Hot wire type of air flow meter and internal combustion engine using the same

Abstract: The present invention whose object is to provide a hot wire type of flow meter (1) capable of accurately detecting a flow rate under various conditions required for satisfying a desire for low fuel consumption of an engine system and space saving in an engine room, the present invention being characterized in a hot wire type of air flow meter including a main flow passage (21) forming an air intake flow passage of an internal combus­ tion engine, a hot wire element (2a) measuring intake air, and a bypass (12) having the hot wire element therein and disposed in the main flow passage (21), the hot wire type of air flow meter comprising: a flow passage (31a) forming the bypass, and formed in the axial direc­ tion of the main flow passage (21); and a flow passage (31c) forming the bypass and formed in the radial direction of the main flow passage, the flow passage having a structure designed in such a manner that the upper stream portion of an outlet portion thereof projects over the lower stream portion of the same.

Description: The present invention relates to a hot wire type of the kind referred to in the preamble portion of claim 1. The invention also relates to an internal combustion engine comprising such an air flow meter. Hitherto, as disclosed in JP-A-50-50520, JP-A-50-146369 and JP-A-55-69021, there is a hot wire type of air flow meter for an internal combustion engine so constituted that a straight-shaped bypass (branch pipe) is disposed in the central portion of a main pipe and hot wire elements are disposed therein. However, in the structure disposed in JP-A-50-50520, such hot-wire element is not protected from reverse heat due to backfire caused from disorder of the engine ignition timing. To this end, a protecting structure for a hot wire element against the backfire has been disclosed, for example, in JP-A-55-69021. However, a hot wire element generally has a characteristics such that the output therefrom is reduced although the average flow velocity increases in a case where the same is disposed in the flow having large pulsation due to its nonlinear heat transfer coefficient thereof. Therefore, any of the above-described structure cannot correctly detect the flow rate of a pulsation flow. As disclosed in Japanese Utility Model Unexamined Publication No. 56-135127, a bypass, in which a hot wire element is disposed, is disposed in the main pipe, and in order to perform protection against the above-described backfire or correctly detect the flow rate of the pulsation flow, the fluid resistance at the bypass in the lower stream to the hot wire element is enlarged and as well the outlet and inlet ports of the bypass are formed in parallel or substantially in parallel to the main stream. That is, the dynamic pressure acting to the outlet and inlet ports of the bypass is reduced even if a reverse flow is generated and as well the flow reaching the hot wire element is damped so that backfire resistance is improved. Since the outlet port of the bypass opens directly and substantially in parallel to the main stream, the flow in the bypass is slightly changed due to the statical pressure change caused from mixture of flows at this portion. It leads to generation of noise in the output from the hot wire element. Although high frequency noise can be reduced by a filter disposed in the circuit, the above-described type of noise due to the change causes a system control problem when, for example, the engine is operated at a low speed. Furthermore, on the viewpoint of hardware, the structure arises productionality (cost and weight) and reliability (the number of parts) problems since the axial length is too long and the component parts for the bypass are difficult to be mounted. On the other hand, structures have been disclosed, for example, a structure disclosed in Japanese Patent Unexamined Publication No. 56-76012, the structure being characterized in that a bypass in which a hot wire element is disposed is formed outside the main stream in order to prevent the above-described type of backfire and stabilize the outputs. However, these structures arise, as pointed out in JP-A-56-76012, problems that flow rate detection error enlarges due to the thermal conditions such as thermal conduction from the engine, heating of the hot wire element, in an automobile case, engine heat and rise in temperature on the engine room inside due to solar radiation. That is, since the bypass portion provided with the hot wire element has a large thermal capacity and is formed with a relatively tight width in the inner portion of a body wall which has not large area for conducting the heat of the intake air. Furthermore, the bypass is formed to perform a good thermal conduction of the air flow passing therein. Therefore, the temperature of the air flow in the bypass is affected by the temperature of the passage wall of the bypass, causing a large temperature difference from that of the main flow. It leads to enlargement of the error in measurement of the intake air flow rate. Some of the disclosures have the structure which cannot withstand strong engine backfire and incapable of correctly detecting the average flow rate of the pulsation flow, so that it cannot be put to practical use. Some of them are incapable of correctly measuring the flow rate under certain thermal conditions and as well be incapable of being sufficiently controlled to have the engine operated at the most suitable air-fuel ratio due to increase in the noise in the output. Therefore, it interrupts cleaning of the exhaust gas from the engine, improvement in fuel consumption and operability. On the other hand, some of them are insufficient in reduction of the axial length of the main flow meter body, that is, they are insufficient in reduction of the length of the intake pipe, weight of the body and manufacturing cost. Therefore, problems arise that the pressure loss in the intake pipe increases, weight of the system including the engine becomes heavier, causing improvement in engine fuel consumption and reduction of the engine room space to be interrupted. US-A- 4 304 129 discloses a hot wire type air flow meter including a main flow passage forming an air intake flow passage of an internal combustion engine, a probe holder block formed integral with a circuit unit having a bypass measuring element therein and disposed in said main flow passage. EP-A-0 173 946 discloses in Figures 12-14 a hot wire type air flow meter as defined in the preamble of claim 1. Also postpublished document EP-A- 295 647 discloses in Fig. 1 such a flow meter. An object of the present invention is to provide a hot wire type flow meter capable of correctly detecting flow rate under a variety of conditions needed for achieving low fuel consumption of the engine system and small space in an engine room. Another object of the present invention is to provide an internal combustion engine capable of controlling the most suitable air-fuel ratio by using the above-described type of hot wire type of air flow meter. These above-desribed objects, are achieved with a hot wire type flow meter according to claim 1. Dependent claims are directed on features of preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view illustrating an internal combustion engine employing a hot wire type of air flow meter without the features of the present invention for the purpose of illustration; Figs. 20, 21 and 22 illustrate a first embodiment of the present invention; Fig. 23 is a vertical cross-sectional view illustrating a second embodiment of the present invention; Fig. 24 is a cross-sectional view taken along the line XXIV-XXIV in Fig. 23; Fig. 1 illustrates a system of an internal combustion engine equipped with an electronic fuel injection device to which an hot wire type of air flow meter for an internal combustion engine without the features of the present invention is applied. Air is taken in through an air filter 503, and is supplied to an internal combustion engine (cylinder) 500 after it has passed through a connecting pipe 504, a flow meter 1 and an air intake manifold 501. A bypass 22 projecting to a main passage 11 is formed in the flow meter 1. A hot wire element 2a and a temperature compensation element 2b formed integral with a circuit unit 2 are provided within the bypass 12 for the purpose of obtaining an output corresponding to the total quantity of intake air by detecting air flow velocity at this portion. A throttle valve 3 for controlling a quantity of intake air is provided within the passage of the flow meter 1, this throttle valve 3 being designed to act in synchronization with the accelerator pedal of an automobile. The flow meter 1 is further provided with an idle-speed control (ISC) valve 8 for controlling a flow rate when the throttle valve 3 is completely closed (idling). On the other hand, fuel introduced from a fuel chamber 505 by a pump 506 is injected into an air intake manifold 501 by an injector 507 so that the fuel is supplied to the engine 500 together with the air. The control unit 510 receives an output signal from the hot wire circuit unit 2, a signal representing the rotational angular degree of the throttle valve 3, an output signal from an oxygen density sensor 508 disposed in an exhaust manifold 507, an output signal from an engine revolution speed sensor 509 and so forth, whereby a quantity of fuel to be injected and the degree of an ISC valve 8 to be opened are calculated. In response to these calculations, the control unit 510 controls the injector 507 and the ISC valve 8. Figs. 20 and 21 illustrate a hot wire type of flow meter according to a first embodiment of the present invention. A probe holder block 203 is individually formed from a body 200, the inside thereof being provided with a bypass 202b formed in parallel to the main stream and a radial bypass 202c.This bypass 202c is formed in the lower stream of the main stream by a groove having a square cross sectional shape and a cover 205. An outlet port 202d of the bypass comprises a wind shield wall 204 formed by extending the probe holder block 203 in the upper stream of the main stream and as well a wind shield wall 206 formed by extending a cover 204 in the lower stream. In this embodiment, the width of the wind shield wall 206 is made smaller than that of the bypass 202c. The reason for this is to prevent the bypass flow at the outlet port thereof from being excessively disordered by the cover 206. This measurement is a critical factor to enhance the effect of the upper stream wind shield. The wind shield wall 204 for the outlet port 202d in the upper stream of the main stream is effective to reduce noise at the time of normal state, that is, when the flow is normal. On the other hand, the wall 206 in the lower stream can significantly reduce the introduction force of the reversed flow due to backfire or blowback into the bypass. That is, the flow can be separated into two currents by this wind shield 206, and the two currents interferes with each other in front of the bypass outlet 202d so that the introducing force can be weakened. This type of structure exhibits an excellent intake air pulsation damping characteristics when used in an engine involving occurrence of frequently blowback. Fig. 22 illustrates a hot wire type of flow meter according to a minor change of the first embodiment of the present invention. A cover 225 is designed in such a manner that the width thereof is maintained to outlet ports 222d and 222e at which side walls 223a and 223b forming a radial bypass 222c are cut off. Therefore, the width of the portion corresponding to the wind shield wall 204 shown in Fig. 20 is made large for the purpose of forming wind shield walls 224a and 224b to serve for the bypass outlets 222d and 222e. Figs. 23 and 24 illustrate a hot wire type of flow meter according to a second embodiment of the present invention. This embodiment is characterized in that the structure shaped like the first embodiment is formed in a probe holder block formed integral with a body 230. A radial bypass 232c is bored from outside of the body, and its outlet port 232d is also machined by an end mill or the like in the same direction. At this time, in order to form a wind shield wall 234 in the upper stream of the main stream and a wind shield wall 236 in the wer stream of the same, a probe holder block 233 is formed. A plug 235 plugs unnecessary hole after the machining has been completed. An effect obtained in this embodiment is, in principle, the same as that obtained in the first embodiment, but the structure can be performed easier, causing cost to be reduced. As described above, according to the present invention, an accurate measurement of a flow rate can be performed even under various conditions since the above-described structure prevents the reversed flow to a bypass due to backfire or blowback to the bypass and thereby disorder of the flow at the upper stream portion of the flow meter.

Claims: A hot wire type air flow meter including a main flow passage forming an air intake flow passage of an internal combustion engine, a hot wire element for measuring intake air, and a bypass having said hot wire element therein and disposed fully in said main flow passage, and said bypass consisting of a first passage formed in the axial direction of said main flow passage and a second presage formed in the radial direction of said main flow passage, the radially facing outlet rim of the bypass outlet port at the end of the radial passage having a wind shield wall (204; 224; 234) radially extending from the upper stream side of said outlet rim into said main flow and a further wind shield wall (206; 236) radially extending from the downstream side of said bypass outlet rim into said main stream, characterised in that the width of said wind shield wall (236) provided at the downstream side of said bypass outlet port (232d) is smaller than the width of said upstream wind shield wall (234) said widths being measured perpendicularly to said axial and radial directions. A hot wire type air flow meter according to claim 1, wherein the width of said wind shield wall (206,236) provided at the downstream side of said bypass outlet port (232d) is smaller than the width of the outlet rim (202d, 232d) of said bypass (202c, 232c). A hot wire type air flow meter according to claim 1, wherein a member (230) forming said main flow passage and a member for forming said wind shield walls (234, 236) are formed integral with each other. An internal combustion engine comprising: a hot wire type of air flow meter according to claim 1, a speed sensor for detecting the engine speed, a fuel injection device for injecting fuel to intake air, and a control device for obtaining, in response to a quantity of intake air detected by said hot wire type of air flow meter and the engine speed detected by said speed sensor, a corresponding amount of fuel to be injected, and outputting an instruction to inject fuel by the thus-obtained quantity to said fuel injection device.

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