Exhaust gas recirculation system for internal combustion engines

Abstract

Apparatus for electronically controlling exhaust gas recirculation in an internal combustion diesel engine to reduce nitrogen oxide emissions. An exhaust gas recirculation control valve is driven by a feedback circuit which compares the valve position with the engine load. A maximum amount of exhaust gas is recirculated under a no-load condition. As the load increases to 100%, the amount of recirculated exhaust gas is decreased down to zero percent. The amount of recirculated exhaust gas also may be affected by the engine speed to minimize smoke in the engine exhaust.

Description

BACKGROUND OF THE INVENTION

This invention relates to controlling exhaust gas emissions from internal combustion diesel engines and in particular to controlling the emission of oxides of nitrogen in such exhaust gas. The worldwide population increase and the uncontrolled increased use of mechanization in everyday living has caused increased concern about the environment. Governments have only recently been regulating to protect the environment from pollutants such as those produced by internal combustion engines. The exhaust from internal combustion engines consists of various constituents including fully oxidized products of combustion such as carbon dioxide and water plus undesirable pollutants such as partially oxidized, cracked and other hydrocarbons, carbon monoxide, oxides of nitrogen and traces of miscellaneous other pollutants. The carbon dioxide and water emissions are unharmful. However, the other exhaust emission constituents are considered highly undesirable.

It is generally understood that the presence of nitrogen oxides in engine exhaust is determined by the combustion temperature. An increase in combustion temperature causes an increase in the amount of nitrogen oxides present in the engine exhaust. It is therefore desirable to control the combustion temperature to limit the oxides of nitrogen present in the exhaust of an internal combustion engine. One method suggested in the prior art for limiting or controlling the combustion temperature has been to recirculate a portion of the exhaust gas back to the engine air intake. Since the exhaust gas is low in oxygen, this will result in a richer combustion mixture which will burn at a lower temperature. The lower combustion temperature will, in turn, reduce the amounts of nitrogen oxides produced during combustion.

The operating conditions which result in the highest combustion temperatures depend upon the type of internal combustion engine. In a spark-ignited gasoline engine, for example, the combustion temperature will be at a low point during idle. The temperature will also be at a low point at wide open throttle since engines of this type will normally have a rich fuel mixture under this condition. Ideally, there should be no exhaust gas recirculation while the engine is idling. From an idle, the amount of recirculated exhaust gas should increase up to a partial load condition and decrease from such partial load condition to a wide open throttle. In a diesel engine, on the other hand, a maximum combustion temperature occurs during a no-load condition at idle. Therefore, it is desirable to have a maximum amount of exhaust gas recirculation during idle and to decrease this amount as the load increases to 100% of the rated load.

Various types of controls for exhaust gas recirculation have been suggested in the prior art. U.S. Pat. No. 2,456,213 which issued on Dec. 14, 1948 to Plec, for example, teaches an early type control system for use with a diesel engine. In this system, the recirculated exhaust gas flows through two separate series connected valves. One valve is pneumatically controlled in response to the intake manifold vacuum and the other valve is mechanically controlled in response to engine speed. The net result of the two controls is that the recirculated exhaust gas is controlled in response to engine load, which is a function of both intake vacuum and engine speed. In another prior art recirculation control system, such as is shown in U.S. Pat. No. 3,703,164 which issued on Feb. 19, 1971 to Weaving, a third valve is provided for each cylinder for introducing exhaust gas directly into the cylinder. A mechanical valve is provided in series between the engine exhaust and all of the valves which introduce the exhaust gas into the cylinders to control the amount of exhaust gas recirculated. Various systems have also been adapted to spark-ignited internal combustion engines. However, each of the prior art recirculation systems has incorporated a mechanical control operating from devices such as cams, centrifugal speed sensors, pneumatic vacuum or pressure sensors, and the like. Although prior art systems reduce the nitrogen oxide components in engine exhaust gas, they generally will not produce an accurate control resulting in a minimum amount of nitrogen oxide under all operating conditions and all use mechanical controls which can be unreliable.

SUMMARY OF THE INVENTION

According to the present invention, an electronic control is provided for exhaust gas recirculation in internal combustion engines. The control is particularly suitable for meeting the various requirements of a diesel engine to reduce nitrogen oxide emissions. The control includes a sensor which measures the load on the engine. The engine load signal is used as a primary control over the position of an exhaust gas recirculation valve. Preferably, a feedback circuit is provided for sensing the position of the exhaust gas recirculation valve. The valve position is compared with the engine load for generating a signal which operates a valve drive motor. The control circuit is also provided with means for maintaining the recirculation valve wide open or in a maximum recirculation condition whenever the load is at a minimum or the engine is idling.

The control may also be modified to further increase its efficiency in reducing nitrogen oxide and other emissions. An engine speed sensor may be provided for modifying at higher engine speeds the rate at which the exhaust gas recirculation valve is closed under increasing load conditions. The speed sensor may further be adapted to assure that the recirculation valve is closed when a predetermined maximum engine speed is exceeded and the load on the engine is simultaneously at a minimum, as when the engine is coasting at a high speed. Such controls will have the advantage of not only minimizing the nitrogen oxide emissions, but also minimizing the amount of smoke produced by the engine which, although not considered highly harmful, is also undesirable.

Accordingly, it is a preferred object of the invention to provide an electronic control over exhaust gas recirculation in internal combustion engines to minimize nitrogen oxide emissions.

A further object of the invention is to provide an improved exhaust gas recirculation system for diesel engines.

Still another object of the invention is to provide an improved exhaust gas recirculation control for diesel engines which operates at maximum efficiency under various conditions of load and engine speed for reducing nitrogen oxide emissions.

Other objects and advantages of the invention will become apparent from the following detailed description, with reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an internal combustion engine incorporating an exhaust gas recirculation system constructed in accordance with the present invention;

FIG. 2 is a detailed schematic block diagram of an exhaust gas recirculation control constructed in accordance with a first embodiment of the invention;

FIG. 3 is a graph showing typical signals generated for operating the drive motor for the exhaust gas recirculation valve;

FIG. 4 is a chart showing typical operating characteristics for a second embodiment of a control for an exhaust gas recirculation system according to the present invention; and

FIG. 5 is a detailed schematic block diagram of the second embodiment of an exhaust gas recirculation control according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings and particularly to FIG. 1, a block diagram is shown for an exhaust gas recirculation system 10 embodying the principles of the present invention. The system 10 is shown connected to a conventional diesel engine 11 which, for example, may be of the type used in trucks, construction machinery and the like. The diesel engine 11 is provided with an air intake 12 and an exhaust gas outlet 13. The exhaust gas outlet 13 is connected through a valve 14 to an exhaust pipe 15 which leads to the atmosphere. The exhaust pipe 15 may include a sound muffling system (not shown). The valve 14 includes an adjustable flap 16 which diverts a controlled portion of the exhaust gas through a return pipe 17 to the air intake 12.

One or more sensors 18 are connected to the engine 11 for measuring at least the engine load and, in a preferred embodiment, also the engine speed. The output of the sensors 18 is applied to an electronic controller 19 which drives a motor 20 for positioning the valve flap 16. A feedback circuit is preferably provided for assuring a positive operation of the exhaust gas recirculation system 10. The feedback circuit includes a valve position sensor 21 which applies a signal to the electronic controller 19 which corresponds to the position of the valve flap 16. The electronic controller 19 is designed to compare the signal corresponding to the actual position of the valve flap 16 with a desired position as determined by the engine load conditions monitored by the sensors 18 and to use a resultant signal for driving the valve drive motor 20. Generally, the electronic controller 19 drives the motor 20 to position the valve flap 16 for permitting a predetermined maximum amount of exhaust gas recirculation under a no-load condition and to linearly decrease the amount of exhaust gas recirculation by closing the valve flap 16 as the engine load increases until at 100% rated load substantially no exhaust gas is recirculated through the return pipe 17. The electronic controller 19 is also provided with means for assuring that the valve flap 16 is at its maximum open position for recirculating a maximum amount of exhaust gas whenever either the engine load or the engine speed is less than a predetermined amount. This will assure that the valve flap 16 will remain open during idle of or minimum load on the engine.

Referring now to FIG. 2, a schematic block diagram is shown for the exhaust gas recirculation system 10 with details of connections for an electronic controller 19 for performing the above-described functions. As previously indicated, it is necessary to sense the load on the engine. For a diesel engine, the load may be readily measured from the position of the rack which controls the amount of fuel injected into each cylinder. The rack position may be measured by various conventional manners. For example, a linear voltage differential transformer may be connected to have a core move by the rack. However, this arrangement has the disadvantage of placing a load on the rack which may affect the engine operation, particularly where the rack position is adjusted by a sensitive governor. Preferably, the rack position is determined by a proximity detector or other device which need not be connected directly to the rack.

As shown in FIG. 2, the load sensor 18 consists of a proximity probe 25 which senses the location of a cam surface 26 carried by the rack (not shown). An oscillator 27 is connected to provide a relatively low frequency alternating current signal to an input of the proximity probe 25. The alternating current signal may, for example, be on the order of 3 kHz., although the frequency is not critical and will depend upon the probe design. The proximity probe 25 will then have an output 28 of the same frequency as that of the oscillator 27 and of a magnitude which is inversely dependent upon the spacing from the cam 26.

The output 28 from the probe 25 is connected through a buffer amplifier 29 to a demodulator 30 which may, for example, consist of a half wave rectifier. The demodulator 30 has an output 31 which is applied through a low pass filter 32 to one input of a compare circuit 33. The low pass filter 32 should have a sufficiently low cut-off as to filter any engine vibrations present on the cam 26. The low pass filter 32 is designed to only partially filter the demodulator output 31, leaving a ripple on the signal applied to the compare circuit 33 which is of the same frequency as the output from the oscillator 27. The output 31 from the demodulator 30 is also connected through a diode 34 and a variable resistor 35 to ground. The variable resistor 35 may be used for adjusting the gain or load span over which the exhaust gas recirculation valve 14 is operated.

The output from the oscillator 27 is also connected to the valve position sensor 21 which consists of a proximity probe 36. A cam 37 is connected to be driven by the valve drive motor 20 along with the adjustable valve flap 16. The proximity probe 36 senses the spacing to the cam 37 and generates an output 38 which is inversely proportional to such spacing. Thus, the proximity probe output 38 will be of the same frequency as the output from the oscillator 27 and will increase in value as the cam 37 approaches the proximity probe 36.

The proximity probe output 38 is connected through a buffer amplifier 39 to a demodulator 40. A steady DC voltage is also applied to the demodulator 40 from a potentiometer 41 which is connected between a voltage source and ground. The steady DC voltage biases the demodulator 40 for establishing a zero point for the system 10. The output of the demodulator 40 is connected through a low pass filter 42 to a second input of the compare circuit 33 for comparison with the output from the low pass filter 32. In a typical diesel engine, the peak engine vibration is on the order of between 20 Hz. and 50 Hz. Such vibrations may cause the valve flap 16 to flutter and therefore should be filtered from the output 38 from the proximity probe 36. It has been found that a cut-off frequency of 33 Hz. for the low pass filter 42 is effective for applying a substantially constant signal to the compare circuit 33. This signal has a magnitude which is determined by the position of the valve flap 16.

The output of the compare circuit 33 is applied through an amplifier 43 to a bipolar power switch 44 which controls power to the valve drive motor 20. The operation of the compare circuit 33 and the bipolar power switch 44 for operating the motor 20 may be more clearly understood by referring now to FIG. 3. Graph A in FIG. 3 shows typical inputs appearing on the comparator 33. The low pass filter 32 applies a signal 48 to the comparator 33 while the low pass filter 42 applies a signal of the type shown as 49a, 49b or 49c to the comparator 33. The comparator 33 may consist of a Schmitt trigger which acts as a threshold detector and has one of two outputs, depending upon which of its two inputs is highest. If during the majority of the time the valve position signal is above or nearest the highest ripple level of the load signal 48 as shown at 49a, then a signal of the type shown in FIG. 3B is applied to the bipolar power switch 44. If the ripple level of the load signal 48 is 50% above and 50% below the valve position signal as shown at 49b, then a signal of the type shown in FIG. 3C is applied to the bipolar power switch 44. If the majority of the ripple in the engine load signal 48 is above the valve position signal, as shown at 49c, then a signal of the type shown in FIG. 3D is applied to the bipolar power switch 44. The bipolar power switch 44 will apply a signal to the valve drive motor 20 similar to those shown in FIGS. 3B, C and D. These signals alternate between equal positive and negative voltages. If the output of the bipolar power switch 44 is of the type shown in FIG. 3C, then there will be a 50% duty cycle and the motor 20 will stand still since the motor 20 is not capable of following the three kilohertz pulses and therefore will not oscillate. However, if a comparison of the valve position and the load indicates that the valve should be opened or closed, the duty cycle will change from that shown in FIG. 3C towards one of those shown in FIGS. 3B or 3D. When the power applied to the motor 20 is of the format shown in FIG. 3B, a negative input will be applied to the motor 20 for a greater period of time than a positive input and the motor 20 will be driven in one direction. When the power applied to the motor 20 is changed to the format shown in FIG. 3D, the signal on the motor 20 will be positive for a greater time than it is negative and the motor will be driven in the opposite direction. As the valve flap 16 is driven to a position satisfying the demands of the engine load, the duty cycle of the power applied to the motor 20 will approach 50--50 and the motor torque and speed will decrease.

In general, the valve 14 should be operated in a linear fashion with linear load changes. It is desirable to have maximum recirculation of exhaust gas at no-load and to have a minimum recirculation at 100% rated load with the amount of recirculation varying linearly between these points. Although the valve 14 may be of a linear type, it is less expensive to use a non-linear valve. Non-linearities in the operation of the valve 14 may then be compensated for by providing an appropriately shaped surface on the cam 37 which is rotated with the valve flap 16.

As previously discussed, it is desirable to maintain maximum exhaust gas recirculation when the engine is supplying a minimum load. This may be accomplished by the addition of a second comparator 50 to the circuit of FIG. 2. The comparator 50 compares the output of the low pass filter 32 with a fixed voltage obtained from a potentiometer 51. The potentiometer 51 is used for establishing a trip point at which the recirculation valve 14 is driven to a fully open condition. When the engine fuel rack moves below the trip point, the output from the comparator 50 changes. This output is applied to the comparator 33 along with the valve position signal from the low pass filter 42. The comparator 33 will then have a constant output regardless of its input from the low pass filter 32 and will cause the motor 20 to drive the valve 14 to its fully open state.

By adding additional controls over the positioning of the exhaust gas recirculation valve 14, the exhaust gas recirculation system 10 may be made even more efficient. For example, a diesel engine is typically operated in the range of 1,800 to 2,800 rpm. If the engine is used, for example, in a truck, the maximum normal operating speed may be exceeded when the truck is coasting down a hill. During coasting, the driver will normally let up on the accelerator. Since the rack indicates a no-load condition, the exhaust gas recirculation valve 14 will normally be driven fully open. Under these conditions, the engine may emit smoke when the driver again hits the accelerator. Therefore, a control may be added to drive the exhaust gas recirculation valve 14 to a fully closed position when the engine is under a no-load condition and the engine speed exceeds a preselected maximum speed, such as 3,000 rpm. A control may also be provided to drive the exhaust gas recirculation valve 14 to a fully open position at a preselected minimum engine speed, such as 1,200 rpm, regardless of the load on the engine. Still another control may be provided to minimize smoke in the normal operating range, which, although being appreciably less harmful than nitrogen oxide emissions, is undesirable. It has been determined that less exhaust gas recirculation can be tolerated at lower engine speeds than at higher engine speeds to prevent the engine from smoking. This is due primarily to the fact that at higher engine speeds a considerably higher volume of air passes through the engine. Therefore, a control may also be added to shift the linear curve on which the exhaust gas recirculation valve 14 is operated in response to a predetermined engine speed. For example, smoking may be minimized if the exhaust gas recirculation valve 14 recirculates between 35% and 0% of the exhaust gas when the engine is operating between zero load and full load below 1,950 rpm and to recirculate between 50% and 0% of the exhaust gas when the engine is operating between no-load and full load above 1,950 rpm. This type of operation of the exhaust gas recirculation valve 14 is shown in the graph in FIG. 4.

Turning now to FIG. 5, a detailed block diagram is shown for apparatus 52 including the above-described controls. The apparatus 52 includes an oscillator 53 having an output applied through a buffer amplifier 54 to the inputs of proximity probes 55 and 56. The proximity probe 55 is mounted to measure the position of a cam or lobe 57 carried by the fuel rack (not shown) which controls the fuel injectors in a diesel engine. The output of the proximity probe 55 is in the form of an alternating current signal having a level inversely proportional to the spacing to the cam 57 and the same frequency as the oscillator 53. This output is applied through an amplifier 58 to a demodulator 59. The output of the demodulator 59 is filtered by means of a low pass filter 60 and applied to one input of a comparator 61. The gain of the signal applied to the comparator 61 may be controlled by means of a diode 62 and a variable resistor 63 connected between the input of the low pass filter 60 and ground in a manner similar to that described above for FIG. 2.

The proximity probe 56 is connected to measure the spacing to a cam 64 which is driven by the valve drive motor 20 along with the flap 16 of the exhaust gas recirculation valve 14. The alternating current output from the proximity probe 56, which is indicative of the position of the valve 14, is applied through an amplifier 65 to a demodulator 66. A DC voltage is applied from a potentiometer 67 to the demodulator 66 for zeroing the apparatus 52. The output from the demodulator 66 is applied through a low pass filter 68 to a second input of the comparator 61. The output of the comparator 61, which alternates between two levels, is applied through an amplifier 69 and a bipolar switch 70 for driving the valve motor 20. The operation of the circuitry described so far for FIG. 5 is the same as that described above for the circuitry shown in FIG. 2. However, a number of additional controls are added for further reducing both smoke and nitrogen oxide emissions.

The additional controls added in FIG. 5 require a signal indicative of the speed at which the engine is running. Therefore, a lobed cam 75 is mounted to be driven by the engine. A magnetic pickup 76 is positioned adjacent the lobed cam 75 for generating a pulse train having a frequency proportional to the engine speed. The output of the magnetic pickup 76 is applied to a frequency-to-voltage converter 77 which generates a DC output 78 having a voltage level proportional to the engine speed.

The converter output 78 is applied to one input of a comparator 79. A second input to the comparator 79 is a fixed voltage determined by a potentiometer 80. The comparator 79 will have one of two outputs which depend upon whether or not the converter output 78 is above or below the voltage set by the potentiometer 80. The potentiometer is adjusted such that the output from the comparator 79 will change levels at a relatively low engine speed, such as 1,200 rpm. The output from the comparator 79 is applied along with the output from the low pass filter 60 to the comparator 61. When the output from the comparator 79 changes due to the engine dropping below the preset speed, a signal is applied to the comparator 61 to drive the valve 14 to a fully opened condition, allowing a maximum amount of exhaust gas recirculation regardless of the position of the engine fuel rack. When the preset speed is exceeded, the comparator 79 will not affect operation of the exhaust gas recirculation valve 14. Thus, the magnetic speed pickup 76, the converter 77 and the comparator 79 function to assure that the valve 14 is fully opened whenever the engine is idling, regardless of load on the engine.

As previously indicated, another desirable operating condition is to have the valve 14 maintained in a fully closed position whenever the speed of the engine exceeds it normal operating range and simultaneously the fuel injection rack is at a minimum or low load setting. This is accomplished by means of a pair of comparators 81 and 82 and an AND gate 83. The output 78 from the converter 77, which is proportional to engine speed, is applied to one input of the comparator 81. A potentiometer 84 applies a fixed voltage to the second input of the comparator 81 for determining the engine speed at which the output from the comparator 81 changes. Typically, the potentiometer 84 will be adjusted such that the output from the comparator 81 changes levels when the engine speed slightly exceeds its normal operating range, such as 3,000 rpm. The output from the comparator 81 is applied to one input of the AND gate 83. The comparator 82 has one input which is connected to the output of the low pass filter 60, which output is indicative of the position of the fuel injection rack, and a second input connected to a fixed DC voltage source such as a potentiometer 85 connected between a voltage source and ground. The potentiometer 85 is adjusted such that the output of the comparator 82 will change when the fuel injection rack is positioned for a minimum load. When the output of the comparator 81 indicates that the engine speed has exceeded the preselected maximum value and the output of the comparator 82 indicates that the fuel injection rack is at a predetermined minimum load position, the AND gate 83 will apply a signal to the comparator 61 for causing the motor 20 to drive the valve to a fully closed position. Thus, there will be minimal exhaust gas recirculation under these conditions. This will prevent the engine from emitting a cloud of smoke when the engine is again accelerated.

The output 78 from the converter 77 is also connected to a third comparator 86. A potentiometer 87 applies a fixed DC voltage to a second input of the comparator 86. The potentiometer 87 is adjusted such that the output from the comparator 86 changes when the engine passes through a preselected midrange speed, such as 1,950 rpm. When this preselected speed is exceeded, the comparator 86 sinks current from an attenuator 88 and reduces the oscillator signal to the proximity probe 56, thereby shifting the linear curve along which the valve 14 is operated. As previously discussed, two typical curves are shown in the graph of FIG. 4 for the operation of the valve 14. When the engine is run below 1,950 rpm, the maximum exhaust gas recirculation for one engine was found to be about 35% for no-load for providing a good balance between smoke and nitrogen oxide pollutants. Thus, at idle or any speed below 1,950 rpm, about 35% of the exhaust gas is recirculated for a no-load condition of the engine. As the load on the engine increases to 100% of its rated value, the recirculated exhaust gas is linearly decreased to 0% or to some other predetermined low level. At higher engine speeds, additional exhaust gas may be recirculated without having adverse affects on engine smoking. Therefore, above 1,950 rpm, about 50% of the exhaust gas is recirculated for a no-load condition and this is decreased linearly to 0% recirculation for a 100% load. It will, of course, be appreciated that these percentages may vary for any given engine as well as the engine speed at which the curves are shifted. Furthermore, there may be several shifts in the recirculation curve for different engines or the curve may vary continuously with engine speed.

Although not shown in the drawings, it will be appreciated that various other modifications may be made in the above-described exhaust gas recirculation system. For example, it may be desirable to protect the motor when the valve is driven to a fully open or a fully closed position. This could be accomplished, for example, by the addition of circuitry which places a limit on the maximum time at which the motor can be operated at full power in either direction and to then reduce the current. This will not only protect the motor when it is driven fully closed or fully opened, but is would also protect the motor should the valve become jammed.

It should also be appreciated that the above-described exhaust gas recirculation system may also be adapted for use with other types of internal combustion engines, such as spark-ignited gasoline engines. The conditions under which the exhaust gas recirculation is controlled will, of course, be modified for the requirements of any particular engine. For example, a spark-ignited gasoline engine may require maximum recirculation at a mid-speed range rather than at minimum load and idle conditions.

Various other modifications and changes may be made to the present invention without departing from the spirit and the scope of the following claims.

Claims

What we claim is:

1. For an internal combustion engine having an air intake and an exhaust, an exhaust gas emission control system comprising, in combination, valve means for selectively recirculating a portion of the exhaust gas to the air intake, said valve means moving between a first position wherein a predetermined maximum amount of the exhaust gas is recirculated and a second position wherein a predetermined minimum amount of the exhaust gas is recirculated, means for generating an electric signal indicative of a load on the engine, and electrical means positioning said valve means at predetermined intermediate positions between said first and second positions in response to such signal for reducing the oxides of nitrogen present in the engine exhaust gas.

2. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 1, wherein said valve positioning means includes electrical control means for moving said valve means to said first position when the load on the engine is less than a predetermined value and for moving said valve means progressively towards said second position as the load progressively increases above such predetermined value.

3. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 2, and further including means for moving said valve means to said first position when the engine is running at less than a predetermined minimum speed.

4. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 3, and including means responsive to such electric signal and to the engine speed for moving said valve means to said second position when the load on the engine is less than a predetermined value and simultaneously the engine exceeds a predetermined speed.

5. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 4, and further including means responsive to the speed of the engine for modifying said predetermined maximum amount of exhaust gas recirculated when said valve means is in said first position.

6. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 1, including means for generating an electric signal indicative of the position of said valve means, and wherein said valve positioning means includes an electric motor connected to drive said valve means, means for comparing said engine load signal and said valve position signal and means responsive to such comparison for controlling said valve drive motor.

7. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 6, wherein said valve positioning means further includes means responsive to said engine load signal for generating a third signal when the load on the engine is less than a predetermined minimum, and means for causing said valve drive motor to move said valve means to said first position in response to said third signal.

8. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 6, and further including means for generating an electric signal when the engine is running at less than a predetermined minimum speed, and means for causing said valve drive motor to move said valve means to said first position in response to said minimum speed signal.

9. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 6, and further including means for generating an electric signal when the load on the engine is less than a predetermined minimum, means for generating an electric signal when the engine speed exceeds a predetermined maximum, and means responsive to the simultaneous occurrence of said minimum load signal and said maximum speed signal for causing said valve drive motor to move said valve means to said second position.

10. An exhaust gas emission control system for an internal combustion engine, as set forth in claim 6, and further including means for generating a third electric signal when the engine exceeds a predetermined speed, and means responsive to said third signal for increasing said predetermined maximum amount of exhaust gas recirculated when said valve means is in said first position.