Flow measuring device measuring air flow through a bypass passage throttled by a throttle portion

Abstract

A flow measuring device includes a housing including a bypass passage and a flow sensing chip located in the bypass passage and including a sensing surface portion. A throttle portion that is a part of a flowing passage wall facing a sensing surface portion throttles a cross-sectional area of a bypass passage. A position where the throttle portion starts is referred to as a start point position, and a position of the throttle portion where a distance between a gravity center of the sensing surface portion and the throttle portion is shortest is referred to as an end point position. The start point position and the end point position define an imagination line, and the imagination line and the flowing direction define an angle that is in a range from 0 degrees to 20 degrees.

What is claimed is: 
     
         1 . A flow measuring device comprising:
 a housing including a bypass passage that introduces a part of an air flowing through a duct; and   a flow sensing chip located in the bypass passage and including a sensing surface portion that generates an electrical signal in response to a flow volume of the air in the duct by a heat transmission between the sensing surface portion and the air flowing through the bypass passage, wherein   the bypass passage includes:
 a main-bypass passage introducing the air into the housing through an inlet and discharging the air from the housing through an outlet; and 
 a sub-bypass passage branching off from the main-bypass passage, the sub-bypass passage introducing a part of the air flowing through the main-bypass passage through a sub-bypass inlet and discharging the air from the housing through a sub-bypass outlet, 
   the flow sensing chip is disposed in the sub-bypass passage so that the sensing surface portion extends along a flowing direction in which the air flows through the sub-bypass passage,   the sub-bypass passage is throttled by a throttle portion such that a cross-sectional area of the sub-bypass passage perpendicular to the flowing direction decreases in accordance with a decrease in distance from a gravity center of the sensing surface portion to a center of the cross-sectional area in a direction parallel to the flowing direction,   the throttle portion is a part of a flowing passage wall facing the sensing surface portion, the throttle portion throttles the cross-sectional area of the bypass passage such that a distance from the sensing surface portion to the flowing passage wall at the cross-sectional area in a direction perpendicular to the sensing surface portion decreases in accordance with a decrease in distance from the gravity center of the sensing surface portion to the center of the cross-sectional area in the direction parallel to the flowing direction,   a position where the throttle portion starts is referred to as a start point position, and a position of the throttle portion where a distance between the gravity center of the sensing surface portion and the throttle portion is shortest is referred to as an end point position, and   the start point position and the end point position define an imagination line, and the imagination line and the flowing direction define an angle that is in a range from 0 degrees to 20 degrees.   
     
     
         2 . The flow measuring device according to  claim 1 , wherein
 the throttle portion includes a surface that is a planar surface or a curved surface.   
     
     
         3 . The flow measuring device according to  claim 1 , wherein
 another throttle portion that is different from the throttle portion is disposed in the sub-bypass passage.   
     
     
         4 . The flow measuring device according to  claim 1 , wherein
 the throttle portion extends in the direction parallel to the flowing direction, the throttle portion includes a first part located upstream of the gravity center of the sensing surface portion in the flowing direction and a second part located downstream of the gravity center of the sensing surface portion in the flowing direction.   
     
     
         5 . A flow measuring device comprising:
 a housing including a bypass passage that introduces a part of an air flowing through a duct; and   a flow sensing chip located in the bypass passage and including a sensing surface portion that generates an electrical signal in response to a flow volume of the air in the duct, wherein   the bypass passage includes:
 a main-bypass passage introducing the air into the housing through an inlet and discharging the air from the housing through an outlet; and 
 a sub-bypass passage branching off from the main-bypass passage, the sub-bypass passage introducing a part of the air flowing through the main-bypass passage through a sub-bypass inlet and discharging the air from the housing through a sub-bypass outlet, 
   the sub-bypass passage is throttled by a throttle portion such that a cross-sectional area of the sub-bypass passage perpendicular to a flowing direction in which the air flows through the sub-bypass passage decreases in accordance with a decrease in distance from a gravity center of the sensing surface portion to a center of the cross-sectional area in a direction parallel to the flowing direction,   the flow sensing chip is disposed in the sub-bypass passage and the throttle portion is a part of a flowing passage wall facing the sensing surface portion,   a position where the throttle portion starts is referred to as a start point position, the start point position is located upstream of the sensing surface portion in the flowing direction, and   the throttle portion includes a surface that is a planar surface, the surface of the throttle portion and the flowing direction define an angle that is in a range from 0 degrees to 20 degrees.   
     
     
         6 . A flow measuring device comprising:
 a housing including a bypass passage that introduces a part of an air flowing through a duct along a main-flow direction; and   a flow sensing chip located in the bypass passage and including a sensing surface portion that generates an electrical signal in response to a flow volume of the air in the duct by a heat transmission between the sensing surface portion and the air flowing through the bypass passage, wherein   the bypass passage includes:
 a main-bypass passage introducing the air into the housing through an inlet and discharging the air from the housing through an outlet; and 
 a sub-bypass passage branching off from the main-bypass passage, the sub-bypass passage introducing a part of the air flowing through the main-bypass passage through a sub-bypass inlet and discharging the air from the housing through a sub-bypass outlet, 
   the flow sensing chip is disposed in the sub-bypass passage so that the sensing surface portion extends along a flowing direction in which the air flows through the sub-bypass passage,   the sub-bypass passage is throttled by a throttle portion such that a cross-sectional area of the sub-bypass passage perpendicular to the flowing direction decreases in accordance with a decrease in distance from a gravity center of the sensing surface portion to a center of the cross-sectional area in a direction parallel to the flowing direction,   the throttle portion is a part of a flowing passage wall that is included in the housing and faces the sensing surface portion, the throttle portion throttles the cross-sectional area of the sub-bypass passage such that a distance from the sensing surface portion to the flowing passage wall at the cross-sectional area in a direction perpendicular to the sensing surface portion decreases in accordance with a decrease in distance from the gravity center of the sensing surface portion to the center of the cross-sectional area in the direction parallel to the flowing direction,   a position where the throttle portion starts is referred to as a start point position, and a position of the throttle portion where a distance between the gravity center of the sensing surface portion and the throttle portion is shortest is referred to as an end point position,   the start point position and the end point position define an imagination line, and the imagination line and the flowing direction define an angle that is in a range from 0 degrees to 30 degrees,   the sub-bypass outlet of the sub-bypass passage discharges the air that passed through the sensing surface portion into the duct from the sub-bypass passage, and   the sub-bypass outlet opens along the main-flow direction.   
     
     
         7 . The flow measuring device according to  claim 6 , wherein
 the throttle portion is an upstream throttle portion that is located at a position in the sub-bypass passage upstream of the sensing surface portion and extends in the flowing direction to throttle the sub-bypass passage, and   the sensing surface portion includes a center part that is located between the start point position and the end point position in the flowing direction.   
     
     
         8 . The flow measuring device according to  claim 7 , further comprising:
 a support portion being a plate shape, the support portion supporting the sensing surface portion, the support portion extending to a position upstream of the throttle portion.   
     
     
         9 . The flow measuring device according to  claim 6 , wherein
 a distance from the end point position to a support portion supporting the sensing surface portion is shorter than a distance from the start point position to the end point position, in an orthogonal direction perpendicular to the flowing direction.   
     
     
         10 . The flow measuring device according to  claim 6 , wherein
 the flowing passage wall is a first flowing passage wall, the housing includes a second flowing passage wall facing the first flowing passage wall, a support portion supporting the sensing surface portion is located between the first flowing passage wall and the second flowing passage wall,   the sensing surface portion is located on a surface of the support portion facing the first flowing passage wall,   a distance from the start point position of the first flowing passage wall to the support portion is shorter than a distance from the support portion to the second flowing passage wall.   
     
     
         11 . The flow measuring device according to  claim 1 , wherein
 the sub-bypass inlet of the sub-bypass passage opens in a direction angled relative to a direction along which the air flows through the main-bypass passage.   
     
     
         12 . The flow measuring device according to  claim 1 , wherein
 the sub-bypass passage circumferentially extends inside the housing.   
     
     
         13 . The flow measuring device according to  claim 5 , wherein
 the sub-bypass inlet of the sub-bypass passage opens in a direction angled relative to a direction along which the air flows through the main-bypass passage.   
     
     
         14 . The flow measuring device according to  claim 5 , wherein
 the sub-bypass passage circumferentially extends inside the housing.   
     
     
         15 . The flow measuring device according to  claim 6 , wherein
 the sub-bypass inlet of the sub-bypass passage opens in a direction angled relative to a direction along which the air flows through the main-bypass passage.   
     
     
         16 . The flow measuring device according to  claim 6 , wherein
 the sub-bypass passage circumferentially extends inside the housing.