U.S. patent application number 10/195580 was filed with the patent office on 2003-01-09 for exhaust system.
Invention is credited to Hartick, Johannes.
Application Number | 20030005686 10/195580 |
Document ID | / |
Family ID | 24813667 |
Filed Date | 2003-01-09 |
United States Patent
Application |
20030005686 |
Kind Code |
A1 |
Hartick, Johannes |
January 9, 2003 |
Exhaust system
Abstract
An exhaust system or an engine (12) includes a lean NOx
catalytic device (18), and a heat exchanger (70) positioned
upstream of the catalytic device (18). Control means (44, 46)
controls a valve (36) to regulate exhaust gas flow through the heat
exchanger (70) or along a bypass path (26). The heat exchanger (70)
can cool the exhaust gases to ensure that the maximum operating
temperature of the catalytic device (1) is not exceeded. During
use, the heat exchanger (70) can be bypassed to allow high
temperature purge cycles.
Inventors: |
Hartick, Johannes;
(Lancashire, GB) |
Correspondence
Address: |
BARNES & THORNBURG
2600 CHASE PLAZA
10 LASALLE STREET
CHICAGO
IL
60603
|
Family ID: |
24813667 |
Appl. No.: |
10/195580 |
Filed: |
July 15, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10195580 |
Jul 15, 2002 |
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09700484 |
Feb 21, 2001 |
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6422007 |
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Current U.S.
Class: |
60/298 ;
60/320 |
Current CPC
Class: |
F01N 9/005 20130101;
B01D 53/9431 20130101; F01N 3/0885 20130101; F01N 3/0871 20130101;
Y02T 10/26 20130101; F01N 3/043 20130101; F01N 2570/04 20130101;
F01N 3/0814 20130101; F01N 3/2046 20130101; F01N 13/009 20140601;
F01N 2410/00 20130101; Y02T 10/40 20130101; B01D 53/9495 20130101;
F01N 3/0842 20130101; F01N 2240/02 20130101; Y02T 10/47 20130101;
Y02T 10/12 20130101; F01N 3/05 20130101 |
Class at
Publication: |
60/298 ;
60/320 |
International
Class: |
F01N 003/00; F01N
003/02; F01N 005/02 |
Claims
The claimed invention is:
1. A method of operating an exhaust system, the system comprising a
catalytic device and means for cooling exhaust gases in the system,
the method comprising the steps of: controlling the means for
cooling of the exhaust so as to achieve an exhaust temperature
within a first operating range for the catalytic device appropriate
to a first engine cycle; and controlling the means for cooling of
the exhaust so as to achieve an exhaust temperature within a second
operating range for the catalytic device appropriate to a second
engine cycle, the second operating range having a higher maximum
temperature than the first operating range.
2. A method of operating an exhaust system according to claim 1 in
which the first engine cycle is a lean cycle.
3. A method of operating an exhaust system according to claim 1 or
2 in which the second engine cycle is a stoichiometric cycle.
4. A method of operating an exhaust system according to claim 1 or
2 in which the second engine cycle is a sulphur purge cycle.
5. A method of operating an exhaust system according to any
preceding claim, the method comprising the further step of
controlling the means for cooling of the exhaust so as to revert to
the first operating range.
6. A method of operating an exhaust system according to claim 1 in
which the first operating range of exhaust temperature is from
about 200 degrees C. to 450 degrees C.
7. A method of operating an exhaust system according to claim 1 in
which the second operating range of exhaust temperature is from
approximately 350 degrees C. to 750 degrees C.
8. A method of operating an exhaust system according to claim 1 in
which the second operating range of exhaust temperature is from
about 600 degrees C. to 750 degrees C.
9. An exhaust system for an internal combustion engine comprising a
catalytic device for purifying exhaust gases, a heat exchanger in a
gas flow path upstream of the catalytic device, control means for
controlling the cooling of the exhaust gases by the heat exchanger,
the control means controlling the cooling of the gases so as to
maintain the exhaust gases at a temperature within a first
operating range and, when required, controlling the cooling of the
exhaust gases by the heat exchanger so that the exhaust temperature
lies within a second operating range having a higher maximum
temperature than the first operating range.
Description
[0001] This application is a continuation of U.S. utility patent
application Ser. No. 09/700,484, filed on Feb. 21, 2001.
BACKGROUND AND SUMMARY
[0002] The present invention relates to an exhaust system for an
internal combustion engine, in particular to an exhaust system
employing a catalytic device for purifying the exhaust gases. The
invention is especially suitable for a system for a lean burn
engine (employing a lean NOx catalytic device), but it is not
limited exclusively to this.
[0003] In general terms, the need to operate a catalytic device
above a minimum operating temperature is well known in the art. For
example, EP-A-0460507, GB-A-2278068 and WO 96/27734 describe
arrangements for routing the exhaust along an appropriate exhaust
path if the gas is not at an optimum high temperature, or if the
catalytic devices have not yet reached their optimum
temperatures.
[0004] The increasing cost of fuel and the concern over CO.sub.2
emissions has lead a drive for engines with improved fuel economy.
Lean burn engines have been developed using gasoline direct
injection and port injection techniques.
[0005] Under these lean operating conditions the standard 3-way
catalyst is very efficient for CO and hydrocarbon (HC) oxidation,
but the reduction of oxides of nitrogen NOx (NO and NO.sub.2) to
di-nitrogen (N.sub.2) is considerably more difficult. Catalytic
converters and traps are being developed which can operate under
lean conditions. The "lean" problem is that, there is generally an
insufficient quantity of hydrocarbons in the exhaust gas to enable
efficient conversion of all of the NOx to di-nitrogen at the
catalytic device. One type of lean burn engine uses a lean cycle
and an intermittent stoichiometric or rich cycle. A catalytic trap
can be used which absorbs the excess NOx gases during the lean
cycle, and then converts the NOx to NO.sub.2 in the presence of
more hydrocarbons during the rich cycle. The rich cycle is
sometimes referred to as the "purge" cycle.
[0006] Although lean NOx catalytic converters and traps offer
potentially enormous emissions benefits, it has been extremely
difficult to attain the full potential of the catalytic devices,
especially under conditions in which the engine is working hard
(for example, for high speed vehicle cruising). The reason is that,
under such conditions, the temperature of the exhaust gas entering
the catalytic trap often exceeds the optimum operating range for
the catalytic device. For example, FIG. 17 illustrates the typical
temperature characteristics for a lean NOx trap. The catalytic
material has a coating for absorbing the excess NOx, but this is
only effective up to about 450.degree. C. On the other hand, the
reduction of the oxides in the presence of hydrocarbons is only
effective at temperatures above about 200.degree. C. This creates a
useful temperature window from approximately 22-450.degree. C. in
which the lean NOx conversion can occur. At temperatures outside
this window (for example, caused by high engine speed), the
catalytic trap will not operate efficiently. Lean NOx catalytic
converters also operate in a similar temperature range.
[0007] Broadly speaking, one aspect of the present invention is to
provide a cooling heat exchanger unit upstream of a catalytic
device, and a control device for providing selective cooling of the
exhaust gas upstream of the catalytic device, using the heat
exchanger.
[0008] With the invention, the heat exchanger unit can provide
sufficient cooling to cool the hot exhaust gases to a desired
catalytic operating temperature, or to within a desired operating
temperature window, for efficient catalytic operation.
[0009] Moreover, cooling of the exhaust gases provides other
performance advantages, specifically by reducing the volume of the
gas, and thus the volume flow rate through the exhaust system. This
can help reduce the backpressure within the exhaust system, and can
also help reduce flow noise through the system, especially at high
engine speeds and loads. These are significant problems associated
with lean NOx catalytic devices, which tend to require relatively
large substrates for efficient lean NOx operation. The use of large
substrates can cause undesirable backpressure build up. The
reduction in backpressure will help to improve fuel economy and
reduce CO.sub.2 emissions.
[0010] The heat exchanger unit may be a gas cooled unit (for
example, air cooled), or it may be liquid cooled. The latter is
preferred for the following reasons:
[0011] (a) A liquid-cooled heat exchanger can avoid the occurrence
of transient temperature drops which air-cooled exchangers can
cause. Initially, an air-cooled heat exchanger will be much colder
than the hot exhaust gases and, when the hot gases first pass
through the exchanger, the large temperature difference can causes
a very efficient heatsink effect to occur. Such large transiens can
cause the temperature to fall below an optimum operating range of
the catalytic device until the heat exchanger heats up to near the
exhaust gas temperature.
[0012] (b) A liquid-cooled heat exchanger remains at the
temperature of the coolant, and never heats up to the exhaust gas
temperature. Heat transfer is achieved through the large heat
capacity of the liquid, and does not depend (at least to much
extent) on the precise temperature of the coolant itself. In
contrast, an air-cooled exchanger necessarily heats up to near the
exhaust gas temperature, and dissipates heat by being much hotter
than the surroundings. This can cause design problems for placement
on a vehicle away from hazardous (temperature sensitive) areas, and
also requires the presence of a cooling air flow, in use
[0013] (c) A liquid-cooled heat exchanger can enable the use of an
open-loop control system for controlling the cooling operation
without having to measure directly the temperature of the exhaust
gas in the exhaust system. Most vehicles are not equipped with an
exhaust temperature sensor, and the addition of such a sensor able
to withstand harsh exhaust conditions represents additional
expense. With a liquid-cooled system, the exhaust gas temperature
can be predicted using the outputs from conventional vehicle
sensors for sensing, for example, the engine inlet air temperature,
the engine coolant temperature, the engine speed, the air mass flow
entering the engine, and the fuel:air mixture (measured using a
lambda sensor).
[0014] (d) A liquid heat exchanger can generally be made more
compact than an air-cooled heat exchanger.
[0015] If a liquid heat exchanger is used, then preferably, this is
coupled to an existing coolant circuit of a vehicle, such as, for
example, the engine coolant circuit.
[0016] If a gas-cooled heat exchanger is used, then the arrangement
should comprise a gas inlet tube, a heat exchanger unit coupled to
the inlet tube, and an outlet tube exiting the heat exchanger unit,
the heat exchanger unit having a greater heat dissipation effect
than the inlet and outlet tubes.
[0017] In either type of system, the exhaust system preferably
comprises a first flow path through the heat exchanger for cooling
the gas in the first path, and a second flow path bypassing the
heat exchanger. The second flow path may flow through the housing
of the heat exchanger along a substantially non-heat exchange (or
at least a low-heat exchange) path.
[0018] In another broad aspect, the invention provides a method,
and also a control apparatus, for controlling operation of a
cooling device for cooling exhaust gas upstream of a catalytic
exhaust purification device.
[0019] In one preferred aspect, the method includes predicting the
exhaust gas temperature from a plurality of characteristics which
are each not directly indicative of the exhaust temperature, and
controlling cooling operation in response to the predicted exhaust
gas temperature.
[0020] In another preferred aspect, the method includes controlling
the cooling during a first engine cycle to achieve an exhaust
temperature within a first operating range for the catalytic
device, and during a second engine cycle to achieve an exhaust
temperature within a second operating range for the catalytic
device.
[0021] The second operating range (achieved after the first
operating range) may include a higher maximum temperature than the
first operating range. For example, the second operating range may
correspond to a shoichiometric cycle, or to a sulphur purge cycle.
The first cycle may correspond to a lean cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Embodiments of the invention are now described by way of
example only, with reference to accompanying drawings, in
which:
[0023] FIG. 1 is a schematic view illustrating a first embodiment
of an exhaust system for a lean bum engine;
[0024] FIG. 2 is a schematic view illustrating the heat exchanger
in more detail;
[0025] FIG. 3 is a schematic view illustrating a comparative prior
art exhaust system;
[0026] FIG. 4 is a graph illustrating gas temperatures during
steady state cruising;
[0027] FIG. 5 is a graph illustrating the improvement in catalytic
conversion efficiency;
[0028] FIG. 6 is a graph illustrating the behaviour of the system
of FIG. 1 during a drive cycle;
[0029] FIG. 7 is a graph comparing engine torque and power in the
heat exchanger valve-open and valve-closed positions;
[0030] FIG. 8 is a schematic view illustrating a second embodiment
of exhaust system;
[0031] FIG. 9 is a schematic section through the heat exchanger
used in FIG. 8;
[0032] FIG. 10 is a plan view in isolation of a baffle for the heat
exchanger of FIG. 9;
[0033] FIG. 11 is a plan view in isolation of an end plate of the
heat exchanger of FIG. 9;
[0034] FIG. 12 is a graph illustrating the performance of the
second embodiment;
[0035] FIG. 13 is a more detailed view of a portion of FIG. 12
illustrating the effect of coolant temperature;
[0036] FIG. 14 is a schematic section through an alternative design
of heat exchanger usable in the embodiment of FIG. 7;
[0037] FIG. 15 is a flow diagram illustrating the steps used to
control operation of the exhaust system;
[0038] FIG. 16 is a schematic diagram illustrating a control
algorithm; and
[0039] FIG. 17 illustrates conversion efficiency of a conventional
lean NOx catalytic trap.
DETAILED DESCRIPTION OF THE DRAWINGS
[0040] Referring to FIG. 1, a test exhaust system 10 is illustrated
for a lean bum engine, identified schematically at 12. The exhaust
system comprises an exhaust manifold 14 coupled to the exhaust
ports of the engine 12, a conventional light-off catalytic
converter 16 arranged close to the engine 12 to provide catalytic
purification when the engine is first run, and a lean NOx catalytic
device 18 arranged downstream of the light-off converter 16. The
lean NOx device 18 may either be a catalytic trap, or a lean
catalytic converter, to suit the engine 12.
[0041] Arranged between the light-off converter 16 and the lean NOx
device 18 is a cooling arrangement 20 which consists of a heat
exchanger unit 22 arranged in a first gas flow path 24, and a
second gas flow path 26 bypassing the heat exchanger unit 22.
[0042] Referring to FIG. 2, in this embodiment the heat exchanger
unit 22 is air cooled, and comprises a linear radiator arrangement
of nine steel exchanger tubes 28 extending between an inlet
manifold tube 30 and an outlet manifold tube 32. The exchanger
tubes 28 are cooled by moving air, represented by the fan 34 (FIG.
1).
[0043] In the illustrated test arrangement, the exchanger tubes 28
are approximately 600 nun long, with an inside diameter of about 22
mm. The fan 34 provides an ambient air speed of about 2.5 m/s over
the heat exchanger unit 22.
[0044] Flow through the first and second paths 24 and 26 is
controlled by a valve 36 situated in the first flow path 24
downstream of the heat exchanger unit 22. The flow resistance of
the second path 26 relative to the first path 24 is such that, when
the valve 36 is open, a substantial portion of the gas flows
through the first path 24 through the heat exchanger 22. When the
valve 36 is closed, the gas has to flow through the second path 26,
and thereby bypasses the heat exchanger 22. The flow rates through
the first and second paths are selected such that neither path
presents too high an impedance, which would otherwise cause
undesirable back pressure in the exhaust path.
[0045] In the test arrangement illustrated in FIG. 2, the impedance
of the second path 26 is made adjustable by means of a replaceable
constriction assembly 38. The assembly 38 consists of two flanges
40 between which is received an exchangeable disc 42 having an
orifice of a predetermined size.
[0046] The valve 36 is a vacuum controlled butterfly valve, which
is controlled by means of an electrical solenoid 44. The solenoid
is controlled by a control unit 46, described further below.
[0047] The above valve control arrangement is preferred, as it
avoids the need to place a valve in the direct flow of very hot
exhaust gases. Instead, the valve 36 is placed downstream of the
heat exchanger unit, and so is exposed to less hot exhaust gas.
This can increase valve life, and enable a less expensive valve to
be used. However, it will be appreciated that in other embodiments,
a flow switching valve may be used in the second flow path 26 if
desired, or at one of the junctions between the first and second
flow paths 24 and 26 if desired. The valve may be a butterfly type
or other type of valve, as appropriate.
[0048] In this embodiment, a temperature sensor 48 measures the
exhaust gas temperature upstream of the lean NOx catalytic device
18. For example, the temperature sensor 48 may be located at the
inlet to the device 18, or upstream of the heat exchanger unit 22.
The control unit 46 may, for example, be a straightforward
threshold sensing unit (with hysteresis if desired) which controls
the valve 36 to open when the exhaust gas exceeds a threshold
temperature, so that the temperature is maintained in a desired
temperature window. Alternatively, the control unit 46 may include
a predictive control 6 algorithm representing a thermal model of
the exhaust system to predict the exhaust gas temperature depending
on the load conditions of the engine.
[0049] To test the effect of the heat exchanger, the same exhaust
system was also used in a conventional test arrangement, as
illustrated in FIG. 3. Referring to FIG. 3, features described
above are denoted by the same reference numerals, where
appropriate. In this conventional test arrangement, the heat
exchanger of FIG. 1 is replaced by a steel tube approximately 750
mm long. This is equivalent to the path length the exhaust gas
travels when the valve 36 of FIG. 1 is closed. This pipe length is
also representative of the typical distance between a close coupled
(light-off) catalytic converter in a vehicle engine bay, and an NOx
trap in an underfloor position on a vehicle.
[0050] FIGS. 4, 5 and 6 illustrate the performance comparisons
between the arrangements of FIGS. 1 and 3. The engine used was a
1.8 liter four-cylinder homogeneous lean bum engine coupled to a
100 KW DC dynamometer, to simulate appropriate loading on the
engine.
[0051] FIG. 4 illustrates the exhaust gas temperature at the inlet
of the lean NOx catalytic device 18 at an engine speed and load
corresponding to vehicle cruising at a speed of 120 Km/h (about 75
mph). Bar 50 represents the temperature for the conventional system
of FIG. 3, reaching about 600.degree. C., which is well outside the
operating window of 200-450'C for the catalytic device 18. With the
heat exchanger unit 22 in place, and the control valve 36 open, the
temperature is reduced to about 424'C as illustrated by bar 52,
which is inside the optimum temperature range.
[0052] FIG. 5 illustrates the NOx conversion efficiency of the lean
NOx device 18 for the above conditions. For an exhaust gas
temperature of about 600'C, bar 54 shows that the conversion
efficiency is less than 10%, resulting in high NOx pollution.
However, for the lower exhaust gas temperature achieved with the
heat exchanger unit 22, bar 56 shows that the conversion efficiency
approaches 50%.
[0053] FIG. 6 illustrates the exhaust gas temperature (at the inlet
to the lean NOx catalytic device 18) over the first 1200 seconds of
the standard reference European drive cycle.
[0054] Line 58 illustrates the temperature for the conventional
exhaust arrangement of FIG. 3. In the urban drive cycle (portion
60), the temperature reaches the minimum operating 7 temperature of
2001C for the lean NOx catalytic device 18 after about 150 seconds.
The temperature remains below the maximum threshold of 450'C
throughout the urban portion of the drive cycle (portion 60).
However, during the extra urban portion (portion 62), the
temperature quickly exceeds the maximum threshold of 450'C.
[0055] Line 64 illustrates the catalytic device inlet temperature
for the exhaust arrangement of FIG. 1. In the urban drive cycle
portion 60, the temperature reaches the minimum lean NOx catalytic
operating temperature after about 250 seconds, the gas exhaust
temperature being about 50'C below that with the exchanger unit 22
removed, even though during this portion of the cycle the valve 36
is closed. This temperature reduction is believed to be a result of
direct heat conduction through the metal tubes of the exhaust
system, resulting in some heat loss through the heat exchanger unit
22. In the extra urban portion 62 of the cycle, the temperature
begins to rise, resulting in the valve 36 opening to allow gas
through the heat exchanger unit 22. The temperature falls abruptly,
and remains below the 450"C threshold.
[0056] As described previously, the gas flow rates through the
first and second flow paths 24 and 26 (FIGS. 1 and 2) are designed
such that the flow distribution can be controlled by a single valve
36 downstream of the heat exchanger unit 22. FIG. 7. illustrates a
comparison of the engine power and torque curves for the open and
closed conditions of the valve 36. Any large variation in engine
performance would be very undesirable, as this would affect the
drivability of the vehicle, depending on whether the valve were to
be open or closed. However, as can be seen, there is very little
change in the engine performance when the valve is switched.
[0057] It will be appreciated that the cooling arrangement
illustrated above can provide significantly better NOx conversion
performance compared to a conventional exhaust arrangement. The use
selective cooling (provided above by two flow paths) can ensure
that cooling is only used when needed, i.e. when the exhaust gas
temperature becomes elevated. During initial running of the engine
(and during NOx purge and sulphur purge cycles), the cooling can be
bypassed, to ensure that the lean NOx catalytic device 18 reaches
the desired operating temperature, or purge temperature,
quickly.
[0058] 8 A further and important benefit in cooling the exhaust
gases is that it inherently reduces the volume of the gas, and the
thus the volume flow rate of the gas through the exhaust system.
Such a reduction can reduce back-pressure and also the flow noise
in the exhaust system. Back-pressure in a lean NOx system is a very
important consideration, because the catalytic substrates used for
the lean NOx catalytic devices generally have to be relatively
large to provide good performance in lean conditions. Such large
substrates can result in a back-pressure increase, and so any means
of reducing the back-pressure is highly desirable.
[0059] One of the features of the air-cooled heat exchanger system
described above is that there tends to be a large transient
temperature drop when the control valve 36 is switched to the open
condition. Such a transient drop is visible in FIG. 6 at point 66.
This is a result of the heat exchanger unit 22 being initially very
cool (since it is cooled by the fan 34), and acting as a very
efficient heatsink when the exhaust gas is first passed through the
heat exchanger 22. As more exhaust gas passes through the heat
exchanger 22, the heat exchange tubes 28 heat up, and provide a
lesser rate (by dissipating the heat in the air stream provided by
the fan 34). Such a transient may be undesirable, as it can cause
the exhaust gas temperature to fall below the minimum activation
temperature for the lean NOx catalytic device 18 (about 200'C), for
example as illustrated by the point 66 in FIG. 6.
[0060] FIGS. 8-11 illustrate a second embodiment, which can provide
all of the advantages of the first embodiment, and also addresses
the transient problem. Where appropriate, the same reference
numerals have been used to denote features equivalent to those
described previously.
[0061] The principle difference in FIG. 8 is the use of a
liquid-cooled heat exchanger unit 70 in place of the air-cooled
heat exchanger unit 22 of FIG. 1. The liquid-cooled heat exchanger
70 consists generally of a hollow housing 72 which, in this
embodiment, is cylindrical and contains an arrangement of gas
carrying tubes 74 arranged as a uniform "bundle", with spacing
between adjacent tubes to allow thermal contact with the
surrounding coolant liquid. The tubes 74 extend between two end
plates 76 which are 3o apertured to define an openings 77 into
which each tube 74 opens at its end. The ends of 9 the tubes 74 are
welded to the end plates in a liquid-tight manner. Outside the end
plates 76, the housing defines an inlet chamber 78 to allow the
incoming exhaust gas to be distributed to flow into the tubes 74,
and an outlet chamber 80 for the re-collimation of the gas flowing
out of the tubes 74.
[0062] The housing 72 defines a liquid-tight chamber surrounding
the tubes 74. Liquid coolant is received through a coolant inlet
port 82 and is circulated in the housing before exiting through a
coolant outlet port 84. In order to ensure optimum flow of the
coolant in contact with the tubes 74, the housing includes a
plurality of internal baffles 86. Each baffle is similar to the end
plates 76 in that it consists of a wall with openings 88 through
which the tubes 74 pass. However, each baffle includes a "cut-away"
portion to define a passage between the edge of the baffle and the
housing to permit the flow of liquid around the baffle. As best
seen in FIG. 9, the baffles 86 are arTanged alternately to define a
tortuous sinusoidal flow path for the coolant liquid between the
inlet and outlet ports 82 and 84.
[0063] In the present embodiment, the heat exchanger 70 is made of
steel, and is relatively compact, including 19 tubes 74 each of
length 440 mm and diameter 14 mm.
[0064] The housing has a diameter of about 88 mm, and the baffles
each have a "height" of about 60 mm. The baffles are arranged with
a uniform spacing of about 110 mm, and are secured in position by
being spot welded to, for example, three of the tubes 74.
[0065] Liquid coolant circulated through the heat exchanger 70 by a
liquid coolant circuit 90 which includes a heat dissipating
radiator 92 and a coolant pump 94. The coolant circuit may be a
dedicated circuit in the vehicle, but in this preferred embodiment,
the coolant circuit is part of an existing coolant circuit on the
vehicle, for example, the usual engine coolant circuit and using
the engine radiator (92) and the engine coolant pump (94). This can
avoid the additional space and cost of using an independent cooling
circuit.
[0066] FIG. 12 illustrates the performance of the exhaust system
with the liquid-cooled heat exchanger, and using a similar engine
and test arrangement as that described previously. In FIG. 12:
[0067] the line 96 represents the temperature of the exhaust gases
at the inlet to the heat exchanger (equivalent to the exhaust gas
temperature reaching the lean NOx catalytic device 18 if the heat
exchanger were to be omitted); the line 98 represents the
temperature of the exhaust gas leaving the heat exchanger
(equivalent to the temperature of the exhaust gas entering the lean
NOx device 18 when the control valve 36 is open); the line 100
represents the temperature of the liquid coolant being circulated
through the heat exchanger; and the line 102 represents the mass
flow of the exhaust gases.
[0068] The graph illustrates the measured characteristics over a
cycle including three different engine settings, the first portion
104 being at an engine speed of 1000 rpm at % throttle, the second
portion 106 being at an engine speed of 2000 rpm at 50% throttle,
and the third portion 108 being at an engine speed of 4000 rpm at
100% throttle.
[0069] As can be seen from the graph, the relatively compact heat
exchanger provides adequate cooling to maintain the exhaust gas
temperature below about 450'C even at elevated inlet temperatures,
and high mass flow.
[0070] Moreover, the liquid heat exchanger does not produce any
transients when the flow of the exhaust gas is switched from the
bypass path to the heat exchanger path. This is because, unlike
air-cooling, the wall temperature does not vary much. Rather, it is
the high specific heat capacity of the coolant liquid which enables
heat to be absorbed by the coolant, with little resultant
temperature dependency. For example, referring to FIGS. 12 and 13,
in the portion 106 of the test cycle described above, the water
temperature in the heat exchanger fluctuates between about 80'C and
90'C. However, there is virtually no resultant change in the gas
outlet temperature from the heat exchanger (line 98).
[0071] A further advantage with a liquid coolant heat exchanger is
that, in contrast to an air-cooled exchanger, the exchanger does
not heat up to the high exhaust gas temperatures. The exchanger
remains at the temperature of the coolant. This can avoid the need
to provide high temperature dissipation devices in the exhaust
system, which might prove hazardous or position critical for
underfloor exhaust systems, or for engine-bay exhaust components.
The lack of any requirement for a cooling air flow over the 11
exchanger also permits the designer greater flexibility in
positioning the exchanger on a vehicle.
[0072] FIG. 14 illustrates an alternative design of liquid coolant
heat exchanger 110, which incorporates the bypass, non-heat
exchange path, within the housing 112 of the heat exchanger 110.
This avoids the need to employ separate conduits for the exhaust
bypass path. Referring to FIG. 14, the housing 112 through which
the coolant flows has a generally annular shape, and the heat
exchange tubes 74 are arranged in an annular configuration within
the housing 112. The central hollow of the housing provides the
bypass path 26 with little, or no, thermal contact with the coolant
medium. The heat exchange and non-heat exchange paths join at
either end of the housing 112 at an inlet chamber 114 and an outlet
chamber 116. The valve 36 is arranged within the bypass path and,
in this embodiment, can be an integral part of the heat exchanger
unit.
[0073] If desired, it is possible to concatenate the above heat
exchanger 110 with a catalytic device within a common housing, to
provide a single unit which contains a catalytic device and a
temperature regulating mechanism.
[0074] FIG. 15 illustrates a typical control process loop 120 for
controlling the valve 36 during the lean, rich and sulphur purge
cycles of the engine. Step 122 determines whether the engine is
running and, if not, the process branches to a termination step
124.
[0075] If the engine is running, step 126 determines whether a
sulphur purge is necessary to clear the exhaust system of a build
up of sulphur oxides. In. some countries, fuel contains a fairly
high sulphur content, and the sulphur oxides tend to collect in the
catalytic devices (and act in competition to the conversion of
nitrogen oxides). The build up of sulphur oxides is countered by a
high temperature purge. If a sulphur purge is necessary, then step
126 branches to step 128 at which a target temperature window
defined by Tmax, Tmin is set to correspond to the desired high
temperature for a sulphur purge, generally between about 600"C and
750'C. Step 130 controls the valve 36 to try to achieve a
temperature within the window. Generally, the desired temperature
is so high that the valve 36 will remain closed during this period
to allow the exhaust temperature to reach maximum levels.
[0076] 12 Step 132 determines whether the sulphur purge has been
completed. If not, the process loops back to repeat steps 128 and
132 until completion of the sulphur purge.
[0077] Once the sulphur purge has been completed, or if no sulphur
purge was determined to be necessary at step 126, the process
proceeds to step 134 which determines whether the engine is
currently running lean. If the engine is running lean, then the
process proceeds through step 136 at which a target temperature
window defMed by Tmax, Tmin is set to correspond to the temperature
range for lean NOx catalytic operation, generally between about
200'C and 450'C. If the engine is not running lean, then the target
temperature window is set at step 138 to correspond to
stoichiometric NOx catalytic operation, generally between 350'C and
750'C.
[0078] The process then proceeds to step 137 which controls the
valve 36 to try to achieve a temperature within the target window.
Thereafter, the process loops back to step 122 described above.
[0079] The valve 36 may be controlled either to be fully open of
fully closed.
[0080] Alternatively, the valve 36 may be controlled to be open by
a controllable amount, through the use of proportion control, for
example PID (proportional integral differential) control.
[0081] The valve 36 may be controlled simply through the use of a
temperature sensor which measures directly the temperature of the
gas in the exhaust system (closed loop feedback). However, the use
of a liquid cooled heat exchanger system also permits an open loop
control to be used which predicts the temperature of the exhaust
gas without having to measure the exhaust temperature directly.
This can provide cost savings in not having to use a relatively
expensive exhaust gas temperature sensor.
[0082] An open loop system is illustrated, for example, in FIG. 16.
The system uses the outputs of sensors which are provided as
standard sensors on most modem vehicles.
[0083] These are: an air temperature sensor 140 which provides a
signal indicative of the inlet air temperature to the engine; a
coolant temperature sensor 142 which provides a signal indicative
of the engine coolant temperature; an engine speed sensor 144 which
provides an indication of the rpm engine speed (as measured or as
deduced from the engine control system); and air mass flow sensor
146 which provides a signals indicative of the air mass 13 flow
into the engine; and a lambda sensor 148 which provides a signal
indicative of the air: ftiel ratio as measured from the exhaust
gases.
[0084] An engine map/model 150 is used to calculate the exhaust gas
temperature and the exhaust gas mass flow from the engine, and an
exhaust system thermal model 152 is then used to calculate the
amount of cooling required to bring the exhaust gas temperature to
within the target temperature window, based on the liquid coolant
temperature (for example, the same as the engine coolant
temperature if a common system).
[0085] The engine map/model 150, and the thermal model 152 of the
exhaust system (including the heat exchanger), can be implemented
relatively easily using a computer based control system, for
example, a micro controller.
[0086] It will be appreciated that the invention, particularly as
described in the preferred embodiments, can provide a system for
controlling the temperature of exhaust gases to within the desired
operating temperature window for a catalytic device.
[0087] It will be appreciated that the above description is merely
illustrative of preferred embodiments of the invention, and that
many modifications may be made within the scope of the invention.
Features believed to be of particular importance are defined in the
appended claims. However, the Applicant claims protection for any
novel feature or aspect described herein and/or illustrated in the
drawings, whether or not emphasis has been placed thereon.
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