Patent Publication Number: US-2016245155-A1

Title: Control device for internal combustion engine

Description:
BACKGROUND 
     1. Field of the Application 
     The present application relates to a control device for an internal combustion engine, and more particularly to a control device for an internal combustion engine in which it is possible for a local minimum point of a pulsation of an intake air pressure and an opening period of an intake valve to overlap. 
     2. Background Art 
     A turbo-supercharger is driven by exhaust gas to perform work with respect to intake air. The energy of exhaust gas that is received by the turbo-supercharger periodically changes according to the cycle of the internal combustion engine, and becomes a local maximum when an exhaust stroke arrives in the respective cylinders. Consequently, the power of the turbo-supercharger with respect to the intake air also changes periodically, and this produces periodic changes in the pressure, that is, pulsation, of the intake air. Pulsation of the intake air affects the volumetric efficiency of air that enters a cylinder. 
     The lower the intake air pressure is when an intake valve is open, the lower the volumetric efficiency of a cylinder will be. Therefore, in a case where a local minimum point of a pulsation of intake air pressure exists within an opening period of the intake valve, the volumetric efficiency will decrease in comparison to a case where there is no pulsation. In order to suppress the negative influence of a pulsation on the volumetric efficiency, it is desirable to suppress the pulsation itself. As a method for suppressing a pulsation that arises in an intake passage, for example, as disclosed in JP2005-344642A, a method is known in which a variable chamber whose capacity is variable is attached to an independent intake passage that is provided for each cylinder, and the capacity of the variable chamber is changed based on the load and the rotational speed of the internal combustion engine. 
     SUMMARY OF THE APPLICATION 
     However, the technology disclosed in JP2005-344642A requires space for providing a variable chamber for each cylinder, and thus places constraints on the layout of the internal combustion engine. A method may be considered in which the intake air volume is increased by designing the intake passage to have a large effective volume to thereby attenuate pulsations, however, an increase in the intake air volume leads to a decrease in response. In addition, a method may be considered in which pulsations are attenuated by mechanically extending the length of the intake passage, however, changing the length of the intake passage is not easy. 
     The present application is made in the light of the problem as described above, and has an object to provide a control device for an internal combustion engine that is configured to suppress a decrease in volumetric efficiency due to the influence of a pulsation of the intake air pressure that the turbo-supercharger generates. 
     A control device for an internal combustion engine according to the present disclosure is a control device that is applied to an internal combustion engine including: a first supercharger that is driven by exhaust gas; a second supercharger that is disposed downstream relative to the first supercharger in an intake passage, and is driven by a motive power other than exhaust gas; a bypass passage that bypasses the second supercharger; and a bypass valve that opens and closes the bypass passage. The second supercharger may be an electric supercharger that is driven by an electric motor. The present control device is configured to determine whether or not the internal combustion engine is in a specific operating state where a local minimum point of a pulsation of an intake air pressure exists within an opening period of an intake valve. The present control device is also configured to operate the second supercharger and the bypass valve based on a determination result of the determination means. The control device is configured so that, in a case where it is determined that the internal combustion engine is in the specific operating state, the control device actuates the second supercharger and also opens the bypass valve. 
     According to the present control device that is configured as described above, under circumstances in which a local minimum point of a pulsation of the intake air pressure exists within an opening period of the intake valve, since the second supercharger is actuated and the bypass valve is opened, intake air passes through both of the intake passage in which the second supercharger is disposed and the bypass passage. By this means, the total flow path length through which the intake air actually flows is extended, and thus the intake air volume is increased artificially. Attenuation of a pulsation of the intake air pressure that the first supercharger generates is promoted by this artificial increase in the intake air volume, and thus a decrease in the volumetric efficiency due to the influence of the pulsation is suppressed. 
     A rotational speed of the second supercharger in the specific operating state may be set to a lower rotational speed than a rotational speed in a case of assisting supercharging by the first supercharger by means of supercharging by the second supercharger. Since the above described effect is one that is obtained as a result of intake air passing through the second supercharger, the rotational speed of the second supercharger may be low in the specific operating state. Consumption of energy is suppressed by suppressing the rotational speed of the second supercharger. 
     The present control device may be applied to an internal combustion engine that is equipped with a cylinder halting device that halts one or more cylinders. Intake air pulsations are created by the first supercharger being driven by exhaust gas, and the intake air pressure increases in a period in which exhaust gas is flowing into the first supercharger, and decreases in a period in which exhaust gas is not flowing into the first supercharger. When a certain cylinder is halted by the cylinder halting device, exhaust gas does not come out from the halted cylinder, and therefore the number of local maximum points of pulsations of the intake air pressure in a single cycle is reduced by an amount corresponding to the number of halted cylinders. Therefore, a period in which the intake air pressure decreases between one local maximum point and the proximate local maximum point of a pulsation is lengthened, and a period in which the intake air pressure is decreasing is liable to overlap with an opening period of the intake valve. In particular, in a case where two cylinders are halted in a four-cylinder internal combustion engine, since the exhaust stroke of one operating cylinder will not overlap with the intake stroke of the other operating cylinder, a local minimum point of a pulsation of the intake air pressure will definitely exist within an opening period of the intake valve. Hence, when it is determined that the one or more cylinders are being halted by the cylinder halting device, it may be determined that the engine is in the specific operating state, that is, a local minimum point of a pulsation of the intake air pressure exists within an opening period of the intake valve. 
     Further, the phase of a pulsation with respect to an opening period of the intake valve varies depending on the rotational speed of the internal combustion engine. Depending on the rotational speed region, it is also possible for a local minimum point of a pulsation of the intake air pressure to exist within an opening period of the intake valve. Hence, when it is determined that the internal combustion engine is rotating at a speed that is in a predetermined rotational speed region, it may be determined that the engine is in the specific operating state, that is, a local minimum point of a pulsation of the intake air pressure exists within an opening period of the intake valve. 
     In this connection, the influence of a pulsation on the volumetric efficiency decreases relatively as the load of the internal combustion engine increases. Hence, a configuration may be adopted so as to actuate the second supercharger and also perform an operation to open the bypass valve in a case where a local minimum point of a pulsation of the intake air pressure exists within an opening period of the intake valve and the load of the internal combustion engine is less than a predetermined value. That is, a determination may be performed taking a fact that the load of the internal combustion engine is less than a predetermined value as a second condition for determining that the internal combustion engine is in a specific operating state. In a case where the load of the internal combustion engine is equal to or greater than the predetermined value, the bypass valve may be closed and the second supercharger may be caused to operate at a rotational speed that is in accordance with the load, to thus assist supercharging by the first supercharger. 
     According to the control device for an internal combustion engine of the present disclosure, in an operating state in which a local minimum point of a pulsation of intake air pressure that is generated by a first supercharger exists within an opening period of an intake valve, a second supercharger is actuated and a bypass valve is also opened, and therefore intake air passes through both of the intake passage in which the second supercharger is disposed and the bypass passage. By this means, the intake air volume is increased artificially and thus attenuation of the pulsation of the intake air pressure that is generated by the first supercharger is promoted, and a decrease in the volumetric efficiency due to the influence of the pulsation is suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a configuration of a system according to an embodiment; 
         FIG. 2  is a table showing strokes of respective cylinders in a cylinder halting operation; 
         FIG. 3  is a view illustrating the influence of the intake air volume on intake air pulsations; 
         FIG. 4  is a view illustrating the relation between an intake air volume and a volumetric efficiency ratio; 
         FIG. 5  is a view illustrating a flow of intake air when an electric supercharger is stopped; 
         FIG. 6  is a view illustrating a flow of intake air in an assisting operation; 
         FIG. 7  is a view illustrating the flow of intake air in a pulsation suppressing operation; 
         FIG. 8  is a flowchart illustrating a control procedure of the embodiment; and 
         FIG. 9  is a time chart illustrating operations of the system according to the control procedure of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present disclosure is described hereunder with reference to the accompanying drawings. However, it is to be understood that even when the number, quantity, amount, range or other numerical attribute of an element is mentioned in the following description of the embodiment, the present disclosure is not limited to the mentioned numerical attribute unless it is expressly stated or theoretically defined. Further, structures or steps or the like described in conjunction with the following embodiment are not necessarily essential to the present disclosure unless expressly stated or theoretically defined. 
     1. Configuration of System 
       FIG. 1  is a view illustrating the configuration of a system of an embodiment of the present disclosure. The system shown in  FIG. 1  includes an internal combustion engine  10 . The internal combustion engine  10  is a diesel engine, and is mounted in a vehicle and used as a power apparatus thereof. The internal combustion engine  10  is an inline four-cylinder engine that includes four cylinders in an engine main body  12 . A cylinder halting device  60  that halts the operation of a cylinder is provided in some cylinders (for example, a second cylinder #2 and a third cylinder #3) among the four cylinders #1, #2, #3, #4 of the engine main body  12 . Cylinder halting is realized by fully closing both an intake valve and an exhaust valve, and stopping fuel injection by a fuel injector. 
     An intake passage  14  and an exhaust passage  16  are connected to the engine main body  12  of the internal combustion engine  10 . An air cleaner  18  is provided at an inlet of a main intake passage  14   a  of the intake passage  14 . A compressor  22   a  of a turbo-supercharger  22 , as a first supercharger for supercharging intake air, is disposed in the main intake passage  14   a  at a position on a downstream side relative to the air cleaner  18 . Hereunder, the compressor  22   a  is referred to as “turbo-compressor  22   a ”. The turbo-supercharger  22  includes a turbine  22   b  in the exhaust passage  16 . The turbo-compressor  22   a  is integrally connected to the turbine  22   b  through a connecting shaft, and is driven by exhaust gas that flows into the turbine  22   b.    
     An intercooler  28  for cooling intake air that was compressed by the turbo-compressor  22   a  is disposed in the main intake passage  14   a  at a position on the downstream side relative to the turbo-compressor  22   a.  Further, a compressor  24   a  of an electric supercharger  24  as a second supercharger is also disposed in the main intake passage  14   a  on the downstream side relative to the intercooler  28 . Hereunder, the compressor  24   a  is referred to as “electric compressor  24   a ”. The electric compressor  24   a  is driven by an electric motor  24   b.  Electric power from a battery (not shown in the drawings) is supplied to the electric motor  24   b.    
     The intake passage  14  includes a bypass passage  14   b  that bypasses the electric compressor  24   a.  A bypass valve  26  that opens and closes the bypass passage  14   b  is arranged in the bypass passage  14   b.  Although in  FIG. 1  the bypass valve  26  is provided at the center of the bypass passage  14   b,  as long as the bypass passage  14   b  cane be opened and closed, the bypass valve  26  may be provided at a branch point or a merging point with the bypass passage  14   b.  The main intake passage  14   a  is connected to an intake manifold  14   c  on a downstream side of a position at which the bypass passage  14   b  merges with the main intake passage  14   a.  The intake manifold  14   c  distributes intake air to each cylinder. 
     Exhaust gas from the respective cylinders is collected by an exhaust manifold  16   a  of the exhaust passage  16 , and sent to the turbine  22   b.  The exhaust manifold  16   a  is connected to the main intake passage  14   a  at a position between the intercooler  28  and the electric compressor  24   a  by an EGR passage  50 . An EGR cooler  54  for cooling EGR gas is disposed in the EGR passage  50 . An EGR valve  52  that opens and closes the EGR passage  50  is disposed in the EGR passage  50  at a position that is on the downstream side relative to the EGR cooler  54 . 
     The system illustrated in  FIG. 1  includes a control device  100  that controls the internal combustion engine  10 . The control device  100  is an ECU. The control device  100  includes at least an input/output interface, a memory and a CPU. The input/output interface is provided in order to take in sensor signals from various sensors installed in the internal combustion engine  10  and the vehicle in which the internal combustion engine  10  is mounted, and to also output actuating signals to actuators provided in the internal combustion engine  10 . Various control programs and maps for controlling the internal combustion engine  10  are stored in the memory. The CPU reads out a control program from the memory and executes the control program, and generates actuating signals based on sensor signals that were taken in. 
     2. Issue in Cylinder Halting Operation 
     Strokes of the respective cylinders during a cylinder halting operation in the internal combustion engine  10  are shown in a table in  FIG. 2 . For a first cylinder (#1) and a fourth cylinder (#4) that are operating cylinders, an intake stroke, a compression stroke, an expansion stroke and an exhaust stroke are repeated in that order. In contrast, for a second cylinder (#2) and a third cylinder (#3) that are halted cylinders, a compression stroke and an expansion stroke are repeated. By halting some cylinders in this manner, pumping loss, friction loss, cooling loss and the like decrease and fuel consumption improves. 
     However, on the other hand, the following issue exists with regard to the cylinder halting operation. 
     Exhaust gas flows into the turbine  22   b  of the turbo-supercharger  22  during the exhaust stroke of each cylinder, and work with respect to intake air is performed by the turbo-compressor  22   a  as a result of the turbine  22   b  being rotated by exhaust energy and the turbo-compressor  22   a  being driven. Consequently, the power of the turbo-compressor  22   a  with respect to the intake air periodically changes according to the cycle of the internal combustion engine  10 , and becomes a maximum when the exhaust stroke of the respective cylinders arrives. As a result, with a cycle at which an exhaust stroke arrives, a pulsation is generated in the supercharging pressure that is the pressure of the intake air downstream of the turbo-compressor  22   a.    
     In the case of the internal combustion engine  10  that is an inline four-cylinder engine, because the explosion interval for the respective cylinders is staggered by 180 degrees for each cylinder, the exhaust stroke of a certain cylinder overlaps with an intake stroke of some other cylinder. Therefore, a pulsation that arises in intake air due to the action of the turbo-supercharger  22  has an effect of promoting the flow of intake air into a cylinder and thereby improving the volumetric efficiency. However, when a cylinder halting operation is being performed, as shown in the table in  FIG. 2 , an exhaust stroke of the respective operating cylinders does not overlap with an intake stroke of another cylinder. 
     In  FIG. 3 , a waveform (indicated by a broken line) of the supercharging pressure in four-cylinder operation, a waveform (indicated by a solid line) of the supercharging pressure in two-cylinder operation with the cylinder halting operation, and opening periods of an exhaust valve and an intake valve are drawn on the same crank angle axis. Although a plurality of waveforms of the supercharging pressure in two-cylinder operation are drawn in the figure, the capacity of the intake manifold  14   c  is different for the respective waveforms. A plurality of intake manifolds  14   c  of differing capacities were prepared, and waveforms of the supercharging pressure in two-cylinder operation were measured for each capacity. The plurality of waveforms shown in  FIG. 3  were obtained in this way. The waveforms of the supercharging pressure were measured in the vicinity of the intake valve. 
     In the waveform of the supercharging pressure during four-cylinder operation, four local maximum points can exist in a single cycle in correspondence to the exhaust strokes of the respective cylinders. Since a time lag exists from a time that the exhaust valve opens until the supercharging pressure rises, the mountain-like shape of the waveform of the supercharging pressure and the opening period of the intake valve do not overlap at all. However, if even a part of the mountain-like shape of the waveform of the supercharging pressure overlaps with a part of the opening period of the intake valve, the flow of intake air into a cylinder during that period will be promoted and the volumetric efficiency will increase. 
     On the other hand, in the waveforms of the supercharging pressure during two-cylinder operation, there are only two local maximum points of the supercharging pressure in a single cycle, which corresponds to the exhaust strokes of the two operating cylinders that are separated by an interval of 360 degrees. Further, the exhaust stroke of one of the operating cylinders and the intake stroke of the other operating cylinder deviate from each other by 540 degrees. Therefore, a local maximum point of the supercharging pressure is separated by a large amount from an opening period of the intake valve and, conversely, a negative effect that lowers the volumetric efficiency due to the local minimum point of the supercharging pressure entering the opening period of the intake valve is larger. 
     In  FIG. 4 , a plurality of polygonal lines are drawn that show the relation between the volumetric efficiency ratio and the engine rotational speed in two-cylinder operation in a case where the volumetric efficiency in four-cylinder operation is taken as 1. The capacity of the intake manifold  14   c  is different for each polygonal line. That is, the plurality of polygonal lines shown in  FIG. 4  represent results obtained when a plurality of the intake manifolds  14   c  having different capacities to each other were prepared and the relation between the volumetric efficiency ratio and the engine rotational speed in two-cylinder operation was determined for each capacity. 
     Based on  FIG. 4 , it is found that, over approximately the entire region of the engine rotational speed, the volumetric efficiency decreases in two-cylinder operation compared to four-cylinder operation. In the example illustrated in  FIG. 4 , there is a noticeable decrease in the volumetric efficiency in a normal rotational speed region of a diesel engine, and there is a small decrease in the volumetric efficiency in a high rotation region and an extremely low rotation region. The reason that the volumetric efficiency changes in this way depending on the engine rotational speed is that a time lag from a time that the exhaust valve opens until the supercharging pressure rises, particularly, a time lag before exhaust gas arrives at the turbine  22   b,  changes depending on the engine rotational speed, and consequently the phase of a local maximum point of the supercharging pressure with respect to the opening period of the intake valve changes. A decrease in the volumetric efficiency in a diesel engine leads to a decrease in EGR gas that enters the cylinders, and consequently raises the in-cylinder temperature and increases cooling loss. 
     3. Pulsation Suppression Control 
     Differences in the capacity of the intake manifold  14   c  result in differences arising in the volume of intake air that actually flows along a flow path from the outlet of the turbo-compressor  22   a  to the intake valve. As will be understood by comparing the respective waveforms shown in the above-described  FIG. 3 , the larger that the intake air volume is, the greater the degree to which attenuation of intake air pulsations proceeds and the smaller that the amplitude of local minimum points of the supercharging pressure in the opening period of the intake valve becomes. Therefore, as shown in the above-described  FIG. 4 , the larger that the intake air volume is, the greater the degree to which a decrease in the volumetric efficiency that is due to the influence of pulsations is suppressed. 
       FIG. 5  is a view that illustrates the flow of intake air at a time that the electric supercharger  24  is stopped (or a time of preliminary rotation). In a case where the electric supercharger  24  is stopped or a case where the electric supercharger  24  is performing preliminary rotation (operating at a rotational speed of a level that does not produce an air flow), the bypass valve  26  is placed in a fully open state. By this means, as indicated by a dashed arrow line in  FIG. 5 , the intake air flows through the bypass passage  14   b.    
     When an assisting operation is performed by the electric supercharger  24 , the bypass valve  26  is placed in a fully closed state. An assisting operation is an operation performed by the electric supercharger  24  which has a fast response in order to compensate for turbo lag of the turbo-supercharger  22 , and is performed in a case where there is an acceleration request from the driver.  FIG. 6  is a view that illustrates the flow of intake air during an assisting operation by the electric supercharger  24 . As shown by a dashed arrow line in  FIG. 6 , during the assisting operation, by placing the bypass valve  26  in a fully closed state, all of the intake air flows through the main intake passage  14   a  in which the electric supercharger  24  is disposed. 
     In the case of the system of the embodiment, when the flow path of the intake air shown in  FIG. 5  and the flow path of the intake air shown in  FIG. 6  are compared, it is found that the flow path that passes through the electric supercharger  24  has a longer total flow path length. Hence, the volume of the intake air that actually flows along the flow path from the outlet of the turbo-compressor  22   a  to the intake valve is increased by actuating the electric supercharger  24  so that the intake air flows through the main intake passage  14   a.  However, according to the configuration of the system of the embodiment, there is room to further increase the intake air volume. In the system of the embodiment, by performing pulsation suppression control that operates the electric supercharger  24  and the bypass valve  26  in the following manner, the intake air volume is further increased to suppress pulsations. 
       FIG. 7  is a view that illustrates the flow of intake air when pulsation suppression control is performed. According to the pulsation suppression control, the electric supercharger  24  is operated at a rotational speed that is equal to or higher than the speed of preliminary rotation, and the bypass valve  26  is opened. By this means, as indicated by a dashed arrow line in  FIG. 7 , the intake air flows through both the bypass passage  14   b  and the main intake passage  14   a  in which the electric supercharger  24  is disposed. 
     By realizing the flow of intake air shown in  FIG. 7 , the total flow path length of the flow path from the outlet of the turbo-compressor  22   a  to the intake valve is extended by an amount corresponding to the amount of the bypass passage  14   b  in comparison when performing the assisting operation. The volume of intake air that actually flows through the flow path is increased by an amount corresponding to the amount by which the total flow path length is extended. A certain amount of attenuation occurs in a pulsation before the intake air reaches the intake valve from the outlet of the turbo-compressor  22   a.  The attenuation of the pulsation is promoted as a result of the intake air volume being increased. 
     Further, there is a difference in distance between the distance in the main intake passage  14   a  from the branch point with the bypass passage  14   b  to the merging point therewith, and the distance in the bypass passage  14   b.  Consequently, a phase difference arises between a pressure waveform of a pulsation that passes through the main intake passage  14   a  and a pressure waveform of a pulsation that passes through the bypass passage  14   b,  that depends on the difference in distance and the wavelengths of the pulsations. At the position at which the bypass passage  14   b  merges with the main intake passage  14   a,  mutual interference arises between the pulsations of the two paths between which the phase difference exists, and attenuation of the pulsations is further promoted. 
     As described above, according to the pulsation suppression control, an effect that promotes attenuation of pulsations is obtained by increasing the intake air volume, and furthermore, an effect that promotes attenuation of pulsations is also obtained by mutual interference. By means of these effects, a decrease in volumetric efficiency that is due to the influence of pulsations is suppressed by implementing the pulsation suppression control when performing a cylinder halting operation. 
     Note that, it is not necessary to fully open the bypass valve  26  to create the flow of intake air shown in  FIG. 7 , and it is sufficient to open the bypass valve  26  by even a small amount. It is sufficient for the electric supercharger  24  to rotate to a degree whereby intake air flows from upstream to downstream. Naturally, the electric supercharger  24  may rotate at a rotational speed that is equal to the rotational speed when performing the assisting operation, and in that case intake air may flow backward through the bypass passage  14   b . As long as the configuration allows pulsations to be propagated along the two respective paths of the bypass passage  14   b  and the electric supercharger  24 , the above-described effects produced by the pulsation suppression control are obtained. 
     4. Control Procedure 
       FIG. 8  is a flowchart that illustrates a control procedure of the embodiment. A control program corresponding to the control procedure is stored in the memory of the control device  100 . The flow of intake air shown in  FIG. 7  is realized by controlling the electric supercharger  24  and the bypass valve  26  in accordance with this control procedure. 
     According to the control procedure shown in  FIG. 8 , first, the control device  100  checks whether or not the cylinder halting operation is being performed (step S 2 ). A fact that the cylinder halting operation is being performed is one condition for determining that the internal combustion engine  10  is in a specific operating state that requires pulsation suppression control. If the cylinder halting operation is not being performed, the control device  100  performs operation according to normal control (step S 8 ). In the normal control, operation of the electric supercharger  24  and the bypass valve  26  is performed in accordance with a map that takes the engine load and engine rotational speed as parameters. A map for four-cylinder operation and a map for two-cylinder operation are prepared as maps, and the maps are switched depending on whether or not the cylinder halting operation is being performed. 
     If the cylinder halting operation is being performed, the control device  100  determines whether the load of the internal combustion engine  10  is less than a predetermined value (for example, about 10%) (step S 4 ). The influence of pulsations on the volumetric efficiency decreases relatively as the load of the internal combustion engine  10  increases. At a time of a low load when the influence of pulsations on the volumetric efficiency is large, energy efficiency is increased by suppressing pulsations by performing the pulsation suppression control. However, in a case where the load is large to a certain extent, rather than suppressing pulsations, performing the assisting operation by the electric supercharger  24  improves the energy efficiency of the overall system. Hence, the load being less than a predetermined value is a second condition for determining that the internal combustion engine  10  is in a specific operating state that requires pulsation suppression control. If the load is equal to or greater than the predetermined value, the control device  100  performs operation by normal control (step S 8 ). 
     In a case where the cylinder halting operation is being performed and the load is less than the predetermined value, the control device  100  performs the pulsation suppression control by actuating the electric supercharger  24  and also opening the bypass valve  26  (step S 6 ). As long as a flow of intake air is secured, the rotational speed of the electric supercharger  24  at this time may be a fixed rotational speed that does not depend on the load. 
     5. Operation of the System 
       FIG. 9  is a time chart illustrating operation of the system in a case where the above-described control procedure is implemented. A chart on the first (upper) level shows the status of a halt flag that indicates whether or not the cylinder halting operation is being executed. A chart on the second level from the top shows the load. A chart on the third level from the top shows the opening degree of the bypass valve  26 . A chart on the fourth level from the top shows the rotational speed of the electric supercharger  24 . 
     According to the time chart, the cylinder halting operation is not performed until a time t 1 . Further, the load during this period is low, and the assisting operation by the electric supercharger  24  is not being performed. The bypass valve  26  is opened fully, and the electric supercharger  24  is caused to perform preliminary rotation. 
     When the cylinder halting operation is performed at the time t 1 , the pulsation suppression control is performed. The bypass valve  26  is opened to a previously set opening degree for pulsation suppression, and the electric supercharger  24  is rotated at a previously set rotational speed for pulsation suppression. The rotational speed for pulsation suppression is set to a lower rotational speed than the rotational speed during an assisting operation. Since an effect that suppresses pulsations will be obtained if intake air passes through the electric supercharger  24 , the rotational speed for pulsation suppression may be low, and consumption of energy can thus be suppressed by suppressing the rotational speed. Note that the setting for the rotational speed for pulsation suppression may be changed in accordance with the remaining capacity of the battery. 
     Subsequently, upon the load reaching a predetermined value at a time t 2 , the operation mode of the electric supercharger  24  is switched from the pulsation suppressing operation to the assisting operation. The bypass valve  26  is fully closed, and the electric supercharger  24  is rotated at a rotational speed in accordance with the engine load and the engine rotational speed. By switching to the assisting operation by the electric supercharger  24  when the load increases, the efficiency is increased in comparison to the case of continuing the pulsation suppression control. 
     6. Miscellaneous 
     In the above-described embodiment, whether or not to execute pulsation suppression control is determined based on the load. However, a configuration may be adopted that determines whether or not to execute pulsation suppression control based on the engine rotational speed. Since the phase of a pulsation with respect to the opening period of the intake valve changes depending on the engine rotational speed of the internal combustion engine  10 , a position of a local minimum point of the intake air pressure in an opening period of the intake valve also changes depending on the engine rotational speed. Therefore, as shown in  FIG. 4  that is described above, a decrease in the volumetric efficiency is noticeable in a specific rotational speed region. Thus, consumption of energy is suppressed by limiting execution of the pulsation suppression control to a rotational speed region in which a decrease in the volumetric efficiency is particularly noticeable. 
     Although in the above-described embodiment the second cylinder and third cylinder are halted in an inline four-cylinder engine, the total number of cylinders and the number of halted cylinders is not limited according to the present disclosure. As long as a part of the cylinders of the internal combustion engine are halted, a decrease in volumetric efficiency due to the influence of pulsations is suppressed by performing pulsation suppression control. 
     In addition, execution of pulsation suppression control also exerts an effect when operating on all cylinders, and not just when performing a cylinder halting operation. There is always a trough in a pulsation of intake air, and the phase of a pulsation with respect to the opening period of the intake valve changes depending on the engine rotational speed of the internal combustion engine  10 . Consequently, even when operating on all cylinders, there are cases where, depending on the rotational speed region, a local minimum point of the intake air pressure enters an opening period of the intake valve. For example, in a waveform of the supercharging pressure during four-cylinder operation that is shown in  FIG. 3  also, a local minimum point of the intake air pressure due to pulsations is positioned within the opening period of the intake valve. By performing pulsation suppression control in such a case, a decrease in the volumetric efficiency due to the influence of pulsations is suppressed. 
     Note that, although an electric supercharger is used as the second supercharger in the above-described embodiment, the second supercharger may be a mechanical supercharger that is driven by the internal combustion engine. The rotational speed of the mechanical supercharger is controlled by interposing a clutch between an output shaft of the internal combustion engine and the mechanical supercharger, and adjusting the engagement state of the clutch. Further, the present disclosure can also be applied to a spark-ignition lean-burn engine, and not just to a diesel engine.