Abstract:
A vehicle is provided with an exhaust gas recirculation system and an automatic transmission which shifts in accordance with a plurality of shift patterns including a shift pattern in which upshift occurs at relatively low vehicle speeds and a shift pattern in which upshift occurs at relatively high vehicle speeds. The exhaust gas recirculation system includes an exhaust gas recirculation control mechanism which reduces the amount of exhaust gas to be returned to the combustion chamber when the former shift pattern is selected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an exhaust gas recirculation system for an automatic transmission vehicle, and more particularly to an exhaust gas recirculation system for a vehicle provided with an automatic transmission having a plurality of shift patterns. 
     2. Description of the Prior Art 
     There has been in wide use an automatic transmission in which gear-shifting is automatically effected in accordance with a shift pattern predetermined according to the vehicle speed and the throttle opening. Recently, there has been put into practice an automatic transmission in which gear-shifting is automatically effected in accordance with a shift pattern chosen from a plurality of shift patterns by the driver. For example, when a shift pattern in which upshift occurs at relatively low vehicle speeds is selected, fuel economy can be improved since the engine runs mainly in relatively low rpm ranges. On the other hand, when a shift pattern in which upshift occurs at relatively high vehicle speeds is selected, the engine runs mainly in relatively high rpm ranges and, accordingly, a powerful driving force can be obtained. 
     However, when the vehicle having a plurality of shift patterns is provided with an exhaust gas recirculation system, there arises a problem that the operating condition of the engine is adversely affected by the exhaust gas returned to the combustion chamber when the shift pattern in which upshift occurs at relatively low vehicle speeds is selected. That is, in the exhaust gas recirculation system, a part of the exhaust gas, which is inert, is returned to the combustion chamber to lower the combustion temperature when the combustion temperature is high, thereby reducing formation of NOx. However, when the amount of exhaust gas returned to the combustion chamber is too large, the operating performance of the engine is adversely affected. Therefore, the amount of exhaust gas to be returned to the combustion chamber must be controlled according to the operating condition of the engine. However, conventionally the amount of exhaust gas to be returned to the combustion chamber has not been controlled in accordance with the rpm range to be mainly used, and accordingly, when the shift pattern in which a relatively low rpm range is mainly used is selected, the operating condition of the engine is deteriorated since the operating condition of the engine is more apt to be affected when the engine runs at low rpm. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing observations and description, the primary object of the present invention is to provide an exhaust gas recirculation system for a vehicle provided with an automatic transmission which shifts in accordance with a plurality of shift patterns in which system the amount of exhaust gas to be returned to the combustion chamber is controlled so as not to adversely affect the operating performance of the engine when the shift pattern in which upshift occurs at relatively low vehicle speeds is selected. 
     In accordance with the present invention, a vehicle is provided with an exhaust gas recirculation system and an automatic transmission which shifts in accordance with a plurality of shift patterns including a shift pattern in which upshift occurs at relatively low vehicle speeds (which shift pattern will be referred to as &#34;economy pattern&#34;, hereinbelow), and a shift pattern in which upshift occurs at relatively high vehicle speeds (which shift pattern will be referred to as &#34;power pattern&#34;, hereinbelow). The exhaust gas recirculation system includes an exhaust gas recirculation control means which reduces the amount of exhaust gas to be returned to the combustion chamber when said economy pattern is selected. The exhaust gas recirculation system may reduce to zero the amount of exhaust gas returned to the combustion chamber or stop recirculation of exhaust gas, if necessary. For example, the exhaust gas recirculation system controls the negative pressure for driving a recirculation control valve of the exhaust gas recirculation system to reduce the amount of exhaust gas to be returned upon receipt of a signal indicating that the economy pattern is selected. 
     Since combustion temperature is low and formation of NOx is inherently little in the low engine rpm range, reduction of the amount of exhaust gas returned to the combustion chamber in the low engine rpm range does not increase NOx emission. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic view for illustrating an internal combustion engine provided with an exhaust gas recirculation system in accordance with an embodiment of the present invention, 
     FIG. 2 shows an example of the power pattern and the economy pattern, 
     FIG. 3 is an enlarged cross-sectional view of the EGR valve employed in the engine shown in FIG. 1, 
     FIG. 4 is an enlarged cross-sectional view of the modulator valve employed in the engine, 
     FIG. 5 is a flow chart for illustrating the operation of the control circuit employed in the engine, and 
     FIG. 6 is a flow chart for illustrating the operation of the control circuit employed in the engine in accordance with a modification of the engine shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, an internal combustion engine 10 is provided with an exhaust gas recirculation system 20 in accordance with an embodiment of the present invention and an automatic transmission 50 which shifts in accordance with a plurality of shift patterns. 
     The control system of the automatic transmission 50 includes a gear-shifting control circuit 53 and a lockup control circuit 54. A throttle opening signal A and a turbine rpm signal B are input to the gear-shifting control circuit 53 and the lockup control circuit 54 respectively from a throttle opening sensor 51 which detects the opening degree of the throttle valve 12 in the intake passage 11 of the engine 10 and a turbine rpm sensor 52 which detects the rotational speed of the output shaft of the torque convertor (turbine shaft) in the automatic transmission 50. The control system further includes a shift pattern changing device 55 which delivers a shift pattern signal C to the control circuits 53 and 54. 
     In this particular embodiment, the automatic transmission 50 shifts in accordance with a power pattern shown in FIG. 2-(I) or an economy pattern shown in FIG. 2-(II). As can be understood from FIG. 2, upshifts from first to second, from second to third and from third to fourth for a given throttle opening occur at lower speeds in accordance with the economy pattern than in accordance with the power pattern. At the same time, the lockup ranges for the third and the fourth in accordance with the economy pattern are lower than those in accordance with the power pattern. Thus, the engine 10 generally runs in a lower rpm range when the economy pattern is used than when the power pattern is used. 
     A hydraulic circuit 56 for the automatic transmission 50 includes first to fourth solenoids 57 1  to 57 4 . The gear-shifting control circuit 53 delivers a gear-shifting control signal D to the first to third solenoids 57 1  to 57 3 , and the lockup control circuit 54 delivers a lockup control signal E to the fourth solenoid 57 4 . That is, the automatic transmission 50 is shifted and the lockup clutch of the torque converter is engaged or disengaged according to the throttle opening and the turbine rpm (representing the vehicle speed) represented by the signals A and B and in accordance with the shift pattern selected by way of the shift pattern changing device 55. 
     The exhaust gas recirculation system 20 comprises an EGR (exhaust gas recirculation passage) 21 connecting the exhaust passage (not shown) with the intake passage 11 downstream of the throttle valve 12, an EGR (exhaust gas recirculation valve) 22 provided in the EGR passage 21 and a modulator valve 23 for controlling the negative pressure to be fed to the EGR valve 22. A first negative pressure introducing passage 24 directly communicates a portion of the intake passage immediately upstream of the throttle valve 12 with the modulator valve 23 so that negative pressure is introduced into the modulator valve 23 according to the throttle opening. A second negative pressure introducing passage 25 communicates a portion of the intake passage 11 immediately upstream of the throttle valve 12 with the modulator valve 23 by way of a three-way switching valve 26. The modulator valve 23 and the EGR valve 22 are communicated with each other by a control negative pressure feed passage 27, and an exhaust pressure introducing passage 28 communicates a portion of the EGR passage 21 immediately upstream of the EGR valve 22 with the modulator valve 23. 
     When the economy pattern is selected, the shift pattern changing device 55 delivers an economy pattern signal F to the three-way switching valve 26. The three-way switching valve 26 opens the second negative pressure introducing passage 25 to an atmosphere upon receipt of the economy pattern signal F. 
     As shown in FIG. 3, the EGR valve 22 comprises a valve body 31 which is movable to open and close the EGR passage 21, a diaphragm 32 to which the valve body 31 is connected, and a spring 33 which urges the diaphragm 32 in the closing direction of the valve body 31. When a negative pressure is introduced into the negative pressure chamber 34 above the diaphragm 32 (as seen in FIG. 3) by way of the control negative pressure feed passage 27, the diaphragm 32 is displaced in the direction of the arrow a overcoming the spring 33 by an amount corresponding to the negative pressure introduced into the negative pressure chamber 34, whereby the opening degree of the EGR valve 22 is controlled. 
     As shown in FIG. 4, the modulator valve 23 comprises a negative pressure chamber 42 and an exhaust pressure chamber 43 divided by a diaphragm 41. Said second negative pressure introducing passage 25 is connected to the negative pressure chamber 42 and said exhaust pressure introducing passage 28 is connected to the exhaust pressure chamber 43. A spring 44 for urging the diaphragm 41 toward the exhaust pressure chamber 43 is provided in the negative pressure chamber 42. Further, said first negative pressure introducing passage 24 is communicated with the control negative pressure feed passage 27 in the modulator valve 23, and a branch passage 45 branches off from the junction thereof. The free end of the branch passage 45 projects into the negative pressure chamber 42 and is opposed to the center of the diaphragm 41. 
     When negative pressure is introduced into the modulator valve 23 through the first negative pressure introducing passage 24 only (i.e., when the opening degree of the throttle valve 12 is relatively small), the negative pressure acts on the diaphragm 41 by way of the branch passage 45 and exhaust pressure acts on the diaphragm 41 by way of the exhaust pressure introducing passage 28, whereby the diaphragm 41 is displaced in the direction of the arrow b overcoming the force of the spring 44 to close the open end of the branch passage 45. Accordingly, the negative pressure in the first negative pressure introducing passage 24 is fed to the EGR valve 22 through the control negative pressure feed passage 27, thereby moving the valve body 31 to open the EGR passage 21 to permit recirculation of exhaust gas into the intake passage 11. When the EGR passage 21 is opened, the exhaust pressure introduced into the exhaust pressure chamber 43 of the modulator valve 23 through the exhaust pressure introducing passage 28 is lowered, and the diaphragm 41 is displaced in the direction opposite to the direction of the arrow b under the force spring 44, whereby the branch passage 45 is opened to relieve the negative pressure. Accordingly, the negative pressure introduced into the EGR valve 22 is lowered and the valve body 31 of the EGR valve 22 closes the EGR passage 21. The EGR valve 22 thus repeats opening and closure of the EGR passage 21 and exhaust gas is returned to the intake passage 11 through the EGR passage 21 according to the period of the opening and closure of the EGR passage 21. When the negative pressure is introduced into the modulator valve 23 through both the first and second negative pressure introducing passages 24 and 25 (i.e., when the opening degree of the throttle valve 12 is large), the diaphragm 41 is pressed against the open end of the branch passage 45 to completely close the branch passage 45 by the negative pressure introduced into the negative pressure chamber 42 through the second negative pressure introducing passage 25, and accordingly, the negative pressure in the first negative pressure introducing passage 24 is constantly fed to the EGR valve 22 through the control negative pressure feed passage 27, whereby the EGR valve 22 is held open and the amount of exhaust gas returned to the intake passage 21 is increased. That is, as the opening degree of the throttle valve 12 is increased and negative pressure comes to be introduced into the modulator valve 23 also through the second negative pressure introducing passage 25, the amount of exhaust gas returned to the intake passage 11 is increased. 
     The relation between the shift pattern selected by way of the shift pattern changing device 55 and the amount of exhaust gas to be returned to the intake passage 11 will be described with reference to the flow chart shown in FIG. 5 which shows the operation of the gear-shifting control circuit 53 and the lockup control circuit 54. As shown in FIG. 5, the control circuits 53 and 54 determine which shifting pattern has been selected, the economy pattern or the power pattern. (Steps S2 and S3) When it is determined that the power pattern has been selected, a power pattern map such as one shown in FIG. 2-(I) is read and upshift control, downshift control and lockup control are effected in accordance with the power pattern. (Steps S4, S6, S7 and S8) When the power pattern is selected, the amount of exhaust gas to be returned to the intake passage 11 is increased with the increase in the throttle opening in the manner described above. On the other hand, when it is determined that the economy pattern has been selected, an economy pattern map such as one shown in FIG. 2-(II) is read and upshift control, downshift control and lockup control are effected in accordance with the economy pattern. (Steps S5, S6, S7, and S8) When the economy pattern is selected, the shift pattern changing device 55 delivers an economy pattern signal F to the three-way switching valve 26 as described above. The three-way switching valve 26 opens the second negative pressure introducing passage 25 to the atmosphere upon receipt of the economy pattern signal F. Accordingly, even when the opening degree of the throttle valve 12 is large, negative pressure cannot be introduced into the negative pressure chamber 42 of the modulator valve 23 through the second negative pressure introducing passage 25, and the EGR valve 22 repeats opening and closure of the EGR passage 21. Thus in the case where the economy pattern is selected, the amount of exhaust gas to be returned to the intake passage 11 is not increased with increase in the throttle opening. 
     In the embodiment described above, when the economy pattern is selected by the shift pattern changing device 55, the shift pattern is locked to the economy pattern irrespective of the operating condition of the vehicle. In the embodiment shown in FIG. 6, even if the economy pattern is selected by the shift pattern changing device 55, the shift pattern is changed to the power pattern when the vehicle is accelerated and ascends or descends a slope. In this case, the economy pattern signal F is input into the three-way switching valve 26 from the shift pattern changing device 55 by way of a gate 60 as shown by a dotted line in FIG. 1 so that the economy pattern signal F is actually input into the three-way switching valve 26 only when an economy pattern permission signal G is generated from the gear-shifting control circuit 53. The operation of the control circuits 53 and 54 will be understood from the flow chart shown in FIG. 6.