Hot-wire type air flow meter.

Abstract: A compact hot-wire type air flow meter for an internal combustion engine with a high measuring accuracy is provided with a primary flow path(3)forming an intake air passage and an auxiliary flow path(4)incorpo­ rating therein a hot-wire element(10)for measuring the intake air. The auxiliary flow path(4)is defined by a flowpath(4a)in an axial direction of the primary flow path(3)and a flow path(5)in a radial direction of the primary flow path(3).

Description: BACKGROUND OF THE INVENTION The present invention relates to a hot-wire type air flow meter, and more particularly to a hot-wire type flow meter for an automotive internal combustion engine, which constitutes an intake system of the internal combustion engine, and adapted to detect and control an flow rate of intake air. As shown in, for example, Japanese Utility Model Unexamined Publication 56-135127 and Japanese Patent Unexamined Publication 60-185118, there has been provided a conventional passage structure for a hot-wire type air flow meter for an automotive engine, in which an auxiliary flow path is formed in an intake pipe; a hot-wire element is arranged in the auxiliary flow path; an obstacle or a complicated bent flow path that is long in the axial direction is provided downstream of the hot-wire element for the purpose of protecting the hot-wire element against a backfire or a backblow of the engine and for the purpose of preventing an abnormal output of the hot-wire element caused by the pulsation of the engine. In such a flow meter, since the auxiliary path portion including the hot-wire element is formed in such manner as to expose to the primary flow, an output error caused by the temperature increase of the flow meter body is small. However, this arrangement requires a long physical length in the axial direction and a large number of mechanical parts with a degraded mounting property. Therefore, this arrangement suffers from defects in compactness and cost. Also, as disclosed in, for example, Japanese Patent Unexamined Publication Nos. 57-23818 and 57-113926, there has been proposed an arrangement in which a hot-wire type air flow meter and a throttle valve means are disposed close to each other in an integral body. In Japanese Patent Unexamined Publication No. 57-23818, the same techniques as those in the foregoing two publications are adopted in the arrangement in which the auxiliary passage within which the hot-wire element is disposed is defined by a straight pipe and is formed in the central portion of the primary passage. However, in the publication '818, there is no protection means for the hot-wire element against the backfire and backblow of the engine. The throttle valve downstream of the primarily flow might serve as the protection means under the condition it is almost closed but the throttle valve might have no function as the protection means under the full or almost full open condition. Also, in addition to this problem, this arrangement suffers from another problem in which the flow within the auxiliary flow path is affected by the movement of the throttle valve to become unstable. In Japanese Patent Unexamined Publication No. 57-113926, within a body wall having a large thermal capacity and having no wide relative transfer area, an auxiliary flow path in which a hot-wire element is disposed is defined in an L-shape by a first flow path parallel to a primary flow and a second flow path perpendicular to the first flow path. With such an arrangement, it is possible to protect the hot-wire element against the blowback or backfire of the engine. However, due to the structure of the auxiliary flow path, since air of the primary flow cannot flow around the auxiliary flow path wall, a temperature of the auxiliary flow path wall is highly increased due to the heat generated by the hot-wire element as well as the heat transferred from the engine. As a result, the air within the auxiliary flow path is heated so that a difference in temperature between the air in the auxiliary flow path and the air in the primary flow path is large. Thus, it is impossible to exactly measure the flow rate of the intake air. EP-A-0 173 946 shows various examples of a hot wire type air-flow meter as defined in the preamble of claim 1. US-A-4 527 423 discloses a hot wire type air-flow meter having an auxiliary flow path with an axially extending portion for the sensor and a radially extending portion. US-A-4 393 697 shows a similar meter and in Figure 6, the use of a backfire preventing wall. The foregoing prior art is silent with respect to the reduction of the pipe path length between the hot-wire type air flow meter and the throttle valve means. Therefore, the prior art suffers from the problems of increase of pressure loss in the intake passage and of increase of weight and cost of the equipment.Moreover, the prior art encounters the following difficulties: a heat generation of the hot-wire element; a temperature increase of the auxiliary flow path wall around the hot-wire element due to the thermal invasion from the outside, that is, an error due to a difference between a temperature of the actual intake air and a temperature of the air flowing through the auxiliary flow path while impinging against the hot-wire element and a temperature compensation element; a countermeasure against a change of the flow rate distribution ratio between the primary flow path and the auxiliary flow path due to the swirl or change of the intake air or the change of flow downstream of the flow meter, even if the constant distribution is intended; a reduction of flow turbulence within the auxiliary flow path, that is, the reduction of the output noises; a protection for the elements against the counterflow due to the backblow or backfire and the pulsation; and a countermeasure against the abnormal output. Accordingly, it is the object of the present invention is to provide a hot-wire type air flow meter which is compact and has a high measuring precision and improved back-fire protection. The above object is achieved according to claim 1. The dependent claims relate to prefered embodiments. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a cross-sectional view showing one embodiment of the invention; Fig. 2 is a view showing a system of an electronic fuel injection means employing the meter of the present invention. (check valve not shown). Fig. 1 shows an embodiment of the invention, in which an auxiliary flow path 252b in parallel to the primary flow path 251 and an auxiliary flow path 252c perpendicular to the auxiliary flow path 252b are formed in a projecting portion 250d of the body 250, and further an outlet opening 252d of the auxiliary flow path is formed so as to be directed in the downstream direction of the primary flow with a check valve 254. Since the outlet opening 252d is perpendicular to the primary flow, if the counter flow due to the backblow or backfire is produced, without any modification, the counter flow within the auxiliary flow path is remarkable in which the outlet surface of the auxiliary flow path is in parallel with the primary flow. This is avoided by the check valve 254. The check valve 254 made of thin plate material is supported by a retainer 255 that is shorter in length than the check valve 254 and fixed thereto by a bolt 256. Also, in order to largely hinder the flow from the auxiliary flow path outlet 252, the check valve 254 is constructed so that it is normally opened toward the retainer 255 as shown in Fig. 1. When the counter flow is generated, the dynamic pressure is applied to the check valve 255 to thereby clog the auxiliary flow path 252d to prevent the counter flow from entering into the auxiliary flow path 252. The fluid resistance of the auxiliary flow path 252 of this embodiment is composed of a passage configuration resistance of the two right-angled bends and a passage frictional resistance Because of the provision of the check valve, this embodiment is effective against the backblow or backfire. The embodiment is advantageous against the contamination due to a long service life as described in conjunction with the eleventh embodiment. Incidentally, the auxiliary flow path 252c of this embodiment is formed in a circular cross section from the outside of the body 250. Blind plugs 253 and 257 are provided for the respective flow path formations. The internal combustion engine in which the meter of the invention is employed will be described with reference to Fig. 2. Fig. 2 shows a system of the internal combustion engine provided with an electronic control type fuel injection unit to which the automotive hot-wire type air flow meter according to the invention is applied. Air for cylinders 500 is sucked through an air filter 503 and is made to flow through a connector pipe 504, a flow meter 1 and an intake manifold 501. The flow meter 1 is provided with an auxiliary flow path 22 projected into a primary flow path 21. A hot-wire element 2a and a temperature compensation element 2b that is in unison with a circuit unit 2 are provided within the auxiliary flow path 22, thereby detecting the flow rate of air through this portion to obtain an output relative to the overall intake air flow rate. A throttle valve 3 for controlling the intake air flow rate, that is associated with an acceleration pedal of an vehicle is provided in the passage of the flow meter 1. Furthermore, an idle speed control (ISC) valve 8 for controlling a flow rate at the throttle valve fully closed condition (idle speed) is disposed in the flow meter 1. On the other hand, fuel is injected into the intake manifold 501 from an injector 507 by an injection pump 506 coupled to a fuel reservoir 505 and is supplied to the engine 500 together with the air. Into a control unit 510, there are inputted an output signal of the hot-wire element circuit unit 2, a rotational angle signal of the throttle valve 3, an output signal of an oxygen concentration sensor 508 provided in an exhaust manifold 511, an output signal of an engine rpm sensor 509 and the like. Thus, a fuel injection amount and the ISC valve opening degree are calculated. In response to the calculation results, the injector 507, the ISC valve 8 and the like are controlled. Also, a data table of the fuel injection amounts corresponding to the intake air flow rate and the rpm is stored in the control unit 510, so as to immediately determine the intake air flow rate on the basis of the hot-wire element and the fuel injection amount on the basis of the rpm from the rpm sensor, thus controlling the fuel injection amount to be injected from the injection unit.

Claims: Hot wire type air-flow meter, comprising: a primary flow path (251,21) which is formed, in a body (1), an auxiliary flow path (252,22) which is formed in a projecting portion (250d) provided inside the primary flow path (251) and consists of an axially extending inlet portion (252b) opening to the upstream side, and a radially extending outlet portion (252c) which is provided with an outlet opening (252d) located downstream of the inlet opening of the inlet portion (252b) and a flow sensor (2a,2b,10,12) provided in the inlet portion (252b), characterized in that a check valve (254) is mounted on the downstream side of the projecting portion (250d) being capable of closing the outlet opening (252d) in case of backfiring. Air-flow meter according to claim 1, characterized in that the check valve (254) is supported by a retainer (255). Air-flow meter according to claim 1 or 2, characterized in that the projecting portion (250d) is formed integrally with the body (250). Air-flow meter according to one of claims 1 to 3, characterized in that the inlet portion (252b) is provided near the inner wall of the body (250). Air-flow meter according to one of claims 1 to 4, characterized in that a throttle is located downstream of the air-flow sensor (2a,2b,10,12) in the axially extending inlet portion (262). Air-flow meter according to one of claims 1 to 5, characterized in that the outlet opening (252d) is extending axially. Air-flow meter according to one of claims 2 to 6, characterized in that the check-valve (254) is made of thin-plate material, and the retainer (274) is shorter in length than the check valve (254) and is mounted downstream of the check valve (254). Air-flow meter according to one of claims 1 to 7, characterized in that a throttle is provided downstream of the the flow sensor (2a,2b,10,12).

Inventors: ARAI NOBUKATSU, OSAWA TADAO, SEKINE YOSHIHITO, TOKUDA HIROATSU, TSUTSUI MITSUKUNI, USUI TOSHIFUMI, ARAI, NOBUKATSU, OSAWA, TADAO, SEKINE, YOSHIHITO, TOKUDA, HIROATSU, TSUTSUI, MITSUKUNI, USUI, TOSHIFUMI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: G01F1
IPC codes: G01F1, F02D35, F02D41, G01P5, G01F5