U.S. patent application number 10/915380 was filed with the patent office on 2005-02-24 for exhaust gas purifying apparatus.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Kakinohana, Masaru, Tsuchiya, Jiro.
Application Number | 20050039441 10/915380 |
Document ID | / |
Family ID | 34055948 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050039441 |
Kind Code |
A1 |
Kakinohana, Masaru ; et
al. |
February 24, 2005 |
Exhaust gas purifying apparatus
Abstract
The present invention relates to an exhaust gas purifying
apparatus comprising; a plasma-assist PM purifying reactor; a
detecting device for detecting an amount of PM accumulated in the
PM purifying reactor; a power supply for the PM purifying reactor;
and a controlling device for controlling the power supply on the
basis of signals from the detecting device. The controlling device
controls the power supply to start supplying the electric power to
the PM purifying reactor, or to increase the amount of electric
power which is supplied to the PM purifying reactor, when the
amount of accumulated PM detected by the detecting device exceeds a
predetermined amount.
Inventors: |
Kakinohana, Masaru;
(Susono-shi, JP) ; Tsuchiya, Jiro; (Susono-shi,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
34055948 |
Appl. No.: |
10/915380 |
Filed: |
August 11, 2004 |
Current U.S.
Class: |
60/275 ; 60/295;
60/297 |
Current CPC
Class: |
F01N 9/002 20130101;
F01N 3/0275 20130101; F01N 2240/28 20130101 |
Class at
Publication: |
060/275 ;
060/297; 060/295 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 19, 2003 |
JP |
2003-207896 |
Claims
1. An exhaust gas purifying apparatus comprising; a plasma-assist
PM purifying reactor; a detecting device for detecting an amount of
PM accumulated in said PM purifying reactor; a power supply for
said PM purifying reactor; and a controlling device for controlling
said power supply on the basis of signals from said detecting
device, said controlling device controls said power supply to start
supplying the electric power to said PM purifying reactor, or to
increase the amount of electric power which is supplied to said PM
purifying reactor, when the amount of accumulated PM detected by
said detecting device exceeds a predetermined amount.
2. The exhaust gas purifying apparatus according to claim 1,
wherein the detecting device comprises a differential pressure
gauge which detects a differential pressure across the PM purifying
reactor.
3. The exhaust gas purifying apparatus according to claim 1,
wherein said PM purifying reactor comprises electrodes and an
insulative honeycomb structure having a number of cell
passages.
4. The exhaust gas purifying apparatus according to claim 3,
wherein the electrodes generate an electric field which is not
parallel to the direction of the cell passages of said honeycomb
structure.
5. The exhaust gas purifying apparatus according to claim 4,
wherein the electrodes comprises a center electrode and an outer
electrode surrounding the center electrode, and the honeycomb
structure is positioned between the center and the outer
electrodes.
6. The exhaust gas purifying apparatus according to claim 4,
wherein said honeycomb structure has opposite outer surfaces, and
said electrodes comprise a pair of plate electrodes respectively
placed on the opposite outer surfaces of said honeycomb structure,
and sets of said honeycomb structure and said pair of plate
electrodes are aligned in parallel to each other.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an apparatus for purifying
an exhaust gas emitted from an internal combustion engine and, in
particular, to an apparatus for purifying PM (particulate matter)
emitted from a diesel engine.
[0003] 2. Description of Related Art
[0004] Diesel engines are usually used for motor vehicles, and
particularly, for large motor vehicles. The diesel engine must now
have a decreased emission of nitrogen oxide (NO.sub.x), carbon
monooxide (CO) and hydrocarbon (HC) as well as PM. Therefore, it is
desired to develop not only the technique which decreases the
amount of generated PM by improving engine itself or optimizing
combustion condition thereof, etc., but also the technique which
purifies the PM in the exhaust gas.
[0005] It is possible to trap the PM in the exhaust gas by the use
of a so-called diesel particulate filter (DPF). However, the DPF is
plugged by the trapped PM as time goes by and increases the
differential pressure across the DPF and this causes a larger load
on the engine.
[0006] In the prior art, it is well-known to burn out the trapped
PM by heating the DPF with a heater, etc.
[0007] Recently, as a more efficient procedure, it is also proposed
to use plasma energy in order to burn out PM which is trapped in
the DPF. For example, Japanese Unexamined Patent Publication (JPP)
No. 2002-276333 proposes to combine the DPF and a plasma generator,
thereby generating NO.sub.2 and active oxygen to promote oxidation
of the trapped PM. The document also proposes promoting the
oxidation of PM by generating plasma when the DPF is at a low
temperature.
[0008] JPP No. 2002-129949 describes combining a catalyst and
plasma generator for purifying HC, CO and NO.sub.x, and supplying a
requisite minimum amount of electric power to the plasma generator
on the basis of a engine speed, a temperature of the catalyst,
etc.
[0009] According to these prior arts utilizing plasma, it is
possible to effectively oxidize and remove trapped PM even when the
DPF has a lower temperature as in the case of starting an engine.
However, even though the DPF has a higher temperature, a large
amount of PM may be accumulated in the DPF due to the change of
combustion conditions in the engine.
[0010] Once such a large amount of PM is accumulated in DPF, the
accumulated PM makes the generation of plasma unstable and lowers
the PM oxidation ability, and therefore, PM is further accumulated
in the DPF. This becomes a vicious cycle.
[0011] A large amount of heat is generated, if the large amount of
accumulated PM is burned out at once to remove. The large amount of
heat shortens the life of the DPF, and may break the DPF.
[0012] This problem can be partially solved by controlling
generation of plasma on the basis of not only the temperature of
DPF, but also the engine speed, etc. as described in above JPP'949.
However, changes in the combustion conditions in the engine are not
always pradictable. Therefore, in some cases, plasma generation
controlled on the basis of engine speed, etc. may be not enough in
order to properly remove PM.
BRIEF SUMMARY OF THE INVENTION
[0013] An exhaust gas purifying apparatus of the present invention
comprises a plasma-assist PM purifying reactor (also referred
simply as a "PM purifying reactor"); a detecting device for
detecting an amount of PM accumulated in the PM purifying reactor;
a power supply for the PM purifying reactor; and a controlling
device for controlling the power supply on the basis of signals
from the detecting device. The controlling device controls the
power supply to start supplying the electric power to the PM
purifying reactor, or to increase the amount of electric power
which is supplied to the PM purifying reactor, when the amount of
accumulated PM detected by the detecting device is above a
predetermined amount.
[0014] According to the present invention, the power supply is
controlled on the basis of "the amount of accumulated PM" in the PM
purifying reactor rather than "the amount of generating PM" which
relates to parameters such as engine speed. Therefore, it is
possible to avoid the destabilizing of plasma generation and the
intensive burning of PM caused by an unexpectedly accumulated large
amount of PM. Further, it is, naturally, also possible to save the
electric energy needed to remove PM, comparing to supplying a
constant amount of energy to the PM purifying reactor.
[0015] In one embodiment of the present exhaust gas purifying
apparatus, the detecting device comprises a differential pressure
gauge which detects a differential pressure across the PM purifying
reactor.
[0016] According to this embodiment, it is possible to determine
the pressure loss at the PM purifying reactor on the basis of the
differential pressure across it, and then more correctly determine
the amount of PM actually accumulated in the PM purifying reactor
on the basis of the pressure loss. It is because that the pressure
loss at the PM purifying reactor increases with the amount of PM
accumulated in the PM purifying reactor.
[0017] In the case that DPF is heated by use of heater, etc. to
burn PM in the PM purifying apparatus out, the PM contacting to a
catalyst is mainly burned out. Therefore, as shown in FIG. 5(a),
when a large amount of PM 61 is deposited on the catalyst 63 on the
substrate 65, the PM is burned out only near the catalyst 63, and
only SOF (soluble organic fraction) is burned out at a point
distant from the catalyst 63. Therefore, PM 61 may be left in a
part away from the catalyst 63 as shown in FIG. 5(b). In the
situation of FIG. 5(b), the interrelationship between the pressure
loss and the amount of PM accumulated in the PM purifying reactor
differs from the that in the situation of FIG. 5(a), and therefore,
it is sometimes difficult to accurately determine the amount of PM
accumulated in the PM purifying reactor on the basis of the
pressure loss at the reactor.
[0018] Contrary to heating the DPF by the use of heater to burn PM
out, in the case that PM is burned out by the use of plasma in the
PM purifying reactor, the deposited PM is burned out and removed
also at the surface layer by the electrons which are emitted from
negative-electrode wall of the cell in DPF, etc., and by NO.sub.2
and/or active oxygen which are generated by plasma. Further, an
electric current passing through the deposited PM by application of
electric voltage leads to the burning out of PM also at a point
other than the interface of catalyst and PM.
[0019] Therefore, contrary to heating the DPF by the use of heater
to burn PM out, the burning out of the PM by the use of plasma
makes it easier to determine the amount of accumulated PM on the
basis of the a differential pressure across the PM purifying
reactor.
[0020] In one embodiment of the present invention, the PM purifying
reactor comprises electrodes and an insulative honeycomb structure
having a number of cell passages.
[0021] According to this embodiment, the PM in the exhaust gas
passing through the cell passages of the honeycomb structure is
deposited onto the sidewalls of the cell passages of the honeycomb
structure, and is burned out thereon.
[0022] In this embodiment, the electrodes may make an electric
field which is non-parallel, particularly at the angle of at least
45 or 60 degrees, more particularly substantially perpendicular, to
the direction of the cell passages of the honeycomb structure.
[0023] Further, the electrodes may comprise a center electrode and
an outer electrode surrounding the center electrode, and the
honeycomb structure may be positioned between the center and the
outer electrodes.
[0024] Alternatively, the reactor may have two or more honeycomb
structures having opposite outer surfaces, and the electrodes may
comprise two or more pairs of plate electrodes respectively placed
on the opposite outer surfaces of the two or more honeycomb
structures, and the sets of the honeycomb structure and the pair of
plate electrodes may be aligned in parallel to each other. In this
case, the plate electrode between the honeycomb structures may be
shared among the adjacent sets.
[0025] In these cases, the electrodes generate an electric field
which is non-parallel with the direction of the cell passages.
Therefore, the Coulomb force between the PM and the electric field
improves trapping of the PM. Further, in this situation, the PM
deposited in the honeycomb structure is burned with the use of not
only thermal energy of an exhaust gas but also the plasma and an
electrical current that passes through the deposited PM rather than
the insulative honeycomb structure.
[0026] These and other objects, features and advantages of the
present invention will become apparent to a person with ordinary
skilled in the art upon studying the following detailed description
and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows a block diagram of one embodiment of the
present apparatus.
[0028] FIG. 2 shows a flow chart describing a way to control the
electric power supply of the present apparatus.
[0029] FIG. 3 shows a perspective view of an plasma assist plasma
purifying reactor which can be used for the present exhaust gas
purifying apparatus.
[0030] FIG. 4 shows a perspective view of another plasma assist
plasma purifying reactor which can be used for the present exhaust
gas purifying apparatus.
[0031] FIGS. 5(a) and (b) respectively show a schematic diagram of
the shape of the PM accumulated in the DPF before and after being
burned by heating the DPF.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention is described using embodiments and
drawings which are not intended to limit the scope of the present
invention shown in the claims.
[0033] FIG. 1 schematically shows the exhaust gas purifying
apparatus of the present invention. In this FIG. 1, an exhaust gas
from the engine (ENG) passes through the plasma assist PM purifying
reactor 10 and any other exhaust gas purifying device (not shown)
such as NO.sub.x. storage-reduction catalyst downstream of the
plasma assist PM purifying apparatus 10 to the atmosphere in the
direction of arrow 9.
[0034] In this exhaust gas purifying apparatus, PM purifying
reactor 10 is located downstream of the engine. The detecting
device for detecting an amount of PM accumulated in the PM
purifying reactor comprises the pressure gauges 1 ahead of and
behind the reactor 10, the signal line 2 and the calculating device
3, and is connected to the controlling device 5 via the signal line
4. The calculating device 3 may, naturally, be an ECU (Engine
Control Unit). The power controlling device 5 is connected to the
power supply 7 via the signal line 6, and the power supply 7 is
connected to the PM purifying reactor 10 via the power line 8.
[0035] The power-supply controlling device 5 controls the power
supply 7 on the basis of the amount of PM accumulated in the PM
purifying reactor 10, the amount being determined using the
detecting device 1, 2, 3. In this case, when the amount of
accumulated PM, which is detected by the detecting device 1, 2, 3,
exceeds a predetermined amount, the controlling device 5 controls
the power supply 7 via controlling line 6 such that the power
supply 7 commences to supply the electric power or increases the
amount of electric power supplied to the PM purifying reactor
10.
[0036] In this case, the detecting device comprising the pressure
gauges 1, the signal line 2 and the calculating device 3 is used as
a detecting device for detecting an amount of PM accumulated in
plasma assist PM purifying reactor 10. However, any other device
can be used in order to detect an amount of PM accumulated in the
PM purifying reactor. Therefore, it is also possible to determine
the amount of accumulated PM on the basis of the changes of
electric resistance, weight, degree of blackening, or concentration
of NO.sub.2 or O.sub.2 in exhaust gas having passed through the
reactor (the concentration of NO.sub.2 and O.sub.2 are reduced when
the amount of accumulated PM is increased, because they are
consumed in the burning of the PM).
[0037] FIG. 2 shows the one embodiment of use of the present
exhaust gas purifying apparatus of FIG. 1. As shown in FIG. 2, when
the engine starts to run, the PM purifying reactor is powered, and
the exhaust gas from the engine is passed through the plasma assist
PM purifying reactor 10, whereby the PM in the exhaust gas is
trapped by the reactor 10. After that, if the detecting device 1,
2, 3 detects that a predetermined amount of PM is accumulated in
the PM purifying reactor 10, the controlling device 5 controls the
power supply 7 in order to increase an amount of electric power
supplied to the PM purifying reactor 10 for a predetermined
duration or until the amount of PM detected by the detecting device
become less than a predetermined amount. If the detecting device 1,
2, 3 detects that the accumulated PM in the PM purifying reactor 10
is less than the predetermined amount, the controlling device 5
controls the power supply 7 in order to keep the amount of electric
power supplied to the PM purifying reactor 10 stable, and then the
detecting device 1, 2, 3 check the amount of accumulated PM
again.
[0038] Although the electric power supply starts with the operation
of the engine, it is also possible to control the power supply 7 by
the use of the controlling device in order not to supply electric
power normally, and to commence supplying electric power only when
the amount of accumulate PM exceeds a predetermined amount.
Further, it is also possible to control the power supply on the
basis of the condition of the engine as described in JPP'949, i.e.
on the basis of the amount of generating PM, in addition to the
amount of accumulated PM as in the present invention. According to
this, it is possible to normally control the power supply on the
basis of the condition of the engine, and to control the power
supply according to the present invention only when an amount of
accumulated PM exceeds a predetermined amount.
[0039] The plasma assist PM purifying reactor which can be used for
the present exhaust gas purifying apparatus may be ones that
generate plasma therein and the plasma promotes combustion of PM.
Therefore, the plasma assist PM purifying reactor may be ones
described in above JPP'333 and JPP'949.
[0040] Further, the plasma assist PM purifying reactor may be ones
seen in FIG. 3 which shows perspective views thereof.
[0041] In FIG. 3, 10 indicates a straight-flow type insulative
honeycomb structure having a number of cell passages, 14 indicates
a center electrode, 16 indicates an outer electrode, 18 indicates
needle electrodes on the center electrode 14, and 110 indicates a
power supply. The insulative honeycomb structure 10 is positioned
between the center 14 and the outer 16 electrodes such that these
electrodes are electrically insulated. An exhaust gas containing PM
flows from the left side to the right side of the FIG. 3 as
indicated by an arrow 100, and passes through the cell passages of
the honeycomb structure 10 surrounded by the outer electrode
16.
[0042] In the use of the PM purifying reactor shown in FIG. 1, the
electric power supply 110 applies a voltage between the center
electrode 14 and the outer electrode 16 to generate a radial
electric field in the honeycomb structure 10.
[0043] The components of the PM purifying reactor shown in FIG. 3
are described below in more detail.
[0044] The insulative honeycomb structure 10 may be made of a
ceramic material, e.g. cordierite. The honeycomb structure may be a
straight-flow type (i.e. a honeycomb structure of which cell
passages are substantially not plugged) or wall-flow type (i.e. a
honeycomb structure of which cell passages are alternatively
plugged, a so-called "Diesel Particulate Filter (DPF)"). According
to this embodiment, the straight-flow type honeycomb structure is
preferable for gas-flow resistance, and can achieve a sufficient PM
trapping. Further, the wall-flow type honeycomb structure is
preferable for producing a PM path, and then burning the trapped PM
by the electric current therethrough. The insulative honeycomb
structure may be sufficiently more insulative than PM in order to
make sure that more electric current passes through the deposited
PM than through the honeycomb structure to burn out the PM. The
honeycomb structure may carry a catalyst such as NO.sub.x purifying
catalyst.
[0045] A voltage applied between the electrodes may usually be more
than 5 kV, preferably more than 10 kV. The pulse period of the
applied voltage is preferably less than 10 ms (milli-second), more
preferably less than 1 ms. A direct current (DC) voltage,
alternating current (AC) voltage, a voltage having a periodic
waveform, etc. may be applied between the electrodes. Preferably, a
DC pulse voltage is applied since it can generate a stable corona
electric discharge. The applied voltage, pulse width and pulse
period of the DC pulse voltage may be optionally determined as long
as it generates a corona electric discharge. Preferably, the
applied voltage and pulse period are respectively a high voltage
and short period in order to generate a corona electric discharge,
though those parameters may be restricted by the design of the
apparatus, an economical interest, etc.
[0046] In FIG. 4, 50 indicates a straight-flow type insulative
honeycomb structure having a number of cell passages, 54 to 58
indicate mesh plate electrodes, 110 indicates an electric power
supply. Among the plate electrodes 54 to 58, the plate electrodes
55 and 57 are connected to the electric power supply 110, and the
plate electrodes 54, 56 and 58 are grounded. Each of the plate
electrodes 54 to 58 is electrically insulated with adjacent ones by
the insulative honeycomb structure 50 therebetween. An exhaust gas
containing PM passes through the cell passages of the insulative
honeycomb structures 50 sandwiched between the plate electrodes 54
to 58, as shown in an arrow 100.
[0047] In the use of the PM purifying reactor shown in FIG. 4, the
electric power supply 110 applies a voltage between the plate
electrodes 54, 56 and 58, and the adjacent electrodes 55 and 57 to
generate an electric field in the honeycomb structure 50. In any
case, the electric field crosses the cell passages of the honeycomb
structure 50 through which an exhaust gas flows. The electric field
forces the PM in the exhaust gas to be deposited onto the sidewall
of the cell passages of the honeycomb structure 50 by the Coulomb
force in order to improve a trapping of the PM.
[0048] Regarding the components of the reactor of FIG. 4, refer to
the description for the reactor of FIG. 3.
[0049] In the following examples, the effects of the PM purifying
reactors are disclosed. These reactors can be used for the present
exhaust gas purifying apparatus and are as disclosed in FIGS. 3 and
4.
EXAMPLE 1
[0050] A PM purifying reactor was provided as shown in FIG. 3. That
is, in this reactor, around the circumference surface of a
straight-flow type cordierite honeycomb structure (diameter: 30 mm
and length: 50 mm, cell density: 200 cells/square inch, porosity:
65%, and average pore size: 25 .mu.m (micro meters)), a stainless
steel mesh (width: 40 mm, SUS 304, 300 mesh) was surrounded to be
an outer electrode. On the center axis of the honeycomb structure,
a center electrode (bar electrode) having needle electrodes was
fixed.
EXAMPLE 2
[0051] The exhaust gas purifying apparatus of this example was the
same as that of the example 1, except that a wall-flow type
cordierite honeycomb structure (cell density: 300 cells/square
inch, porosity: 65%, and average pore size: 25 .mu.m) was used in
place of the straight-flow type cordierite honeycomb of the example
1.
[0052] Performance Evaluation: PM trapping
[0053] Each of the reactors of the examples 1 and 2 was surrounded
by an alumina mat and inserted in a quartz tube having an inner
diameter of 37 mm (milli-meters). The center electrode was
electrically connected to an electric power supply, and the outer
electrode was grounded. To the exhaust gas purifying apparatus, a
part of the exhaust gas (100 L/minute) from a direct-injection
system diesel engine having a displacement volume of 2400 cc was
pumped, and a voltage of 4 kV (input electric power of about 3 W)
was applied. The contents of the PM in the exhaust gas were
determined at the upstream and downstream of the apparatus by the
use of an ELPI (Electric Low Pressure Impactor). A PM purifying
rate was determined from the difference between the contents of the
PM at the upstream and downstream of the apparatus. The higher this
value is, the better the performance of the reactor is. In all
cases, the engine was idling (at 700 rpm).
[0054] Performance Evaluation: PM Oxidization
[0055] After sufficiently depositing PM in the honeycomb structures
of the examples 1 and 2, the honeycomb structures were dried for 24
hours at the temperature of 120.degree. C. in a dryer, and then
weighed. The obtained weight is an initial weight. The each reactor
was inserted in the quartz tube as stated above (atmosphere: air),
and the center electrode was power at 15 kV for 15 minutes. The
resulted honeycomb structure was dried for 24 hours at the
temperature of 120.degree. C., and then weighed. The obtained
weight is an after-treatment weight. The PM oxidation (combustion)
amount is obtained from the difference between the initial weight
and the after-treatment weight. The PM oxidation energy was
calculated by dividing the PM oxidation amount by the input energy
(voltage.times.electric current.times.time) from the electric power
supply. The lower this value is, the better the PM oxidation
performance is. The input energy required for oxidizing PM by mere
heating is about 290 kJ/g.
1 TABLE 1 PM trapping with electric without electric PM oxidation
energy field (%) field (%) (kJ/g) Ex. 1 11 69 65 (straight-flow)
Ex. 2 45 68 67 (wall-flow)
[0056] The PM trapping performances in Table 1 show that the
electric field in the honeycomb structure improves the PM trapping,
and that the straight-flow type and wall-flow type honeycomb
structures achieve the similar PM trapping results on being
provided with the electric power. The PM oxidizing performances in
Table 1 show that the electric current reduces the required PM
oxidation energy relative to mere heating.
EXAMPLE 3
[0057] A PM purifying reactor was provided as shown in FIG. 4. That
is, straight-flow type cordierite honeycomb structures in
rectangular parallelepiped form (cell density: 200 cells/square
inch, porosity: 65%, average pore size: 25 am, height: 15 cells,
width: 5 cells, and length: 50 mm) are sandwiched with stainless
steel mesh electrodes (SUS 304, height of 24 mm, length of 45 mm,
and 300 or 30 mesh).
[0058] In the experiment, the exhaust gas passes through the
reactors in a direction indicated by an arrow 100 in FIG. 4. The
mesh electrodes are alternatively connected to an electric power
supply and to the ground. The electrodes connected to the electric
power supply are anodes, and the grounded electrodes are
cathodes.
[0059] Performance Evaluation: PM Trapping
[0060] A PM trapping rate was determined as in examples 1 and 2,
except that the reactor of the example 3 was surrounded by an
alumina mat and inserted in an acrylic tube having a profile of
34.times.48 mm, and that a DC electric power of 4 kV and about 3 W
was applied to the electrodes.
[0061] Performance Evaluation: PM Oxidization
[0062] The PM oxidation energy is determined as in Examples 1 and 2
except that the honeycomb structures of example 3 was inserted in
the acrylic tube as stated above (atmosphere: air), and powered at
10 kV for 20 minutes.
2 TABLE 2 PM trapping with electric without electric PM oxidation
energy field (%) field (%) (kJ/g) Ex. 3 19 67 70
[0063] The PM trapping performances in Table 2 show that the
electric field in the honeycomb structure improves the PM trapping.
The PM oxidizing performances in Table 2 show that the electric
current reduces the required PM oxidation energy relative to mere
heating.
[0064] Although the present invention has been fully described by
way of the example with reference to the accompanying drawings, it
should be understood that various changes and modifications will be
apparent to those skilled in the art. Therefore, such changes and
modifications can be made within the scope of the present invention
as hereinafter defined.
* * * * *