Abstract:
An intake and exhaust device for a multi-cylinder engine includes: a first intake and exhaust system corresponding to a first cylinder group and having an exhaust passage and an intake passage; a second intake and exhaust system corresponding to a second cylinder group and having an exhaust passage and an intake passage; a communication passage through which the exhaust passage of the first intake and exhaust system and the exhaust passage of the second intake and exhaust system communicate with each other; an adjusting valve for opening and closing the communication passage; an EGR unit for causing exhaust gas to circulate between the exhaust passage and the intake passage of the first intake and exhaust system and between the exhaust passage and the intake passage of the second intake and exhaust system respectively; and a control unit for controlling opening and closing of the adjusting valve and circulation of exhaust gas by the EGR unit based on an operating condition of the engine.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an intake and exhaust device for a multi-cylinder engine, and more particularly to an intake and exhaust device for a multi-cylinder engine equipped with two intake and exhaust systems each having a turbocharger.  
         [0003]     2. Description of the Related Art  
         [0004]     In a V-type multi-cylinder engine, turbochargers are disposed corresponding to both banks respectively, and energy of exhaust gas discharged from cylinders in the respective banks is recovered by turbines of the turbochargers to drive a compressor, whereby compressed intake air is supplied to the cylinders. In a V-type multi-cylinder engine equipped with such turbo chargers, as disclosed in, for example, JP 2002-522687 A, exhaust passages of both banks are coupled to each other to balance pressures in both the turbochargers, and a coupling portion between the exhaust passages is further connected to an intake passage common to both the banks by an EGR passage, so that part of exhaust gas is caused to recirculate (EGR) to an intake system. An on/off state of EGR is controlled by opening and closing an EGR valve disposed in the EGR passage.  
         [0005]     However, since the EGR valve is disposed in a coupling portion between the exhaust passages for both the banks and the EGR passage, the exhaust passages for both the banks communicate with each other when the EGR valve is open, namely, when EGR is on. In contrast, it is impossible to establish communication between the exhaust passages for both the banks when EGR is off. Thus, in a high-load high-revolution range where EGR is generally off so as to achieve enhancement of output performance, the exhaust passages for both the banks are shut off from each other, and a difference in revolution speed between both the turbochargers, which occurs as a result of a difference in pressure between the exhaust passages for both the banks, is not eliminated. If the difference is small, revolution speeds of both the turbochargers can be made close to their maximums, so that a high output can be generated. However, if the difference is large, the revolution speed of one of the turbochargers may be still lower than its maximum even when the revolution speed of the other turbocharger is close to its maximum. This causes a problem in that it is difficult to sufficiently enhance an output.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention has been made to solve the conventional problem mentioned above. It is an object of the present invention to provide an intake and exhaust device for a multi-cylinder engine which makes it possible to sufficiently enhance an output in all ranges including a high-load high-revolution range.  
         [0007]     An intake and exhaust device for a multi-cylinder engine having a first cylinder group and a second cylinder group according to the present invention includes: a first intake and exhaust system corresponding to the first cylinder group and having an exhaust passage and an intake passage; a second intake and exhaust system corresponding to the second cylinder group and having an exhaust passage and an intake passage; a communication passage through which the exhaust passage of the first intake and exhaust system and the exhaust passage of the second intake and exhaust system communicate with each other; an adjusting valve for opening and closing the communication passage; EGR means for causing exhaust gas to circulate between the exhaust passage and the intake passage of the first intake and exhaust system and between the exhaust passage and the intake passage of the second intake and exhaust system respectively; and control means for controlling opening and closing of the adjusting valve and circulation of exhaust gas by the EGR means based on an operating condition of the engine. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a diagram showing an overall construction of an intake and exhaust device for a multi-cylinder engine according to a first embodiment of the present invention;  
         [0009]      FIG. 2  is a map used in the first embodiment and showing an EGR-on range and an EGR-off range;  
         [0010]      FIG. 3  is a map used in the first embodiment and showing an open range and a closed range of an adjusting valve;  
         [0011]      FIG. 4  is a diagram showing an overall construction of an intake and exhaust device for a multi-cylinder engine according to a second embodiment of the present invention; and  
         [0012]      FIG. 5  is a diagram showing an overall construction of an intake and exhaust device for a multi-cylinder engine according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings.  
       First Embodiment  
       [0014]      FIG. 1  shows an overall construction of an intake and exhaust device for a multi-cylinder engine according to a first embodiment of the present invention. In a V-8 cylinder engine, to which the first embodiment is applied, eight cylinders are divided into two groups of four cylinders. The cylinders in one of the groups are arranged in a left bank  1   a,  and those in the other group are arranged in a right bank  1   b.  Two intake and exhaust systems  2   a  and  2   b  are disposed corresponding to the left and right banks  1   a  and  1   b  respectively.  
         [0015]     The intake and exhaust system  2   a  corresponding to the left bank  1   a  is provided with a turbocharger  3   a.  An exhaust manifold  5   a  is connected to an inlet port of a turbine  4   a  of the turbocharger  3   a,  and a diesel particulate filter (DPF)  6   a  is connected to an outlet port of the turbine  4   a.  An air cleaner  9   a  is connected to a suction port of a compressor  7   a  of the turbocharger  3   a  via an airflow meter  8   a,  and an intake manifold  11 , which is common to both the intake and exhaust systems  2   a  and  2   b,  is connected to a discharge port of the compressor  7   a  via an intercooler  10 .  
         [0016]     Similarly, the intake and exhaust system  2   b  corresponding to the right bank  1   b  is provided with a turbocharger  3   b.  An exhaust manifold  5   b  is connected to an inlet port of a turbine  4   b  of the turbocharger  3   b,  and a DPF  6   b  is connected to an outlet port of the turbine  4   b.  An air cleaner  9   b  is connected to a suction port of a compressor  7   b  of the turbocharger  3   b  via an airflow meter  8   b,  and the intake manifold  11  is connected to a discharge port of the compressor  7   b  via the intercooler  10 .  
         [0017]     The exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  communicate with each other through a communication passage  12 . An adjusting valve  13  for opening and closing the communication passage  12  is disposed in the communication passage  12 .  
         [0018]     An EGR introduction passage  14   a  for the intake and exhaust system  2   a  and an EGR introduction passage  14   b  for the intake and exhaust system  2   b  are formed. The EGR introduction passage  14   a  branches off from the communication passage  12  at a position between the exhaust manifold  5   a  for the left bank  1   a  and the adjusting valve  13 , and leads to the intake manifold  11 . The EGR introduction passage  14   b  branches off from the communication passage  12  at a position between the exhaust manifold  5   b  for the right bank  1   b  and the adjusting valve  13 , and leads to the intake manifold  11 . In other words, the two EGR introduction passages  14   a  and  14   b  are disposed on opposite sides with respect to the adjusting valve  13 . EGR valves  15   a  and  15   b  are disposed in the EGR introduction passages  14   a  and  14   b  respectively.  
         [0019]     A control portion  16 , which controls the opening and closing of the adjusting valve  13  and the EGR valves  15   a  and  15   b,  is connected thereto. Moreover, various sensors including a revolution speed sensor  17  for detecting an engine revolution speed and an accelerator pedal position sensor  18  for detecting a position of an accelerator pedal are connected to the control portion  16 . Detection signals indicating operating conditions of the engine such as an engine revolution speed and a position of the accelerator pedal are inputted to the control portion  16  from those sensors. A fuel injection amount is determined on the basis of the detected position of the accelerator pedal.  
         [0020]     The control portion  16  can control a state of communication/shutoff between the exhaust manifolds  5   a  and  5   b  of both the intake and exhaust systems  2   a  and  2   b  and an on/off state of EGR for the intake and exhaust systems  2   a  and  2   b  independently of each other, by opening and closing the adjusting valve  13  and the two EGR valves  15   a  and  15   b.    
         [0021]     Although not shown, an intake throttle, an EGR cooler, a bypass passage for the EGR cooler, a switching valve installed in the bypass passage, and the like are also provided in addition to the aforementioned construction.  
         [0022]     Next, an operation of the first embodiment will be described.  
         [0023]     Exhaust gas discharged from the respective cylinders in the left bank  1   a  causes the turbine  4   a  of the turbocharger  3   a  to rotate and then is delivered to the DPF  6   a.  The compressor  7   a  of the turbocharger  3   a  is thereby driven, so that air, which has been taken in via the air cleaner  9   a  and the airflow meter  8   a,  is compressed by the compressor  7   a  and then delivered to the intake manifold  11  via the intercooler  10 .  
         [0024]     Similarly, exhaust gas discharged from the respective cylinders in the right bank  1   b  causes the turbine  4   b  of the turbocharger  3   b  to rotate and then is delivered to the DPF  6   b.  The compressor  7   b  of the turbocharger  3   b  is thereby driven, so that air, which has been taken in via the air cleaner  9   b  and the airflow meter  8   b,  is compressed by the compressor  7   b  and then delivered to the intake manifold  11  via the intercooler  10 .  
         [0025]     At this moment, the control portion  16  determines an on/off state of EGR according to a preset map as shown in  FIG. 2 , based on operating conditions such as an engine revolution speed and a position of the accelerator pedal, which have been detected by the revolution speed sensor  17  and the accelerator pedal position sensor  18  respectively. Furthermore in an EGR-on range, the control portion  16  calculates an optimal EGR rate for each of the intake and exhaust system  2   a  on the side of the left bank  1   a  and the intake and exhaust system  2   b  on the side of the right bank  1   b  according to operating conditions of the engine, and adjusts opening degrees of the EGR valves  15   a  and  15   b  such that the calculated EGR rates are achieved. Thus, part of exhaust gas taken out from the exhaust manifold  5   a  on the side of the left bank  1   a  and part of exhaust gas taken out from the exhaust manifold  5   b  on the side of the right bank  1   b  are caused to recirculate to the intake manifold  11 .  
         [0026]     The control portion  16  also controls the opening and closing of the adjusting valve  13  according to a preset map as shown in  FIG. 3 . In this case, the adjusting valve  13  is closed in a low-to-medium-revolution EGR-off range. Thus, gas flow between the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  is shut off. As a result, internal pressures of the exhaust manifolds  5   a  and  5   b  rise, which leads to enhancement of supercharging pressures of the turbochargers and thus to enhancement of an output.  
         [0027]     In the EGR-on range, on the other hand, the adjusting valve  13  is opened. Thus, the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  communicate with each other, and exhaust gases in both the exhaust manifolds mix with each other. Therefore, EGR distribution in the left bank  1   a  and the right bank  1   b  becomes favorable, and a difference between the EGR rate of the intake and exhaust system  2   a  on the side of the left bank  1   a  and the EGR rate of the intake and exhaust system  2   b  on the side of the right bank  1   b  is decreased. Thus, the amount of NOx can be reduced by carrying out EGR favorably while holding an output drop at its minimum.  
         [0028]     The adjusting valve  13  is opened also in a high-load high-revolution EGR-off range, and the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  communicate with each other. A difference Do in revolution speed between the left and right turbochargers  3   a  and  3   b  at the time when the adjusting valve  13  is opened to establish communication between the left and right exhaust manifolds  5   a  and  5   b  is smaller than a difference Dc in revolution speed between the left and right turbochargers  3   a  and  3   b  at the time when the adjusting valve  13  is closed to shut off gas flow between the left and right exhaust manifolds  5   a  and  5   b.  This is because the internal pressures of the left and right exhaust manifolds  5   a  and  5   b  have been balanced by opening the adjusting valve  13 . This makes it possible to raise revolution speeds of both the turbochargers  3   a  and  3   b  to their maximums and thus to achieve enhancement of an output.  
         [0029]     As described hitherto, it is possible to sufficiently enhance an output not only in a low-to-medium-revolution range but also in a high-load high-revolution range.  
         [0030]     As shown in  FIG. 3 , as is the case with the high-load high-revolution range, the adjusting valve  13  is opened to establish communication between the left and right exhaust manifolds  5   a  and  5   b  also in a low-load high-revolution range. In general, however, since there is no great need to further enhance an output in this range, the adjusting valve  13  may also be closed to shut off gas flow between the left and right exhaust manifolds  5   a  and  5   b  in the low-load high-revolution range.  
         [0031]     Although the adjusting valve  13  may fully open and close the communication passage  12 , it is also possible to use a valve whose opening degree is adjustable so that a degree of communication between the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  can be adjusted.  
       Second Embodiment  
       [0032]      FIG. 4  shows an overall construction of an intake and exhaust device for a multi-cylinder engine according to a second embodiment of the present invention. In the second embodiment, instead of the intake manifold  11  and the intercooler  10  that are common to the left bank  1   a  and the right bank  1   b  in the device of the first embodiment shown in  FIG. 1 , an intake manifold  11   a  and an intercooler  10   a  on the side of the left bank  1   a  and an intake manifold  11   b  and an intercooler  10   b  on the side of the right bank  1   b  are disposed independently of each other.  
         [0033]     Air compressed by the compressor  7   a  of the turbocharger  3   a  on the side of the left bank  1   a  is supplied to the intake manifold  11   a  via the intercooler  10   a  and then delivered to the respective cylinders in the left bank  1   a.  On the other hand, air compressed by the compressor  7   b  of the turbocharger  3   b  on the side of the right bank  1   b  is supplied to the intake manifold  11   b  via the intercooler  10   b  and then delivered to the respective cylinders in the right bank  1   b.    
         [0034]     When EGR is on, part of exhaust gas taken out from the exhaust manifold  5   a  on the side of the left bank  1   a  is caused to recirculate to the intake manifold  11   a  via the EGR valve  15   a.  On the other hand, part of exhaust gas taken out from the exhaust manifold  5   b  on the side of the right bank  1   b  is caused to recirculate to the intake manifold  11   b  via the EGR valve  15   b.    
         [0035]     In this construction as well, as is the case with the aforementioned first embodiment, it is possible to sufficiently enhance an output in the low-to-medium-revolution range and in the high-load high-revolution range, to decrease a difference in EGR rate between the left and right intake and exhaust systems  2   a  and  2   b,  and to achieve a reduction in amount of NOx while holding an output drop at its minimum.  
       Third Embodiment  
       [0036]      FIG. 5  shows an overall construction of an intake and exhaust device for a multi-cylinder engine according to a third embodiment of the present invention. The third embodiment is obtained by modifying the device of the first embodiment shown in  FIG. 1  such that the intake manifold  11  is coupled to the communication passage  12  via an EGR introduction passage  14  common to the left bank  1   a  and the right bank  1   b,  that an adjusting valve  23  is disposed in a coupling portion between the communication passage  12  and the EGR introduction passage  14 , and that an EGR valve  15  is disposed in the EGR introduction passage  14 . When the adjusting valve  23  is opened, the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  communicate with each other via the communication passage  12 , which communicates with the EGR introduction passage  14 . On the other hand, when the adjusting valve  23  is closed, gas flow between the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  is shut off, and the EGR introduction passage  14  is also closed regardless of whether the EGR valve  15  is open or closed.  
         [0037]     The control portion  16  closes the adjusting valve  23  in the low-to-medium-revolution EGR-off range according to a preset map as shown in  FIG. 3 . Thus, gas flow between the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  is shut off. As a result, internal pressures of the exhaust manifolds  5   a  and  5   b  rise, which leads to enhancement of supercharging pressures of the turbochargers.  
         [0038]     In the EGR-on range, on the other hand, the control portion  16  opens the adjusting valve  23 . As a result, the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  communicate with each other, and the communication passage  12  and the EGR introduction passage  14  communicate with each other. In this case, the control portion  16  calculates optimal EGR rates for both the intake and exhaust systems  2   a  and  2   b  according to operating conditions of the engine, and adjusts an opening degree of the EGR valve  15  such that the calculated EGR rates are achieved. Thus, exhaust gases in both the systems  2   a  and  2   b  mix with each other, which makes it possible to carry out EGR favorably while suppressing an output drop, and to reduce an amount of NOx.  
         [0039]     In the high-load high-revolution EGR-off range, the control portion  16  opens the adjusting valve  23  and the exhaust manifold  5   a  for the left bank  1   a  and the exhaust manifold  5   b  for the right bank  1   b  communicate with each other, but the EGR valve  15  is closed. Thus, internal pressures of the left and right exhaust manifolds  5   a  and  5   b  are balanced, which makes it possible to enhance an output.  
         [0040]     Thus, in the third embodiment as well, as is the case with the aforementioned first and second embodiments, it is possible to sufficiently enhance an output in the low-to-medium-revolution range and in the high-load high-revolution range, and to reduce an amount of NOx while holding an output drop at its minimum.  
         [0041]     In the first to third embodiments, it is not absolutely necessary to install the DPF  6   a  or  6   b  in each of the intake and exhaust systems. It is also possible to dispense with the DPF or to dispose a catalyst instead of the DPF.  
         [0042]     It is not absolutely required that the adjusting valve and the EGR valve be electrically opened and closed by the control portion  16 . For instance, a valve that is opened and closed through introduction of a pressure in an exhaust passage or an intake passage may be used.  
         [0043]     As a method of controlling the adjusting valve, it is also adoptable to establish communication between the exhaust manifold for the left bank and the exhaust manifold for the right bank by opening the adjusting valve when EGR is on, or when the engine revolution speed is equal to or higher than a predetermined value (e.g., 2000 rpm), and to shutoff gas flow between the exhaust manifold for the left bank and the exhaust manifold for the right bank by closing the adjusting valve when EGR is off and the engine revolution speed is lower than the predetermined value (e.g., 2000 rpm).  
         [0044]     Although the intake and exhaust device applied to the V-8 cylinder engine has been described in each of the aforementioned first to third embodiments, the present invention should not be limited thereto. The present invention is widely applicable to an engine having two intake and exhaust systems, for example, a V-type multi-cylinder engine other than a V-8 cylinder engine, a non-V-type engine such as a horizontally-opposed engine or an inline engine, or the like.  
         [0045]     According to the present invention, it is possible to sufficiently enhance an output in all ranges including a high-load high-revolution range.