Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2001-10-24
2003-04-01
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06539793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates mainly to a flow rate detector of thermal type such as the one employed for detecting amount of intake air in vehicle engines, and more particularly to a method for improving precision in detection of a pulsating flow in vehicle engines.
2. Background Art
Generally in vehicle engines, a mixed gas of fuel and intake air is combusted in a combustion chamber of an engine, and a rotating power is generated utilizing a combustion pressure produced by the combustion. Therefore it is essential to precisely detect a flow rate of intake air in order to properly control injection amount at the time of combustion. For that purpose, various thermal type flow rate detectors for detecting a flow rate of intake air have been heretofore proposed, including a flow rate detector disclosed in the Japanese Patent Publication (unexamined) No.22563/1982.
Flow rate of intake air in vehicle engines varies depending on driving conditions of an engine. For example, when engine speed is constant, airflow-resistance in a throttle valve decreases, as opening of the throttle valve becomes larger. As a result, pressure in intake manifold increases thereby increasing a flow rate, and amplitude of pulsating flow becomes larger.
With respect to such pulsating flow, the relation between flow rate and output of detection is non-linear, and thermal response of an exothermic resistor is delayed. Therefore, when converting every detected output into pulsating flow using a thermal type flow rate detector, an average value of the detected flow rate becomes smaller than actual flow rate (this phenomenon is hereinafter referred to as leaning error). This leaning error augments as engine speed becomes higher and as pulsation amplitude becomes larger.
FIG. 7
is a characteristic diagram showing a relation between pressure P of intake manifold according to extent of opening of throttle valve and flow rate (average of flow rate signals) Qav of intake air.
Referring to
FIG. 7
, reference code Ca shows a curve of actual flow rate with increasing opening of a throttle valve under a constant engine speed. On the other hand, code Cb in
FIG. 7
shows a curve of average flow rate detected by a conventional thermal flow rate detector. It is understood that a leaning error &Dgr;l is produced between the mentioned curves Ca and Cb. In case that a flow rate detecting characteristic appears as shown by the mentioned curve Cb, an identical flow rate may be detected under two loading conditions different from each other, which brings about a disadvantage of making it impossible to uniquely determine a flow rate.
In view of the foregoing disadvantage that a leaning error &Dgr;l is produced due to a lower detected flow rate than actual amount of intake air, a flow dividing type thermal flow rate detector of was proposed, as disclosed in the Japanese Patent Publication (unexamined) No.19510/1983 (hereinafter referred to as Prior Art 1).
In this Prior Art 1, a passage for divided flow, used as flow detecting tube in which an exothermic resistor is disposed, is formed longer than a passage for main flow, in order to increase inertia of air-flow in the mentioned passage for divided flow. As a result, an average flow rate through the passage for divided flow becomes larger than a flow rate of steady flow, and moreover pulsation amplitude of flow velocity in the passage for divided flow decreases. Therefore the mentioned detecting error due to delay in thermal response of exothermic resistor can be offset by properly establishing a ratio of inertia length of the passage for divided flow and the main passage.
Also, the Japanese Patent Publication (unexamined) No.205915/2000 discloses a thermal flow rate detector which compensates flow detecting error of a pulsating flow, using gain compensating means incorporated in a flow rate detector (hereinafter referred to as Prior Art 2).
This flow rate detector of Prior Art 2 comprises a flow computing circuit for computing a flow-detecting signal according to a flow rate on the basis a resistance value of a thermo-sensitive resistor disposed in a passage of an object fluid, and gain compensating means for amplifying a flow-detecting signal from the mentioned flow computing circuit, thereby compensating a gain lowered by a thermal time constant of a thermo-sensitive resistor.
FIG. 5
is a characteristic diagram showing a relation of gain and frequency characteristic of gain compensating means. Referring to
FIG. 5
, axis of ordinates stands for gain G of the gain compensating circuit, axis of abscissas stands for frequency f, and code To shows a curve of frequency characteristic of gain compensating means disclosed in the Prior Art 2.
Accordingly, in the flow rate detector of the Prior Art 2, when the pulsating flow frequency is below the first predetermined frequency f
1
B, the flow computing circuit outputs a flow-detecting signal of which value is substantially corresponding to the pulsating flow. At this time, the mentioned gain compensating means amplifies the flow-detecting signal by a predetermined constant gain G
1
.
On the other hand, when the pulsating flow frequency is higher than the first predetermined frequency f
1
B, the output of a flow-detecting signal from the flow computing circuit becomes lower due to thermal time constant of a thermo-sensitive resistor. The output of a flow-detecting signal becomes lower also due to non-linear relation between flow rate and flow-detecting signal. At this time, the gain compensating means amplifies flow-detecting signals by a gain G
2
that corresponds to the frequency. As a result, the delay caused by thermal time constant of the thermo-sensitive resistor is compensated, and the gain compensating means outputs a waveform close to the variation in actual flow rate.
Further, when the pulsating flow frequency is higher than the second predetermined frequency f
2
B, the gain compensating means amplifies a flow-detecting signal outputted by flow computing circuit by a gain G
3
(>G
1
). As a result, a flow-detecting signal on the higher frequency side, which has little influence on the flow detection, is not amplified more than is required. As a result, a flow detecting error on the higher frequency side is not caused, and precision in flow detection is improved.
It is certain that the device according to the Prior Art 1 offsets a detecting error of a pulsating flow caused by delay in thermal response of an exothermic resistor. But a complicated structure is required to secure a necessary length of passage for divided flow, and the flow tends to be disorderly or turbulent in the proximity of the exothermic resistor. Moreover, there is a disadvantage that sufficient flow detection sensitivity cannot be secured because flow velocity in the proximity of the exothermic resistor for detecting flow rate becomes slower than the velocity of main flow.
Also, it is certain that gain compensating means according to the Prior Art 2 can compensate a decline of a flow-detecting signal caused by thermal time constant of the thermo-sensitive resistor. But a decline in flow-detecting signal still may take place depending on the waveform of pulsation or structure of thermo-sensitive resistor and flow detecting tube in which a thermo-sensitive resistor is disposed, in the pulsating flow having a frequency lower than f
1
B predetermined by thermal time constant of thermo-sensitive resistor. This may result in a large leaning error from actual flow rate. This point is now described in more detail.
As one of the important causes of a leaning error occurring even in the detection of a pulsating flow of a lower frequency than f
1
B predetermined by thermal time constant of thermo-sensitive resistor, there is a difference in flow rate between main tube and flow detecting tube, depending upon whether it is a pulsating flow or a steady flow. For example, in case that a flow detecting tube is contracted toward the outlet in order to improve the stability of flow, the area of outlet passage is smaller t
Taniguchi Shinji
Tanimoto Koji
Toyama Ryuji
Yamakawa Tomoya
Thompson Jewel V.
Williams Hezron
LandOfFree
Flow rate detector does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Flow rate detector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Flow rate detector will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3099815