Hot wire air flow meter arrangement for monitoring intake air flow rate in internal combustion engine

Abstract: A hot wire air flow meter arrangement comprises a primary (3a) and an auxiliary (3b) air flow path defined in the air induction passage (3). A hot wire (7) is provided in the primary air flow path (3a) for monitoring air flow rate therethrough. A flow restricting means (9) is provided in the auxiliary air flow path (3b) for restricting air flow therethrough. The flow restricting means is selectively operative so that it is effective for restricting air flow in a low engine load condition to force the major part of induction air to flow through the primary air flow path (3a) and is not effective at high engine load range so as to minimize flow resistance for induction of sufficient amount of air to an internal combustion engine (1).

Description: The present invention relates to a hot wire flow meter arrangement in accordance with the prior art portion of claim 1. Hot wire air flow meters, to which the present invention concerns, have already been widely known in the automotive technologies. The hot wire air flow meters are known as advantageous in comparison with the conventionally known flap type air flow meter because of lesser flow resistance against the flow of intake air flowing through an air induction passage. In general, the hot wire air flow meter comprises an electrically conductiv resistor body in the form of a wire, a strip, a tape or so forth. The resistor body is heated at a predetermined reference temperature so as to set is resistance at a reference value. The heated resistor body is placed in the air flow, the temperature of the resistor body is lowered in a magnitude corresponding to the air flow rate or air flow velocity through the air induction passage. According to temperature variation, resistance of the resistor body varies holding a known relationship with variation of the temperature of the resistor body. The resistor body is connected to a detector circuit for monitoring variation of the resistance in the resistor body and thereby derives the air flow rate in the air induction passage. The intake air flow rate is used in an engine operation control, such as a fuel injection control, air/fuel ratio control, spark ignition timing control and so forth, as an engine load condition representative parameter. In such hot wire flow meters, a difficulty is encountered in maintaining satisfactorily high accuracy in a substantially low engine load condition. Namely, at a substantially low engine load condition, such as that in the engine idling condition, flow velocity of the intake air is substantially low. In the low engine load range, the substantially low air flow velocity results in substantially small resistance variation to the extent that the noise component in various source and introduced in the detector circuit may cause substantial influence in deriving the air flow rate. Particularly, in the case of a high performance engine which has relatively large path area induction system, air flow velocity in the induction system at the low engine load condition becomes smaller to increase difficulty in accurately measuring the intake air flow rate. In addition, the hot wire flow meter of the conventional construction is also defective at engine full load condition. Namely, when a throttle valve is fully open to provide a maximum intake air flow area for maximum amount of intake air, partial concentration of air flow tends to occur when lowering the measured intake air flow rate across a value which should be obtained in relation to the actual intake air flow rate. Influence of partial concentration of intake air can be avoided by providing flow restrictive flow regulating member, such as a flow restriction ring, in an orientation upstream of the position of the hot wire. Such an arrangement may improve accuracy of measurement of the intake air flow rate. However, the flow restriction may cause degradation of performance of air induction. GB-A-1318497 discloses a hot wire flow meter arrangement of the above-mentioned type. This prior art hot wire flow meter comprises an intake manifold wherein a cross-section controlling member is arranged and which further comprises a bypass in the intake manifold which is arranged that such the cross-section controlling member is circumvented by the bypass. The electrically conductive resistor body is located in the bypass. The angular position of the cross-section control member is controlled such that the output voltage of the electrically conductive resistor body keeps almost constant. Due to this kind of feedback-controlling of the angular position of the cross-section controlling member, the angular position thereof can be used for deriving the air flow rate. In the case of a step-like variation of the air flow rate, it takes some time until the cross-section controlling member is operated such that the output of the resistor body again comes close to the predetermined feedback controlling target value. Thus, this system suffers from time delays in detecting sudden changes of the air flow rate. Therefore, this prior art air flow meter cannot provide a sufficient measuring accuracy in the case of dynamically changing intake air flow rates. Further, this system creates an undesirable high flow resistance under high engine load conditions. US-A-4648270 discloses another type of air hot wire flow meter arrangement having a main channel and a bypass channel in which one heated thermistor and two passive thermistors placed upstream and downstream of the heated thermistor are arranged at equal distances. The voltages developed across the two outside thermistors due to heat transfer are fed to a differential amplifier to generate a mass flow indicative signal. However, systems of this type suffer from an insufficient measuring accuracy in the case of low air flow velocities and cause a degradation of the air induction efficiency due to their essential flow resistance. Therefore, the present invention is based on the object of providing a hot wire air flow meter arrangement of the above-mentioned type having an improved accuracy in measuring the intake air flow rate without negatively affecting the air induction efficiency. This object is achieved by a hot wire flow meter arrangement having the features indicated in claim 1. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment, but are for explanation and understanding only. In the drawings: Fig. 1 is a fragmentary diagram of an air induction system for an internal combustion engine, in which the preferred embodiment of a hot wire flow meter, according to the present invention, is employed; Fig. 2 is a sectional view of the portion of the air induction system, where the preferred embodiment of the hot wire air flow meter is provided; Fig. 3 is a graph showing relationship between an output voltage of the preferred embodiment of the hot wire air flow meter and an intake air flow rate represented by the output voltage; Fig. 4 is a graph showing relationship between a pressure loss and intake air flow rate; and Fig. 5 is a graph showing variation of output voltage of the conventional hot air flow meter and an intake air flow rate at a constant engine speed. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1 , the preferred embodiment of a hot wire flow meter arrangement, according to the present invention, is applied to an air induction system of an internal combustion engine 1 . The air induction system includes an air cleaner 2 and an intake duct 3 defining an air intake passage. A throttle valve 4 is disposed within the intake duct 3 for adjusting an intake air amount to introduce the engine 1 . An engine speed sensor 5 is provided for the engine 1 is provided for monitoring a revolution speed N of the engine. The engine speed sensor 5 may comprise a crank angle sensor for monitoring a crankshaft angular position. As is well known, the crank angle sensor produces a crank reference signal produced at every predetermined crankshaft angular position and a crank position signal produced at every given angular displacement. The engine speed can be derived on the basis of an interval of occurrence of the crank reference signal, or, in the alternative, by counting the crank position signal. Manner of deriving the engine speed N based on the crank reference signal or crank position signal is known in the art and thus do not need to discuss in detail. A throttle angle sensor 6 , such as a potentiometer, is provided for monitoring angular position the throttle valve 4 to produce a throttle angle indicative signal TVO . A hot wire 7 is provided in the intake duct 3 at an orientation downstream of the air cleaner 2 and upstream of the throttle valve 4 . The hot wire 7 is made of white gold wire, a nickel strip or tape or so forth. The portion of the intake duct 3 where the hot wire 7 is disposed, is partitioned and separated into a primary passage 3a and an auxiliary passage 3b . The hot wire 7 is disposed in the primary air passage 3a for monitoring intake air flow rate flowing through the primary passage. The hot wire 7 is connected to a control circuit 8 . The control circuit 8 supplies heating power to the hot wire 7 for maintaining the temperature of the wire at a predetermined temperature. Therefore, the output voltage VQ supplied to the hot wire 7 for maintaining the wire temperature constant is variable depending upon the intake air flow rate of the induction air flowing through the primary passage 3a . The control circuit 8 further outputs the output voltage VQ . As seen from Fig. 1 , a flow restricting annular wall 11 is provided at the upstream end of the primary passage 3a . The flow restricting annular wall 11 is designed so as not to serve for restricting intake air flow in the substantial magnitude and so as to regulate the air flow through the primary passage 3a . This flow restricting annular wall 11 is effective for minimizing pulsation of the intake air flow, which pulsation is caused by variation of intake vacuum in synchronism with the engine revolution. The flow restricting annular wall 11 also serves for preventing error in measurement of the intake air flow rate at the engine full load condition. This is advantageously introduced technology in view of performance of measurement of the intake flow rate near or at the engine full load range. Namely, as shown in Fig. 5 , unless the flow restriction is provided at the orientation upstream of the hot wire, the measured intake air flow amount at the engine full load range tends to lower than that actually flowing through the air intake duct. For instance, in the example of Fig. 5 , when the engine speed N is maintained constant at 1200 rpm and the pressure in the air intake duct is varied from 500 mmHg to the pressure WOT at the throttle valve fully open position, actually flowing intake air flow rate varies as shown by solid line. Whereas, the air flow rate derived on the basis of the output of the hot wire air flow meter arrangement varies as shown by broken line. According to the shown embodiment, since the flow restricting annular wall 11 serves for providing flow resistance, whereby for compressing the intake at an orientation upstream of the hot wire 7 and subsequently allow expansion to the initial volume, air flow in the primary passage can be regulated. As a result, the measured air flow rate becomes substantially coincide with the actual intake air flow rate. A butterfly valve 9 is pivotally provided in the partition for pivotal movement at a valve shaft 9a . As seen from Fig. 2 , the butterfly valve 9 is fixed to the valve shaft 9a for rotation therewith. The valve shaft 9a extends transversely to the axis of the intake duct 3 . One end of the valve shaft 9a is connected to an actuator 10 , such as an electric motor, for driving the butterfly valve 9 together with the valve shaft 9a between an open position and a closed position. The butterfly valve 9 employed in the preferred embodiment of the hot wire air flow meter, according to the invention, will not subject substantial pressure difference between upstream and downstream thereof. Therefore, in the shown embodiment, the butterfly valve 9 may be made of synthetic resin, for example. The resin valve may be advantageous for easy production. The engine speed sensor 5 , the throttle angle sensor 6 and the control circuit 8 are connected to a control unit 12 which may comprise a microprocessor. The control unit 12 processes the output voltage of the control circuit 8 for deriving the intake air flow rate indicative signal. On the other hand, the control unit 8 monitors the throttle valve angular position and the engine speed for deriving the valve position of the butterfly valve 9 so as to selectively close the valve at a predetermined engine load condition as derived based on the throttle valve angular position. It should be appreciated that the control unit 12 may perform not only derivation of the intake air flow rate and control of the butterfly valve position, but also perform the fuel injection control, spark ignition timing control and so forth. Operation of the above-mentioned preferred embodiment of the hot wire air flow meter arrangement, according to the invention will be discussed herebelow. At first, it is assumed that the engine is driven at a relative heavy engine load. At this condition, the control unit 12 controls the actuator 10 so as to maintain the butterfly valve position at a fully open position. At this condition, the intake air past the air cleaner 2 passes both of the primary and auxiliary air passages 3a and 3b for minimizing flow resistance against the intake air. According to the shown embodiment of the hot wire air flow meter arrangement, the pressure loss caused by flow restricting in the hot wire air flow meter arrangement can be maintained within an acceptable range even at the maximum flow rate, as shown in Fig. 4 . At this time, the relationship between the intake air flow rate and the output voltage VQ of the control circuit 8 becomes as illustrated by line A in Fig. 3 . By setting the output voltage VQ variation in relation to variation of the intake air flow rate in a form of data map or look-up table in a memory (not shown) in the control unit 12 , the air flow rate can be easily derived in terms of the output voltage VQ of the control circuit 8 . The control unit 12 checks the engine driving range on the basis of the engine speed N and the throttle valve angular position TVO . The control unit 12 detects the engine driving range to discriminate low engine load range and high engine load range. Namely, when the engine speed is lower than or equal to a predetermined engine speed threshold N ref and the throttle valve open angle is smaller than or equal to a predetermined angle TVO ref , the control unit outputs an actuator control signal to operate the latter to place the butterfly valve 9 at the fully closed position. In the alternative, the control unit 12 compares the output voltage VQ of the control circuit 8 with a predetermined threshold voltage V₀ which represents a criteria of low engine load range where accuracy in measurement of intake air flow rate can be lowered due to substantially low intake air flow velocity. When the output voltage VQ is lower than or equal to the threshold voltage V₀ , then the control unit 12 operates the actuator 10 to shift the butterfly valve 10 to the fully closed position. By positioning the butterfly valve 9 at the fully closed position, the intake air flows only through the primary passage 3a . Therefore, intake air flow velocity is increased. The magnitude of variation of the output voltage VQ of the control circuit 8 in relation to variation of the intake air flow rate in the low engine load range, becomes greater. While the butterfly valve 9 is held at fully closed position, the control unit 12 utilizes a characteristics as illustrated by broken line B in Fig. 3 for deriving the intake air flow rate based on the output voltage VQ . By this, noise which can superimpose on the output voltage will not cause significant influence for the resultant intake air flow rate obtained as a result of measurement. Though the shown embodiment detects the low engine load condition by detecting the engine condition based on the engine speed and the throttle valve angular position or in the alternative based on the output voltage of the control circuit, it may be possible to detect the low engine load condition by means of a mechanical switch associated with the throttle valve to detect the throttle valve open angle smaller than a predetermined angle.

Claims: A hot wire air flow meter arrangement for monitoring an intake air flow rate flowing through an air induction system of an internal combustion engine, comprising: means for dividing said air induction system (3) into a primary passage (3a) where the intake air flow rate is to be measured and an auxiliary passage (3b); an electrically conductive resistor body (7) disposed within said primary passage (3a); an electric circuit (8) connected to said resistor body (7) for supplying a controlled voltage to said resistor body (7) for heating same at a predetermined temperature, said electric circuit (8) generating an output signal representative of the voltage supplied to said resistor body (7); auxiliary flow path area varying means (9) for varying the auxiliary flow path area of said auxiliary passage (3b); actuator means (10,12) for actuating said auxiliary flow path area varying means (9); and means (12) for deriving said intake air flow rate, characterized in that said actuator means (10,12) is arranged such that it responds to engine load condition representative parameter for either opening or closing said auxiliary passage by placing said auxiliary flow path area varying means (9) either in a first or in a second position; and said air flow rate deriving means (12) derives said air flow rate based on said output signal of said electric circuit (8) according to a first characteristic when said auxiliary flow path area varying means (9) is placed in its first position or according to a second characteristic when said auxiliary flow path area varying means (9) is placed in its second position. A hot wire air flow meter arrangement as set forth in claim 1, characterized in that said primary passage (3a) is formed by partitioning an intake duct (3) of said air induction system into said primary and auxiliary passages (3a, 3b). A hot wire air flow meter arrangement as set forth in claim 1 or 2, characterized by means (11) for restricting intake air flowing into said primary passage (3a). A hot wire air flow meter arrangement as set forth in claim 1, 2 or 3, characterized by detecting means (5,6,12) for detecting said engine load condition representative parameter on the basis of angular position (TVO) of a throttle valve (4) disposed in said air induction system (3). A hot wire air flow meter arrangement as set forth in claim 4, characterized in that said detecting means (5,6,12) detects said engine load condition on the further basis of the engine revolution speed. A hot wire air flow meter arrangement as set forth in any of the claims 1 to 5, characterized in that said detecting means (5,6,12) detects said engine load condition on the basis of said output signal (VQ) of the electric circuit (8). A hot wire air flow meter arrangement as set forth in any of the claims 1 to 6, characterized in that said auxiliary flow path area varying means is a butterfly valve (9).

Inventors: FUJIWARA KOICHI C O JAPAN, FUJIWARA, KOICHI C/O JAPAN
Applicants: JAPAN ELECTRONIC CONTROL SYST, JAPAN ELECTRONIC CONTROL SYSTEMS CO., LTD.
Application year: 1989
Main classification: G01F1
IPC codes: F02D41, G01F1, F02D35, G01F7, G01F5, F02D45