface member, and the other end portion of the first arc portion covers one end portion of the second arc portion in the circumferential direction; and

the one end portion of the second arc portion in the circumferential direction covers the outer surface of the outer surface member, and the other end portion of the second arc portion covers the one end portion of the first arc portion in the circumferential direction.

8. The gas sensor as claimed in claim 6 , wherein

a diameter of the first arc portion is greater than a diameter of the second arc portion;

the first arc portion has a center angle greater than 180 degrees; and

end portions of the second arc portion on opposite sides in the circumferential direction are covered by the first arc portion.

9. The gas sensor as claimed in claim 1 , wherein

the fixation portion has elasticity;

when viewed in the axial direction, the fixation portion has the shape of an arc having a center angle greater than 180 degrees and an opening;

the minimum spacing of the opening in the circumferential direction is smaller than the outer diameter of the fixation region; and

the fixation portion has guide portions located at opposite ends of the opening, and the spacing between the guide portions in the circumferential direction expands from the minimum spacing.

10. A protection member for a gas sensor including a sensor element extending in an axial direction, a metallic shell surrounding a circumference of the sensor element and holding the sensor element, and a tubular outer surface member attached directly or indirectly to the metallic shell, constituting an outer surface of the sensor element, and extending toward a rear end side in relation to the metallic shell,

the protection member being characterized in that

the protection member is detachably fixed to a fixation region of an outer circumference of the outer surface member;

the protection member includes a first arc portion, a second arc portion, and a fixation portion which is brought into contact with and is fixed to the outer surface member; and

when the protection member fixed to the outer circumference of the outer surface member by fixing the fixation portion to the outer surface member is viewed in the axial direction, the first arc portion and the second arc portion cover in total at least 90% of the entire circumference of the outer surface of the outer surface member,

wherein the protection member is composed of two members which have the first arc portion and the second arc portion, respectively, and individual fixation portions connected to the first arc portion and the second arc portion, respectively, and the first arc portion and the second arc portion can be separated from each other.