EP3730771A1

EP3730771A1 - Engine control method, computer program product, engine control device, and engine system

Info

Publication number: EP3730771A1
Application number: EP20162076.2A
Authority: EP
Inventors: Takuya Ohura
Original assignee: Mazda Motor Corp
Current assignee: Mazda Motor Corp
Priority date: 2019-04-22
Filing date: 2020-03-10
Publication date: 2020-10-28
Also published as: JP2020176592A

Abstract

When an engine is operated in a first range, a stoichiometric/rich all-cylinder operation execution step is executed where an air-fuel ratio in each cylinder is set to a stoichiometric air-fuel ratio or less, and air-fuel mixture is combusted in all of the cylinders, when a determination is made that the engine is operated in a second range, a lean reduced-cylinder operation execution step is executed where the air-fuel ratio in each cylinder is set higher than the stoichiometric air-fuel ratio, and air-fuel mixture is combusted only in part of the cylinders and, during a predetermined period after a determination is made that an operation point of the engine is shifted from the first range to the second range, a switching step is executed where an amount of air to be introduced into each cylinder is increased such that the air-fuel ratio in each cylinder becomes higher than the stoichiometric air-fuel ratio, and air-fuel mixture is combusted in all of the cylinders.

Description

[Technical Field]

The present invention relates to an engine control method, a computer program product, an engine control device, and an engine system.

[Background Art]

As disclosed in patent document 1, a technique has been studied where, in an engine including a plurality of cylinders, a reduced-cylinder operation is executed where air-fuel mixture is combusted only in part of the cylinders in some operating ranges. In the reduced-cylinder operation, operation of part of the cylinders is stopped and hence, fuel efficiency of the entire engine can be increased.
Further, as the configuration for increasing fuel efficiency, a configuration is also known where air-fuel mixture is combusted in a state where the air-fuel ratio of the air-fuel mixture is set higher than the theoretical air-fuel ratio (lean state).

[Citation List]

[Patent Literature]

[Patent Literature 1] Japanese Patent Laid-Open No. 2006-183556

[Summary of Invention]

[Technical Problem]

It is considered that when a reduced-cylinder operation is executed and, further, an air-fuel ratio in a cylinder to be operated during the reduced-cylinder operation (a cylinder where air-fuel mixture is combusted) is set higher than the stoichiometric air-fuel ratio, fuel efficiency can be remarkably increased. However, in the reduced-cylinder operation in a state where an air-fuel ratio in a cylinder is set higher than the stoichiometric air-fuel ratio, engine torque which can be acquired is relatively small and hence, in some operating ranges, it is necessary to set the air-fuel ratio in the cylinder to a value in the vicinity of the stoichiometric air-fuel ratio. Accordingly, in the configuration where the reduced-cylinder operation is executed in a state where the air-fuel ratio in the cylinder is set higher than the stoichiometric air-fuel ratio, it is necessary to switch between an operation mode where the reduced-cylinder operation is executed in a state where the air-fuel ratio in the cylinder is set higher than the stoichiometric air-fuel ratio and an operation mode where the air-fuel ratio in the cylinder is controlled to a value in the vicinity of the stoichiometric air-fuel ratio. Even if opening or the like of a throttle valve is changed, the amount of air to be introduced into the cylinder is not immediately varied. Accordingly, immediately after the operation mode is switched from the operation mode where the air-fuel ratio in the cylinder is controlled to a value in the vicinity of the stoichiometric air-fuel ratio to the operation mode where the reduced-cylinder operation is executed in a state where the air-fuel ratio in the cylinder is set higher than the stoichiometric air-fuel ratio, there is a possibility that the sufficient amount of air is not introduced into each cylinder so that appropriate combustion is not realized and, eventually, an appropriate engine torque is not realized.
The present invention has been made under such circumstances, and it is an object of the present invention to appropriately ensure an engine torque while increasing fuel efficiency.

[Solution to Problem]

To overcome the problem, the present invention is defined in independent claims. Preferred embodiments are defined in dependent claims.
An aspect of the present invention provides a method for controlling an engine including a plurality of cylinders. The method includes an operating range determination step of determining a range in which the engine is operated, a stoichiometric/rich all-cylinder operation execution step of setting an air-fuel ratio in each cylinder to a stoichiometric air-fuel ratio or less, and combusting air-fuel mixture in all of the cylinders, in a case where a determination is made in the operating range determination step that the engine is operated in a predetermined first range, a lean reduced-cylinder operation execution step of executing a lean reduced-cylinder operation in which the air-fuel ratio in each cylinder is set higher than the stoichiometric air-fuel ratio, and the air-fuel mixture is combusted only in part of the cylinders, in a case where a determination is made in the operating range determination step that the engine is operated in a predetermined second range, and a switching step of increasing an amount of air to be introduced into each cylinder such that the air-fuel ratio in each cylinder becomes higher than the stoichiometric air-fuel ratio, and combusting the air-fuel mixture in all of the cylinders. The switching step is executed for a predetermined period before the lean reduced-cylinder operation is started in a case where a determination is made in the operating range determination step that an operation point of the engine is shifted from the first range to the second range.
Particularly, the method includes: an operating range determination step where a range in which an engine is operated is determined; a stoichiometric/rich all-cylinder operation execution step where an air-fuel ratio in each cylinder is set to a theoretical air-fuel ratio (or stoichiometric air-fuel ratio) or less, and air-fuel mixture is combusted in all of the cylinders, the stoichiometric/rich all-cylinder operation execution step being executed in a case where a determination is made in the operating range determination step that the engine is operated in a predetermined first range; a lean reduced-cylinder operation execution step where a lean reduced-cylinder operation is executed where the air-fuel ratio in each cylinder is set higher than the stoichiometric air-fuel ratio, and the air-fuel mixture is combusted only in part of the cylinders, the lean reduced-cylinder operation execution step being executed in a case where a determination is made in the operating range determination step that the engine is operated in a predetermined second range; and a switching step where an amount of air to be introduced into each cylinder is increased such that the air-fuel ratio in each cylinder becomes higher than the stoichiometric air-fuel ratio, and the air-fuel mixture is combusted in all of the cylinders, the switching step being executed for a predetermined period before the lean reduced-cylinder operation is started in a case where a determination is made in the operating range determination step that an operation point of the engine is shifted from the first range to the second range. (Claim1)
In the above method, in the second range, the lean reduced-cylinder operation is executed where an air-fuel ratio in the cylinder is set higher than the stoichiometric air-fuel ratio, and air-fuel mixture is combusted only in part of the cylinders. With such a configuration, fuel efficiency can be increased with certainty.
Further, in the above method, when the operating range of the engine is shifted from the first range, in which the stoichiometric/rich all-cylinder operation is executed where an air-fuel ratio in each cylinder is set to the stoichiometric air-fuel ratio or less, and air-fuel mixture is combusted in all of the cylinders, to the second range, where the lean reduced-cylinder operation is executed, during a predetermined period until the lean reduced-cylinder operation is started, a switching step is executed where air-fuel mixture is combusted in all of the cylinders while the amount of air to be introduced into each cylinder is increased such that the air-fuel ratio in each cylinder becomes higher than the stoichiometric air-fuel ratio. That is, during a period which is immediately after the operating range of the engine is shifted, and there is a great shortage of the amount of air (a shortage with respect to an amount suitable for combustion in the second range) in each cylinder, the reduced-cylinder operation is not executed, but the all-cylinder operation is executed where combustion is performed in all of the cylinders. Accordingly, it is possible to avoid a situation where, in a state where there is a great shortage of the amount of air so that appropriate combustion is not easily realized, the number of operating cylinders is reduced so that an appropriate engine torque cannot be acquired. Therefore, it is possible to appropriately ensure an engine torque.
The predetermined period may be set to a period until an end of combustion performed at least one time in all of the cylinders from shift of the operation point of the engine from the first range to the second range.
As the configuration different from the above, the predetermined period may be set to a period until an amount of intake air to be introduced into each cylinder reaches a predetermined reference amount from shift of the operation point of the engine from the first range to the second range.
With these configurations, when the amount of intake air to be introduced into each cylinder, that is, the amount of air in each cylinder, reaches the amount of air suitable for the reduced-cylinder operation, the reduced-cylinder operation can be started. Accordingly, appropriate combustion can be realized with more certainty.
Particularly, the method further includes performing spark ignition combustion and/or compression ignition combustion in the first range.
Further particularly, the method further includes performing combustion in which spark ignition combustion and compression ignition combustion are combined in the first range.
Further particularly, the method further includes performing combustion in which spark ignition combustion and compression ignition combustion are combined in the second range.
Further particularly, a computer program product comprising computer-readable instructions which, when loaded and executed on an ECU), perform the above method.
A aspect of the present invention also provides a control device for an engine including a plurality of cylinders, the control device including: an air amount changer configured to change an amount of air to be introduced into each cylinder; a fuel supplier configured to individually supply fuel to each cylinder; and a controller configured to control the air amount changer and the fuel supplier such that an operational state of each cylinder is switched between an all-cylinder operational state, where air-fuel mixture is combusted in all of the cylinders, and a reduced-cylinder operational state where the air-fuel mixture is combusted only in part of the cylinders, wherein in a state where the engine is operated in a predetermined first range, the controller controls the air amount changer such that the air-fuel ratio in each cylinder becomes a stoichiometric air-fuel ratio or less, and the controller controls the fuel supplier such that the operational state of each cylinder assumes/is the all-cylinder operational state, in a state where the engine is operated in a predetermined second range, the controller controls the air amount changer such that the air-fuel ratio in each cylinder becomes higher than the stoichiometric air-fuel ratio, and the controller controls the fuel supplier such that the operational state of each cylinder assumes/is the reduced-cylinder operational state, and in a state where an operation point of the engine is shifted from the first range to the second range, during a predetermined period after the operation point of the engine is shifted to the second range and before the operational state of each cylinder is switched to the reduced-cylinder operational state, the controller controls the fuel supplier such that the operational state of each cylinder assumes/is the all-cylinder operational state, the controller causes the air amount changer to increase the amount of air to be introduced into each cylinder such that the air-fuel ratio in each cylinder becomes higher than the stoichiometric air-fuel ratio, and the controller controls the fuel supplier such that the operational state of each cylinder assumes/is the reduced-cylinder operational state after the predetermined period is elapsed after the operation point of the engine is shifted to the second range.
Also with this device, in the same manner as the above-mentioned method, it is possible to appropriately ensure an engine torque while increasing fuel efficiency.
The predetermined period may be set to a period until an end of combustion performed at least one time in all of the cylinders from shift of the operation point of the engine from the first range to the second range.
As the configuration different from the above, the predetermined period may be set to a period until an amount of intake air to be introduced into each cylinder reaches a predetermined reference amount from shift of the operation point of the engine from the first range to the second range.
With these configurations, in the same manner as the above-mentioned method, the reduced-cylinder operation can be started when the amount of intake air to be introduced into each cylinder, that is, the amount of air in each cylinder reaches an amount of air suitable for the reduced-cylinder operation. Accordingly, it is possible to realize appropriate combustion in each cylinder with more certainty.
Particularly, the controller is configured to perform spark ignition combustion and/or compression ignition combustion in the first range.
Further particularly, the controller is configured to perform combustion in which spark ignition combustion and compression ignition combustion are combined in the first range.
Further particularly, the controller is configured to perform combustion in which spark ignition combustion and compression ignition combustion are combined in the second range.
Further particularly, an engine system includes an engine including a plurality of cylinders, and the above control device.

[Advantageous Effect of Invention]

As has been described above, according to the above engine control method and the above engine control device, it is possible to ensure an appropriate engine torque while increasing fuel efficiency.

[Brief Description of Drawings]

[Fig. 1] Fig. 1 is a system diagram schematically showing an overall configuration of an engine according to one embodiment of the present invention.
[Fig. 2] Fig. 2 is a block diagram showing the control system of the engine.
[Fig. 3] Fig. 3 is a map view where an operating range of the engine is classified based on a difference in operation mode.
[Fig. 4] Fig. 4 is a graph showing a waveform of a heat generation rate during SPCCI combustion.
[Fig. 5] Fig. 5 is a view showing an injection pattern and a waveform of a heat generation rate during the SPCCI combustion.
[Fig. 6] Fig. 6 is a flowchart showing the flow of control when the operating range is shifted.
[Fig. 7] Fig. 7 is a view showing variation over time of respective parameters when the operating range is shifted from a second operating range to a lean SPCCI range.

[Description of Embodiment]

(1) Overall configuration of engine

Fig. 1 is a system diagram schematically showing the overall configuration of an engine to which an engine control method and an engine control device of the present invention are applied. An engine system shown in Fig. 1 is mounted on a vehicle, and includes an engine body 1 forming a power source for traveling. In this embodiment, a four-cycle gasoline direct injection engine is used as the engine body 1. The engine system includes, in addition to the engine body 1, an intake passage 30 through which intake air to be introduced into the engine body 1 flows, an exhaust passage 40 through which an exhaust gas discharged from the engine body 1 flows, and an EGR device 50 which causes a portion of the exhaust gas flowing through the exhaust passage 40 to reflux to the intake passage 30.
The engine system as shown in Fig. 1 is an example of a system having an engine including a plurality of cylinders. All of the features of the engine system as shown in Fig. 1 are not necessarily essential. Particularly, a supercharger may not be essential to the invention.
The engine body 1 includes a cylinder block 3 in which cylinders 2 are formed, a cylinder head 4 mounted on the upper surface of the cylinder block 3 to close the cylinders 2 from above, and pistons 5 respectively inserted into the cylinders 2 such that the pistons 5 are reciprocally slidable/movable. The engine body 1 is of a multi-cylinder type which includes a plurality of cylinders 2 (for example, four cylinders 2 arranged in a direction orthogonal to a paper surface on which Fig. 1 is drawn). However, for the sake of simplification, the description will be made by focusing on only one cylinder 2. Other cylinders are substantially identical to the cylinder 2 as shown in Fig. 1, respectively.
A combustion chamber 6 is defined above the piston 5, i.e., between the piston 5 and the cylinder head 4, and fuel, containing gasoline as a main component, is particularly supplied to the combustion chamber 6 by the injection from an injector 15 described later. Then, the supplied fuel is mixed with air and combusted in the combustion chamber 6 so that the piston 5 which is pushed down by an expansion force caused by such combustion performs a reciprocating motion in an up-down direction. For fuel to be injected into the combustion chamber 6, fuel containing gasoline as a main component is used. This fuel may contain subcomponent, such as bioethanol, in addition to gasoline. In the embodiment, the injector 15 corresponds to "fuel supplier" in Claims.
A crankshaft 7, forming the output shaft of the engine body 1, is provided below the piston 5. The crankshaft 7 is coupled with the piston 5 via a connecting rod 8, and is rotationally driven about the center axis thereof corresponding to the reciprocating motion (vertical motion) of the piston 5.
The geometric compression ratio of the cylinder 2, that is, the ratio of the volume of the combustion chamber 6 when the piston 5 is at the top dead center to the volume of the combustion chamber 6 when the piston 5 is at the bottom dead center is set to 13 or more and 30 or less, which is a value suitable for SPCCI combustion (partial compression ignition combustion) described later.
A crank angle sensor SN1 is provided to the cylinder block 3, the crank angle sensor SN1 detecting the rotation angle (crank angle) of the crankshaft 7 and the rotational speed (engine speed) of the crankshaft 7. An engine coolant temperature sensor SN2 is also provided to the cylinder block 3, the engine coolant temperature sensor SN2 detecting the temperature of engine coolant which flows through a water jacket formed on the cylinder block 3 to cool the engine body 1.
The cylinder head 4 is provided with an intake port 9 and an exhaust port 10 which are open to the combustion chamber 6, an intake valve 11 which opens and/or closes the intake port 9, and an exhaust valve 12 which opens and/or closes the exhaust port 10. Note that the valve type of the engine in this embodiment is a four-valve type of two intake valves and two exhaust valves so that two intake ports 9, two exhaust ports 10, two intake valves 11 and two exhaust valves 12 are provided to each one cylinder 2. In this embodiment, an openable swirl valve 18 is provided to one of the two intake ports 9 connected to one cylinder 2 so that the intensity of swirl flow (a swirl flow which swirls about the cylinder axis) in the cylinder 2 can be changed. The swirl valve 18 may not be essential to the invention.
The intake valve 11 and the exhaust valve 12 are opened and/or closed by valve trains 13, 14, including a pair of camshafts and the like provided to the cylinder head 4, in an interlocking manner with the rotation of the crankshaft 7.
An intake VVT 13a, which can change at least the opening timing of the intake valve 11, is incorporated in the valve train 13 for the intake valve 11. In the same manner, an exhaust VVT 14a, which can change at least closing timing of the exhaust valve 12, is incorporated in the valve train 14 for the exhaust valve 12. The intake VVT 13a and/or the exhaust VVT 14a may not be essential to the invention.
In this embodiment, these intake VVT 13a and exhaust VVT 14a are particularly controlled such that the closing timing of the exhaust valve 12 is set to timing on the retard angle side of the valve opening timing of the intake valve 11 so that valve overlap is realized where both the intake valve 11 and the exhaust valve 12 are brought into an open state for a predetermined period. Further, with control of the intake VVT 13a and the exhaust VVT 14a, a valve overlap period can be changed which is a period during which both the intake valve 11 and the exhaust valve 12 are brought into an open state. When the intake valve 11 and the exhaust valve 12 are driven to realize the valve overlap, internal EGR is performed where a burned gas is discharged from the combustion chamber 6 to at least one of the intake passage 30 and the exhaust passage 40 and, thereafter, this burned gas is introduced into the combustion chamber 6 again. With this internal EGR, the burned gas (internal EGR gas) remains in the combustion chamber 6. The amount of internal EGR gas, which is a burned gas remaining in the combustion chamber 6, varies depending on the valve overlap period so that the amount of internal EGR gas is adjusted by adjusting the above-mentioned valve overlap period. Note that the intake VVT 13a (the exhaust VVT 14a) may be a variable mechanism of a type which changes only closing timing (opening timing) while opening timing (closing timing) of the intake valve 11 (the exhaust valve 12) is fixed, or may be a variable mechanism of a phase type which changes opening timing and closing timing of the intake valve 11 (the exhaust valve 12) simultaneously.
The cylinder head 4 is provided with the injectors 15 which inject fuel (mainly gasoline) into the combustion chamber 6, and spark plugs 16 which ignite air-fuel mixture of fuel injected into the combustion chamber 6 from the injector 15 and air introduced into the combustion chamber 6. The cylinder head 4 is also provided with a cylinder inner pressure sensor SN3, which detects a cylinder inner pressure which is a pressure in the combustion chamber 6.
The injector 15 is particularly an injector of a multinozzle hole type which has a plurality of nozzle holes at the distal end portion of the injector 15. The injector 15 can radially inject fuel from the plurality of nozzle holes. The injector 15 is provided such that the distal end portion of the injector 15 opposes the center portion of the crown surface of the piston 5. Note that although omitted from the drawing, in this embodiment, the crown surface of the piston 5 has a cavity formed by causing a region including the center portion of the crown surface to be recessed to the side opposite to the cylinder head 4 (to the lower side). The injector 15 may have a single nozzle hole at the distal end portion of the injector 15.
The spark plug 16 is disposed at a position slightly displaced toward the intake side with respect to the injector 15.
The intake passage 30 is connected to one side surface of the cylinder head 4 to communicate with the intake port 9. Air (intake air, fresh air) taken into the intake passage 30 from the upstream end of the intake passage 30 is introduced into the combustion chamber 6 through the intake passage 30 and the intake port 9.
The intake passage 30 is provided with, in order from the upstream side of the intake passage 30, an air cleaner 31, which removes foreign substances in intake air, an openable throttle valve 32, which adjusts the flow rate of intake air, a supercharger 33, which compresses and feeds the intake air, an inter-cooler 35, which cools the intake air compressed by the supercharger 33, and a surge tank 36.
The respective portions of the intake passage 30 are provided with an airflow sensor SN4, which detects the flow rate of intake air (the amount of intake air), and an intake air temperature sensor SN5, which detects the temperature of the intake air (intake air temperature). The airflow sensor SN4 is provided to the portion of the intake passage 30 between the air cleaner 31 and the throttle valve 32, and detects the flow rate of intake air which passes through the portion. The intake air temperature sensor SN5 is provided to the surge tank 36, and detects the temperature of intake air in the surge tank 36.
The supercharger 33, which may not be essential to the invention, is a mechanical supercharger which is mechanically linked with the engine body 1. Although the specific type of the supercharger 33 is not particularly limited, for example, any of known superchargers, such as a lysholm-type supercharger, a root-type supercharger, or a centrifugal-type supercharger, may be used as the supercharger 33. An electromagnetic clutch 34, which may not be essential to the invention and which can electrically switch between an engaged state and a released state, is interposed between the supercharger 33 and the engine body 1. When the electromagnetic clutch 34 is engaged, a driving force is transmitted to the supercharger 33 from the engine body 1 so that boosting is performed by the supercharger 33. Whereas, when the electromagnetic clutch 34 is released, transmission of the driving force is shut off so that boosting performed by the supercharger 33 is stopped.
A bypass passage 38 for bypassing the supercharger 33, which many not be essential to the invention, is provided to the intake passage 30. The bypass passage 38 connects the surge tank 36 and an EGR passage 51 described later with each other. An openable bypass valve 39, which may not be essential to the invention, is provided to the bypass passage 38. The bypass valve 39 is a valve for adjusting the pressure of intake air to be introduced into the surge tank 36, that is, a boost pressure. For example, with an increase in opening of the bypass valve 39, the flow rate of intake air which backflows to the upstream side of the supercharger 33 through the bypass passage 38 increases and, as a result, a boost pressure reduces.
The exhaust passage 40 is connected to the other side surface of the cylinder head 4 to communicate with the exhaust port 10. A burned gas (exhaust gas) generated in the combustion chamber 6 is discharged to the outside through the exhaust port 10 and the exhaust passage 40.
A catalyst converter 41 is provided to the exhaust passage 40. The catalyst converter 41 incorporates, in order from the upstream side, a three-way catalyst 41a for purifying an exhaust gas of harmful components (HC, CO, NOx) contained in the exhaust gas, and a GPF (gasoline particulate filter) 41b for collecting particulate matter (PM) contained in the exhaust gas.
An exhaust gas temperature sensor SN6, which detects the temperature of exhaust gas (exhaust gas temperature), is provided to the exhaust passage 40. The exhaust gas temperature sensor SN6 is provided to the portion of the exhaust passage 40 on the upstream side of the catalyst converter 41.
The EGR device 50 includes the EGR passage 51, which connects the exhaust passage 40 and the intake passage 30, and an EGR cooler 52 and an EGR valve 53, which are provided to the EGR passage 51. The EGR passage 51 connects the portion of the exhaust passage 40 on the downstream side of the catalyst converter 41 and the portion of the intake passage 30 between the throttle valve 32 and the supercharger 33. The EGR cooler 52 cools, by heat exchange, an exhaust gas (external EGR gas) which is caused to reflux to the intake passage 30 from the exhaust passage 40 through the EGR passage 51. The EGR valve 53 is provided to the EGR passage 51 on the downstream side of the EGR cooler 52 (the side closer to the intake passage 30) in an openable manner, and adjusts the flow rate of exhaust gas which flows through the EGR passage 51.
A differential pressure sensor SN7 is provided to the EGR passage 51, the differential pressure sensor SN7 detecting a difference between a pressure on the upstream side of the EGR valve 53 and a pressure on the downstream side of the EGR valve 53.

(2) Control system

Fig. 2 is a block diagram showing the control system of the engine. An ECU 100 shown in Fig. 2 is a micro processing unit for centrally controlling the engine, and is formed of well-known CPU, ROM, RAM and the like.
Detection signals from various sensors are inputted to the ECU 100. For example, the ECU 100 is electrically connected with one or more sensors, particularly the above-mentioned crank angle sensor SN1, engine coolant temperature sensor SN2, cylinder inner pressure sensor SN3, airflow sensor SN4, intake air temperature sensor SN5, exhaust gas temperature sensor SN6, and differential pressure sensor SN7. Accordingly, information (that is, crank angle, engine speed, engine coolant temperature, cylinder inner pressure, the amount of intake air, intake air temperature, exhaust gas temperature, and differential pressure between the upstream side and the downstream side of the EGR valve 53) detected by these sensors is sequentially inputted to the ECU 100. Further, a vehicle is provided with an accelerator sensor SN8, which detects the opening of an accelerator pedal operated by a driver driving the vehicle, and a detection signal from this accelerator sensor SN8 is also inputted to the ECU 100.
The ECU 100 controls respective portions of the engine while performing one or more various determinations, arithmetic operations and the like based on input signals from the respective sensors. That is, the ECU 100 is electrically connected with the intake VVT 13a, the exhaust VVT 14a, the injector 15, the spark plug 16, the swirl valve 18, the throttle valve 32, the electromagnetic clutch 34, the bypass valve 39, the EGR valve 53 and the like, and the ECU 100 outputs control signals to each equipment based on the results of the arithmetic operation and the like. This ECU 100 corresponds to "controller" in Claims. Further, in this embodiment, an external EGR gas and an internal EGR gas can be introduced into the combustion chamber 6, and the amount of air to be introduced into the combustion chamber 6 is changed also by the amount of these gases in addition to the opening of the throttle valve 32. Accordingly, the throttle valve 32, the EGR valve 53, which can change the amount of external EGR gas to be introduced into the combustion chamber 6, and at least one of the intake VVT 13a and the exhaust VVT 14a which can vary the amount of internal EGR gas remaining in the combustion chamber 6 correspond to "air amount changer " in Claims.

(3) Basic control

Fig. 3 is a map view for describing a difference in operation mode which corresponds to an engine speed and an engine load. The map as shown in Fig. 3 may not be essential to the invention. One or more maps other than the map as shown in Fig. 3 may be used. As shown in Fig. 3, the operating range of the engine is roughly classified into a plurality of ranges, particularly three operating ranges, that is, a first operating range A, a second operating range B, and a third operating range C.
The third operating range C is a range where an engine speed is equal to or more than a rotational speed N4 at which a predetermined SI is executed. The first operating range A is a range obtained by excluding, from a range where an engine speed is less than the rotational speed N4, at which SI is executed, a range on the high load side, a range on the high rotational speed side, and a range on the extremely low load side. The second operating range B is a remaining range excluding the first operating range A and the third operating range C.
The first operating range A is further divided into a plurality of ranges, particularly a reduced-cylinder lean SPCCI range A1 on the low load side, and an all-cylinder lean SPCCI range A2 on the high load side. To be more specific, the reduced-cylinder lean SPCCI range A1 is a range where an engine load is less than a switching load T3 and equal to or more than a reduced-cylinder operation starting load T1, which is the lower limit load of the first operating range A, and an engine speed is a first rotational speed N1 or more and less than a second rotational speed N2. The all-cylinder lean SPCCI range A2 is a range obtained by excluding the reduced-cylinder lean SPCCI range A1 from the first operating range A. That is, the all-cylinder lean SPCCI range A2 is a range obtained by excluding a portion on the low load side from a range where an engine load is equal to or more than an all-cylinder lean starting load T2, which is lower than the switching load T3, and less than a stoichiometric starting load T4, and an engine speed is less than a third rotational speed N3.

(3-1) SPCCI combustion

In the first operating range A and the second operating range B, that is, in the reduced-cylinder lean SPCCI range A1, the all-cylinder lean SPCCI range A2, and the second operating range B, compression ignition combustion which is mixture of Spark Ignition (SI) combustion and Compression Ignition (CI) combustion (hereinafter, such combustion is referred to as "SPCCI combustion") is performed. Note that "SPCCI" in SPCCI combustion is an abbreviation of "Spark Controlled Compression Ignition". The SPCCI combustion may not be essential to the invention. Particularly, The SI combustion or the CI combustion may not be essential to the invention.
SI combustion is a mode where air-fuel mixture is ignited by the spark plug 16, and the air-fuel mixture is forcibly combusted by flame propagation where a combustion region is expanded from such an ignition point to the surrounding. CI combustion is a mode where air-fuel mixture is combusted by self-ignition under an environment where a temperature and a pressure are increased to high values due to the compression performed by the piston 5. Further, SPCCI combustion, which is a mixture of these SI combustion and CI combustion, is a combustion mode where a portion of air-fuel mixture in the combustion chamber 6 is caused to perform SI combustion by spark ignition, which is performed under an environment immediately before air-fuel mixture is self-ignited and, after the SI combustion, air-fuel mixture remaining in the combustion chamber 6 is caused to perform CI combustion by self-ignition (by a further increase in temperature and pressure to high values caused with SI combustion).
Fig. 4 is a graph showing a variation in heat generation rate (J/deg) with respect to a crank angle when SPCCI combustion occurs. In SPCCI combustion, heat is generated more gently during SI combustion than during CI combustion. For example, in the waveform of the heat generation rate when SPCCI combustion is performed, as shown in Fig. 4, the gradient of the rise is relatively small. Pressure fluctuation (that is, dP/dθ:P is the cylinder inner pressure, and θ is the crank angle) in the combustion chamber 6 is also gentler during SI combustion than during CI combustion. In other words, the waveform of the heat generation rate during SPCCI combustion is formed such that a first heat generation rate portion (a portion indicated by "M1") and a second heat generating portion (a portion indicated by "M2") are continued in this order, the first heat generation rate portion being formed by SI combustion, and having a relatively small gradient of the rise, and the second heat generating portion being formed by CI combustion, and having a relatively large gradient of the rise.
When a temperature and a pressure in the combustion chamber 6 are increased due to SI combustion, unburned air-fuel mixture is self-ignited with the increase in temperature and pressure and hence, CI combustion is started. As exemplified in Fig. 4, at this timing of self-ignition (that is, timing at which CI combustion is started), the gradient of the waveform of the heat generation rate varies from a small gradient to a large gradient. That is, the waveform of the heat generation rate during SPCCI combustion has an inflection point (X in Fig. 4) which appears at timing at which CI combustion is started.
After CI combustion is started, SI combustion and CI combustion are performed in parallel. Heat generation in CI combustion is greater than that in SI combustion and hence, the heat generation rate is relatively large. However, CI combustion is performed after the compression top dead center so that there is no possibility that the gradient of the waveform of the heat generation rate becomes excessively large. That is, after the piston 5 reaches the compression top dead center, a motoring pressure is reduced due to the downward movement of the piston 5, thus suppressing the elevation of the heat generation rate. As a result, it is possible to avoid that dP/dθ during CI combustion becomes excessively large. As described above, due to the nature where CI combustion is performed after SI combustion in SPCCI combustion, dP/dθ, which is the index of combustion noise, does not easily become excessively large and hence, it is possible to suppress combustion noise compared with simple CI combustion (the case where all fuel is caused to perform CI combustion).
SPCCI combustion ends with the end of CI combustion. A combustion speed of CI combustion is higher than that in SI combustion and hence, it is possible to make combustion end timing earlier than that of simple SI combustion (the case where all fuel is caused to perform SI combustion). In other words, in SPCCI combustion, it is possible to set combustion end timing close to the compression top dead center in expansion stroke. Accordingly, in SPCCI combustion, fuel efficiency can be enhanced compared with simple SI combustion.

(3-2) First operating range

In the first operating range A, that is, in the reduced-cylinder lean SPCCI range A1 and the all-cylinder lean SPCCI range A2, to increase fuel efficiency, SPCCI combustion is executed while an air-fuel ratio (A/F) in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio of 14.7 (set to a lean state). In this embodiment, in the first operating range A, an air-fuel ratio in the combustion chamber 6 is increased to an extent that the amount of raw NOx, which is NOx to be generated in the combustion chamber 6, is sufficiently reduced. For example, the air-fuel ratio in the combustion chamber 6 is set to approximately 30 in the first operating range A. Hereinafter, SPCCI combustion of air-fuel mixture where an air-fuel ratio is higher than the stoichiometric air-fuel ratio is referred to as "lean SPCCI combustion".
In the first operating range A, the respective portions of the engine are driven as follows so as to realize lean combustion, particularly lean SPCCI combustion.
In the first operating range A, the injector 15 injects fuel into the combustion chamber 6 at an amount at which an air-fuel ratio (A/F) in the combustion chamber 6 is higher than the stoichiometric air-fuel ratio as described above. In this embodiment, the injector 15 is driven such that substantially the whole amount of fuel to be supplied to the combustion chamber 6 in one cycle is injected into the combustion chamber 6 during an intake stroke. For example, as shown in Fig. 5, most fuel is injected during the intake stroke (Q1) in the first operating range A, and remaining fuel is injected during compression stroke such that the remaining fuel is divided into two times (Q2, Q3). No fuel may be injected in the compression stroke. Furthermore, fuel may be injected one time during the compression stroke.
In the first operating range A, as shown in Fig. 5, the spark plug 16 ignites air-fuel mixture in the vicinity of the compression top dead center, particularly before the compression top dead center. SPCCI combustion is started with this ignition used as a trigger, and a portion of air-fuel mixture in the combustion chamber 6 is caused to perform combustion (SI combustion) by flame propagation and, thereafter, the remaining air-fuel mixture is caused to perform combustion (CI combustion) by self-ignition. Note that, to activate air-fuel mixture, ignition may be additionally performed prior to the ignition executed in the vicinity of the compression top dead center.
In the first operating range A, the opening of the throttle valve 32 is particularly set to opening at which the throttle valve 32 is fully opened or opening close to the opening at which the throttle valve 32 is fully opened.
In the first operating range A, the EGR valve 53 is particularly brought into a fully closed state so that the amount of external EGR gas to be introduced into the combustion chamber 6 is set to zero or substantially zero.
The reason that the amount of external EGR gas is set to zero is to cause a portion of air-fuel mixture to appropriately perform CI combustion in the first operating range A. Specifically, in the first operating range A, an engine load is low so that small heat energy is generated in the combustion chamber 6 and hence, a temperature in the combustion chamber 6 is liable to become low. If the temperature in the combustion chamber 6 is low, the temperature of air-fuel mixture is not sufficiently increased so that the combustion speed of SI combustion is reduced whereby it becomes difficult to cause CI combustion at an appropriate timing. Further, as described above, an external EGR gas is cooled by the EGR cooler 52 so that the external EGR gas has a relatively low temperature. Accordingly, when an external EGR gas with a low temperature is introduced into the combustion chamber 6 in the first operating range A, there is a possibility that the temperature of air-fuel mixture is not sufficiently elevated and hence, appropriate CI combustion is not realized. In view of the above, in the first operating range A, introduction of an external EGR gas into the combustion chamber 6 is stopped.
In the first operating range A, the intake VVT 13a and the exhaust VVT 14a drive the intake valve 11 and the exhaust valve 12 such that these intake valve 11 and exhaust valve 12 realize valve overlap. In this embodiment, the intake valve 11 and the exhaust valve 12 are driven to open for a predetermined period while over the exhaust top dead center. When the valve overlap of the intake valve 11 and the exhaust valve 12 is executed, as described above, internal EGR is performed so that burned gas with high temperature remains in the combustion chamber 6. When the burned gas with high temperature remains in the combustion chamber 6, the temperature of air-fuel mixture is increased and hence, it is possible to cause air-fuel mixture to appropriately perform CI combustion. For this reason, in the first operating range A, the intake VVT 13a and the exhaust VVT 14a (the intake valve 11 and the exhaust valve 12) are controlled as described above. In the first operating range A, for example, a valve overlap period of the intake valve 11 and the exhaust valve 12 is set to approximately 50 to 70° CA (crank angle), and a substantially constant angle is maintained in the entire first operating range A.
In the first operating range A, the swirl valve 18 is fully closed or closed to have low opening close to opening at which the swirl valve 18 is fully closed.
In the first operating range A, the driving of the supercharger 33 is stopped. That is, the electromagnetic clutch 34 is released so that coupling between the supercharger 33 and the engine body 1 is released, and the bypass valve 39 is brought into a fully open state and hence, boosting performed by the supercharger 33 is stopped.
Hereinafter, the above-described control executed in the first operating range A is referred to as "lean SPCCI control" when appropriate.

(All-cylinder operating range A2 and reduced-cylinder operating range A1)

In both the reduced-cylinder lean SPCCI range A1 and the all-cylinder lean SPCCI range A2, the above-mentioned lean SPCCI control is executed so that air-fuel mixture is caused to perform SPCCI combustion while the air-fuel ratio of the air-fuel mixture is set higher than the stoichiometric air-fuel ratio. However, the reduced-cylinder lean SPCCI range A1 and the all-cylinder lean SPCCI range A2 differ from each other in the number of operating cylinders 2. In the all-cylinder lean SPCCI range A2, the all-cylinder operation is executed where combustion is executed in the combustion chamber 6 of each of the cylinders 2 so that all of the cylinders 2 are operated. On the other hand, in the reduced-cylinder lean SPCCI range A1, the reduced-cylinder operation is executed where combustion is executed only in the combustion chambers 6 of part of the cylinders 2 so that only part of the cylinders 2 is operated. For example, in an engine including four cylinders 2, only two cylinders 2 are operated, and operation of remaining two cylinders 2 is stopped. Hereinafter, when appropriate, the cylinder 2 which is operated during the reduced-cylinder operation is referred to as "operating cylinder", and the cylinder 2 operation of which is stopped during the reduced-cylinder operation is referred to as "rest cylinder".
As described above, in the reduced-cylinder lean SPCCI range A1, lean SPCCI control is executed and the reduced-cylinder operation is executed. On the other hand, in the all-cylinder lean SPCCI range A2, lean SPCCI control is executed, and the all-cylinder operation is executed. In the reduced-cylinder lean SPCCI range A1, as described above, the reduced-cylinder operation is executed while the air-fuel ratio of air-fuel mixture in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio. That is, in this reduced-cylinder lean SPCCI range A1, the lean reduced-cylinder operation is executed where an air-fuel ratio in each cylinder 2 is set higher than the stoichiometric air-fuel ratio, and air-fuel mixture is combusted only in part of the cylinders 2. Accordingly, in this embodiment, the reduced-cylinder lean SPCCI range A1 corresponds to "second range" in Claims.
In this embodiment, the reduced-cylinder operation is realized such that the driving of the injectors 15 of the rest cylinders 2 is stopped so that the supply of fuel to the rest cylinders is stopped, while driving of only the injectors 15 of the operating cylinders is maintained so that fuel is supplied only to operating cylinders. However, the amount of fuel to be supplied to the combustion chamber 6 of each cylinder 2 during the reduced-cylinder operation is set larger than the amount of fuel to be supplied to the combustion chamber 6 of each cylinder 2 assuming that the all-cylinder operation is executed. Further, in the reduced-cylinder lean SPCCI range A1, lean SPCCI control is executed even during the reduced-cylinder operation and hence, the injector 15 of the operating cylinder 2 injects fuel at an amount at which an air-fuel ratio in the combustion chamber 6 becomes leaner than the stoichiometric air-fuel ratio as described above. In this embodiment, in the reduced-cylinder lean SPCCI range A1, even during the reduced-cylinder operation, the intake valves 11 and the exhaust valves 12 of the rest cylinders 2 are controlled by the above-mentioned lean SPCCI control so that the intake valves 11 and the exhaust valves 12 of all cylinders 2 are opened and/or closed.

(3-3) Second operating range B

In a range where an engine load and/or an engine speed is extremely low, combustion energy generated in the combustion chamber 6 is low so that the temperature of air-fuel mixture is suppressed to a low temperature. In a range where an engine speed is high, a time of the valve overlap period of the intake valve 11 and the exhaust valve 12 is short so that the amount of burned gas remaining in the combustion chamber 6 is small and hence, the temperature of air-fuel mixture is suppressed to a low temperature. Accordingly, in these ranges, SI combustion is performed slowly and hence, it becomes difficult to cause CI combustion to occur at appropriate timing. However, when the air-fuel ratio of air-fuel mixture is set to the stoichiometric air-fuel ratio or less, the combustion speed of SI combustion is increased so that it is possible to cause CI combustion to occur at appropriate timing, and NOx can be purified by the three-way catalyst 41a. Further, in a range where an engine load is high, the amount of fuel supplied into the combustion chamber 6 is large and hence, it is difficult to set the air-fuel ratio of air-fuel mixture to a lean state. Accordingly, in a range where a load is extremely low, a range where an engine load is higher than that in the first operating range A, and the second operating range B which includes a range where an engine speed is higher than that in the first operating range, air-fuel mixture is caused to perform SPCCI combustion while an air-fuel ratio in the combustion chamber 6 is set to the stoichiometric air-fuel ratio or less. In this embodiment, the air-fuel ratio of air-fuel mixture is set to substantially the stoichiometric air-fuel ratio in the second operating range B.
In the second operating range B, the opening of the throttle valve 32 is set such that the amount of air which corresponds to an engine load is introduced into the combustion chamber 6. In this embodiment, in the second operating range B, the opening of the throttle valve 32 is set to opening close to a fully open state.
In the second operating range B, the injector 15 injects fuel into the combustion chamber 6 at an amount at which an air-fuel ratio assumes/is the stoichiometric air-fuel ratio as described above. In this embodiment, the injector 15 is driven such that most of fuel to be injected during one cycle is injected during intake stroke, and remaining fuel is injected during compression stroke.
Also in the second operating range B, the spark plug 16 ignites air-fuel mixture in the vicinity of the compression top dead center. Also in the second operating range B, SPCCI combustion is started with this ignition used as a trigger, and a portion of air-fuel mixture in the combustion chamber 6 is caused to perform combustion (SI combustion) by flame propagation and, thereafter, remaining air-fuel mixture is caused to perform combustion (CI combustion) by self-ignition.
In the second operating range B, to reduce NOx to be generated in the combustion chamber 6, the EGR valve 53 is opened to introduce an external EGR gas into the combustion chamber 6. However, when an engine load is high, a large amount of air is required to be introduced into the combustion chamber 6 and hence, it is necessary to reduce the amount of external EGR gas to be introduced into the combustion chamber 6. Accordingly, in the second operating range B, the opening of the EGR valve 53 is controlled such that the amount of external EGR gas to be introduced into the combustion chamber 6 is reduced toward the high load side, and the EGR valve 53 is brought into a fully closed state in a range where an engine load is maximum.
Also in the second operating range B, the intake VVT 13a and the exhaust VVT 14a drive the intake valve 11 and the exhaust valve 12 such that these intake valve 11 and exhaust valve 12 realize valve overlap.
In the second operating range B, the swirl valve 18 is opened to appropriate intermediate opening which excludes the fully closed state/fully open state. The higher the engine load, the larger the opening of the swirl valve 18 is set.
The supercharger 33 is stopped on the side of the second operating range B where both an engine speed and an engine load are low. Meanwhile, the supercharger 33 is operated in other ranges in the second operating range B. That is, the electromagnetic clutch 34 is engaged, and the supercharger 33 and the engine body 1 are coupled. At this point of operation, the opening of the bypass valve 39 is controlled such that a pressure (boost pressure) in the surge tank 36 agrees with a target pressure predetermined for each operation condition (rotational speed/load).
Hereinafter, the above-described control executed in the second operating range B is referred to as "stoichiometric SPCCI control" when appropriate. In the second operating range B, as described above, the all-cylinder operation is executed while the air-fuel ratio of air-fuel mixture in the combustion chamber 6 is set to the stoichiometric air-fuel ratio or less. That is, in the second operating range B, the stoichiometric/rich all-cylinder operation is executed where an air-fuel ratio in each cylinder 2 is set to the stoichiometric air-fuel ratio or less, and air-fuel mixture is combusted in all of the cylinders 2. In this embodiment, the second operating range B corresponds to "first range" in Claims.

(3-4) Third operating range C

In the third operating range C, relatively orthodox SI combustion is performed. To realize this SI combustion, in the third operating range C, the injector 15 injects fuel over a predetermined period which overlaps with at least intake stroke. The spark plug 16 ignites air-fuel mixture in the vicinity of the compression top dead center. In the third operating range C, SI combustion is started with this ignition used as a trigger, and all air-fuel mixture in the combustion chamber 6 is combusted by flame propagation.
The supercharger 33 is operated in the third operating range C. The throttle valve 32 is brought into a fully open state. The opening of the EGR valve 53 is controlled such that an air-fuel ratio in the combustion chamber 6 becomes the stoichiometric air-fuel ratio or less. For example, in the third operating range C, the opening of the EGR valve 53 is controlled such that an air-fuel ratio in the combustion chamber 6 becomes the stoichiometric air-fuel ratio or a value slightly smaller than the stoichiometric air-fuel ratio. In the third operating range C, the swirl valve 18 is brought into a fully open state. In the third operating range C, the all-cylinder operation is executed.
Hereinafter, control executed in this third operating range C is referred to as "SI control" when appropriate.
In this embodiment, also in the third operating range C, the all-cylinder operation is executed while the air-fuel ratio of air-fuel mixture in the combustion chamber 6 is set to the stoichiometric air-fuel ratio or less. The term "first range" in what is claimed also includes the third operating range C.

(4) Switching control for operation modes

The flow of control executed by the ECU 100 will be described with reference to flowchart in Fig. 6.
First, in step S1, the ECU 100 acquires values detected by the respective sensors SN1 to SN8.
Next, in step S2, the ECU 100 determines the operating range where the engine is currently operated, that is, the ECU 100 determines which operating range A1, A2, B, C includes the current operation point (hereinafter, referred to as "current operating range" when appropriate). Specifically, the ECU 100 calculates a current engine load, that is, required engine torque (an engine torque which is required), based on the opening of an accelerator pedal detected by the accelerator sensor SN8, an engine speed detected by the crank angle sensor SN1 and the like, and the ECU 100 determines the current operating range from the calculated engine load and the current engine speed.
Next, in step S3, the ECU 100 determines whether or not the current operating range determined in step S2 is the reduced-cylinder lean SPCCI range A1.
When the determination in step S3 is NO where the current operating range is not the reduced-cylinder lean SPCCI range A1, the process proceeds to step S8. In step S8, it is determined whether or not the current operating range is the all-cylinder lean SPCCI range A2. When this determination is YES where the current operating range is the all-cylinder lean SPCCI range A2, the process proceeds to step S9. In step S9, the ECU 100 executes lean SPCCI control, and executes the all-cylinder operation and, then, ends the process (the process returns to step S1).
On the other hand, when the determination in step S8 is NO where the current operating range is not the all-cylinder lean SPCCI range A2, the process proceeds to step S10. In step S10, the ECU 100 determines whether or not the current operating range is the second operating range B (stoichiometric SPCCI range) where stoichiometric SPCCI control is executed. When the determination in step S10 is YES where the current operating range is the second operating range B, the process proceeds to step S11. In step S11, the ECU 100 executes stoichiometric SPCCI control and executes the all-cylinder operation and, then, ends the process (the process returns to step S1). On the other hand, when the determination in step S10 is NO where the current operating range is not the second operating range B, the process proceeds to step S12. In step S12, the ECU 100 executes SI control, and executes the all-cylinder operation and, then, ends the process (the process returns to step S1).
Returning to step S3, when the determination in step S3 is YES where the current operating range is the reduced-cylinder lean SPCCI range A1, the process proceeds to step S4.
In step S4, the ECU 100 determines whether or not the current operating range is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1. The stoichiometric/rich range is an operating range where air-fuel mixture in the combustion chamber 6 is set to the stoichiometric air-fuel ratio or less, and the stoichiometric/rich range means the second operating range B and the third operating range C. That is, in step S4, it is determined whether or not the operation point of the engine is shifted from the second operating range B or the third operating range C to the reduced-cylinder lean SPCCI range A1. To be more specific, in step S4, the ECU 100 determines whether or not conditions are established where the engine is operated in the second operating range B or the third operating range C in the previous arithmetic operation cycle, and the engine is currently operated in the reduced-cylinder lean SPCCI range A1.
When the determination in step S4 is NO where the operation point is not shifted from the stoichiometric range to the reduced-cylinder lean SPCCI range A1, for example, when the operation point is shifted from the all-cylinder lean SPCCI range A2, or when a switching time described later is already elapsed after the operation point is shifted to the reduced-cylinder lean SPCCI range A1, the process proceeds to step S7.
In step S7, the ECU 100 executes lean SPCCI control, and executes the reduced-cylinder operation (in the case where the lean SPCCI control and the reduced-cylinder operation are already executed, the ECU 100 continuously executes the lean SPCCI control and the reduced-cylinder operation).
On the other hand, when the determination in step S4 is YES where the operation point is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1, the process proceeds to step S5.
In step S5, the ECU 100 executes lean SPCCI control, and executes the all-cylinder operation. Note that the all-cylinder operation is always executed in a range excluding the reduced-cylinder lean SPCCI range A1 so that the all-cylinder operation is maintained in step S5.
As described above, in the lean SPCCI control, the respective portions of the engine are controlled such that the air-fuel ratio in the combustion chamber 6 becomes higher than the stoichiometric air-fuel ratio. To the contrary, in the stoichiometric/rich range (the second operating range B and the third operating range C), the air-fuel ratio in the combustion chamber 6 is set to a value in the vicinity of the stoichiometric air-fuel ratio. Accordingly, in step S5, the respective portions of the engine are controlled such that the amount of air to be introduced into the combustion chamber 6 is increase. For example, the throttle valve 32 is set to the open side, or the EGR valve 53 is closed.
After step S5, the process proceeds to step S6. In step S6, the ECU 100 determines whether or not the predetermined switching time is elapsed after the operating range is shifted from the stoichiometric range to the reduced-cylinder lean SPCCI range A1. In this embodiment, the switching time is set to a time required to perform combustion one time in all of the cylinders. For example, in the case of a four-cylinder four cycle engine, a switching time is a time required for four cycles (720° CA: CA being a crank angle). The ECU 100 calculates a switching time from the current engine speed, and use the switching time when making the determination in step S6.
When the determination in step S6 is NO where the switching time is not yet elapsed after the operating range is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1, the ECU 100 returns to step S5 and executes operations. That is, the ECU 100 continuously executes lean SPCCI control, and continuously performs the all-cylinder operation.
On the other hand, when the determination in step S6 is YES where the switching time is elapsed after the operating range is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1, the process proceeds to step S7. In step S7, the ECU 100 executes lean SPCCI control, and executes the reduced-cylinder operation and, then, ends the process (the process returns to step S1). That is, the ECU 100 starts the reduced-cylinder operation while continuously executing lean SPCCI control.
As described above, in this embodiment, when the operating range is shifted from the stoichiometric range to the reduced-cylinder lean SPCCI range A1, the all-cylinder operation is maintained while lean SPCCI control is executed and, only after the switching time is elapsed after the operating range is shifted from the stoichiometric range to the reduced-cylinder lean SPCCI range A1, the reduced-cylinder operation is started.
In this embodiment, a range where the engine is operated is determined in the above-mentioned step S2 so that step S2 corresponds to "operating range determination step" in Claims. Further, in step S11, stoichiometric SPCCI control is executed so that an air-fuel ratio in the combustion chamber 6 is set to the stoichiometric air-fuel ratio or less, and the all-cylinder operation is executed. This step S11 corresponds to "stoichiometric/rich all-cylinder operation execution step" in Claims. Further, in step S7, lean SPCCI control is executed so that an air-fuel ratio in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio, and the reduced-cylinder operation is executed. This step S7 corresponds to "lean reduced-cylinder operation execution step" in Claims. Further, in step S5, lean SPCCI control is executed so that an air-fuel ratio in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio, and the all-cylinder operation is executed. This step S5 corresponds to "switching step" in Claims.

(5) Manner of operation and the like

Fig. 7 shows variation over time of respective parameters when the operating range is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1. Fig. 7 shows an example where the operation point of the engine is shifted from a point P1 to a point P2 as indicated by an arrow Y1 in Fig. 3, and the operating range of the engine is shifted from the second operating range B to the reduced-cylinder lean SPCCI range A1. Fig. 7 also exemplifies the case where, in the engine including four cylinders 2, operation of two cylinders 2 is stopped during the reduced-cylinder operation.
Until a point of time t1, the all-cylinder operation is executed so that four operating cylinders are used. Further, stoichiometric SPCCI control is executed until the point of time t1. As described above, in the stoichiometric SPCCI control, external EGR is executed so that, during a period until the point of time t1, the EGR valve 53 is brought into an open state. In an example in Fig. 7, since the engine load at the operation point P1 is relatively low, the opening of the throttle valve 32 is set to opening at which the throttle valve 32 is closed from the fully open state until the point of time t1.
When a pressing amount of the accelerator pedal (accelerator opening) is reduced at the point of time t1 so that the operating range is shifted to the reduced-cylinder lean SPCCI range A1, lean SPCCI control is started, and the opening of the throttle valve 32 is increased toward a fully open state, and the EGR valve 53 is closed to assume/be a fully closed state. With such operations, the amount of intake air, which is the amount of air to be introduced into the combustion chamber 6, is increased from the point of time t1. However, the throttle valve 32 and the EGR valve 53 have driving delay. Further, these valves 32, 53 are away from the combustion chamber 6 so that even if openings of these valves 32, 53 are changed, the amount of intake air is not immediately changed. Accordingly, even if lean SPCCI control is started, the amount of intake air is not immediately increased and, during a predetermined period after the point of time t1, a state is brought about where the amount of intake air is insufficient with respect to a target value of the amount of intake air at an operation point after the shift indicated by a broken line in Fig. 7, that is, the amount of intake air necessary for realizing appropriate SPCCI combustion at the operation point after the shift.
Appropriate SPCCI combustion is not realized in a state where the amount of intake air is insufficient as described above. Accordingly, if the reduced-cylinder operation is started in such a state so that the number of operating cylinders is reduced, there is a possibility that an engine output is significantly reduced.
To the contrary, in this embodiment, also after the point of time t1, four operating cylinders are maintained so that the all-cylinder operation is continued. Accordingly, combustion energy can be acquired from all of the cylinders 2 and hence, a reduction in engine output can be avoided.
Further, when the switching time is elapsed after lean SPCCI control is started (at a point of time t2), the operating cylinders are reduced to two operating cylinders so that the reduced-cylinder operation is started.
At the point of time t2 where the switching time is elapsed after lean SPCCI control is started, the amount of intake air is an amount in the vicinity of a target value and hence, even when the reduced-cylinder operation is started, an engine output is ensured. In other words, the switching time is set to a time from the start of the lean SPCCI control until a point of time when the amount of intake air reaches an amount in the vicinity of the target value so that engine output can be ensured even if the reduced-cylinder operation is started. Further, it is known that this switching time is substantially equal to a time required to perform combustion one time in all of the cylinders. Accordingly, in this embodiment, as described above, this switching time is set to a time required to perform combustion one time in all of the cylinders.
As described above, in this embodiment, in the reduced-cylinder lean SPCCI range A1, a combustion mode is adopted where air-fuel mixture is combusted in a state where an air-fuel ratio in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio, and the reduced-cylinder operation is executed where the number of operating cylinders is reduced. Accordingly, fuel efficiency can be increased with certainty. Particularly, in this embodiment, in the reduced-cylinder lean SPCCI range A1, SPCCI combustion having high fuel efficiency is executed and hence, fuel efficiency can be remarkably increased. Further, in the second operating range B and the third operating range C where an engine load is high, an air-fuel ratio in the combustion chamber 6 is set to a value in the vicinity of the stoichiometric air-fuel ratio, and a large amount of air is introduced into the combustion chamber 6 and hence, it is also possible to realize a high engine torque which corresponds to an engine load. Further, even in a range or the like where an engine load is extremely low, an air-fuel ratio in the combustion chamber 6 is set to a value in the vicinity of the stoichiometric air-fuel ratio and hence, it is possible to prevent that combustion stability is deteriorated in such a range.
However, as described above, the reduced-cylinder lean SPCCI range A1 where air-fuel mixture is combusted in a state where an air-fuel ratio in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio, and the reduced-cylinder operation is executed and a range where an air-fuel ratio in the combustion chamber 6 is set to the stoichiometric air-fuel ratio or less, that is, the stoichiometric/rich range are present in a mixed manner. Accordingly, when the operating range is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1, as described above, there is a possibility that the reduced-cylinder operation is executed in a state where the amount of air in the combustion chamber 6 is insufficient and hence, an engine output may be reduced with such insufficient amount of air. However, in this embodiment, during a period until the switch period is elapsed from the above-mentioned shift, lean SPCCI control is executed so that the amount of air to be introduced into the respective combustion chambers 6 is increased to set an air-fuel ratio in each combustion chamber 6 becomes higher than the stoichiometric air-fuel ratio by bringing the EGR valve 53 into a fully closed state or by setting the throttle valve 32 to an open side, and the all-cylinder operation is continued to generate combustion energy in a larger number of cylinders 2. With such a configuration, it is possible to avoid that an engine output is rapidly reduced with the shift.

(6) Modification

In the above-mentioned embodiment, the description has been made with respect to the case where air-fuel mixture is caused to perform SPCCI combustion in the operating range where the air-fuel ratio in the combustion chamber 6 is set higher than the stoichiometric air-fuel ratio, and the reduced-cylinder operation is executed. However, the combustion mode of air-fuel mixture in the above-mentioned operating range is not limited to SPCCI combustion. However, as described above, executing SPCCI combustion can remarkably increase fuel efficiency. Further, the combustion mode in another operating range is not limited to the mode described in the above-mentioned embodiment.
Further, in the above-mentioned embodiment, the description has been made with respect to the case where the all-cylinder operation is executed while lean SPCCI control is executed in both the case where the operating range is shifted from the second operating range B to the reduced-cylinder lean SPCCI range A1 and the case where the operating range is shifted from the third operating range C to the reduced-cylinder lean SPCCI range A1. However, it may be configured such that the all-cylinder operation is executed while lean SPCCI control is executed only in the case where the operating range is shifted from one of the second operating range B or the third operating range C to the reduced-cylinder lean SPCCI range A1.
Further, in the above-mentioned embodiment, the description has been made with respect to the case where the switching time is set to a time required to perform combustion one time in all of the cylinders 2. However, the switching time is not limited to such a time, and may be set to a time which is set in advance. Further, the switching time may be set to a time required for the amount of intake air to be introduced into the combustion chamber 6, that is, the amount of air to be introduced into the combustion chamber 6, to reach a predetermined amount from when an operating range is shifted from the stoichiometric/rich range to the reduced-cylinder lean SPCCI range A1. With such a configuration, the reduced-cylinder operation can be started when the sufficient amount of intake air is ensured with more certainty and hence, air-fuel mixture can be combusted in each cylinder 2 in an appropriate state with certainty whereby it is possible to avoid a reduction in engine output with certainty. Particularly, when the predetermined amount is set to a target value in the shift destination, that is, the reduced-cylinder lean SPCCI range A1, the reduced-cylinder operation can be started in a state where the amount of intake air is appropriately ensured with more certainty and hence, appropriate combustion can be realized with more certainty. Further, the number of cylinders of the engine is not limited to four, and this embodiment is applicable to an engine including a plurality of cylinders 2.

[Reference Signs List]

1: engine body
2: cylinder
6: combustion chamber
15: injector (fuel supplier)
32: throttle valve (air amount changer)
53: EGR valve (air amount changer)
100: ECU (controller)
A1: reduced-cylinder lean SPCCI range (second range)
B: second operating range (first range)
C: third operating range (first range)

Claims

A method for controlling an engine including a plurality of cylinders (2), the method comprising:
an operating range determination step of determining a range (A1, B, C) in which the engine is operated;

a stoichiometric/rich all-cylinder operation execution step of setting an air-fuel ratio in each cylinder (2) to a stoichiometric air-fuel ratio or less, and combusting air-fuel mixture in all of the cylinders (2), in a case where a determination is made in the operating range determination step that the engine is operated in a predetermined first range (B, C);

a lean reduced-cylinder operation execution step of executing a lean reduced-cylinder operation in which the air-fuel ratio in each cylinder (2) is set higher than the stoichiometric air-fuel ratio, and the air-fuel mixture is combusted only in part of the cylinders (2), in a case where a determination is made in the operating range determination step that the engine is operated in a predetermined second range (A1); and

a switching step of increasing an amount of air to be introduced into each cylinder (2) such that the air-fuel ratio in each cylinder (2) becomes higher than the stoichiometric air-fuel ratio, and combusting the air-fuel mixture in all of the cylinders (2), the switching step being executed for a predetermined period before the lean reduced-cylinder operation is started in a case where a determination is made in the operating range determination step that an operation point of the engine is shifted from the first range (B, C) to the second range (A1).
The method for controlling an engine according to claim 1, wherein
the predetermined period is a period until an end of combustion performed at least one time in all of the cylinders (2) from shift of the operation point of the engine from the first range (B, C) to the second range (A1).
The method for controlling an engine according to claim 1, wherein
the predetermined period is a period until an amount of intake air to be introduced into each cylinder (2) reaches a predetermined reference amount from shift of the operation point of the engine from the first range (B, C) to the second range (A1).
The method for controlling an engine according to any one of the preceding claims, further comprising
performing spark ignition combustion and/or compression ignition combustion in the first range (B, C).
The method for controlling an engine according to any one of the preceding claims, further comprising
performing combustion in which spark ignition combustion and compression ignition combustion are combined in the first range (B, C).
The method for controlling an engine according to any one of the preceding claims, further comprising
performing combustion in which spark ignition combustion and compression ignition combustion are combined in the second range (A1).
A computer program product comprising computer-readable instructions which, when loaded and executed on an ECU (100), perform a method of any one of the preceding claims.
A control device for an engine including a plurality of cylinders (2), the control device comprising:
an air amount changer (32, 53) configured to change an amount of air to be introduced into each cylinder (2);

a fuel supplier (15) configured to individually supply fuel to each cylinder (2); and

a controller (100) configured to control the air amount changer (32, 53) and the fuel supplier (15) such that an operational state of each cylinder (2) is switched between an all-cylinder operational state, where air-fuel mixture is combusted in all of the cylinders (2), and a reduced-cylinder operational state where the air-fuel mixture is combusted only in part of the cylinders (2), wherein

in a state where the engine is operated in a predetermined first range (B, C), the controller (100) is configured to control the air amount changer (32, 53) such that an air-fuel ratio in each cylinder (2) becomes a stoichiometric air-fuel ratio or less, and the controller (100) is configured to control the fuel supplier (15) such that the operational state of each cylinder (2) is the all-cylinder operational state,

in a state where the engine is operated in a predetermined second range (A1), the controller (100) is configured to control the air amount changer (32, 53) such that the air-fuel ratio in each cylinder (2) becomes higher than the stoichiometric air-fuel ratio, and the controller (100) is configured to control the fuel supplier (15) such that the operational state of each cylinder (2) is the reduced-cylinder operational state, and

in a state where an operation point of the engine is shifted from the first range (B, C) to the second range (A1), during a predetermined period after the operation point of the engine is shifted to the second range (A1) and before the operational state of each cylinder (2) is switched to the reduced-cylinder operational state, the controller (100) is configured to control the fuel supplier (15) such that the operational state of each cylinder (2) is the all-cylinder operational state, the controller (100) is configured to cause the air amount changer (32, 53) to increase the amount of air to be introduced into each cylinder (2) such that the air-fuel ratio in each cylinder (2) becomes higher than the stoichiometric air-fuel ratio, and the controller (100) is configured to control the fuel supplier (15) such that the operational state of each cylinder (2) is the reduced-cylinder operational state after the predetermined period is elapsed after the operation point of the engine is shifted to the second range (A1).
The control device for an engine according to claim 8, wherein
the predetermined period is a period until an end of combustion performed at least one time in all of the cylinders (2) from shift of the operation point of the engine from the first range (B, C) to the second range (A1).
The control device for an engine according to claim 8, wherein
the predetermined period is a period until an amount of intake air to be introduced into each cylinder (2) reaches a predetermined reference amount from shift of the operation point of the engine from the first range (B, C) to the second range (A1).
The control device for an engine according to any one of claims 8 to 10, wherein
the controller (100) is configured to perform spark ignition combustion and/or compression ignition combustion in the first range (B, C).
The control device for an engine according to any one of claims 8 to 11, wherein
the controller (100) is configured to perform combustion in which spark ignition combustion and compression ignition combustion are combined in the first range (B, C).
The control device for an engine according to any one of claims 8 to 12, wherein
the controller (100) is configured to perform combustion in which spark ignition combustion and compression ignition combustion are combined in the second range (A1).
An engine system comprising:
an engine including a plurality of cylinders (2); and

the control device according to any one of claims 8 to 13.