U.S. patent number 3,802,402 [Application Number 05/239,670] was granted by the patent office on 1974-04-09 for internal combustion engines.
Invention is credited to Peter Phillimore Swatman.
United States Patent |
3,802,402 |
Swatman |
April 9, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
INTERNAL COMBUSTION ENGINES
Abstract
An exhaust gas recirculation system for an internal combustion
engine comprises a metering orifice in the exhaust system of the
engine opening into a recirculation conduit which leads to the
inlet manifold of the engine, a pressure-control valve assembly
which controls communication between a location in said conduit
immediately downstream of the metering orifice and the inlet
manifold in response to changes in pressure at said location such
as to maintain said location at substantially atmospheric pressure.
The rate of recirculation through said conduit is therefore a
substantially constant percentage of the mass air-flow rate of the
engine. A pressure sensing valve assembly is responsive to a
pressure in the engine supply system which indicates certain
operating conditions of the engine and automatically closes the
valve of the pressure-control valve assembly to prevent exhaust gas
circulation under said operation conditions.
Inventors: |
Swatman; Peter Phillimore
(Solihull, Warwickshire, EN) |
Family
ID: |
26217017 |
Appl.
No.: |
05/239,670 |
Filed: |
March 30, 1972 |
Current U.S.
Class: |
123/676;
123/568.29 |
Current CPC
Class: |
F02M
26/58 (20160201); F02M 26/68 (20160201); F02M
26/61 (20160201); F02M 26/56 (20160201); F02M
26/39 (20160201); F02M 2026/0025 (20160201) |
Current International
Class: |
F02M
25/07 (20060101); F02m 025/06 () |
Field of
Search: |
;123/119A
;60/278,279 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Smith; Al Lawrence
Assistant Examiner: Rutledge, Jr.; W. H.
Attorney, Agent or Firm: Brisebois & Kruger
Claims
I claim:
1. An exhaust gas recirculation system for an internal combustion
engine equipped with an inlet manifold and an exhaust system, said
recirculation system comprising
a metering orifice in said exhaust system opening into a
recirculation conduit which leads to said inlet manifold, and
a pressure-control valve assembly comprising a pressure-sensitive
member subjected to atmospheric pressure on one side and to the
pressure in said conduit at a location immediately downstream of
the exhaust gas metering orifice on its other side, an
exhaust-admission valve in said recirculation conduit, and a
pneumatic servo connected to operate said exhaust-admission valve
in response to movement of said pressure sensitive member
and thereby maintain substantially atmospheric pressure at said
location whenever said exhaust gas is recirculated, so that said
metering orifice governs the rate of recirculation through said
conduit as a substantially constant percentage of the mass air-flow
rate of the engine.
2. An internal combustion engine according to claim 1, in which the
exhaust-admission valve is of disc-like form and is operated by a
remote valve stem which is so located that it is removed from the
influence of the recirculated exhaust gas.
3. An internal combustion engine according to claim 1, in which the
exhaust-admission valve is rigidly connected to the remote valve
stem by a hook-like yoke which operates within a chamber that leads
from the inlet manifold at a location upstream of that at which the
recirculated exhaust gas enters this manifold.
4. An internal combustion engine according to claim 1, pg,19 in
which the exhaust gas metering orifice is established by an annular
seat receiving a reciprocatable plug-like valve which penetrates
the orifice and which, whenever it moves, exerts on the surrounding
edge of the orifice a rubbing action which ensures maintenance of
the original effective area of the orifice.
5. An internal combustion engine according to claim 1, in which the
exhaust-admission valve is of circular form and is rigidly
connected to one end of a hollow valve stem which passes, through a
sliding pressure seal, into a casing containing three
diaphragm-chambers formed by two diaphragms of which one
constitutes the pressure-sensitive member and the other constitutes
the pneumatic servo; the hollow valve stem extending through the
servo-diaphragm into the intermediate diaphragm-chamber, which is
in permanent communication with atmosphere.
6. An internal combustion engine according to claim 5, in which the
end of the hollow valve stem remote from the exhaust-admission
valve has an orifice which is located within the intermediate
diaphragm-chamber and which coacts with a disc-like valve fixed to
the pressure-sensitive diaphragm; the valve stem has a second
orifice placing its interior in permanent communication with the
diaphragm-chamber formed by the servo-diaphragm; and a restrictor
is fitted in the valve stem either adjoining the exhaust-admission
valve or at a location between that valve and the said second
orifice.
7. An internal combustion engine according to claim 1, in which the
servo-diaphragm is spring loaded in the direction to close the
exhaust-admission valve, and the relation between the spring-load
and the effective area of this diaphragm is such that the exhaust
valve is opened when a depression slightly less than that
prevailing when the engine is operating at full load acts on the
servo-diaphragm.
8. An exhaust gas recirculation system as claimed in claim 1 which
comprises a pressure-sensing valve assembly connected to respond to
the depression obtaining at the location of a vacuum-advance
drilling in a carburetter supplying said engine and to prevent the
pressure-control valve assembly from permitting recirculation when
said depression indicates that said engine is idling, overrunning,
or operating at full load.
9. An exhaust gas recirculation system according to claim 8
comprising thermally responsive means connected to prevent said
pressure-control valve assembly from permitting recirculation when
the temperature of said engine is below about 60.degree. F.
Description
This invention is concerned with means for reducing atmospheric
pollution by exhaust emissions from internal combustion
engines.
Exhaust gas recirculation is well established as a means of
reducing the emission of oxides of nitrogen (commonly represented
as NO.sub.x) by the exhaust system of an internal combustion
engine. But exhaust gas recirculation systems employed hitherto
leave much to be desired, as regards reducing NO.sub.x emissions,
simply because the control exercised on the amount of exhaust gas
recirculated is too imprecise, and also varies with the operating
conditions of the engine.
The invention, which is applicable both to an engine with
carburetter equipment and to one with a petrol-injection system, is
aimed at providing an exhaust gas recirculation system which: (1)
controls the amount of exhaust gas recirculated so that this amount
is a substantially constant proportion (e.g. from about 12 to about
17 percent) of the mass air-flow rate of the engine; and (2)
preferably is controlled automatically to effect recirculation only
when the operating conditions of the engine are such as to require
it. With regard to 2, recirculation is not required in any of the
following operating conditions: idling, overrunning, full load, or
when the engine is running at a temperature which is below a
predetermined level. In particular, recirculation is not required
at cold-starting, nor during the initial phase of warming-up, but
is required when the vehicle is being driven with the engine at a
temperature of 60.degree.F. or more.
There are several different ways of determining the mass air-flow
rate of any given internal combustion engine, but I consider that
the most practical method in this context for application to motor
vehicles is to utilize the back-pressure of the exhaust gas as a
criterion. The mass of exhaust gas produced by the engine per unit
time is directly proportional to the mass air-flow rate of the
engine.
The principle underlying the invention can readily be appreciated
by notionally regarding the exhaust system of the engine as
comprising two simple orifices, primary and secondary respectively;
each discharging exhaust gas into the atmosphere. That is to say,
the respective effluxes occur into zones of identically the same
pressure. Consequently, the mass flow rate through each orifice
will be proportional to the effective cross-sectional area of the
individual orifice. Therefore, if the effective cross-sectional
area of the secondary orifice were about 15 percent of that of the
primary orifice (which, as already postulated, discharges into
atmospheric pressure), and the efflux from the secondary orifice
was discharged into a zone of atmospheric pressure before entering
the induction system of the engine, then the mass rate of exhaust
gas recirculation would be substantially 15 percent of the mass
air-flow rate of the engine, irrespective of the temperature of the
exhaust gas.
It will be apparent, from the foregoing, that, having established
experimentally the correct percentage of the exhaust gas mass flow
rate that should be recirculated for a given engine installation,
it merely remains to provide means effective automatically either
to engender recirculation or to prevent it (as the case may be),
depending upon the operating conditions of the engine at any given
time.
According to this invention an internal combustion engine is
equipped with an exhaust gas recirculation system in which: a
metering orifice in parallel with the exhaust system governs the
rate of recirculation as a substantially constant percentage of the
mass air-flow rate of the engine; and a pressure-control valve
assembly maintains substantially atmospheric pressure at a location
immediately downstream of the metering orifice and also controls
communication between that location and the inlet manifold. A
subsidiary feature of the invention is that a pressure-sensing
valve assembly, responsive to the operating conditions of the
engine, automatically signals the pressure-control valve assembly
to permit exhaust gas recirculation only when this is
necessary.
More specifically, an internal combustion engine conforming to the
invention and having means for controlling the rate of mass air
flow into its induction system, is equipped with an exhaust gas
recirculation system which effects automatic control of the
recirculation at a substantially constant proportion of the mass
air-flow rate only when the operating conditions of the engine are
such as to require it, and which comprises: a metering orifice in
parallel with the exhaust system and serving to govern the rate of
recirculation as a substantially constant percentage of the mass
air-flow rate of the engine; a recirculation conduit receiving the
efflux of the metering orifice and leading to the inlet manifold
from a location immediately downstream of that orifice; a
pressure-control valve assembly that includes an exhaust-admission
valve which controls communication between the recirculation
conduit and the inlet manifold, and which, by its movements,
maintains substantially atmospheric pressure in the recirculation
conduit immediately downstream of the metering orifice whenever
exhaust gas recirculation is required; and a pressure-sensing valve
assembly which is responsive to the depression obtaining at the
location of the usual vacuum ignition-advance drilling, and which
is effective automatically to signal the pressure-control valve
assembly to permit recirculation only when the operating conditions
of the engine are such that exhaust gas recirculation is
necessary.
In the preferred embodiment of the invention the pressure-control
valve assembly comprises: a pressure-sensitive member that senses
atmospheric pressure at one side and, at its other side, the
pressure immediately downstream of the exhaust gas metering
orifice; and a pneumatic servo which operates the exhaust-admission
valve in response to movement of the pressure-sensitive member.
Operation of the pressure-sensing valve assembly to signal the
pressure-control valve assembly, in order to engender exhaust gas
recirculation, is precluded by thermally-responsive means whenever
the engine temperature is below about 60.degree.F.
Referring to the accompanying drawings:
FIG. 1 illustrates schematically the general arrangement of an
exhaust gas recirculation system of an internal combustion engine
incorporating the invention;
FIG. 2 illustrates schematically a system basically similar to that
of FIG. 1, but in which some of the corresponding components are
constructed and arranged differently; and
FIG. 3 is a sectional elevation of an alternative arrangement of a
pressure-control valve assembly which is employed in each of the
systems illustrated in FIGS. 1 and 2.
In the arrangement depicted in FIG. 1, an internal combustion
engine 1 has a carburetter 2, an air cleaner 3, an induction pipe 4
(forming part of an inlet manifold 5) and an exhaust pipe 6. The
carburetter 2 (which in this instance is of the well-known
controllable jet, automatic variable-choke type) has its mixture
passage 7 fitted, as usual, with a throttle disc 8.
The exhaust gas recirculation system comprises: a metering orifice
9 in the exhaust pipe 6, a pressure-control valve assembly 10 and a
pressure-sensing valve assembly 11. The valve assembly 10, which
controls communication between the induction pipe 4 (downstream of
the throtte disc 8) and a recirculation conduit 12 that leads from
the metering orifice 9, serves to maintain substantially
atmospheric pressure in the conduit 12 immediately downstream of
the metering orifice 9 when recirculation is in progress. The
pressure-sensing valve assembly 11 determines when exhaust gas
recirculation is to take place, by exercising a triggering effect
upon the pressure-control valve assembly 10.
The pressure-control valve assembly 10 has a casing 13 containing
three diaphragm-chambers 14, 15 and 16 which are formed by two
diaphragms 17 and 18 arranged as shown. The intermediate chamber 15
is in permanent communication with atmosphere, by way of a duct 19
and an air cleaner 20. It should perhaps be pointed out that the
separate air cleaner 20 is necessary because the supply of air to
the chamber 15 must be at atmospheric pressure, and cannot be taken
from downstream of the engine air cleaner 3.
Exhaust gas recirculation is, so to speak, turned `on` and `off` by
an exhaust-admission valve 21. When this valve is moved off its
seating 22, the efflux of the metering orifice 9 passes from the
recirculation conduit 12 into the inlet manifold 5, so that exhaust
gas recirculation then takes place.
The exhaust-admission valve 21 is of annular form and is rigidly
fixed to one end of a hollow valve stem 23, which passes into the
casing 13 through a flexible grommet-like gland 24 that affords a
sliding pressure seal with the valve stem. The latter is attached
to the diaphragm 17, but extends into the diaphragm-chamber 15 so
that an orifice 25 at the top of the hollow valve stem 23 is able
to coact with a disc-like valve 26 fixed centrally to the diaphragm
18. The interior of the valve stem 23 is in permanent communication
with the diaphragm-chamber 14 through an orifice 27; and a
restrictor 28 is fitted in the hollow valve stem 23, either (as
shown) adjoining the valve 21 or at a location between that valve
and the orifice 27.
The exhaust-admission valve 21 moves under the influence of the
diaphragm 17 which senses manifold depression (or a controlled part
of it) applied to the chamber 14 by way of the restrictor 28, the
hollow valve stem 23 and the orifice 27.
The diaphragm 17, which constitutes a pneumatic servo, is acted
upon by a helical compression spring 29 which permits its required
travel to open the valve 21 fully under a depression of
approximately 21/2 inches Hg (which is not quite the value of the
full-load inlet manifold depression of the engine). The effective
area of the diaphragm 17 has to be such that this depression,
acting on it, overcomes the spring 29. It should perhaps also be
pointed out that, when no exhaust gas recirculation is taking
place, the spring 29 has to be strong enough to ensure that the
valve 21 is held to its seating 22 against such manifold depression
or exhaust gas pressure as may be tending to unseat this valve.
That is to say, the spring 29 has to apply a given load for a given
area of the valve 21.
A compression spring 30, trapped between the diaphragms 17 and 18,
is just strong enough to support the weight of the diaphragm 18
when the control valve assembly 10 is disposed vertically, as
shown.
The depression acting on the diaphragm 17 is a function of: the
depression existing in the inlet manifold 5, the effective area of
the restrictor 28 and the effective area of the orifice 25 opened
to the atmospheric connection 19 by the valve 26 on the diaphragm
18.
Movement of the valve 26, which varies the area of the orifice 25,
is controlled by movement of the diaphragm 18 when it senses the
pressure immediately downstream of the metering orifice 9 via a
pipe 31, connecting the diaphragm-chamber 16 to the
pressure-sensing valve assembly 11, and a pipe 32 which leads to
that valve assembly; the pipe 32, which emerges from the
recirculation conduit 12, having an orifice 33 located immediately
downstream of the metering orifice 9.
When the pressure in the diaphragm-chamber 16 is greater than
atmospheric, the diaphragm 18 is deflected so that its valve 26
closes the orifice 25. Consequently, the available manifold
depression now acts on the diaphragm 17 which moves against its
spring 29 and causes the valve 21 to open, so that exhaust gas
recirculation occurs.
When the pressure in the diaphragm-chamber 16 is less than
atmospheric, the orifice 25 is opened and, therefore, more nearly
atmospheric pressure obtains in the diaphragm-chamber 14. Under the
action of its spring 29, the diaphragm now causes the valve 21 to
close: the movement of this valve being automatically adjusted
between the `open` and `closed` positions so that a greater or
lesser depression is created in the conduit 12 to maintain
substantially atmospheric pressure downstream of the metering
orifice 9, irrespective of the exhaust gas pressure acting on that
orifice and irrespective of the inlet manifold depression acting on
the valve seating 22.
The various operating conditions of the engine in which exhaust gas
recirculation is not required have been specified above, and need
not be repeated. As already indicated, the purpose of the
pressure-sensing valve assembly 11 is to signal the
pressure-control valve assembly 10 either to effect exhaust gas
recirculation or to preclude this, depending upon the operating
conditions of the engine at any time.
The pressure-sensing valve assembly 11 has a casing 34 containing
two diaphragm-chambers 35 and 36 which are formed by a diaphragm
37. This is loaded by a helical compression spring 38, and has an
adjustable stop 39 limiting its outward travel. From a
throttle-edge drilling 40 of the carburetter 2, a pipe 41 is
connected to the diaphragm-chamber 35. The drilling 40 is similar
in function to the so-called vacuum ignition-advance drilling
employed in modern carburetters. A duct 42 connects the
diaphragm-chamber 36 with a valve housing 43 which has alternative
annular seatings 44 and 45 for a disc-like valve 46. The latter is
mounted on one end of a stem 47 that passes through the duct 42,
the other end of this stem being articulated to the
thermally-deflectable limb of a substantially U-shaped bi-metallic
element 48 of which the other limb is rigidly fixed to the central
part of the diaphragm 37. Thus, the valve 46 is movable both by the
diaphragm 37 and by thermal deflection of the interposed element
48. In the manner shown, the pipes 31 and 32 (referred to earlier)
respectively connect the valve housing 43 to the diaphragm-chamber
16 of the pressure-control valve assembly 10, and to the orifice 33
located immediately downstream of the exhaust gas metering orifice
9. And a pipe 49 interconnects the duct 42 and the region 7 lying
between the variable choke (not shown) and the throttle disc 8 of
the carburetter 2. As this region 7 is one in which a substantially
constant depression exists whenever a carburetter of the
above-described type here involved is operating, it is commonly
designated simply the constant depression region.
The load imposed by the spring 38 in association with the effective
area of the diaphragm 37 is such that the latter makes a full
excursion to the stop 39 under a depression of approximately 1 psi
in the chamber 35; the other diaphragm-chamber 36 being in
communication, via the pipe 49, with the constant depression region
7 of the carburetter 2.
When the throttle 8 of the carburetter 2 is virtually shut, as at
idling, only the depression existing in the constant depression
region 7 acts in the diaphragm-chamber 35, and the diaphragm 37
does not move. This diaphragm is now in a position such that the
valve 46 is on its seating 45, sealing the end of the pipe 32; and
the diaphragm-chamber 16 of the pressure-control valve assembly 10
is connected via the pipes 31 and 49 to the constant depression
region 7 of the carburetter. As a result, the diaphragm 18 of the
valve assembly 10 lifts the valve 26 off the top of the hollow
valve stem 23, so that the diaphragm-chamber 14 is at atmospheric
pressure (by way of the orifices 25 and 27) and the servo-diaphragm
17 keeps the valve 21 closed, thus precluding exhaust gas
recirculation.
When the throttle disc 8 is opened, the chamber 35 of the valve
assembly 11 is subjected to an increasing degree of inlet manifold
depression which, acting on the diaphragm 37, causes the valve 46
to move onto its seating 44 so that it closes off the connection to
the constant depression region 7 (by way of the pipe 49). At the
same time, the valve 46 has opened communication between the pipes
31 and 32. Consequently, the diaphragm-chamber 16 of the valve
assembly 10 is now pressurized from the exhaust gas orifice 33, and
recirculation ensues.
When the throttle disc 8 is opened so widely that the engine is on
full load, the depression acting in the chamber 35 of the valve
assembly 11 is insufficient to hold the diaphragm 37 against the
thrust of its spring 38, and consequently the valve 46 is moved off
its seating 44, so that the pipes 31 and 49 effect communication
between the constant depression region 7 of the carburetter 2 and
the diaphragm-chamber 16 of the valve assembly 10. Then, in the way
already described, the servo-diaphragm 17 closes the valve 21 which
turns off the recirculation.
Under cold conditions the thermally-deflectable limb of the
bi-metallic element 48 assumes the dotted line position, with the
result that when the engine temperature is below approximately
60.degree.F. the diaphragm 37 comes against the stop 39 without
moving the valve 46, and recirculation does not occur.
In practice, a maximum depression occurs in the constant depression
region 7 of the carburetter when the engine is running at its
maximum speed on full load, and when the air cleaner 3 is so
clogged with dirt that the restriction which it imposes on the air
flow is also at a maximum. The strength of the spring 38 is
selected so that movement of the diaphragm 37 does not occur under
this worst condition.
In overrun conditions the throttle disc 8 is shut, inadequate
depression obtains at the throttle-edge drilling 40 (and hence in
the diaphragm-chamber 35) to move the valve 46 onto its seating 44,
and the diaphragm-chamber 16 of the pressure-control valve assembly
10 is put into communication (by the pipes 31 and 49) with the
constant depression region 7 of the carburetter; with the result
that exhaust gas recirculation is precluded by closure of the valve
21 in the manner previously explained.
Referring now to FIG. 2, those components which are constructed and
arranged in the same manner as in FIG. 1 are identified by the same
reference numerals. The chief differences between the two systems
lie in the exhaust gas metering orifice and in the pressure-sensing
valve assembly.
In the case of the system depicted in FIG. 2, the exhaust pipe 6
has a stub-like branch 50 in which there is a metering orifice 9A
penetrated by a reciprocatable plug-like valve 51. The metering
orifice 9A, which is established by an annular seating 52 for the
valve 51, is kept free from incrustation by particulate deposits
because, each time it moves, the valve 51 exerts (on the
surrounding edge of the orifice 9A) a rubbing action which ensures
maintenance of the original effective area of the metering
orifice.
The exhaust valve 51, which functions either fully open or fully
closed, is carried by a stem 53 which passes slidably through a
chamber 54 and into a casing 55. The latter contains a diaphragm 56
loaded by a helical compression spring 57 which is located in a
diaphragm-chamber 58. The valve stem 53, which is attached
centrally to the diaphragm 56, is fitted with a stop device 59
which, by contacting the top of the chamber 54, limits the opening
movement of the exhaust valve 51 and also seals that chamber. When
the exhaust valve 51 is open, the efflux of the metering orifice 9A
passes from the chamber 54 to the inlet manifold 5 by way of a
recirculation conduit 12A and the valve 21.
When the diaphragm-chamber 58 is subjected to the inlet manifold
depression (in a manner which will be described later), the
diaphragm 56 normally opens the exhaust valve 51. But the
relationship between the effective area of the diaphragm 56 and the
load imposed by the spring 57 is such as to preclude opening of the
exhaust valve 51 when the diaphragm-chamber 58 is subjected to the
maximum depression obtaining in the inlet manifold 5 at full-load
operation of the engine.
The diaphragm 18 of the pressure-control valve assembly 10 senses
the pressure immediately downstream of the exhaust gas metering
orifice 9A through a pipe 60, which interconnects the
diaphragm-chamber 16 and the exhaust gas chamber 54. A pipe 61
connects the diaphragm-chamber 58 to a pressure-sensing valve
assembly 11A that includes a disc-like valve 46A having alternative
annular seatings 44A and 45A in a valve block 62.
The valve 46A is mounted on a stem 63 which passes slidably (and in
gas-tight manner) through one end of the valve block 62 and into a
casing 64, of which the adjoining zone is open to atmospheric
pressure. The casing 64 contains a diaphragm 65 loaded by a helical
compression spring 66 which is located in a diaphragm-chamber 67.
The valve stem 63, which is attached centrally to the diaphragm 65,
is fitted with a stop device 68 that coacts with the
thermally-deflectable limb of a substantially U-shaped bi-metallic
element 69, the other limb of which is rigidly fixed at 70. The
arrangement is such that whenever the engine temperature is below
about 60.degree.F. the free end of the bi-metallic element 69
prevents movement of the stop device 68 to the right, as shown, so
that the valve 46A cannot be moved off its seating 45A. But when
the engine temperature exceeds about 60.degree.F. the
thermally-deflectable limb of the element 69 is in the position
indicated by the dotted line, and no longer prevents the valve 46A
from being moved off its seating 45A.
The valve block 62 has a duct 71 which is permanently open to
atmosphere, and which is so located with respect to the valve
seatings 44A and 45A that when the valve 46A (as depicted) is off
its seating 44A the pipe 61 communicates atmospheric pressure to
the diaphragm-chamber 58. The exhaust valve 51 is then kept closed
by the spring 57.
When the operating conditions are such as to require exhaust gas
recirculation, the increased depression communicated to the
diaphragm-chamber 67 of the pressure-sensing valve assembly 11A
causes the diaphragm 56 to move the valve 46A off the seating 45A
and onto the seating 44A. In consequence, by way of a pipe 72
connecting the inlet manifold 5 to the valve block 62 at a location
open to the bore of the valve seating 45A, the pipe 61 now
communicates the inlet manifold depression to the diaphragm-chamber
58. The diaphragm 56 thereupon moves against its spring 57 and
opens the exhaust valve 51. As a result, the pressure immediately
downstream of the exhaust gas metering orifice 9A is transmitted
from the chamber 54, by way of the pipe 60, to the
diaphragm-chamber 16 of the pressure-control valve assembly 10. The
latter now admits exhaust gas into the inlet manifold 5 (in the
manner already described for FIG. 1), thus maintaining
substantially atmospheric pressure downstream of the metering
orifice 9A.
Referring now to FIG. 3, this depicts an alternative arrangement of
part of the pressure-control valve assembly 10 of FIGS. 1 and 2;
the purpose of this alternative arrangement being to obviate, as
far as possible, the likelihood of the valve stem 23 (FIGS. 1 and
2) eventually sticking in its gland 24 due to accretion of
particulate carbon deposited on the valve stem 23 by the exhaust
gas to which it is exposed. Accordingly, in the alternative
arrangement (FIG. 3), the previous valve stem 23 is replaced by a
similar hollow valve stem 23A which is remote from the
exhaust-admission valve, and which is removed from the influence of
the exhaust gas whenever this is delivered to the inlet manifold 5
from the recirculation duct 12 or 12A. The previous annular
exhaust-admission valve 21 is replaced by a disc-valve 21A which
has a seating 22A at the outlet of the recirculation duct 12 or
12A, and which is rigidly connected to the remote valve stem 23A by
a hook-like yoke 73. This yoke is a T-section casting having at its
ends accurately aligned holes 74 and 75 into which the valve 21A
and the valve stem 23A are respectively pressed.
The yoke 73 operates within a chamber 76, to which access is
afforded by a removable sealing plug 77. The base of the assembly
shown in FIG. 3 is bolted directly to the inlet manifold (FIGS. 1
and 2), downstream of the throttle disc 8, with the chamber 76
lying nearest to the throttle disc 8. As a result, the remote valve
stem 23A becomes exposed only to fuel vapour in the chamber 76, and
does not suffer contamination by exhaust gas when the
exhaust-admission valve 21A is open, because the chamber 76 leads
from the inlet manifold 5 (FIGS. 1 and 2) at a location upstream of
that at which the recirculated exhaust gas enteres this
manifold.
* * * * *