DE102007037512A1 - Air mass sensor for use in internal combustion engine, has ultrasonic transducers arranged coaxially opposite to each other in pipe under angle to pipe central axis, and is coupled over coupling sound channel to surrounding - Google Patents

Abstract

The air mass sensor has two ultrasonic transducers (3,4), which are arranged coaxially opposite to each other in a pipe (2) under an angle to a pipe central axis (M). The ultrasonic transducer is coupled over a coupling sound channel (9) to the surrounding. The coupling sound channel is conducted as horn, particularly as exponential horn and is covered for frequencies of approximately 400 kilo Hertz to 500 kilo hertz. The coupling sound channel from the transducer is upstream of a pressure chamber (11).

Description

The The present invention relates to an air mass sensor.

In known manner in internal combustion engines, an air-fuel mixture brought under compression to combustion. The power output the internal combustion engine depends on the ratio from fuel mass to air mass. The measurement of a respective Air mass is carried out with an air mass sensor, which sits in the intake of the internal combustion engine. by virtue of the high economic importance of the automotive sector is hereinafter without limitation of use according to the invention and generally applicable flow sensors only on the Application for the determination of a sucked or in another way received an internal combustion engine supplied air mass.

numerous Modern internal combustion engines are today with an exhaust turbocharger equipped, which causes a pre-compression of the air mass. Was already at the beginning of the development of internal combustion engines the attempt of precompression of an internal combustion engine to be supplied with the aim of an increase the engine power by increasing the air flow and fuel flow rate per work cycle, so today is the charge of gasoline engines no longer primarily under seen in terms of performance, but as a way to Saving fuel and reducing pollutants. there is in a known manner a respective exhaust stream energy for pre-compression the air mass flow through a turbine running in the exhaust stream withdrawn with mechanically coupled fresh air compressor, so that, for example, a diesel engine is no longer considered a naturally aspirated engine, but as a charged engine with charge air pressures of up to 1.5 bar or even 2.5 bar with significant increase in performance and reduced pollutant emissions works. This is of course, a respective fuel mass one Add air mass in a predetermined ratio, so that an air mass sensor has a significant importance in terms of cost-effectiveness and pollutant reduction of an internal combustion engine.

There it in the chemical process of combustion in any operating condition an internal combustion engine on the mass ratios of fuel and air is the mass flow rate of the intake / charge air also to measure continuously as accurately as possible. The maximum to be measured air mass flow is depending on the engine power of the internal combustion engine in the range from 400 to approx. 1000 kg / h. Due to the low idle demand modern internal combustion engines is the ratio a minimum to a maximum air flow between 1:90 to about 1: 100.

One Air mass sensor can be used as a mass flow sensor after a thermal Principle work, with a release of heat output of a sensor wire heated by the flow of electric current compared to a thermally insulated sensor wire over a resistance bridge circuit as a measure of a respective flow rate is evaluated.

An alternative approach, which clearly saves electrical energy compared to the electrothermal measurement, is generally discussed in the article. "Flow Measurement - An Overview", in the journal "Technisches Messen tm", 1979, No. 4, pages 145-149 , has been described. For this purpose, it is known to build up a flow measurement on the basis of the so-called drift principle using a transmitting and a receiving ultrasonic measuring head. The two ultrasound measuring heads serve as a transmitter and a receiver and require a corresponding transceiver before signal evaluation.

Furthermore, it is off the Kraft Paperback / Bosch, 23, updated and expanded edition, Verlag Vieweg, 1999, ISBN 3-528-03876-4, page 115 , a method for ultrasonic flow measurement, according to which a transit time t of a sound pulse through a measuring medium, such as air, at the helix angle α with the same measuring section 1 once against an air flow and once in the air flow direction. A resulting transit time difference of the sonic pulse is directly proportional to the volume flow.

In order to influence the air flow as little as possible and to minimize the space compared to other known arrangements, the ultrasonic transducers are arranged in known arrangements coaxially and obliquely opposite, such. B. in the DE 33 31 519 C2 disclosed. However, this leads to a reduction in the measuring effect with increasing angle of attack of the transducer to the horizontal. This results in a compromise of an installation length dependent on the angle of the transducers to the horizontal and the measuring effect of the arrangement. The diagonally opposite arrangement of the transducers offers the advantage of averaging the drift over a flow velocity distribution of the entire pipe cross-section.

Reinstating on a signal drift, which occurs due to the drift of sound through the fluid flow, are for example in the DE 103 44 895 A1 a method and apparatus for ultrasonic flow velocity measurement has been disclosed. Here, sound pulses are transmitted from an ultrasonic transmitter to an ultrasonic receiver, which are formed together as a transducer array and arranged in parallel aligned in a pipe wall. The sound waves pass through the entire pipe cross-section twice when reflected on the opposite pipe wall. The sound beam emitted by a transmitter part of the transducer array is also bundled so that it is focused on the receiver region of the array substantially at one point. Dependent on a respective typical mean flow velocity from about 0.5 m / s to about 50 m / s occur at different levels of drift, which are detected by a division of the receiver area into individual small receiving cells of the array. Both arrangement principles mentioned above are also in E. Schrüfer "Electrical Measurement", 6th edition, Carl Hanser Verlag Munich, Vienna, 1982, pages 455-457 summarized.

All of the abovementioned solutions have in common the problem that the low signal strength of the received ultrasonic signal leads to low reliability of the measuring principle, especially at high flow velocities. In order to achieve a sufficient signal level in the received signal, the transmitter side to drive the piezo transducers pulses high electrical voltage of z. B. 200 V used. However, a fundamental problem with the use of piezoelectric transducers is their low efficiency: Known piezoelectric materials have a characteristic impedance that differs by more than six orders of magnitude from the characteristic impedance of the surrounding air: medium Acoustic impedance Z [kg / (m ² · s)] Air (20 ° C) 416 water 1.48 · 10 ^-61 quartz 15.2 x 10 ^-6 PZT-5 28.0 · 10 ^-6

von T ≈ 6·10^–5. Der weit überwiegende Teil des erzeugten Ultraschallfalls wird also an der Grenzschicht in das piezoelektrische Material reflektiert und ist für die eigentliche Messung verloren.With PZT-5, a composition ratio of lead zirconate titanate which is particularly frequently used in the art for ultrasonic volume oscillators is referred to here. Thus, at a boundary layer between PZT-5 and air such a serious mismatch of the acoustic radiation resistance Z _{L given} to the acoustic impedance Z _{w of} the transducer that only a very small part of the vibrations generated in the piezoelectric material transmits via the boundary layer to the air becomes, namely only a portion

of T ≈ 6 · 10 ^-5 . The vast majority of the ultrasound generated is thus reflected at the boundary layer in the piezoelectric material and is lost for the actual measurement.

In order to achieve an improved effectiveness in the piezoelectric transducers themselves, the transducer crystals are provided in the form of the usual volume oscillator with a matching layer of plastics with an adapted acoustic impedance, which then at least as a λ / 4 transformer at a fixed frequency f _o = c / λ causes a sufficiently good adaptation of the acoustic radiation resistance Z _L to the acoustic impedance Z. of the transducer. Nevertheless, the signal strength of the received signal, especially at high mass flows, currently so low that the measurement is not yet reliable.

It The object of the present invention is a low-cost as well as reliable device for flow measurement also in the area of high and very high volume flows.

These The object is achieved by the features of the independent claim solved. Advantageous developments are the subject of Dependent claims.

A basis of the present invention is the recognition that further constructive measures for impedance matching for the air operation of operating in the ultrasonic range piezoelectric transducer materials are provided. According to the invention is therefore in an air mass sensor with two in a pipe at an angle to the pipe center axis, coaxially opposite one another arranged ultrasonic transducers at least one of the ultrasonic transducers via a Schallka signal to the environment kopkop pelt. In one embodiment of the invention, this coupling sound channel is a horn, in particular an exponential horn. Volume oscillators when coupled to air reach their limit at a frequency of about 500 kHz, since here the attenuation in the air increases considerably and this influence can no longer be compensated by ordinary means for air adaptation or makes disproportionate demands on a manufacturing process of such ultrasonic transducers. When using RF ultrasound with frequencies of preferably f> 400 kHz only a minimum characteristic length of 1 <0.135 mm perform, resulting in a very practical design, which is well integrated as a mold in an injection molding tool.

In Another embodiment is the horn of the transducer her seen upstream of a pressure chamber.

Further Features and advantages of the invention will be described below of embodiments with reference to the figures indicated in the drawing. In the drawing show in schematic Presentation:

1 a section in the axial direction through an intake pipe with two mutually inclined in a plane opposite and arranged interconnected via a Schallausbreitungsweg ultrasonic transducers with coupling sound channel in the form of an exponential horn;

2 : An illustration of a known device analogous to the arrangement of 1 ,

about the different illustrations will become consistent below same reference numerals and designations for identical functional or assemblies used.

The picture of 2 shows a simplified longitudinal section through a known device 1 with a suction pipe 2 in which two ultrasonic transducers 3 . 4 are provided at an angle α to a pipe center axis M arranged coaxially opposite one another. The two ultrasonic transducers 3 . 4 are hereinafter with an electronics not shown 5 connected. The flow through a pipe generally has no constant value, in particular at higher flow velocities and / or high volume flows over a pipe cross-section. This is indicated above by the indication of the velocity v (y, z) with the dependencies on the mutually orthogonal coordinates y, z. From the path of the ultrasound results a measurement of an average value of the location-dependent speed v (y, z) of the substantially in the x-direction along a center axis M of the intake pipe 2 directed flow. To amplify this effect are the two ultrasonic transducers 3 . 4 in terms of their dotted line drawn transmission / reception axes 6 to each other at an angle a obliquely to the tube center axis M arranged so that the acting of a to a subsequent period as a transmitter ultrasonic transducer 3 . 4 emitted ultrasonic waves 7 out of the range of a respective holder 8th out into each acting as a receiver ultrasonic transducer 3 . 4 be transmitted. The running distance of the sound is predominantly in direct contact with the flow. The sound largely shading mounts 8th are fluid and gas tight at the intake manifold 2 arranged.

The ultrasonic transducers 3 . 4 are with the electronics 5 connected to the control and signal evaluation. Through this electronics, the ultrasonic transducers 3 . 4 operated in the manner indicated alternately as a transmitter and receiver. It always forms a pair of a transmitter and a receiver in each case reversed course of the sound signal, as shown by the two arrows in 1 indicated. The results of one in electronics 5 based on the output signals of the ultrasonic transducers 3 . 4 Evaluation performed are forwarded via a connector interface not shown to a subsequent engine management.

Since known piezoelectric materials have a characteristic impedance which differs by more than six orders of magnitude from the characteristic impedance of the surrounding air, such a serious mismatch of the acoustic radiation resistance Z _L occurs at the interface between PZT-5 and air to the acoustic impedance Z _{w of} the transducer 3 . 4 in that only a very small part of the vibrations generated in the piezoelectric material is transmitted via the boundary layer to the air. A fundamental problem with the use of piezo transducers is thus their low efficiency. As a countermeasure, high voltages of z. B. created 200V.

Furthermore, measuring arrangements are known in which the ultrasonic transducers are arranged very close to an evaluation electronics. By the smallest possible cable length between the ultrasonic transducers 3 . 4 and the electronics 5 the additional influence of the electrical signal attenuation is reduced.

In order to achieve improved efficiency in the piezoelectric transducers themselves, the transducer crystals in the form of the usual volumetric oscillators are occasionally provided with an adaptation layer made of plastics with a matched acoustic impedance, which is then at least as λ / 4 transformer at a fixed frequency f ₀ = c / λ causes a sufficiently good adaptation of the acoustic radiation resistance Z _L to the acoustic impedance Z _{w of} the transducer. However, this approach is ver relatively expensive and is therefore technically almost never used.

According to the invention now more constructive measures for impedance matching for the air operation of operated in the ultrasonic range provided piezoelectric transducer materials. From the underlying Transducer principle are below next to the common volume oscillators also bending or membrane oscillator and in particular capacitive micromechanical Membrane transducer can be used for ultrasonic generation.

1 shows an example of a section in the axial direction through the intake manifold 2 with two inclined to each other in a plane opposite and arranged over a sound propagation path 6 interconnected ultrasonic transducers 3 . 4 according to 2 with a coupling sound channel 9 in the form of an exponential horn 10 , Although ultrasound is defined in a range from 20 kHz to 1 GHz, a range between 20 kHz and 10 MHz is used for flow measurement. When using HF ultrasound with a frequency of here f = 400 kHz results in a minimum characteristic length of 1 0.135 mm, resulting in a horn length of a few millimeters to a few centimeters and thus to a very practical design that fits well in an injection molding tool for one in an intake pipe 2 module to be inserted can be integrated.

As an additional or alternative usable measure is to the converter 4 subsequently and the exponential horn 10 upstream of a pressure chamber 11 been provided. In terms of effect, this approach follows the principle of the so-called sound box of the gramophone.

in the The result is the measures described above in an arrangement can be integrated, which is very compact. A the above Embodiment corresponding sound horn, z. B. as Made of plastic injection molded part, is significantly cheaper than a Electronics that achieve a comparable effect.

There are also the following advantages:
The improved matching leads to an amplification of the transmitted signal, so that the reliability of the measurement can be improved and possibly the electronic generation of the transmission signal and the electronic processing of the received signal can be simplified, since it is possible to work with lower electrical transmission powers or with lower reception amplifications. In addition, the following side effect is observed: The improved adaptation of the acoustic radiation resistance to the transducer increases the mechanical attenuation of the transducer as a result of the sound radiation, resulting in temporally shorter turn-on and Ausschwing phases of the transducer and thus to an improved detectability of the signal propagation time.

Preferably, in one embodiment of the invention, the intake pipe with at least one coupling sound channel 9 designed as an injection molded part in one piece and very compact. It is only the ultrasonic transducers 3 . 4 as well as the electronics 5 to mount and electrically connect with each other and an external logic or a motor management. In comparison to the prior art significantly reduced effort in the production and simultaneous limitation of electrical power so better and in particular more reliable results are delivered with an arrangement according to the present invention.

1

contraption

2

Pipe / intake

3

ultrasound transducer

4

ultrasound transducer

5

electronics

6

Transmission / reception axis

7

ultrasonic wave

8th

bracket

9

Source attach sound channel

10

exponential

11

pressure chamber

α

Inclination angle of the transmission axis 6 with respect to the center axis M of the intake pipe 2

f

frequency wavelength

λ

wavelength

M

Pipe center axis

v

flow rate

x

x-coordinate Right-hand system, parallel to M running

y

y coordinate in the plane of the pipe cross-section

z

z-coordinate, in the plane of the pipe cross-section

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 3331519 C2 [0008]
• DE 10344895 A1 [0009]

Cited non-patent literature

• - "Flow Measurement - An Overview", in the journal "Technisches Messen tm", 1979, No. 4, pages 145-149 [0006]
• - the Automotive Handbook / Bosch, 23, updated and upgrade edition, Verlag Vieweg, 1999, ISBN 3-528-03876-4, page 115 [0007]
• E. Schrüfer "Electrical Measurement", 6th edition, Carl Hanser Verlag Munich, Vienna, 1982, pages 455-457 [0009]

Claims (5)

Air mass sensor with two ultrasonic transducers ( 3 . 4 ) in a pipe ( 2 ) are arranged coaxially opposite one another at an angle (α) to a pipe central axis (M), characterized in that at least one of the ultrasonic transducers ( 3 . 4 ) via a coupling sound channel ( 9 ) is coupled to the environment. Air mass sensor according to claim 1, characterized in that the coupling sound channel ( 9 ) is designed as a horn, in particular as an exponential horn ( 10 ). Air mass sensor according to one of the preceding claims, characterized in that the coupling sound channel ( 9 ) is designed for frequencies (f) of about 400 kHz <f <500 kHz. Air mass sensor according to one of the preceding claims, characterized in that the coupling sound channel ( 9 ) from the converter forth a pressure chamber ( 11 ) is connected upstream. Air mass sensor according to one of the preceding claims, characterized in that it comprises at least one coupling sound channel ( 9 ) is manufactured as a plastic injection molded part.