SUPERCHARGER SYSTEM

[Technical Field]

The present invention relates to an apparatus and method for controlling a twin electric supercharger system, and more specifically relates to an apparatus and method for controlling a twin electric supercharger system which ensure that an optimal control mode among the control modes for supercharging in the twin electric supercharger system can be determined.

[Background Art of the Invention]

Generally, superchargers (or charger systems) are apparatuses which produce maximum torque and output that are higher than in a natural suction engine having the same displacement, by allowing intake air to be supplied into the cylinders of the engine at a pressure higher than atmospheric pressure. That is to say,

superchargers (or charger systems) are used in order to improve the output of vehicles.

Such superchargers (or charger systems) include turbochargers and

superchargers.

It should be noted that turbochargers operate using a method in which the exhaust gases generated in the engine rotate the turbine of the turbocharger, thereby operating a compressor through the rotational power of the turbine and so compressing air and sending same to the engine. On the other hand, superchargers operate using a method in which a compressor/blower is rotated using the power of the engine (crank shaft), thereby forcibly supplying external air to the combustion chamber of the engine.

Also, the electric supercharger related to an embodiment of the present invention operates using a method in which the turbine is turned using the force of an electric motor rather than the power of the engine to supercharge the engine. Now, in order to compensate for the drawback, whereby, due to the characteristics of electric superchargers, sufficient pressure cannot be generated as the supercharging flow rate is inferior to that of turbochargers, a twin electric supercharger system using two superchargers has been proposed.

But, because of the characteristics of the twin electric supercharger which supercharges the engine using the force of electric motors rather than the power of the engine as described above, a battery power source is used, and thus there is a problem in that the weight or cost increases in order to ensure sufficient battery capacity and so the cost-effectiveness (cost vs performance) may deteriorate.

Background art of the present invention is introduced in the "INTEGRATED

TWINCHARGER SYSTEM" in Korean Laid-Open Patent Gazette No.

10-2012-0055882 (01 June 2012).

[Details of the Invention]

[Problem to be Solved]

According to an aspect of the present invention, the present invention has been devised to solve the problems such as the above, and has the objective of providing an apparatus and method for controlling a twin electric supercharger system which ensure that an optimal control mode, among the control modes for supercharging in the twin electric supercharger system, can be determined on the basis of the electric power consumption of each of the electric superchargers which constitute the twin electric supercharger system.

[Means of Solving the Problem]

The apparatus for controlling a twin electric supercharger system according to one aspect of the present invention is characterized by comprising: a first and second supercharger which compress air; an airflowpath forming portion which forms an air flowpath such that air from an air cleaner is compressed by means of at least one of the first and second superchargers and is supplied to the engine; and a control unit which detects predesignated operating region information and controls the first and second superchargers and the air flowpath forming portion in accordance with the control mode determined based on said information, and also determines the optimal control mode, which has the lowest electric power consumption, on the basis of the electric power consumption of each of the plurality of control modes for supercharging in the twin electric supercharger system. A characterizing feature in the present invention is that said air flowpath forming portion comprises: a first bypass pipe which supplies air from said air cleaner to said first supercharger; a first bypass valve which controls the amount of air supplied through said first bypass pipe; an air supply pipe which supplies air from said air cleaner to said second supercharger; a second bypass pipe which supplies air from said second supercharger to said first supercharger; a second bypass valve which controls the amount of air supplied through said second bypass pipe; an air suction pipe which supplies the air delivered from at least one of said first supercharger and said second supercharger to the engine; a third bypass pipe which supplies air from said second supercharger to said air suction pipe; and a third bypass valve which controls the amount of air supplied through said third bypass pipe.

A characterizing feature in the present invention is that the control unit calculates the electric power consumption (EPC) with respect to each control mode by using a predesignated look-up table or using a predesignated arithmetic expression.

A characterizing feature in the present invention is that said control mode includes at least one of: a general mode or limp home mode in which said first, second and third bypass valves of the air flowpath forming portion are completely open; a single mode in which the air supplied through said first bypass pipe of the air flowpath forming portion is compressed by said first supercharger so as to be supplied to the engine through said air suction pipe; a serial mode in which the air is compressed by said second supercharger so as to be delivered through said second bypass pipe to said first supercharger and the air delivered from said second supercharger is compressed by said first supercharger and then supplied to the engine through said air suction pipe; and a parallel mode in which air is compressed by said second supercharger so as to be supplied through said air suction pipe to the engine, and the air supplied through said first bypass pipe is compressed by said first supercharger so as to be supplied through said air suction pipe to the engine.

A characterizing feature in the present invention is that, in order to set the control mode of the twin electric supercharger system to the normal mode, when preset conditions for switching to a limp home mode are not detected and also preset conditions for implementing supercharging are not detected, the control unit outputs the rotational speed of the supercharger and bypass valve on/off values as control values corresponding to the normal mode.

A characterizing feature in the present invention is that, when the preset conditions for implementing supercharging are detected, the control unit checks whether the plurality of control modes for supercharging in the twin electric supercharger system are over the choke line, and the control modes over the choke line are excluded (inhibited) from being subject to control mode determination, and also, the control unit checks whether the plurality of control modes for supercharging in the twin electric supercharger system are over the surge line, and the control modes over the surge line are excluded (inhibited) from being subject to control mode determination.

A characterizing feature in the present invention is that the choke line means an imaginary line that connects and displays the maximum supercharging flow with respect to the compression ratio, on the mechanical efficiency characteristics map of the compressor in the twin electric supercharger system, while the surge line means an imaginary line that connects and displays the maximum compression ratio with respect to the supercharging flow, on the mechanical efficiency characteristics map of the compressor in the twin electric supercharger system. A characterizing feature in the present invention is that the control unit is implemented in such a way that, when checking whether the plurality of control modes for supercharging in the twin electric supercharger system are over the choke line, the control unit does not perform the over-the-choke-line check for at least one predesignated control mode.

A characterizing feature in the present invention is that the control mode that is predesignated so as not to perform the over-the-choke-line check comprises a parallel mode. A characterizing feature in the present invention is that the control unit is implemented in such a way that, when checking whether the plurality of control modes for supercharging in the twin electric supercharger system are over the surge line, the control unit does not perform the over-the-s urge-line check for at least one predesignated control mode.

A characterizing feature in the present invention is that the control mode that is predesignated so as not to perform the over-the-surge-line check comprises a serial mode. A characterizing feature in the present invention is that, for each control mode that does not go over both the choke line and the surge line among the plurality of control modes of the twin electric supercharger system, the control unit calculates and compares the electric power consumption and determines that the control mode with the lowest electric power consumption is the optimal control mode. A characterizing feature in the present invention is that, for the calculation and comparison of the electric power consumption for each of the control modes, the control unit performs a comparison of the last electric power consumption, to which a preset hysteresis value of the current control mode has been applied, and the electric power consumption of other control modes, and then, when the electric power consumption of another control mode is lower than the last electric power consumption, in which a hysteresis value has been applied to the electric power consumption of the current control mode, makes a decision to switch the current control mode to the other control mode.

A characterizing feature in the present invention is that, when the control mode having the lowest electric power consumption is a single mode, the control unit outputs control values corresponding to the single mode; when the control mode having the lowest electric power consumption is a serial mode, the control unit outputs control values corresponding to the serial mode; when the control mode having the lowest electric power consumption is a parallel mode, the control unit outputs control values corresponding to the parallel mode, and the twin electric supercharger system is driven in accordance with the control values corresponding to each set control mode, and the control values comprise the rotational speed of the supercharger and bypass valve on/off values.

The method for controlling a twin electric supercharger system according to another aspect of the present invention is characterized by comprising steps in which: the control unit detects the current control mode among a plurality of control modes for supercharging in a twin electric supercharger system; when preset conditions for implementing supercharging are detected in the current control mode, the control unit checks whether the plurality of control modes for supercharging in the twin electric supercharger system respectively go over the choke line or the surge line; the control unit calculates and compares the electric power consumption of each of the control modes which go over neither the choke line nor the surge line; and, depending on the results of comparing the electric power consumption, the control unit determines that the control mode with the lowest electric power consumption is the optimal control mode.

A characterizing feature in the present invention is that the method also comprises a step in which: when the preset conditions for switching to a limp home mode are not detected and also preset conditions for implementing supercharging are not detected in the current control mode, the control unit sets the control mode of the twin electric supercharger system to the normal mode. A characterizing feature in the present invention is that in the step of checking whether the plurality of control modes go over the choke line or the surge line, the control unit excludes (inhibits) any control mode which is over the choke line or the surge line from being subject to control mode determination.

A characterizing feature in the present invention is that the control unit is implemented in such a way that, when checking whether the plurality of control modes for supercharging in a twin electric supercharger system are over the choke line, the over-the-choke-line check is not performed with respect to at least one predesignated control mode, and the control mode that is predesignated so as not to perform the over-the-choke-line check comprises a parallel mode.

A characterizing feature in the present invention is that the control unit is implemented in such a way that, when checking whether the plurality of control modes for supercharging in a twin electric supercharger system are over the surge line, the over-the-s urge-line check is not performed with respect to at least one predesignated control mode, and the control mode that is predesignated so as not to perform the over-the-surge-line check comprises a serial mode.

A characterizing feature in the present invention is that, for the calculation and comparison of the electric power consumption for each of the control modes, the control unit performs a comparison of the last electric power consumption, to which a preset hysteresis value of the current control mode has been applied, and the electric power consumption of other control modes, and then, when the electric power consumption of another control mode is lower than the last electric power consumption, in which a hysteresis value has been applied to the electric power consumption of the current control mode, makes a decision to switch the current control mode to the other control mode.

A characterizing feature in the present invention is that, in the step of determining that the control mode with the lowest electric power consumption is the optimal control mode: when the control mode having the lowest electric power consumption is a single mode, the control unit outputs control values corresponding to the single mode; when the control mode having the lowest electric power consumption is a serial mode, the control unit outputs control values corresponding to the serial mode; when the control mode having the lowest electric power consumption is a parallel mode, the control unit outputs control values corresponding to the parallel mode, and the twin electric supercharger system is driven in accordance with the control values corresponding to each set control mode, and the control values comprise the rotational speed of the supercharger and bypass valve on/off values. [Advantages of the Invention]

According to one aspect of the present invention, the present invention ensures that an optimal control mode, among the control modes for supercharging in the twin electric supercharger system, can be determined on the basis of the electric power consumption of each of the electric superchargers which constitute the twin electric supercharger system.

[Brief Description of the Drawings]

FIG. 1 is an illustrative drawing schematically showing features of the apparatus for controlling the twin electric supercharger system according to one embodiment of the present invention.

FIG. 2 is an illustrative drawing for explaining the air flow in the normal mode or limp home mode in FIG. 1.

FIG. 3 is an illustrative drawing for explaining the air flow in the single mode in FIG. 1.

FIG. 4 is an illustrative drawing for explaining the air flow in the serial mode in FIG. 1.

FIG. 5 is an illustrative drawing for explaining the air flow in the parallel mode in FIG. 1.

FIG. 6 is a flowchart for explaining the method for controlling the twin electric supercharger system according to one embodiment of the present invention.

FIG. 7 is an illustrative drawing showing the mechanical efficiency characteristics map of the compressor in the twin electric supercharger system according to the present embodiment.

[Detailed Disclosure for Implementing the Invention]

Hereinafter, an embodiment of the apparatus and method for controlling a twin electric supercharger system according to the present invention will be described with reference to the accompanying drawings. In this process, aspects such as the thickness of lines and the size of components shown in the drawings may be exaggerated for clarity of description and

convenience. In addition, the terms which are mentioned below are defined in relation to their functions in the present invention, and can vary depending on the intention of the user or operator, or on customary practice. Thus, definitions of such terms should be made based on content across the whole of this specification.

FIG. 1 is an illustrative drawing schematically showing features of the apparatus for controlling the twin electric supercharger system according to one embodiment of the present invention.

As shown in FIG. 1 , the apparatus for controlling the twin electric supercharger system according to the present embodiment comprises: an air cleaner (10); a first supercharger (20); a second supercharger (30); an air flowpath forming portion (60); and a control unit (70).

The first supercharger (20) compresses the air supplied through a first bypass pipe (62) from the air cleaner (10) in accordance with the control signal of the control unit (70).

The second supercharger (30) compresses the air supplied through an air supply pipe (61 ) from the air cleaner (10) in accordance with the control signal of the control unit (70).

The air cleaner (10) filters out foreign matter contained in the external air flowing in from the outside.

The air flowpath forming portion (60) forms an air flowpath such that air from an air cleaner (10) is compressed by means of at least one of the first supercharger (20) and second supercharger (30) and is supplied to the engine (80).

The air flowpath forming portion (60) comprises: an air supply pipe (61 ), a first bypass pipe (62), a second bypass pipe (63), a third bypass pipe (64), an air suction pipe (65), a first bypass valve (66), a second bypass valve (67) and a third bypass valve (68).

The air supply pipe (61 ) has one side connected to the air cleaner (10) and the other side connected to the second supercharger (30) so as to supply the air discharged from the air cleaner (10) to the second supercharger (30). The first bypass pipe (62) has one side connected to the air cleaner (10) and the other side connected to the first supercharger (20) so as to supply the air discharged from the air cleaner (10) to the first supercharger (20). The first bypass valve (66) is installed in the first bypass pipe (62) and regulates the air flow supplied through the first bypass pipe (62) to the first supercharger (20). That is to say, the first bypass valve (66) regulates the air flow supplied through the first bypass pipe (62) by regulating its degree of opening in accordance with the control signal of the control unit (70).

The second bypass pipe (63) has one side connected to the second supercharger (30) and the other side connected to the first supercharger (20) so as to supply the air discharged from the second supercharger (30) to the first supercharger (20). The second bypass valve (67) is installed in the second bypass pipe (63) and regulates the air flow supplied from the second supercharger (30) through the second bypass pipe (63) to the first supercharger (20). That is to say, the second bypass valve (67) regulates the air flow supplied through the second bypass pipe (63) to the first supercharger (20) by regulating its degree of opening in accordance with the control signal of the control unit (70).

The third bypass pipe (64) has one side connected to the second supercharger (30) and the other side connected to the air suction pipe (65) so as to supply the air discharged from the second supercharger (30) to the air suction pipe (65).

The third bypass valve (68) is installed in the third bypass pipe (64) so as to regulate the air flow supplied through the third bypass pipe (64) to the air suction pipe (65). That is to say, the third bypass valve (68) regulates the airflow supplied through the third bypass pipe (64) to the air suction pipe (65) by regulating its degree of opening in accordance with the control signal of the control unit (70).

The air suction pipe (65) has one side connected to the first supercharger (20) and the other side connected to the engine (80). Here, an intercooler (40) and a throttle valve (50) can be additionally installed on the other side of the air suction pipe (65).

The intercooler (40) cools the air supplied through the air suction pipe (65).

The intercooler (40) can adopt both a water cooling method which uses cooling water to cool air or an air cooling method which uses air for cooling. The throttle valve (50) is installed on the air suction pipe (65) at the rear end of the intercooler (40) so as to undertake load control when the first supercharger (20) and the second supercharger (30) are not operating, whilst, when the first supercharger (20) or the second supercharger (30) is operating, the throttle valve (50) is controlled to be opened to the maximum so that the pumping loss of the first supercharger (20) or the second supercharger (30) is minimized.

The control unit (70) detects operating region information for controlling the supercharging flow rate to the engine (80), and the operating region information may include the number of engine revolutions, target intake air amount, target suction to compression ratio (target suction to compression ratio of the front and rear ends of the compressor), engine torque and suction air pressure. Here, a separate means for detecting the operating region information may also be included. The control unit (70) determines a control mode in accordance with the detected operating region information and controls the first supercharger (20), the second supercharger (30) and the bypass valves (66, 67, 68) of the air flowpath forming portion (60) in accordance with the determined control mode. In this case, the control unit (70) controls the rotational speed of the first supercharger (20) and the second supercharger (30) and the degree of opening of the first to third bypass valves (66-68) in accordance with the determined control mode (e.g. normal mode or limp home mode, single mode, serial mode and parallel mode).

In particular, in the present embodiment, the control unit (70) determines the optimal control mode, from among the three types of control mode for supercharging in a twin electric supercharger system, on the basis of the operating region target values of the engine (e.g. compression ratio and supercharging flow rate) and the consumed power predicted from the mechanical and electrical efficiencies of each of the electric superchargers that constitute the twin electric supercharger system from the operating region target values in question.

Hereinbelow, the control modes of the twin electric supercharger system according to the present embodiment will be described with reference to FIGS. 2 to 5. FIG. 2 is an illustrative drawing for explaining the air flow in the normal mode or limp home mode in FIG. 1 ; FIG. 3 is an illustrative drawing for explaining the air flow in the single mode in FIG. 1 ; FIG. 4 is an illustrative drawing for explaining the air flow in the serial mode in FIG. 1 ; and FIG. 5 is an illustrative drawing for explaining the air flow in the parallel mode in FIG. 1. As shown in FIG. 2, the general mode (normal mode) or limp home mode is a control mode in which air is supplied through the first bypass pipe (62), the second bypass pipe (63), the third bypass pipe (64) and the air suction pipe (65) to the engine (80), by completely opening the first bypass valve (66), the second bypass valve (67) and the third bypass valve (68).

Here, when the limp home mode satisfies the predesignated conditions for entering the limp home mode from the general mode (normal mode), single mode, serial mode and parallel mode, the limp home mode can be entered.

As shown in FIG. 3, the single mode is a control mode in which the air supplied through the first bypass pipe (62) is compressed by the first supercharger (20) and then the air is supplied through the air suction pipe (65) to the engine (80).

As shown in FIG. 4, the serial mode is a control mode in which the air is compressed by the second supercharger (30) and is delivered through the second bypass pipe (63) to the first supercharger (20), and the air delivered from the second

supercharger (30) is compressed by the first supercharger (20) and then the air is supplied through the air suction pipe (65) to the engine (80).

As shown in FIG. 5, the parallel mode is a control mode in which the air is compressed by the second supercharger (30) and is supplied through the air suction pipe (65) to the engine (80), and, in addition, the air supplied through the first bypass pipe (62) is compressed by the first supercharger (20) and is supplied through the air suction pipe (65) to the engine (80).

As mentioned above, when any control mode among the plurality of control modes is determined, the control unit (70) controls the rotational speed of the first supercharger (20) and the second supercharger (30) in accordance with the control mode in question, and controls the degree of opening of at least one of the first to the third bypass valves (66-68), thereby enabling operation in the control mode in question.

Hereinbelow, a method is described in which the control unit (70) switches to (or determines) the optimal control mode corresponding to the state of the vehicle in the current control mode.

FIG. 6 is a flowchart for explaining the method for controlling the twin electric supercharger system according to one embodiment of the present invention. With reference to FIG. 6, when in the state (S101 ) which assumes that the normal mode is the current control mode of the twin electric supercharger system according to the present embodiment, if for any reason a failure is detected (YES in S102) (i.e. if the conditions which have been set for switching to the limp home mode are satisfied), the control unit (70) sets (S103) the control mode of the twin electric supercharger system to the limp home mode. That is to say, the control unit (70) outputs the control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the limp home mode. But, when the vehicle is not in the initial state, the current control mode is not the normal mode but may be any control mode among the three types of supercharging mode (e.g. single mode, serial mode and parallel mode).

Also, in the case where no failures are detected (NO in S102), a check is made as to whether the preset conditions for implementing supercharging (i.e. conditions for increasing engine output) are satisfied, and if the conditions for implementing supercharging are not satisfied (NO in S104), the control unit (70) sets (S105) the control mode of the twin electric supercharger system to the normal mode. That is to say, the control unit (70) outputs the control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the normal mode.

Meanwhile, in the case in which the preset conditions for implementing

supercharging (i.e. conditions for increasing engine output) are satisfied (YES in S104), the control unit (70) checks (S106) whether the three types of control mode

(e.g. single mode, serial mode and parallel mode) of the twin electric supercharger system according to the present embodiment are all over the choke line.

Here, as shown in FIG. 7, the choke line means the imaginary line that connects and displays the maximum supercharging flow (volume flow) with respect to the compression ratio (pressure ratio), on the mechanical efficiency characteristics map of the compressor in the twin electric supercharger system according to the present embodiment. Here, in the case in which the three types of control mode (e.g. single mode, serial mode and parallel mode) of the twin electric supercharger system are all over the choke line (YES in S106), the control unit (70) sets (S107) the control mode of the twin electric supercharger system to a predesignated allowed mode (e.g. parallel mode). For example, at least one control mode predesignated as the allowed mode (e.g. parallel mode) can be selected so as to avoid performing the over-the-choke-line check. The reason for this is because, if the parallel mode is the control mode used to correspond to several supercharging flow rates, and if one assumes a case in which even the parallel mode is over the choke line and excluded (inhibited) when the other control modes are over the choke line, the control mode can be switched to the normal mode. Therefore, the over-the-choke-line check cannot be performed for at least one of the allowed modes. Furthermore, when the three types of control mode are all over the choke line, the control mode can be set to at least one predesignated allowed mode (e.g. parallel mode).

That is to say, the control unit (70) outputs the control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the parallel mode. But it should be noted that the allowed mode (e.g. parallel mode) is an illustrative mode and the present invention is not limited thereto.

Furthermore, in the case in which the three types of control mode (e.g. single mode, serial mode and parallel mode) of the twin electric supercharger system are not over the choke line (or for the control mode that is not over the choke line or for the remaining control modes apart from the modes that are over the choke line (i.e. inhibited modes)) (NO in S106), the three types of control mode or the control modes that are not over the choke line are all checked (S108) to see whether they are over the surge line.

Here, as shown in FIG. 7, the surge line means the imaginary line that connects and displays the supercharging flow (volume flow) with respect to the maximum compression ratio (pressure ratio), on the mechanical efficiency characteristics map of the compressor in the twin electric supercharger system according to the present embodiment.

By way of example, at least one control mode predesignated as the allowed mode (e.g. serial mode) can be selected so as to avoid performing the over-the-surge-line check. The reason for this is because, if the serial mode is the control mode used for several boostings, and if one assumes a case in which even the serial mode is over the surge line and excluded (inhibited) when the other control modes are over the surge line, the control mode can be switched to the normal mode. Therefore, the over-the-surge-line check cannot be performed for at least one of the allowed modes. Furthermore, when the three types of control mode are all over the surge line, the control mode can be set to at least one predesignated allowed mode (e.g. serial mode).

Here, in the case in which the three types of control mode or the control mode that is not over the choke line are all over the surge line (YES in S108), the control unit (70) sets (S109) the control mode of the twin electric supercharger system to a predesignated allowed mode (e.g. serial mode). That is to say, the control unit (70) outputs the control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the serial mode. But it should be noted that the allowed mode (e.g. serial mode) is an illustrative mode and the present invention is not limited thereto.

Meanwhile, in the case in which the three types of control mode (or three types of supercharging mode) of the twin electric supercharger system according to the present embodiment are all not over the choke line and surge line, the control unit(70) calculates and compares (S1 10) the electric power consumption (EPC) for each control mode, with respect to the three types of control mode (or with respect to the control modes that are not over the choke line and surge line).

Here, when comparing the electric power consumption (EPC) for each control mode, the control unit (70) applies a preset hysteresis value of the current control mode and carries out comparison.

For example, the hysteresis value can be understood to be a type of upper/lower (+,-) margin value.

As an example, when the current control mode is assumed to be the single mode, the final electric power consumption, in which the hysteresis value has been applied to the electric power consumption (EPC) of the single mode, is compared to the electric power consumption (EPC) of another control mode (e.g. serial mode, parallel mode).

That is to say, in order to switch the current control mode (e.g. single mode) to another control mode (e.g. serial mode, parallel mode), the electric power consumption (EPC) of the other control mode (e.g. serial mode, parallel mode) must be smaller than the final electric power consumption (EPC) in which the hysteresis value has been applied to the electric power consumption (EPC) of the single mode. Here, in order to calculate the electric power consumption (EPC) for each control mode, the calculation can be performed by using a predesignated look-up table or a predesignated arithmetic expression (or function).

The control mode with the lowest electric power consumption (EPC) according to the results of calculating and comparing the electric power consumption (EPC) for each control mode is set (S1 12, S1 13, S1 14).

That is to say: when the control mode having the lowest electric power consumption (EPC) is the single mode (S1 12), the control unit (70) outputs control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the single mode (S1 12); when the control mode having the lowest electric power consumption (EPC) is the serial mode (S1 13), the control unit (70) outputs control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the serial mode (S1 13); and when the control mode having the lowest electric power consumption (EPC) is the parallel mode (S 1 14), the control unit (70) outputs control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the parallel mode (S1 14).

Consequently, the control unit (70) drives (S 1 15) the twin electric supercharger system according to the present embodiment by using control values (i.e. the rotational speed of the supercharger (speed setpoint) and on/off of the bypass valves) corresponding to the set control mode.

As described above, the present embodiment ensures that an optimal control mode, among the control modes for supercharging in the twin electric supercharger system, can be determined on the basis of the electric power consumption of each of the electric superchargers which constitute the twin electric supercharger system.

Furthermore, the present embodiment determines the optimal control mode, in which the driving power of the compressor system can be minimized, on the basis of the current operating region target values and mechanical and electrical efficiencies of the compressor, and thus the effort put into engine dynamometer testing can be minimized relative to the optimal control mode determination method based on conventional testing, and also the loss of power consumed by driving the electric superchargers can be minimized so as to allow an improvement in fuel economy for driving the engine by applying the optimal control mode determination method for a twin electric supercharger system in an engine operating region which is driven in accordance with the demands of the driver. The present invention has been described above with reference to the embodiment depicted in the drawings, but the embodiment is merely illustrative, and it will be understood by those of ordinary skill in the art from the embodiment that various modifications and other equivalent embodiments are possible. Therefore, the scope of technical protection of the present invention should be determined by the following claims. Furthermore, the embodiment described in the present specification can be realized as, for example, a method or process, apparatus, software program, data stream or signal. Despite being discussed only in the context of a single form of embodiment (for example, discussed only as a method), embodiments of the features that have been discussed can be realized in other forms (for example, an apparatus or program). The apparatus can be realized as appropriate hardware, software, firmware etc. The method can be realized in apparatuses such as processors which are generally called processing devices and, for example, include computers, microprocessors, integrated circuits and programmable logic devices. Processors include communication devices such as computers for easy communication of information between end users, cell phones, mobile/personal information terminals (personal digital assistants“PDA”) and other devices.

[Description of the Reference Numerals]

10: Air cleaner 20: First supercharger

30: Second supercharger 40: Intercooler

50: Throttle valve 60: Air flowpath forming portion

61 : Air supply pipe 62: First bypass pipe

63: Second bypass pipe 64: Third bypass pipe

65: Air suction pipe 66: First bypass valve

67: Second bypass valve 68: Third bypass valve

70: Control unit 80: Engine