Exhaust Gas Recirculation Flow Control System

FIG. 1 is a diagramatic view of the exhaust gas recirculation flow control system.

FIG. 2 is a cross-sectional view of the preferred embodiment of the exhaust gas flow sensing means and its interrelationship with the standard EGR type valve.

FIG. 3 is a schematic view of a carburetor normally associated with an internal combustion engine and illustrating the engine air flow sensing means.

FIG. 4 is a cross-sectional view of the control valve assembly of the preferred embodiment of this invention.

FIG. 5 is a cross-sectional view of the control valve assembly shown in FIG. 1 and illustrating an alternate embodiment of the invention which incorporates a limiting function which cuts off the recirculated exhaust gas when intake manifold vacuum is low and engine air flow is high.

FIG. 6 graphically illustrates the test results of the invention as applied to an existing internal combustion engine.

The exhaust gas recirculation flow control system according to the invention is shown in FIG. 1. The control system is comprised of the engine air flow sensing means (40), the exhaust gas flow sensing means (30), and the control assembly (20) including the control valve member, and its related structure which controls the exhaust gas recirculation (EGR) valve (10). An example of the temperature responsive member (50) which may be used in combination with the inventive system is shown in FIG. 1.

Referring to FIG. 3, a carburetor (41) of the down draft type is shown having an air-fuel induction passage (45) with an atmospheric air inlet (44) at one end and connected to the engine's intake manifold (47) at the opposite end. Passage (45) contains a fixed area venturi (42) and a throttle valve (46). The latter is rotatably mounted on a part of the carburetor body across passage (45) in a manner to control the flow of air-fuel mixture into the intake manifold. Fuel is indicated from a nozzle, (not shown) projecting into or adjacent venturi (42).

Throttle valve (46) is shown in its engine idle speed position essentially closing induction passage (45) and is rotatable to a nearly vertical position essentially unblocking passage (45). A port or static tap (45) is provided in the throat of the venturi (42) which in combination with the venturi (42) and a conduit passage (48) mounted adjacent to static tap (49) comprises the engine air flow sensing means. The pressure or vacuum level at the static tap (49) is a function of the total air flow passing through venturi (42). That is, engine air flow is regulated by the relative position of throttle valve (46). As the throttle valve opens air flow increases which in turn causes the pressure at static tap (49) to decrease with respect to atmospheric pressure, or any pressure at an upstream stagnation point which is not shown. It has been shown that the stagnation point may also be between the static tap and the throttle plate in an area where the flow stream velocity is near zero. The vacuum or pressure which is created at static tap (49) for purposes of discussion will be labelled as P.sub.v. Pressure signal P.sub.v is communicated to the control valve housing (21) shown in FIG. 1 through conduit member (48). The cross-sectional area of the throat of the venturi (42) is designated as A.sub.5. The cross-sectional area A.sub.5 is a design parameter which must be known since the disclosed system regulates EGR flow as a partial function of the air flow which passes through this venturi. A conduit passage (36) in the intake manifold (47) communicates the intake manifold pressure P.sub.m sensed at port (36') to the control valve housing (21).

A spark port (43) is provided at a point just above the idle position of throttle valve (46), which is traversed by the throttle valve (46) as it rotates to unblock passage (45). The vacuum or pressure level at spark port (43) will vary as a function of the rotational movement of the throttle valve, the spark port (43) reflecting essentially atmospheric pressure upon closure of the throttle valve. The vacuum or pressure sensed at port (43) is communicated through conduit passage member (115) to the vacuum source (122) shown in FIG. 4. With this modification incorporated into the system, exhaust gas recirculation will be inhibited at small throttle openings, as well as at low engine air flow for reasons which will become more apparent from the following description.

Although the engine air flow sensing means (40) as herein described relates to a down draft type carburetor (41), element (40) can also be referred to more generally as any air throttling body since the disclosed system can be used in combination with any fuel metering system which senses air flow or in which an air flow sensor has been added.

Referring to FIG. 2, the exhaust flow responsive means (30) is shown. A housing (31) has a passage (32) formed therein to which is connected a pipe (33) from the exhaust manifold and a pipe (34) connected to the intake manifold. Disposed within the passage (32) and fixedly secured thereto is a valve seat (38) having a cross-sectional area A.sub.2. A conduit passage (37) is located between the valve seat (38) and an orifice (35) disposed downstream of valve seat (38). The orifice (35) has a cross-sectional area A.sub.1. Arrow E shows the direction of recirculated exhaust gas flow.

An EGR valve (10) which is of standard design is fixedly secured to the housing (31) by any conventional means such as bolting. The EGR valve is of the spring biased diaphragm type having a rod (12) fixedly secured to diaphragm (13). Diaphragm (13) divides the housing (11) of the EGR valve into two separate chambers (14 and 15), chamber (14) being exposed to ambient air through an opening (19) and chamber (15) communicating with control valve housing (21) (shown in FIG. 1) through conduit passage (16). Valve body (18) which is fixedly secured or integrally formed on rod (12) is biased against the valve seat (38) of housing (31) by spring (17) which has a predetermined spring force F.sub.1. It is to be understood that as the pressure varies across the diaphragm (13) the rod and therefore the valve body (18) move towards or away from the valve seat (38) thereby controlling the amount of exhaust gas recirculated through passage (32) into the intake manifold (47).

Exhaust flow sensing is accomplished by measuring the pressure drop across restriction (35). Conduit passage member (37) which has an opening (37') senses the exhaust gas pressure P.sub.e in passage (32), between EGR valve (10) and restriction (35) and communicates this pressure to control valve housing (21). As discussed above, a conduit passage member (36) in the intake manifold (47) communicates the intake manifold pressure P.sub.m at opening (36') to the control valve assembly (21). The differential pressure established by pressures P.sub.e and P.sub.m increases with the flow rate of exhaust gas passing through orifice (35) in a manner like the differential pressure between atmospheric pressure P.sub.a and venturi pressure P.sub.v increases with the flow rate of air passing through carburetor venturi (42) as described above. Orifice (35) having a cross-sectional area A.sub.1 serves the additional function of limiting the maximum amount of exhaust gas that can be recirculated to the intake manifold. By sensing exhaust gas flow downstream of EGR valve (10) the exhaust gas recirculated to the intake manifold can become a function of not only engine air flow, but also a function of intake manifold vacuum which will become apparent from the description which follows below.

Referring now to FIG. 4, the control valve assembly (20) of the exhaust gas recirculation flow control system is shown. The control valve housing (21) is of a laminated construction with sections (21a) and (21b) retaining a diaphragm (60) and sections (21b) and (21c) retaining a diaphragm (70) and sections (21c and (21d) retaining a diaphragm (80) therebetween. These four sections form a cavity (22) which because of the diaphragms (60), (70) and (80) is divided into separate chambers (22a), (22b), (22c) and (22d). These sections are secured to each other by any suitable means such as riveting, with cover section (21e) completing the housing assembly. Disposed within cavity (22) and fixedly secured, through the use of distribution members (61) and (71), to the diaphragm (60), (70) and (80) is a control valve member (23). The portions of diaphragm (60), (70) and (80) which are included within the cavity (22) have "effective" cross-sectional areas A.sub.6, A.sub.7 and A.sub.8 respectively. The "effective" area of a diaphragm is that area which is defined by the ratio of the resultant force to the applied pressure and therefore, the effective area is a function of both the outside diameter of the diaphragms within cavity (22) and the outside diameter of the pressure distribution members (61) and (71).

Control valve member (23) is comprised essentially of three main parts, a stem portion (24), an adjustment member (25) and a piston member (26). Stem portion (24) is fixedly secured to diaphragms (60), (70) and (80) through distribution members (61) and (71). Adjustment member (25) is threadably disposed within stem portion (24). Stem portion (24) has a portion of its inner diameter (29) formed for receiving piston member (26). Piston (26) is slidably assembled into the formed portion (29) and the disposition of diaphragms (60), (70) and (80) as well as distribution members (61) and (71) within cavity (22) relative to piston member (26) is determined by adjustment member (25). The lower most extension of piston (26) has a valve (27) fixedly secured thereto. Valve (27) may be integrally formed with piston (26). Valve (27) is adapted to seat against valve seat (92) as the piston (26) translates within stem portion (24). The pressure signal P.sub.EGR which is communicated from cavity (90) to the EGR valve port (16) is a function of the relative position of valve (27) and may be of a magnitude equal to atmospheric (P.sub.a) or the source pressure (P.sub.s) in cavity (122) or some value in between.

Control valve member (23) is biased by spring members (130) and (140). Housing cover (21e) is adapted to receive spring (140) so that the force generated by spring (140) can act in an axial direction upon piston (26). Similarly, housing chamber (21a) is adapted to receive spring member (130) so that an adjustment screw (131) may be threadedly secured to housing cavity (21a) and through spring retainer member (132) generate a force axially aligned with control valve member (23). Utilization of the biasing forces generated by spring member (130) and spring member (140) permits adjustment of the complete control valve assembly to compensate for tolerance variations between the subassembly of distribution members (61) and (71), stem (24) and diaphragms (60), (70) and (80). Any suitable means biasing the control valve member (23) with an adjustment feature can be utilized to accomplish a similar result.

Housing section (21a) is adapted to receive pressure signal P.sub.v and by means of passage (21a') and conduit passage (48), chamber (22a) communicates with static tap (49) in venturi throat (45). Housing section (21b) is adapted to receive pressure signal P.sub.m from the intake manifold and through passage (21b') and conduit passage (36), chamber (22b) communicates with the intake manifold (47). Housing section (21c) is adapted to receive pressure signal P.sub.e and chamber (22c) communicates, through passage (21c') and conduit passage (37), with the exhaust gas passage (32). Chamber cavity (22d) is subject to a filtered source of atmospheric pressure P.sub.a communicated to cavity (22d) through passage (21d') formed in housing section (21d). Chamber (22d) is also connected through passage (91) to a second cavity (90) formed in housing section (21d). The lower portion of passage (91) forms the valve seat (92) projecting into cavity (90). Passage (91) has a cross-sectional area designated A.sub.9. Housing cavity (90) communicates with chamber (15) of EGR valve (10) through EGR conduit passage (16) (see FIG. 2) and housing section (21d) passage (21d.sub.3 '). Cavity (90) is formed by securing housing section cover (21e) to section (21d).

Cavity (90) has a second passage (95) which communicates with housing chamber (22d). The lower portion of this passage terminates in a valve seat (96). A bimetallic strip (50) is mounted in cavity (90) secured to housing cover (21e) by any conventional method such as riveting (51). The bimetallic strip (50) senses temperature in the vicinity of the control valve assembly (20) i.e., the engine compartment temperature, and in response to a predetermined temperature valve member (97) will seat upon valve seat (96) and terminate the communication of pressure signal P.sub.a from chamber (22d) to cavity (90) through second passage (95). By terminating pressure signal P.sub.a through passage (95), the pressure in cavity (90) becomes a function of P.sub.s when valve (27) is closed. This function will be communicated to chamber (15) of EGR valve (10) and generate a force F.sub.2 across diaphragm (13). When F.sub.2 becomes greater than F.sub.1 (spring force generated by spring (17)) valve (18) will begin to open and exhaust gas will begin to flow through passage (32) into the intake manifold. This will be discussed in more detail in the operational discussion that follows. Other temperature responsive means can also be incorporated in this invention so that the control valve assembly 20 could also be made responsive to such temperature variables, as for example, the temperature of the engine coolant.

The combination of control valve cover (21e), housing section (21d) and cover member (125), creates a third cavity (122) within housing section (21d). Cavity (122) communicates with cavity (90) through orifice (111). Housing section (21d) is adapted to receive spark port signal P.sub.s sensed at port (43) of induction passage (45) and cavity (122) communicates with the carburetor's induction passage (45) through housing passage (21d.sub.2 ') and conduit passage (115). A check valve (124) of standard design is disposed about the openings (123) of cavity (122) in order to insure an adequate power signal source regardless of variation in induction passage signal. An alternate source of power can be selected, namely, the intake manifold signal.

Referring to FIG. 5 an alternate embodiment of the invention is shown incorporating a limiting function which terminates EGR flow when intake manifold pressure P.sub.m approaches atmospheric pressure P.sub.a and engine air flow is high. The portions of the control valve assembly (20) which are the same as those shown in FIG. 1 carry the same reference numerals in FIG. 5. Chamber cavity (22d) is subject to a filtered source of atmospheric pressure P.sub.a communicated to chamber (22d) through passages (221) and filter element (222). For purposes of clarity only that portion of passage (21d.sub.3 ') that intersects with cavity (90) is shown. It is understood that as in FIG. 1, passage (21d.sub.3 ') communicates with chamber (15) of the EGR valve.

Control valve member (223) which is an alternate embodiment is referred to as a receiving or summing means and is comprised again essentially of three main parts, a stem portion (224), an adjustment member (225) and a second stem portion (226) incorporating a valve body (227). Stem portion (224) is fixedly secured to diaphragms (70) and (80) through distribution members (61), (71) and (261). Stem portion (226) is secured to diaphragm (60) through distribution member (61) and at its lower extremity incorporates valve body (227). Adjustment member (225) is threadedly disposed on one end of stem (224). The opposite end or upper portion of stem (224) includes valve seat (228). Stem (224) also has a central passage (229) which permits any signal in housing chamber (22b) to communicate with cavity (90) when valve body (227) is unseated from valve seat (228). It can readily be understood that when the force generated across diaphragm (60) acts in an upward direction, with a sufficient magnitude to overcome the spring force (130) valve body (227) will unseat from valve seat (228) and permit the signal present and chamber (22b) P.sub.m to be communicated through passage (229) to cavity (90) and through passage (21d.sub.3 ') to (10). (15) of EGR valve 10.

To simplify the understanding of the operation of the exhaust gas recirculation flow control system, the operation of the system immediately before the EGR flow control valve begins to open will first be considered. From this discussion it will be obvious to one skilled in the art that the amount of EGR recirculated to the intake manifold is a function of both engine air flow and intake manifold pressure. The function related to intake manifold pressure is obtained through controlling the relationship of the effective areas of the diaphragms. Therefore, once EGR begins to flow the overall effect of this function upon the control valve assembly can only be determined by looking at the net forces generated to open or close valve (27). It will be obvious to those skilled in the art from the discussion that follows as to how the above engine parameters affect the net force generated on the control valve assembly to thereby control the recirculation of exhaust gas into the intake manifold.

For discussion purposes, manifold vacuum, venturi vacuum, etc., will be regarded as pressures, recognizing that they are normally negative with respect to atmospheric pressure.

The control valve assembly serves to regulate the pressure P.sub.EGR which controls the opening and closing of the EGR valve. The position of the valve (27) with respect to the valve seat (92) is determined by the summation of forces on valve stem (24). These forces consist of spring forces (130) and (140), biasing the valve stem, and forces exerted by the three diaphragms (60), (70) and (80) under the action of the differential pressures across said diaphragms. As indicated above, diaphragm (60) has an effective area equal to A.sub.6, diaphragm (70) has an effective area equal to A.sub.7 and diaphragm (80) has an effective area of A.sub.8.

Carburetor venturi pressure P.sub.v is communicated from the carburetor through conduit passage (48) and housing passage (21a') to the top side of diaphragm (60) in chamber (22a). The pressure signal P.sub.v decreases as the engine air flow increases through the carburetor. The intake manifold pressure, P.sub.m, is communicated through conduit passage (36) and housing passage (21b') to chamber (22b) and to the underside of diaphragm (60) or top side of diaphragm (70). A filtered source of atmospheric pressure, P.sub.a, is communicated through passage (21d') to chamber (22d) or to the underside of diaphragm (80). Recirculated exhaust gas pressure, P.sub.e, is communicated from exhaust gas passage (32) between exhaust gas valve seat (38) and restriction (35), to the top side of diaphragm (80) in chamber (22c) through conduit passage (37) and housing passage (21c'). A pressure source, normally the spark port pressure, provides pressure signal P.sub.s to chamber (122) through housing passage (21d.sub.2 ') and conduit passage (115). This pressure source P.sub.s is maintained at the lowest attainable pressure at spark port (43) by check valve (124) which permits air flow only towards the intake manifold.

In the control valve assembly, an orifice (111) separates pressure P.sub.s from chamber (90) and therefore P.sub.EGR is some function of P.sub.s, said function being determined by the position of valve (27) relative to valve seat (92), or the net force on valve stem (24).

When valve (27) opens fully, P.sub.EGR becomes nearly equal to atmospheric pressure P.sub.a. Since the cross-sectional area (A.sub.9) of passage (91) is substantially greater than the cross-sectional area of orifice (111), atmospheric pressure, P.sub.a, is communicated from chamber (22d) through passage (91) and into cavity (90). When valve (27) is fully closed, only P.sub.s is communicated to cavity (90) and therefore, P.sub.EGR will equal P.sub.s. As pressure signal P.sub.EGR decreases and approaches P.sub.s, the force generated across diaphragm (13) of the EGR valve (10) increases sufficiently to overcome the biasing force F.sub.1 due to the spring (17) and the EGR valve will begin to open permitting exhaust gas to be recirculated into the intake manifold. When P.sub.EGR equals P.sub.s, the EGR valve will be fully open.

The functional relationship between recirculated exhaust gas flow and air flow through the engine can easily be seen by considering the effect of only air flow on the control valve assembly when the EGR valve is closed. Under this condition, P.sub.e is equal to P.sub.m and the net force generated on control valve member (23) is directly related to the venturi pressure P.sub.v in cavity (22a) and the atmospheric pressure P.sub.a in cavity (22d). The pressure differential P.sub.a - P.sub.v increases with increasing air flow through the carburetor. The effective areas of diaphragms (60) and (80) are essentially equal A.sub.6 = A.sub.8 and all three diaphragms act directly on the valve stem. Diaphragm (70) not having a differential pressure across it will therefore not generate a force. Thus, an increase in air flow increases the differential P.sub.a - P.sub.v and causes an increase in the upward force on the valve stem (24), tending to close valve (27) and thereby start EGR flow by reducing the influence of P.sub.a on P.sub.EGR.

When exhaust gas begins to flow in passage (32), a feedback signal tending to null the effect caused by the air flow through the carburetor is generated. The nulling effect is such that for a given air flow a certain EGR flow will be obtainable regardless of the variations in the EGR valve design parameters. This nulling or feedback effect is illustrated by considering the effect of exhaust gas flow when the EGR valve begins to open. As soon as the EGR valve opens and exhaust gases are permitted to flow in exhaust gas passage (32) past orifice (35), P.sub.e, the pressure upstream of the metering orifice, no longer equals P.sub.m, the pressure downstream of the metering orifice or intake manifold pressure. This results in a differential pressure P.sub.e - P.sub.m across diaphragm (70). This differential pressure P.sub.e - P.sub.m relates to EGR flow in the same way that P.sub.a - P.sub.v relates to air flow. The essential difference, however, is that P.sub.e and P.sub.m act not only on diaphragm (70) but also on diaphragms (80) and (60) respectively. Since the effective area of diaphragm (80) is greater than the effective area of diaphragm (70), an increase in P.sub.e will result in a downward force on valve stem (23) thereby tending to move valve (27) from its valve seat (92) open and permit pressure P.sub.a from chamber (22d) to be communicated to cavity (90) and through passage (21d.sub.3 ') to the EGR valve. The pressure in chamber (15) of the EGR valve will then increase and approach atmospheric pressure which will cause the EGR valve to close. The intake manifold pressure, P.sub.m, in cavity (22b) has a similar influence across diaphragm (60) but in the opposite direction of the force generated across diaphragm (80). It can therefore be seen that an increase in EGR flow, which results in an increase in the pressure differential P.sub.e - P.sub.m, which in turn will increase the downward force on the control valve member (23) causing valve (27) to move away from valve seat (92) thereby permitting pressure signal P.sub.a to be communicated to chamber (90) resulting in P.sub.EGR becoming a greater function of P.sub.a rather than P.sub.s and thereby beginning to close the EGR valve by reducing the force generated across diaphragm (13) in the EGR valve. The above described exhaust gas flow sensing function is referred to as the closed loop control of the exhaust gas flow recirculation system.

An additional function of the restriction (35) in the exhaust gas passage (32) is to limit the maximum amount of exhaust gas recirculated to the intake manifold. When the EGR flow becomes limited by the effect of orifice (35) the pressure differential P.sub.e - P.sub.m remains relatively constant and as a result further increases in air flow increases the closing force on the valve. Under these conditions the spark port pressure will continue to decrease and approach atmospheric pressure. Check valve (124) is designed to maintain P.sub.s at its former level rather than permit it to decrease and approach atmospheric pressure present at the spark port. The effect of check valve (124) maintaining P.sub.s at its former level results in the EGR valve maintaining its former position. Therefore, the combination of reservoir (122) and check valve (124) maintains an adequate pressure source regardless of the variations in engine air flow or engine manifold pressure when EGR flow is maximized and regardless of whether the reservoir is controlled by spark port pressure or by manifold pressure. The EGR valve is thereby enabled to be operative at very low pressure levels while still having the capability of delivering EGR flow at pressure levels nearly equal to atmospheric pressure.

The functional relationship between recirculated exhaust gas flow and intake manifold pressure is obtained by permitting the effective area of diaphragm (60) to be different than the effective area of diaphragm (80). This will become clear to one skilled in the art considering a condition where the effective areas of diaphragm (60) and diaphragm (80) have equal effective areas. Under this condition manifold pressure P.sub.m will have little effect on the EGR flow. Before EGR begins to flow P.sub.e equals P.sub.m, and the force contribution of diaphragm (70) to the overall net force on valve stem (23) is zero. If the area of diaphragm (60) is greater than that of diaphragm (80), an increase in manifold pressure P.sub.m (equal to P.sub.e) will contribute an upward force to the net force on valve steam (23) and assist in closing valve (27) causing EGR to start flowing at a lower air flow. As air flow increases, the effect is for EGR flow to remain higher at high values of intake manifold pressure P.sub.m (low vacuum) than at lower levels of intake manifold pressure.

It can therefore be easily seen that the net force operating on valve stem (23) can be independently controlled by selecting diaphragm (60) to have a larger effective area than diaphragm (80) and thereby cause intake manifold pressure in chamber (22b) to independently affect the force required to move valve (27) relative to valve seat (92). This is effectively illustrated in FIG. 6. Note that at six inches of manifold vacuum represented by curve D the exhaust gas recirculated to the intake manifold begins at a substantially lower air flow than that shown by curve E where the intake manifold vacuum is 14 inches of vacuum. Therefore, the percentage of EGR flow recirculated to the intake manifold is not only a function of air flow as described above, but also a function of intake manifold vacuum.

By modifying and changing the various design parameters shown herein and, for example, by using intake manifold pressure instead of spark port pressure as a source pressure in reservoir (122), different criteria for affecting "cut in " and "cut off" of EGR flow can be accomplished.

The alternate embodiment of control valve (20) shown in FIG. 6 provides the feature of a complete cut off function when the intake manifold pressure approaches atmospheric pressure and engine air flow is high. This is desirable to permit full utilization of the power potential of the engine which is not normally associated with urban driving. Under these conditions normally the engine air flow is high and the manifold pressure is relatively close to atmospheric pressure. At high engine air flows the pressure signal P.sub.v in chamber (22a) will be substantially lower than the pressure signal P.sub.m in chamber (22b). The net effect of the summation of these two pressure signals will be to cause an upward force on diaphragm (60). As diaphragm (60) deflects upwards valve (227) is caused to unseat from valve seat (228) thereby uncovering a passage through stem (224) which allows pressure signal P.sub.m to be communicated to chamber (90). The effect of communicating pressure signal P.sub.m, which is approaching atmospheric pressure, to chamber (90) will cause pressure signal P.sub.EGR to become a greater function of P.sub.m. Pressure signal P.sub.EGR is communicated to chamber (15) of EGR valve (10) and as P.sub.EGR approaches P.sub.m the balance of forces across diaphragm (13) of EGR valve (10) will equalize each other thereby permitting spring (17) to bias valve stem (12) and valve body (18) towards valve seat (38). The effect of this cut off function is to stop the recirculation of the exhaust gases through exhaust gas passage (32) to the intake manifold when P.sub.m approaches P.sub.a and engine air flow is high. Therefore, under these conditions full utilization of the engine's power potential is available and in effect provide more economical operational characteristics. As the air flow declines, the pressure differential P.sub.v - P.sub.m across diaphragm (60) will be insufficient to keep valve (227) unseated, and therefore valve (227) will seat on valve seat (228) and permit the net forces across the control valve assembly to again be a function of the engine's air flow and intake manifold pressure as earlier discussed.

As was described above, the temperature responsive element (50), as shown in FIG. 4, can be caused to react making the control valve responsive to low and/or high ambient or engine compartment temperature in order to inhibit EGR flow when either of these conditions exist. That is, as the engine compartment temperature decreases a predetermined temperature is eventually reached which will cause the bimetal strip to bend downwardly away from valve seat (96), thereby opening passage (95) to cavity (90) and permitting pressure signal P.sub.a to communicate with the pressure signal P.sub.EGR in cavity (90). This results in P.sub.EGR being a greater function of P.sub.a than P.sub.s and since P.sub.EGR is communicated through passage (21d.sub.3 ') and passage (16) to chamber (15) of the EGR valve, the EGR valve will begin to close. Temperature element (50) will maintain an open position until ambient temperatures have reached a predetermined level thereby inhibiting EGR flow below this predetermined level.

Whenever valve (27) as well as valve (97) is not in contact with its respective valve seat, there is a continuous flow of fresh air through passage (21b'), (91) or (95) into cavity (90) and into cavity (122). If this fresh air was permitted to continue to travel through passage (21d.sub.2 ') and into the spark port area of the carburetor these large amounts of fresh air would create air fuel ratio errors and adversely affect the operation of the engine. Orifice (111) is designed to serve the function of preventing large amounts of fresh air to flow into the carburetor spark port area. Check valve (124) as shown in FIG. 4, serves the purpose of preventing the flow of air from the spark port passage or alternately from the intake manifold should the intake manifold be used as a source pressure, to chamber (122). As earlier discussed, it also serves the function of maintaining P.sub.s at a level different than the spark port pressure.

Referring to FIG. 7, actual test results are graphically illustrated. Lines A, B and C represents plots of an EGR system designed for EGR flows of exactly 10, 20 and 30 percent of the engine air flow respectively. Line D is a plot of the disclosed system wherein intake manifold vacuum is 6 inches of mercury. Line E is a plot of the exhaust gas recirculated to the intake manifold at a vacuum level of 14 inches of mercury. As discussed above, the curves illustrated indicate the objects of the invention, since EGR is inhibited at low engine air flows and also EGR flow is illustrated as a function of not only engine air flow, but also intake manifold pressure.

Although only one preferred embodiment and an alternate embodiment showing the cut off function has been illustrated in the accompanying figures and description of the foregoing specification, it is especially understood that various changes may be made to the embodiment shown and described without departing from the spirit and scope of the invention as will now be apparent to those skilled in the art. Also, it will be apparent that changes may be made to the invention as set forth in the appended claims and in some instances certain features of the invention may be used to advantage without corresponding use of other features. For example, it was pointed out that the control of the control valve member (23) can be easily modified by changing some of the design parameters, (i.e. size of certain of the pressure sensitive elements contained in the housing (21)). Also, on a qualitative basis, control valve (23) can control the EGR flow directly. However, since the venturi pressure signals are relatively low and since the EGR valve must have a significant cross sectional area, this approach would require very large diaphragms and would thus be impractical. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.