U.S. patent application number 12/495365 was filed with the patent office on 2009-10-22 for automotive turbocharger systems.
Invention is credited to Matthew Gerard Beasley, Richard C.E. Cornwell, Malcolm Fry, Richard King.
Application Number | 20090265080 12/495365 |
Document ID | / |
Family ID | 29226684 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090265080 |
Kind Code |
A1 |
Fry; Malcolm ; et
al. |
October 22, 2009 |
AUTOMOTIVE TURBOCHARGER SYSTEMS
Abstract
A turbocharger system comprising first and second turbochargers,
configured in series, where the turbochargers hand over control
from one turbocharger to the other, which incorporates switching
adjustment terms at the point of transition to ensure a smooth
transition.
Inventors: |
Fry; Malcolm; (West Sussex,
GB) ; Cornwell; Richard C.E.; (West Sussex, GB)
; King; Richard; (Hove, GB) ; Beasley; Matthew
Gerard; (West Sussex, GB) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
29226684 |
Appl. No.: |
12/495365 |
Filed: |
June 30, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10570877 |
Oct 25, 2006 |
|
|
|
PCT/GB04/03809 |
Sep 6, 2004 |
|
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12495365 |
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Current U.S.
Class: |
701/103 ; 60/602;
60/612 |
Current CPC
Class: |
F02B 37/183 20130101;
F02B 37/162 20190501; F02B 37/16 20130101; F02B 37/002 20130101;
F02D 23/00 20130101; F02B 37/18 20130101; F02D 2041/1409 20130101;
F02D 41/0007 20130101; F02B 37/013 20130101; Y02T 10/144 20130101;
Y02T 10/12 20130101 |
Class at
Publication: |
701/103 ; 60/612;
60/602 |
International
Class: |
F02D 43/00 20060101
F02D043/00; F02B 33/44 20060101 F02B033/44; F02D 23/00 20060101
F02D023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 8, 2003 |
GB |
0320986.3 |
Claims
1. A turbocharger system for an automotive engine comprising: an
air inlet duct; an exhaust gas duct; a first turbocharger; a second
turbocharger, the first turbocharger being smaller than the second
turbocharger, said first turbocharger and said second turbocharger
each including an exhaust turbine situated in the exhaust duct and
a blower wheel situated in the inlet duct, said blower wheels being
arranged in series along said inlet duct; a bypass duct, said
bypass duct being connected to the exhaust duct on each side of the
exhaust turbine of the first turbocharger and having no more than a
single inlet port from an exhaust manifold and no more than a
single outlet port leading to the second turbo-charger; and the
bypass duct comprising a selectively operable butterfly shut-off
valve, said butterfly shut-off valve comprising a valve flap
pivotally mounted within a housing, wherein an internal wall of the
housing has two oppositely directed semi-annular sealing surfaces
extending transversely to the direction of the exhaust gas flow,
the valve flap being positionable between an open position in which
the bypass duct is substantially unrestricted to gas flow and a
closed position in which the valve flap is in sealing engagement
with the two oppositely directed semi-annular sealing surfaces.
2. The system of claim 1, wherein the two oppositely directed
semi-annular sealing surfaces are offset from one another in the
direction of exhaust gas flow through the housing by a distance
substantially equal to a thickness of the valve flap.
3. The system of claim 1, wherein the two oppositely directed
semi-annular sealing surfaces comprise opposed side surfaces of two
semi-annular sealing projections provided on the internal surface
of the valve housing.
4. The system of claim 1, wherein the interior surface of the
housing is smoothly continuous throughout with the exception of two
discontinuities at which the respective two oppositely directed
semi-annular sealing surfaces are defined.
5. A method of controlling a turbo charger system having a
controller and first and second turbochargers with respective first
and second operational ranges, wherein said first and second
turbochargers include respective first and second compressors, said
method comprising: issuing a control command to open a bypass
around said first compressor when a downstream pressure with
respect to the first compressor is less than or equal to an
upstream pressure with respect to the first compressor; and
comparing one or more engine operating parameters to a threshold
and issuing a control command to close said bypass when said
comparison indicates that operation of the first compressor is
required.
6. The method of claim 5, wherein the set of one or more engine
operating parameters that may be compared to a threshold comprises
engine speed, engine load, pressure, exhaust gas mass flow,
fuelling, and valve position.
7. The method of claim 5, wherein closing the bypass comprises
fully or partially closing the bypass.
8. The method of claim 5, wherein the first and second compressors
are in a series sequential configuration and the first compressor
is in a high pressure location in such configuration.
9. The method of claim 5, further comprising keeping the bypass
around the first compressor closed until a control command to
re-open the bypass is received.
10. The method of claim 9, wherein keeping the bypass closed
comprises preventing issuance of a control command to open the
bypass if, after closing the bypass, said downstream pressure
becomes less than or equal to said upstream pressure.
11. A computer-readable medium encoded with a program configured to
implement a method of controlling a turbo charger system having a
controller and first and second turbochargers with respective first
and second operational ranges, wherein said first and second
turbochargers include respective first and second compressors, said
method comprising: issuing a control command to open a bypass
around said first compressor when a downstream pressure with
respect to the first compressor is less than or equal to an
upstream pressure with respect to the first compressor; and
comparing one or more engine operating parameters to a threshold
and issuing a control command to close said bypass when said
comparison indicates that operation of the first compressor is
required.
12. The computer-readable medium of claim 11, further comprising an
engine control unit.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/570,877, filed Oct. 25, 2006, under 35
U.S.C. .sctn. 371 claiming priority to Great Britain application
ser. no. 0320986.3, filed Sep. 8, 2003, and international
application number PCT/GB2004/003809, filed Sep. 6, 2004, the
entirety of each is hereby incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to turbocharger systems for
automotive engines.
BACKGROUND
[0003] Turbochargers are of course well known devices which include
a compressor or blower wheel, typically an impeller, which is
situated in an engine inlet duct and is connected to an exhaust
turbine, which is situated in the engine exhaust duct and arranged
to be rotated at high speed by the engine exhaust gases. Rotation
of the exhaust turbine results in rotation of the blower wheel
which produces a boost pressure, that is to say it increases the
pressure in the inlet duct to a superatmospheric value. The result
of this increased inlet pressure is that a greater amount of air is
admitted into each cylinder of the engine during the induction
stroke of the pistons in the cylinders, which results in an
increased power output from the engine.
[0004] The power absorbed from the exhaust gases by a turbocharger
exhaust turbine is proportional to the cube of the speed of the
exhaust gases, which means that although the blower wheel rotates
very rapidly and thus produces a substantial boost pressure at high
engine speed, it does not rotate at all or only at negligible speed
at low engine speed. This means that no boost pressure is available
at a time when maximum engine power is frequently needed, i.e. when
accelerating rapidly from engine idle speed.
[0005] One way of overcoming this problem is to increase the speed
of the exhaust gases past the exhaust turbine. This can be done by
providing guide vanes of variable pitch in the exhaust duct to
enable the local exhaust gas speed to be increased and thus the
power output of the turbine wheel to be increased, even at low
engine speed. However, such a construction is complex and expensive
and subject to failure as a result of lubrication problems. Simply
making the turbocharger physically smaller, thereby increasing the
exhaust velocity through it, would substantially improve the
characteristics of the turbocharger at low engine speeds, but at
high engine speeds the exhaust turbine would constitute an
unacceptable flow restriction for the exhaust gases and would be
liable to failure as a result of being driven at an unacceptably
high speed.
[0006] It has been proposed that an automotive engine be provided
with a turbocharger system comprising two turbochargers, one
relatively small and the other relatively large. The two blower
wheels are provided in series in the engine inlet duct and the two
exhaust turbines are provided in series in the exhaust duct. Since
the small turbocharger is inappropriate at high engine speeds and
would be liable to failure if used at such speeds, the smaller
exhaust turbine and the smaller blower wheel are provided with
respective bypass passages incorporating respective shut-off valves
operated under the control of the engine management system.
[0007] The operation of such a system is supposed to be as follows:
The two bypass valves are shut at low engine speeds. The relatively
small volume of exhaust gas flows through the exhaust turbine of
the smaller turbocharger at a substantial speed due to the
relatively small dimension of the duct in which the turbine is
situated. The smaller exhaust turbine is thus rotated at a
substantial speed and this rotation is transmitted to the smaller
blower wheel, which thus creates a significant boost pressure in
the inlet duct. The exhaust gas also flows through the exhaust
turbine of the larger turbocharger, but at a significantly lower
speed due to its greater size. The larger exhaust turbine is thus
rotated very slowly, if at all, and the larger blower wheel thus
plays effectively no part in the creation of the boost pressure. As
the engine speed and/or load rises, the engine management system
opens the two bypass valves. The exhaust gas now flows through the
passage bypassing the smaller exhaust turbine and then flows
through the larger exhaust turbine where it now reaches a
substantial speed due to the increased flow rate of exhaust gas.
The larger exhaust turbine is thus rotated at high speed and this
rotation is transmitted to the larger blower wheel, which creates a
boost pressure in the inlet duct. The bypass duct around the
smaller blower wheel has larger flow area than that of the smaller
blower and thus does not constitute an unacceptable flow
restriction in the inlet duct.
[0008] Accordingly, such a composite turbocharger system should
provide a solution to the problem of inadequate boost pressure at
low engine speeds. However, it is found in practice that it does
not do so and tests have indicated that an engine fitted with such
a turbocharger system has a power output of only about two-thirds
of that which would be expected at low engine speeds.
[0009] In addition difficulties are encountered in controlling
operation of the individual turbochargers and in particular
airflow. For example the larger turbocharger has a turbine bypass
valve (for bypassing the larger turbine in an overboost or
overspeed condition) and control of the smaller turbine bypass and
larger turbine bypass must be achieved without competition between
the control strategies. Yet a further problem is that the smaller
compressor can act as a restriction on airflow from the larger
compressor whilst producing no pressure rise at higher engine
speeds/loads.
[0010] It is, therefore, the object of the invention to provide a
turbocharger system of the type incorporating two turbochargers
which does provide a substantial boost pressure at substantially
all engine speeds and enables the engine to produce a significantly
enhanced power output at low engine speeds.
SUMMARY
[0011] According to the present invention, a turbocharger system
for an automotive engine comprises an air inlet duct, an exhaust
gas duct and first and second turbochargers, the first turbocharger
being substantially smaller than the second turbocharger, each
turbocharger including an exhaust turbine situated in the exhaust
duct and a blower wheel situated in the inlet duct, a bypass duct
being connected to the exhaust duct on each side of the exhaust
turbine of the first turbocharger, the bypass duct including a
selectively operable butterfly shut-off valve including a valve
flap pivotally mounted within a housing, the internal wall of the
housing carrying two oppositely directed semi-annular sealing
surfaces extending transversely to the direction of the exhaust gas
flow, the valve flap being movable between an open position in
which the bypass duct is substantially unrestricted and a closed
position in which it is in sealing engagement with the two sealing
surfaces.
[0012] Exhaustive tests on the known turbocharger system including
two turbochargers have revealed that the reason why it does not
produce a satisfactory boost pressure at low engine speeds is that
the bypass valve is inherently leaky and a substantial proportion
of the exhaust gas thus flows through the bypass passage and not
through the smaller exhaust turbine, even when the bypass valve is
nominally closed. Although numerous different types of shut-off
valve are known, the high pressures and temperatures and aggressive
conditions which prevail in an automotive exhaust duct mean that
one type of valve that is practicable is a butterfly valve.
However, in order to avoid the valve flap becoming jammed against
the wall of the housing, particularly as a result of the
differential thermal expansion which occurs, it is, as a matter of
practice, necessary to make the valve flap significantly smaller
than the housing in which it is pivotally accommodated. This means
that there is in practice a significant gap between the internal
wall of the housing and the outer edge of the valve flap, when the
valve is closed. This gap constitutes the leakage path through
which a significant proportion of the exhaust gas escapes and thus
does no work in the exhaust turbine.
[0013] It has thus been appreciated that what is needed is to
substantially improve the gas tightness of the bypass valve, when
closed, and this is achieved by the two semi-annular sealing
surfaces in the present invention. These two sealing surfaces will
in practice be offset in the housing in the direction of exhaust
gas flow through it by a distance substantially equal to the
thickness of the valve flap. Thus when the valve is closed, a seal
is created not between the outer edge surface of the valve flap and
the inner surface of the valve housing, as previously, but between
the outer portion of one flat surface of one half of the valve flap
and one of the sealing surfaces and between the outer portion of
the other flat surface of the other half of the valve flap and the
other of the sealing surfaces.
[0014] In one embodiment, two semi-annular sealing projections are
provided on the internal surface of the housing, opposite side
surfaces of which constitute respective sealing surfaces.
Alternatively, the interior surface of the bypass valve housing may
be effectively smoothly continuous throughout with the exception of
two discontinuities at which the respective sealing surfaces are
defined. In this latter embodiment, the two portions of the gas
flow passage through the housing on opposite sides of the valve
flap are effectively slightly offset from one another in a
direction transverse to the direction of gas flow through it,
whereby the two opposed sealing surfaces are afforded at the
discontinuities, that is to say at the positions where the offset
portions of the flow passage merge into one another. The flow
passage through the housing may of course be of any shape
conventional with butterfly valves, e.g. circular or
rectangular.
[0015] The provision of the opposed sealing surfaces with which the
valve flap co-operates in the closed position results in the valve
forming a very effective seal. Little or no exhaust gas thus leaks
through the bypass passage when the valve is closed which results
in substantially all of the exhaust gas flow flowing past the
turbine wheel of the smaller turbocharger at low engine speeds,
whereby the blower wheel of the smaller turbocharger may produce a
substantial boost pressure in the air inlet duct. The power output
of the engine is therefore substantially increased at low engine
speeds by comparison with engines with dual turbocharger systems of
known type.
[0016] The present invention also embraces an automotive engine
including a turbocharger system of the type referred to above.
Further aspects of the invention are set out in the claims.
[0017] Further features and details of the invention will be
apparent from the following description of one specific embodiment
which is given by way of example with reference to the accompanying
drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a highly diagrammatic view of an automotive engine
including a turbocharger system in accordance with the
invention;
[0019] FIG. 2 is a view from one end of the exhaust gas bypass
valve housing, from which the valve flap has been omitted for the
sake of clarity;
[0020] FIG. 3 is a sectional side view of the exhaust gas bypass
valve;
[0021] FIG. 4 is a schematic diagram showing in more detail the
components of an engine with a two-stage turbocharger;
[0022] FIG. 5 is a schematic block diagram showing a closed loop
control scheme for a bypass valve;
[0023] FIG. 6 is a flow diagram showing operation of the control
scheme for handover of control between turbine bypass valves;
and
[0024] FIG. 7 is a block diagram showing a control scheme for a
bypass valve scheme for a bypass valve following handover of
control.
DETAILED DESCRIPTION
[0025] FIG. 1 diagrammatically illustrates an automotive engine 2,
which in this case has four cylinders 4. The cylinders 4
communicate via one or more respective inlet valves with an inlet
manifold 6 which communicates with the atmosphere at an air inlet 8
via an inlet duct 10, which includes an intercooler 12. The
cylinders 4 of the engine also communicate via one or more
respective exhaust valves with an exhaust gas manifold 14 which
communicates with the atmosphere at an outlet 16 via an exhaust gas
duct 18.
[0026] The engine includes a turbocharger system comprising two
turbochargers, each of which includes an exhaust gas turbine
situated in the exhaust duct 18 and an air blower wheel or
compressor which is connected thereto and is situated in the air
inlet duct 10. One of these turbochargers is substantially larger
than the other, which is to say that its exhaust gas turbine and
its air blower wheel and the passages in which these are situated
are substantially larger than those of the smaller turbocharger.
More specifically, the smaller turbocharger includes an exhaust gas
turbine 20 in the exhaust duct 18 connected to an air blower wheel
22 in the inlet duct 10. The larger turbocharger has an exhaust
turbine wheel 24 in the exhaust duct 18 connected to an associated
blower wheel 26 in the inlet duct 10. Connected to the exhaust gas
pathway upstream and downstream of the smaller exhaust gas turbine
20 is a bypass passage 28. Situated in this bypass passage is a
butterfly shut-off valve 30 connected to be rotated between an open
and a closed position by an actuator 32 which is actuated in
response to signals produced by a control system, typically the
engine management system with which most modern automotive engines
are now provided. As discussed above, it is crucial that the
butterfly valve 30 forms a reliable seal, when in the closed
position, and its detailed construction will be discussed below.
The smaller turbocharger can be termed a high pressure turbocharger
and the larger one a low pressure turbocharger, with the individual
components named accordingly.
[0027] Connected to the inlet duct 10 upstream and downstream of
the blower wheel 22 of the smaller turbocharger is a further bypass
passage 34. Situated in this passage is a further butterfly
shut-off valve 36, which is again connected to an actuator 38 under
the control of the engine management system. The pressure
differentials and temperature variations in the inlet duct are very
much smaller than those in the exhaust duct and the ability of the
butterfly valve 36 to form a reliable seal, when in the closed
position, is very much less important than in connection with the
exhaust shut-off valve 30. Accordingly, the bypass valve 36 may be
of the same construction as the bypass valve 30, to be described
below, or it may be of conventional construction.
[0028] As shown in FIG. 2, the exhaust butterfly valve 30 comprises
a housing 40, through which a flow passage 42 extends and which is
connected at its two ends to the exhaust duct 18. Pivotally mounted
within the circular flow passage 42 is a valve flap 44. As may be
seen in FIG. 3, the diameter of the valve flap 44 is significantly
less than that of the portion of the flow passage 42 in which it is
accommodated, thereby ensuring that differential expansion does not
result in the valve flap 44 becoming jammed within the passage. The
wall surface defining the flow passage 42 is smooth and circular
but has two semi-annular discontinuities formed in it at positions
which are spaced apart in the direction of the length of the flow
passage by a distance equal to the width of the valve flap 44.
These discontinuities constitute two oppositely directed,
semi-annular sealing surfaces 46, one of which is visible when
looking through the flow passage from one end and the other of
which is visible when looking through the flow passage from the
other end. The valve flap 44 is mounted on two stub shafts 48
accommodated in respective openings 50 in the valve housing 40. One
of these stub shafts 48 is connected to the actuator 32. This
actuator is arranged to rotate the valve flap 44 under the control
of the engine management system between an open position, in which
the valve flap extends substantially parallel to the axis of the
flow passage 42 and the flow passage 42 is therefore substantially
unobstructed, and a closed position, which is illustrated in FIG.
3, in which the valve flap 44 closes the flow passage 42. As may be
seen in FIG. 3, when the valve flap is in the closed position, it
engages the two sealing surfaces 46 with its opposed side surfaces.
The valve flap thus forms a reliable seal with the wall surface of
the flow passage and thus reliably closes the flow passage.
[0029] In use, at low engine speeds, the high pressure turbine
bypass valve 30 and high pressure compressor bypass 36 are both
closed. The turbine bypass valve 30 forms a reliable seal and all
the exhaust gas is thus directed through the smaller exhaust gas
turbine 20. Due to the relatively small size of this turbine, the
gas flowing through it reaches a relatively high speed and rotates
the exhaust turbine and thus also the air blower 22 attached to it
at a relatively high speed. The blower wheel 22 thus produces a
substantial boost pressure in the inlet duct 10. The exhaust gases
also flow through the exhaust gas turbine 24 of the larger
turbocharger but, due to its substantially larger area, the larger
exhaust gas turbine is rotated only at low speed. It does, however,
constitute only a negligible flow resistance. It is necessary (and
a practical limitation often overlooked in two stage and sequential
turbocharging schemes) to maintain low speed rotation of the "idle"
turbocharger in order to keep the bearings and oil seals of the
turbocharger in order. When the engine speed reaches a higher value
predetermined by the engine management system, the two bypass
valves 30 and 36 are opened. Due to the fact that the area of the
bypass passage 28 is substantially greater than that of the duct
leading to the smaller exhaust gas turbine, substantially all the
exhaust gas bypasses the smaller turbine 20 and flows through the
bypass passage 28. It then flows through the larger exhaust gas
turbine 24 and rotates it and thus also the larger air blower wheel
26. The air blower wheel 26 thus produces a boost pressure in the
inlet duct 10. Since the flow passage through the smaller air
blower 22 is relatively small, this would constitute a significant
flow restriction and it is for this reason that the further bypass
passage 34 is provided. As mentioned above, the high pressure
compressor bypass valve 36 is opened at higher engine speeds and
due to the fact that the flow area of the bypass passage 34 is
significantly greater than that of the larger air blower wheel 22,
substantially all the inlet air bypasses the smaller blower wheel
22 at higher engine speeds and flows through the bypass passage
34.
[0030] A turbocharger system in accordance with the invention can
thus produce a substantial boost pressure in the inlet duct not
only of high engine speeds but also at low engine speeds and
therefore overcomes the traditional problem that turbochargers are
largely ineffective at low engine speeds.
[0031] Referring to FIG. 4 an engine system of the type described
in FIG. 1 is shown but with additional components now described.
Common reference numerals denote common components and will not be
described further for the avoidance of repetition.
[0032] In particular it will be seen that an additional bypass 100
is provided around the low pressure turbine 24 controlled by a
controller 102. As the aim of the system as a whole is to achieve a
desired boost it will be seen that control of the high pressure and
low pressure turbine bypass or valves 30, 101, by respective
controllers 32, 102 must be achieved without competition between
the control strategies which could give rise to unstable or
inefficient operation.
[0033] It will be appreciated that the various components described
herein including the turbocharger components and control components
can be of any appropriate form as will be apparent to the skilled
reader. For example, control can be affected by appropriate
software implemented on an engine control unit (ECU).
[0034] As engine speed/load is increased from idle, all three
valves are shut and the system uses both turbines and compressors
in series. Because the high pressure turbocharger is substantially
smaller than the low pressure turbocharger, it is this machine
which will provide the majority of the boost pressure at low engine
speeds when the exhaust gas flow rate is low. At medium/low speeds
the high pressure turbocharger 20 starts to over-boost or
over-speed and exhaust gas must be bypassed around the high
pressure turbine 20 to control the output of the high pressure
turbocharger, allowing exhaust gas to feed the low pressure turbine
24 directly. At higher engine speeds the exhaust gas massflow is
substantially greater than the flow capacity of the high pressure
turbine and therefore the bypass valve 30 opens fully to completely
bypass the high pressure turbine (subject to previous comments
regarding maintaining seals and bearings in satisfactory order). On
the inlet side, the high pressure compressor bypass valve 36 is
opened when the (slowly rotating) high pressure compressor [[ ]] 26
acts as a restriction, owing to the high pressure turbine being
substantially bypassed. At still higher engine speeds the low
pressure turbocharger 24 starts to over-boost or over-speed and it
must be bypassed.
[0035] In particular closed loop control of the high pressure
turbine bypass valve 30 and the low pressure turbine bypass valve
101 is required with hand over of control from one to the other
effected so as to provide a smooth transition. A basic control
scheme is shown in FIG. 5 in which each valve 30, 101 is controlled
on the basis of a feedback loop, enclosed loop operation, based on
an open loop set point map 110. The open loop set point map 110
comprises a mapping of engine speed and load (e.g. fuel) at input
112 and 114, which are measured by any appropriate sensor (not
shown). These variables are mapped to an output set point 116
comprising valve position. The target variable is a desired boost
pressure at the engine and so boost pressure deviation 118 is
obtained from inputs of achieved boost (for example measured at the
intake manifold 6 by a sensor (not shown)) as against desired boost
for example as derived from the set point or from the engine
control unit from the engine speed and load inputs 112, 114. The
achieved boost and desired boost are input at 120, 122 respectively
to the boost pressure deviation block 118. The deviation is output
at 124 to a proportional integral derivative (PID) controller 126
of the type that will be well known to the skilled person. The PID
controller 126 output comprises a correction 128, which is input to
a differencer at 130 together with the valve set point output 116
and these are output to the duty cycle 132 controlling operation of
the valve and in particular the valve position. As a result closed
loop control is provided in which the valve position converges on
the position providing the desired boost.
[0036] The required position of each valve is very non-linear, for
example the high pressure turbine bypass valve 30 must open slowly
to control the high pressure turbo 20, but then open quickly as the
turbine needs to be completely bypassed. To a certain extent the
high pressure turbine bypass valve must fulfil two functions, that
of bypassing (wastegating) the high pressure turbine to modulate
boost at low engine speeds, and that of completely bypassing the
turbine at higher engine speeds. It is therefore important that the
closed loop controller uses the calibrated open loop maps as a
basis for closed loop control (to control to a desired boost
pressure for a given speed and load).
[0037] The specific scheme according to which individual PID
controllers 126 are provided for each bypass valve on the turbine
side can be understood with reference to FIG. 6.
[0038] In low speed/load conditions, when all the valves are shut,
the system is run in open loop control at block 140, each
individual controller for high and low pressure turbine bypass
valves using the set points from its calibrated open loop map. It
is possible in the initial state to run both controllers in open
loop control as the bypass valves at low engine speeds are in fact
typically at their fully closed positions such that closed loop
control is not required. It will be appreciated that the open loop
can be calibrated in any appropriate manner, for example during an
initial pre-production calibration phase.
[0039] As the engine reaches the speed and load conditions when the
high pressure turbine 30 approaches over-speed/boost, the high
pressure turbine bypass switches to closed loop control at block
144. As discussed above with reference to FIG. 5, during closed
loop control the high pressure turbine bypass valve is controlled
to a desired boost pressure based on speed and load.
[0040] As the high pressure bypass valve 30 moves towards its
maximum opening position it becomes necessary to prepare to hand
over control to the low pressure bypass valve 101 in order to
maintain closed loop control turbine bypass valve. This point is
identified when the open loop set point valve position for the high
pressure bypass valve reaches a threshold value corresponding to a
point at which the high pressure turbine bypass valve 30 can no
longer regulate boost due to being substantially open. At this
stage the position of the low pressure turbine bypass valve is
controlled as a function of the high pressure turbine bypass valve
30 position setpoint using a map of engine speed and high pressure
turbine bypass valve position as also shown in block 144.
[0041] The high pressure turbine bypass valve continues to open to
a fully open position at which point (or earlier) control must be
passed to the low pressure turbine bypass valve in order to
maintain closed loop control. Accordingly, at block 148 control is
now switched to a second closed loop controller of the type shown
in FIG. 5 which uses the low pressure turbo charger open loop map
as the input together with boost pressure deviation and a PID
controller as described above. It will be appreciated that a smooth
transition is desirable at this stage to avoid abrupt movements of
either turbine bypass valve position setpoint at the point of
handover. This could occur where the set point low pressure valve
position during slaved control by the high pressure valve position
differed from the set point position from the open-loop map for the
low pressure turbine bypass valve. The manner in which the
transition is managed is described in more detail below but for
completeness the remainder of the operation of the control strategy
as the engine reduces speed/load is first discussed with reference
to FIG. 6.
[0042] As the speed/load conditions are reduced from their maxima,
(in a manner as described with reference to block 144) also at
block 148, whilst continuing to control the boost pressure with
closed loop control of the low pressure turbine bypass valve, the
high pressure turbine bypass valve is then controlled as a function
of the low pressure turbine bypass valve bypass valve position.
Once again this is done using a map of engine speed and low
pressure turbine bypass valve positions. The low pressure turbine
bypass valve 101 starts to approach a fully closed position at
which point (or earlier) control must be passed to the high
pressure turbine bypass valve (at block 150) in order to maintain
closed loop control and finally enters open loop control again at
block 152.
[0043] The transition strategy by which closed loop control is
handed from one controller to the other as described with reference
to FIG. 6, blocks 148 and 150, will now be described with reference
to FIG. 7. In overview, the strategy achieves smooth transition
firstly by compensating for any deviation between the expected
valve position under slaved control from the other valve (prior to
switching control) and the open loop set point (after transition).
In addition the system allows for different PID gains to exist for
the low and high pressure turbine bypass valve controllers (thereby
allowing valves with significantly different time constants to be
controlled and also at different engine speed/load conditions). The
following discussion deals with the transition from closed loop
control on the high pressure turbo charger to closed loop control
on the low pressure turbo charger but it will be appreciated that
it applies equally, mutatis mutandis, to transition in the other
direction.
[0044] A key operating principle behind this aspect of the
invention is as follows: in any transition from one closed loop
controller to the other, the controller corrects the output of the
open loop map for the destination controller to match the current
position. This correction is in two parts, firstly modification of
the open loop: map output of the destination controller (to which
control is handed) allowing for current position of the destination
valve, and secondly modification of the open loop map output of the
destination controller allowing for any differences in proportional
gains between the two controllers.
[0045] Just before the change from closed loop control on the high
pressure turbo charger to closed loop control on the low pressure
turbo charger, the difference between the current position of the
low pressure turbine bypass valve (according to the position
determined as a function of the high pressure valve position from
the mapping described above) and the position according to the low
pressure turbine open loop map (for example map 110 shown in FIG.
5) is calculated. Referring now to FIG. 7 which corresponds to FIG.
5 and in which like numerals designate like components which are
not described here to avoid repetition, this discrepancy or
"switching adjustment term 1 (SAT 1)" is input at block 134 to an
adder 136 allowing compensation of the output 116 of the open loop
set-point map to provide a corrected value 138 to the subtractor
130. As a result removal of the discrepancy from the open loop map
is achieved prior to the open loop term being changed by the PID
controller. As a result there will be no sudden jump to a revised
position on handover.
[0046] The second step is a further "switching adjustment term 2
(SAT 2)" input at block 135 to compensate for any difference
between the P gains of each controller, which is input to an adder
137 downstream of the adder 136. As a general point, the integral
term and differential (if used) term in the controller 126 should
be reset to zero at all times when the controller is not being
used. Accordingly the integral term in the low pressure PID
controller 126 is set to zero ensuring that there is not over
compensation for any initial error terms. The product of the
proportional gain from the destination controller and the current
boost pressure deviation is SAT 2 and must be added to the open
loop map prior to the open loop term being changed by the PID
controller At the moment of changeover of the high pressure closed
loop controller to the low pressure closed loop controller, these
two SATs must be frozen for the duration of closed loop control of
the low pressure turbine bypass valve.
[0047] As a result it will be seen that a sophisticated, smooth and
rapidly converging control strategy is provided for the
turbocharger system, which allows seamless tracking of the desired
boost pressure even during handover of control between the high
pressure and low pressure bypass valves and vice versa.
[0048] As discussed above a high pressure compressor bypass valve
36 is also provided to prevent the high pressure compressor
becoming a restriction to airflow during operation of the low
pressure compressor 26. The use of a bypass channel in the context
of a two stage turbocharging system with a valve of unspecified
type between the interstage compressor position and the HP
compressor outlet is known and described in EP0416520010411
(D/C/BorgWarner R2S patent), U.S. Pat. No. 5,408,979--in which a
butterfly High Pressure Compressor Bypass valve may be
electronically controlled but is a non return valve and U.S. Pat.
No. 4,930,315 in which a two stage charging system includes a
charging bypass from low pressure (LP) outlet to high pressure (HP)
charging pipe, bypassing the high pressure compressor with passive
check valve (to prevent reverse flow) in the charging bypass.
[0049] In addition, other engines/supercharging systems in
production use a mechanical/electrical supercharger in series with
a turbocharger, with a passive or actively controlled non-return
valve around the mechanical/electrical supercharger. An example of
this is the Volvo Penta (passive non return valve (NRV) flap
valve).
[0050] A problem with existing systems is that passively operated
(self acting) non return valves have known problems with
instability/response to pulsations/noise.
[0051] According to a further aspect of the invention, a method of
operating an actively controlled butterfly valve used as an air
side bypass in a two stage turbocharging system is provided.
[0052] As discussed above the two stage system is configured such
that the HP stage and LP stage differ considerably in size (and
therefore flow capacity), and the HP turbine 20 is bypassed
(turbine bypass valve) at higher engine speeds such that its speed
drops to very low levels. Consequently the HP compressor 22
produces no pressure rise, and in fact, if left in circuit, would
act as a restriction to the airflow to the engine. The HP
compressor bypass valve 36 is therefore necessary to provide an
alternative route for air to bypass the HP compressor 22 when it is
producing no pressure rise.
[0053] The valve is arranged to be commanded open when the
pressures upstream and downstream of the HP compressor are equal
(as detected by appropriate sensors). However once such a butterfly
valve is commanded to be open, any resumption of operation of the
HP compressor (i.e. by closing of the HP turbine bypass valve) will
merely result in the airflow recirculating around the HP compressor
via the bypass channel. Because the airflow is recirculating no
pressure rise can be generated upstream of the HP compressor and
the pressures upstream and downstream remain equal, therefore the
bypass valve will remain open under the command of the pressure
differential signal.
[0054] When in open loop boost control the HP compressor bypass
operates in open loop mode from a calibrated speed and fuel map in
a similar manner to that discussed above in relation to the
turbine. When the closed loop boost pressure controller is in
operation, the two turbine bypass valves (i.e. including the low
pressure turbine bypass valve--not shown) can deviate significantly
from their open loop maps, and, concurrently, the airflow behaviour
can deviate significantly from steady state condition. Therefore
the compressor bypass cannot by suitably controlled by the open
loop maps.
[0055] During closed loop control therefore the pressures from the
exit of the low pressure compressor (LPC) 26 and the entry to the
intercooler 12 (post HP compressor 22) are compared. The post HP
compressor 22 pressure (i.e. the pre-intercooler pressure) is
estimated from the boost pressure with a correction for the
estimated pressure drop across the intercooler 12.
[0056] The system embodied in the invention operates in the
following manner:
[0057] When the LP compressor 26 out pressure exceeds the
pre-intercooler pressure (i.e. there is a pressure drop across the
HP compressor 22) the HP compressor bypass 36 is requested to open,
as per the known technique.
[0058] When the HP bypass 36 closed loop setpoint closes beyond a
threshold the compressor bypass 36 is requested to close.
Hysteresis is included in the form of a timer to prevent the valve
opening immediately again even if the pressure differential
criteria is met. As a result a pressure differential can form,
avoiding the problem identified above of the bypass valve
flip-flopping open and closed rapidly.
[0059] The above disclosure reflects exemplary embodiments and
should not be considered limiting of the invention, which is
limited only by the claims.
* * * * *