Thermo-type air flow meter

Abstract: An axial flow thermo-type air flow meter for internal combustion engines has a spider (3, 3A, 3C, 3D, 3E, 3F, 3G, 3I) disposed in a main air passage (2) formed in a body (1) and having formed therein an auxiliary air flow passage (4) including an axial auxiliary air flow passage portion (4b, 40 , 41-47) accommodating a heat-generating resistor element (7) for detecting a rate of engine intake air flow. The auxiliary air flow passage also includes more than one radial air flow passage portions (4e, 4f, 40a, 40b, 40c, 41a, 42a, 43a, 44a, 45a, 46a, 47a) which are connected to the axial air flow passage portion and curved and turned at their radially outer ends and then extend radially inwardly to outlet ends open to the main air passage, whereby the total length of the auxiliary air flow passage is increased to decrease the influence of pulsation of engine intake air flow on the air flow meter. Positioning of the outlet openings of the auxiliary air flow passage within a range of a radius from the axis of the main air flow passage, which radius is 1/2 of the radius of the main air flow passage, is effective to reduce fluctuation of the electric output of the air flow meter due to an unbalanced air flow velocity distribution occurring upstream of the air flow meter.

Description: The present invention relates to a thermo-type air flow meter, and more particularly, to an axial flow thermo-type air flow meter suitably used to measure the rate of intake air flow in internal combustion engines for automobiles, the air flow meter being of the type in which an auxiliary air flow passage with a heat generating resistor element for detecting the rate of engine intake air flow accommodated therein is disposed within the main air flow passage. Thermo-type (hot-wire type) air flow meters have been proposed in, for example, JP-A-57-64109 and 61-65053. These thermo-JP-A-57-64109 and 61-65053. These thermo-type air flow meters each have an auxiliary air flow passage which extends in a direction substantially perpendicular to the main air flow passage and which has a portion for changing the direction of air flow by 180°. In the air flow meter disclosed in JP-A- 57-64109, a radial auxiliary air flow passage is separated into two portions by a partitioning wall-extending in the radial direction, and the connecting portion of these two separate portions is formed by an axial air flow passage. As the air flow passage is separated into two parts by the partitioning plate, the structure of the air flow meter mounted on an engine is complicated. Furthermore, since the corner portion of the air flow passage which changes the direction of air flow by 180° is made of a thin plate, the pressure loss in the air flow passage is increased, thus decreasing the air flow velocity and increasing changes in the air flow velocity, which results in noises in the output of the air flow meter. In the air flow meter disclosed in JP-A-61-65053, the direction of the air flow in the auxiliary air flow passage is changed by 180° in total by using two right-angled elbow tubes each of which changes the direction of the air flow passage by 90°. In this example, the air flow passage is not formed in the same plane, as in the former type of air flow meter. Consequently, the structure of the air flow meter mounted on the engine is complicated. Furthermore, the pressure loss at the right-angled elbow tube portions is large, and the same problems as those of the former type thus occur. JP-A-1-206223 discloses an axial thermo-type air flow meter in which the auxiliary flow passage is formed within the main air flow passage. In this air flow meter, the auxiliary air flow passage includes an axial auxiliary air flow passage portion having an inlet open to the main air flow passage and extending parallel to the main air flow passage, and two radial auxiliary air flow passage portions formed in a straight line in directions perpendicular to the direction of the main air flow passage and in the radial directions and respectively having outlets open to the main air flow passage in the vicinity of the peripheral wall of the main air flow passage. A hot-wire element is disposed within the axial auxiliary air flow passage portion. US-A-4,709,581 discloses a thermo-type flow meter in which an auxiliary air flow passage is located in the main air flow passage. The auxiliary air flow passage comprises an air inlet, a first or sensor mounting passage portion extending parallel to the main air flow passage, a second passage portion extending in a direction perpendicular to the first portion, a third passage portion extending substantially in a direction perpendicular to the second portion, and a fourth passage portion extending substantially in a direction perpendicular to the third portion, and an outlet opening to the main air flow passage. In accordance with this construction, the second, third and fourth passage portions of the auxiliary air flow passage are formed in a plane parallel to the axis of the main air flow passage. An air flow meter comprising an auxiliary air flow passage portion which extends in a direction perpendicular to the main air flow passage and is curved by more than 180° is described in EP-A-87,621. The auxiliary air flow passsage is composed of a rectilinear portion parallel to the main passage in which the hot wire type resistor is provided and a curved portion provided in the circumferential part of the main air flow passage. EP-A-295,647 discloses a thermo-type air flow meter comprising an auxiliary air flow passage consisting of an axial inlet portion provided in the main air flow passage and comprising the sensor arrangement, a portion extending in a direction perpendicular to the main air flow passage, and two curved portions bent by about 180° and carrying the outlet openings of the auxiliary air flow passage which are also extending in a direction perpendicular to the axis of the main air flow passage. In this prior art embodiment, however, the curved portions are provided within the wall of the main air flow passage. In the four cylinder engines, the two-value phenomenon of the average output voltage (inversion of the output) of the air flow meter caused by pulsations in the engine intake air flow are a serious problem. Occurrence of this phenomon may be reduced by attenuating the magnitude of the pulsations which is achieved by increasing the total length of the auxiliary air flow passage. However, in the conventional axial thermo-type air flow meters, it is not always possible to provide a sufficiently long radial auxiliary air flow passage portion because the provision of such a passage may be hindered due to the specification of the engine or the configuration of the tube of the air intake system. A primary object of the present invention is to provide a thermo-type air flow meter which is less affected by pulsations of the engine intake air flow which occur downstream of the meter. Upstream of the air flow meter are disposed an air cleaner and a duct which may be the cause of the occurrence of separation and swirled air flow. Consequently, an air flow velocity distribution in which the air flow velocities are varying at the radial and circumferential positions of the main air flow passage, that is, an unbalanced air flow velocity distribution, occurs on the upstream side of the air flow meter due to the separation or swirled air flow. This unbalanced air flow velocity distribution pattern is variable depending on the position and shape of the air cleaner and the duct. Conventional thermo-type air flow meters in which the outlets of two radial auxiliary air flow passage portions are open in the vicinity of the peripheral wall of the main air flow passage are readily affected by changes in such an unbalanced air flow velocity distribution, and their measured values thus change as the unbalanced air flow distribution changes. In the design of the passages of the air intake system, the position and shape of the air cleaner and the duct vary depending on the type of vehicle. Therefore, the provision of an air flow meter which is readily affected by changes in the air flow velocity distribution requires adjustment for an accurate measurement of the air flow rate. Such air flow meters cannot be generally used without specific adjustment. A second object of the present invention is to provide a thermo-type air flow meter which is less affected by changes in an unbalanced air flow velocity distribution which occurs upstream of the air flow meter. To achieve the above-described objects, the present invention provides an axial flow thermo-type air flow meter as claimed in claim 1. The dependent claims relate to prefered embodiments. In a preferred form, the inlet opening of the axial auxiliary air flow passage portion is located at the center of the main air flow passage, and the radial auxiliary air flow passage portion is formed by a single air flow passage portion connected to the axial auxiliary air flow passage portion and either a plurality of air flow passage portions which branch from the single air flow passage portion and extend to the respective outlet openings or a single air flow passage portion which extends to the outlet opening thereof. In another preferred form, the outlet opening of the radial auxiliary air flow passage portion is located within a range of a radius from the axis of the main air flow passage, which radius is 1/2 of the radius of the main air flow passage. In a further preferred form, the radial auxiliary air flow passage portion is formed in the radial wall portion such that the outlet opening thereof is located at the central portion of the main air flow passage. In the present invention, the radial auxiliary air flow passage portion is curved at least at one portion thereof in a single plane substantially perpendicular to the main air flow passage so that it changes the direction of the air flow by about 180°. Consequently, the length of the radial auxiliary air flow passage portion can be increased, thereby reducing pulsations sufficiently. Furthermore, since the air flow passage is formed in a single plane, the structure of the air flow meter can be simplified. Furthermore, if the portion for changing the direction of the air flow by 180° is smoothly curved in order to achieve a reduction in the pressure loss, a reduction in the air flow velocity will be prevented, and the generation of noises in the output due to changes in time cased by separated air flow which is generated at the corner portion of the curved portion will also be reduced. In consequence, the output of the air flow meter can be made stable, and the two-value phenomena due to pulsations in the engine intake air flow will not be readily generated. Changes in the unbalanced air flow velocity distribution which occur upstream of the air flow meter due to changes in the position and the shape of the air cleaner and the duct are larger in the peripheral portion of the main air flow passage than at the central portion thereof. Thus, positioning of the inlet and outlet openings of the auxiliary air flow passage within a range of a radius from the axis of the main air flow passage, which radius is 1/2 of the radius of the main air flow passage, will be effective to reduce the influence of changes in the unbalanced air flow velocity distribution. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an end view, taken in the direction indicated by the arrow I in Fig. 2, of a first embodiment of a thermo type air flow meter according to the present invention, showing the state thereof in which a passage cover is removed; Fig. 2 is a sectional view of the thermo-type air flow meter taken along the line II - II of Fig. 1; Fig. 3 shows the positional relation of the outlet openings of the auxiliary air flow passage with the main air flow passage; Figs. 4A to 4D respectively show different unbalanced air flow velocity distribution patterns to illustrate changes of such air flow velocity distributions and their influence on the air flow meter; Fig. 5 shows the two-value phenomenon; Fig. 6 is an end view similar to Fig. 1, but showing a second embodiment of the air flow meter according to the present invention; and Figs. 7 to 13 are end views similar to Fig. 1 but showing third to ninth embodiments of the air flow meter according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to Figs. 1 to 5. Referring first to Figs. 1 and 2, a body 1 made by aluminum die casting comprises the main air flow passage 2. In the main air flow passage 2, a radial wall portion (spider) 3 formed integrally with the body 1 is disposed across the main air flow passage 2 in the radial direction thereof. The radial wall portion 3 has in it an auxiliary air flow passage 4 through which a part of the intake air passes. The auxiliary air flow passage 4 includes an inlet opening 4a open to-the central portion of the main air flow passage 2, an axial auxiliary passage portion (first portion) 4b connected to the opening 4a and extending parallel to the axis of the main air flow passage 2, and two radial auxiliary passage portions (second portions) 4e and 4f extending in directions perpendicular to the main air flow passage 2 and in the radial directions, which are separated from each other by 180°, and respectively having outlet openings 4c and 4d open to the main air flow passage 2. The radial auxiliary passage portions 4e and 4f each are in the form of a groove which opens to the end face of the radial wall portion 3. The grooves which form the radial auxiliary passage portions 4e and 4f are closed by a passage cover 5 which is fixed to the end face of the radial wall portion 3 by means of attaching screws 6. Fig. 1 shows the state in which the passage cover 5 is removed. Reference numerals 6a denote screw holes into which the attaching screws 6 are screwed. The two radial auxiliary passage portions 4e and 4f respectively have substantially semi-circular, smoothly curved portions 4g and 4h which change the directions of air flows by substantially 180°. The outlet openings 4c and 4d of these radial auxiliary passage portions 4e and 4f are each located within the range of a radius from the axis of the main air flow passage which radius is 1/2 of the a radius 'd' of the main air flow passage, as shown in Fig. 3. The radial wall portion 3 having the two radial auxiliary air flow passage portions 4e and 4f divides the main air flow passage 2 having the circular cross-section into two main air flow passage portions 2a and 2b. The outlet openings 4c and 4d of the radial auxiliary air flow passage portions 4e and 4f are respectively open to the main air flow passage portions 2a and 2b. Within the axial auxiliary air flow passage portion 4b are disposed a heat-generating resistor element 7 and a temperature resistor element 8 for detecting the rate of engine intake air flow An electronic control module 9 for converting the rate of engine intake air flow detected by the resistor elements 7 and 8 into an electric signal is mounted on the outer surface of the body 1. After air enters the body 1, it flows into both the main air flow passage 2 and the auxiliary air flow passage 4, as indicated by arrows in Fig. 2. The air which flows into the auxiliary air flow passage 4 flows first through the axial auxiliary air flow passage portion 4b where it comes into contact with the heat-generating resistor element 7 and the temperature resistor element 8 and then flows into both the radial auxiliary air flow passage portions 4d and 4f. In these radial auxiliary air flow passage portions 4d and 4f, air flows first 5 outwardly, then passes through the curved portions 4g and 4h which are curved by about 180° near the periphera wall of the main air flow passage 2, then flows inwardly in the radial direction and finally flows out of the auxiliary air flow passage from the outlet openings 4c and 4d into the main air flow passage portions 2a and 2b at the central portion if the main air flow passage 2. It is to be noted that the body 1 may also be made of materials other than aluminum, such as plastics. In the hot-wire type air flow meter designed in the manner described above, if it is assumed that the pressure loss of the auxiliary air flow passage 4 is equal to the pressure loss of the main air flow passage 2 between the inlet and outlet openings of the auxiliary air flow passage 4, the relation between the main air flow passage 2 and the auxiliary air flow passage 4 is expressed as follows: Lm dU dt + Cm U 2 2 = lb du dt + Cb u 2 2 Lm: The equivalent length of the main air flow passage U: The air flow velocity in the main air flow passage Cm: The resistance coefficient of the main air flow passage lb: The equivalent length of the auxiliary air flow passage u: The air flow velocity in the auxiliary air flow passage Cb: The resistance coefficient of the auxiliary air flow passage. Therefore, using Cm = U 2 / 2= Cb U 2 / 2, the dynamic relation between the velocity of the main air flow and the velocity of the auxiliary air flow is expressed as follows: Lm = dU dt = lb du dt . Thus, du = dU Lm lb . Thus, it is possible to reduce the pulsating changes in the velocity of the auxiliary air flow relative to dynamic pulsating changes in the velocity of the main air flow by increasing the overall length 1b of the auxiliary air flow passage relative to the length Lm of the main air flow passage between the inlet and outlet openings of the auxiliary air flow passage. In the four-cylinder engines, the two-value phenomenon of the output voltage of the air flow meter caused due to pulsations in the engine intake air is a serious problem. More specifically, as the boost pressure (vacuum) is increased by increasing the opening of the throttle valve, the air flow rate detected by the air flow meter increases. However, when the boost pressure is increased to a value higher than a certain level, which is achieved by opening the throttle valve almost fully, the air flow rate detected by the air flow meter decreases, as shown in Fig. 5. This means that there are two boost pressures (openings) corresponding to the airflow rate detected when the boost pressure is higher than the certain level. Hence, the control unit cannot determine the corresponding operational condition when it receives electric signals representing those two values. This is the two-value phenomenon. Since an increase in lb with respect to Lm is effective to decrease pulsations in the auxiliary air flow passage, as described above, the increase in lb is very effective for preventing the two-value phenomenon. In the present embodiment, in order to realize the structure in which the outlet openings 4c and 4d of the radial auxiliary air flow passage portions 4e and 4f are each located substantially at the central portion of the main air flow passage, the curved portions 4g and 4h for changing the directions of air flows by substantially 180° are respectively provided in the radial auxiliary air flow passage portions 4e and 4f. In this way, the length of each of the radial auxiliary air flow passage portions 4e and 4f can be made greater than the radius of the main-air flow passage 2. Particularly, since the outlet openings 4c and 4d of the radial auxiliary air flow passage portions 4e and 4f are located within the range of 1/2 of the radius 'd' of the main air flow passage in this embodiment, the length of each of the radial auxiliary air flow passage portions 4e and 4f can be made about two times that of the radius of the main air flow passage 2. As a result, pulsations in the intake air flow in the auxiliary air flow passage can be reduced, to thereby decrease the occurrence of the two-value phenomenon caused by the pulsations in the engine intake air flow. Next, the static relation between the velocity of the main air flow and the velocity of the auxiliary air flow under steady state conditions is expressed as follows: Lm = dU dt = lb du dt = 0 Therefore, Cm U 2 2 = Cb u 2 2 . Hence, u=U (Cm/Cb) So, it is apparent that the velocity of the auxiliary air flow is dependent upon the velocity of the main air flow. As stated above, the distribution of the air flow velocity on the upstream side of the air flow meter changes in accordance with the structure of the air cleaner and the shape of the duct. Figs. 4A to 4D show examples of the air flow velocity distribution pattern. In the pattern shown in Fig. 4A, the velocity of air flow is substantially uniform in all the radial and circumferential positions. In the patterns shown in Figs. 4B to 4D, the velocity of air flow is distributed non-uniformly in the radial and circumferential directions. Similar air flow velocity distributions occur in the vicinity of the outlet openings of the auxiliary air flow passage. As is clear from Figs. 4A to 4D, the changes in the air flow velocity caused in accordance with the changes in an unbalanced air flow distribution pattern are greater in the vicinity of the peripheral wall which defines the body 1 than in the central portion of the main air flow passage. When the conventional air flow meter shown in Fig. 4A in which outlet openings 10a and 10b of the auxiliary air flow passage are provided in the vicinity of the peripheral wall of the main flow passage is subjected to the above-described changes in the air flow velocity distribution, the air flow velocity 'u' in the auxiliary air flow passage easily changes because of relatively large changes in the air flow velocity 'U' in the main air flow passage in the vicinity of the wall. Particularly, this tendency increases in the case in which the auxiliary air flow passage has a single outlet opening. Even when the auxiliary air flow passage has two outlet openings, as shown in Fig. 5, if the air flows in a distribution pattern unbalanced to one side of the center of the main air flow passage, as in the cases shown in Figs. 4B and 4C, the changes in the passage resistance are greater than in the cases shown in Figs. 4A and 4D in which the air flows at both sides of the center, thus readily generating changes in the air flow velocity. In the air flow meter, changes in the air flow velocity in the vicinity of the center of the main air flow passage are smaller than those in the vicinity of the peripheral wall except for special cases. Therefore, the positioning of the outlet openings 4c and 4d of the radial auxiliary air flow passage portions 4e and 4f in the central portion of the main air flow passage 2, as in the described embodiment, is effective to decrease fluctuation of the electric output of the air flow meter which occurs when the distribution of the air flow velocity (the unbalanced air flow) changes in the vicinity of the inlet opening. In this embodiment, since not only the inlet opening 4a of the auxiliary air flow passage 4 but also the outlet openings 4c and 4d thereof are disposed at the central portion of the main air flow passage 2, the air flow meter is less affected by changes in the air flow velocity distribution, and can thus be used in air intake systems designed in various manners. The present inventors have confirmed through experiments that the aforementioned advantages can be similarly assured when the inlet and outlet openings of the auxiliary air flow passage 4 are located within the range of a radius from the central axis of the main air flow passage which radius is 1/2 of the radius of the main air flow passage. A second embodiment of the present invention will be described below with reference to Fig. 6 which is a view similar to that of Fig. 1 and shows the state in which the passage cover is removed. In this embodiment, three radial auxiliary air flow passage portions are formed in different radial directions. In the structure shown in Fig. 6, the radial wall portion (spider) 3A has a shape which divides the main air flow passage 2 into three portions. The spider 3A has in it an axial auxiliary air flow passage portion 40 whose inlet opening is located at the central portion of the main air flow passage 2, and three radial auxiliary air flow passage portions 40a, 40b and 40c having the same form as that of the radial auxiliary air flow passage portions 4e and 4f in the first embodiment. That is, the radial auxiliary air flow passage portions 40a, 40b and 40c respectively have curved portions 40g, 40h and 40i for changing the directions of the air flow by about 180°, and the outlet openings 40d, 40e and 40f thereof are located at the central portion of the main air flow passage 2. The afore-mentioned advantages of the first embodiment can also be assured in the second embodiment. The radial auxiliary air flow passage portions may also be provided in a number of four or more. Other embodiments of the present invention will be described with reference to Figs. 7 to 9 which are views similar to that of Fig. 1 and show air flow meters with removed passage covers. In the third embodiment shown in Fig. 7, a cantilever type radial wall portion 3B has an axial auxiliary air flow passage portion 41 whose inlet opening is located at the central portion of the main air flow passage 2, and one radial auxiliary air flow passage portion 41a having the same form as that of the radial auxiliary air flow passage portions 4e and 4f in the first embodiment. That is, the radial auxiliary air flow passage portion 41a has a curved portion 41c for changing the direction of the air flow by about 180°, and the outlet opening 41b thereof is located at the central portion of the main air flow passage 2. The afore-mentioned advantages of the first embodiment can also be assured in this third embodiment. Furthermore, in this embodiment, since the radial wall portion 3B is of the cantilever type, the resistance to the air flow in the main air flow passage is reduced, thereby decreasing the pressure loss. Furthermore, in this embodiment, a sensor unit consisting of the radial wall portion 3B formed separately from the body 1 and an integral electronic control module mounted on the radial wall portion 3B, may be detachably mounted on the body 1. In this way, the structure of the body 1 which defines the main air flow passage is simplified. Furthermore, since the sensor unit can be handled as a single unit, initial adjustment and maintenance of the electronic control module are facilitated. In the fourth embodiment shown in Fig. 8, a radial wall portion 3C is disposed across the main air flow passage 2, like the radial wall portion 3 shown in Fig. 1. This radial wall portion 3C comprises an axial auxiliary air flow passage portion 42 whose inlet opening is located at the central portion of the main air flow passage 2, and a radial auxiliary air flow passage portion 42a having a single outlet opening 42b at the central portion of the main air flow passage 2 and two curved portions 42c and 42d for respectively changing the direction of air flow by about 180°. Consequently, the length of the auxiliary air flow passage is increased to about four times that of the radius of the main air flow passage. In this embodiment, since the length of the auxiliary air flow passage is further increased, the two-value phenomenon caused by pulsations of the engine intake air flow downstream of the air flow meter can be further reduced. This embodiment is particularly effective when it is applied to four-cylinder engines whose intake air flow pulsates to a large degree. In the fifth embodiment shown in Fig. 9, the radial wall portion 3D is of the cantilever type, as in the case of the third embodiment shown in Fig. 7. It comprises an axial auxiliary air flow passage portion 43 whose inlet opening is located at the central portion of the main air flow passage 2, and a composite radial auxiliary air flow passage portion 43e consisting of a single radial air flow passage portion 43a connected to the axial auxiliary air flow passage portion 43 and two divided air flow passage portions 43b and 43c branching from the radial air flow passage portion 43a. The air flow passage portions 43b and 43c of the radial auxiliary air flow passage portion 43e respectively have outlet openings 43f and 43g at the central portion of the main air flow passage. The portions of the divided air flow passage portions 43b and 43c which are branched from the single air flow passage portion 43a are curved to form curved portions 43h and 43i for changing the direction of the air flow by about 180°. In this embodiment, since the length of the radial auxiliary air flow passage portion is further increased as compared with the embodiment shown in Fig. 7, the occurrence of the two-value phenomenon can be further reduced. Furthermore, it is possible to form a sensor unit that can be detachably mounted on the body 1 by the radial wall portion 3D and the electronic control module 9, as in the case of the embodiment shown in Fig. 7. Such a structure facilitates the initial adjustment and the maintenance of the electronic control module. Sixth to ninth embodiments of the present invention will be described with reference to Figs. 10 to 13. In the sixth embodiment shown in Fig. 10, the radial wall portion 3E is disposed across the main air flow passage 2, like the radial wall portion 3 shown in Fig. 1. The radial wall portion 3E comprises an axial auxiliary air flow passage portion 44 whose inlet is located at the central portion of the main air flow passage 2, and a single radial auxiliary air flow passage portion 44a having a curved portion 44c for changing the direction of the air flow by about 180°. The outlet opening 44b of the radial auxiliary air flow passage portion 44a is located at a position offset from the axis of the main air flow passage 2. In the seventh embodiment of the present invention shown in Fig. 11, the radial wall portion 3F is disposed across the main air flow passage 2. The radial wall portion 3F comprises an axial auxiliary air flow passage portion 45 whose inlet is located at a position offset from the axis of the main air flow passage 2, and a single radial auxiliary air flow passage portion 45a having a curved portion 45c for changing the direction of the air flow by 180°. The outlet opening 45b of the radial auxiliary air flow passage portion 45a is also located at a position offset from the axis of the main air flow passage. In the eighth embodiment shown in Fig. 12, the radial wall portion 3G is disposed across the main air flow passage 2, like the radial wall portion 3 shown in Fig, 1. The radial wall portion 3G comprises an axial auxiliary air flow passage portion 46 whose inlet opening is located at a position offset from the axis of the main air flow passage 2, and a single radial auxiliary air flow passage portion 46a having a curved portion 46c for changing the direction of air flow by about 180°. The outlet opening 46b of the radial auxiliary air flow passage 46a is located at the central portion of the main air flow passage 2. Consequently, the length of the auxiliary air flow passage can be increased to about three times that of the radius of the main air flow passage. In the ninth embodiment shown in Fig. 13, the radial wall portion 3I is disposed across the main air flow passage 2, like the radial wall portion 3G shown in Fig. 12. This radial wall portion 3G comprises an axial auxiliary air flow passage portion 47 whose inlet opening is located at a position offset from the axis of the main air flow passage 2, and a single composite radial auxiliary air flow passage portion 47e consisting of a single radial air flow passage portion 47a connected to the axial auxiliary air flow passage portion 47, and two air flow passage portions 47b and 47c branching from the air flow passage portion 47a, as in the case of the embodiment shown in Fig. 9. The air flow passage portions 47b and 47c respectively have outlet openings 47f and 47g at the central portion of the main air flow passage. The portions of the air flow passage portions 47b and 47c which branch from the air flow passage portion 47a are curved to form curved portions 47h and 47i for changing the direction of the air flow by about 180°. In each of the embodiments shown in Figs. 10 to 13, the length of the auxiliary air flow passage is increased, as in the preceding embodiments. Consequently, the two-value phenomenon caused by pulsations in the intake air flow which occur downstream of the air flow meter can be prevented very effectively. In the embodiments shown in Figs. 11 to 13, the inlet opening of each of the axial auxiliary air flow passage portions 45, 46 and 47 is offset from the center of the main air flow passage. Thus, the inlet opening may have an elliptical form in order to enhance the stability thereof against unbalanced air flow. As will be understood from the foregoing description, it is possible according to the present invention to provide a thermo-type air flow meter which is less affected by changes in the unbalanced air flow distribution which occurs upstream of the radial wall portion and which can be applied to air intake systems designed in various manners. It is also possible to provide a thermo-type air flow meter which reduces the occurrence of the two-value phenomenon caused by pulsations in the engine intake air flow and which is hence highly reliable.

Claims: Axial flow thermo-type air flow meter comprising a body (1) having formed therein a main air flow passage (2) through which the intake air flows, a radial wall portion (3) being disposed in the main air flow passage (2), an auxiliary air flow passage (4) which is formed in the radial wall portion (3) and accommodates a heat generating resistor element (7) and a temperature resistor element (8) for detecting the rate of engine intake air flow, said auxiliary air flow passage comprising (a) a first portion (4b) having an inlet opening (4a) open to the main air flow passage (2) and extending parallel to the axis of the main air flow passage (2) in which the resistor elements (7, 8) are disposed, (b) at least one second portion (4f) extending in a direction substantially perpendicular to the main air flow passage (2), and (c) an outlet opening (4d) open to the main air flow passage (2), the second portion (4f) of said auxiliary air flow passage comprises the following three sections along its length: (A) a first section (4j; 43a; 47a) extending from the downstream end of said first portion (4b) to a point adjacent to the inner wall of the body (1), (B) a second section (4h;43h;47h;42c) arranged adjacent to the inner wall of the body (1) forming an approximately 180° bend in said second portion, and (C) a third section (4k;43b;47b) extending parallel to the first section and directed inwardly towards the central region of the main air flow passage (2), and connected to said outlet opening, and all portions of the auxiliary air flow passage (4) being disposed within the main air flow passage (2). characterised in that said first, second and third sections are formed with their central axes in one single plane perpendicular to the axis of the main air flow passage (2). Air flow meter according to claim 1, characterized in that the first portion (4b) of said auxiliary air flow passage is located at the center of the main air flow passage (2). Air flow meter according to claim 1, characterized in that the first portion (4b) of said auxiliary air flow passage is located at a position offset from the center of the main air flow passage (2). Air flow meter according to claim 1 or 2, characterized in that the auxiliary air flow passage (4) comprises a plurality of second portions extending in a direction perpendicular to the main air flow passage (2). Air flow meter according to claim 1, 2 or 4, characterized in that the auxiliary air flow passage (4) comprises two second portions (4e, 4f) extending in a direction perpendicular to the main air flow passage (2). Air flow meter according to claim 1, 2 or 4, characterized in that the auxiliary air flow passage (4) comprises three second portions (40a, 40b, 40c) extending in a direction perpendicular to the main air flow passage (2). An axial flow thermo-type air flow meter according to claim 2 or 3, characterized in that said second section adjacent to the inner wall of the main air flow passage (2) is branched into two curved parts (43h; 43i; 47h; 47i) so as to form said approximately 180° bend and a second approximately 180° bend, and a further section (43c,47c) of said second portion adjoins said second approximately 180° bend and extends parallel to the first section (43a; 47a). Air flow meter according to one of claims 1, 2 and 4 to 7, characterized in that the outlet opening (4c, 4d) of the auxiliary air flow passage (4) is located within a range which is delimited by a circle being arranged coaxially with the axis of the main air flow passage (2) and having a radius which is one half of the radius of the main air flow passage (2). Air flow meter according to claim 8, characterized in that both the inlet opening (4a) and the outlet opening (4d) of the auxiliary air flow path (4) are arranged within the range delimited by said circle. Air flow meter according to claim 2, characterized in that said second portion of said auxiliary air flow passage (4a) comprises a third section extending across said main passage and a further approximately 180° bend (42d) adjacent the opposite liner wall of the body, a further section being connected to said bend being directed towards the central region of the main passage and connected to the outlet opening (42b).

Inventors: ARAI NOBUKATSU, KOBAYASHI CHIHIRO, MIYAZAKI ATSUSHI, ARAI, NOBUKATSU, KOBAYASHI, CHIHIRO, MIYAZAKI, ATSUSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: G01F1
IPC codes: G01F1, G01F5