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
turbine wheel rotates a compressor wheel mounted on the other end of the
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.
According to a first aspect of the invention, there is provided a sensor
apparatus comprising a housing having an inner perimeter which defines an
area through which gas may flow, the housing being provided with a first
chamber which extends around the area through which gas may flow, an
entrance being distributed around the first chamber, and a second chamber
which extends around the area through which gas may flow, an exit being
distributed around the second chamber, the first chamber being arranged to
be upstream of the second chamber in use, wherein the sensor apparatus
further comprises one or more sensors arranged to measure a pressure
difference between pressure in the first chamber and pressure in the second
chamber.
The first chamber may have a cross-sectional area which is sufficiently large
that in use the pressure of gas within the first chamber substantially equalises
during operation of the sensor.
66284305-1-proberts
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The entrance to the first chamber may be narrower in the vicinity of the sensor
and wider further away from the sensor.
The entrance may extend intermittently around the first chamber. Open
portions of the entrance may occupy a smaller proportion of the entrance in
the vicinity of the sensor than open portions of the entrance further away from
the sensor.
The second chamber may have a cross-sectional area which is sufficiently
large that in use the pressure of gas within the second chamber substantially
equalises during operation of the sensor.
The exit from the second chamber may be narrower in the vicinity of the
sensor and wider further away from the sensor.
The exit may extend intermittently around the second chamber. Open
portions of the exit may occupy a smaller proportion of the exit in the vicinity
of the sensor than open portions of the exit further away from the sensor.
The sensor apparatus may further comprise a sensing channel which is
connected between the first chamber and the second chamber such that gas
flows through the sensing channel in use, and wherein the one or more
sensors are located in the sensing channel.
The first chamber may be shaped such that there is no direct flow path
between the entrance of the first chamber and the sensing channel.
The one or more sensors may comprise a sensing device which is at least
partially located within the sensing channel.
The sensing device may be at least partially located within a flow restrictor
which is provided in the sensing channel.
66284305-1-proberts
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The sensing device may comprise two bipolar transistors, one of the bipolar
transistors being electrically heated.
The sensing device may further comprise a circuit configured to provide
substantially constant power to the heated bipolar transistor and to measure a
difference between base emitter voltages of the bipolar transistors.
The sensing device may further comprise a circuit configured to maintain a
substantially constant temperature difference between the bipolar transistors,
and to measure the power used to heat the heated transistor.
The circuit may be further configured to measure the temperature of the
bipolar transistor which is not electrically heated.
The flow restrictor may be not formed integrally with other parts of the sensing
apparatus.
The flow restrictor may be formed from a material which is different to the
material used to form the housing of the sensor apparatus.
The one or more sensors may comprise a strain gauge which is connected
between the first chamber and the second chamber. The first and second
chambers may be configured such that there is no bleed of gas between them
when a strain gauge is used.
The strain gauge may be provided in a sensing channel which is connected
between the first chamber and the second chamber.
The sensor apparatus may further comprise an additional chamber located
between the first and second chambers, the additional chamber being
connected to the first chamber or to the second chamber, an additional sensor
66284305-1-proberts
5
being located within the additional chamber. The additional chamber may be
configured to shelter the additional sensor from the effects of airflow. The
additional sensor may be an ambient air temperature sensor.
The first chamber and the second chamber may be not connected, and the
one or more sensors may comprise a pressure sensor located in the first
chamber and a pressure sensor located in the second chamber.
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 sensor apparatus of the first aspect of the invention is provided in
an inlet of the compressor.
According to a third aspect of the invention there is provided a method of
measuring the mass flow rate of a gas using a sensor apparatus comprising a
housing having an inner perimeter which defines an area through which the
gas may flow, the method comprising receiving gas in a first chamber which
extends around the area through which gas may flow, an entrance being
distributed around the first chamber, receiving downstream gas in a second
chamber which extends around the area through which gas may flow, an exit
being distributed around the second chamber, and using one or more sensors
to measure a pressure difference between pressure in the first chamber and
pressure in the second chamber.
The gas may be flowing into a compressor of a 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;
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6
Figure 2 schematically depicts a compressor inlet of a turbocharger which
includes a sensor apparatus according to an embodiment of the invention;
Figure 3 schematically depicts the sensor apparatus in transverse crosssection;
Figure 4 schematically depicts part of the sensor apparatus in cross section
and a sensor which forms part of the sensor apparatus;
Figure 5 schematically depicts the sensor apparatus in cross-section;
Figure 6 schematically depicts part of the sensor apparatus in cross-section;
Figure 7 schematically depicts in cross-section part of an inlet of the sensor
apparatus;
Figure 8 schematically depicts in cross-section part of an outlet of the sensor
apparatus;
Figure 9 is a schematic circuit diagram of a sensing device which forms part
of an embodiment of the invention;
Figure 10 is a schematic circuit diagram of the sensing device of Figure 9 in
greater detail;
Figure 11 schematically depicts a sensor of the sensor apparatus according to
an alternative embodiment of the invention;
Figure 12 schematically depicts part of the sensor apparatus in partial crosssection;
Figure 13 schematically depicts an entrance of the sensor apparatus
according to an embodiment of the invention; and
Figure 14 schematically depicts an entrance of the sensor apparatus
according to an alternative embodiment of the invention.
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
66284305-1-proberts
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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 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 8 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
15 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.
66284305-1-proberts
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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.
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).
An upper portion of Figure 2 schematically depicts a modified cross-section
through a compressor housing 2a which has a similar construction to the
compressor housing 2 of the turbocharger shown in Figure 1. A sensor
apparatus 30 according to an embodiment of the invention is provided at an
air inlet 22a of the compressor housing 2a. The sensor apparatus 30
comprises an annular housing 29 containing a sensing channel 31, an inlet 32
and an outlet 33. The annular housing 29 has an inner perimeter which
defines an area through which air may flow into the compressor. Also shown
in Figure 2 is a wall structure 62 which is also in the air inlet 22a of the
compressor housing 2a. The wall structure may be connected to the sensor
apparatus 30.
66284305-1-proberts
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A lower portion of Figure 2 is an enlarged view of an encircled part of the
upper portion of Figure 2. Referring to the lower portion of Figure 2, the inlet
32 comprises an annular chamber 35 (which may be referred to as a first
chamber or inlet chamber) to which an entrance 36 is connected. The
entrance 36 is defined by walls 37, 38 which extend at a non-zero angle
relative to a central axis V-V of the turbocharger. The entrance 36 is annular
and extends around the inlet chamber 35. Consequently, the inlet chamber
35 receives air from around the circumference of the sensor apparatus 30.
The entrance 36 may for example be a slit, an opening, a plurality of slits or a
plurality of openings. The entrance 36 may extend continuously or
substantially continuously around the inlet chamber 35. The entrance 36 may
extend intermittently around the inlet chamber 35. The entrance 36 may be
distributed around the inlet chamber 35.
The outlet 33 of the sensor apparatus 30 comprises an annular chamber 43
(which may be referred to as a second chamber or outlet chamber) to which
an exit 44 is connected. The exit 44 extends around the outlet chamber 43,
and thus allows air to leave the outlet chamber from around an inner
perimeter of the sensor apparatus. The exit 44 may for example be a slit, an
opening, a plurality of slits or a plurality of openings. The exit 44 may extend
continuously or substantially continuously around the outlet chamber 43. The
exit 44 may extend intermittently around the outlet chamber 43. The exit 44
may be distributed around the outlet chamber 43.
An annular bracket 45 which is connected to an interior wall of the
compressor housing 2a supports the sensor apparatus 30. The annular
bracket 45 may form a wall of one or more of the inlet chamber 35, outlet
chamber 43, and sensing channel 31 (e.g. as shown). Alternatively, the
annular bracket may merely support the sensor apparatus 30 and not form
part of a wall of the sensor apparatus.
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Figure 3 is a schematic cross-section of the sensor apparatus 30 viewed from
one end. The inlet chamber 35 receives air from around the circumference of
the sensor apparatus 30, as is represented schematically by eight arrows
distributed around Figure 3 which point into the inlet chamber. The sensing
channel 31 has an entrance 40 which opens into the inlet chamber 35. Thus,
air passes from the inlet chamber 35 via the entrance 40 into the sensing
channel 31 (as represented by a solid black arrow). The air in the sensing
channel 31 passes a sensor 41 (described further below) which is provided in
the sensing channel and which may be used to measure mass air flow. The
sensing channel 31 is provided with an exit 42 which opens into the outlet
chamber 43 of the sensor apparatus. Air passes from the sensing channel 31
into the outlet chamber 43 via the exit 42 (as represented by a solid black
arrow). Air may pass out of the outlet chamber 43 from around the inner
perimeter of the sensor apparatus 30, as is represented schematically by
eight arrows distributed around Figure 3 which point out of the outlet chamber.
The sensing channel 31 limits flow of air between the inlet chamber 35 and
the outlet chamber 43 such that a pressure differential exists between them
(i.e. pressure does not equalise between the inlet and outlet chambers 35,
43).
The sensor apparatus 30 may be configured such that less than 1/50th of the
mass air flow passing into the compressor travels through the sensing
channel 31. The sensor apparatus 30 may be configured such that less than
1/100th, less than 1/200th or as little as 1/400th of the mass air flow passing
into the compressor travels through the sensing channel 31. The majority of
the mass air flow passes through an area 90 defined by the inner perimeter
91 of the sensor apparatus 30. The inner perimeter 91 of the sensor
apparatus 30 may for example have a diameter of around 60mm. The inner
perimeter 91 of the sensor apparatus 30 may for example define an area of
around 3600mm2 through which air may travel to the compressor.
66284305-1-proberts
11
The sensing channel 31 occupies only a small fraction of the circumference of
the sensor apparatus 30. Thus, space remains in which sensors arranged to
measure properties other than mass air flow may be provided. For example,
in Figure 3 an additional chamber 92 is provided adjacent to the sensing
channel 31. The additional chamber 92 is connected via an entrance 93 to
the outlet chamber 43 but is not connected to the inlet chamber 35. As a
result, there is no flow of air through the additional chamber. Consequently,
there is little or no ‘swirl’ of air in the additional chamber 92 (the additional
chamber is sheltered from the effects of airflow). This is particularly the case
at ends of the additional chamber which are located away from the entrance
93. Air in the additional chamber 92 has the same pressure as air in the
outlet chamber 43. A temperature sensor 94 (e.g. a thermistor) is provided
adjacent to one end of the additional chamber 92. Since there is relatively
little movement of air at the end of the additional chamber 92, the temperature
sensor 94 provides a measurement of the air temperature which is
substantially unaffected by the rate of flow of air through the compressor inlet
(this may be considered to be a measurement of the ambient air
temperature). An air density sensor 95 is also provided in the additional
chamber 9, the air density sensor 95 being provided at an opposite end of the
additional chamber 92 from the temperature sensor 94. There is relatively
little movement of air at the end of the additional chamber 92, and therefore
the measurement provided by the air density sensor 95 is substantially
unaffected by the rate of flow of air through the compressor inlet. Any suitable
sensor may be provided in the additional chamber. For example, a humidity
sensor or a gas spectrometry sensor may be provided.
Although only one additional chamber 92 is shown in Figure 3, two or more
additional chambers may be provided. The additional chamber(s) may be
provided at any suitable location in the sensor apparatus 30.
Figure 4 schematically shows part of the sensor apparatus 30 of Figure 3. In
Figure 4 the sensing channel 31, entrance 40 and exit 42 may all be seen. An
66284305-1-proberts
12
enlarged cross-sectional view of the sensor 41 is shown to the right hand side
of Figure 4. As may be seen, the sensor 41 comprises a flow restrictor 46, a
sensing device 47 being provided inside the flow restrictor. Because the
pressure of air is higher at an input side of the sensor 41 (and in the inlet
chamber 35) than the pressure at an output side of the sensor (and the outlet
chamber 43) air flows through the flow restrictor 46. The flow of air through
the flow restrictor is indicated schematically by dashed arrow A. The sensing
device 47 measures the mass flow rate of air flowing through the flow
restrictor 46. Although the illustrated flow restrictor 46 has a particular form,
any suitable form of flow restrictor may be used. An orifice plate may be used
as the flow restrictor.
The flow of air through the sensor 41 is determined by the difference in
pressure on either side of the sensor 41 and by the diameter of the flow
restrictor 46. The internal diameter of the flow restrictor 46 may be uniform
(or substantially uniform) in order to ensure that pressure within the flow
restrictor is equal throughout the length of the flow restrictor. The length of
the flow restrictor 46 may have an insignificant effect upon the flow of air
through the flow restrictor, provided that the flow restrictor is relatively short.
The flow restrictor 46 may for example have a length of around 1 cm or less.
The pressures on either side of the sensor 41 correspond respectively with
the pressures in the inlet chamber 35 and the outlet chamber 43 (the sensing
channel 31 may have a diameter which is sufficiently large that it does not
significantly affect these pressures). The diameter and length of the flow
restrictor 46 may be accurately controlled during manufacture of the flow
restrictor. Thus, the mass flow rate of air measured by the sensor 41 may be
used to provide an accurate determination of the mass flow rate of air
travelling through the compressor inlet 22a (see Figure 2).
The flow restrictor 46 acts as a Venturi. The mass flow rate of air through the
flow restrictor 46 is proportional to the difference in pressure at the input and
66284305-1-proberts
13
output sides of the flow restrictor. Therefore, by measuring the mass flow rate
of air through the flow restrictor 46, a measurement of the pressure differential
is effectively being performed. The pressure differential in the inlet and outlet
chambers 35, 43 is itself determined by the mass flow rate of air passing
through the compressor inlet 22a. Consequently, the mass flow rate
measured by the sensing device 47 provides an indication of the mass flow
rate of air passing through the compressor inlet 22a. The mass flow rate of
air passing through the sensor 41 may be directly proportional to the mass
flow rate of air passing through the compressor inlet 22a. Alternatively, some
other relationship may exist between the mass flow rates of air in the sensor
and air passing through the compressor inlet. A microprocessor or other
control apparatus (not shown) may store information regarding the
relationship between the mass flow rates of air through the sensor 41 and air
passing through the compressor inlet, thereby allowing a measurement
performed using the sensor to be converted to a mass flow rate of air passing
through the compressor inlet.
The sensing device 47 may for example comprise transistors 48, 49 which are
connected to a circuit that measures their temperatures (the circuit is
described further below). The measured temperatures of the transistors 48,
49 may be used to determine the mass flow rate of air flowing through the
flow restrictor 46.
The flow restrictor 46 provides a number of benefits. The flow restrictor 46
may for example dampen pressure within the inlet and outlet chambers 35,
43. This damping may improve equalisation of pressure within the inlet
chamber 35, and similarly improve equalisation of pressure within the outlet
chamber 43. This may improve the accuracy of the air mass flow rate
measured by the sensor 41. Similarly, because the flow of air through the
sensor 41 is restricted by the flow restrictor 46, the entrance 36 (see Figure 6)
of the inlet chamber 35 may be widened to cover a greater intake area without
giving rise to increased turbulence at the entrance (the flow restrictor 46 limits
66284305-1-proberts
14
the inflow of air). This may improve equalisation of pressure in the inlet
chamber 35 with pressure in the compressor inlet. Similarly, widening the exit
44 of the outlet chamber 43 may improve equalisation of pressure in the outlet
chamber 43 with pressure in the compressor inlet. Finally, the flow restrictor
46, because it is not formed integrally with other parts of the sensor
apparatus, can be manufactured with greater accuracy than the entire sensor
apparatus 30, or than the sensing channel 31 (for a given cost of
manufacture). In other words, because the flow restrictor 46 can be
manufactured with an accurately dimensioned inner diameter, the tolerances
with which the other parts of the sensor apparatus 30 such as the sensing
channel 31 are manufactured may be greater (compared with the situation if
the flow restrictor were not present). The flow restrictor 46 may for example
be manufactured from a material which has stable dimensions over a given
range of temperatures (e.g., the temperature range which is expected during
normal operation of the compressor). The flow restrictor 46 may for example
be made using ceramic, aluminium or a suitable glass polymer.
The flow restrictor 46 may have any suitable cross-sectional shape. The flow
restrictor 46 may for example have a circular cross-sectional shape or a
rectangular cross-sectional shape. It may be easier to provide the sensing
device 47 and the transistors 48, 49 in a flow restrictor with a rectangular
cross-sectional shape.
The effect of the flow restrictor 46 may be expressed in terms of the airflow
that would pass between the inlet chamber 35 and the outlet chamber 43 if
there were no restriction between them. The flow restrictor may for example
restrict this airflow to between around 50% and around 95% of the
unrestricted airflow. The cross-sectional area of the flow restrictor 46 may be
selected to provide a desired restriction of the airflow between the inlet and
outlet chambers 35, 43. The cross-sectional area of the flow restrictor 46 may
be selected based on a desired flow rate of air through the flow restrictor
using the following equation:
66284305-1-proberts
15

2( 1 2) 2 P P
Q A


where Q is the flow rate of air , A is the cross-sectional area of the flow
restrictor, P1 and P2 are air pressures on either side of the flow restrictor, and
ρ is the density of the air.
In a conventional compressor inlet operating in a conventional manner, a flow
restrictor cross-sectional area of around 7 mm2 may for example provide a
volume flow rate of air of around 5 ml/s. The cross-sectional area of around 7
mm2 could for example be provided by a circular flow restrictor with a
diameter of around 3mm, or could for example be provided by a rectangular
flow restrictor with internal dimensions of around 1.5 x 5 mm. For a volume
flow rate of air of around 10 ml/s the cross-sectional area of the flow restrictor
may be scaled up accordingly, e.g. to around 14 mm2. The flow restrictor 46
may have any suitable cross-sectional area.
Although the sensing channel 31 has a particular length in Figures 3 and 4,
the sensing channel may have any suitable length. Lengthening the sensing
channel 31 will tend to straighten air which is travelling through the sensing
channel, thereby reducing turbulence in the air. However, in embodiments in
which the flow restrictor 46 is used, the flow rate through the flow restrictor
may be sufficiently low that turbulence is not present to a significant degree.
Where this is the case, a lengthened sensing channel 31 is not required, and
the sensing channel may merely be long enough to accommodate the flow
restrictor 46 and to connect the inlet and outlet chambers 35, 43.
An advantage of the sensor apparatus 30 is that, because it receives air from
around the perimeter of the area 90 enclosed by the annular housing 29, the
sensing device 47 measures a pressure which corresponds to the pressure
which exists around the annular housing. In other words, the pressure is
66284305-1-proberts
16
effectively sampled around the perimeter of the area 90 enclosed by the
sensor apparatus 30. It is conventional to provide a pressure sensor on a rod
which extends radially inwardly from a wall of a compressor inlet. When the
pressure is sensed in this conventional manner only the pressure at one
location in the compressor inlet is measured. The distribution of pressure
within the compressor inlet may vary as the airflow into the inlet changes, for
example with the distribution of peaks of pressure moving around the inlet as
the airflow into the inlet increases or decreases. This may cause prior art
pressure sensors to provide incorrect or inconsistent pressure measurements.
This problem is avoided by the sensor apparatus 30 according to the
embodiment of the invention because it samples air from around an area 90
enclosed by the sensor apparatus 30 in the compressor inlet 22a.
The cross-sectional area of the inlet chamber 35 may be sufficiently large that
the pressure of air within the inlet chamber can equalise (or substantially
equalise) around the inlet chamber. The cross-sectional area of the inlet
chamber may for example be around 100mm2 or more, and may for example
be greater than around 300mm2. The cross-sectional area of the inlet
chamber 35 which will provide pressure equalisation may depend upon the
diameter of the inlet chamber. In an embodiment, the inlet chamber 35 may
have an outer diameter of around 90mm, for example with a cross-sectional
area of around 100mm2. The inlet chamber 35 may have any suitable outer
diameter.
The pressure of air at the entrance 40 to the sensing channel 31 may be the
equalised pressure of air around the inlet chamber 35. Therefore, a local
increase of the pressure at an entrance 36 location which for example
happens to be close to the entrance 40 of the sensing channel 31 will not give
rise to a significant measurement error, because the local pressure increase
will be distributed around the inlet chamber 35.
66284305-1-proberts
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The cross-sectional area of the outlet chamber 43 may be sufficiently large
that air pressure can equalise (or substantially equalise) around the outlet
chamber. This prevents pressure measurements being influenced for
example by pressure fluctuations which happen to occur adjacent to the outlet
chamber 43 in the vicinity of the exit 42 of the sensing channel 31. The effect
of such pressure fluctuations is equalised around the outlet chamber 43. In
practice, pressure fluctuations may be smaller in the vicinity of the outlet
chamber 43 than in the vicinity of the inlet chamber 35 (the pressure becomes
more uniform as the air moves further into the compressor inlet).
The cross-sectional area of the outlet chamber may for example be around
100mm2 or more, and may for example be greater than around 200mm2. The
cross-sectional area of the outlet chamber 43 which will provide pressure
equalisation may depend upon the diameter of the outlet chamber. In an
embodiment, the outlet chamber 35 may have an outer diameter of around
70mm, for example with a cross-sectional area of around 100mm2. The outlet
chamber 43 may have any suitable outer diameter.
The entrance 36 shown in Figures 2 and 3 is annular and extends around the
entire inlet chamber 35. The entrance 36 may be considered to be a slit.
However, the entrance to the inlet chamber may have any suitable form. For
example the entrance may comprise a series of openings which are
distributed around the circumference of the inlet chamber 35. The openings
may for example be distributed with substantially equal separations (or may
be distributed with different separations). A slit and a distributed series of
openings may both be considered to be examples of an entrance which is
distributed around the inlet chamber 35.
Similarly, the exit 44 shown in Figures 2 and 3 is annular and extends around
the entire outlet chamber 43. The exit 44 may be considered to be a slit.
However, the exit from the outlet chamber may have any suitable form. For
example the exit may comprise a series of openings which are distributed
66284305-1-proberts
18
around the circumference of the outlet chamber 43. The openings may for
example be distributed with substantially equal separations (or may be
distributed with different separations). A slit and a distributed series of
openings may both be considered to be examples of an exit which is
distributed around the annular housing.
Figure 5 shows schematically in transverse cross-section the sensor
apparatus 30 secured by the annular bracket 45 to the compressor inlet wall
22a. The direction of flow of air A through the compressor inlet is indicated by
arrows in Figure 5. The sensor apparatus 30 corresponds with that shown in
Figure 2 but has a slightly modified shape. The inlet chamber 35 has a
modified cross-sectional shape, and the annular housing of the sensor
apparatus 30 has a more pronounced slope in the direction of flow of air A.
Due to the slope of the annular housing, the entrance 36 of the inlet chamber
35 is upstream of the air flow A from the exit 44 of the outlet chamber 43. The
sloping configuration of the annular housing of the sensor also directs airflow
into the compressor.
Unlike in Figure 2, the entrance 40 to the sensing channel 31 is shown, and
consequently the inlet chamber 35 shown in Figure 5 has a slightly different
appearance from the inlet chamber shown in Figure 2. A baffle 39 extends
between the entrance 36 and the entrance 40 to the sensing channel 31. The
baffle forces the air to change direction after passing through the entrance 36
and before entering the sensing channel 31, thereby preventing direct
injection of air from the entrance 36 into the sensing channel entrance 40.
The inlet chamber 31 is shaped such that there is no direct route from the
entrance 36 to the sensing channel 31. The change of direction of the air in
the inlet chamber 31 promotes equalisation of pressure around the inlet
chamber 35 (the air may be said to diffuse into the inlet chamber). At
locations around the sensor apparatus 30 where the sensing channel
entrance 40 is not provided, the baffle 39 is extended such that it forms a wall
which extends fully across the inlet chamber 35 (as shown in Figure 2).
66284305-1-proberts
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An outer perimeter of the sensor apparatus 30 is not pressed against the
compressor inlet wall 22a, but instead a gap 54 exists between the sensor
and the pressure inlet wall. The gap 54 may for example be 1-2mm wide.
One or more openings 52 are provided in the bracket 45. The openings 52
may also be 1-2 mm wide, and may be distributed around the bracket 45.
The bracket 45 presses against the compressor inlet interior wall 22a. The
gap 54 between the sensor apparatus 30 and the compressor inlet wall 22a,
together with the openings 52, form a generally annular channel through
which air may flow. A so-called boundary layer of air may exist at the wall 22a
of the compressor inlet, the boundary layer having properties which are not
representative of the main body of air passing through the compressor inlet.
This boundary layer of air passes through the gap 54 and openings 52 and
thus does not influence the pressure as measured by the sensor. This may
be desirable because, as mentioned, the boundary layer may not be
representative of the pressure of the general body of air passing through the
compressor inlet. The sensor apparatus 30 includes a sloping portion 51 at
an upstream end which slopes downstream and towards the compressor wall
22a. The sloping portion 51 acts to push the boundary layer away from the
entrance 36 of the sensor, and thereby prevents the boundary layer from
influencing the measured pressure (or reduces the extent to which the
boundary layer may influence the measured pressure).
Also shown in Figure 5 is a wall structure 62 (which may be referred to as a
baffle) which may also be located in the compressor inlet. The sensor
apparatus 30 may have an innermost diameter which is substantially equal to
or greater than the inner diameter of the wall structure 62. This may ensure
that air which passes through the area encircled by the sensor apparatus 30
is not then obstructed as it travels beyond the sensor.
The wall structure 62 may include a sloping portion 53 which is configured to
direct boundary layer air that has passed through gaps 54, 52 such that it
66284305-1-proberts
20
rejoins the main airflow of the compressor (i.e. the airflow indicated by arrows
A). The wall structure 62 may provide a map width enhancement to a
turbocharger (as is known from the prior art). The wall structure may reduce
audible noise during operation of a turbocharger (as is known from the prior
art).
The sensing channel 31 may include one or more components which are
arranged to modify the flow of air within the sensing channel. For example,
one or more air straighteners 55 (e.g. air straightening tubes) may be used to
provide a non turbulent flow of air in the sensing channel 31. An example of
this is shown schematically in Figure 6. The air straighteners may for
example comprise an array of substantially parallel tubes which are provided
in the sensing channel 31. The air straighteners may condition the flow of air
into a constant stream, removing fluctuations from the air. This may be
beneficial if the flow of air through the sensing channel 31 is sufficiently high
that turbulence may be generated in the air. In an embodiment in which a
flow restrictor is provided in the sensing channel 31, the flow of air may be
sufficiently low that significant turbulence will not occur, in which case air
straighteners may be omitted.
Figure 7 shows schematically in cross-section an inlet portion of the sensor
apparatus 30. As is represented schematically by arrows, the annular inlet
chamber 35 is shaped to induce a swirling action in air which enters the inlet
chamber. This swirling action will tend to fling oil droplets and other
contamination outwards towards surfaces of the inlet chamber 35. This is
advantageous because oil droplets which are received on walls of the inlet
chamber 35 will not travel to the sensing channel 31 and therefore will not
contaminate the pressure sensing device in the sensing channel. One or
more drains (not shown) may be provided in the inlet chamber 35 to allow oil
to flow from the inlet chamber. The inlet chamber 35 has a relatively large
surface area when viewed in cross-section. This is advantageous because it
provides more opportunities for oil to come into contact with and adhere to a
66284305-1-proberts
21
surface (compared with a relatively small surface area). The surface area of
the inlet chamber 35 when viewed in cross-section is larger than for example
would be the case if the inlet chamber were to be circular or rectangular in
cross-section (for a given inlet chamber size).
In general operation of the compressor, air flow is as indicated by arrows A in
Figure 5. However, under engine braking there may be some flow of gas in
the opposite direction, such that the pressure at the exit 44 of the sensor
apparatus 30 is greater than the pressure at the entrance 36 of the sensor
apparatus. Where this is the case, gas may flow into the exit 44 and thus into
the outlet chamber 43. This gas may have travelled from an internal
combustion engine and may thus carry a substantial amount of oil. It would
therefore be disadvantageous for that oily gas to be incident upon the
pressure sensing device since this would cause contamination build-up on the
pressure sensing device. As shown schematically in Figure 8, a relatively
tortuous path must be followed by such back pressure oily gas in order to
reach the sensing channel 31. Furthermore, the shape of the outlet chamber
43 induces swirling action in the gas in the outlet chamber. This swirling
action will tend to fling oil droplets and other contamination outwards towards
surfaces of the outlet chamber 43. The relatively tortuous path to the sensing
channel and the swirling action of gas induced by the shape of the outlet
chamber 43 both restrict the extent to which oily gas will reach the sensor 41,
thereby limiting contamination of the sensor.
Figure 9 shows schematically in more detail the sensing device 47 shown in
Figure 4. The sensing device 47 comprises first and second bipolar junction
transistors 48, 49 which are arranged such that the temperature of the
transistors is affected by air which flows through the flow restrictor 46 of the
sensor 41 (see Figure 4). The flow of air over the transistors 48, 49 is
represented schematically in Figure 9 by an arrow A. Although arrow A is
shown to one side of the transistors 48, 49 and displaced away from one of
the transistors 48, this is merely a consequence of the schematic nature of the
66284305-1-proberts
22
figure. Dotted lines extending from the arrow A to the transistors 48, 49 are
intended to indicate that the air is in thermal contact with the transistors. The
transistors 48, 49 may be surface mount transistors. If the surface mount
transistors include a thermal barrier, then the surface mount transistors may
be mounted such that the thermal barrier is on an opposite side of the
transistors from the airflow passing through the flow restrictor 46. The
transistors 48, 49 may be arranged such that they lie flush (or substantially
flush) with an inner surface of the flow restrictor 46. Alternatively, the
transistors 48, 49 may be arranged such that they partially project from an
inner surface of the flow restrictor 46. The partial projection may be
sufficiently small that it does not induce a significant amount of turbulence into
air flowing through the flow restrictor (i.e., turbulence which would have a
significantly detrimental effect upon airflow measurements is not induced).
The sensing device 47 is a thermal flow meter that makes use of King’s Law,
which states that the heat energy removed from a hot body is proportional to
the mass flow rate of air passing over the hot body. A comparison is made by
the sensing device 47 between a hot body and an unheated body. In this
case the hot body is a heated transistor (second transistor 49). A constant
amount of power is delivered to the second transistor 49, thereby heating the
second transistor to a temperature which is above the temperature of the first
transistor 48 (the first transistor is not heated and is the unheated body). The
difference in temperatures between the first and second transistors 48, 49 will
depend upon the mass flow rate of the air flowing over the first and second
transistors.
An operational amplifier 70 generates an output VOut which corresponds to the
difference between the base emitter voltage (VBE1) of the first transistor 48
and the base emitter voltage (VBE2) of the second transistor 49. The base
emitter voltage of a silicon bipolar transistor has a linear dependence upon
the temperature of the transistor (-1.79mV/ ºC). Therefore, the output Vout of
the operational amplifier 70 is directly proportional to the temperature
66284305-1-proberts
23
difference between the first and second transistors 48, 49. The linear
relationship between the base emitter voltage and temperature applies over
the expected operational temperature range of the sensing device 47 (this
may be from -50 ºC to 150 ºC). Thus, the output Vout of the operational
amplifier 70 may be used to determine the temperature difference between
the first and second transistors 48, 49 over the expected operational
temperature range of the sensing device 47.
A power control circuit 71 controls the delivery of power to the second
(heated) transistor 49. The power delivered to the second transistor 49 may
heat the second transistor such that, in the absence of airflow, the second
transistor 49 is held at a temperature which is around 27 ºC above the
temperature of the first transistor 48. The thermal resistance between the
junction of a silicon bipolar transistor and the case of the silicon bipolar
transistor may for example be around 340 ºC per watt. Thus, since the
second transistor 49 is a silicon bipolar transistor, the temperature of the
second transistor may be kept at around 27 ºC above the temperature of the
first transistor 48 by dissipating 80 mW in the second transistor (in the
absence of cooling by an airflow). When air is flowing over the transistors 48,
49, the cooling effect of the air on the second transistor will be significant (due
to the elevated temperature of the second transistor). Consequently, a
significant amount of heat will be lost from the second transistor 49. Since the
power delivered to the second transistor 49 is not increased (it is maintained
at 80 mW), the temperature difference between the first and second
transistors is reduced. The output Vout of the operational amplifier 70
measures the reduced temperature difference. The output Vout of the
operational amplifier 70 therefore provides a measurement of the mass flow
rate of air over the first and second transistors 48, 49.
Since the temperature variation of the base emitter voltage of a silicon bipolar
transistor is known and is linear, the base emitter voltage VBE1 of the first
transistor 48 provides an indication of the temperature of the sensing device
66284305-1-proberts
24
47. When no air is flowing through the flow restrictor 46, the temperature
measured using VBE1 will correspond to the ambient temperature of the
sensor apparatus 30.
The sensing device 47 is shown in more detail in Figure 10. The first
transistor 48 is connected via a resistor R1 to a voltage supply Vs. The
voltage supply Vs may for example be a 5V supply, and the resistor R1 may
for example have resistance of around 10k. The emitter of the first
transistor 48 is connected via a resistor R2 to ground. The resistor R2 may for
example have a resistance of around 30Ω. The base and collector of the first
transistor 48 are connected together. An operational amplifier 72 has an
inverting input connected to the base and collector of the first transistor 48,
and has a non-inverting input connected to the emitter of the transistor. The
operational amplifier 72 thus provides an output which is indicative of the base
emitter voltage VBE1 of the first transistor 48, and which is thus indicative of
the temperature of the first transistor. Negligible power is dissipated through
the first transistor 48, and the temperature of the first transistor thus
corresponds with the general temperature of the sensing device 47. This
depends upon the ambient temperature of the sensor apparatus 30 and the
flow of air through the flow restrictor 46.
The second transistor 49 is connected between the voltage supply Vs and the
second resistor R2. The power control circuit 71 is indicated by a dotted line.
The power control circuit 71 is configured to maintain the power delivered to
the second transistor 49 at 80mW. The power control circuit automatically
reduces or increases the current supplied to the second transistor 49 to
compensate for variation of the voltage across the second transistor. The
power control circuit 71 comprises an operational amplifier 73 which has an
output connected to the base of the second transistor 49. An inverting input
of the operational amplifier 73 is connected to the emitter of the second
transistor 49. The power control circuit 71 further comprises an operational
amplifier 75 with an output which is connected to a non-inverting input of the
66284305-1-proberts
25
operational amplifier 73. An inverting input of the operational amplifier 75 is
connected between two resistors R3, R4. The resistor R3 may for example
have a resistance of around 70kΩ and the resistor R4 may for example have a
resistance of around 10Ω. A non-inverting input of the operational amplifier
75 is connected between a resistor R5 and a diode D1. The resistor R5 may
for example have a resistance of 10kΩ. The power control circuit 71 may
adjust the current delivered to the second transistor 49 between 15 mA and
22 mA to compensate for changes of the voltage across the second transistor,
thereby maintaining the power delivered to the second transistor at 80 mW.
By delivering a constant amount of power to the second transistor 49, the
power control circuit 71 ensures that the voltage VOut output of the operational
amplifier 70 indicates the mass flow rate of air B which is flowing through the
flow restrictor 46 (see Figure 4). As mentioned further above, the output VBE1
of the operational amplifier indicates the temperature in the flow restrictor 46.
Due to manufacturing tolerances, the 27ºC offset between the temperatures of
the first and second transistors 48, 49 which is referred to above may not be
provided in every case by the 80 mW power delivered to the second
transistor. Instead, there may be some variation between the temperature
offsets of different sensing devices 47. A measurement of the temperatures
of the first and second transistors 48, 49 may be performed when there is no
air flowing through the sensing device 47. This measurement provides an
accurate indication of the temperatures, irrespective of manufacturing
tolerances, because the temperatures are governed by the -1.79mV/ºC
temperature dependence of the base emitter voltage silicon bipolar transistors
(a material property which is unaffected by manufacturing tolerances). The
measured temperature differential in the absence of airflow may be stored
and used to calibrate subsequent airflow measurements. The calibration may
for example be performed by a microprocessor or other control or monitoring
apparatus.
66284305-1-proberts
26
In the above description of the sensing device in connection with Figures 9
and 10, specific values of resistance, current, voltage and power are referred
to. These values are merely examples, and other suitable values may be
used. Similarly, although a particular circuit is shown in Figure 10, other
circuits may be used.
The sensing device 47 of Figures 9 and 10 is merely an example of a sensing
device which may be used by embodiments of the invention. Other sensing
devices may be used, for example in conjunction with a flow restrictor 46, or
without a flow restrictor.
An alternative sensor may comprise a flow restrictor and two transistors, one
of the transistors being heated to a temperature which is higher than the
temperature of the other transistor (which may be unheated). This may be
done for example by using a modified circuit to hold VBE2 at the second
transistor 49 at a constant value (the constant value corresponding to a
transistor temperature which is higher than the maximum expected air
temperature). The base current drawn by the second (heated) transistor 49
will then provide an indication of the mass flow rate of air passing over the
second transistor. The current will increase as the flow of air increases
because more current will be needed to keep the second transistor at the
constant temperature. In general, a temperature difference between
transistors may be maintained during flow of air through the flow restrictor, the
power required to maintain the temperature difference providing an indication
of the mass flow rate of air passing over the transistors.
The transistors 48, 49 of the sensing device 47 are both provided within the
flow restrictor 46 (e.g. as shown in Figure 4) and both are arranged such that
air flowing through the flow restrictor flows over them. In an alternative
arrangement (not illustrated), a first transistor is provided within the flow
restrictor 46 and a second transistor is provided outside of the flow restrictor.
The second transistor may for example have a surface located in the inlet
66284305-1-proberts
27
chamber 35 such that it provides a measurement of the air temperature in the
inlet chamber. In this arrangement the sensing device may be considered to
be partially within the sensing channel and partially outside of the sensing
channel. Power may be provided to the first transistor in order to raise the
temperature of the first transistor. The resulting temperature of the first
transistor (as indicated by the base emitter voltage of that transistor) may be
compared with the temperature of the second transistor (as indicated by the
base emitter voltage of that transistor) in order to determine the mass flow
rate of air through the flow restrictor. Where this arrangement is used, the
first and second transistors may be substantially thermally isolated from each
other, such that heat delivered to the second transistor does not cause
significant heating of the first transistor (heat of the first transistor could
reduce the accuracy of mass flow rate measurements). This may be done for
example by separating the first transistor from a circuit board on which the
second transistor is provided. Thermal insulation may be provided between
the first transistor and the circuit board. Alternatively, the second transistor
may be separated from the circuit board. Thermal insulation may be provided
between the second transistor and the circuit board. In general, thermal
insulation may be provided between the first and second transistors.
In an embodiment, the power provided to the first transistor may be adjusted
in order to keep the first transistor at a constant elevated temperature. The
power required to keep the transistor at the constant elevated temperature will
provide a measurement of the mass flow rate of air through the flow restrictor.
An alternative sensor may measure mass flow rate by measuring transfer of
heat between two devices. A heat source of the sensor may be a self heating
device such as a thermistor or any device that generates heat. A device such
as a thermistor or metal which changes its resistivity according to temperature
may be provided adjacent to the heat source. Airflow transports heat from the
heater and warms the device, the extent to which the device is warmed being
determined by the mass flow rate of the air. Devices may be provided either
66284305-1-proberts
28
side of the heat source so that the mass flow rate of air in either direction can
be measured (thereby allowing for example measurement of back pressure).
The above described sensors include a flow restrictor which is manufactured
separately from the sensing channel. The flow restrictor may for example be
manufactured from a material which differs from the material used to make
the sensing channel and other parts of the sensor apparatus (and may be
made with more accurately controlled dimensions). In an alternative
approach, a separately fabricated flow restrictor may be omitted, with the
sensing channel itself being provided with a narrow diameter such that it acts
to restrict airflow. Where this approach is used, the dimensions of the sensing
channel should be accurately controlled. This may be more difficult to
achieve than accurate control of dimensions of a separately fabricated flow
restrictor. When the flow restrictor is omitted the sensing device 47 may be
provided on an inner wall of the sensing channel.
Figure 11 shows an alternative sensor in the sensing channel. The alternative
sensor comprises a hot wire mass flow rate sensor 58. The hot wire mass
flow rate sensor comprises a wire 59 which extends across the sensing
channel 31. A current is passed through the wire 59, thereby heating the
wire. The wire is cooled by the flow of air over the wire 59. The resistance of
the wire, which is linked to the temperature of the wire, therefore provides an
indication of the mass flow of air through the sensing channel.
A vortex generating device 60 may be provided in the sensing channel 31, as
shown schematically in Figure 7. The vortex generating device 60 may for
example have a conical upstream surface which includes a helix or generally
helical structure and which is arranged to induce a vortex in air flowing
through the sensing channel 31. The vortex will tend to push oil droplets or
other contaminants away from a central axis of the sensing channel 31, as is
represented schematically by dotted arrows B.
66284305-1-proberts
29
The vortex generating device 60 may be located axially centrally within the
sensing channel 31. The wire 59 may intersect a central axis of the sensing
channel 31. Radially outer ends of the wire 59 may be covered with an
insulating material 61, the insulating material being arranged such that the
flow of air over the insulating material does not significantly affect the
temperature of the wire. Since the vortex tends to push oil droplets or other
contaminants away from the central axis of the sensing channel 31, the oil
droplets or other contaminants are directed away from a sensing portion of
the wire 59, and instead bypass the wire or are incident upon the insulating
material 61. The effect of oil droplets and other contaminants on the mass
flow rate sensed by the wire 59 is thereby reduced.
The vortex generating device 60 may be used in conjunction with other forms
of sensor.
In a further alternative approach a Karman vortex sensor may be used as the
sensor. The Karman vortex sensor works by disrupting a laminar airflow
using a bow which extends across the airflow. A resulting wake in the airflow
consists of an oscillatory pattern of Karman vortices. The frequency of the
pattern is proportional to the air velocity and the amplitude of the pattern is
proportional to the density of the airstream. The oscillatory pattern of Karman
vortices may for example be measured using a pressure detector.
In a further alternative approach an ionising Karman vortex sensor may be
used as the sensor. The ionising Karman vortex sensor corresponds with a
conventional Karman vortex sensor except that a voltage is applied to the bow
which extends across the airflow, the bow thus causing ionisation of the air
which passes over it. Since the air is ionised the oscillatory pattern of Karman
vortices can be detected using electrodes located downstream of the bow.
Referring to Figure 11, the vortex generating device 60 may be replaced with
a bow to which a voltage is applied, and the wire 59 may be replaced with
electrodes (e.g. an electrode being provided on either side of the sensing
66284305-1-proberts
30
channel 31). An output signal from the electrodes will provide a measurement
of the oscillatory pattern of Karman vortices, thereby allowing the mass flow
rate of air in the sensing channel 31 to be determined.
In a further alternative approach, the sensor 41 may be a strain gauge
pressure sensor (e.g. a piezoresistive strain gauge). The piezoresistive strain
gauge may for example be provided in the sensing channel 31, blocking the
sensing channel such that pressure on one side of the strain gauge
corresponds with (or is related to) the pressure in the inlet chamber 35 and
pressure on the opposite side of the strain gauge corresponds with (or is
related to) pressure in the outlet chamber 43. Other suitable pressure
sensors which block the sensing channel 31 may be used. Where this
approach is used there is no flow of air through the sensing channel. The
chamber 35 may thus be referred to as the first chamber rather than the inlet
chamber, and the chamber 43 may thus be referred to as the second
chamber rather than the outlet chamber. The strain gauge could be placed in
an opening between the first chamber 35 and the second chamber 43 (i.e.
such that the strain gauge closes the opening), with the sensing channel 31
being omitted.
In a further alternative approach, there may be no connection between the
first and second chambers 35, 43. A pressure sensor may be used to
measure the pressure in the first chamber, and a pressure sensor may be
used to measure the pressure in the second chamber. The difference
between these measurements will correspond with the pressure difference
between the first and second chambers, thereby allowing the mass flow rate
of air flowing through the compressor inlet to be determined.
Figure 12 shows the sensor apparatus 30 in partial cross-section, together
with part of the compressor housing 2a. Also shown in Figure 12 is part of the
wall structure 62 which is located in the compressor inlet. The wall structure
may for example be formed from plastic. The sensor apparatus may be
66284305-1-proberts
31
formed from plastic. The sensor apparatus 30 and the wall structure 62 may
be fixed together.
As shown in Figure 12, the sensor apparatus 30 and wall structure 62 are
inserted into the compressor inlet. A circlip 80 (or other securing device)
holds the sensor apparatus 30 and wall structure 62 in place in the
compressor inlet. The sensor apparatus 30 includes an arm 81 which
extends from the generally annular part of the sensor apparatus. The arm 81
has some flexibility (or is hinged) such that an end of the arm which is distal
from a main portion of the sensor apparatus may be moved radially inward (as
shown). A socket 82 projects radially outwardly from the arm 81. A hole 83 is
provided in the compressor housing 2a, the hole having a shape which
corresponds with the exterior perimeter of the socket 82. To fit the sensor
apparatus 30, the sensor apparatus and wall structure 62 may be inserted into
the compressor inlet, following which the socket 82 may be drawn into the
hole 83. The socket 82 and hole 83 may help to position the sensor
apparatus 30 and wall structure 62 correctly in the compressor inlet, and may
help to retain the sensor apparatus and wall structure in the compressor inlet.
Wires (or other electrical connectors) extend within the arm 81 and to the
socket 82. The wires are connected to the sensor 41 (described further
above). A plug (not shown) may be plugged into the socket 82 thereby
providing electrical connection to the sensor 41 (see Figure 4). Output signals
may thus be taken from the sensor 41, and for example passed to a
microprocessor or other control or monitoring apparatus. The socket 82 may
also be used to deliver power to the sensor 41.
The arm 81 and socket 82 are merely examples of one way in which the
electrical connection may be made to the sensor. Other ways of electrically
connecting to the sensor will be apparent to those skilled in the art.
66284305-1-proberts
32
As will be appreciated from Figure 12, the sensor apparatus 30 may be easily
and conveniently fitted to a compressor inlet. Similarly, the sensor apparatus
may be easily and conveniently removed from the compressor inlet (e.g. to
allow repair or replacement of the sensor apparatus).
As described further above, Figure 2 shows an entrance 36 to the inlet
chamber 35, the entrance being annular and extending around the inlet
chamber 35. The entrance 36 may extend with a uniform width around the
inlet chamber 35. Alternatively, the width of the entrance 36 may be nonuniform
around the inlet chamber. For example, the width of the entrance 36
may vary as a function of circumferential position around the inlet chamber
35.
Figure 13 shows schematically the entrance 36 defined by outer 37 and inner
38 entrance walls. Other parts of the sensor apparatus are omitted in order to
avoid complicating the figure. However, the location of the entrance to the
sensing channel from the inlet chamber is indicated by an arrow 40. As may
be seen from Figure 13, the entrance 36 varies in width, having a minimum
width in the vicinity of the sensing channel entrance 40. The width of the
entrance 36 increases as the circumferential distance from the sensing
channel entrance 40 increases. In an embodiment, the width of the entrance
36 may continue to increase with circumferential distance from the sensing
channel entrance 40, such that the entrance has a maximum width on an
opposite side of the inlet chamber from the sensing channel entrance.
Alternatively, the width of the entrance 36 may increase to a maximum width
at a given circumferential distance from the sensing channel entrance 40 (e.g.
90° from the entrance), with the remainder of the entrance having the
maximum width. In general, the entrance 36 may be narrower in the vicinity
of the sensing channel entrance 40 and wider further away from the sensing
channel entrance. In this context the terms “narrower” and “wider” are not
intended to imply particular absolute sizes, merely to indicate a relative
difference in size.
66284305-1-proberts
33
Because the entrance 36 to the inlet chamber is narrower in the vicinity of the
entrance 40 to the sensing channel, the flow of air into the inlet chamber is
reduced in the vicinity of the sensing channel entrance. This reduces the
extent to which pressure in the compressor inlet 2a in the vicinity of the
sensing channel entrance 40 affects measurements obtained using the
sensor apparatus 30. If perfect equalisation of pressure in the inlet chamber
occurs then this reduction effect provides no benefit (in which case a uniform
inlet chamber entrance 36 may be used). If equalisation of pressure in the
inlet chamber is not perfect then this reduction effect may improve the
accuracy of measurements obtained using the sensor apparatus 30, by
preventing or reducing compressor inlet pressure in the vicinity of the sensing
channel entrance 40 from disproportionately affecting the measured pressure.
Imperfect equalisation of pressure may for example occur if flow through the
sensing channel 31 (see Figure 3) is not small enough to amortise any
differences in pressure in the inlet chamber 35. This could occur for example
if the flow restrictor 46 allows a substantial flow of gas (or if a flow restrictor is
not present).
In an alternative approach, schematically pictured in Figure 14, the entrance
36 to the inlet chamber has a uniform width around the inlet chamber (see
Figure 3), but portions of the entrance are closed. Closed portions 34a-d of
the inlet chamber entrance 36 are shaded black in Figure 14 and open
portions 36a-d of the entrance are white. As may be seen, a closed entrance
portion 34a is provided in the immediate vicinity of the sensing channel
entrance 40. Open portions 36a are provided either side of the closed portion
34a. Additional closed portions 34b are provided at outer ends of the open
portions 36a, followed by additional open portions 36b. Additional closed
portions 34c, open portions 36c and closed portions 34d follow. Finally, an
open portion 36d is provided on an opposite side of the inlet chamber from the
sensing channel entrance 40. The length of the open portions 36a-d
increases with circumferential distance from the sensing channel entrance 40.
66284305-1-proberts
34
Thus, the open portions 36a-d of the entrance 36 occupy a smaller proportion
of the entrance 36 in the vicinity of the sensing channel entrance 40 than on
an opposite side of the first chamber from the sensing channel entrance. This
reduces the flow of air into the inlet chamber 35 in the vicinity of the sensing
channel entrance 40. The flow of air into the inlet chamber 35 increases as
circumferential distance from the sensing channel entrance 40 increases.
This reduces the extent to which pressure in the compressor inlet 2a in the
vicinity of the sensing channel entrance 40 affects measurements obtained
using the sensor apparatus 30. As explained above, this may be beneficial if
equalisation of pressure in the inlet chamber 35 is not perfect.
The arrangement shown in Figure 14 may be considered to be an example of
the entrance 36 extending intermittently around the inlet chamber. It may be
considered to be an example of the entrance 36 being distributed around the
inlet chamber.
Although Figures 13 and 14 and the above description relate to the inlet
chamber entrance, the illustrated and described features may also be applied
to the exit 44 of the outlet chamber 43 (see Figures 2 and 3).
Embodiments of the invention sample a fraction of the air passing through the
compressor inlet, and generate an air mass flow rate measurement using that
sampled fraction. This is advantageous compared with a conventional air
mass flow rate sensor apparatus which extends across the compressor inlet,
because the conventional apparatus receives the full force of air flowing
through the inlet and is therefore more likely to become damaged or
contaminated.
In the above description, where a component is described as being annular
this may be interpreted as meaning that the component has a generally
annular shape, and encompasses for example a discontinuous annular
shape. For example, the annular entrance 36 may be discontinuous. The
66284305-1-proberts
35
annular entrance 36 could for example include structural elements which are
distributed around the entrance and which block the entrance at those
distributed locations.
In the above description, the term “circumference” may be interpreted as
referring to a path which extends around an annular component at any radial
position (i.e. it is not limited to the outer perimeter of the annular component).
Although the above description refers to air passing into the compressor inlet,
other gases may pass into the compressor inlet. For example, recirculated
exhaust gas may pass into the compressor inlet.
Embodiments of the invention may provide measurements of the mass flow
rate of gas out of the compressor inlet (i.e. flow of gas in the opposite
direction to the direction indicated in Figure 5).
Although the above description describes the sensor apparatus in the context
of a compressor inlet, the sensor apparatus may be provided at any suitable
location. For example, the sensor apparatus may be provided at some other
location in an internal combustion engine or may be connected to an internal
combustion engine.
Although the sensor apparatus described above has a generally annular
shape, the sensor apparatus may have any suitable shape. For example, the
sensor apparatus may be oval, or may be substantially rectangular (e.g. with
rounded corners to promote pressure equalisation through corners of
chambers of the sensor apparatus). Embodiments of the invention may
include a first chamber which receives gas from an entrance distributed
around the first chamber. The first chamber may be shaped to allow
substantial equalisation of pressure within the first chamber. Embodiments of
the invention may include a second chamber which receives gas from an
entrance distributed around the second chamber. The second chamber may
66284305-1-proberts
36
be shaped to allow substantial equalisation of pressure within the second
chamber. The first chamber may be upstream of the second chamber in use.
The first and second chambers may be connected by a sensing channel.
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.

WE CLAIM: (Voluntarily Amended Claims)
1. A sensor apparatus comprising a housing having an inner perimeter
which defines an area through which gas may flow, the housing being
provided with:
a first chamber which extends around the area through which gas may
flow, an entrance being distributed around the first chamber; and
a second chamber which extends around the area through which gas
may flow, an exit being distributed around the second chamber;
the first chamber being arranged to be upstream of the second
chamber in use;
wherein the sensor apparatus further comprises one or more sensors
arranged to measure a pressure difference between pressure in the first
chamber and pressure in the second chamber.
2. The sensor apparatus of claim 1, wherein the first chamber has a
cross-sectional area which is sufficiently large that in use the pressure of gas
within the first chamber substantially equalises during operation of the sensor.
3. The sensor apparatus of any preceding claim, wherein the second
chamber has a cross-sectional area which is sufficiently large that in use the
pressure of gas within the second chamber substantially equalises during
operation of the sensor.
4. The sensor apparatus of any preceding claim, wherein the sensor
apparatus further comprises a sensing channel which is connected between
the first chamber and the second chamber such that gas flows through the
sensing channel in use, and wherein the one or more sensors are located in
the sensing channel.
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5. The apparatus of claim 4, wherein the first chamber is shaped such
that there is no direct flow path between the entrance of the first chamber and
the sensing channel.
6. The sensor apparatus of claim 4 or claim 5, wherein the entrance to the
first chamber is narrower in the vicinity of the sensor and wider further away
from an entrance of the sensing channel.
7. The sensor apparatus of claim 4 or claim 5, wherein the entrance
extends intermittently around the first chamber, and wherein open portions of
the entrance occupy a smaller proportion of the entrance in the vicinity of an
entrance of the sensing channel than open portions of the entrance further
away from the entrance of the sensing channel.
8. The sensor apparatus of any of any of claims 4 to 7, wherein the exit
from the second chamber is narrower in the vicinity of an exit of the sensing
channel and wider further away from an exit of the sensing channel.
9. The sensor apparatus of any of claims 4 to 7, wherein the exit extends
intermittently around the second chamber, and wherein open portions of the
exit occupy a smaller proportion of the exit in the vicinity of an exit of the
sensing channel than open portions of the exit further away from the exit of
the sensing channel.
10. The sensor apparatus of any of claims 4 to 9, wherein the one or more
sensors comprises a sensing device which is at least partially located within
the sensing channel.
11. The sensor apparatus of claim 10, wherein the sensing device is at
least partially located within a flow restrictor which is provided in the sensing
channel.
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12. The sensor apparatus of claim 10 or claim 11, wherein the sensing
device comprises two bipolar transistors, one of the bipolar transistors being
electrically heated.
13. The sensor apparatus of claim 12, wherein the sensing device further
comprises a circuit configured to provide substantially constant power to the
heated bipolar transistor and to measure a difference between base emitter
voltages of the bipolar transistors.
14. The sensor apparatus of claim 12, wherein the sensing device further
comprises a circuit configured to maintain a substantially constant
temperature difference between the bipolar transistors, and to measure the
power used to heat the heated transistor.
15. The sensor apparatus of claim 13 or claim 14, wherein the circuit is
further configured to measure the temperature of the bipolar transistor which
is not electrically heated.
16. The sensor apparatus of claim 15, wherein the bipolar transistor which
is not electrically heated is located within the flow restrictor.
17. The sensor apparatus of claim 11, wherein the flow restrictor is not
formed integrally with other parts of the sensing apparatus.
18. The sensor apparatus of any of claim 11 or claim 17, wherein the flow
restrictor is formed from a material which is different to the material used to
form the housing of the sensor apparatus.
19. The sensor apparatus of any of claims 1 to 7, wherein the one or more
sensors comprises a strain gauge which is connected between the first
chamber and the second chamber.
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20. The sensor apparatus of claim 19, wherein the strain gauge is provided
in a sensing channel which is connected between the first chamber and the
second chamber.
21. The sensor apparatus of any preceding claim, further comprising an
additional chamber located between the first and second chambers, the
additional chamber being connected to the first chamber or the second
chamber, wherein an additional sensor is located within the additional
chamber.
22. The sensor apparatus of any of claims 1 to 7, wherein the first chamber
and the second chamber are not connected, and wherein the one or more
sensors comprise a pressure sensor located in the first chamber and a
pressure sensor located in the second chamber.
23. The sensor apparatus of any preceding claim, wherein a bracket
extends from the sensor apparatus, the bracket having openings through
which gas may flow.
24. The sensor apparatus of any preceding claim, wherein the sensor
apparatus includes a sloping portion configured to push boundary layer gas
away from the entrance of the first chamber.
25. A turbocharger comprising a turbine connected via a shaft to a
compressor, wherein the sensor apparatus of claim 1 is provided in an inlet of
the compressor.
26. The turbocharger of claim 25, wherein a gap is provided between the
sensor apparatus and a wall of the inlet of the compressor.
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27. A method of measuring the mass flow rate of a gas using a sensor
apparatus comprising a housing having an inner perimeter which defines an
area through which the gas may flow, the method comprising:
receiving gas in a first chamber which extends around the area through
which gas may flow, an entrance being distributed around the first chamber;
receiving downstream gas in a second chamber which extends around
the area through which gas may flow, an exit being distributed around the
second chamber; and
using one or more sensors to measure a pressure difference between
pressure in the first chamber and pressure in the second chamber.
28. The method of claim 27, wherein a boundary layer of gas passes
around an outer perimeter of the housing and thereby bypasses the entrance
of the first chamber.
29. The method of claim 27 or claim 28, wherein the gas is flowing into a
compressor of a turbocharger.