Abstract:
A protective cover assembly of a gas sensor which is made up of an inner and an outer cover. The inner cover is disposed within the outer cover coaxially through a given clearance. The dimension of the clearance is defined within a range suitable for improving the response rate of the gas sensor while ensuring the effect of avoiding the damage such as cracks in a sensor element arising from wetting thereof with moisture contained in a measurement gas.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine to determine the concentration of O 2 , NOx, HC, or CO in exhaust emissions, and more particularly to an improved structure of such a type of gas sensor designed to ensure a quick response to a change in, for example, concentration of a gas to be measured without sacrificing the effect of avoiding breakage of a sensor element. 
     2. Background Art 
     Conventionally, gas sensors are used for burning control of internal combustion engines for automotive vehicles. As a typical example, a gas sensor is installed in an exhaust pipe of an automotive engine to measure the concentration of a specified gas contained in exhaust emissions of the engine. A gas sensor of this type consists essentially of a gas sensor element disposed within a hollow cylindrical housing, an air cover installed on a base portion of the housing, and a protective cover assembly installed on a tip portion of the housing. The protective cover assembly has a double-walled structure made up of an inner and an outer cylindrical cover. The inner cover is smaller in diameter than the outer cover and disposed inside the outer cover coaxially. 
     The protective cover assembly has formed in the inner and outer covers a plurality of gas inlets through which the exhaust emissions enter a gas chamber defined in the cover assembly. The sensor element measures the concentration of the specified gas such as oxygen in the exhaust emissions admitted into the gas chamber. 
     Accurate measurement of the concentration of the specified gas subject to change cyclically requires use of gas sensors designed to provide a quick response to such a change. Further, the gas sensor element is apt to be wetted with moisture contained in the exhaust emissions, and may be broken. In order to avoid this, it is required to minimize the quantity of water entering the gas chamber through the gas inlets of the inner and outer covers. 
     Increasing the response rate of the gas sensors requires increasing the size of the gas inlets of the inner and outer covers, but it results in ease of intrusion of the moisture into the gas chamber. This is objectionable in avoiding the wetting of the gas sensor. 
     Single-walled protective covers are also known which are designed to simplify the flow of gas into the gas chamber for improving the response rate of the gas sensor, but it causes the sensor element to get wet directly with the moisture, thus accelerating the breakage such as cracks in the sensor element. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an improved structure of a gas sensor designed to ensure a rapid response to a change in, for example, concentration of a gas to be measured and to avoid undesirable wetting of a sensor element leading to the breakage of the sensor element. 
     According to one aspect of the invention, there is provided a gas sensor which may be employed in measuring the concentration of a specified gas contained in exhaust emissions of an internal combustion engine of an automotive vehicle. The gas sensor comprises: (a) a hollow cylindrical housing; (b) a gas sensor element retained within the housing which has a sensing portion working to sense a specified gas; and (c) a cover assembly installed on an end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted. The cover assembly is made up of an inner and an outer hollow cylindrical cover which are different in diameter and have gas inlets through which the specified gas passes. The inner cover is disposed within the outer cover coaxially with each other with a given clearance therebetween which lies within a range of 0.2 mm to 0.6 mm 
     In the preferred mode of the invention, each of the gas inlets of the inner and outer covers of the cover assembly has an area within a range of 0.2 mm 2  to 20 mm 2 . 
     Each of the inner and outer covers has a bottom in which a hole is formed. 
     Each of the inner and outer covers has a rounded corner formed between the bottom and a side wall thereof. 
     The sensor element has an electrode. The gas inlet of the inner cover of the cover assembly faces the electrode for directing the specified gas into the electrode. 
     The distance between a tip of the sensing portion of the sensor element remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     The distance between an end of the outer cover of the cover assembly remote from the housing and the gas inlet of the outer cover is within a range of 1.5 mm to 15 mm. 
     The clearance between the inner and outer covers preferably lies within a range of 0.2 mm to 0.55 mm 
     According to the second aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a gas sensor element retained within the housing which has a sensing portion working to sense a specified gas; and (c) a cover assembly installed on an end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted. The cover assembly is made up of an inner and an outer hollow cylindrical cover which are arranged coaxially, different in diameter, and have gas inlets through which the specified gas passes. If a circle having the same area as a cross sectional area of the outer cover is defined as S 1 , and a circle having the same area as a cross sectional area of the inner cover is defined as S 2 , a difference in radius between the circles S 1  and S 2  is defined within a range of 0.2 mm to 0.6 mm. 
     In the preferred mode of the invention, each of the gas inlets of the inner and outer covers of the cover assembly has an area within a range of 0.2 mm 2  to 20 mm 2 . 
     Each of the inner and outer covers has a bottom in which a hole is formed. 
     Each of the inner and outer covers has a rounded corner formed between the bottom and a side wall thereof. 
     The sensor element has an electrode. The gas inlet of the inner cover of the cover assembly faces the electrode in order to direct the specified gas to the electrode. 
     The distance between a tip of the sensing portion of the sensor element remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     The distance between an end of the outer cover of the cover assembly remote from the housing and the gas inlet of the outer cover is within a range of 1.5 mm to 15 mm. 
     The difference in radius between the circles S 1  and S 2  lies preferably within a range of 0.2 mm to 0.55 mm. 
     According to the third aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a gas sensor element retained within the housing which has a sensing portion working to sense a specified gas; and (c) a cover assembly installed on an end of the housing, the cover assembly being made up of an inner and an outer hollow cylindrical cover which are arranged coaxially and different in diameter. The cover assembly defines therein a gas chamber in which the sensing portion of the sensing element is disposed and into which a specified gas is admitted through gas inlets formed in the outer and inner covers. The volume of the gas chamber is defined within a range of 800 mm 3  to 1600 mm 3 . 
     In the preferred mode of the invention, if the volume of the gas chamber is defined as V 1 , and a volume of a clearance defined between the inner and outer covers of the cover assembly is defined as V 2 , a relation of V 2 /V 1 ≦0.25 is met. 
     Each of e gas inlets of the inner and outer covers of the cover assembly has an area within a range of 0.2 mm 2  to 20 mm 2 . 
     Each of the inner and outer covers has a bottom in which a hole is formed. 
     Each of the inner and outer covers has rounded corner formed between the bottom and a side wall thereof. 
     The sensor element has an electrode. The gas inlet of the inner cover of the cover assembly faces the electrode in order to direct the specified gas to the electrode. 
     The distance between a tip of the sensing portion of the sensor element remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     The distance between an end of the outer cover of the cover assembly remote from the housing and the gas inlet of the outer cover is within a range of 1.5 mm to 15 mm. 
     According to the fourth aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a gas sensor element retained within the housing which has a sensing portion working to sense a specified gas; and (c) a cover assembly installed on an end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted. The cover assembly is made up of an inner and an outer hollow cylindrical cover which are arranged coaxially, different in diameter, and have gas inlets through which the specified gas passes. Each of the gas inlets of the outer cover is defined by a portion of a peripheral side wall which is cut and bent outward. 
     In the preferred mode of the invention, each of the gas inlets of the inner cover is defined by a portion of a peripheral side wall which is cut and bent outward. 
     Each of the gas inlets of the inner and outer covers of the cover assembly has an area within a range of 0.2 mm 2  to 20 mm 2 . 
     Each of the inner and outer covers has a bottom in which a hole is formed. 
     Each of the inner and outer covers has a rounded corner formed between the bottom and a side wall thereof. 
     The sensor element has an electrode. The gas inlet of the inner cover of the cover assembly faces the electrode in order to direct the specified gas to the electrode. 
     The distance between a tip of the sensing portion of the sensor element remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     The distance between an end of the outer cover of the cover assembly remote from the housing and the gas inlet of the outer cover is within a range of 1.5 mm to 15 mm. 
     According to the fifth aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a gas sensor element retained within the housing which has a sensing portion working to sense a specified gas; and (c) a cover assembly installed on an end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted, the cover assembly being made up of an inner and an outer hollow cylindrical cover which are arranged coaxially, different in diameter, and have gas inlets through which the specified gas passes. The inner cover has a peripheral wall made up of a shoulder and a straight portion. The shoulder has a diameter increasing toward the end of the housing on which the cover assembly is installed and is located closer to the end of the housing than the gas inlets. The straight portion has a constant diameter. 
     In the preferred mode of the invention, the distance between one of ends of the shoulder remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 0.2 mm to 2 mm. 
     The distance between one of ends of the shoulder remote from the housing and a bottom of the outer cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     Each of the gas inlets of the inner and outer covers of the cover assembly has an area within a range of 0.2 mm 2  to 20 mm 2 . 
     Each of the inner and outer covers has a bottom in which a hole is formed. 
     Each of the inner and outer covers has a rounded corner formed between the bottom and a side wall thereof. 
     The sensor element has an electrode for picking up a sensor signal. The gas inlet of the inner cover of the cover assembly faces the electrode in order to direct the specified gas to the electrode. 
     The distance between a tip of the sensing portion of the sensor clement remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     The distance between an end of the outer cover of the cover assembly remote from the housing and the gas inlet of the outer cover is within a range of 1.5 mm to 15 mm. 
     According to the sixth aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow cylindrical housing; (b) a gas sensor element retained within the housing which has a sensing portion working to sense a specified gas; and (c) a cover assembly installed on an end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted, the cover assembly being made up of an inner and an outer hollow cylindrical cover which are arranged coaxially, different in diameter, and have gas inlets through which the specified gas passes. The inner cover is disposed within the outer cover coaxially with each other with a given clearance therebetween which has a volume lying within a range of 50 mm 3  to 200 mm 3 . 
     In the preferred mode of the invention, if a volume of the gas chamber is defined as V 1 , and the volume of the clearance between the inner and outer covers of the cover assembly is defined as V 2 , a relation of V 2 /V 1 ≦0.25 is met. 
     Each of the gas inlets of the inner and outer covers of the cover assembly has an area within a range of 0.2 mm 2  to 20 mm 2 . 
     Each of the inner and outer covers has a bottom in which a hole is formed. 
     Each of the inner and outer covers has a rounded corner formed between the bottom and a side wall thereof. 
     The sensor element has an electrode for picking up a sensor output. The gas inlet of the inner cover of the cover assembly faces the electrode for direct the specified gas to the electrode. 
     The distance between a tip of the sensing portion of the sensor element remote from the housing and the gas inlet of the inner cover of the cover assembly is within a range of 1.5 mm to 15 mm. 
     The distance between an end of the outer cover of the cover assembly remote from the housing and the gas inlet of the outer cover is within a range of 1.5 mm to 15 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
       In the drawings: 
         FIG. 1  is a longitudinal sectional view which shows a gas sensor equipped with a protective cover assembly according to the first embodiment of the invention; 
         FIG. 2  is a sectional view which shows an internal structure of the protective cover assembly in  FIG. 1 ; 
       FIG.  3 ( a ) is a partially enlarged view of  FIG. 2 ; 
       FIG.  3 ( b ) is a partially enlarged view which shows a positional relation between gas inlets and a gas sensor element; 
         FIG. 4  is an illustration which shows a velocity profile of a gas flow passing through an exhaust pipe of an internal combustion engine; 
         FIG. 5  is a graph which shows the temperature of an outer electrode of a sensor element and the distance of a temperature measurement point from a tip of the sensor element; 
         FIG. 6  is an illustration which shows a positional relation between an inner cover of a cover assembly and a head portion of a sensor element; 
         FIG. 7  is a longitudinal sectional view which shows a gas sensor equipped with a protective cover assembly according to the second embodiment of the invention; 
         FIG. 8  is a longitudinal sectional view which shows a protective cover assembly according to the third embodiment of the invention; 
       FIG.  9 ( a ) is a partially sectional view which shows a first example of installation of the protective cover assembly of  FIG. 8  to a housing of a gas sensor; 
       FIG.  9 ( b ) is a partially sectional view which shows a second example of installation of the protective cover assembly of  FIG. 8  to a housing of a gas sensor; 
         FIG. 10  is a transverse sectional view which shows a protective cover assembly according to the fourth embodiment of the invention; 
       FIG.  11 ( a ) is a partially side view which shows an outer cover of a protective cover assembly according to the fifth embodiment of the invention; 
       FIG.  11 ( b ) is a partially transverse view, as taken along the line A—A in FIG.  11 ( a ); 
       FIG.  12 ( a ) is a transverse sectional view which shows a first modification of a protective cover assembly in the fifth embodiment; 
       FIG.  12 ( b ) is a transverse sectional view which shows a second modification of a protective cover assembly in the fifth embodiment; 
         FIG. 13  is a graph which shows a change in frequency of an output of a gas sensor in response to a change in concentration of oxygen for difference values of a clearance between inner and outer covers of a cover assembly; 
         FIG. 14  is a longitudinal sectional view which shows a gas sensor equipped with a protective cover assembly according to the sixth embodiment of the invention; and 
         FIG. 15  is a longitudinal sectional view which shows a gas sensor equipped with a protective cover assembly according to the seventh embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown a gas sensor  1  according to the first embodiment of the invention which may be employed in a burning control system for automotive engines to measure the concentration of a gas component such as O 2 , NOx, HC, or CO contained in exhaust gasses of the engine. 
     The gas sensor  1  generally includes a gas sensor element  4 , a metallic hollow cylindrical housing  10 , an air cover  12 , and a protective cover assembly  11 . The gas sensor element  4  is retained in the cylindrical housing  10  in a liquid tight fashion and has a head portion or sensing portion exposed outside the housing  10 . The cover assembly  11  is attached to a head of the housing  10  and has a longitudinal center line  119 , as shown in FIG.  3 ( a ), extending in alignment with a longitudinal center line of the gas sensor  1  (i.e., the gas sensor element  4 ). The cover assembly  11  consists of a hollow cylindrical outer cover  3  and a hollow cylindrical inner cover  2  and has a flange (will also be referred to as a base below) installed in an annular groove formed in the end wall  101  of the housing  10  to define a gas chamber  112  within which the sensing portion of the gas sensor element  4  is disposed and into which a gas to be measured (will also be referred to as a measurement gas below) is admitted through gas inlets  241 ,  242 ,  341 , and  342 , as clearly shown in  FIG. 2 , formed in the outer and inner covers  2  and  3 , respectively. 
     The inner cover  2  is smaller in diameter than the outer cover  3  and disposed inside the outer cover  3  coaxially. The inner cover  2  has a shoulder  23  to define a clearance  111  between an outer side wall of the inner cover  2  and an inner side wall of the outer cover  3 . The clearance  111  is, as indicated by C in FIG.  3 ( a ), defined within a range of 0.2 mm to 0.6 mm, and preferably within a range of 0.2 mm to 0.55 mm. The reason for setting the clearance  11  within such a range will be described later in detail. 
     The following discussion will refer to an example in which the gas sensor  1  is, as shown in  FIG. 4 , installed in a mount hole formed in a peripheral wall  90  of an exhaust pipe  9  of an automotive engine in contact of the housing  10  with the mount hole to measure the concentration of oxygen contained in exhaust emissions for use in the air-fuel ratio control. 
     Usually, an exhaust gas flow has a velocity profile  91 , as shown in  FIG. 4 , within a flow passage  900  of the exhaust pipe  9 . The velocity profile  91  is defined by illustrated velocity vectors V. Specifically, the velocity of a central stream of the gas flowing through the flow passage  900  is the greatest, and the velocity of a peripheral stream of the gas along an inner wall of the exhaust pipe  9  is the lowest. 
     Referring back to  FIGS. 1 and 2 , the air cover  12  is fitted on a boss of the housing  10 . An outer cover  121  is provided around the air cover  12  and staked or crimped to retain a cylindrical water-repellent filter  120  on the periphery of the air cover  12 . The air cover  12  and the outer cover  121  have formed therein air inlets through which air is admitted as a reference gas into an air chamber defined inside the air cover  12 . 
     The cover assembly  11  has a double-walled structure made up of the inner and outer covers  2  and  3 . The outer cover  3  has, as described above, the gas inlets  341  and  342  formed therein. Similarly, the inner cover  2  has the gas inlets  241  and  242  formed therein. The gas inlets  341 ,  342 ,  241 , and  242  are of circular shape and identical in area with each other. In this embodiment, the area of each of the gas inlets  341 ,  342 ,  241 , and  242  is 3.14 mm 2 . The gas inlets  342 ,  342 ,  241 , and  242  are all opposed to an outer electrode  41 , as will be described later in detail, which is installed on the gas sensor element  4 . 
     The outer and inner covers  3  and  2  are, as clearly shown in  FIG. 2 , formed by hollow cylinders with bottoms  36  and  26 . The bottoms  36  and  26  have circular holes  361  and  261 . The outer and inner covers  3  and  2  have bottom corners  35  and  25  rounded between peripheries of the bottoms  335  and  25  and side walls  369  and  269 , respectively. 
     The inner cover  2  is, as shown in FIGS.  2  and  3 ( a ), made up of the mount flange  21 , the contact wall  22 , the shoulder  23 , the straight wall  24 , the rounded corner  25 , and the bottom  26 . The mount flange  21  is, as can be seen in  FIG. 1 , staked in an annular groove formed on an end wall of the housing  10  to mount the inner cover  2  on the housing  10 . The straight wall  24 , the shoulder  23 , and the contact wall  22  forms the above described side wall  269  of the inner cover  2 . 
     The straight wall  24  has a constant diameter. The shoulder  25  has a diameter increasing toward the contact wall  22 . The contact wall  22  has an outer diameter substantially identical with an inner diameter of the outer cover  3  and is contact with the inner wall of the outer cover  3 . Numeral  232  in FIG.  3 ( a ) indicates an end perimeter of the contact wall  22  leading to the shoulder  23 . 
     The outer cover  3  is, as clearly shown in FIGS.  2  and  3 ( a ), made up of the mount flange  31 , the straight wall  34 , the rounded corner  35 , and the bottom  36 . The mount flange  31  is, like the mount flange  21  of the inner cover  2 , staked in the annular groove formed on the end wall of the housing  10  to mount the outer cover  3  on the housing  10  together with the inner cover  2 . The straight wall  34  has a constant diameter and forms the side wall  369 . 
     The straight walls  24  and  34  have formed therein the gas inlets  241 ,  242 ,  341 , and  342 , respectively. The gas inlets  241  of the inner cover  2  are the closest to the housing  10 . The gas inlets  342  of the outer cover  3  are the farthest from the housing  10 . The gas inlets  341  of the outer cover  3  are closer to the housing  10  than the gas inlets  242  of the inner cover  2 . 
     The gas inlets  241  are implemented by six holes (only two are shown in  FIG. 2  for the brevity of illustration) which are formed at regular intervals in the peripheral wall of the inner cover  2  and located at the same distance from the end (i.e., the mount flange  21 ) of the inner cover  2 . The same is true for the gas inlets  242 ,  341 , and  342 . The gas inlets  241  of the inner cover  2  do not face the gas inlets  341  of the outer cover  3 . Similarly, the gas inlets  242  of the inner cover  2  do not face the gas inlets  342  of the outer cover  3 . 
     The distance D 1 , as shown in FIG.  3 ( a ), between an inner surface  344  of the bottom  36  of the outer cover  3  and an end perimeter  231  of the shoulder  23  of the inner cover  2  leading to the straight wall  24  is preferably within a range of 1.5 mm to 15 mm (10 mm in this embodiment). The distance D 2  between an end of each of the gas inlets  241  of the inner cover  2 , as viewed in the drawing, close to the housing  10  and the end perimeter  231  of the shoulder  23  of the inner cover  2  is preferably within a range of 0.2 mm to 2 mm (1 mm in this embodiment). The distance C between the inner surface of the outer cover  3  and the outer surface of the inner cover  2  (i.e., the thickness of the clearance  111 ) is set to 0.5 mm in this embodiment. 
     Referring back to  FIG. 1 , the air cover  12  is, as described above, fitted on the boss of the housing  10 . The outer cover  121  is provided around the air cover  12  and staked or crimped to retain the water-repellent filter  120  on the periphery of the air cover  12 . An elastic insulating holder  14  is fitted in an end of the air cover  12  to hold therein a plurality of leads  15  connected electrically to the gas sensor element  4 . 
     The gas sensor element  4  consists of a cup-shaped solid electrolyte body. The gas sensor element  4  has the outer electrode  41  and an inner electrode (not shown) formed on outer and inner wall thereof, respectively. The gas sensor element  4  has formed therein a cavity into which a ceramic heater  45  is inserted and which defines a reference gas chamber into which the air is introduced as a reference gas through the water-repellent filter  120 . The outer electrode  41  is electrically connected to the lead  15  through a lead  43 , a terminal  46 , and a connector  47 . Similarly, the inner electrode is electrically connected to the lead  15  through a terminal  46  and a connector  47 . All the leads  15  are connected to a control circuit (not shown) which works to pick up an output of the gas sensor element  4  to determine the concentration of oxygen (also referred to as a measurement gas below) contained in the exhaust gasses admitted into the gas chamber  112  of the cover assembly  11 . 
     The ceramic heater  45  has installed therein a heating element coupled with the leads  15  through terminals  451  and connectors  452 . The ceramic heater  45  is supplied with power through the leads  15  and works to heat the gas sensor element  4  up to a desired activation temperature. 
     The connectors  47  and  452  are disposed in the porcelain insulator  13  installed in the air cover  12 . 
     The operation of the gas sensor  1  of this embodiment will be described below. 
     The cover assembly  11  has, as described already, the clearance  111  defined between the inner and outer covers  2  and  3  within a specified range. This causes the flow velocity of the measurement gas to be different greatly between the clearance  111  and the inside of the inner cover  2 , thus resulting in an increased pressure within the clearance  111 , thereby facilitating entrance of the measurement gas into the inner cover  2 . A change in concentration of measurement gas occurring outside the gas sensor  1  is, therefore, transmitted quickly into the inner cover  2 , thereby providing for a quick response rate of the gas sensor  1 . 
     The clearance  111  also serves to facilitate evaporation of drops of water which have passed through the clearance  111  and stuck to the inner cover  2  by heat dissipation from the covers  2  and  3 , thereby minimizing wetting of the gas sensor element  4 . 
     The shoulder  23  of the inner cover  2  defines a wedge-shaped portion of the clearance  111  which works to reduce an upward flow of the measurement gas, as viewed in  FIGS. 1 and 2 , to the housing  10 , thereby straightening the flow of the measurement gas into the gas inlets  241  and  242  of the inner cover  2 , which results in quick entrance of the measurement gas into the gas chamber  112 . 
     The distance D 2  between the ends of the gas inlets  241  of the inner cover  2  close to the housing  10  and the end perimeter  231  of the shoulder  23  of the inner cover  2  is, as described above, within a specified range of 0.2 mm to 2 mm. This causes the measurement gas to stay between the shoulder  23  and the gas inlets  241 , which functions as a buffer promoting the entrance and exit of the measurement gas into and from the gas chamber  112 , thus resulting in an improved response rate of the gas sensor  1 . If the distance D 2  is less than 0.2 mm, it becomes difficult to machine the gas inlets  241  of the inner cover  2 . Conversely, if it is more than 2 mm, it will cause the measurement gas to stay within the clearance  111  for a relatively long time, which results a time delay in the entrance and exit of the measurement gas into and from the gas chamber  112 . In a case where the gas inlets  241  are located at different levels in the lengthwise direction of the cover assembly  11 , then the distance D 2  is defined as an interval between the end of one of the gas inlets  241  closest to the housing  10  and the end perimeter  231  of the shoulder  23  of the inner cover  2 . 
     The distance D 1 , as shown in FIG.  3 ( a ), between the inner surface  361  of the bottom  36  of the outer cover  3  and the end perimeter  231  of the shoulder  23  of the inner cover  2  leading to the straight wall  24  is, as described above, within a specified range of 1.5 mm to 15 mm. This allows the volume of the outer cover  3  to be minimized, thus minimizing the volume of the clearance  111 . This results in a decrease in time required for the measurement gas to pass through the clearance  111  and enter the gas chamber  112 . If the distance D 1  is less than 1.5 mm, it results in increased difficulty of the measurement gas in entering the gas chamber  112  through the inner cover  2 . Conversely, if it is more than 15 mm, it results in a time delay in the entrance and exit of the measurement gas into and from the gas chamber  112 . 
     The gas inlets  241 ,  242 ,  341 , and  342  of the inner and outer covers  2  and  3  are, as described above, identical in area with each other. The area of the gas inlets  241 ,  242 ,  341 , and  342  is preferably within a range of 0.2 mm 2  to 20 mm 2 . This results in uniformity of streams of the measurement gas from the clearance  111  into the gas chamber  112  through the gas inlets  241  and  242  of the inner cover  2 , thus facilitating the replacement of the measurement gas in the gas chamber  112 , which provides for a quick response of the gas sensor  1 . If the area of the gas inlets  241 ,  242 ,  341 , and  342  is less than 0.2 mm 2 , it results in an increased resistance of a gas flow through each of the gas inlets. If it is more than 20 mm 2 , it facilitates ease of intrusion of water into the gas chamber  112 . The gas sensor element  4 , thus, gets wet, thus increasing output errors of the gas sensor  1 . 
     The inner and outer covers  2  and  3  have formed in the bottoms  25  and  35  the circular holes  261  and  361  which work as gas outlets to form flow paths of the measurement gas from the gas inlets  242  and  342  to the outside of the cover assembly  11 , thereby facilitating the entrance and exit of the measurement gas into and from the gas chamber  112  and the clearance  111 . 
     The inner and outer covers  2  and  3  have the corners  25  and  35  rounded, thus resulting in an increase in service life of a press used in forming the inner and outer covers  2  an  3 , which results in a decrease in manufacturing cost of the gas sensor  1 . 
     The gas inlets  242  of the inner cover  2  face the outer electrode  41  of the gas sensor element  4 , thus facilitating hits of streams of the measurement gas on the electrode  41 , which increases the response rate of the gas sensor  1 . 
       FIG. 5  is a graph which represents a relation between the temperature of the outer electrode  41  activated by the heating of the heater  45 . The heater  45  in this embodiment is installed in the sensor element  4  in direct contact of a head thereof with an inner head surface of the sensor element  4 . A head portion of the sensor element  4  is, thus, well heated by the heater  45 . U.S. Pat. No. 5,956,841, published Sep. 28, 1999, assigned to the same assignee of this application discloses such a type of gas sensor, disclosure of which is incorporated herein by reference. L indicates the distance between the tip  491  of the sensor element  4 , as shown in  FIG. 6 , and a temperature-measured point P. In  FIG. 6 , M indicates an area of the inner cover  2  on which the distance L is projected. The broken line  492  indicates a longitudinal center line of the sensor element  4 . The graph of  FIG. 5  shows that the temperature of the outer electrode  41 , as measured at the point P, rises as the distance L increases. It is, thus, preferable that the area M is defined within a desired range of activation temperatures of the outer electrode  41 , the gas inlets  241  and  242  are formed within the area Min order to direct the measurement gas to a well-activated portion of the outer electrode  41 . 
     The distance E 1 , as shown in FIG.  3 ( b ), between the tip  491  of the gas sensor element  4  and ends  246  of the gas inlets  242  of the inner cover  2  close to the tip  491  is preferably within a range of 1.5 mm to 15 mm (4 mm in this embodiment). A portion of the gas sensor element  4  which is opposed to a portion of the heater  45  (i.e., the head of the heater  45  in this embodiment) producing the highest temperature is the most reactive with the measurement gas and contributes to determination of the concentration of the measurement gas greatly. The distance E 1  falling within the above range serves to improve the efficiency of hits of streams of the measurement gas on the portion of the gas sensor element  4  that is the highest reactive with the measurement gas. 
     The distance E 2 , as shown in FIG.  3 ( a ), between the inner surface  361  of the bottom  36  of the outer cover  3  and ends  343  of the gas inlets  342  which are the closest to the bottom  36  is within a range of 1.5 mm to 15 mm (1.5 mm in this embodiment). As described in  FIG. 4 , the central stream of the exhaust gas flowing through the flow passage  900  of the exhaust pipe  9  is the highest in flow velocity, while the peripheral stream is the lowest. It is, thus, advisable that the gas inlets  342  of the outer cover  3  be located near the central stream of the exhaust gas for increasing the efficiency of entrance of the measurement gas into the gas chamber  112 . The distance E 2  within the above range meets this condition in the structure of the gas sensor  1  as illustrated in FIG.  1 . 
       FIG. 7  shows the gas sensor  1  according to the second embodiment of the invention which includes the gas sensor element  5  implemented by a laminated plate made up of a base portion and a sensing portion exposed directly to the measurement gas. For example, U.S. Pat. No. 5,573,650, issued on Nov. 12, 1996 to Fukaya et al. teaches a typical laminated sensor element, disclosure of which is incorporated herein by reference. 
     The gas sensor  1  consists essentially of the metallic hollow cylindrical housing  10 , the gas sensor element  5 , the air cover  12 , the first insulation porcelain  51 , the second insulation porcelain  13 , and the protective cover assembly  11 . The gas sensor element  5  is retained within the housing  10  hermetically. The air cover  12  is fitted on the base portion  102  of the housing  10  to define the air chamber into which the air is, like the first embodiment, admitted through the water-repellent filter  120 . 
     The first insulation porcelain  51  is fitted within the housing  10  and holds therein the gas sensor element  5  in an air-tight fashion using a sealing member  52  made of a glass material, for example. The second insulation porcelain  13  is mounted on the first insulation porcelain  51  in alignment with each other and surrounds the base portion of the gas sensor element  5 . The air cover  12  covers the second insulation porcelain  13 . 
     The second insulation porcelain  13  has disposed therein four leads  152  (only two are shown for the simplicity of illustration) each of which is made of a wire folded elastically to make an electric contact at one end with an electrode terminal (not shown) formed on a base end (i.e., an upper end, as viewed in the drawing) of the gas sensor element  5 . The leads  152  extend at the other end through holes formed in an end of the second insulation porcelain  13  and connect with the leads  15  through the connectors  53 , respectively, for transmission of sensor signals between the gas sensor element  5  and an external control circuit and supply of electric power to a ceramic heater installed in the gas sensor element  5 . 
     The gas sensor element  5  is, as described above, made of the laminated plate which is formed by a sensing section made up of a solid electrolyte layer on which electrodes are formed and insulating layers and the ceramic heater equipped with a heating member. 
     The cover assembly  11  is, like the first embodiment, made up of the inner and outer covers  2  and  3 . The inner cover  2  is identical in structure with the one in the first embodiment, but the outer cover  3  is different from the first embodiment in that a shoulder  33  is formed slightly beneath the shoulder  23  of the inner cover  2 , as viewed in the drawing. Other arrangements are identical with those of the first embodiment, and explanation thereof in detail will be omitted here. 
       FIGS. 8 ,  9 ( a ), and  9 ( b ) show the protective cover assembly  11  according to the third embodiment of the invention. 
     The cover assembly  11  is, like the above embodiments, made up of the inner and outer covers  2  and  3  which are formed by straight side walls  24  and  34 , respectively. 
     The inner cover  2  also includes the mount flange  219 , the rounded corner  25 , and the bottom  26 . The gas inlets  242  are formed in the side wall  24  slightly above the rounded corner  25 . 
     The outer cover  3  also includes the mount flange  31 , the rounded corner  35 , and the bottom  36 . The gas inlets  341  and  342  are formed in the side wall  34  and shifted from the gas inlets  241  and  242  of the inner cover  2  toward the mount flange  31 . Specifically, the gas inlets  341  and  342  are in misalignment with the gas inlets  241  and  242  in a radius direction of the cover assembly  11 . The gas inlets  241 ,  242 ,  341 , and  342  are identical in size and shape with each other. 
     The installation of the cover assembly  11  may be achieved in a manner, as illustrated either in FIGS.  9 ( a ) or  9 ( b ). 
     In FIG.  9 ( a ), the mount flange  31  of the outer cover  3  is staked in an annular groove formed in the end wall  101  of the housing  10 . The mount flange  219  of the inner cover  2  is affixed to an inner wall of the outer cover  3 . 
     In FIG.  9 ( b ), the housing  10  has a boss  109 . The outer cover  3  is put on the boss  109  and installed at the mount flange  31  in the same manner as in FIG.  9 ( a ). The inner cover  2  is secured on the end surface  108  of the boss  109 . 
     In the case where the cover assembly  111  is installed on the housing  10  in the manner as illustrated in FIG.  9 ( a ), the volume V 1  of the inside of the cover assembly  11  (i.e. the gas chamber  112 ), as indicated by a hatched area in the drawing, defined by the inner cover  2  and the front end of the housing  10  is 846 mm 3 . The broken line P 1  indicates a plane extending over the front surface of the housing  10 . 
     In the case where the cover assembly  11  is installed on the housing in the manner as illustrated in FIG.  9 ( b ), the volume V 1  of the inside of the cover assembly  11 , as indicated by a hatched area, is 808 mm 3 . In this case, the volume V 1  is equivalent to the volume of the inner cover  2 . The broken line P 2  indicates a plane extending over the front surface (i.e., the end surface  108 ) of the housing  10 . 
     The structure of the cover assembly  11 , as illustrated either in FIGS.  9 ( a ) or  9 ( b ), results in an increased velocity of flow of the measurement gas in the lengthwise direction of the cover assembly  11  (i.e., the longitudinal direction in the drawing), thus improving the response rate of the gas sensor  1 . 
     The volume V 1  may lie within a range of 800 mm 3  to 1600 mm 3 , which results in a decrease in amount of water entering the inner cover  2  to avoid wetting of the gas sensor element  5 . If the volume V 1  is more than 1600 mm 3 , it will result in a decrease in ability to replace or freshen the measurement gas in the gas chamber  112 . 
     If the volume of the clearance  111  between the inner and outer covers  2  and  3  is defined as V 2 , it preferably meets a condition of V 2 /V 1 ≦0.25. This range provides two effects: one is to improve the response rate of the A/F sensor  30  resulting from a difference in flow velocity between the clearance  111  and the inside of the inner cover  2  increased by minimizing the volume V 2  of the clearance  111  and the other is to avoid wetting of the sensor element resulting from maximizing of the volume V 1  of the gas chamber  112 . Note that a lower limit of V 2 /V 1  is more than zero (0) since the volume V 2  of the clearance &gt;0. 
     Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
       FIG. 10  shows the protective cover assembly  11  according to the fourth embodiment of the invention. 
     The outer cover  3  is, like the above embodiments, circular in cross section, while the inner cover  2  is triangular in cross section. 
     If a circle having the same sectional area as that of the outer cover  3  is defined as S 1 , and a circle having the same sectional area of the inner cover  2  is defined as S 2 , the inner and outer covers  2  and  3  are so designed that a difference in radius between the circles S 1  and S 2  may be within a range of 0.2 mm to 0.6 mm, and preferably within a range of 0.2 mm to 0.55 mm (0.4 mm in this embodiment). The structure of the cover assembly  11  of this embodiment serves to facilitate evaporation of drops of water which have passed through the clearance  111  and stuck to the inner cover  2  by heat dissipation from the covers  2  and  3 , thereby minimizing wetting of the gas sensor element  4 . If the difference in radius between the circles S 1  and S 2  is less than 0.2 mm, it is difficult to machine the inner and outer covers  2  and  3  with required dimensional accuracy. Alternatively, if it is more than 0.6 mm, it results in reduction in ability of the cover assembly  11  to replace or freshen the measurement gas in the gas chamber  112 . 
     Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIGS.  11 ( a ) and  11 ( b ) show the outer cover  3  of the protective cover assembly  11  according to the fifth embodiment of the invention. The inner cover  2  is identical in structure with the one in the first embodiment. 
     The outer cover  3  has formed in a side wall thereof a plurality of gas inlets  345  each of which is formed by cutting a portion of the side wall to a square tab  346  and bending it at a non-cut side  347  thereof inwardly. All the square tabs  346 , as clearly shown in FIG.  11 ( a ), have the non-cut side  347  on the same side. 
     The square tabs  346  projecting inwardly of the outer cover  3  work to form a plurality of streams of the measurement gas in the same direction, thus facilitating ease of entrance of the measurement gas into the outer cover  3 . 
     The inner cover  2 , as illustrated in FIGS.  12 ( a ) and  12 ( b ), may alternatively have formed in a side wall thereof gas inlets  245  which are identical in shape with the gas inlets  345  of the outer cover  3 . Each of the gas inlets  245  is formed by cutting a portion of the side wall to a square tab  246  and bending it at a non-cut side  247  thereof inwardly. All the square tabs  246  of the inner cover  2  and the square tabs  346  of the outer cover  3  are preferably oriented either in a counterclockwise direction, as illustrated in FIG.  12 ( a ), or a clockwise direction, as illustrated in FIG.  12 ( b ), for forming streams of the measurement gas in the same direction to facilitate the ease of entrance of the measurement gas into the gas chamber  112 . 
     Instead of the square tabs  246  and  346 , circular or polygonal tabs may be used. 
     Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
       FIG. 13  is a graph which shows experimental test results indicating a change in frequency of an output of the gas sensor  1  of the first embodiment in response to a change in concentration of oxygen contained in exhaust gasses of the automotive engine for different values of the clearance  111  between the inner and outer covers  2  and  3  of the cover assembly  11 . Tests were performed by changing the concentration of oxygen of the exhaust gasses from lean to rich and from rich to lean side sequentially at a time when the output of the gas sensor  1  indicated 0.45V. The texts were made four times for each value of the clearances  111 . Maximum, minimum, and average values of outputs of the gas sensor  1  are plotted in the graph. 
     The graph shows that the frequency of the output of the gas sensor  1  is greatly decreased when the clearance  111  is less than 0.2 mm, and when the clearance  111  is within a range of 0.2 mm to 0.6 mm, it results in an increased velocity at which the exhaust gasses enter and go out of the gas chamber  112 , thereby increasing a response rate of the gas sensor  1 . If the clearance  111  is more than 0.6 mm, it results in a decreased difference in flow velocity between the inside of the outer cover  3  and the inside of the inner cover  2 , thus decreasing the response rate of the gas sensor  1 . 
       FIG. 14  shows the gas sensor  1  according to the sixth embodiment of the invention which is a modification of the one shown in FIG.  1 . 
     The inner and outer covers  2  and  3  have the straight walls  24  and  34  and the shoulders  23  and  33 , respectively. The gas inlets  241  and  242  of the inner cover  2  are in misalignment with the gas inlets  341  and  342  of the outer cover  3  in the radius direction of the cover assembly  11 . The gas inlets  241  and  242  are located closer to the housing  10  than the gas inlets  341  and  342 . 
     The shoulder  23  of the inner cover  2  and the shoulder  33  of the outer cover  3  are substantially flush with each other in the longitudinal direction of the cover assembly  11 . Other arrangements are substantially identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
       FIG. 15  shows the gas sensor  1  according to the seventh embodiment in which the cover assembly  11  identical in structure with the one in  FIG. 14  is installed on the gas sensor  1 , as illustrated in  FIG. 7 , equipped with the laminated sensor element  5 . Other arrangements are identical with those in  FIG. 7 , and explanation thereof in detail will be omitted here. 
     The structure of the cover assembly  11  of each of the gas sensors  1  in  FIGS. 14 and 15  works to provide two different flow paths A and B. The flow path A forms a flow of the measurement gas which is directed from one of the gas inlets  341  and  342  of the outer cover  3  to another of the gas inlets  341  and  342  through the clearance  111 . The flow path B forms a flow of the measurement gas which rises from one of the gas inlets  341  and  342  of the outer cover  3  while circulating through the clearance  111 , enters one of the gas inlets  241  and  242  of the inner cover  2 , and goes out of the gas outlets  261 . The gas flow along the flow path A serves to discharge drops of water heaver than the measurement gas out of the outer cover  3 , thereby minimizing the wetting of the gas sensor element. 
     While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.