Exhaust gas recirculation system

A pressure regulating unit modulates the vacuum present in the vacuum chamber of a vacuum motor operatively connected to an EGR control valve, in response to the pressure differential between the pressure in the EGR passage, between a restriction and the EGR control valve, and the pressure immediately downstream of a restriction in the exhaust conduit, the latter restriction being located downstream of the junction of the EGR passage and the exhaust conduit.

BACKGROUND OF THE INVENTION 
This invention relates to a motor vehicle powered by an internal combustion 
engine which is equipped with an EGR system (a system for recirculating 
portion of the exhaust gases to the induction manifold of the engine for 
the purpose of reducing NOx emissions from said engine.) and more 
particularly to an improved EGR system. 
It is well known to recirculate a portion of the exhaust gases emitted from 
the engine to the induction manifold to produce an air/EGR-gas/fuel 
mixture which exhibits a reduced rate of combustion in the combustion 
chamber or chambers of the engine. This reduced rate of combustion reduces 
the peak combustion temperatures to accordingly maintain the combustion 
temperature at a level where NOx is virtually not formed. However to 
achieve effectively the desired reduction of the NOx generation during 
combustion it is necessary to recirculate rather large amounts of exhaust 
gas. Combustion chambers have been developed which permit very high rates 
of EGR to be employed without loss of power output of the engine. However 
during certain modes of engine operation such as high speed and low load 
NOx is inherently low and accordingly a reduced rate of EGR can be used 
without any increase in NOx emissions. However an EGR system for 
controlling the rate of EGR has not been developed yet which can 
adequately proportion the amount of exhaust gases introduced into the 
engine during all modes of engine operation when a very high rate of EGR 
is employed. Systems which have attempted to achieve the above mentioned 
degree of control have suffered from various drawbacks such as over 
sensitivity to variations in exhaust gas pressure in the exhaust manifold 
and associated exhaust conduit. This so-called over sensitivity has not 
presented any large problems in EGR systems which recirculate exhaust 
gases in the order of a few percent. A variation in the amount of EGR gas 
supplied under the afore mentioned condition produces only a slight change 
in engine performance. However a small change when using upwards of a 50% 
EGR rate results in a large change in the amount or volume of exhaust 
gases in fact introduced into the combustion chamber or chambers. Erratic 
engine performance immediately results from this rather large change in 
the volume of EGR gas supplied thereinto. Thus despite the development of 
combustion chambers having the capacity to combust mixtures containing 
large quantities of EGR gas the engine as a whole has been unable to 
produce the desired performance. 
There are several possible sources of the afore mentioned pressure 
variation, these include; accidents which bend or dent the exhaust pipe 
and or manifold to a degree where an increased back pressure results 
causing a higher than intended pressure to be present in the EGR passage; 
carbon deposites in critical places causing a reduction in the effective 
cross sectional areas of, for example part of the EGR passage; and 
variations in the dimensions of the exhaust manifold, EGR passage etc. 
during mass production of same. The latter of course always exists and 
there is inevitably a car to car variation in the pressure existing in the 
EGR passage immediately downstream of the valve controlling the flow of 
the gases through said passage. 
Thus there still remains a need for an EGR control system which operates to 
adequately proportion the amount of exhaust gases recirculated during all 
modes of engine operation, which is not effected by the above described 
pressure variations and therefore controls the supply of large amounts of 
EGR gas to the engine in a manner which ensures optimal output of the 
engine with greatly reduced NOx emissions. 
SUMMARY OF THE INVENTION 
In view of the foregoing an EGR system designed to recirculate large 
quantities of exhaust gas has been developed which uses the rather 
considerable exhaust gas pressure as a control signal indicative of the 
amount of induced air rather than the vacuum existing in the venturi 
portion of the induction system. The latter requiring amplification via 
complex apparatus before being suitable as the afore mentioned signal. In 
a detail, the pressure regulating unit of the EGR system is connected to a 
source of vacuum (the induction manifold at a location remote or distal 
from the venturi portion) and arranged modulate the degree of vacuum 
present in the vacuum motor utilized for opening and closing a valve which 
controls the flow of exhaust gases from the exhaust manifold to the 
induction manifold, in accordance with two pressure signals from the 
exhaust system. The first originates in the EGR passage between a 
restriction and the valve seat of the afore mentioned valve and the second 
immediately downstream of a restriction in the exhaust conduit downstream 
of the branching of the EGR passage and the exhaust conduit. Suitable 
selection of the diameters of the restriction orifices produces a suitable 
ratio between the two pressures which remains constant despite the 
previously described variation of the pressure in the exhaust manifold. 
Thus it is an object of the present invention to provide an EGR system 
which is simple in construction, senses the exhaust gas pressure 
prevailing in two different locations, one in the exhaust manifold, and 
the other in the EGR passage, and accordingly permits a flow of EGR gas 
into the induction manifold which is very closely proportioned to the 
amount of inducted air despite variations of the exhaust gas pressure with 
respect to the amount of inducted air, with the passing of time and from 
vehicle to vehicle due to mass production dimension variation. 
It is also an object of the present invention to provide an EGR system 
which reduces the amount of EGR gas normally recirculated under 
predetermined operating modes of the engine such as high speed low load 
operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before considering the preferred embodiments of the invention let us turn 
to FIG. 1 and consider the prior art EGR system shown therein. The numeral 
1 denotes an EGR passage in which a restriction 2 is disposed upstream of 
an EGR control valve 3 (consisting of a valve stem 3a valve head 3b and 
valve seat 3c). A chamber is defined between the valve seat and the 
restriction 2 which is as shown fluidly communicated with a chamber of a 
pressure regulating unit 4 through a conduit 13. Operatively connected to 
the valve stem 3a is a diaphragm 12 of a vacuum motor 14. Disposed in the 
vacuum chamber 14a between the casing of the vacuum motor and the 
diaphragm is a spring 11 which is arranged to bias the diaphragm 12 into 
the atmospheric chamber 14b and thus urge the valve head 3b into sealing 
engagement with the valve seat 3c. A conduit 7b fluidly connected at one 
end to the vacuum chamber 14a of the vacuum motor 14 is arranged to 
fluidly communicate with two other conduits 10 and 7a. The first conduit 
10 which has a restriction (no numeral) disposed therein is arranged as 
shown to communicate with a portion of the induction manifold at a 
location which is immediately upstream of the throttle valve 9, rotatably 
disposed in the induction manifold downstream of the venturi portion, when 
it is in a fully closed position. Hence when the throttle valve is fully 
closed a positive pressure or atmospheric pressure is fed therein and when 
it is open a negative pressure or vacuum is fed therein. The second 
conduit 7a interconnects conduits 10 and 7b with the atmospheric chamber 
of the pressure regulating unit 4, and is arranged to project into afore 
mentioned chamber so as to juxtapose a member formed on the diaphragm 6 
disposed in the pressure regulating unit. The diaphragm 6 divides the 
casing of the pressure regulating unit 4 sealingly into the afore 
mentioned atmospheric chamber and the pressure chamber 5 which is exposed 
to the pressure prevailing in the chamber defined between the restriction 
2 and the valve seat 3c. 
In operation when the throttle valve is closed a positive pressure is fed 
to the vacuum chamber 14a of the vacuum motor 14 via conduits 10 and 7b. 
This urges the valve head 3b firmly against the valve set 3c to cut the 
flow of exhaust gases into the induction manifold (connection no shown). 
Thus the pressure in the chamber defined between the valve seat and the 
restriction 2 rises to urge the diaphragm 6 to position where the member 
formed thereon closed the end of the conduit 7a to ensure a positive 
pressure prevails in the vacuum chamber 14a. 
When the throttle valve is opened the positive pressure disappears and is 
replaced by a negative pressure pressure so that the valve 3 is opened to 
permit the flow of EGR gas via the resulting flexing of the diaphragm 12 
into the vacuum chamber 14a. The pressure Pe in the afore-mentioned 
chamber (between the restrictor and the valve seat) drops accordingly, and 
the upon reaching a given value the diaphragm 6 is permitted to move away 
from the position in which it closes the conduit 7a thus permitting the 
vacuum being fed from the induction manifold to be modulated by the 
introduction of atmospheric air into the conduits 7a and 7b. Thus 
variations in the pressure Pe and the degree of vacuum prevailing in the 
induction manifold at the point or location where the conduit 10 joins 
same, control the amount of EGR gas introduced into the induction 
manifold. Accordingly it is necessary to design the apparatus around 
parameters such as the exhaust pressure developed for a given amount of 
inducted air and the degree of vacuum present in the induction manifold at 
this time. 
Now with the passing of time should the pressure prevailing in the exhaust 
manifold for a given volume of inducted air change for any of the reasons 
set forth under the heading of "description of the prior art" the volume 
of exhaust gases in fact recirculated will change accordingly. Hence the 
afore-described problem will be encountered if the EGR rate is not within 
a few percent. 
Referring now to FIG. 2 of the drawings, a first preferred embodiment of an 
EGR (Exhaust Gas Recirculation) system according to the present invention 
is shown and in which the numeral 20 indicates an exhaust conduit 
operatively connected to an internal combustion engine (not shown) for 
receiving exhaust gases thereinto. Fluidly communicating with the exhaust 
conduit 20 at a first end thereof is an EGR passage 21. Although not 
shown, the other or second end of the EGR passage 21 fluidly communicates 
with the induction manifold or intake passage 29 of the engine to 
introduce exhaust gases, transferred through the EGR passage 21, 
thereinto. As usual, the intake passage 29 is connectable to the 
combustion chamber or combustion chambers (not shown) of the engine to 
feed the combustion chamber with air-fuel mixture prepared by known 
air-fuel mixture preparing device (no numeral) such as a carburetor. The 
air-fuel mixture preparing device may be one including a fuel injection 
system. A part of the intake passage 29 is formed in the air-fuel mixture 
preparing device of which throttle valve V is rotatably disposed in the 
above-mentioned part of the passage 29. Operatively mounted in the passage 
21 is an EGR control valve generally denoted by the numeral 37. As shown 
the valve consists of a valve stem 37a, a valve head 37b and a valve seat 
37c. Formed or disposed in the EGR passage 21 upstream of the valve seat 
37c is a restriction or orifice 22. Another restriction or orifice 24 is 
disposed in the exhaust conduit 20 at a location downstream of the 
intersection, or branching of the EGR passage 21 and the exhaust conduit 
20. A vacuum motor denoted by the numeral 23 is mounted on the EGR passage 
21 in such a manner that a diaphragm 36 thereof is connected to the valve 
stem 37a so that flexing thereof directly induces reciprocating motion of 
the valve stem and head to open and close the valve 37. The vacuum chamber 
31 has a spring 38 disposed therein to bias the diaphragm into the 
atmospheric chamber (no numeral) of the vacuum motor 23 (i.e. away from 
the vacuum chamber). 
A pressure regulating unit 25 consists of a casing (no numeral) sealingly 
divided into three chambers 27a, 27b and 27c by two diaphragms 26a and 
26b. 
The first chamber 27a is communicated with the atmosphere through an 
opening (no numeral) formed in the casing. A conduit 30 is arranged as 
shown to pass through the casing so that the end thereof juxtaposes the 
first diaphragm 26a in such a manner that upward flexing of the diaphragm 
26a toward the portion of the casing through which the conduit 30 is 
disposed reduces the communication between the holes providing 
communicating with the atmosphere and the inlet orifice of the 
afore-mention conduit 30 and finally cuts same. The conduit 30 is branched 
to fluidly communicate with two other conduits 30a and 30b. The conduit 
30a communicates with the induction manifold or passage 29 at a location 
downstream of the throttle valve so as to tap into a variable source of 
vacuum. A restriction 30' is disposed in the conduit 30a between the 
induction manifold and the branched conduit 30b. The conduit 30b is 
arranged to fluidly communicate with the vacuum chamber 31 of the vacuum 
motor 23 to supply the vacuum tapped from the induction manifold 
thereinto. The vacuum in fact reaching the vacuum chamber 31 is modified 
by the degree of opening between the end of the conduit 30 which opens 
into the first chamber 27a and the first diaphragm 26a. 
The second chamber 27b is communicated with the exhaust conduit 20 at a 
location downstream of the restriction 24 through a conduit 28a. Thus the 
chamber 27b is exposed to the pressure which exists downstream of the 
restriction 24 which is denoted by P.sub.1 in FIG. 2. 
The third chamber 27c is communicated with the chamber defined between the 
valve seat 37c and the restriction 22 so as to be exposed to the pressure 
prevailing therein. This pressure is denoted by P.sub.2. Now it should be 
pointed out that the afore mentioned pressures P.sub.1 and P.sub.2 
normally have absolute values lower than atmospheric i.e. are normally 
vacuums. This situation is brought about by the increase of velocity of 
the exhaust gases as they pass through the orifices formed in the 
restrictions. As shown the first diapjragm 26a is smaller in effective 
surface area than the second diaphragm 26b. A spring 35 is disposed 
between the partition (no numeral) to which the first diaphragm 26a is 
fixedly attached and the second diaphragm 26b to bias the latter in the 
direction of the third chamber 27c. 
The two diaphragms 26a and 26b are as clearly shown interconnected by a rod 
(no numeral) attached at its ends to the diaphragms at substantially their 
mid-points to ensure their simultaneous movement. 
At this time, it is considered advantageous to consider the fundamentals of 
the operation of the present invention before describing the actual 
operational steps. 
It will be appreciated that it is extremely important to select the 
diameters of the orifices in the restrictions 22 and 24 according to the 
present invention because these determine the flow rates through the EGR 
passage 21 and the exhaust conduit 20. As can be seen in FIG. 2 the square 
of the rate of flow of the exhaust gases through the exhaust conduit 20 is 
proportional to (P.sub.o - P.sub.1) or the pressure differential on either 
side of the restriction 24 and the flow rate of the exhaust gases through 
the EGR passage 21 is proportional to (P.sub.o - P.sub.2) or the pressure 
differential on either side of the restriction 22. It will also be 
appreciated that the flow rate of exhaust gases actually being exhausted 
from the engine through the exhaust conduit 20, i.e. .sqroot.(P.sub.o - 
P.sub.1), is proportional to the rate of air inducted into the induction 
manifold 29 viz. the volume of gases inducted deformines the amount of 
gases in fact exhausted. 
Now if the amount of exhaust gases recirculated is controlled, as is highly 
desirable with respect to the volume of inducted air (in this case 
proportional to the exhaust gas flow rate or .sqroot.(P.sub.o - P.sub.1), 
a desirable volume of exhaust gases will be introduced into the engine. 
Since the pressure regulating unit 25 is sensitive to pressures P.sub.1 
and P.sub.2 which are in fact representative of the flow rates represented 
by (P.sub.o - P.sub.1) and (P.sub.o - P.sub.2) controlling the pressure 
regulating unit with respect to a preselected ratio of P.sub.1 and 
P.sub.2, the flow rates through the EGR passage 21 and the exhaust conduit 
20 will be similarly proportional. This of course is quite different from 
the prior art where recirculation is controlled with respect to the 
absolute value of the exhaust gas pressure only. 
Thus it will be obvious that, by selecting the diameters of the orifices in 
the restrictions 22 and 24 and selecting a suitable ratio of the effect of 
each of the pressures P.sub.1 and P.sub.2 (i.e. selecting appropriate 
diameters and effective working areas of the diaphragms in the pressure 
regulating unit 25), an adequate proportioning of the exhaust gases will 
be assured despite pressure variations of the nature set forth earlier in 
this disclosure. 
In operation when the engine is in an idling mode of operation and the 
pressure in the exhaust conduit 20 is relatively low and the vacuum in the 
induction manifold 29 is relatively high, the diaphragms 26a and 26b will 
be flexed in the direction of the chamber 27c by the action of the spring 
35 to open the conduit 30. The action of the spring 35 directly influences 
the first diaphragm 26a due to the connection via the rod interconnecting 
same with the second diaphragm 26b. Hence, as the spring urges the second 
diaphragm 26b toward the third chamber 27c, the rod draws the first 
diaphragm 26a in the same direction. This permits substantially 
atmospheric pressure to prevail in the vacuum chamber 31, since the 
magnitude of the vacuum in the induction manifold is all but bled off by 
the atmospheric air passing through the conduit 30. Thus, at this time, 
the EGR control valve 37 is closed under the influence of the spring 
housed in the vacuum chamber 31. Now as long as the absolute values of the 
pressures P.sub.1 and P.sub.2 remain above given levels, the above 
described condition will continue. However as the throttle valve opens and 
the volume of inducted air rises, the pressure of the exhaust gases will 
also rise. Hence as pressure P.sub.1 and P.sub.2 fall, the diaphragms in 
the pressure regulating unit 25 will be urged against the biasing force of 
the spring 35. Simultaneously the degree of vacuum prevailing in the 
induction manifold 29 rises and is fed via conduits 30a and 30b to the 
vacuum chamber 31. The amount of vacuum bled off is reduced as the 
pressures P.sub.1 and P.sub.2 fall. The thus modulated vacuum in the 
vacuum chamber opens the EGR control valve to permit the flow of exhaust 
gases. 
During steady state operation of the engine, should the engine begin to 
receive too much EGR gas, the pressure P.sub.2 in the chamber defined 
between the EGR control valve 37 and the restriction 22 will fall with 
respect to the pressure P.sub.1 in the exhaust conduit. Hence at this 
time, the diaphragms 26a and 26b will flex due to the change in the 
pressure differential (P.sub.1 - P.sub.2) to increase the amount of air 
permitted into the conduit 30 from the atmospheric chamber 27a. The degree 
of vacuum in the vacuum chamber 31 will drop permitting the valve head 37b 
to approach the valve seat 37c to reduce the volume of exhaust gases 
passing therethrough. The pressure P.sub.2 will immediately begin to rise 
until the predetermined ratio of P.sub.1 to P.sub.2 (or pressure 
differential P.sub.1 - P.sub.2) in the pressure regulating unit is 
satisfied whereupon the diaphragms will again flex to reduce the amount of 
air entering the conduit 30. Thus via the above described feedback 
operation any deviation in pressure in the EGR passage with respect to 
that in the exhaust conduit 20 (and vice versa) will be quickly rectified. 
Let us now turn to FIG. 3 wherein second preferred embodiment similar in 
basic operation to the first preferred embodiment is shown. However the 
second preferred embodiment is equipped with an additional circuit for 
modifying the operation of the pressure regulating unit during high speed 
and low load operation of the engine. As previously described it is 
unnecessary to recirculate large quantities of EGR gas due the inherent 
low production of NOx during this mode. 
Since the system of the second preferred embodiment is basically the same 
except for a few additional components, a detailed description of the 
entire system will be omitted for previty. Now as seen in the figure a 
third diaphragm 26c is provided within the casing to define, in this case, 
four chambers 27a, 27b, 27c and 27d. Only the last is new, the others 
being functionally the same as in the first preferred embodiment. The 
diaphragm 27d is fixedly connected to the other diaphragms through a rod 
(no numeral) for simultaneous movement therewith. The chamber 27d is 
communicated with the atmosphere through a conduit 32 which has a 
restriction 32a disposed therein to limit said communication. Fluidly 
connected to the conduit 32 at a location between the restriction 32a and 
the chamber 27d is a conduit 34. The conduit 34 fluidly communicates with 
the afore-described conduit 30a at a location between the branching of the 
conduits 30, 30a and 30b and the restriction 30'. Operatively disposed in 
the conduit 34 is a check or relief valve 33. Another restriction 30a' is 
disposed in the conduit 30a between the junction of the conduit 34 with 
said conduit 30a and the junction or branching of the conduits 30, 30a and 
30b. 
The operation of the system shown in FIG. 3 is identical with that shown in 
the FIG. 2 up until a predetermined engine speed when the vacuum 
prevailing in the induction manifold 29 exceeds a predetermined value to 
overcome the force closing the check valve 33. Prior to the opening of the 
check valve 33 atmospheric pressure prevails in the fourth chamber 27d. 
The selection of the diameters of the orifices in the restrictions may be 
arranged to compensate for the effect of the fourth chamber. 
Now when the check valve 33 opens, the vacuum prevailing in the induction 
manifold 29 is fed through the conduits 30a and 34 (and simultaneously 
through conduits 30b and 30) is partially bled off by the atmospheric air 
entering the conduit 32 through the restriction 32a and finally fed into 
the fourth chamber 27d via conduit 32. The pressure having changed from 
atmospheric to less than same induces the flexing of the third diaphragm 
37d into the fourth chamber 27d. This, by way of the rods interconnecting 
the other diaphragms, causes the first diaphragm to flex so as to increase 
the amount of air permitted to enter the vacuum chamber 31 via conduits 30 
and 30b. The reduction of the vacuum prevailing in the chamber 31 permits 
a reduction of the opening of the EGR valve 37 via the action of the 
spring disposed in said vacuum chamber. Thus, as long as the engine runs 
at the aforementioned high speed and low load, an equilibrium between the 
pressure P.sub.1, P.sub.2 and the pressure in the fourth chamber 27d will 
reduce the amount of EGR gases introduced in the engine. 
The afore-described reduction in the volume of the EGR gases is highly 
advantageous because it permits the engine to run stably at high speeds 
using less fuel (than normally required if the high rate of EGR is 
continued) and simultaneously produce very little NOx (because some EGR is 
maintained to suppress the little formation which would noramlly take 
place in the case of no EGR).