Carburetor venturi vacuum and engine manifold vacuum controlled exhaust gas recirculating
United States Patent 3878823

The engine has a duct connecting the exhaust gas crossover passage to the intake manifold, the duct normally being closed by a load and speed sensitive valve whenever manifold vacuum acting on the valve exceeds an amplified carburetor venturi vacuum acting on the valve in the opposite direction; the duct being opened when the amplified venturi vacuum exceeds manifold vacuum except at high speed and load conditions.

Application Number:
Publication Date:
Filing Date:
Ford Motor Company (Dearborn, MI)
Primary Class:
International Classes:
F02M25/07; (IPC1-7): F02M25/06
Field of Search:
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Primary Examiner:
Myhre, Charles J.
Assistant Examiner:
Richter, Sheldon
Attorney, Agent or Firm:
Mccollum, Robert Zerschling Keith E. L.
I claim

1. An exhaust gas recirculating system for an internal combustion engine having a carburetor induction passage containing a venturi, comprising a duct connecting the exhaust gases to the engine intake manifold, and a speed and load responsive valve means movable by the differential force between engine manifold vacuum and an amplified carburetor venturi vacuum operatively acting on the valve to open the duct whenever amplified venturi vacuum exceeds manifold vacuum, a servo means for moving the valve, the servo having a first flexible diaphragm dividing the servo into a venturi vacuum chamber and an ambient air pressure chamber, a second flexible diaphragm further dividing the servo means into a manifold vacuum chamber in back-to-back relationship to the venturi vacuum chamber and a second ambient air pressure chamber, a third flexible diaphragm between the manifold and venturi vacuum chambers, and means connecting the diaphragms to the valve, the diaphragms being so constructed and arranged in effective area that the venturi vacuum acting against the diaphragms is amplified whereby an increase of the amplified venturi vacuum to a level exceeding the manifold vacuum moves the valve to open the duct.

This invention relates, in general, to an internal combustion engine. More particularly, it relates to a system for controlling the recirculation of exhaust gases back into the engine through the intake manifold.

Devices are known for recirculating a portion of the engine exhaust gases back through the engine to control the emission of unburned hydrocarbons and lower the output of oxides of nitrogen. These devices usually consist of a single valve that is spring seated to prevent recirculation of the exhaust gases at undesired times and opened by vacuum from a port above the carburetor throttle valve controlled by movement of the throttle valve so that recirculation is prevented during engine idle and wide-open throttle operations. This is desirable from an emissions output standpoint, because at engine idle exhaust gas scavenging is inefficient, while at wide-open throttle position, maximum power is limited by the availability of oxygen.

On the other hand, test results have shown that engine drivability or the engine's tolerance to exhaust gas recirculation (EGR) is best when the EGR is a fixed percentage of the airflow. Since airflow varies as a function of both speed and load, then the EGR flow should also vary in this manner.

The prior art devices, however, generally vary the EGR as a function of load only, by varying EGR flow as a function of manifold vacuum levels. Therefore, at light load operation, when only a small percentage of EGR is necessary for emission control, the maximum rate of EGR flow is obtained because ported vacuum is high causing the EGR valve to open wide. Similarly, at high engine speeds and loads, a high EGR flow rate can be tolerated without deteriorating engine drivability, and yet in the prior art devices, the low manifold vacuum present would only permit a low EGR flow rate.

Therefore, it is an object of this invention to provide an EGR device that varies EGR flow as a function of both speed and load so that the flow is a more nearly constant percentage of the airflow.

Airflow through a carburetor varies both as a function of speed and load. Manifold vacuum varies in inverse proportion to load, while carburetor venturi vacuum is nearly proportional to speed. Therefore, a device that will vary EGR flow as a function of the changes in both venturi and manifold vacuums can provide a nearly constant percentage of EGR flow to airflow.

It is another object of the invention, therefore, to provide an EGR device that varies EGR flow as a function of the differential in forces between carburetor venturi vacuum and manifold vacuum to provide good emission control with a minimum of sacrifice in engine drivability. Since venturi vacuum is far less than manifold vacuum at low speeds and loads, the venturi vacuum is amplified by the construction to be described so that changes in venturi vacuum can offset the manifold vacuum forces.

Other objects, features and advantages of the invention would become more apparent upon reference to the succeeding detailed description thereof, and to the drawings illustrating the preferred embodiment thereof, wherein;

FIG. 1 is a cross-sectional view of a portion of an internal combustion engine and associated carburetor embodying the invention;

FIG. 2 is a cross-sectional view taken on a plane indicated by and viewed in the direction of the arrows 2--2 of FIG. 1; and,

FIG. 3 is a chart plotting the change in carburetor vacuum with airflow.

FIG. 1 illustrates a portion 10 of one-half of a two-barrel carburetor of a known downdraft type. It has an air horn section 12, a main body portion 14, and a throttle body 16, joined by suitable means not shown. The carburetor has the usual air/fuel induction passage 18 open at their upper ends 20 to fresh air from the conventional air cleaner, not shown. The passages 18 have the usual fixed area venturies 22 cooperating with booster venturies 23 through which the main supply of fuel is induced, by means not shown.

Flow of air and fuel through induction passages 18 is controlled by a pair of throttle valve plates 24 each fixed on a shaft 25 rotatably mounted in the side walls of the carburetor body.

The induction passages also contain a manifold vacuum sensing port 26 and a venturi vacuum sensing port 28.

The throttle body 16 is flanged as indicated for bolting to the top of the engine intake manifold 30, with a spacer element 32 located between. Manifold 30 has a number of vertical risers or bores 34 that are aligned for cooperation with the discharge end of the carburetor induction passages 18. The risers 34 extend at right angles at their lower ends 36 for passage of the mixture out of the plane of the figure to the intake valves of the engine.

The exhaust manifolding part of the engine cylinder head is indicated partially at 38, and includes an exhaust gas crossover passage 40. The latter passes from the exhaust manifold, not shown, on one side of the engine to the opposite side beneath the manifold trunks 36 to provide the usual "hot spot" beneath the carburetor to better vaporize the air/fuel mixture.

As best seen in FIG. 2, the spacer 32 is provided with a worm-like recess 42 that is connected directly to crossover passage 40 by a bore 44. Also connected to passage 42 is a passage 46 alternately blocked or connected to a central bore or passage 48 communicating with the risers 34 through a pair of ports 50. Mounted to one side of the spacer is a cup-shaped boss 52 forming a chamber 54 through which passages 46 and 48 are interconnected.

As described above, it is necessary and desirable to provide some sort of control to prevent the recirculation of exhaust gases at undesirable times. For this purpose, passage 46 normally is closed by a valve 56 that is sensitive to both speed and load and moved to an open position by a servo 58.

FIG. 3 shows a typical carburetor flow curve plotting the changes in vacuum with airflow. Points A to B represent road load manifold vacuum changes and points C to D venturi vacuum changes at the same speeds and loads. The maximum flow capacity is represented by points D and B. At these points, the vehicle is at its maximum speed and power.

After reaching maximum speed/load, any additional load on the vehicle, such as climbing a hill, causes both the venturi and manifold vacuum to start dropping to point C. The throttle plate is wide open from D to C and B to C. The venturi vacuum generated by the airflow has the one curve C-D-C. The intake manifold vacuum can vary anywhere between curve A-B and curve B-C. For example, at 80 CFM, the part throttle vacuum is 13 inches Hg., and venturi vacuum 2 inches Hg. An increase of vehicle load will require the throttle plate be moved to the wide open position. As the throttle is opened, the manifold vacuum follows the curve E to T. However, with the speed (airflow) held constant, the venturi vacuum remains at 2 inches Hg. Thus, it will be seen that an exhaust gas recirculation system need be controlled as a function of changes in both speed and load if a finite control is desired, and one that regulates EGR flow rate correctly.

To meet this objective, the exhaust gas recirculation system controlling movement of valve 56 consists of two separate systems; a vacuum control system, and a valve body. The forces required to control the valve lift are the venturi vacuum and the manifold vacuum. Venturi vacuum is a function of vehicle speed (airflow through the carburetor), whereas manifold vacuum is a function of engine load at any airflow. Since the venturi signal is far less than the manifold vacuum at low speeds and loads, a magnification of the forces created by the venturi vacuum will be required.

As seen in FIG. 2, the servo 58 includes an upper shell 64 divided by an edge mounted flexible diaphragm 66 into two chambers 68 and 70. Chamber 68 is an air chamber, being vented at 72. Chamber 70 is a manifold vacuum chamber, and is connected to the manifold vacuum sensing port 26 in FIG. 1 by a passage 74. A larger lower shell 76 has an opening 78 in its upper surface, and is also divided by an edge mounted flexible diaphragm 79 into a vacuum chamber 80 and an air chamber 82. The vacuum chamber in this case is connected to venturi vacuum sensing port 28 in FIG. 1 by a passage 84. The opening 78 is closed by a further edge mounted flexible diaphragm 86, and there is a light positioning spring 87. All of the diaphragms are fixed to the valve plunger or stem 88 of valve 56, which slidably and sealingly projects through a plate 90 closing chamber 54. The valve per se is a spool like valve having spaced lands 92 and 94 for a purpose to be described.

The vacuum system thus consists of two vacuum chambers; manifold vacuum chamber 70 and venturi vacuum chamber 80, separated by three diaphragms 66, 86 and 79. The total area of these diaphragms are important in determining the magnification of forces. Since the venturi vacuum signal is extremely low at light speeds and load, a predetermined magnification of this force is required to offset the forces created by the manifold vacuum. This magnification is dependent upon EGR requirement for NOx reduction and good driveability.

Assume, for example, that to move valve stem 88 upwards at a predetermined speed and load requirement, it will require a magnification of ten of the forces created by the venturi vacuum. This factor of ten is established as follows: ##EQU1## For Example: ##SPC1##

if D(86) = 0.5 inch

then D(79) = 2.785 and D(66) - 1 inch

These are the sizes of the three diaphragms (66), (86) and (79) that will establish the size of this valve.

In operation, at idle with high manifold vacuum and low venturi signal, the forces on valve 56 are downwards forcing the valve land 92 on seat 96 and preventing flow of gas through passage 48. The forces created by spring 87 are chosen to be negligible. As the vehicle speed and load increases, the increasing venturi vacuum force with its ten magnification equals and then finally exceeds the forces created by the manifold vacuum. This moves valve stem 88 upwards and allows exhaust gas to flow to the intake manifold through passage 48. The valve 56, therefore, remains open as long as the venturi vacuum forces exceed the forces created by the intake manifold vacuum, up to wide open throttle position. The lower valve land 94 will prevent flow of exhaust gas through passage 48 at high speed and load requirements. As valve stem 88 moves upwards to its maximum flow position, land 94 will engage seat 98 and prevent flow of gas.

From the foregoing, it will be seen that the higher the load and speed, the more EGR is added, without materially reducing engine drivability. Thus, at idle speed, no EGR occurs; at light loads and low speeds, only a small amount of EGR occurs, at heavier loads/higher speeds, a maximum amount of EGR is added, and, finally, at wide open throttle/heavy loads/high speed operation, no EGR occurs because the valve land 94 shuts off flow.

While the invention has been shown and described in its preferred embodiment, it will be clear to those skilled in the arts to which it pertains, that many changes and modifications may be made thereto without departing from the scope of the invention.