U.S. patent application number 11/711657 was filed with the patent office on 2008-08-28 for engine exhaust treatment system.
This patent application is currently assigned to Caterpillar Inc.. Invention is credited to James Joshua Driscoll, Amit Jayachandran.
Application Number | 20080202097 11/711657 |
Document ID | / |
Family ID | 39386467 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080202097 |
Kind Code |
A1 |
Driscoll; James Joshua ; et
al. |
August 28, 2008 |
Engine exhaust treatment system
Abstract
An engine exhaust treatment system is provided. The system may
include a catalyst-based device including a catalyst configured to
reduce an amount of NO.sub.x in exhaust gases produced by an
engine, by using ammonia stored in the catalyst-based device. The
system may also include a controller configured to control one or
more operating parameters of the engine to, under predetermined
operating conditions of the engine or catalyst-based device,
increase an amount of NO.sub.x produced by the engine to prevent at
least some of the ammonia stored in the catalyst-based device from
being released from the catalyst-based device into the exhaust.
Inventors: |
Driscoll; James Joshua;
(Dunlap, IL) ; Jayachandran; Amit; (Peoria,
IL) |
Correspondence
Address: |
CATERPILLAR/FINNEGAN, HENDERSON, L.L.P.
901 New York Avenue, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
Caterpillar Inc.
|
Family ID: |
39386467 |
Appl. No.: |
11/711657 |
Filed: |
February 28, 2007 |
Current U.S.
Class: |
60/274 ;
60/286 |
Current CPC
Class: |
B01D 53/9431 20130101;
F01N 2610/02 20130101; F01N 2560/06 20130101; Y02T 10/12 20130101;
Y02T 10/40 20130101; F01N 2570/18 20130101; F01N 3/208 20130101;
Y02T 10/47 20130101; B01D 2251/206 20130101; F01N 13/009 20140601;
F01N 11/002 20130101; F01N 3/2066 20130101; F01N 13/0097 20140603;
F01N 13/0093 20140601; B01D 53/9495 20130101; Y02T 10/24
20130101 |
Class at
Publication: |
60/274 ;
60/286 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Claims
1. An engine exhaust treatment system, comprising: a catalyst-based
device including a catalyst configured to reduce an amount of NOx
in exhaust gases produced by an engine, by using reductant stored
in the catalyst-based device; and a controller configured to
control one or more operating parameters of the engine to, in
response to a rate of increase of a temperature associated with the
catalyst-based device, increase an amount of NOx produced by the
engine to prevent at least some of the reductant stored in the
catalyst-based device from being released from the catalyst-based
device into the exhaust.
2. The exhaust treatment system of claim 1, wherein the controller
is configured to control one or more operating parameters of the
engine in response to the rate of increase of the temperature
associated with the catalyst-based device being faster than a
threshold rate of temperature increase.
3. The exhaust treatment system of claim 2, wherein, in response to
the rate of increase of the temperature associated with the
catalyst-based device being faster than a threshold rate of
temperature increase, the controller is further configured to
reduce an amount of urea injected into the catalyst-based
device.
4. The exhaust treatment system of claim 1, wherein the controller
is configured to increase the amount of NOx produced by the engine
by advancing injection timing.
5. The exhaust treatment system of claim 1, wherein the controller
is configured to increase the amount of NOx produced by the engine
by reducing exhaust gas recirculation flowrate.
6. The exhaust treatment system of claim 1, wherein the controller
is configured to increase the amount of NOx produced by the engine
by increasing injection pressure.
7. The exhaust treatment system of claim 1, wherein the controller
is configured to increase the amount of NOx produced by the engine
by changing valve actuation strategy.
8. A method of exhaust treatment, comprising: storing reductant in
a catalyst-based device, wherein the catalyst-based device includes
a catalyst configured to reduce an amount of NOx in exhaust gases
produced by an engine by using the reductant stored in the
catalyst-based device; and controlling one or more operating
parameters of the engine to, in response to a rate of increase of a
temperature associated with the catalyst-based device, increase an
amount of NOx produced by the engine to prevent at least some of
the reductant stored in the catalyst-based device from being
released from the catalyst-based device into the exhaust.
9. The method of claim 8, wherein controlling one or more operating
parameters of the engine occurs in response to the rate of increase
of the temperature associated with the catalyst-based device being
faster than a threshold rate of temperature increase.
10. The method of claim 8, further including reducing an amount of
urea injected into the catalyst-based device, in response to the
rate of increase of the temperature associated with the
catalyst-based device being faster than a threshold rate of
temperature increase.
11. The method of claim 8, wherein increasing the amount of NOx
produced by the engine is accomplished at least in part by
advancing injection timing.
12. The method of claim 8, wherein the increasing the amount of NOx
produced by the engine is accomplished at least in part by reducing
exhaust gas recirculation flowrate.
13. The method of claim 8, wherein the increasing the amount of NOx
produced by the engine is accomplished at least in part by
increasing injection pressure.
14. The method of claim 8, wherein the increasing the amount of NOx
produced by the engine is accomplished at least in part by changing
valve actuation strategy.
15. A power system, comprising: an exhaust producing engine; and an
engine exhaust treatment system configured to reduce an amount of
one or more constituents of the exhaust produced by the engine, the
engine exhaust treatment system including: a catalyst-based device
including a catalyst configured to reduce an amount of NOx in
exhaust gases produced by the engine, by using reductant stored in
the catalyst-based device; and a controller configured to control
one or more operating parameters of the engine to, in response to a
rate of increase of a temperature associated with the
catalyst-based device being faster than a threshold rate of
temperature increase, increase an amount of NOx produced by the
engine to prevent at least some of the reductant stored in the
catalyst-based device from being released from the catalyst-based
device into the exhaust;
16. The power system of claim 15, wherein, in response to the rate
of increase of the temperature associated with the catalyst-based
device being faster than the threshold rate of temperature
increase, the controller is further configured to reduce an amount
of urea injected into the catalyst-based device.
17. The power system of claim 15, wherein the controller is
configured to increase the amount of NOx produced by the engine by
advancing injection timing.
18. The power system of claim 15, wherein the controller is
configured to increase the amount of NOx produced by the engine by
reducing exhaust gas recirculation flowrate.
19. The power system of claim 15, wherein the controller is
configured to increase the amount of NOx produced by the engine by
increasing injection pressure.
20. The power system of claim 15, wherein the controller is
configured to increase the amount of NOx produced by the engine by
changing valve actuation strategy.
Description
TECHNICAL FIELD
[0001] The present disclosure is directed to a engine exhaust
treatment system and, more particularly, to an engine exhaust
treatment system configured to prevent ammonia slip.
BACKGROUND
[0002] Engines, including diesel engines, gasoline engines, natural
gas engines, and other engines known in the art, may exhaust a
complex mixture of air pollutants. The air pollutants may be
composed of both solid material, such as, for example, particulate
matter, and gaseous material, which may include, for example,
oxides of nitrogen, such as NO.sub.2 and NO.sub.3 (commonly
referred to collectively as "NO.sub.x").
[0003] Due to increased environmental concerns, exhaust emission
standards have become more stringent. The amount of particulate
matter and gaseous pollutants emitted from an engine may be
regulated depending on the type, size, and/or class of engine. In
order to meet these emissions standards, engine manufacturers have
pursued improvements in several different engine technologies, such
as fuel injection, engine management, and air induction, to name a
few.
[0004] In addition, engine manufacturers have developed devices for
treatment of engine exhaust after it leaves the engine (sometimes
referred to as "after-treatment"). For example, engine
manufacturers have employed exhaust treatment devices that utilize
catalysts to convert one or more components of the exhaust to
different, more environmentally-friendly compounds. Catalyst-based
devices have been developed for reducing or removing NO.sub.x from
the exhaust stream. In some systems, NO.sub.x may be reduced by
selective catalytic reduction (commonly referred to as "SCR"). In
such systems, urea may be added to a catalyst-based device, where
it is broken down into ammonia (NH.sub.3) that is stored in (or on)
the catalyst. The ammonia stored in the catalyst reacts with
NO.sub.x in the exhaust to thereby convert the NO.sub.x to Nitrogen
(N.sub.2) and water (H.sub.2O).
[0005] The amount of ammonia that can be stored in a catalyst may
depend on the temperature of the catalyst. Generally, the hotter
the catalyst, the less ammonia that can be stored in it. Therefore,
one problem with such systems is that if operating conditions cause
an increase in catalyst temperature that happens rapidly, then some
of the ammonia in the catalyst may be released into the exhaust and
carried downstream from the catalyst-based device and exhausted
into the atmosphere with the exhaust gases. This is commonly
referred to as "ammonia slip."
[0006] Systems have been developed to prevent ammonia slip. For
example, some systems attempt to precisely control the amount of
ammonia in the catalyst to correspond to the amount of NO.sub.x
produced by the engine. These systems operate by selectively
injecting urea (which breaks down into ammonia) into the catalyst
in varying amounts/rates depending on the operating conditions of
the engine and/or catalyst. Generally, the injection of urea is
controlled, based on the amount of NO.sub.x produced by the engine,
in order to maintain a predetermined maximum amount of ammonia in
the catalyst, while taking into account that less ammonia may be
stored in the catalyst as catalyst temperatures rise. For example,
as catalyst temperatures rise, ammonia injection may be reduced or
stopped. However, even with no ammonia injection, the consumption
of ammonia already in the catalyst (i.e., via the reaction with
NO.sub.x) may be relatively slow compared to the rate at which the
capacity of the catalyst to retain ammonia is diminished.
Therefore, such systems must predict NO.sub.x output by predicting
future engine operating parameters, because in order to prevent
ammonia slip when catalyst temperature is increasing, the urea
injection must be reduced or stopped early to allow time for the
amount of ammonia already in the catalyst to be consumed before the
decrease in the capacity of the catalyst to retain ammonia leads to
excess ammonia being released by the catalyst (i.e., ammonia
slip).
[0007] One system has been developed that not only controls
injection of NO.sub.x reducing agents, but also regulates the
amount of NO.sub.x produced by the engine by controlling engine
operating parameters to produce less NO.sub.x under certain
operating conditions. U.S. Pat. No. 5,845,487 issued to Fraenkle et
al. ("the '487 patent") discloses a system configured to reduce
both injection of NO.sub.x reducing agents and NO.sub.x output by
the engine during cold-start warm up. Once operating temperatures
exceed a predetermined threshold, the system switches over to a
higher engine NO.sub.x output operating condition and increased
injection of NO.sub.x reducing agents.
[0008] While the control strategy employed by the '487 patent may
attempt to address certain emissions concerns during cold-start
conditions, the control strategy does not address the issue of
ammonia slip, particularly during conditions other than cold-start.
In addition, the control strategy of the '487 patent modifies the
engine operating parameters under some conditions in a manner that
reduces fuel efficiency.
[0009] The present disclosure is directed at solving one or more of
the problems discussed above.
SUMMARY OF THE INVENTION
[0010] In one aspect, the present disclosure is directed to an
engine exhaust treatment system. The system may include a
catalyst-based device including a catalyst configured to reduce an
amount of NO.sub.x in exhaust gases produced by an engine, by using
ammonia stored in the catalyst-based device. The system may also
include a controller configured to control one or more operating
parameters of the engine to, under predetermined operating
conditions of the engine or catalyst-based device, increase an
amount of NO.sub.x produced by the engine to prevent at least some
of the ammonia stored in the catalyst-based device from being
released from the catalyst-based device into the exhaust.
[0011] In another aspect, the present disclosure is directed to a
method of exhaust treatment. The method may include storing ammonia
in a catalyst-based device, wherein the catalyst-based device
includes a catalyst configured to reduce an amount of NO.sub.x in
exhaust gases produced by the engine by using the ammonia stored in
the catalyst-based device. The method may also include controlling,
with a controller, one or more operating parameters of the engine
to, under predetermined operating conditions of the engine or the
catalyst-based device, increase an amount of NO.sub.x produced by
the engine to prevent at least some of the ammonia stored in the
catalyst-based device from being released from the catalyst-based
device into the exhaust.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a diagrammatic illustration of a machine according
to an exemplary disclosed embodiment.
DETAILED DESCRIPTION
[0013] FIG. 1 illustrates a machine 10. Machine 10 may include a
frame 12, an operator station 14, one or more traction devices 16,
and a power system 17. Power system 17 may include an exhaust
producing engine 18 and an engine exhaust treatment system 20
configured to reduce the amount of one or more constituents of the
exhaust produced by engine 18.
[0014] Although machine 10 is shown as a truck, machine 10 could be
any type of machine having an exhaust producing engine.
Accordingly, traction devices 16 may be any type of traction
devices, such as, for example, wheels, as shown in FIG. 1, tracks,
belts, or any combinations thereof.
[0015] Engine 18 may be mounted to frame 12 and may include any
kind of engine that produces an exhaust flow of exhaust gases. For
example, engine 18 may be an internal combustion engine, such as a
gasoline engine, a diesel engine, a natural gas engine or any other
exhaust gas producing engine. Engine 18 may be naturally aspirated
or, in other embodiments, may utilize forced induction (e.g.,
turbocharging or supercharging).
[0016] System 20 may include, among other things, a controller 22,
an exhaust conduit 26, and one or more after-treatment devices 28.
These and other components of system 20 will be discussed in
greater detail below.
[0017] Controller 22 may include any means for receiving machine
operating parameter-related information and/or for monitoring,
recording, storing, indexing, processing, and/or communicating such
information. These means may include components such as, for
example, a memory, one or more data storage devices, a central
processing unit, or any other components that may be used to run an
application.
[0018] Although aspects of the present disclosure may be described
generally as being stored in memory, one skilled in the art will
appreciate that these aspects can be stored on or read from types
of computer program products or computer-readable media, such as
computer chips and secondary storage devices, including hard disks,
floppy disks, optical media, CD-ROM, or other forms of RAM or ROM.
Various other known circuits may be associated with controller 22,
such as power supply circuitry, signal-conditioning circuitry,
solenoid driver circuitry, communication circuitry, and other
appropriate circuitry.
[0019] Controller 22 may be configured to perform multiple
processing and controlling functions, such as, for example, engine
management (e.g., controller 22 may include an engine control
module, a.k.a. an ECM), monitoring/calculating various parameters
related to exhaust output and after-treatment thereof, etc. In some
embodiments, machine 10 may include multiple controllers (a
configuration not shown), each dedicated to perform one or more of
these or other functions. Such multiple controllers may be
configured to communicate with one another.
[0020] After-treatment devices 28 may include a catalyst-based
device 30 (e.g., a catalytic converter). Catalyst-based device 30
may include a catalyst 32 configured to convert (e.g., via
oxidation or reduction) one or more gaseous constituents of the
exhaust stream produced by engine 18 to a more environmentally
friendly gas and/or compound to be discharged into the atmosphere.
For example, catalyst 32 may be configured to chemically alter at
least one component of the exhaust flow. Catalyst-based device 30
may be configured for one or more various types of conversion, such
as, for example, select catalytic reduction (SCR), diesel oxidation
(e.g., a diesel oxidation catalyst, DOC), and/or adsorption of
nitrous oxides (NO.sub.x; e.g., a NO.sub.x adsorber).
[0021] After-treatment devices 28 may also include a particulate
trap 34. Particulate trap 34 may include any type of
after-treatment device configured to remove one or more types of
particulate matter, such as soot and/or ash, from an exhaust flow
of engine 18. Particulate trap 34 may include a filter medium 36
configured to trap the particulate matter as the exhaust flows
through it. Filter medium may consist of a mesh-like material, a
porous ceramic material (e.g., cordierite), or any other material
and/or configuration suitable for trapping particulate matter.
[0022] In some embodiments, after-treatment devices 24 may include
combinations of these types of devices. For example,
after-treatment devices 28 may include one or more catalytic
particulate traps (not shown), which may include a catalytic
material integral with filter medium 36. For example, catalyst 32
may be packaged with, coated on, or otherwise associated with
filter medium 36. In some embodiments, filter medium 36 may,
itself, be a catalytic material. In some embodiments, there may be
only a single after-treatment device 28 that is a combined
particulate trap/catalyst-based device. In addition, although
system 20 is shown with a single catalyst-based device 30 and a
single particulate trap 34, system 20 may include more than one of
either or both. In other embodiments, system 20 may include more
than one catalytic particulate trap. Such multiple after-treatment
devices may be positioned in series (e.g., along exhaust conduit
26) or in parallel (e.g., in dual exhaust conduits; not shown). In
some embodiments, catalyst-based device 30 may be positioned
downstream from particulate trap 34. In other embodiments,
catalyst-based device 30 may be positioned upstream from
particulate trap 34.
[0023] In some embodiments, catalyst-based device 30 may be
configured for selective catalytic reduction (SCR). In such
embodiment, system 20 may include a urea injector 38 configured to
inject urea into exhaust conduit 26 and/or directly into
catalyst-based device 30. The injected urea may be broken down into
ammonia, which may be retained within catalyst-based device 30. The
ammonia stored in catalyst-based device 30 may reduce the amount of
NO.sub.x in the exhaust gases passing through catalyst-based device
30. Alternatively or additionally, other agents suitable for
reducing NO.sub.x may be injected into exhaust conduit 26 and/or
catalyst-based device 30.
[0024] In embodiments configured for SCR, catalyst 32 may be
configured to reduce the amount of NO.sub.x in the exhaust gases
produced by engine 18 by using ammonia stored in catalyst-based
device 30. Controller 22 may be configured to control one or more
operating parameters of engine 18 to, under predetermined operating
conditions of engine 18 or catalyst-based device 30, increase an
amount of NO.sub.x produced by engine 18 to prevent at least some
of the ammonia stored in catalyst-based device 30 from being
released from catalyst-based device 30 into the exhaust.
[0025] System 20 may include a temperature sensor 40 configured to
detect a temperature associated with catalyst-based device 30. In
one embodiment, temperature sensor 40 may be configured to detect
inlet temperature of catalyst-based device 30, as illustrated in
FIG. 1. Alternatively, temperature sensor 40 may be positioned at
any location suitable to detect a temperature associated with
catalyst-based device 30. For example, temperature sensor 40 may be
located along exhaust conduit 26 somewhat upstream from
catalyst-based device 30, such that the temperature of
catalyst-based device 30 may be predicted based on the upstream
exhaust gas temperature detected by temperature sensor 40. In other
embodiments, temperature sensor 40 may be what may be referred to
as a "virtual sensor." For example, a virtual temperature sensor
may be derived by controller 22 by monitoring one or more engine
and/or exhaust operating parameters and predicting the inlet
temperature of catalyst-based device 30 based on the monitored
operating parameters.
[0026] In some embodiments, controller 22 may be configured to
monitor the rate of temperature increase measured by temperature
sensor 40. Controller 22 may be configured to increase the amount
of NO.sub.x produced by engine 18 when the rate of increase of a
temperature associated with the catalyst-based device is faster
than a predetermined rate of temperature increase. Rapid rates of
catalyst temperature increase may occur when exhaust temperatures
rise quickly, such as upon increased engine load. Increases in
engine load may occur, for example, due to acceleration of engine
18 and/or acceleration of machine 10 or due to machine 10 suddenly
starting to drive uphill, thus increasing the load-on engine
18.
[0027] In one embodiment, when the inlet temperature of
catalyst-based device 30, as measured by temperature sensor 40, is
increasing faster than a predetermined rate of increase, the
capacity of catalyst-based device 30 to retain ammonia may be
decreasing at a rate fast enough to cause ammonia to be released
(i.e., ammonia slip) before it may be used to react with NO.sub.x
in the exhaust flowing through catalyst-based device 30. In order
to prevent the ammonia from being released from catalyst-based
device 30 due to the decrease in capacity, controller 22 may
increase the amount of NO.sub.x produced by engine 18, when the
catalyst inlet temperature is increasing faster than a
predetermined rate of increase. The extra NO.sub.x in the exhaust
gases may react with more ammonia, causing the ammonia to be
"consumed" in the NO.sub.x reduction reaction before the ammonia
slip occurs.
[0028] System 20 may effectuate the increased NO.sub.x output of
engine 18 in any number of different ways. Controller 22 may be
configured to increase NO.sub.x output of engine 18 by changing one
or more operating parameters of engine 18. For example, controller
22 may be configured to increase the amount of NO.sub.x produced by
engine 18 by advancing injection timing, reducing exhaust gas
recirculation flowrate, increasing injection pressure, and/or
changing valve actuation strategy. In addition to reducing or
preventing ammonia slip, these changes in engine operating
parameters may also improve fuel efficiency of engine 18.
Alternatively or additionally, other changes to operating
parameters of engine 18 that cause an increase in NO.sub.x
production may be employed.
[0029] In addition to increasing NO.sub.x output of engine 18 under
the predetermined operating conditions (e.g., rapid increases in
catalyst temperature), the controller may also be configured to
reduce the amount of urea injected into catalyst-based device 30,
under such conditions. In some embodiments, reducing the urea
injection may include completely ceasing urea injection under
predetermined operating conditions.
INDUSTRIAL APPLICABILITY
[0030] The disclosed system may be suitable to enhance exhaust
emissions control for engines. The disclosed system may be used for
any application of an engine. Such applications may include
supplying power for machines, such as, for example, stationary
equipment such as power generation sets, or mobile equipment, such
as vehicles. The disclosed system may be used for any kind of
vehicle, such as, for example, automobiles, construction machines
(including those for on-road, as well as off-road use), and other
heavy equipment.
[0031] Not only may the disclosed system be applicable to various
applications of an engine, but the disclosed system may be
applicable to various types of engines as well. For example, the
disclosed system may be applicable to any exhaust producing engine,
which may include gasoline engines, diesel engines, natural gas
engines, hydrogen engines, etc. The disclosed system may also be
applicable to a variety of engine configurations, including various
cylinder configurations, such as "V" cylinder configurations (e.g.,
V6, V8, V12, etc.), inline cylinder configurations, and
horizontally opposed cylinder configurations. The disclosed system
may also be applicable to engines with a variety of induction
types. For example, the disclosed system may be applicable to
normally aspirated engines, as well as those with forced induction
(e.g., turbocharging or supercharging). Engines to which the
disclosed system may be applicable may include combinations of
these configurations (e.g., a turbocharged, inline-6 cylinder,
diesel engine).
[0032] The disclosed system may also be applicable to various
exhaust path configurations. For example, the disclosed system may
be applicable to exhaust systems that employ a single exhaust
conduit (e.g., the exhaust from each cylinder ultimately feeds into
a single conduit, such as after an exhaust manifold). The disclosed
system may also be applicable to dual exhaust systems (e.g.,
different groups of cylinders may feed into separate exhaust
conduits). In such systems, many of the components of the disclosed
system may be provided in duplicate (e.g., one catalyst-based
device for each exhaust conduit, one particulate trap for each
conduit, etc.).
[0033] Further, where appropriate, the disclosed system may provide
more than one of certain components that have been shown and
discussed herein as singular components. For example, in any given
embodiment, the disclosed system may include more than one
catalyst-based device, regardless of the exhaust configuration
utilized in that embodiment.
[0034] An exemplary method of exhaust treatment using the disclosed
system may include storing ammonia in a catalyst-based device,
wherein the catalyst-based device includes a catalyst configured to
reduce an amount of NO.sub.x in exhaust gases produced by the
engine by using the ammonia stored in the catalyst-based device.
The exemplary method may also include controlling, with a
controller, one or more operating parameters of the engine to,
under predetermined operating conditions of the engine or the
catalyst-based device, increase the amount of NO.sub.x produced by
the engine to prevent at least some of the ammonia stored in the
catalyst-based device from being released from the catalyst-based
device into the exhaust. In some embodiments, the method may
include reducing an amount of urea injected into the catalyst-based
device, under the predetermined operating conditions. In some
embodiments, increasing the amount of NO.sub.x produced by the
engine may be accomplished at least in part by advancing injection
timing, reducing exhaust gas recirculation flowrate, increasing
injection pressure, and/or changing valve-actuation strategy.
[0035] In addition to preventing ammonia slip, the disclosed system
may provide other advantages, such as improving fuel economy. As
discussed above, operating an engine in a manner that produces more
NO.sub.x may be more fuel efficient. Further, the disclosed system
may also make it possible to use smaller catalyst-based devices
because, with the disclosed system, more ammonia may be stored in
the catalyst at any given time without risking ammonia slip upon
subjecting the catalyst to rapid temperature increases.
[0036] It will be apparent to those having ordinary skill in the
art that various modifications and variations can be made to the
disclosed exhaust treatment system without departing from the scope
of the invention. Other embodiments of the invention will be
apparent to those having ordinary skill in the art from
consideration of the specification and practice of the invention
disclosed herein. It is intended that the specification and
examples be considered as exemplary only, with a true scope of the
invention being indicated by the following claims and their
equivalents.
* * * * *