GB2511312A

GB2511312A - Sensor apparatus and turbocharger

Info

Publication number: GB2511312A
Application number: GB1303461.6A
Authority: GB
Inventors: Calvin Howard Cox
Original assignee: Cummins Ltd
Current assignee: Cummins Ltd
Priority date: 2013-02-27
Filing date: 2013-02-27
Publication date: 2014-09-03
Anticipated expiration: 2033-02-27
Also published as: GB201303461D0; GB2511312B

Abstract

A sensor apparatus for measuring the mass flow rate of a fluid, the sensor apparatus comprising an ion cloud generator configured to generate a cloud of ions in the fluid, an electrode located at least partially downstream from the ion cloud generator such that during flow of the fluid the ion cloud extends from the ion cloud generator to the electrode, and an output connected to the electrode, which provides an output signal indicative of overlap between the ion cloud and the electrode. The sensor apparatus may comprise a second electrode separated from the first electrode in a direction transverse to a direction of flow of the fluid and a detection circuit configured to obtain differential measurement using output from the electrode and the second electrode. The mass flow rate sensor may be used in a turbocharger compressor inlet.

Description

SENSOR APPARATUS AND TURBOCHARGER

The present invention relates to a sensor apparatus and to a turbocharger.

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost) pressure.

A conventional turbocharger typically comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the thrbine wheel roiaies a compressor wheel mouriied on the oiher end of ihe shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.

It may be desirable to measure the mass flow rate of air flowing through an inlet of a compressor and/or the speed of the compressor wheel.

According to a first aspect of the invention, there is provided a sensor apparatus for measuring the mass flow rate of a fluid, the sensor apparatus comprising an ion cloud generator configured to generate a cloud of ions in the fluid, an electrode located downstream from the ion cloud generator such thai during flow of the fluid ihe ion cloud exiends from ihe ion cloud generator to the electrode, and an output connected to the electrode, which provides an output signal indicative of overlap between the ion cloud and the electrode.

The sensor apparatus may further comprise a second electrode located downstream from the ion cloud generator such that during flow of the fluid the ion cloud extends from the ion cloud generator to the second electrode.

The electrode and the second electrode may be separated from one another in a direction transverse to a direction of flow of the fluid.

The sensor apparatus may further comprise a detection circuit configured to obtain a differential measurement using output from the electrode and the second electrode.

The sensor apparatus may further comprise a processor configured to measure an amplitude of a signal output from the electrode or electrodes, and to use the measured amplitude to provide an output indicating the mass flow rate of fluid passing the sensor apparatus.

The amplitude measured by the processor may be an amplitude of the differential measurement.

The sensor apparatus may further comprise a processor configured to measure a frequency of a signal output from the electrode or electrodes.

The electrode or electrodes may be located fully downstream from the ion cloud generator.

The sensor apparatus may further comprise an additional electrode located upstream from the ion cloud generator such that during reverse-flow of the fluid the ion cloud extends from the ion cloud generator to the additional electrode, and an output connected to the additional electrode, which provides an output signal indicative of overlap between the ion cloud and the additional electrode.

The sensor apparatus may further comprise a processor configured to measure an amplitude of a signal output from the additional electrode, and to use the measured amplitude to provide an output indicating the mass flow rate of fluid passing the sensor apparatus during reverse-flow of the fluid.

The ion generator may comprise a structure with a radius of curvature which is sufficiently small to cause ionisation of air molecules when a pre-determined voltage is applied to the needle or other structure.

The ion generator may comprise one or more nano-tubes.

The sensor apparatus may further comprise a memory within which calibration data linking output signals to mass flow rates is stored.

The sensor apparatus may further comprise a processor configured to convert frequency measurements to speed of rotation of a compressor wheel.

The fluid may be a gas. The gas may be air.

According to a second aspect of the invention there is provided a turbocharger comprising a turbine connected via a shaft to a compressor, wherein the ion generator and the one or more electrodes of the sensor apparatus of the first aspect of the invention are provided in an inlet of the compressor.

The ion generator and the one or more electrodes may be located on an insert which is provided in an inlet of the compressor.

The second aspect of the invention may incorporate any of the features of the first aspect of the invention.

According to a third aspect of the invention there is provided a method of measuring the mass flow rate of a flowing fluid, the method comprising using an ion cloud generator to generate a cloud of ions in the fluid, the cloud of ions extending from the ion cloud generator and overlapping with an electrode located downstream from the ion cloud generator, and providing an output signal indicative of overlap between the ion cloud and the electrode, the output signal being directly linked to the mass flow rate of the fluid.

The method may further comprise comparing the output signal with calibration data and using the calibration data to convert the output signal to a mass flow rate measurement.

The ion cloud generator and the electrode may be located in a compressor inlet of a turbocharger.

A frequency of the output signal may be used to provide a measurement of a speed of rotation of a compressor wheel of the turbocharger.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: Figure 1 schematically depicts an axial cross-section through a variable geometry turbocharger; Figure 2 schematically depicts a compressor of a turbocharger which includes a sensor apparatus according to an embodiment of the invention; Figure 3 schematically depicts an ion generator and electrodes of the sensor apparatus; Figures 4-6 schematically depict measurement of elongation of the ion cloud under various operating conditions; Figure 7 schematically depicts output signals which may be generated using outputs from the electrodes shown in Figures 4-6; Figure 8 schematically depicts measurement of elongation of the ion cloud under a different operating condition; Figure 9 is a schematic circuit diagram of a detection circuit which forms part of an embodiment of the invention;

S

Figures 10 and 11 are circuit diagrams showing parts of the detection circuit of Figure 9; Figure 12 is a circuit diagram showing an alternative to part of the detection circuit of Figure 9; and Figure 13 schematically depicts an alternative arrangement of an ion generator and electrodes of the sensor apparatus.

Figure 1 illustrates a variable geometry turbocharger comprising a variable geometry turbine housing 1 and a compressor housing 2 interconnected by a central bearing housing 3. A turbocharger shaft 4 extends from the turbine housing 1 to the compressor housing 2 through the bearing housing 3. A turbine wheel 5 is mounted on one end of the shaft 4 for rotation within the turbine housing 1, and a compressor wheel 6 is mounted on the other end of the shaft 4 for rotation within the compressor housing 2. The shaft 4 rotates about turbocharger axis V-V on bearing assemblies located in the bearing housing 3.

The turbine housing 1 defines an inlet volute 7 to which exhaust gas from an internal combustion engine (not shown) is delivered, for example via one or more conduits (not shown). The exhaust gas flows from the inlet chamber 7 to an axial outlet passageway S via an annular inlet passageway 9 and turbine wheel 5. The inlet passageway 9 is defined on one side by the face 10 of a radial wall of a movable annular wall member 11, commonly referred to as a "nozzle ring", and on the opposite side by an annular shroud 12 which forms the wall of the inlet passageway 9 facing the nozzle ring 11. The shroud 12 covers the opening of an annular recess 13 in the turbine housing 1.

The nozzle ring 11 supports an array of circumferentially and equally spaced inlet vanes 14 each of which extends across the inlet passageway 9. The vanes 14 are orientated to deflect gas flowing through the inlet passageway 9 towards the direction of rotation of the turbine wheel 5. When the nozzle ring 11 is proximate to the annular shroud 12, the vanes 14 project through suitably configured slots in the shroud 12, into the recess 13. In another embodiment (not shown), the wall of the inlet passageway may be provided with the vanes, and the nozzle ring provided with the recess and shroud.

The position of the nozzle ring 11 is controlled by an actuator assembly, for example an actuator assembly of the type disclosed in US 5,868,552. An actuator (not shown) is operable to adjust the position of the nozzle ring 11 via an actuator output shaft (not shown), which is linked to a yoke 15. The yoke in turn engages axially extending moveable rods 16 that support the nozzle ring 11. Accordingly, by appropriate control of the actuator (which control may for instance be pneumatic, hydraulic, or electric), the axial position of the rods 16 and thus of the nozzle ring 11 can be controlled.

The nozzle ring 11 has axially extending radially inner and outer annular flanges 17 and 18 that extend into an annular cavity 19 provided in the turbine housing 1. Inner and outer sealing rings 20 and 21 are provided to seal the nozzle ring 11 with respect to inner and outer annular surfaces of the annular cavity 19 respectively, whilst allowing the nozzle ring 11 to slide within the annular cavity 19. The inner sealing ring 20 is supported within an annular groove formed in the radially inner annular surface of the cavity 19 and bears against the inner annular flange 17 of the nozzle ring 11. The outer sealing ring 20 is supported within an annular groove formed in the radially outer annular surface of the cavity 19 and bears against the outer annular flange 18 of the nozzle ring 11.

Exhaust gas flowing from the inlet chamber 7 to the outlet passageway 8 passes over the turbine wheel 5 and as a result torque is applied to the shaft 4 to drive the compressor wheel 6. Rotation of the compressor wheel 6 within the compressor housing 2 pressurises air present in an air inlet 22 and delivers the pressurised air to an air outlet volute 23 from which it is fed to an internal combustion engine (not shown), for example via one or more conduits (not shown).

Figure 2 schematically depicts a cross-section through a compressor which has a similar construction to the compressor of the turbocharger shown in Figure 1. The compressor may for example form part of the turbocharger shown in Figure 1. The compressor includes an air inlet 22a within which an insert 29 is provided. The insert may for example be formed from plastic.

Part of a sensor apparatus according to an embodiment of the invention is represented schematically by a box 30, and comprises electrodes and an ion generator located on the insert 29. The sensor apparatus further comprises a detection circuit (not shown), and a socket 28 which is electrically connected to the electrodes and ion generator 30. The detection circuit may be located away from the compressor, and may be connected to the electrodes and ion generator 30 via the socket 28. Although the box 30 schematically representing the electrodes and ion generator projects from a wall of the insert 29, in practice the electrodes may be substantially flush with the wall of the insert 29. The ion generator may project slightly from the wall of the insert 29.

In the absence of airflow through the compressor inlet 22a, an ion cloud generated by the ion generator will extend towards the central axis VV of the compressor. When airflow through the inlet 22a occurs, this will draw the ion cloud downwards and cause it to be elongated. The overlap of the ion cloud with electrodes of the sensor apparatus allows the mass flow rate of air flowing through the compressor inlet 22a to be determined. The ion cloud also allows the speed of rotation of the compressor wheel 6a to be determined.

The part of the sensor apparatus schematically represented by the box 30 in Figure 2 is shown viewed from above in Figure 3. First, second and third electrodes 31-33 are shown, as is an ion generator 34. The ion generator 34 is a plurality of nanotubes which extend from a wall of the insert 29 (see Figure 2). The nanotubes may for example be formed from a metal oxide (e.g. ZnO). The ion generator 34 is held at a positive voltage by a voltage source (not shown).

The ion generator 34 may for example be held at a voltage of around 20-30 V. Air molecules are ionised when air comes into contact with tips of the nanotubes of the ion generator 34. The tip of each nanotube is the part of the nanotube with the smallest radius of curvature r and the density d of charge at the tip is: d= (1) 4,zr Where 0 is the quantity of charge at the tip. The charge density causes a force f perpendicular to the surface of: f = 21 (2) When the density of charge d reaches a particular level, the force f is sufficiently large to overcome the natural attraction between ions which constitute air molecules. Thus, molecules of air surrounding the tip are pulled apart by the force 1, thereby generating a cloud of ions. The volume of the ion cloud is limited by the amount of ionisation energy that is applied by the ion generator 34. Ions will migrate in directions which tend to neutralise the external field. Thus, ions with an opposite charge to the ion generator will migrate towards the ion generator, and ions with the same charge as the ion generator will migrate away from the ion generator. An outer boundary of the of the ion cloud is formed when the potential of the ions in the cloud is sufficiently great that it attracts ions of opposite polarity. These ions recombine to form neutral air molecules. The outer boundary is known as the boundary shell.

As explained above, a cloud of ions which is centred upon the ion generator 34 is generated. The ion cloud is generally circular when viewed from above.

The ion cloud is shown schematically as grey area 36 in Figure 4. The volume of the ion cloud 36, and the charge density within the ion cloud, is limited by the amount of applied ionisation energy. This means that a particular voltage applied to the ion generator 34 will consistently generate a particular current at the electrodes 31, 32. This applies when no air is flowing through the compressor inlet. However, when air flows through the compressor inlet the shape of the ion cloud 36 will be modified, thereby changing the current generated at the electrodes 31, 32. Specifically, the size of the ion cloud 36 is increased when air flows through the compressor inlet.

The ion cloud is elongated in the direction of air flow due to movement of the ions caused by the air flow. Although the charge density of the ion cloud 36 is limited by the amount of ionisation energy delivered by the ion generator 34, the volume of the ion cloud is increased and thus the total charge in the ion cloud has increased. The elongation of the ion cloud 36 provides a greater overlap of the ion cloud with either or both of the electrodes 31, 32. The increased charge in the ion cloud 36 and the increased overlap of the ion cloud with the electrodes 31, 32 increases the current which flows through the electrodes. Thus, there is a direct correlation between the current flowing through the electrodes 31, 32 and the air flow (i.e. mass flow rate of air) through the compressor inlet 22a.

As mentioned above, the ion generator 34 is held at a positive voltage. As a result, when air molecules are ionised, electrons are attracted to the ion generator 34, whereas positively charged air ions are repelled by the ion generator. The positive ions are attracted to the electrodes 31-33 of the sensor apparatus, which may for example be held at negative voltages. The positive ions attracted to the electrodes generate an output signal which is indicative of the mass flow rate of the air through the compressor inlet (as is described further below). The use of positive ions to generate the output signal is preferred to the use of negative ions (electrons) because the positive ions have a greater mass and thus move in a more predictable manner. This provides a more consistent relationship between the output signal and the mass flow rate of air than would be the case if electrons were to be used to generate the output signal (although in an alternative embodiment electrons could be used).

There is no requirement for the ion generator 34 to extend by a significant dislance into Ihe compressor inlel 22a. All Ihat is needed is one or more nanotubes with a small radius of curvature which extend from the wall of the insert 29. Although the ion generator could be in the form of nanotubes (or a single nano-tube), it could have any suitable form. For example, the ion generator could be in the form of some other nano-material which provides a plurality of regions with many sharp areas (i.e. areas having a small radius of curvature). The nano-material may for example be a metal oxide, e.g. ZnO, or some other suitable material. The sharpness of the nano-material may be such that a charge density sufficient to break down molecules of surrounding air may be achieved at lower voltages (e.g. around 5-12V).

In a further alternative arrangement the ion generator 34 may for example be a needle. The needle may be blunter Ihan the nanolubes (i.e. have a larger radius of curvature), and thus may require a higher voltage in order to obtain a charge density which is sufficient to break down molecules of surrounding air.

The needle may be held for example at a voltage of between 450 and 800V.

Nanotubes may be preferred over a needle because they provide an equivalent ionisation field at a significantly lower voltage. For example, around 30V applied to nanotubes may provide an ionisation field which is equivalent to that formed by applying around 600V to a needle.

Referring again to Figure 3, the three electrodes 31-33 of the sensor assembly 30 may be seen. The first and second electrodes 31, 32 are positioned such that they are downstream of the ion generator 34 when the turbocharger is in normal use (i.e. when the compressor is being used to deliver compressed air to an engine). The direction of airflow through the compressor is indicated by arrow 38. The first and second electrodes 31, 32 are separated from each other in a transverse direction by a gap 40 (the term "transverse" in this context means substantially perpendicular to the direction of airflow 38). The gap 40 may be sufficiently large to prevent or minimise cross-talk between the first and second electrodes 31, 32.

The first and second electrodes 31, 32 are substantially rectangular, but are missing a corner which would intersect with the ion generator 34. The corner of the first and second electrodes 31, 32 has been cut away by an arc which has a constant radius relative to the centre of the ion generator 34. The electrodes 31, 32 have been cut away in this manner in order to prevent the electrodes from coming into contact with the ion generator 34 (this would generate an electrical short which would prevent the sensor assembly 30 from working correctly). The electrodes 31, 32 may come very close to the ion generator 34, for example to within around 1mm. provided that they do not come into contact with it. The electrodes 31, 32 may be sufficiently close to the ion generator 34 to provide some overlap between the ion cloud 36 and the electrodes 31, 32 when no air is flowing through the compressor inlet.

This is advantageous because it allows auto-calibration of the sensor assembly to be performed when no air is flowing. The auto-calibration may for example comprise adjusting the voltage applied to the ion generator 34 until output signals from the electrodes 31, 32 correspond with desired signals (e.g. signals used to generate calibrated mass flow rate measurements).

Conductors 31 a, 32a extend from the electrodes 31, 32 to nodes 31 b, 32b. A detection circuit (described further below) is connected to the nodes 31 b, 32b.

As may be seen in Figure 3, a third electrode 33 is also provided adjacent to the ion generator 34. This third electrode 33 is upstream of the ion generator 34 and of the second electrode 32. The third electrode 33 is separated from the second electrode 32 in the axial direction by a gap 41. The gap 41 may be sufficiently large to prevent or minimise cross-talk between the second electrode 32 and the third electrode 33. A corner of the third electrode 33 which is closest to the ion generator 34 has been cut away by an arc which has a constant radius from the centre of the ion generator. Again, this prevents the third electrode contacting the ion generator 34 and causing an electrical short. A conductor 33a extends from the third electrode 33 and terminates at a node 33b. The node 33b is also connected to the detection circuit (described further below). In order to accommodate the conductor 32a connected to the second electrode 32, the third electrode 33 has a smaller surface area than the second electrode 32. However, the third electrode 33 may have any suitable size.

Although a particular configuration of electrodes 31-22, conductors 31a-33a and nodes 31b-33b is shown in Figure 3, any suitable configuration of electrodes, conductors and nodes may used.

The manner in which the sensor apparatus measures the mass flow rate of air passing through the compressor inlet 22a may be understood by considering Figures 4a and 4b. Figure 4a shows the situation when there is little or no air flowing through the compressor inlet. The ion generator 34 generates an ion cloud 36 in the manner described further above. The ion generator 34 is circularly symmetric in this embodiment, and thus generates an ion cloud which is also circularly symmetric. The ion cloud 36 extends a limited distance from the ion generator 34. The ion cloud 36 may for example extend a few mm from the ion generator.

Beyond this limited distance the ions recombine to form neutral air molecules.

The ion cloud 36 overlaps with part of the first electrode 31 and with part of the second electrode 32. Ions from the ion cloud 36 are thus received at the first and second electrodes 31, 32. The ions received at the electrodes 31, 32 are predominantly positively charged air ions (as explained further above).

The electrodes 31, 32 are each separately connected to ground via resistors.

Consequently, ions which are received on the electrodes 31, 32 from the ion cloud flow to ground via the resistors. The voltage across each resistor is proportional to the current flowing through the electrode to which that resistor is connected. The voltage at each electrode 31, 32 thus provides an indication of the overlap between the ion cloud 36 and the electrodes.

Referring to Figure 4b, when air is flowing through the compressor the ion cloud 36 is elongated in the direction of airflow (indicated by arrow 38). As a result of this elongation of the ion cloud 36, the area of overlap between the ion cloud and the electrodes 31, 32 is increased. This will cause more ions to be received at the electrodes 31, 32, and therefore generate larger currents and hence larger voltages at the electrodes. The extent to which the ion cloud is elongated by the airflow 38 depends upon the mass flow rate of the air. Thus, the mass flow rate of air through the compressor inlet 22a has a direct correlation with the voltages seen at the electrodes 31, 32. Calibration may be used to determine the relationship between the electrode voltages and the mass flow rate of the air, for example generating a calibration data set The calibration data set may for example be generated by using a test rig to measure the mass flow rate of air through the compressor inlet, and linking the measured mass flow rate to voltages at the electrodes. The calibration data set may be stored in a memory and may then subsequently be used to provide measurements, based on output signals from the electrodes 31, 32 of the mass flow rate of air flowing through a compressor inlet 22a in a vehicle engine during operation of that vehicle engine. A processor may be used to convert the output signals from the electrodes 31, 32 to mass flow rate values, for example using the calibration data set.

Figures 4a and 4b illustrate schematically the position of the ion cloud 36 when there is no rotation of the compressor wheel 6 (see Figure 1). However, in practice there will be rotation of the compressor wheel when there is airflow through the compressor inlet 22a, since it is rotation of the compressor wheel which causes the airflow to occur.

Figure 5 shows the effect of rotation of the compressor wheel upon the ion cloud 36. In Figure 5 the mass flow rate of air through the compressor inlet is low, and hence there is negligible elongation of the ion cloud in the direction of airflow 38. This is indicated schematically by a shortened arrow 38. Figure 5a schematically represents the ion cloud 36 as a blade of the compressor wheel approaches the ion generator 34. The compressor wheel 6 is positioned slightly downstream of the first and second electrodes 31, 32, and thus does not pass directly over the ion generator 34 or the electrodes 31, 32.

Nevertheless, the blades of the compressor wheel 6 pass sufficiently close to the ion generator 34 that this has a significant effect upon the ion cloud 36. In this description references to a compressor blade approaching the ion generator or moving away from the ion generator should not necessarily be interpreted as meaning that the compressor blade passes directly over the ion generator (although the possibility of the compressor blade passing directly over the ion generator is not excluded).

As the compressor blade approaches the ion generator 34, it pushes air away from the ion generator in the direction of travel of the compressor blade (to the right in Figure 5a). This causes the ion cloud 36 to be elongated in the direction of travel of the compressor wheel blade, as shown in Figure 5a. Due to this elongation of the ion cloud 36, the ion cloud overlaps with a larger area of the second electrode 32 and overlaps with a smaller area of the first electrode 31. Thus, the voltage at the second electrode 32 will be greater than the voltage at the first electrode 31.

Referring to Figure 5b, once the compressor wheel blade has moved past the ion generator 34, the ion cloud 36 is not elongated in the direction of travel of the blade, but instead is elongated in the opposite direction. This elongation of the ion cloud in the opposite direction may be caused by a pressure gradient generated by the compressor blade (a region immediately behind the blade may have a low pressure). As a result of this elongation of the ion cloud 36 in the opposite direction to the direction of travel of the blade, the overlap of the ion cloud with the first electrode 31 is significantly larger than the overlap of the ion cloud with the second electrode 32. Thus, the voltage at the first electrode 31 is significantly greater than the voltage at the second electrode 32.

A deteclion circuil (described furlher below) connected 10 Ihe eleclrodes 31, 32 measures the voltages at the electrodes. The detection circuit may provide a differential output (e.g. the voltage of the first electrode 31 minus the voltage of the second electrode 32). This differential output will be modulated each time a compressor blade passes the ion generator 34, due to the displacement of the ion cloud 36 by that compressor blade. The frequency of the modulation of the output may be used to determine the speed of rotation of the compressor wheel 6. The measured frequency of the modulation may be converted into a rotation speed by multiplying it by the number of blades which are provided on the compressor wheel 6. This may be done by a processor.

As will be understood from Figures 4 and 5, the sensor assembly may be used to obtain a measurement of the mass flow rate of air passing through the compressor inlet 22, and in addition may be used to obtain a measurement of the speed of rotation of the compressor wheel 6. Although these measurements have been discussed separately, they may in practice be obtained simultaneously, as explained below in conjunction with Figure 6.

Figure 6a shows the position of the ion cloud 36 when there is considerable mass flow of air through the compressor inlet 22a and when a compressor blade is approaching the ion generator 34. As may be seen from Figure 6a, the ion cloud 36 is elongated in the direction of airflow 38, and is also elongated in the direction of travel of the compressor blade. Thus, the net elongation of the ion cloud 36 is in a diagonal direction as indicated by arrow 42. This diagonal elongation of the ion cloud 36 is such that there is a very large overlap between the ion cloud and the second electrode 32, and almost no overlap between the ion cloud and the first electrode 31. The voltage at the second electrode 32 will thus be considerably greater than the voltage at the first electrode 31.

Figure 6b shows the elongation of the ion cloud 36 when the compressor blade has passed the ion generator 34 and is moving away from the ion generator. The ion cloud 36 continues to be elongated in the direction of airflow 38. However, it is now also elongated in a direction which is opposite to the direction of movement of the compressor blade. The net elongation of the ion cloud 36 is thus in a diagonal direction as indicated by arrow 43. As a result of this diagonal elongation, the overlap of the ion cloud with the first electrode 31 is very large and the overlap of the ion cloud with the second electrode 32 is very small. Thus, the voltage at the first electrode 31 is considerably greater than the voltage at the second electrode 32.

The elongation of the ion cloud by air flowing through the compressor inlet depends upon the speed of flow of the air. The density of charge in the ion cloud depends upon the density of the air. Thus, the charge which is incident upon an electrode 31, 32 (and hence the voltage at that electrode) depends upon a combination of the speed of air flow and the density of the air. The voltage as measured by the electrode therefore depends upon the mass flow rate of the air. This allows the sensor assembly 30 to provide a measurement of the mass flow rate of air flowing through the compressor inlet 22a.

Figure 7 schematically shows a differential output signal generated by the electrodes for the two situations shown in Figures 5 and 6. Figure 7a shows the differential output signal seen when the situation shown in Figure 5 occurs. The differential signal has a period T, which corresponds with the time elapsed between successive compressor blades coming into alignment with the ion generator 34. This period may be multiplied by the number of compressor blades in order to determine the period of a rotation of the compressor wheel (assuming that all of the blades of the compressor wheel have the same separation). The amplitude A of the signal is determined by the difference between the overlap of the ion cloud 36 with the first electrode 32 and the overlap of the ion cloud 31 with the second electrode when the ion cloud is at its maximum elongation 31, 32 (and vice versa). The small amplitude A of the signal in Figure 7a is indicative of a small mass flow of air through the compressor inlet 22a.

Figure 7b schematically shows the differential output signal when the situation shown in Figure 6 applies. In Figure 7b the period T between successive blades being aligned with the ion generator 34 has been reduced considerably. This indicates that the compressor wheel 6 is rotating more rapidly. The amplitude A of the signal is considerably increased, thereby indicating that the mass flow rate of air though the compressor inlet is considerably greater.

Although the differential output signals shown in Figure 7 are symmetric about OV, in practice the differential output signals are likely to be asymmetric about OV. This is because the effect on the ion cloud of a blade approaching the ion generator 34 differs from the effect on the ion cloud of a blade moving away from the ion generator.

As mentioned further above, during normal operation of the turbocharger, air is drawn into the air inlet 22a by rotation of the compressor wheel 6 and is expelled into the volute 23, from where it passes to an intake manifold of an internal combustion engine. However, in some circumstances, air may flow through the compressor in the opposite direction (this may be referred to as reverse-flow). That is, air may flow into the compressor from the volute 23 and out through the inlet 22a. This may for example occur when the internal combustion engine to which the turbocharger is fitted moves quickly from operating at high torque to operating at low torque (this may occur when changing gear). The third electrode 33 of the sensor apparatus 30 may be used to detect reverse-flow of air through the inlet 22a. This is illustrated schematically in Figure 8.

Referring first to Figure 8a, the reverse-flow of gas through the inlet causes elongation of the ion cloud 36. The direction of elongation caused by the flow of air corresponds with the direction of the flow of the air (indicated by arrow 46). The ion cloud 36 is also displaced laterally by blades of the compressor wheel 6. This provides a net elongation of the ion cloud 36 in a diagonal direction which generally corresponds with the position of the third electrode 33 (i.e. in the direction of arrow 48). The mass flow rate of air during reverse-flow is less than the mass flow rate of air during normal operation of the compressor, and the ion cloud 36 will be less elongated in the axial direction.

For this reason, the third electrode 33 extends less far in the axial direction than the first and second electrodes 31, 32.

The third electrode 33 is connected via a resistor to ground, and hence the overlap of the ion cloud 36 with the third electrode 33 causes current to flow through the third electrode 33. A resistor is connected between the third electrode 33 and ground, and as a result a voltage is generated at the third electrode 33 which is proportional to the current flowing through the third electrode 33. The voltage at the third electrode 33 is directly determined by the overlap of the ion cloud 36 with the third electrode, and thus may be used to provide a measurement of the mass flow rate of air during reverse-flow of the air. The relationship between the measured voltage and the mass flow rate of the air may be determined by using a calibration to generate a calibration data set. This calibration data set may then subsequently be used to provide real time measurements of the mass flow rate of air during reverse-flow of the air (this may for example be done by a processor).

Figure Sb shows elongation of the ion cloud 36 after the compressor blade has passed the ion generator. As can be seen from Figure Sb, the combination of the mass airflow of the air and the movement of the compressor blade causes the ion cloud 36 to be elongated in a diagonal direction which is diagonally away from the third electrode 33 (as indicated by arrow 49). Thus, the overlap of the ion cloud 36 with the third electrode 33 is small. There is no fourth electrode positioned to measure the ion cloud when it is elongated in this direction. This is because the space which would be occupied by this electrode is needed in order to provide the conductor 34a that is connected to the ion generator 34 (see Figure 3). Since there is no fourth electrode, a differential signal between the third and fourth electrodes cannot be obtained. Instead, a differential signal between the second and third electrodes 32, 33 may be used. This provides a measurement of the mass flow rate of the air. However, this may have a poorer signal to noise ratio than would be the case if a fourth electrode were to be present, for the reasons set out further below. Nevertheless, the third electrode 33 facilitates measurement of the mass flow rate of air out of the inlet 22a during reverse-f low.

Some overlap of the ion cloud 36 with the first and second electrodes 31, 32 remains during reverse flow. Thus, the first and second electrodes 31, 32 may continue to be used to provide a measurement of the speed of rotation of the compressor wheel during reverse flow.

A detection circuit 50 which may form part of the sensor apparatus is shown schematically in Figure 9. The detection circuit 50 provides outputs which are indicative of the mass flow rate of air travelling through the compressor inlet, the direction of travel of that air, and the speed of rotation of the compressor wheel. The first, second and third electrodes 31-33 are shown in Figure 9, as is the ion generator 34. A first operational amplifier 52 is configured to act as a differential amplifier, and has a non-inverting input connected to the first electrode 31 and an inverting input connected to the second electrode 32.

The first and second electrodes 31, 32 are separately connected to ground by first and second resistors 53, 54. Ions from the ion cloud which are received at the first electrode 31 pass through the first resistor 53 to ground, thereby providing a current which gives rise to a voltage at the non-inverting input of the first operational amplifier 52. Similarly, ions from the ion cloud cause current to flow through the second electrode 32 which gives rise to a voltage at the inverting input of the operational amplifier 52. The resistors 53, 54 may for example each have resistances of the order of 1 Mc»=, and the voltages at the inputs of the first operational amplifier 52 may for example be of the order of one microvolt. The first operational amplifier 52 subtracts the voltage at the second electrode 32 from the voltage at the first electrode 31 and amplifies the result, thereby generating a difference signal. The difference signal may be a modulated wave of the type shown in Figure 7.

The difference signal is passed to a mass flow rate measurement sub-circuit and a frequency measurement sub-circuit 70. The mass flow rate measurement sub-circuit 60 comprises a second operational amplifier 62 which is configured to act as an integrator. The output of the first operational amplifier 52 is connected via a resistor 63 to an inverting input of the second operational amplifier 62. A capacitor 64 is connected between the output of the second operational amplifier 62 and the non-inverting input. A reference voltage V is provided at the non-inverting input of the operational amplifier 62.

The reference voltage V is a mid-rail voltage which biases the operational amplifier 62 to the middle of its operational range.

The mass flow measurement sub-circuit 60 is configured to integrate the modulated difference signal that it receives, and to provide an output which is the integral of the modulated difference signal. lithe modulated difference signal were to be symmetric about 0 volts (e.g. as shown in Figure 7), then the output from the mass flow rate measurement sub-circuit 60 would be zero, irrespective of the amplitude A of the signal. However, referring to Figure 6, the effect of the compressor blade upon the ion cloud 36 as it passes the ion generator 34 is not symmetric. That is, the compressor blade causes a greater overlap of the ion cloud 36 with the second electrode 32 as it approaches the ion generator, and causes a lesser overlap of the ion cloud with the first electrode 31 as it moves away from the ion generator. For this reason, the modulated difference signal will not be symmetric about 0 volts, but instead will be asymmetric. Due to this asymmetry, the output of the mass flow rate measurement sub-circuit 60 will have a non-zero value. This non-zero value is directly linked to the mass flow of air passing through the compressor inlet 22a when a compressor blade passes the ion generator 34.

A frequency measurement sub-circuit 70 comprises a third operational amplifier 72. The modulated difference signal which is output from the first operational amplifier 52 is connected via a capacitor 74 to the inverting input of the third operational amplifier 72. A resistor 73 is connected between the output and the non-inverting input of the third operational amplifier 72. The resistor 73 provides positive feedback to the operational amplifier 72, causing it to saturate to a supply rail voltage at relatively low input voltages. The frequency measurement sub-circuit 70 converts the modulated difference signal into a square wave output signal, the square wave having the same frequency at the input signal. The frequency of the square wave output signal may be measured using a processor (not shown).

The output from the mass flow rate measurement sub-circuit 60 can be converted to a mass flow rate of air passing through the compressor inlet.

This conversion may be performed for example by a processor (not shown) using calibration data stored in a memory. The calibration data may link the outputs of the mass flow rate measurement sub-circuit 60 (and optionally the frequency measurement sub-circuit 70) to the mass flow of air through the compressor inlet. The output of the mass flow rate measurement sub-circuit may then be used to obtain real time measurements of the mass flow rate of air passing into the compressor during operation of an internal combustion engine to which the turbocharger is fitted. 1)

As mentioned further above, the frequency output from the frequency measurement sub-circuit 70 may be converted to a measurement of the rotational frequency of the compressor wheel 6 by dividing the measured frequency by the number of blades on the compressor wheel.

The detection circuit 50 further comprises a reverse-flow measurement sub-circuit 80. The reverse-flow measurement sub-circuit 80 comprises a fourth operational amplifier 82 wiih an inveriing input connecied Ia the Ihird electrode 33 and a non-inverting input connected to the second electrode 32.

A resistor 83 is connected between the third electrode 33 and ground. This resistor generates a voltage at the inverting input of the fourth operational amplifier 82 when the ion cloud 36 overlaps with the third electrode 33, the voltage being directly linked to the extent to which this overlap occurs. The fourth operational amplifier 82 acts as a differential amplifier, and amplifies the difference between the voltage at the second electrode 32 and the voltage at the third electrode 33. This difference will provide a modulated output signal which is not symmetric about 0 volts. The output signal can be integrated in order to obtain a value indicative of the mass flow rate of air during reverse-flow in the inlet 22a. The fourth operational amplifier may be configured such thai a negaiive ouipui value indicaies thai reverse-flow is occurring.

As explained further above in connection with Figure 8, there may be little overlap of the ion cloud 36 with the third electrode 33 when a compressor blade is travelling away from the ion generator 34 (there is no fourth electrode in this embodiment).

As a result, the total signal received by the fourth operational amplifier 82 may be significantly less than the total signal received by the first operational amplifier 52. For this reason, the signal to noise ratio of the output signal from the fourth differential amplifier 82 may be smaller than the signal to noise ratio of the output from the first operational amplifier 52.

The output of the detection circuit 50 may periodically be auto-calibrated (e.g. to zero) when the compressor wheel is stationary (i.e. when no air or other gas is flowing through the compressor inlet 22a). This may prevent or reduce the effect of environmental factors upon the output signals. These environmental factors may for example include residual surface charge at the electrodes 31-33, the ingress of humidity into the sensor assembly 30, etc. Although the ion generator 34 has been described as being negatively charged, in an alternative embodiment the ion generator may be positively charged.

Figure 10 is a circuit diagram which shows one possible embodiment of the first differential amplifier 52. The circuit includes an arrangement of capacitors and resistors which are configured to promote desired characteristics of the first operational amplifier 52, and to avoid saturation of the first operational amplifier. A llOkQ resistor R50 and a 22pF capacitor C28 are connected between the output of the first operational amplifier 52. A lOOnF capacitor 019, 020 and a 33kQ resistor R43, R44 are connected in series to both inputs of the operational amplifier 52. The differential amplifier 52 provides a gain of 3, as determined by the resistors. The differential amplifier also acts as a low pass filter with a cut-off frequency of around 66kHz. This is equivalent to around 600,000 RPM of the compressor wheel (which may be expected to rotate a speeds below this). Providing a low pass filter is advantageous because it removes high frequency noise from the output signal. A voltage divider comprising two 220kQ resistors R51, R52 is connected to the non-inverting input of the differential amplifier 22. The voltage divider provides mid-point bias to the operational amplifier 52 and provides an effective impedance (11 OkQ) which matches the feedback resistance.

Figure 11 is a circuit diagram which shows an embodiment of the frequency measurement sub-circuit 70. A lOOnF capacitor 031 provided before the inputs of the operational amplifier 72 extracts the AC portion of an input signal (removing the DC portion). A 680kQ resistor R55 is connected between the output and the non-inverting input of the operational amplifier 72, thereby providing positive feedback. This positive feedback will tend to cause the output of operational amplifier 72 to saturate to an upper supply rail voltage or to a lower supply rail voltage. A 3OnF capacitor 030 provides a delay, which introduces a phase difference between the inputs of the operational amplifier 72. The non-inverting input will always lead the inverting input by a time constant determined by the capacitor 030. The time constant is selected to be sufficiently large that jitter at the inputs does not influence the output of the operational amplifier 72. Hysteresis generated by the capacitor 030 allows changes of direction of the input voltage (i.e. change from the voltage increasing to the voltage decreasing and vice versa) to be detected by the sub-circuit 70. This provides the advantage that switching between values of a square wave output by the operational amplifier 72 does not depend upon specific threshold levels, but instead arises from the hysteresis. The sub-circuit 70 is thus self-biasing and independent of the amplitude of input values. Although specific component values are shown in Figure 11, components with other suitable values may be used.

The detection circuit 50 of Figure 9 is merely one way of generating signals indicative of overlap between the ion cloud 36 and the electrodes 31-33. One or more parts of the detection circuit may be modified, or other detection circuits may be used. For example, a circuit which measures peak amplitudes may be used instead of the sub-circuit 60 shown in Figure 9. An example of such a circuit is shown in Figure 12.

In the circuit shown in Figure 12, an operational amplifier 90 is configured to generate an output signal which follows an input signal. The output signal passes to a diode 92 which rectifies the output signal. A capacitor 94 connected to the diode 92 is charged by the rectified output signal. A resistor 96 connected in parallel with the capacitor 94 determines a time constant for decay of charge on the capacitor. The capacitor 94 has a capacitance of lOOnF and the resistor has a resistance of 1MQ. This provides a time constant of around lOOms, which is longer than the expected minimum interval between blades passing the ion generator (e.g. longer than 5ms).

Since the decay of charge on the capacitor 94 is longer than the interval between blades passing the ion generator, the circuit provides a substantially constant output value (although the substantially constant output value may include some ripple). The output value may be indicative of the peak amplitude of the signal output from the electrodes 31, 32 (see Figure 9). The output value may be directly linked to the mass flow of air passing through the compressor inlet 22a when a compressor blade passes the ion generator 34.

The configuration of electrodes 31-33 shown in the figures is merely one possible configuration of electrodes. Any suitable electrode configuration may be used.

An alternative ion generator and electrode arrangement is shown schematically in Figure 13. The ion generator 134 is formed from a multiplicity of nanotubes (or some other nanostructure) which are spread out across an area. This forms a larger ion cloud (not shown) for a given applied voltage, thereby improving the signal to noise provided by the sensor apparatus. The electrodes 131-133 have a similar arrangement to the arrangement shown in earlier figures. However, the third electrode 133 is not located to one side of the ion generator 134 but instead extends across the top of the ion generator. This may provide a more stable output value during reverse flow than the arrangement shown in earlier figures. The third electrode 133 does not need to measure the rotation speed of the compressor wheel (this may be measured by the first and second electrodes 131, 132 even during reverse flow). Electrical connection to the ion generator 134 may for example be provided by passing a conductor through a supporting substrate on which the ion generator and electrodes are provided.

In general, an ion cloud generator and an electrode are needed in order to implement the invention. A single electrode, for example located downstream from the ion cloud generator, may be sufficient to allow useful measurements to be performed. The electrode may be positioned and/or dimensioned such that during normal operation of the compressor, the overlap between the ion cloud and the electrode increases and decreases as a compressor blade moves towards and away from the ion generator. The output from such an electrode may be used to obtain a mass flow rate measurement (using the amplitude of the output signal) and may be used to obtain a blade frequency measurement (using the frequency of the measured output). Because a differential signal is not used in this simple embodiment, this embodiment may have a relatively poor signal to noise ratio, and may be more prone to drift of output values. If there is some overlap between the ion cloud and the single electrode when no air is flowing through the compressor, then the single electrode embodiment may be used to obtain a measurement of the air flow during reverse-flow. This is because the degree of overlap between the ion cloud and the single electrode will be reduced by the flow of air out of the compressor inlet.

An electrode may have a size and position which is substantially matched to the size and orientation of the ion cloud during operation of the compressor.

For example, the first electrode 31 may be arranged such that during maximum speed rotation of the compressor wheel, the ion cloud will cover the majority of the area of the first electrode (for a particular position of compressor wheel blade and ion cloud). The first electrode 31 may, for example, be arranged such that when the ion cloud is in this position a distal end of the ion cloud does not extend significantly beyond the first electrode.

Similar considerations may apply for the second and third electrodes 32, 33.

Although the electrodes 31-33 are represented as being substantially rectangular, the electrodes may have any suitable shape.

Although only one ion generator and associated electrodes 30 is shown in Figure 2, more than one ion generator and associated electrodes may be provided. For example, a plurality of ion generators and associated electrodes may be distributed around the inlet 22a. Increasing the number of ion generators and associated electrodes may improve the signal to noise ratio provided by the sensor apparatus.

Although the sensor apparatus is shown in Figure 2 as having an ion generator and electrodes 30 located on an inner surface of an insert 29 in the compressor inlet 2a, the ion generator and electrodes may be provided at any suitable location. For example, the ion generator and electrodes may be provided on an outer surface of the insert. Some air will flow between the insert and a wall of the compressor inlet, and the ion generator and electrodes may measure the mass flow rate of that air.

The ion generator and electrodes of the sensor apparatus may be integrally formed with the insert of the compressor inlet. Alternatively, the ion generator and electrodes may be formed separately, and may then be fitted to the insert. For example, the ion generator and electrodes may be formed inside an open ended tube, which may then be attached to the insert. The tube may be generally aligned with the direction of flow of air through the compressor inlet. The tube may be dimensioned such that is corresponds in shape with a profile of a wall of the insert. This may allow the tube to be securely attached to the insert (either to the inside or the outside of the insert). In use, some of the air flowing through the compressor inlet will flow through the tube, allowing the mass flow rate of the air to be measured (and also allowing the speed of rotation of the compressor wheel to be measured).

In the above description, the term downstream" is intended to be interpreted with reference to the flow direction of air through the compressor inlet 22a during normal operation of the turbocharger (i.e. when air is being compressed by the compressor and delivered to an internal combustion engine).

Although embodiments of the invention have been described in the context of air, the invention may be used for any suitable gas. The invention may also be used to measure the mass flow late of liquids. Thus, the invention may be used to measure the mass flow rate of fluids generally.

Although embodiments of the invention have been described in the context of a compressor inlet, the invention may be used in any suitable location. For example, the invention may be provided at some other location in a turbo-charger, in a turbo-machine, or at some other location in an internal combustion engine (or in an apparatus connected to an internal combustion engine).

Modifications to the structure of the illustrated embodiments of the invention will or may be readily apparent to the appropriately skilled person after assessment of the provided description, claims and Figures, especially in the context of the field of the invention as a whole. Thus, it should be understood that various modifications may be made to the embodiments of the invention described above, without departing from the present invention as defined by the claims that follow.

Claims

CLAIMS1. A sensor apparatus for measuring the mass flow rate of a fluid, the sensor apparatus comprising: an ion cloud generator configured to generate a cloud of ions in the fluid; an electrode located downstream from the ion cloud generator such that during flow of the fluid the ion cloud extends from the ion cloud generator to the electrode; and an output connected to the electrode, which provides an output signal indicative of overlap between the ion cloud and the electrode.
2. The sensor apparatus of claim 1, wherein the sensor apparatus further comprises a second electrode located downstream from the ion cloud generator such that during flow of the fluid the ion cloud extends from the ion cloud generator to the second electrode.
3. The sensor apparatus of claim 2, wherein the electrode and the second electrode are separated from one another in a direction transverse to a direction of flow of the fluid.
4. The sensor apparatus of claim 2 or claim 3, wherein the sensor further comprises a detection circuit configured to obtain a differential measurement using output from the electrode and the second electrode.
5. The sensor apparatus of any preceding claim, wherein the sensor apparatus further comprises a processor configured to measure an amplitude of a signal output from the electrode or electrodes, and to use the measured amplitude to provide an output indicating the mass flow rate of fluid passing the sensor apparatus.
6. The sensor apparatus of claim 4 and claim 5, wherein the amplitude measured by the processor is an amplitude of the differential measurement.
7. The sensor apparatus of any preceding claim, wherein the sensor apparatus further comprises a processor configured to measure a frequency of a signal output from the electrode or electrodes.
8. The sensor apparatus of any preceding claim, wherein the electrode or electrodes are located fully downstream from the ion cloud generator.
9. The sensor apparatus of any preceding claim, wherein the sensor apparatus further comprises an additional electrode located upstream from the ion cloud generator such that during reverse-flow of the fluid the ion cloud extends from the ion cloud generator to the additional electrode; and an output connected to the additional electrode, which provides an output signal indicative of overlap between the ion cloud and the additional electrode.
10. The sensor apparatus of claim 9, wherein the sensor apparatus further comprises a processor configured to measure an amplitude of a signal output from the additional electrode, and to use the measured amplitude to provide an output indicating the mass flow rate of fluid passing the sensor apparatus during reverse-flow of the fluid.
11. The sensor apparatus of any preceding claim, wherein the ion generator comprises a structure with a radius of curvature which is sufficiently small to cause ionisation of air molecules when a pre-determined voltage is applied to the needle or other structure.
12. The sensor apparatus of any preceding claim, wherein the ion generator comprises one or more nano-tubes.
13. The sensor apparatus of any preceding claim, wherein the sensor apparatus further comprises a memory within which calibration data linking output signals to mass flow rates is stored.
14. The sensor apparatus of any preceding claim, wherein the sensor apparatus further comprises a processor configured to convert frequency measurements to speed of rotation of a compressor wheel.
15. The sensor apparatus of any preceding claim, wherein the fluid is gas.
16. A turbocharger comprising a turbine connected via a shaft to a compressor, wherein the ion generator and the one or more electrodes of the sensor apparatus of any preceding claim are provided in an inlet of the compressor.
17. The turbocharger of claim 16, wherein the ion generator and the one or more electrodes are located on an insert which is provided in an inlet of the compressor.
18. A method of measuring the mass flow rate of a flowing fluid, the method comprising: using an ion cloud generator to generate a cloud of ions in the fluid, the cloud of ions extending from the ion cloud generator and overlapping with an electrode located downstream from the ion cloud generator; and providing an output signal indicative of overlap between the ion cloud and the electrode, the output signal being directly linked to the mass flow rate of the fluid.
19. The method of claim 18, further comprising comparing the output signal with calibration data and using the calibration data to convert the output signal to a mass flow rate measurement.
20. The method of claim 18 or claim 19, wherein the ion cloud generator and the electrode are located in a compressor inlet of a turbocharger.
21. The method of any of claims 18 to 20, wherein a frequency of the output signal is used to provide a measurement of a speed of rotation of a compressor wheel of the turbocharger.