阅读说明：本技术 车辆用内燃机的控制方法以及控制装置 (Method and device for controlling internal combustion engine for vehicle ) 是由 越后亮 于 2017-11-29 设计创作，主要内容包括：内燃机(1)具有由车载电池驱动的电动增压器(2),并且能够切换为将理论空燃比附近设为目标空燃比的化学计量燃烧模式、以及将稀薄空燃比设为目标空燃比的稀薄燃烧模式。在需要设为稀薄燃烧模式的稀薄燃烧运转区域(L)的一部分(L2),电动增压器(2)负担进气量的一部分。在从化学计量燃烧运转区域(S)变换为稀薄燃烧运转区域(L)时,如果充电状态(SOC)小于或等于下限值(SOClim),则不向稀薄燃烧模式切换而持续执行化学计量燃烧模式。由此,能避免因中间的空燃比的运转导致的NOx变差。(An internal combustion engine (1) is provided with an electric supercharger (2) driven by an on-vehicle battery, and is capable of switching between a stoichiometric combustion mode in which the vicinity of a stoichiometric air-fuel ratio is set to a target air-fuel ratio and a lean combustion mode in which a lean air-fuel ratio is set to the target air-fuel ratio, wherein the electric supercharger (2) is burdened with a part of an intake air amount in a part (L2) of a lean combustion operation region (L) required to be set to the lean combustion mode, and wherein, when switching from the stoichiometric combustion operation region (S) to the lean combustion operation region (L), if a state of charge (SOC) is less than or equal to a lower limit value (SOClim), the stoichiometric combustion mode is continuously executed without switching to the lean combustion mode.)

1. A control method of an internal combustion engine for a vehicle, comprising: an internal combustion engine that can be switched between a stoichiometric combustion mode in which a stoichiometric air-fuel ratio is set to a target air-fuel ratio and a lean combustion mode in which a lean air-fuel ratio is set to the target air-fuel ratio; and an electric intake air supply device driven by the vehicle-mounted battery and configured to bear a part of the intake air amount at least under a part of the operation conditions in the lean combustion mode,

a stoichiometric combustion operation region in which the stoichiometric combustion mode is set and a lean combustion operation region in which the lean combustion mode is set are set in advance using a torque and a rotational speed of the internal combustion engine as parameters, and,

when the stoichiometric combustion operation region is shifted to the lean combustion operation region, the required amount of electric power of the electric intake air supply device when the mode is switched to the lean combustion mode is determined,

when the state of charge of the battery is insufficient for the requested amount of electric power, the stoichiometric burn mode is continuously executed.

2. The control method of an internal combustion engine for a vehicle according to claim 1,

the requested electric power amount is set in accordance with the fuel increase amount required for NOx treatment accompanying switching from the stoichiometric combustion mode to the lean combustion mode and switching from the lean combustion mode to the stoichiometric combustion mode thereafter, and the duration of the lean combustion mode at the break-and-break point between the fuel decrease amount due to the lean air-fuel ratio.

3. The control method of an internal combustion engine for a vehicle according to claim 1 or 2, wherein,

switching to a lean combustion mode is permitted when switching to an operating point at which a lean air-fuel ratio can be achieved in the lean combustion operating region without depending on the electric intake air supply device.

4. The control method of an internal combustion engine for a vehicle according to any one of claims 1 to 3,

a lower limit value of the SOC of the battery is set according to the required electric power amount of other electronic devices mounted on the vehicle and the required electric power amount,

the lower limit value is compared with the SOC of the battery to determine whether the state of charge of the battery is insufficient.

5. The control method of an internal combustion engine for a vehicle according to claim 1,

the requested electric power amount is obtained in consideration of a predicted vehicle operation mode during automatic operation of the vehicle.

6. The control method of an internal combustion engine for a vehicle according to any one of claims 1 to 5,

comprising: a lean air-fuel ratio map in which a target air-fuel ratio, which is a lean air-fuel ratio, is assigned to each operating point in the lean combustion operating region; a stoichiometric air-fuel ratio map in which a target air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio is assigned to at least each operating point in the stoichiometric combustion operating region; and a 3 rd air-fuel ratio map in which a target air-fuel ratio in the vicinity of a stoichiometric air-fuel ratio or a target air-fuel ratio that is a lean air-fuel ratio is assigned to each operating point in an operating region including both the stoichiometric operating region and the lean operating region on the premise that the electric intake air supply device is stopped,

when the state of charge of the battery is insufficient, the 3 rd air-fuel ratio map is used.

7. A control device for a vehicle internal combustion engine includes: an internal combustion engine that can be switched between a stoichiometric combustion mode in which a target air-fuel ratio is set to a vicinity of a stoichiometric air-fuel ratio and a lean combustion mode in which a target air-fuel ratio is set to a lean air-fuel ratio; an electric intake air supply device driven by an in-vehicle battery and configured to bear a part of an intake air amount under at least a part of operation conditions in a lean combustion mode; and a controller, wherein the controller is configured to, among other things,

the controller includes a control map in which a stoichiometric combustion operation region in the stoichiometric combustion mode and a lean combustion operation region in the lean combustion mode are set in advance using a torque and a rotational speed of the internal combustion engine as parameters,

when a request for a change from the stoichiometric burn operating region to the lean burn operating region is detected, a requested amount of electric power of the electric intake air supply device when switching to the lean burn mode is determined, and when the state of charge of the battery is insufficient for the requested amount of electric power, the stoichiometric burn mode is continuously executed.

Technical Field

The present invention relates to a control method and a control device for a vehicle internal combustion engine that can switch between a stoichiometric combustion mode in which the vicinity of a stoichiometric air-fuel ratio is set to a target air-fuel ratio and a lean combustion mode in which a lean air-fuel ratio is set to the target air-fuel ratio, and more particularly to a control method and a control device for a vehicle internal combustion engine that require operation of an electric intake air supply device under some of the operating conditions of the lean combustion mode.

Background

In order to reduce fuel consumption, an internal combustion engine is known that can switch between a stoichiometric combustion mode in which a stoichiometric air-fuel ratio is set to a target air-fuel ratio and a lean combustion mode in which a lean air-fuel ratio is set to the target air-fuel ratio. In such an internal combustion engine, it is preferable to set the lean combustion mode under a wider range of engine operating conditions (torque and engine rotational speed) in terms of fuel efficiency reduction.

Patent document 1 discloses a technique for supercharging an internal combustion engine by an electric compressor driven by an in-vehicle battery. When the motor temperature of the electric compressor is in the temperature range in which the operation is restricted, the motor becomes substantially non-supercharging (natural air supply) even in the supercharging region.

However, the amount of NOx discharged from the internal combustion engine (so-called NOx discharge amount from the engine) decreases when the air-fuel ratio is sufficiently lean, and increases if the lean degree is insufficient. Further, under such lean combustion, the usual three-way catalyst does not function. Therefore, in order to reduce fuel efficiency and reduce the amount of NOx discharged from the engine, it is preferable to avoid using an intermediate air-fuel ratio between a lean air-fuel ratio that is sufficiently lean and the stoichiometric air-fuel ratio.

In order to obtain a sufficiently high air-fuel ratio, a large amount of air needs to be supplied into the cylinder, and if a sufficient amount of air cannot be secured under atmospheric pressure, some type of supercharging means or intake air supply device may be necessary.

If an electric intake air supply device such as an electric compressor is used as the intake air supply device for lean combustion, the motor rotation speed is reduced when the state of charge of the battery is insufficient, the air supply is insufficient with respect to the target lean air-fuel ratio, and the actual air-fuel ratio may be lower than the target lean air-fuel ratio. In this case, the NOx discharge amount from the engine increases.

Therefore, an object of the present invention is to avoid an increase in the amount of NOx discharged from the engine by eliminating as much as possible an operation at a lean air-fuel ratio that is not an optimal intermediate air-fuel ratio between the lean air-fuel ratio at which the amount of NOx discharged is small and the stoichiometric air-fuel ratio.

Patent document 1: japanese patent laid-open publication No. 2009-228586

Disclosure of Invention

A control method and a control device for a vehicle internal combustion engine according to the present invention include: an internal combustion engine that can be switched between a stoichiometric combustion mode in which a target air-fuel ratio is set to a vicinity of a stoichiometric air-fuel ratio and a lean combustion mode in which a target air-fuel ratio is set to a lean air-fuel ratio; and an electric intake air supply device that is driven by the in-vehicle battery and that is configured to bear a part of the intake air amount under at least a part of the operating conditions in the lean combustion mode.

In the present invention, a stoichiometric burn operating region set as the stoichiometric burn mode and a lean burn operating region set as the lean burn mode are set in advance with a torque and a rotational speed of an internal combustion engine as parameters, and a requested electric power amount of the electric intake air supply device when switching to the lean burn mode is obtained when switching from the stoichiometric burn operating region to the lean burn operating region, and the stoichiometric burn mode is continuously executed when a state of charge of the battery is insufficient for the requested electric power amount.

That is, when the state of charge of the battery is insufficient, the operation near the target air-fuel ratio is continuously executed without switching to the lean combustion mode. This makes it possible to avoid an increase in NOx associated with the NOx discharge associated with the switching to the lean combustion mode and a subsequent decrease in the operation of the electric intake air supply device. If the air-fuel ratio is near the stoichiometric air-fuel ratio, the exhaust gas can be purified by the three-way catalyst.

Drawings

Fig. 1 is a configuration explanatory diagram showing a system configuration of an internal combustion engine as one embodiment of the present invention.

Fig. 2 is an explanatory diagram of a control map in which a stoichiometric combustion operation region and a lean combustion operation region are set.

Fig. 3 is a flowchart showing a control flow of the combustion mode switching.

FIG. 4 is a flowchart showing essential parts of the embodiment having the 3 rd air-fuel ratio map.

Fig. 5 is a timing diagram showing mode switching of one embodiment in comparison with a comparative example.

Detailed Description

An embodiment of the present invention will be described in detail below with reference to the drawings.

Fig. 1 shows a system configuration of an internal combustion engine 1 as an embodiment of the present invention. This embodiment is a structure in which the electric supercharger 2 and the turbocharger 3 are used simultaneously as the supercharging unit. The internal combustion engine 1 is, for example, a 4-stroke cycle spark ignition gasoline internal combustion engine, and is particularly configured to be switchable between a stoichiometric combustion mode in which a vicinity of a stoichiometric air-fuel ratio (that is, an excess air ratio λ ═ 1) is set to a target air-fuel ratio, and a lean combustion mode in which a lean air-fuel ratio (for example, a vicinity of λ ═ 2) is set to a target air-fuel ratio.

An exhaust turbine 4 of the turbocharger 3 is disposed in an exhaust passage 6 of the internal combustion engine 1, and an upstream-side catalytic converter 7 and a downstream-side catalytic converter 8 using, for example, a three-way catalyst are disposed downstream of the exhaust turbine 4. As the upstream side catalytic converter 7 or the downstream side catalytic converter 8, a so-called NOx storage catalyst can be used. An exhaust muffler 9 is provided on the further downstream side of the exhaust passage 6, and the exhaust passage 6 is opened to the outside through the exhaust muffler 9. The exhaust turbine 4 has a known waste gate valve (not shown) for controlling the boost pressure.

The internal combustion engine 1 has a variable compression ratio mechanism using a multi-link mechanism as a piston-crank mechanism, for example, and is provided with an electric actuator 10 for changing a compression ratio. Further, at least one of the intake valve and the exhaust valve may be provided with an electric variable valve timing mechanism or a variable valve lift mechanism.

A compressor 5 of the turbocharger 3 is disposed in an intake passage 11 of the internal combustion engine 1, and an electronically controlled throttle valve 12 that controls an intake air amount is disposed downstream of the compressor 5. The throttle valve 12 is positioned at the inlet of the manifold portion 11a, and the intake passage 11 branches into the cylinders as an intake manifold at a position downstream of the manifold portion 11 a. An intercooler 13 for cooling the supercharged intake air is provided inside the main pipe portion 11 a. The intercooler 13 has a water-cooled structure in which cooling water is circulated between the radiator 32 and the intercooler by the action of the pump 31.

Further, a recirculation passage 35 having a recirculation valve 34 is provided to communicate the outlet side and the inlet side of the compressor 5. The recirculation valve 34 is controlled to be in an open state when the internal combustion engine 1 decelerates, that is, when the throttle valve 12 is rapidly closed, thereby circulating the pressurized intake air to the compressor 5 via the recirculation passage 35.

An air cleaner 14 is disposed in the most upstream portion of the intake passage 11, and an air flow meter 15 for detecting the intake air amount is disposed downstream of the air cleaner 14. The electric supercharger 2 is disposed between the compressor 5 and the main pipe portion 11 a. That is, the compressor 5 of the turbocharger 3 and the electric supercharger 2 are arranged in series with each other in the intake passage 11 so that the electric supercharger 2 is located on the downstream side with respect to each other.

Further, a bypass passage 16 is provided so as not to connect the inlet side and the outlet side of the electric supercharger 2 via the electric supercharger 2. The bypass passage 16 is provided with a bypass valve 17 for opening and closing the bypass passage 16. When the electric supercharger 2 is stopped, the bypass valve 17 is opened.

The electric supercharger 2 includes: a compressor section 2a interposed in the intake passage 11; and a motor 2b for driving the compressor part 2 a. In fig. 1, the compressor portion 2a is shown as a centrifugal compressor similarly to the compressor 5 of the turbocharger 3, but in the present invention, any type of compressor such as a roots blower or a screw compressor can be used. The motor 2b is driven by an on-board battery not shown as a power source. That is, in the present embodiment, the electric supercharger 2 corresponds to an "electric intake air supply device".

An exhaust gas recirculation passage 21 for recirculating a part of the exhaust gas to the intake system is provided between the exhaust passage 6 and the intake passage 11. One end 21a, which is an upstream end, of the exhaust gas recirculation passage 21 branches from the exhaust passage 6 on the downstream side of the exhaust turbine 4, specifically, from between the upstream catalytic converter 7 and the downstream catalytic converter 8. The other end 21b, which is a downstream end, is connected to the intake passage 11 at a position on the upstream side of the compressor 5. An exhaust gas recirculation control valve 22 whose opening degree is variably controlled in accordance with an operating condition is attached to an intermediate portion of the exhaust gas recirculation passage 21, and an EGR gas cooler 23 that cools recirculated exhaust gas is provided on the exhaust passage 6 side of the exhaust gas recirculation control valve 22.

The internal combustion engine 1 is controlled by the engine controller 37. In addition to the air flow meter 15, detection signals of various sensors such as a crank angle sensor 38 for detecting the engine rotation speed, a water temperature sensor 39 for detecting the cooling water temperature, an accelerator opening sensor 40 for detecting the amount of depression of an accelerator pedal operated by the driver as a sensor for detecting the torque request of the driver, a boost pressure sensor 41 for detecting the boost pressure (intake air pressure) of the manifold portion 11a, and an air-fuel ratio sensor 42 for detecting the exhaust air-fuel ratio are input to the engine controller 37. Battery controller 43 that detects the SOC (state of charge), which is the state of charge of the battery, not shown, is connected to engine controller 37, and a signal indicating the SOC is input from battery controller 43 to engine controller 37. The engine controller 37 optimally controls the fuel injection amount, the injection timing, and the ignition timing of the internal combustion engine 1, the opening degree of the throttle valve 12, the operation of the electric supercharger 2, the opening degree of the bypass valve 17, the opening degree of a not-shown wastegate valve, the opening degree of the recirculation valve 34, the opening degree of the exhaust gas recirculation control valve 22, and the like based on the detection signals.

Fig. 2 shows a control map in which a stoichiometric burn operating region S that needs to be set to a stoichiometric burn mode and a lean burn operating region L that needs to be set to a lean burn mode are set with the torque (in other words, the load) and the rotational speed of the internal combustion engine 1 as parameters, the control map is stored in advance in a storage device of the engine controller 37 together with a target air-fuel ratio map described later, the lean burn operating region L is set to a low/intermediate speed region in which the torque is small, the other regions except for the lean burn operating region L are substantially the stoichiometric burn operating region S. in addition, not shown in detail, but in the stoichiometric burn operating region S, the target air-fuel ratio and the stoichiometric air-fuel ratio are slightly lean in a region close to the fully open region, and here, the lean burn operating region L includes a 1 st lean burn operating region L in which the supply of air is not dependent on the electric supercharger 2, and a 2 nd lean burn operating region L in which the electric supercharger 2 makes a part of the supply of air dependent on the lean burn operating region L, that is a low load region, i.e., a lean burn operating region L in which is dependent on the electric supercharger 2.

In the present invention, "vicinity" of the stoichiometric air-fuel ratio refers to an air-fuel ratio range in which a three-way catalytic action can be achieved, and for example, may be a value in the range of 14.5 to 15.0 when the stoichiometric air-fuel ratio is set to 14.7, and may be a value in the range of 14.6 "or a value in the range of 14.6.6" when other operating conditions are considered.

On the other hand, if the operating conditions of the internal combustion engine 1 are within the lean operation region L, a lean air-fuel ratio map is used as the target air-fuel ratio map, and operation is performed in a lean combustion mode in which the fuel injection timing, the ignition timing, and the like are set to be suitable for lean combustion, and the lean air-fuel ratio map assigns the target air-fuel ratio as the lean air-fuel ratio to each operating point of the lean combustion operation region L.

In the 1 st lean burn operation region L1 and the 2 nd lean burn operation region L2 in the lean burn operation region L, the target air-fuel ratio is not greatly different, and the target air-fuel ratio is set in both the vicinity of "λ ═ 2" as the target air-fuel ratio, however, in the 1 st lean burn operation region L1, the target lean air-fuel ratio can be achieved without depending on the electric supercharger 2, whereas in the 2 nd lean burn operation region L2, the target air-fuel ratio is set on the assumption of the operation of the electric supercharger 2, and therefore, if the electric supercharger 2 does not achieve the desired operation, the target lean air-fuel ratio cannot be achieved in the 2 nd lean burn operation region L2.

Here, it is assumed that if the operation is switched to the lean burn mode in the lean burn operation region L, particularly in the 2 nd lean burn operation region L2, when the SOC of the vehicle-mounted battery is low, the electric power supplied to the electric supercharger 2 becomes insufficient in a short period of time, and the intake air supply by the electric supercharger 2 is reduced, and the target lean air-fuel ratio may not be maintained.

Further, when NOx adsorbed to the catalytic converters 7 and 8 is treated by so-called rich stoichiometric treatment, which is a temporary enrichment of the air-fuel ratio, the fuel consumption accompanying the rich stoichiometric treatment increases due to an increase in the amount of NOx discharged from the engine. The fuel efficiency is improved in the operation in the lean combustion mode in which the target air-fuel ratio is made lean as compared with the operation in the stoichiometric combustion mode, but if the operation in the lean combustion mode is performed in a short time with an increase in the NOx emission amount from the engine, the fuel consumption is rather deteriorated due to the request for the rich stoichiometric process.

Therefore, in the present embodiment, when operation in the lean combustion mode is requested, when the SOC of the battery is equal to or less than a predetermined threshold (i.e., a lower limit), the stoichiometric combustion mode is continuously executed without switching to the lean combustion mode, that is, even if the battery is switched to the lean combustion operation region L, the target air-fuel ratio is set to an air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio based on the stoichiometric air-fuel ratio map.

Fig. 3 is a flowchart showing a control flow of such combustion mode switching. The flow shown in this flowchart is repeatedly executed by the engine controller 37 at predetermined calculation cycles. In step 1, various parameters are read based on signals input from the sensors and internal signals calculated in the engine controller 37. Specifically, the accelerator opening (the amount of depression of the accelerator pedal) APO, the rotation speed Ne of the internal combustion engine 1, the torque Te of the internal combustion engine 1, the boost pressure, and the like are read.

In step 2, it is determined whether or not the operating point in the present calculation cycle is within the lean burn operating region L. if it is within the stoichiometric burn operating region S, the routine proceeds from step 2 to step 4, where the stoichiometric air-fuel ratio map is selected as the target air-fuel ratio map, and the routine proceeds to step 5, where the operation in the stoichiometric burn mode is continued.

If it is determined in step 2 that the engine is within the lean combustion operation region L, the routine proceeds to step 3 to determine whether or not the current combustion mode, in other words, the combustion mode up to the present time is the stoichiometric combustion mode, and if it is determined here to be NO, the routine has already changed to the lean combustion mode, and therefore the routine proceeds to step 6 to select a lean air-fuel ratio map as the target air-fuel ratio map, and the routine proceeds to step 7 to continue the operation in the lean combustion mode.

When it is determined in step 3 that the combustion mode up to now is the stoichiometric combustion mode, it means that the stoichiometric combustion operation region S is shifted to the lean combustion operation region L, and therefore the routine proceeds from step 3 to step 8, where it is determined whether or not the SOC of the battery exceeds a lower limit value SOClim, which will be described later.

If it is determined in step 8 that the SOC of the battery exceeds the lower limit value SOClim, the routine proceeds from step 8 to step 6, where a lean air-fuel ratio map is selected as a target air-fuel ratio map, and proceeds to step 7, where the operation in the lean burn mode is performed, that is, the operation is switched from the stoichiometric burn mode to the lean burn mode, and further, when the operation is switched from the stoichiometric burn mode to the lean burn mode, the electric supercharger 2 is driven temporarily in order to compensate for a response delay in the change of the intake air amount even in the 1 st lean burn operation region L1.

On the other hand, if it is determined in step 8 that the SOC of the battery is less than or equal to the lower limit value SOClim, the routine proceeds to step 9, where the stoichiometric air-fuel ratio map is selected as the target air-fuel ratio map, and the routine proceeds to step 10, where the operation in the stoichiometric burn mode is performed, that is, even if the operating point shifts to the lean burn operating region L, the switching to the lean burn mode is prohibited, and the operation in the stoichiometric burn mode is continued as it is.

The lower limit value SOClim is an example, and is set to satisfy the amount of electric power of the electric supercharger 2 that can maintain the target air-fuel ratio in the lean combustion mode in the 2 nd lean combustion operation region L2 for a certain period of time after switching to the lean combustion mode, specifically, the lower limit value SOClim is set based on the sum (i.e., total electric power request) of the amount of electric power of the electric supercharger 2 that is required to maintain the target air-fuel ratio in the lean combustion mode in the 2 nd lean combustion operation region L2 for a certain period of time and the amount of electric power required by other electronic devices including the electronic devices attached to the internal combustion engine 1 such as the electric actuator 10 for the variable compression ratio mechanism, and can be estimated from various parameters including the torque Ne and the rotational speed of the internal combustion engine 1, in relation to the pressure difference between the inlet-side pressure and the outlet-side pressure of the electric supercharger 2 that is requested.

In another example, the NOx process is performed by a rich stoichiometric process, and the lower limit value soclim is set by first finding an operating point in which the difference between the fuel consumption amounts in the lean combustion mode in which the lean air-fuel ratio is set to the target air-fuel ratio and the stoichiometric combustion mode in the vicinity of the stoichiometric air-fuel ratio is the smallest in the 2 nd lean combustion operating region L2, and calculating the difference (Δ F) in the fuel consumption amount per unit time at the operating point.

Then, the operation duration T of the lean combustion mode at the break-and-break point is calculated in which the fuel amount Δ F · T (i.e., the fuel reduction amount due to the lean air-fuel ratio) obtained by multiplying the fuel consumption difference Δ F per unit time by the operation duration T at the operation point exceeds the fuel amount Fa required for the rich stoichiometric process (i.e., the fuel increase amount associated with the switching of the combustion mode). That is, if the lean air-fuel ratio can be maintained longer than this time T, it is advantageous in terms of fuel consumption to perform switching to the lean combustion mode. Conversely, when the lean combustion mode using the electric supercharger 2 cannot be maintained longer than the time T at which the lean branch point is reached, there is a possibility that the fuel consumption will be degraded depending on the operating point, and therefore it is advantageous not to switch the combustion mode.

Finally, the power consumption when the electric supercharger 2 is operated at the maximum output for the time T is determined, and the lower limit SOClim is set based on the power consumption. As described above, it is preferable to consider the power consumption of other electronic devices during the time T.

In another example, in the case where the vehicle is automatically traveling, the amount of power that can be requested to continue the operation in the lean burn mode based on the lean air-fuel ratio, and the lower limit value SOClim of the battery SOC may be set in sequence in consideration of the predicted operation mode of the vehicle, and for example, the amount of requested power required to switch the combustion mode may be obtained by calculating the power consumption of the electric supercharger 2 at each operation point in the 2 nd lean burn operation region L2 cumulatively by using one or more information such as the vehicle speed, the distance between the vehicle and the preceding vehicle followed by the vehicle, the set cruising speed, the road gradient, the road curvature, and the signal condition, and by predicting the change in the operation point of the internal combustion engine 1 within a certain time (for example, 10 seconds) thereafter.

Fig. 5 is a timing chart for explaining the operation of the control described above, where the left row in the drawing shows an example, and the right row in the drawing shows a comparative example, each showing the operation in the case where the transition is made from the stoichiometric burn operation region S to the 1 st lean burn operation region L1 at time t1, and the transition is made from the 1 st lean burn operation region L1 to the 2 nd lean burn operation region L2 at time t2 thereafter, (a) shows a change in the SOC of the battery, (b) shows a change in the electric power supplied to the electric supercharger 2, (c) shows a change in the air excess ratio of the internal combustion engine 1, and (d) shows a change in the NOx emission amount.

First, a comparative example will be described, in the illustrated example, when the battery is switched from the stoichiometric burn operating region S to the 1 st lean burn operating region L1 within time t1, although the SOC of the battery is less than or equal to the lower limit value SOClim, switching from the stoichiometric burn mode to the lean burn mode is performed.

The electric supercharger 2 is operated because the electric supercharger 2 is kept in a stopped state until time t2 in the 1 st lean burn operation region L1, and the target lean air-fuel ratio is maintained because the electric supercharger 2 is stopped because the electric supercharger 2 is shifted from the 1 st lean burn operation region L1 to the 2 nd lean burn operation region L2 within time t2, but the electric supercharger is not sufficiently supplied with electric power because the SOC of the battery is low, and is stopped within a short time in the illustrated example.

In addition, the broken line in the comparative example indicates the characteristic in the case where the stoichiometric combustion mode is forcibly switched to at a stage where the rotation speed of the electric supercharger 2 is reduced to some extent. In this case, when switching from the lean combustion mode to the stoichiometric combustion mode, the air-fuel ratio still passes through the intermediate air-fuel ratio region, and NOx temporarily increases.

In contrast, in the embodiment shown in the left row of the drawing, when the shift is made from the stoichiometric burn operating region S to the 1 st lean burn operating region L1 within time t1, the SOC of the battery is less than or equal to the lower limit value SOClim, and therefore the operation in the stoichiometric burn mode is continued without switching to the lean burn mode.

Next, fig. 4 shows the essential parts of the flowchart of example 2 having a 3 rd air-fuel ratio map for the case where the SOC of the battery is low, which is different from the normal stoichiometric air-fuel ratio map and the lean air-fuel ratio map, and the parts not shown in the flowchart are the same as the flowchart of fig. 3, the 3 rd air-fuel ratio map allocates a target air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio or a target air-fuel ratio as a lean air-fuel ratio on the premise of stopping the electric supercharger 2 for each operating point of the operating region including both the stoichiometric burn operating region S and the lean burn operating region L, for example, in the stoichiometric burn operating region S and the 2 nd lean burn operating region L2, the target air-fuel ratio is substantially in the vicinity of the stoichiometric air-fuel ratio, in the 1 st lean burn operating region L1, the target air-fuel ratio is substantially an air-fuel ratio equivalent to "λ ═ 2", but in the 1 st lean burn operating region L1 and the 2 nd lean burn operating region L2, the target air-fuel ratio is set to be as small as possible (for example, the case where λ 0 is considered).

As shown in FIG. 4, when it is determined in step 8 that the SOC of the battery is less than or equal to the lower limit value SOClim, the process proceeds from step 8 to step 9A, where the 3 rd air-fuel ratio map is selected as the target air-fuel ratio map, and the process proceeds to step 10A, where the internal combustion engine 1 is operated in the lean burn mode or the stoichiometric burn mode in accordance with the value of the target air-fuel ratio assigned to the 3 rd air-fuel ratio map at the operating point at that time.

While one embodiment of the present invention has been described in detail, the present invention is not limited to the above embodiment, and various modifications may be made. For example, in the above-described embodiment, the example in which the air-fuel ratio in the lean combustion mode corresponds to "λ ═ 2" has been described, but the present invention is not limited to this, and an appropriate lean air-fuel ratio may be used. In the above embodiment, the electric supercharger 2 is provided as the electric intake air supply device, but another type of electric intake air supply device such as an electric assist turbocharger that assists rotation of a rotor driven by exhaust energy with an electric motor may be used. Further, the electric supercharger and the electric assist turbocharger may be used together.