US3603297A

US3603297A - Throttle control

Info

Publication number: US3603297A
Application number: US862720A
Authority: US
Inventors: Harry A Sherwin; Ronald E Herman
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-10-01
Filing date: 1969-10-01
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07

Abstract

A carburetor having an induction passage with a rotatable throttle valve therein is provided with a vacuum responsive throttle stop assembly which is effective for engaging the throttle lever in order to at times maintain the throttle valve partly opened during deceleration; a vehicle speed responsive valve member serves to dissipate available vacuum below a predetermined vehicle road speed in order to permit the stop assembly to hold the throttle valve partly opened only during deceleration occurring at or above a predetermined vehicle road speed.

Description

United States Patent '2] [mentors Harry A. Sherwin 2,988.074 6/1961 Lobdellet a], 123/978 11724 Arne] Drive. Warren. Mich. 48093; 3027384 4/1962 Bale, Jrv et a1. .1 123/97 B Ronald E. Herman. 5342 Streekerk Road, 3.289.659 12/1966 Koole .7 123/97 B Warren, Mich. 48092 3,486,491 12/1969 White l 123/97 B X [11 A 1 N 862.720 3,491,737 1/1970 Burnia 123/97 B 1 pp 969 22 Filed ct. 1 1 t 7 Primary Exammer-Wendell E. Burns [45] Paemed Sept 1971 Attorney-Walter Potoroka. Sr.

[54] THROTTLE CONTROL 3 Claims, 5 Drawing Figs.

[52] 123/97 ABSTRACT: A carburetor having an induction passage with a 92/64'92/89'92'99'92H l23/119- rotatable throttle valve therein is provided with a vacuum [51 1 Fozd 9/00 responsive throttle stop assembly which is effective for engag- Folb /01 Folb 19/00 ing the throttle lever in order to at times maintain the throttle [50] held of Search 123/77 valve partly opened during deceleration; a vehicle speed I I 92/89' l l 17 responsive valve member serves to dissipate available vacuum 7 below a predetermined vehicle road speed in order to permit [56] References cued the stop assembly to hold the throttle valve partly opened only UNITED STATES PATENTS during deceleration occurring at or above a predetermined 2.957.463 10/1960 Schnabel 123/97 8 vehicle road speed.

252 a? re 39 52 16 14 THROTTLE CONTROL EACKGROUND OF THE INVENTION Because of recent governmental regulations and a general concern for the elimination or reduction of atmospheric pollutants great efforts have been expended by the automotive industry in devising engine and carburetor improvements which would at least substantially reduce the degree of emission of unburned hydrocarbons into the atmosphere.

One of the major advances of the industry has been the development of what is commonly referred to as a positive crankcase ventilation (PCV) system which, in essence, ventilates or communicates the engine crankcase fumes to the engine intake in order to further burn such fumes instead of merely freely venting such fumes to the atmosphere as has been'done in the past.

However, even with advances in that art as above, a degree of engine exhaust emission does occur and it has been found that such emission often takes place during vehicle deceleration where the vehicle is actually driving the engine. Of course, the higher the vehicle speed at which such deceleration takes place so also the higher the degree of exhaust emission.

Accordingly, the invention as herein disclosed is concerned with the solution of the problem of exhaust emission during engine deceleration as well as other related problems.

SUMMARY OF THE INVENTION According to the invention, a control for a vehicle equipped with an air-consuming internal combustion engine and an apparatus for controlling the rate of flow of said air to said engine, said apparatus including air induction passage means formed therethrough for communication with an air intake portion of said engine and variably openable throttle valve means for controlling the rate of flow of said air through said induction passage means, said control comprising first differential pressure responsive means effective for at certain times permitting said throttle valve means to be moved to a nominally closed position and at other times effective to operatively engage said throttle valve means in order to maintain said throttle valve means in a partially opened position and preclude movement of said throttle valve means to said nominally closed position, a source of vacuum, conduit means for communicating said vacuum to said first differential pressure responsive means, and second means effective during all vehicle speeds below a predetennined vehicle speed for preventing the application of the full degree of said vacuum to said first differential pressure responsive means in order to cause said first differential pressure responsive means to permit said throttle valve means to be moved to said nominally closed position, said second means also being effective during all vehicle speeds at least equal to said predetermined vehicle speed for enabling the application of a greater degree of said vacuum to said first differential pressure responsive means in order to have said first differential pressure responsive means become effective for precluding movement of said throttle valve means to said nominally closed position.

Accordingly, a general object of the invention is to provide means effective during selected periods of engine operation for automatically assuring a minimal opening of the throttle valve.

A more specific object of the invention is to provide means effective during vehicular deceleration at vehicle speeds above a selected vehicle speed, for causing the throttle valve to be partly opened.

Other more specific objects and advantages of the invention will become apparent when reference is made to the following detailed description considered in conjunction with the accompanying drawings.

DESCRIPTION OF. THE DRAWINGS In the accompanying drawings, wherein certain details may, for purposes of clarity, be omitted from one or more views:

FIG. 1 is a somewhat diagrammatic representation of a vehicle equipped with the invention;

FIG. 2 is an enlarged elevational view, with portions broken away and crosshatched, of a carburetor diagrammatically shown in FIG. 1;

FIG. 3 is an enlarged axial cross-sectional view of a vehicle speed sense device also shown in FIG. 1;

FIG. 4 is an enlarged axial cross-sectional view of a throttle modulator assembly, shown also in both FIGS. 1 and 2, in one of its operating positions; and

FIG. 5 is a view similar to FIG. 4 but illustrating the throttle modular assembly in another of its operating positions.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in greater detail to the drawings, FIG. 1 somewhat schematically illustrates a vehicle 10 comprised of an internal combustion engine 12 having an intake manifold 14 with a suitable carburetor assembly 16 situated on the intake manifold and in communication therewith. A transmission assembly 18 connected at one end to the engine 12 has an output shaft 20 operatively connected to ground

engaging driving wheels

22 and 24.

The transmission assembly 18 may be provided with suitable means such as at 26 by which a driving connection may be made with the output shaft means 20 as by suitable motion transmitting cable or shaft means .28 so as to convey a rota tional speed to a speed sensing assembly 30 corresponding to the vehicle speed. As generally illustrated in FIG. 1, the carburetor assembly 16 may be provided with a suitable lever 32 secured for rotation with a carburetor throttle shaft and adapted at times to come into abutting engagement with a pressure responsive stop assembly 34. As will be noted, the speed sense assembly 30 is connected by means of suitable conduitry to a source of engine vacuum, as at 40, the stop as sembly 34 and a source of atmosphere as the interior of the air cleaner assembly 37 situated atop carburetor 16. This may be accomplished as by a conduit 39 communicating with source 40 and having branch conduit portions 35 and 41 respectively leading to the stop device 34 and speed sense 30, while a separate conduit 43 leads from the speed sense 30 to the interior of the air cleaner assembly 37.

FIG. 2 illustrates in somewhat greater detail, the carburetor assembly 16 as being comprised of a carburetor body 42 having an induction passage 44 formed therethrough with the conventional venturi section 46 and an air inlet 48 end in which is situated a rotatably positionable choke valve 60 carried by a choke shaft 52. (The choke valve 50 and shaft 52 may be positioned by any suitable control arrangement operatively connected thereto as by linkage 54 and choke shaft lever 56.)

The discharge end 58 of the induction passage 44 is controlled by a variably positionable throttle valve 60 carried by a throttle shaft 62 so that rotation of throttle shaft 62 causes a corresponding rotation of the throttle valve 60. The carburetor body 42 may include a suitable fuel bowl or reservoir 64 with the usual mechanism associated therewith. The throttle lever 32, secured at one end to throttle shaft 62 for rotation therewith, is operatively connected by suitable linkage means 66 to, for example, the operator controlled foot throttle lever 68 within the associated vehicle 10. Suitable schematically illustrated spring means 70 serves to return the throttle valve toward its nominally closed or curb idle position whenever the foot throttle lever 68 is released. Such spring means 70, of course, may be located wherever practical and need not be located at the foot throttle lever 68.

As generally shown in FIG. 2, whenever the throttle lever 32 is released so as to permit the throttle valve 60 to return toward its curb idle or nominally closed position, a projecting FIG. 3 illustrates the speed sense assembly 30 as comprising a housing 80 with upper and lower-housing sections82 and 84 secured to each other and forming internally thereof a cavity or chamber 86 of a size and configuration to permit the rotation therein of the pneumatic resistor assembly 88.

The pneumatic resistor assembly 88 is comprised generally of a main driving shaft 90 mounted for rotation within housing 80 as by

bearings

92 and 94 within lower housing section 84. A first radially extending generally tubular arm portion 96, provided with aligned apertures 98 and 100 by which it is mounted onto shaft 90, has an end provided with an externally threaded portion 102 by which a second radially directed and generally tubular arm portion 103, having an internally threaded portion 104, is secured thereto.

A generally cylindrical valve member 106, contained within arm section 103, is connected by means of a cross-pin anchor 108 to one end 110 of a tension spring 112 having its other end 114 connected to an adjustably positioned spring anchor 116. As illustrated the spring anchor 116 is externally threaded for coaction with a threaded portion 118 formed internally of arm section 96 and is further provided with a slot 120 for receiving therein the blade of a screwdriver in order to efiect axial adjustment of the anchor 116 by threadable rotation thereof within arm section 96. Once proper adjustment of the spring anchor 1 16 is achieved, the open end ofarm 96 may be closed by a threaded sealing plug assembly 122.

As can be seen, the outermost end of arm section 103 is closed except for a passage 124 formed therethrough which is provided with a valve seat surface internally of the arm section 103 intended to at times coact with a contoured valving portion 126 carried at the radially outer end of valve member 106. As shown, valve member 106 includes a plurality of radially directed conduits 128 formed through the wall thereof so as to communicate with the axial passage 130 within valve member 106 as well as the space between the exterior of the body of valve member 106 and the interior of arm section 103. Normally, the spring end 110, passing through the radial passage 132 formed in driving shaft 90, serves to hold the radially innermost end 134 of valve member 106 against the seat surface 136 of the externally threaded end of arm section 96. In addition to radial passage 132, shaft 90 has an axially extending conduit or passage 138 formed therein communicating at one end with passage 132 and, at another point therealong, with a second radial conduit 140 in continual communication with a chamber 142 within housing section 84 generally circumscribing the shaft 90. The chamber 142 is, in turn, in communication with a source of manifold vacuum via conduit 144. In contrast to this, conduit 146 serves to complete communication between a source of atmospheric pressure, such as within the carburetor air cleaner assembly, and the chamber 86.

The opposite ends of shaft 90 are provided with driving-type sockets 148 and 150 thereby enabling the detachable connection thereto of a first drive cable or shaft 28 leading from the transmission 18 and a second driven cable or shaft 152 at the other end'in order to thereby drive some other associated accessory'siich as the vehicle speedometer. Further, housing section -84 may be provided with an external threaded portion 154 as for connection to a drive cable assembly while housing section 82 may have provided a nut member 156 for securing the cable assembly 152 thereto. If desired, the entire housing :80 may be secured or mounted to any suitable related structure with the use of fasteners and tapped holes 158. Of course, as is evident in FIG: 3, suitable seals and retainers may be provided where deemed to be appropriate.

FIGS..4 and each illustrate the modulating throttle stop assembly in one of its operating positions. Referring in greaterdetail to FIGS. 4 and 5, it can be seen that assembly 34 is generally comprised of a housing assembly 160 formed of housing sections 162, is provided with a necked-down portion 178 so as to define an annular flange or shoulder 180.

Diaphragm plates

170 and 172, formed with accommodating apertures 1751and 177 therethrough, are mounted on the necked-down portion 178 and maintained against the shoulder 180 by means of an oppositely disposed peened-over portion 182. Baselike stem member 174, in addition to threaded portion 78, also has a tubular extension 184 which is adapted to be operatively connected to branch conduit 35 so as to thereby complete communication between conduit 35 and chamber 171 via axial conduit 186 and radial conduit means 188 formed in base or stem member 174. As will be seen, at certain periods of operation, vacuum will be'communicated from conduit 35, via

conduits

186 and 188, to chamber 171; in contrast, chamber 173, preferably, is in constant communication with the ambient atmosphere as by vent means 190.

Housing section 164 is formed to provide a generally cylindrical extension 192, terminating in end 74, having a cylindrical passageway 194 formed therein so as to slidingly receive therein the cylindrical projection 196 of stem member 174. As illustrated at 198, the cylindrical extension or projection 196 may be formed with a flatted portion so as to cooperate with a flatted keylike portion 200 formed within passageway 194. Flatted

portions

198 and 200 cooperate as guide or key means in order to prevent relative rotation between stem member 174 and housing assembly while a compression spring 202, contained within chamber 171 and situated generally about stem member 174, serves to continually urge end surface 204 of cylindrical passageway 194 toward end surface 206 of projection 196. In order to effect a seal between housing section 162 and stem member 174, a bootlike seal 208 may be provided so as to have a first end suitably retained on stem 174 and its other end secured to or retained on the projecting end 210 of housing section 162.

As shown in each of FIGS. 4 and 5, the interior of housing section 164 is provided with a plurality of projecting abutment portions 212, adapted to engage diaphragm plate 172 as in FIG. 4, while the interior of housing section 162 is provided with a plurality of second abutment portions 214, adapted to engage diaphragm plate as in FIG. 5. The entire assembly 34 may be secured to the carburetor body 42 as by end 216 of bracket 76 being retained to threaded stem portion 78 by oppositely disposed

nuts

218 and 220 threadably engaged therewith.

Operation Before discussing the operation of the overall system, it might be best to first consider the operations of the components as shown in FIGS. 3, 4 and 5.

The speed sense unit 30 of FIG. 3 is, in effect, a variable restrictor presenting varying restrictions to flow therethrough (whether such flow be a liquid or gaseous) generally in accordance to the speed sensed within a predetermined speed range. That is, with the elements of FIG. 3 in the positions illustrated, it can be seen that communication is completed between conduits 144 and 146. For example, a flow path exists, starting at conduit 144, through chamber 142, conduits 140, 138 and 132, passageway 130, radial passages 128 in valve body 106, through the clearance between valve body 106 and the interior of arm 103, through aperture 124 into chamber 86 and to conduit 146. Such communication continues until the centrifugal force of valve body 106, due to the increased rate of rotation of shaft 90 and

arm sections

96 and 103, is sufficient to move valve body 106, against the resistance of spring 112, radially outwardly toward aperture 124. As valve body 106 approaches aperture 124, valving portion 126 will tend to restrict the flow through orifice 124 and, of course, if valving portion 126 is moved fully against aperture 124, so as to be seated thereagainst, all flow through orifice 124 will be terminated.

As illustrated in both FIGS. 1 and 3, shaft 90 is rotated, as by drive member 28, so as to have a rotational speed in accordance with and dependent upon vehicle speed, while conduit portion 146 is illustrated as being connected, via conduit 43, to a suitable source 37 of atmospheric pressure and conduit 144 is, via conduit 41 communicating with a source of engine vacuum 40 in a manner so as to be generally in parallel relationship relative to variable stop or throttle modulator assembly 34.

In view of the above, it can be seen that whenever the variable restrictor assembly 30 is rotated at a speed below a predetermined speed, any vacuum applied to conduit 144 from source 40 is dissipated because of the communication with the ambient atmosphere existing through orifice 124. However, once sufficient vehicle speed is attained, the flow through aperture 124 will be restricted and the vacuum from source 40 will then be applied, via

conduits

39, 35, 186 and 188, to chamber 171 of throttle modulator assembly 34.

From FIGS. 4 and 5, it can be seen that the central portion of diaphragm 168 and

plates

170 and 172 is locked against movement to stem 174 while the housing assembly 160 is free to move axially with respect to stem 174. Accordingly, when either no vacuum or an insufficient degree of vacuum is communicated to chamber 171, via conduit 188, spring 202, seated against backing plate 170, urges the housing assembly 160 to the left, as viewed in FIG. 4, with respect to stem 174 and maintains the housing assembly 160 in the position illustrated by having abutment portions 212 engage backing plate 172. For purposes of reference, it can be seen that at this time end surface 206 of stem projection 196 is at a distance D away from end surface 204 of cylindrical passageway 194.

However, when vacuum of sufficient degree is communicated to chamber 171, the resulting pressure differential across housing section 162 becomes sufficient to overcome the force of spring 202 thereby causing the spring 202 to be compressed as the entire housing assembly" 160 moves to the right. Such motion of housing assembly 160 continues until abutment portions 214 engage the opposite backing plate 170 as illustrated in FIG. 5. It should be noted that at this time the distance, D between respective end surfaces 206 and 204 is significantly greater than distance, D,, of FIG. 4.

Referring now to FIG. 2, the throttle modulator assembly 34 is secured to and adjusted relative to support bracket 76 so as to have end 74 of the housing assembly 160 engage the lateral arm portion 72 of throttle lever 32 when the throttle valve 60 has been closed to its nominally closed or curb idle position. This is done with the modulator assembly 34 being in the condition depicted by FIG. 4.

Accordingly, with the various components situated and connected as illustrated by FIGS. 1 and 2, the operation of the overall system is as follows:

When the vehicle is in motion but operating below a predetermined road speed, the throttle valve 60 will be opened, via throttle control 68, linkage 66, throttle lever 32 and throttle shaft 62, to some part throttle position and the vacuum within intake manifold 14 will be communicated via port 40 to conduit 39 and branch conduits 35 and 41. However, since the vehicle is operating below the predetermined road speed, the rotational speed of the variable restrictor 88 is insufficient to cause the valving portion 126 to restrict the flow of atmospheric air through orifice 124. Consequently, the magnitude of the vacuum supplied from port 40 is sufficiently dissipated to preclude movement of the modulator housing assembly 160 from the position shown in FIGS. 2 and 4.

However, when vehicle speed is increased (as by a reduction in the road load or the further opening of the throttle valve 60) to equal or exceed the established predetermined road speed, valving member 126 moves radially outwardly to sufficiently restrict (or even terminate) flow through orifice 124 thereby enabling the application of a sufficient degree of vacuum to chamber 171 of the throttle modulator assembly 34. This, in turn, causes the modulator housing assembly 160 to move to the right assuming a position as shown in FIG. 5.

The housing assembly will remain in such an extended position until the vacuum within chamber 171 is dissipated which, in turn, will only occur when the vehicle speed becomes less than the predetermined road speed at which time valving member 126 and aperture 124 serve to effectively bleed the vacuum to atmosphere.

Accordingly, it can be seen that if throttle actuator 68 is released and throttle valve 60 is permitted to return toward its curb idle position while the vehicle 10 is operating at speeds equal to or above the predetermined road speed, arm portion 72 of throttle lever 32 will strike and abut against end surface 74 of housing assembly 160 while the housing assembly 160 is in an extended position, relative to stem 174, as illustrated in FIG. 5. Consequently, because of such an actuated condition of assembly 34, rotation of throttle lever 32 and throttle valve 60 toward the curb idle position is stopped short of attaining such a curb idle position thereby causing the throttle valve 60 to be held opened some desired amount greater than the opening afforded by the normal curb idle position of the throttle valve. As already indicated, the throttle lever and throttle valve 60 will remain in such a partly opened position during deceleration until the vehicle speed becomes less than the predetermined road speed at which time, the vacuum being dissipated by the variable resistor 88, modulator housing assembly 160 once more moves to the position illustrated in FIG. 4 allowing the throttle lever 32 and throttle valve 60 to rotate further closed to the normal curb idle position.

In view of the above, it can be seen that the purpose of the throttle modulator assembly 34 is to hold the throttle valve opened in a predetermined amount during vehicle decelerations above a predetermined roa-d speed. It has been discovered that such opening of the throttle valve helps to reduce engine exhaust emissions during vehicular deceleration. Generally, the preferred embodiment has two working or operating positions, namely, (1) fully extended, as shown in FIG. 5, when vehicle speeds are above a predetermined road speed and (2) fully retracted when vehicle speeds are below a predetermined road speed or manifold vacuum is below a given value.

Although it has not been shown, for purposes of clarity, it is nevertheless to be understood that the throttle lever 32, or linkage associated therewith, may be provided with the usual throttle idle stop screw and that such stop screw (usually movable with the throttle lever) may be employed for abutting against some fixed stop in order to determine the curb idle position of the throttle valve once the modulator housing assembly 160 has fully retracted to the position shown in FIG. 4.

The use of such an adjustable throttle idle stop screw or the end 74 of modulator housing assembly 160 to determine the curb idle position of the throttle valve 60 is, in view of the disclosure made herein, believed to be a matter of choice either one of which is well within the spirit and scope of the invention.

Although the variable restrictor speed sense assembly 30 has been illustrated, it is nevertheless contemplated that the function of the restrictor or speed sense 30 can be fulfilled by other suitable means. For example, solenoid operated valve means could be employed for relatively venting or applying the actuating vacuum to modulator chamber 171, and related speed responsive switching means could, in turn be employed to create and apply an electrical control signal to the solenoid operated valve.

Although only one preferred embodiment of the invention has been specifically disclosed and described, it is apparent that other embodiments and modifications are possible without exceeding the scope or spirit of the invention.

We claim:

1. A control for a vehicle equipped with an air-consuming internal combustion engine and an apparatus for controlling the rate of flow of said air to said engine, said apparatus including air induction passage means formed therethrough for communication with an air intake portion of said engine and variably openable throttle valve means for controlling the rate of flow of said air through said induction passage means, said control comprising first differential pressure responsive means having at least first and second operating positions and being eflective for at certain times to assume said first operating position and permit said throttle valve means to be moved to a nominally closed position and at other times effective to assume said second operating position and operatively engage said throttle valve means in order to maintain said throttle valve means in a partially opened position and preclude movement of said throttle valve means to said nominally closed position, a source of vacuum, conduit means for communicating said vacuum to said first differential pressure responsive means, and second means effective during all vehicle speeds below a predetermined vehicle speed for preventing the application of the full degree of said vacuum to said first differential pressure responsive means in order to cause said first differential pressure responsive means to permit said throttle valve means to be moved to said nominally closed position, said second means also being effective during all vehicle speeds at least equal to said predetermined vehicle speed for enabling the application of a greater degree of said vacuum to said first differential pressure responsive means in order to have said first differential pressure responsive means become effective for precluding movement of said throttle valve means to said nominally closed position, said apparatus for controlling the rate of flow of said air to said engine comprising a carburetor, said source of vacuum comprising engine vacuum, said variably openable throttle valve means comprising a rotatably positionable throttle valve situated within said induction passage means on a transversely extending pivotally mounted throttle shaft, said first differential pressure responsive means comprising a hollow stemlike base portion fixedly secured with respect to said carburetor, a first flexible wall carried by said base and fixed with respect thereto, a second wall carried by said base 50 as to be movable with respect thereto, connecting means operatively interconnecting said first and second walls so as to define a chamber therebetween exposed to said engine vacuum, a movable abutment portion operatively connected to said second wall and movable therewith, including lever means connected to said throttle shaft for rotating said throttle shaft and throttle valve toward opened and closed positions, said movable abutment portion being placed in the path of travel of said lever means whenever said second wall is moved toward said first wall through the action of said engine vacuum communicated to said chamber through said hollow stemlike base.

2. A pressure responsive motor assembly, comprising a hollow base member adapted to be fixedly secured to related structure, a first flexible wall carried by said base member and fixed against movement with respect thereto, a second wall carried by said base member and situated thereon as to be movable with respect thereto, said first and second walls being operatively connected to each other in order to define an actuating chamber therebetween, and a motion transmitting member operatively connected to said second wall and disposed on an opposite side of said first wall, said second wall being effective, upon experiencing a pressure differential thereacross occasioned by the admission of a relatively low fluid pressure to said chamber through said hollow base, to move toward said first wall and to impart corresponding motion to said motion transmitting member.

3. A pressure responsive motor assembly according to claim 2, wherein said base comprises an axially extending stem member including a threaded portion for adjustably securing said stem to said related structure, wherein said stem includes conduit means for completing communication between an associated source of actuating fluid pressure and said actuating chamber, wherein said first wall includes a flexible sealing portion operatively connected to said second wall, and including keying means for permitting relative axial motion between said second wall and said stem member but precluding relative angular motion therebetween.

Claims

1. A control for a vehicle equipped with an air-consuming internal combustion engine and an apparatus for controlling the rate of flow of said air to said engine, said apparatus including air induction passage means formed therethrough for communication with an air intake portion of said engine and variably openable throttle valve means for controlling the rate of flow of said air through said induction passage means, said control comprising first differential pressure responsive means having at least first and second operating positions and being effective for at certain times to assume said first operating position and permit said throttle valve means to be moved to a nominally closed position and at other times effective to assume said second operating position and operatively engage said throttle valve means in order to maintain said throttle valve means in a partially opened position and preclude movement of said throttle valve means to said nominally closed position, a source of vacuum, conduit means for communicating said vacuum to said first differential pressure responsive means, and second means effective during all vehicle speeds below a predetermined vehicle speed for preventing the application of the full degree of said vacuum to said first differential pressure responsive means in order to cause said first differential pressure responsive means to permit said throttle valve means to be moved to said nominally closed position, said second means also being effective during all vehicle speeds at least equal to said predetermined vehicle speed for enabling the application of a greater degree of said vacuum to said first differential pressure responsive means in order to have said first differential pressure responsive means become effective for precluding movement of said throttle valve means to said nominally closed position, said apparatus for controlling the rate of flow of said air to said engine comprising a carburetor, said source of vacuum comprising engine vacuum, said variably openable throttle valve means comprising a rotatably positionable throttle valve situated within said induction passage means on a transversely extending pivotally mounted throttle shaft, said first differential pressure responsive means comprising a hollow stemlike base portion fixedly secured with respect to said carburetor, a first flexible wall carried by said base and fixed with respect thereto, a second wall carried by said base 50 as to be movable with respect thereto, connecting means operatively interconnecting said first and second walls so as to define a chamber therebetween exposed to said engine vacuum, a movable abutment portion operatively connected to said second wall and movable therewith, incluDing lever means connected to said throttle shaft for rotating said throttle shaft and throttle valve toward opened and closed positions, said movable abutment portion being placed in the path of travel of said lever means whenever said second wall is moved toward said first wall through the action of said engine vacuum communicated to said chamber through said hollow stemlike base.