U.S. patent application number 12/921863 was filed with the patent office on 2011-03-03 for exhaust gas treatment apparatus with improved pressure pulse damping.
Invention is credited to Marco Baccalaro, Ignacio Garcia-Lorenzana Merino, Volker Reusing, Stefan Stein.
Application Number | 20110047996 12/921863 |
Document ID | / |
Family ID | 40559967 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110047996 |
Kind Code |
A1 |
Garcia-Lorenzana Merino; Ignacio ;
et al. |
March 3, 2011 |
EXHAUST GAS TREATMENT APPARATUS WITH IMPROVED PRESSURE PULSE
DAMPING
Abstract
The invention relates to an apparatus for metering at least one
medium for reducing pollutant levels in an exhaust system, in
particular for the introduction of fuel into an exhaust tract for
regenerating an element for reducing pollutant levels in the
exhaust tract. The apparatus includes at least one injection valve,
in particular, a pressure-regulated injection valve and at least
one supply line for supplying the pollutant-reducing medium to the
injection valve. At least one pressure damper is provided in the
supply line upstream of the injection valve.
Inventors: |
Garcia-Lorenzana Merino;
Ignacio; (Bruxelles, BE) ; Baccalaro; Marco;
(Stuttgart, DE) ; Reusing; Volker; (Stuttgart,
DE) ; Stein; Stefan; (Stuttgart, DE) |
Family ID: |
40559967 |
Appl. No.: |
12/921863 |
Filed: |
January 21, 2009 |
PCT Filed: |
January 21, 2009 |
PCT NO: |
PCT/EP09/50653 |
371 Date: |
October 26, 2010 |
Current U.S.
Class: |
60/303 ;
60/324 |
Current CPC
Class: |
Y02T 10/12 20130101;
F02M 55/04 20130101; F01N 2610/03 20130101; F02M 55/02 20130101;
F01N 2610/14 20130101; F01N 3/2033 20130101; Y02T 10/26 20130101;
F01N 2610/1446 20130101; F01N 3/36 20130101 |
Class at
Publication: |
60/303 ;
60/324 |
International
Class: |
F01N 3/10 20060101
F01N003/10; F01N 1/00 20060101 F01N001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 10, 2008 |
DE |
10 2008 013 406.6 |
Claims
1-11. (canceled)
12. An apparatus for metering at least one pollutant-reducing
medium into an exhaust system, in particular for introducing fuel
into an exhaust tract for regenerating an element for reducing
pollutant levels in the exhaust tract, including at least one
injection valve, in particular a pressure-regulated injection
valve, and at least one supply line for supplying the
pollutant-reducing medium to the injection valve, wherein at least
one pressure damper is provided in the supply line, downstream of
the injection valve.
13. The apparatus as defined by claim 12, wherein the pressure
damper has at least one porous element received in the supply
line.
14. The apparatus as defined by claim 13, wherein the porous
element contains at least one of the following materials: a ceramic
material; a metal, in particular a sintered metal; a metal alloy,
in particular a sintered metal alloy; or a composite material.
15. The apparatus as defined by claim 12, wherein the pressure
damper includes at least one hydraulic pressure damper, and the
hydraulic pressure damper includes at least one hydraulic volume of
the pollutant-reducing medium and at least one energy reservoir
that is different from the hydraulic volume and is operatively in
communication with the hydraulic volume.
16. The apparatus as defined by claim 13, wherein the pressure
damper includes at least one hydraulic pressure damper, and the
hydraulic pressure damper includes at least one hydraulic volume of
the pollutant-reducing medium and at least one energy reservoir
that is different from the hydraulic volume and is operatively in
communication with the hydraulic volume.
17. The apparatus as defined by claim 14, wherein the pressure
damper includes at least one hydraulic pressure damper, and the
hydraulic pressure damper includes at least one hydraulic volume of
the pollutant-reducing medium and at least one energy reservoir
that is different from the hydraulic volume and is operatively in
communication with the hydraulic volume.
18. The apparatus as defined by claim 15, wherein the energy
reservoir includes at least one of the following energy reservoirs:
an at least partly elastically deformable plastic; a spring
element; or a compressible, closed-off fluid volume, in particular
a gas volume, in particular air.
19. The apparatus as defined by claim 16, wherein the energy
reservoir includes at least one of the following energy reservoirs:
an at least partly elastically deformable plastic; a spring
element; or a compressible, closed-off fluid volume, in particular
a gas volume, in particular air.
20. The apparatus as defined by claim 17, wherein the energy
reservoir includes at least one of the following energy reservoirs:
an at least partly elastically deformable plastic; a spring
element; or a compressible, closed-off fluid volume, in particular
a gas volume, in particular air.
21. The apparatus as defined by claim 12, wherein the supply line
connects a low-pressure system of a fuel system, in particular a
reservoir-type injection system, to the injection valve.
22. The apparatus as defined by claim 20, wherein the supply line
connects a low-pressure system of a fuel system, in particular a
reservoir-type injection system, to the injection valve.
23. The apparatus as defined by claim 12, wherein at least one
metering unit is also received upstream of the injection valve in
the supply line, and the metering unit has at least one valve for
controlling a procedure of injection of the pollutant-reducing
medium.
24. The apparatus as defined by claim 22, wherein at least one
metering unit is also received upstream of the injection valve in
the supply line, and the metering unit has at least one valve for
controlling a procedure of injection of the pollutant-reducing
medium.
25. The apparatus as defined by claim 23, wherein the metering unit
further includes at least one pressure measuring device, and the
pressure damper is disposed upstream of the pressure measuring
device.
26. The apparatus as defined by claim 24, wherein the metering unit
further includes at least one pressure measuring device, and the
pressure damper is disposed upstream of the pressure measuring
device.
27. The apparatus as defined by claim 25, wherein the pressure
damper is integrated entirely or in part with the metering unit
and/or is disposed entirely or in part upstream of the pressure
measuring device.
28. The apparatus as defined by claim 26, wherein the pressure
damper is integrated entirely or in part with the metering unit
and/or is disposed entirely or in part upstream of the pressure
measuring device.
29. The apparatus as defined by claim 12, further having at least
one overpressure valve, and the overpressure valve is received in a
branch line branching off from the supply line upstream of the
pressure damper.
30. The apparatus as defined by claim 28, further having at least
one overpressure valve, and the overpressure valve is received in a
branch line branching off from the supply line upstream of the
pressure damper.
31. The apparatus as defined by claim 12, further having at least
one damping volume, received in the supply line upstream of the
pressure damper, the damping volume including a damping supply of
the pollutant-reducing medium.
Description
PRIOR ART
[0001] The invention is based on known methods and apparatuses for
posttreatment of exhaust gases, in particular exhaust gases of
internal combustion engines, for instance in the automotive field,
in energy generation, or in similar fields in natural science and
technology. From such fields, techniques are known in which various
pollutant-reducing media, especially fluid media (such as liquids
or gases) are metered, for instance injected into the exhaust gas.
Various techniques and various types of pollutant-reducing media
are employed. Examples of such pollutant-reducing media are
urea-water solutions, which as a reducing agent reduce nitrogen
oxides selectively. Such methods are often also called SCR methods
(SCR: selective catalytic reduction).
[0002] Other methods are based on the injection of hydrocarbons, as
pollutant-reducing media, into exhaust gases. Such methods, which
are often also called HCI methods (HCI: hydrocarbon injection), can
serve various purposes. For one, an injection of fuel, such as
diesel fuel, as a reducing agent can be for instance serve to
reduce nitrogen oxides. Other methods are based on a reaction of
the injected fuel in an oxidation catalytic converter, which for
instance leads to a brief temperature increase in the exhaust
tract. This temperature increase can be employed for instance for
regenerating an exhaust gas posttreatment apparatus, for instance
for regenerating a diesel particle filter by burning off soot.
[0003] Without limiting the further possibilities for embodying the
pollutant-reducing medium, reference will be made hereinafter
essentially to HCI systems. However, it will be pointed out that
other types of pollutant-reducing media, especially liquids, can
also be employed.
[0004] Various apparatuses for introducing the pollutant-reducing
medium into the exhaust gas are known from the prior art. For
instance, German Patent Disclosure DE 10 2005 040 918 A1 describes
a system in which fuel is diverted from a low-pressure part of a
reservoir-type injection system and metered into the exhaust gas.
The low-pressure part has a pressure maintenance valve, for
maintaining a minimum pressure in the low-pressure part.
[0005] In the system shown in DE 10 2005 040 918 A1, the
low-pressure reservoir, with its liquid volume, ensures a certain
calming of pressure fluctuations. Nevertheless, pressure
fluctuations in the low-pressure circuit of an injection system of
that kind can only seldom be avoided. In other types of furnishing
the pollutant-reducing medium as well, such pressure fluctuations
occur. Pressure fluctuations can also, depending on the injection
system, also be generated by the return from the injectors in the
fuel injection system or as a result of pumping.
[0006] To avoid these problems of pressure fluctuations, German
Patent Disclosure DE 10 2005 034 704 A1 discloses an apparatus and
a method for regenerating particle filters. In the apparatus
proposed there as well, a calming volume of fuel is employed in
order to ensure a certain compensation for pressure fluctuations.
It is also proposed that a pressure control valve, which opens and
dissipates the pressure if the supplied fuel exceeds a certain
value, be disposed in a branch line from the calming volume.
[0007] Despite these calming provisions known from the prior art,
it has been demonstrated that under some circumstances, there can
nevertheless be a potential for improvement. For instance, pressure
peaks can still occur and influence the injection of the
pollutant-reducing medium. Moreover, cavitation in the supply line
of the pollutant-reducing medium, for instance in the low-pressure
circuit, can also occur. Such pressure peaks and cavitation can
even lead to damage of the components of the system, such as the
HCI components, and the hydraulic behavior can be adversely
affected.
DISCLOSURE OF THE INVENTION
[0008] An apparatus for metering at least one pollutant-reducing
medium into an exhaust system is therefore proposed which at least
largely avoids the above-described disadvantages of known
apparatuses and systems and which ensures that the injection of the
pollutant-reducing medium is made uniform. With regard to the
embodiment of the pollutant-reducing medium, the above descriptions
of known systems can for instance be referred to, especially HCI
systems. Especially preferably, the apparatus can be used for
regenerating a diesel particle filter, the apparatus being used
such that diesel fuel is injected and catalytically combusted into
an exhaust tract, for instance upstream of an oxidation catalytic
converter. As a result, the temperature in the exhaust system is
actively raised, until the burnoff temperature for the soot
deposited in the diesel particle filter has been reached.
[0009] The proposed apparatus includes at least one injection
valve, in particular a pressure-regulated injection valve, for
injecting the pollutant-reducing medium into the exhaust system.
This can for instance involve a pressure-regulated injection valve,
for instance pressure-regulated injection valves, which is already
being used on a mass production basis for injecting fuels into
combustion chambers of internal combustion engines, and/or for
modifying such valves.
[0010] The apparatus furthermore includes at least one supply line
for supplying the pollutant-reducing medium to the injection
valve.
[0011] To this extent, the system can largely correspond for
instance to the systems described in DE 10 2005 040 918 A1 and/or
DE 10 2005 034 704 A1. However, still other designs are also
possible. In contrast to the systems known from the prior art,
however, in the proposed system at least one pressure damper is
received in the supply line, upstream of the injection valve. The
term "pressure damper" is to be understood here to mean a device,
which damps pressure peaks in the pollutant-reducing medium in the
supply line by providing that the excess energy from these pressure
peaks is dissipated to and at least partly absorbed by an element,
acting as an energy absorber, that is different from the
pollutant-reducing medium and that differs from conventional
provisions on the inlet side, such as pressure control valves or
simple throttle bores. This additional element that receives the
excess energy can, as described below, for instance include a
solid, porous, or elastic, or (although this is less preferable) a
plastic element. Various possibilities are discussed below as
examples.
[0012] By means of the at least one pressure damper provided
according to the invention, the pressure is accordingly efficiently
made uniform, and thus an improvement is brought about in the
process of injecting the pollutant-reducing medium. Dissipating
excess pressure through an additional pressure control valve can be
dispensed with, which can bring about a cost saving as well as
simplification. Such pressure control valves, however, can, as
described below, be provided as additional safety provisions, or
provisions for making the pressure uniform. It is also possible to
dispense with a calming volume of the kind provided in DE 10 2005
034 704 A1 or also, in the form of the low-pressure reservoir, for
instance in DE 10 2005 040 918 A1, or else such a calming volume
may be provided as an additional damping measure.
[0013] It is especially preferred if the pressure damper has at
least one porous element received in the supply line. The porous
element can for instance include a highly porous material of open
porosity, that is, a material in which the pores form continuously
open pore ducts. In particular, a porous element of this kind can
be integrated upstream of components of the apparatus that do not
resist high pressure peaks. The pressure damper can for instance
include a ceramic material, a metal material, a metal alloy, or a
combination of that and/or other materials as the porous element.
In particular, sintered metals, sintered metal alloys, or sintered
ceramics can be used, optionally also in combination.
[0014] The pressure damper and/or the porous element may have
various geometries. The porous element can for instance be
solidified by compaction or molding and ensuing drying and
sintering of ceramic slips and/or metal slurries. The damping
properties can be adapted to the various most frequently occurring
operating conditions, that is, for instance liquid properties,
pressures, temperatures, and/or the like that frequently occur
during operation. An adaptation to the component geometry, for
instance to spatial installation conditions, can also be
purposefully made. By means of a purposeful choice of porosity,
pore size, or similar parameters of the porous element and/or of a
rib thickness of the porous element and/or a length of the porous
element or of the pressure damper, the lowering of the pressure
level can be optimized in a purposeful way.
[0015] Alternatively or in addition to the porous element, the
pressure damper can also include at least one hydraulic pressure
damper. This hydraulic pressure damper should preferably be
arranged in such a way that it includes at least one hydraulic
volume of the pollutant-reducing medium. For instance, this
hydraulic volume can be a closed-off hydraulic volume, which is
received in a widened portion (for instance a pressure vessel).
This pressure vessel can be in communication with the supply line
via an inlet and an outlet, for instance, or integrated with the
supply line.
[0016] The hydraulic pressure damper further includes at least one
energy reservoir that is different from the hydraulic volume and is
in operative communication with the hydraulic volume. While in the
case of the use of the porous element, the porous element itself
acts as an additional element absorbing the excess energy or excess
pressure in the case of pressure peaks, in the case of the
hydraulic pressure damper, the energy reservoir acts as an
additional element for absorbing the excess energy contained in the
pressure peaks and thus for making the pressure uniform.
[0017] The energy reservoir may for instance include a mechanical
energy reservoir, such as an at least partly elastically deformable
plastic or some other elastic element, for instance a spring
element. Alternatively or in addition, the energy reservoir can
also contain at least one compressible closed-off fluid volume, in
particular a gas volume, in particular air. Still other kinds of
energy reservoirs are conceivable. The energy reservoir may also be
designed such that while it absorbs brief pressure peaks,
nevertheless the excess energy of these pressure peaks is returned
to the pollutant-reducing medium again once the pressure peak has
faded. In this way, besides pressure peaks, pressure incursions,
for instance, can also be reduced. The pressure damping properties
can optionally be adapted using throttle elements (such as inflow
throttles and outflow throttles) received in the supply line, with
the pressure level prevailing at the time, and optionally with an
overflow valve or a pressure control valve and/or an overpressure
valve.
[0018] As discussed above, it is especially preferred if the supply
line connects a low-pressure system of a fuel system, in particular
a reservoir-type injection system (such as a diesel common rail
system) with the injection valve.
[0019] Furthermore, upstream of the injection valve in the supply
line, at least one metering unit can also be received, and the
metering unit has at least one valve for controlling a procedure of
injection of the pollutant-reducing medium. This metering unit can
for instance be controlled by a separate controller and/or by a
controller integrated with an engine control unit.
[0020] The metering unit can for instance include a shutoff valve,
which as a whole turns the injection operation on or off.
Alternatively or in addition, the metering unit can include a
metering valve which is operated for instance in clocked fashion
and subjects the injection valve to pressure in clocked fashion, so
that the injection procedure takes place in clocked fashion as
well.
[0021] Moreover, the metering unit can include one or more pressure
measuring devices. For instance, one pressure measuring device can
be provided for determining a metering quantity, for instance
between a shutoff valve and a metering valve. Alternatively or in
addition, one pressure measuring device can be provided between the
metering valve and the injection valve, for instance in the form of
a pressure sensor for detecting leaks. If at least one such
pressure measuring device is provided, then the pressure damper can
in particular be disposed upstream of this at least one pressure
measuring device, for instance upstream of a pressure measuring
device for a metering quantity. In particular, the pressure damper
can be integrated entirely or in part in the metering unit, or it
may also be provided entirely or in part upstream of the metering
unit.
[0022] As described above, in addition to the pressure damper,
further devices can optionally be provided for making the pressure
in the apparatus uniform. In particular, at least one overpressure
valve may for instance be provided, which is received in a branch
line branching off from the supply line upstream of the pressure
damper. Alternatively or in addition, a damping supply of the
pollutant-reducing medium can also be received upstream of the
pressure damper in the supply line, for instance in a widened part
of the supply line and/or in a vessel communicating with the supply
line, such as a pressure vessel. To this extent, the apparatus can
for instance be supplemented with the additional provisions
described in DE 10 2005 034 704 A1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Exemplary embodiments of the invention are shown in the
drawings and described in further detail in the ensuing
description.
[0024] Shown are:
[0025] FIG. 1, a schematic structure of an internal combustion
engine having exhaust gas posttreatment;
[0026] FIG. 2, a schematic detailed view of the exhaust gas
posttreatment of FIG. 1;
[0027] FIG. 3, a first exemplary embodiment of a pressure damper
with a porous element; and
[0028] FIG. 4, a second exemplary embodiment of a pressure damper
with an energy reservoir.
[0029] In FIG. 1, highly schematically, an internal combustion
engine 110 is shown. The internal combustion engine includes a
combustion motor 112, with an air intake tract 114 and an exhaust
tract 116. The combustion motor 112 is designed for instance as a
turbo diesel motor and includes a turbocharger 118 coupled with the
intake tract 114 and the exhaust tract 116. Also provided in the
intake tract 114 are a charge air cooler 120 and a throttle valve
122. The internal combustion engine 110 further has an exhaust gas
recirculation 124, which branches off from the exhaust tract 116
between the combustion motor 112 and the turbocharger 118 and
discharges into the intake tract 114 before the throttle valve 122
and the combustion motor 112. Valves 126 and further coolers 120
can be provided in the exhaust gas recirculation 124.
[0030] In this exemplary embodiment, an oxidation catalytic
converter 128, symbolically represented in FIG. 1 by "DOC", is
disposed downstream of the turbocharger 118 in the exhaust tract
116. Downstream of this oxidation catalytic converter 128 is in
turn a particle filter 130, for instance a diesel particle filter,
which is symbolically indicated in FIG. 1 by "DPF".
[0031] An injection valve 132 is provided between the turbocharger
118 and the oxidation catalytic converter 128. By means of this
injection valve, which is subjected to pollutant-reducing medium,
such as diesel fuel, via a supply line 134, pollutant-reducing
medium 136, which in the HCI process is for instance diesel fuel,
is injected into the exhaust tract 116. The diesel fuel is
catalytically combusted by the oxidation catalytic converter 128,
as a result of which the temperature in the exhaust tract 116 is
actively raised until the burnoff temperature for the soot
deposited in the diesel particle filter 130 is reached. In this
way, regeneration of the diesel particle filter 130 can be
accomplished.
[0032] A metering unit 138 is also disposed in the supply line 134.
This metering unit 138, like the supply line 134 and the injection
valve 132, is a component of an apparatus 140 for metering the
pollutant-reducing medium 136. This apparatus 140 is shown
schematically in further detail in FIG. 2 and will be described in
further detail below.
[0033] Furthermore, in the exemplary embodiment shown in FIG. 1,
the apparatus 140 optionally includes a controller 142, which can
for instance be integrated entirely or in part with an engine
controller (or engine control module, ECM) of the internal
combustion engine 110. As shown in FIG. 1, this controller 142 can
for instance be subjected to various sensor signals, such as
pressure and/or temperature signals, from measurements of various
points in the exhaust tract 116. Signals from various pressure
sensors 146, 148 integrated with the metering unit 138 can also be
sent to the controller 142. The controller 142 generates a first
control signal 150 for a shutoff valve 152 (symbolically
represented in FIG. 1 by "SV"). The controller 142 also generates a
second control signal 154 for triggering a metering valve 156
(symbolically represented by "DV" in FIGS. 1 and 2), downstream of
the shutoff valve 152 in the supply line 134, in the metering unit
138. The control signal 154 is also shown schematically in FIG. 1,
on the left.
[0034] In FIG. 2, highly schematically, the apparatus 140 for
metering the pollutant-reducing medium 136 is shown in a
modification according to the invention. It can be seen, first,
that the supply line 134 connects the injection valve 132,
symbolically represented by "IV", with a low-pressure part 158 of a
fuel system (symbolically represented in FIG. 2 by "LPC"). For
possible details of this optional communication with the
low-pressure part 158, DE 10 2005 040 918 A1 can for instance be
consulted. The fuel, as the pollutant-reducing medium 132 flows via
the supply line 134, via an optional throttle element 160, to the
metering unit 138, which is symbolically represented in FIG. 2 by
"MU". In addition, analogously to the embodiment in DE 10 2005 034
704 A1, for instance, it is optionally possible for a pressure
damper volume, not shown in FIG. 2, to be disposed for instance
between the throttle element 160 and the metering unit 138.
[0035] Inside the metering unit 138, the shutoff 152, at
regeneration intervals, initially interrupts the inflow of
pollutant-reducing medium 136. Optionally, an overpressure valve
162 can be received in the branch line 164, similarly to the
embodiment in DE 10 2005 034 704 A1, for example, which valve
connects the supply line 134 with the tank 166. In this way, a
pressure level can be reduced, and pressure fluctuations can also
be compensated for to a limited extent.
[0036] In the metering unit 138, downstream of the shutoff valve
152, is the first pressure sensor 146, whose signal can be used for
instance for calculating the clocking of the metering valve 156 and
thus for increasing the metering quantity precision. This metering
quantity is then made available via the metering valve 156 and
delivered to the injection valve 132. A second pressure sensor 148,
as a pressure measuring device, for instance for detecting leaks,
can optionally be disposed between the injection valve 132 and the
metering valve 156.
[0037] The injection valve 132 may for instance be a structurally
adapted fuel injection valve, which opens at a defined supply
pressure and injects pollutant-reducing medium 136 into the exhaust
tract. A structurally adapted "K-Jetronic" valve can for instance
be used for this.
[0038] The apparatus 140 shown in FIG. 2 is modified according to
the invention by providing at least one pressure damper 168
upstream of the injection valve 132. For example, a pressure damper
168 of this kind can be disposed at one of the places marked A, B,
or C in FIG. 2, or at some or all of these places. Alternatively or
in addition, such pressure dampers 168 can be fundamentally
disposed at other points in the supply line 138 as well.
[0039] Exemplary embodiments of pressure dampers 168 according to
the invention that can be provided for instance in an apparatus 140
as in FIG. 2 are shown in FIGS. 3 and 4.
[0040] FIG. 3 shows an exemplary embodiment of a pressure damper
168 which functions passively and includes at least one porous
element 170 of open porosity. This porous element 170, which for
instance, as shown above, can include a ceramic, a metal, a metal
alloy, or a combination of these or other materials, is received
for instance in a pressure housing 172. This pressure housing 172
is incorporated into the supply line 134 via an inlet 174 and an
outlet 176.
[0041] The porous element 170 may for instance have nonlinear
properties with regard to the permeability for the
pollutant-reducing medium 136, so that there is a disproportionate
ratio exists for example between the pressure difference at the
inlet 174 and outlet 176 and the delivered quantity of
pollutant-reducing medium 136. This means that pressure peaks can
be intercepted especially effectively by the pressure damper 168.
The excess energy contained in the pressure can be absorbed by the
porous element 170, for instance.
[0042] In FIG. 4, a second possible exemplary embodiment of a
pressure damper 168 is shown. In this case, the pressure damper
again includes a pressure housing 172, with an inlet 174 and an
outlet 176, by way of which inlet and outlet the pressure damper
168 is incorporated into the supply line 134. In the interior of
the pressure housing 172, a hydraulic volume 178 of the
pollutant-reducing medium 136 is received, which is operatively
connected via a ram 180 to a spring element 182, shown in
simplified form, as an energy reservoir 184. Instead of the spring
element 182, other types of energy reservoirs may for instance be
used, as discussed above. A spring chamber 186, in which the spring
element 182 is received, can for instance be pressure-relieved via
a bore, not shown in FIG. 4. All in all, the hydraulic pressure
damper 168 shown in FIG. 4 represents one example of a piston
spring reservoir. Still other types of energy reservoirs may be
employed, however.
* * * * *