Control method and control device for internal combustion engine

Abstract

To reduce NOx in exhaust emission at a start of an internal combustion engine.SOLUTION: In an internal combustion engine 10, when an operation point exceeds a surge limit, combustion becomes instable, and when the operation points exceeds a knock limit, knocking becomes liable to occur. Then, a control unit 13 controls the operation point so as not to exceed the surge limit and the knock limit. That is, the control unit 13 leans an air-fuel ratio up to the surge limit at a start transition, raises an engine rotation number up to a rotation number at which stable combustion becomes possible at an air-fuel ratio which is leaned up to the surge limit, and raises the torque (engine load) of the internal combustion engine 10 up until reaching the knock limit at the engine rotation number at which the stable combustion becomes possible at the air-fuel ratio which is leaned up to the surge limit, and also, at an air-fuel ratio which is leaned up to the knock limit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a control method for an internal combustion engine and a control device for an internal combustion engine. [Background technology]

[0002] For example, Patent Document 1 discloses a technology for a direct injection internal combustion engine in which fuel is injected directly into the combustion chamber, in which the timing of fuel injection at the start of the internal combustion engine is set to the intake stroke, and then the timing of fuel injection is switched to the compression stroke.

[0003] The technology disclosed in Patent Document 1 improves ignition and combustion during cranking, and also improves startability by smoothing the rise in engine speed and transition to an idle state after cranking. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 10-103117 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in an internal combustion engine that is operated at an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio (lean air-fuel ratio), it is important to reduce NOx in the exhaust gas that is generated by combustion.

[0006] However, Patent Document 1 does not take into consideration NOx in the exhaust gas from the start of the internal combustion engine until it is operated at a lean air-fuel ratio, and there is room for further improvement in this regard. [Means for solving the problem]

[0007] The internal combustion engine of the present invention performs power generation operation at a predetermined steady engine speed, a predetermined steady engine load, and a predetermined steady air-fuel ratio for the air-fuel mixture in the cylinder, and is started under operating conditions that are lower than the steady engine speed, lower than the steady load, and richer than the steady air-fuel ratio. After the internal combustion engine is started, during a startup transition period until the engine speed, engine load, and air-fuel ratio reach the steady engine speed, steady load, and steady air-fuel ratio, air-fuel ratio lean processing, which makes the air-fuel ratio leaner while keeping the engine load and engine speed constant, and load increasing processing, which increases the engine load while keeping the air-fuel ratio constant, are repeatedly performed. Furthermore, during the above-mentioned starting transition period, the internal combustion engine of the present invention performs, in addition to the air-fuel ratio lean processing and load increase processing, a rotation speed increase processing for increasing the engine rotation speed while keeping the air-fuel ratio constant, and repeats these processes in the order of air-fuel ratio lean processing, rotation speed increase processing, and load increase processing so as to ensure combustion stability. [Effects of the Invention]

[0008] The internal combustion engine of the present invention can significantly reduce NOx emissions during the starting transition period by leaning the air-fuel ratio while keeping the engine load constant during the starting transition period. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is an explanatory diagram that schematically illustrates an outline of a drive system of a vehicle to which the present invention is applied; [Figure 2] FIG. 4 is an explanatory diagram schematically showing the correlation between the air-fuel ratio and torque during a starting transition period. [Figure 3] 4 is a timing chart showing an example of the behavior of the air-fuel ratio and the like during a start transition period. [Figure 4] 3 is a flowchart showing the flow of control during a start-up transition period of an internal combustion engine in a first embodiment of the present invention. [Figure 5] 5 is a subroutine showing the content of step S1 in FIG. 4. [Figure 6] 5 is a subroutine showing the content of step S2 in FIG. 4. [Figure 7] 5 is a subroutine showing the content of step S3 in FIG. 4. [Figure 8] 3 is a flowchart showing the flow of control during a start-up transition period of an internal combustion engine in a first embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment of the present invention will be described in detail below with reference to the drawings.

[0011] 1 is an explanatory diagram showing a schematic overview of the drive system of a vehicle 1 to which the present invention is applied. The vehicle 1 has a drive unit 3 that drives drive wheels 2 and a power generation unit 4 that generates electricity for driving the drive wheels 2.

[0012] The drive unit 3 has a drive motor 5 as a second electric motor that drives and rotates the drive wheels 2, and a first gear train 6 and a differential gear 7 that transmit the driving force of the drive motor 5 to the drive wheels 2. The drive motor 5 is supplied with power from a battery 8 that is charged with electricity generated by the power generation unit 4, etc.

[0013] The power generation unit 4 has a generator 9 as a first electric motor that generates electricity to be supplied to the drive motor 5, an internal combustion engine 10 that can drive the generator 9, and a second gear train 11 that transmits the rotation of the internal combustion engine 10 to the generator 9.

[0014] The vehicle 1 is a so-called series hybrid vehicle that does not use an internal combustion engine 10 as a power source. For example, when the remaining battery charge of the battery 8 becomes low, the vehicle 1 drives the internal combustion engine 10 to generate electricity with the generator 9 in order to charge the battery 8.

[0015] The traction motor 5 is a direct drive source for the vehicle 1, and is driven by, for example, AC power from a battery 8. The traction motor 5 also functions as a generator when the vehicle 1 decelerates.

[0016] The generator 9 converts the rotational energy generated in the internal combustion engine 10 into electrical energy and charges, for example, the battery 8. The generator 9 also functions as an electric motor that drives the internal combustion engine 10, enabling motoring of the internal combustion engine 10. The generator 9 may also function as a starter motor for the internal combustion engine 10. Note that the electric power generated by the generator 9 may not be charged into the battery 8, but may instead be supplied directly to the drive motor 5 depending on the operating state, for example.

[0017] The internal combustion engine 10 is, for example, a direct-injection internal combustion engine capable of directly injecting fuel into the cylinders, and capable of transmitting the rotation of the crankshaft to the rotor of the generator 9. The internal combustion engine 10 is capable of changing the air-fuel ratio, and can be used by switching between stoichiometric combustion, which is combustion in a first combustion mode, and lean combustion, which is combustion in a second combustion mode. Stoichiometric combustion is combustion in which the target air-fuel ratio is the theoretical air-fuel ratio (stoichiometry). Lean combustion is lean combustion in which the target air-fuel ratio is a lean air-fuel ratio higher than the theoretical air-fuel ratio.

[0018] The internal combustion engine 10 is controlled by a control unit 13 serving as a control section. The control unit 13 is a well-known digital computer equipped with a CPU, a ROM, a RAM, and an input / output interface.

[0019] The control unit 13 receives detection signals from various sensors, such as an air flow meter 14 that detects the amount of intake air, a crank angle sensor 15 that detects the crank angle of a crankshaft (not shown) of the internal combustion engine 10, an accelerator opening sensor 16 that detects the amount of depression of an accelerator pedal (not shown) of the vehicle 1, an A / F sensor 17 that detects the air-fuel ratio, and an oxygen sensor 18.

[0020] The crank angle sensor 15 is capable of detecting the engine speed of the internal combustion engine 10 .

[0021] The accelerator opening sensor 16 is capable of detecting not only the accelerator opening, which is the amount of operation of the accelerator pedal, but also the accelerator change rate, which is the operation speed of the accelerator pedal. In other words, the accelerator opening sensor 16 corresponds to an accelerator operation amount detection unit.

[0022] The A / F sensor 17 is a so-called wide-range air-fuel ratio sensor having an output characteristic that is approximately linear according to the exhaust air-fuel ratio, and is arranged on the inlet side (upstream side) of an exhaust purification catalyst (e.g., a three-way catalyst) not shown that is arranged in an exhaust passage (not shown) of the internal combustion engine 10. In other words, the A / F sensor 17 is arranged in the exhaust passage upstream of the exhaust purification catalyst.

[0023] The oxygen sensor 18 is a sensor whose output voltage changes ON / OFF (rich, lean) within a narrow range near the stoichiometric air-fuel ratio to detect only rich and lean air-fuel ratios, and is disposed on the outlet side (downstream side) of the exhaust catalytic device. In other words, the oxygen sensor 18 is disposed in the exhaust passage downstream of the exhaust purification catalyst.

[0024] Based on the detection signals of various sensors, the control unit 13 optimally controls the amount and timing of fuel injected from the fuel injection valve, the ignition timing of the internal combustion engine 10, the amount of intake air, etc., and also controls the air-fuel ratio of the internal combustion engine 10.

[0025] The control unit 13 uses the detection value of the accelerator opening sensor 16 to calculate the required load of the internal combustion engine 10 (the load of the internal combustion engine 10).

[0026] The control unit 13 controls the internal combustion engine 10 to operate at a predetermined operating point for power generation. Specifically, when generating electricity with the generator 9, the control unit 13 operates the internal combustion engine 10 in a power generation operation where the engine speed is a predetermined steady speed, the engine load is a predetermined steady load, and the air-fuel ratio of the mixture in the cylinder is a predetermined steady air-fuel ratio. In other words, when generating electricity with the generator 9, the control unit 13 operates the internal combustion engine 10 at a power generation operating point where the engine speed is a steady speed, the torque of the internal combustion engine 10 is a predetermined steady torque, and the air-fuel ratio of the mixture in the cylinder is a steady air-fuel ratio.

[0027] The steady air-fuel ratio is an air-fuel ratio used exclusively for charging the battery 8, and is an air-fuel ratio that is significantly leaner than the stoichiometric air-fuel ratio. When the engine load is a steady load, the torque of the internal combustion engine 10 becomes a steady torque.

[0028] The internal combustion engine 10 drives the generator 9 and is started as necessary while the vehicle 1 is traveling. Since the internal combustion engine 10 is started by cranking from a stopped state while the vehicle 1 is traveling, the internal combustion engine 10 is started under operating conditions of a lower rotation speed than the steady-state rotation speed, a lower load than the steady-state load, and an air-fuel ratio that is richer than the steady-state air-fuel ratio and leaner than the stoichiometric air-fuel ratio. After starting, the internal combustion engine 10 changes the air-fuel ratio toward the steady-state air-fuel ratio while ensuring combustion stability.

[0029] Here, HC in the exhaust gas of an internal combustion engine depends on the engine speed and engine load of the internal combustion engine, but is less dependent on making the air-fuel ratio leaner as long as combustion in the internal combustion engine is stable.

[0030] In other words, the HC in the exhaust gas decreases as the engine speed of the internal combustion engine increases and as the engine load of the internal combustion engine increases, but does not change significantly even if the air-fuel ratio is made leaner within the range in which combustion is stable. However, the HC in the exhaust gas of an internal combustion engine increases rapidly if the combustion stability of the internal combustion engine deteriorates.

[0031] The NOx in the exhaust gas of an internal combustion engine depends on the engine speed and engine load of the engine, but is even more dependent on the leaner air-fuel ratio of the engine. In other words, the NOx in the exhaust gas decreases as the engine speed of the engine increases and as the engine load of the engine increases, but the leaner the air-fuel ratio, the greater the decrease (on a logarithmic scale) compared to the changes due to the engine speed and engine load.

[0032] Therefore, after the internal combustion engine 10 is started, the control unit 13 prioritizes leaning the air-fuel ratio toward the steady air-fuel ratio during the startup transition period until the engine speed, engine load (torque of the internal combustion engine 10), and air-fuel ratio reach the steady speed, steady load (steady torque), and steady air-fuel ratio, respectively.

[0033] In other words, after the internal combustion engine 10 is started, during the startup transition period until the engine speed, engine load (torque of the internal combustion engine 10), and air-fuel ratio reach a power generation operating point where they become steady speed, steady load (steady torque), and steady air-fuel ratio, the control unit 13 repeatedly performs an air-fuel ratio lean process that makes the air-fuel ratio leaner while keeping the engine load (torque of the internal combustion engine 10) and engine speed constant, a speed increase process that increases the engine speed while keeping the air-fuel ratio constant, and a load increase process (torque increase process) that increases the torque of the internal combustion engine 10 and increases the engine load while keeping the air-fuel ratio constant.

[0034] In more detail, during the startup transition period, in order to ensure combustion stability, the air-fuel ratio lean processing, rotation speed increase processing, and load increase processing are repeated in this order, and the operating point is changed from the startup operating point described below to the power generation operating point.

[0035] The air-fuel ratio leaning process is performed so that the air-fuel ratio does not exceed the surge limit at which combustion becomes unstable. The rotation speed increasing process increases the engine rotation speed to a rotation speed at which stable combustion is possible with an air-fuel ratio leaned up to the surge limit. The load increasing process increases the torque (engine load) to the knock limit at an engine rotation speed at which stable combustion is possible with an air-fuel ratio leaned up to the surge limit.

[0036] FIG. 2 is an explanatory diagram that schematically illustrates the correlation between air-fuel ratio and torque during the startup transition. The torque, which is the vertical axis of FIG. 2, includes the effects of engine speed and engine load. The solid line P1 in FIG. 2 is a characteristic line that indicates the knocking limit when the engine speed is a predetermined low speed. The solid line P2 in FIG. 2 is a characteristic line that indicates the knocking limit when the engine speed is a predetermined medium speed. The solid line P3 in FIG. 2 is a characteristic line that indicates the knocking limit when the engine speed is a predetermined high speed. The two-dot chain line Q1 in FIG. 2 is a characteristic line that indicates the surge limit when the engine speed is a predetermined low speed. The two-dot chain line Q2 in FIG. 2 is a characteristic line that indicates the surge limit when the engine speed is a predetermined medium speed. The two-dot chain line Q3 in FIG. 2 is a characteristic line that indicates the surge limit when the engine speed is a predetermined high speed. The dashed line R in FIG. 2 is a characteristic line that indicates the surge limit during homogeneous lean combustion.

[0037] In the example shown in Figure 2, the air-fuel ratio lean processing implements homogeneous lean combustion and stratified lean combustion. In homogeneous lean combustion, the air-fuel ratio inside the cylinder is uniform. In stratified lean combustion, fuel is injected into the cylinder twice, for example, in the first half of the intake stroke and the second half of the compression stroke, to create a rich mixture around the spark plug, and then a lean mixture is created around that and burned.

[0038] In the internal combustion engine 10, combustion becomes unstable when the operating point exceeds the surge limit, and knocking is more likely to occur when the operating point exceeds the knock limit. For the same air-fuel ratio, the operating point at the knock limit has a higher load (higher torque) than the operating point at the surge limit. Therefore, the control unit 13 controls the operating point so that it does not exceed the surge limit or the knock limit.

[0039] That is, during the startup transition period, the control unit 13 leans the air-fuel ratio to the surge limit, increases the engine speed to a speed at which stable combustion is possible with the air-fuel ratio leaned to the surge limit, and increases the torque (engine load) of the internal combustion engine 10 until it reaches the knock limit at the air-fuel ratio leaned to the surge limit and the engine speed at which stable combustion is possible with the air-fuel ratio leaned to the surge limit.

[0040] When the air-fuel ratio leaned up to the surge limit is richer than the steady-state air-fuel ratio, the control unit 13 increases the torque up to the knock limit and then further leans the air-fuel ratio.

[0041] This is because, as shown in FIG. 2, the surge limit of the air-fuel ratio expands in the lean direction as the engine speed and the torque of the internal combustion engine 10 increase (as the engine load increases), and the knock limit of the torque of the internal combustion engine 10 expands in the increasing direction as the air-fuel ratio becomes leaner and the engine speed increases.

[0042] In the example shown in FIG. 2, the internal combustion engine 10 is started at an operating point A, and then the air-fuel ratio is made leaner using homogeneous lean combustion up to an operating point B. At the operating point B, the combustion mode is switched from homogeneous lean combustion to stratified lean combustion, and then an air-fuel ratio leaning process is performed to make the air-fuel ratio even leaner up to an operating point C.

[0043] That is, the first air-fuel ratio lean processing is to make the air-fuel ratio lean and move (change) the operating point from A to C.

[0044] The first rotation speed increase process is to move (change) the operating point from C to D while keeping the air-fuel ratio constant.

[0045] The first engine load increase process is to move (change) the operating point from D to E while keeping the air-fuel ratio and engine speed constant.

[0046] The second lean air-fuel ratio process moves (changes) the operating point from E to F while keeping the engine speed and torque constant. In the example shown in Fig. 2, the second lean air-fuel ratio process causes the air-fuel ratio to reach the steady air-fuel ratio, which is the target air-fuel ratio for power generation operation.

[0047] The second rotation speed increase process moves (changes) the operating point from F to G while keeping the air-fuel ratio constant. In the example shown in Figure 2, the engine rotation speed reaches the steady rotation speed, which is the target rotation speed for power generation operation, in the second rotation speed increase process.

[0048] The second engine load increase process moves (changes) the operating point from F to G while keeping the air-fuel ratio and rotation speed constant. In the example shown in Fig. 2, the torque of the internal combustion engine 10 reaches the steady torque in the second engine load increase process, and the torque of the internal combustion engine 10 reaches the steady torque which is the target torque of the operating point for power generation.

[0049] The combustion mode of the internal combustion engine 10 at the operating points C to H is stratified lean combustion.

[0050] The operating point A is a starting operating point used when starting the internal combustion engine 10, which has a lower rotation speed, a lower torque (low engine load), and a richer air-fuel ratio than the operating point for power generation operation.

[0051] The air-fuel ratio at operating point B is set to be as lean as possible within a range that does not exceed the surge limit in homogeneous lean combustion.

[0052] The air-fuel ratio at the operating point C is set to be as lean as possible within a range that does not exceed the surge limit in stratified lean combustion.

[0053] The engine speed and torque of the internal combustion engine 10 at the operating points B and C are the same as the engine speed and torque of the internal combustion engine 10 at the operating point A.

[0054] The engine speed at operating point D is within a range where stable combustion is possible at the air-fuel ratio at operating point C.

[0055] The torque at operating point E is set so that the air-fuel ratio and engine speed at operating point D do not exceed the knock limit.

[0056] The air-fuel ratio at operating point F is set so as not to exceed the surge limit in stratified lean combustion. In the example of Figure 2, the steady-state air-fuel ratio is within a range where the torque at operating point E does not exceed the surge limit in stratified lean combustion, so the air-fuel ratio at operating point F becomes the steady-state air-fuel ratio for power generation operation.

[0057] The engine speed at operating point G is a steady-state speed, which is within the range where stable combustion is possible at the air-fuel ratio of operating point F. In the example of Figure 2, the steady-state speed is within the range where stable combustion is possible at the air-fuel ratio of operating point F, so the engine speed at operating point G is the steady-state speed for power generation operation.

[0058] Operating point H is the operating point for power generation. In the example of Figure 2, the steady-state torque is within the range where the air-fuel ratio and engine speed at operating point G do not exceed the knock limit, so the torque at operating point H becomes the steady-state torque for power generation operation.

[0059] FIG. 3 is a timing chart showing an example of the behavior of the air-fuel ratio and the like during the starting transition period.

[0060] Time t1 is the timing at which fuel injection begins and the internal combustion engine 10 starts.

[0061] In the example of FIG. 3, the internal combustion engine 10 is motored by the generator 9 before firing (sustained operation) of the internal combustion engine 10, and the intake amount (amount of intake air) is ensured before time t1.

[0062] Time t2 is the timing when the lean air-fuel ratio processing starts. The lean air-fuel ratio processing that started at time t2 ends at time t4. In the example of Fig. 3, the period from time t2 to time t3 is the period during which homogeneous lean combustion is performed, and the period from time t3 to time t4 is the period during which stratified lean combustion is performed.

[0063] The rotation speed increasing process starts at time t4 and ends at time t5.

[0064] Time t5 is the timing to start the load increase process.

[0065] 4 is a flowchart showing the flow of control during the start transition (starting) of the internal combustion engine 10 in the first embodiment of the present invention. The internal combustion engine 10 of the first embodiment performs only stratified lean combustion as the air-fuel ratio lean processing.

[0066] When the internal combustion engine 10 is started, in step S1, the air-fuel ratio of the internal combustion engine 10 is set. Step S1 is a step for performing air-fuel ratio lean processing. In step S2, the engine speed of the internal combustion engine 10 is set. Step S2 is a step for performing speed increase processing. In step S3, the load (torque) of the internal combustion engine 10 is set. Step S3 is a step for performing load increase processing. In step S4, it is determined whether the operating point is a power generation operating point. If the operating point is power generation operation in step S4, operation at the power generation operating point is started. If the operating point is not power generation operation in step S4, the process returns to step S1. In other words, during the startup transition period, the processes of steps S1 to S3 are repeatedly performed until the operating point reaches the power generation operating point.

[0067] FIG. 5 is a flowchart showing the subroutine of step S1 in FIG. 4, and showing the control flow of the air-fuel ratio lean processing.

[0068] In step S11, various setting parameters are read from the control unit 13. In step S11, Δrpm_step, which is the difference between the current engine speed and the target speed for the speed increase process immediately after the current air-fuel ratio lean process, A / F_final, which is the steady-state air-fuel ratio, and ΔA / F, which is the amount of change in the air-fuel ratio per time allowed when changing the air-fuel ratio, are read. In step S11, rpm_act, which is the current engine speed (at the start of the current air-fuel ratio lean process), is also read.

[0069] In step S12, the intermediate target rotation speed rpm_mid is calculated. The intermediate target rotation speed is the sum of rpm_act and Δrpm_step, i.e., rpm_mid=rpm_act+Δrpm_step.

[0070] In step S13, various instantaneous parameters (current values) are read in. In step S13, A / F_act, which is the current air-fuel ratio (at the start of the current air-fuel ratio lean processing), Qair_act, which is the current intake air amount, and rpm_act, which is the current engine speed, are read in.

[0071] In step S14, rpm_act, Qair_act, and A / F_act are used to calculate load_act, which is the current engine load (at the start of the current lean air-fuel ratio processing), from a table (load calculation table) pre-stored in the ROM in the control unit 13. load_act=f(rpm_act, Qair_act, A / F_act).

[0072] In step S15, assuming that the engine load is fixed, rpm_mid and load_act are used to calculate A / F_surge, which is the lean process target air-fuel ratio that ensures combustion stability, from a table (surge limit air-fuel ratio calculation table) pre-stored in the ROM in the control unit 13. A / F_surge=f(rpm_mid, load_act).

[0073] In step S16, A / F_int, which is the target air-fuel ratio when leaning the air-fuel ratio toward the lean processing target air-fuel ratio, is calculated. A / F_int is set multiple times while changing its value during the air-fuel ratio lean processing so that the air-fuel ratio during the air-fuel ratio lean processing gradually approaches the lean processing target air-fuel ratio. A / F_int = A / F_act + ΔA / F.

[0074] In step S17, it is determined whether the target air-fuel ratio is leaner than the lean processing target air-fuel ratio. If the target air-fuel ratio is leaner than the lean processing target air-fuel ratio in step S17, the process proceeds to step S18. If the target air-fuel ratio is not leaner than the lean processing target air-fuel ratio in step S17, the process proceeds to step S19.

[0075] In step S19, fuel injection is performed so as to achieve the target air-fuel ratio.

[0076] In step S20, it is determined whether the target air-fuel ratio is the lean processing target air-fuel ratio. If the target air-fuel ratio is the lean processing target air-fuel ratio in step S19, this subroutine is terminated. If the target air-fuel ratio is not the lean processing target air-fuel ratio in step S19, the process returns to step S13, and the air-fuel ratio continues to be made leaner so that the air-fuel ratio becomes the lean processing target air-fuel ratio.

[0077] FIG. 6 is a flowchart showing the subroutine of step S2 in FIG. 4, and is a control flow of the rotation speed increasing process.

[0078] In step S31, various setting parameters are read from the control unit 13. In step S31, a preset Δrpm_inst, which is the amount of change (increase) allowed per change in the engine speed, and Δrpm_step, which is the difference between the current engine speed and the target engine speed for the engine speed increase process immediately after the current lean air-fuel ratio process, are read. In step S31, rpm_act, which is the current engine speed (at the start of the current engine speed increase process), is also read.

[0079] In step S32, the intermediate target rotation speed rpm_mid is calculated. The intermediate target rotation speed is the sum of rpm_act and Δrpm_step, i.e., rpm_mid=rpm_act+Δrpm_step.

[0080] In step S33, various instantaneous parameters (current values) are read in. In step S33, the current (at the start of the current rotation speed increase process) air-fuel ratio A / F_act, the current intake air amount Qair_act, and the current engine rotation speed rpm_act are read in.

[0081] In step S34, rpm_act, Qair_act, and A / F_act are used to calculate load_act, which is the current engine load (at the start of the current lean air-fuel ratio processing), from a table (load calculation table) pre-stored in the ROM in the control unit 13. load_act=f(rpm_act, Qair_act, A / F_act).

[0082] In step S35, assuming that the engine load is fixed, rpm_mid and load_act are used to calculate A / F_surge, which is the rotation speed increase process target air-fuel ratio that ensures combustion stability, from a table (surge limit air-fuel ratio calculation table) pre-stored in the ROM in the control unit 13. A / F_surge=f(rpm_mid, load_act). If the conditions are the same, the rotation speed increase process target air-fuel ratio will be the same value as the lean process target air-fuel ratio.

[0083] In step S36, rpm_int, which is the target engine speed when increasing the engine speed toward the intermediate target engine speed, is calculated. rpm_int is set multiple times while changing its value during the engine speed increasing process so that the engine speed during the engine speed increasing process gradually approaches the intermediate target engine speed. rpm_int = rpm_act + Δrpm_inst.

[0084] In step S37, A / F_int, which is the target air-fuel ratio during the rotation speed increase process, is set to A / F_act, which is the current air-fuel ratio.

[0085] In step S38, it is determined whether the target air-fuel ratio is equal to or less than the rotation speed increase processing target air-fuel ratio. If the target air-fuel ratio is greater (lean) than the rotation speed increase processing target air-fuel ratio in step S38, the process proceeds to step S39. If the target air-fuel ratio is equal to or less (rich) than the rotation speed increase processing target air-fuel ratio in step S38, the process proceeds to step S40.

[0086] In step S39, the target rotation speed is readjusted so that the air-fuel ratio does not exceed the surge limit, that is, rpm_int=rpm_act.

[0087] In step S40, the engine speed is increased to the target speed rpm_int. Steps S36 to S40 correspond to steps of repeatedly adding a preset rotation speed until the engine speed reaches the increase processing target rotation speed, and adjusting the engine speed so that combustion stability is ensured when the engine speed exceeds the increase processing target rotation speed.

[0088] In step S41, fuel injection is performed so that the air-fuel ratio becomes the target air-fuel ratio A / F_int.

[0089] In step S42, it is determined whether the target rotation speed is the intermediate target rotation speed. If the target rotation speed is the intermediate target rotation speed in step S42, this subroutine is terminated. If the target rotation speed is not the intermediate target rotation speed in step S42, the process returns to step S33 and continues the process of increasing the engine rotation speed so that the engine rotation speed becomes the intermediate target rotation speed.

[0090] FIG. 7 is a flowchart showing the subroutine of step S3 in FIG. 4, and is a control flow of the load increasing process.

[0091] In step S51, various setting parameters are read from the control unit 13. In step S51, a preset Δload_inst, which is the amount of change (increment) allowed per change in the engine load, and load_final, which is the steady load, are read.

[0092] In step S52, various instantaneous parameters (current values) are read in. In step S52, the current (at the start of the current rotation speed increase process) air-fuel ratio A / F_act, the current intake air amount Qair_act, and the current engine rotation speed rpm_act are read in.

[0093] In step S53, rpm_act, Qair_act, and A / F_act are used to calculate load_act, which is the current engine load (at the start of the current lean air-fuel ratio processing), from a table (load calculation table) pre-stored in the ROM in the control unit 13. load_act=f(rpm_act, Qair_act, A / F_act).

[0094] In step S54, rpm_act and load_act are used to calculate A / F_knock, which is the knock lower limit air-fuel ratio at which knocking is avoided and combustion stability is ensured, from a table (knock lower limit air-fuel ratio calculation table) pre-stored in the ROM in control unit 13. A / F_knock=f(rpm_act, load_act).

[0095] In step S55, load_int, which is the target load when increasing the engine load, is calculated. load_int is set multiple times while changing its value during the load increasing process so that the engine load during the load increasing process gradually approaches the target load. load_int = load_act + Δload_inst.

[0096] In step S56, it is determined whether the target air-fuel ratio A / F_int during the load increase processing is equal to or greater than the knock lower limit air-fuel ratio A / F_knock. If the target air-fuel ratio during the load increase processing is less than the knock lower limit air-fuel ratio in step S56, the process proceeds to step S57. If the target air-fuel ratio during the load increase processing is equal to or greater than the knock lower limit air-fuel ratio in step S56, the process proceeds to step S58.

[0097] In step S57, the target load is readjusted so that the air-fuel ratio does not fall below the knock lower limit air-fuel ratio, that is, load_int=load_act.

[0098] In step S58, the engine load is increased to load_int.

[0099] Steps S55 to S58 correspond to steps of repeatedly adding a predetermined load until the engine load reaches the target load for increased processing, and adjusting the engine load so that combustion stability is ensured if the engine load exceeds the target load for increased processing.

[0100] In step S59, fuel injection is performed so that the air-fuel ratio becomes the target air-fuel ratio A / F_int.

[0101] In step S60, it is determined whether load_int, which is the target load when increasing the engine load, is equal to load_final, which is the steady load. If the target load is equal to the steady load in step S60, the current routine is terminated. If the target load is not equal to the steady load in step S60, the routine proceeds to step S61.

[0102] In step S61, it is determined whether A / F_int, which is the target air-fuel ratio during the load increase processing, is greater than A / F_knock, which is the knock lower limit air-fuel ratio. If, in step S61, A / F_int, which is the target air-fuel ratio during the load increase processing, is equal to or less than A / F_knock, which is the knock lower limit air-fuel ratio, this subroutine is terminated. If, in step S61, A / F_int, which is the target air-fuel ratio during the load increase processing, is greater than A / F_knock, which is the knock lower limit air-fuel ratio, the process proceeds to step S52, and the process of increasing the engine load continues.

[0103] As described above, the internal combustion engine 10 of the first embodiment can significantly reduce NOx emissions during the starting transition period by leaning the air-fuel ratio while keeping the engine load constant during the starting transition period.

[0104] During the startup transition period, the internal combustion engine 10 leans the air-fuel ratio to a lean processing target air-fuel ratio that ensures combustion stability at the engine load and engine speed at the start of air-fuel ratio lean processing, thereby suppressing an increase in NOx due to an increase in engine speed and engine load.

[0105] When the lean processing target air-fuel ratio is greater (lean) than the steady-state air-fuel ratio, the internal combustion engine 10 can suppress deterioration of combustion stability during the starting transition period by setting the steady-state air-fuel ratio as the lean processing target air-fuel ratio.

[0106] During the rotation speed increase process, the internal combustion engine 10 increases the engine rotation speed at a preset increase rate up to the increase process target rotation speed at which combustion stability can be ensured at the air-fuel ratio at the start of the rotation speed increase process, thereby suppressing an increase in engine load and suppressing an increase in NOx due to an increase in engine load.

[0107] The internal combustion engine 10 repeatedly adds a predetermined rotation speed until the engine rotation speed reaches the increase processing target rotation speed, and when the engine rotation speed exceeds the increase processing target rotation speed, adjusts the engine rotation speed so as to ensure combustion stability. Therefore, the internal combustion engine 10 can suppress deterioration of combustion stability during the rotation speed increase processing in the start transition period.

[0108] When the increase processing target load set in the load increase processing is smaller than the steady load, the internal combustion engine 10 performs air-fuel ratio lean processing after the load increase processing is completed, and performs load increase processing after the air-fuel ratio lean processing is completed. Therefore, it is possible to reach the operating conditions for power generation operation while avoiding operating conditions that result in high NOx emissions, and it is possible to significantly suppress the accumulated amount of NOx during transient operation.

[0109] 8 is a flowchart showing the flow of control during the start-up transition (startup) of the internal combustion engine 10 in a second embodiment of the present invention. The internal combustion engine 10 of the second embodiment performs the same control as the internal combustion engine 10 of the first embodiment described above, except that homogeneous lean combustion is performed prior to stratified lean combustion in the initial air-fuel ratio lean processing at startup.

[0110] When the internal combustion engine 10 of the second embodiment is started, step S71 sets the air-fuel ratio for homogeneous lean combustion in the internal combustion engine 10. Step S72 sets the air-fuel ratio for stratified lean combustion in the internal combustion engine 10. Steps S71 and S72 are steps for performing air-fuel ratio lean processing, and the details thereof are the same as those in FIG. 5 described above.

[0111] Step S73 sets the engine speed of the internal combustion engine 10. Step S73 is a step for carrying out the speed increase process, and the details thereof are the same as those in FIG.

[0112] Step S74 sets the load (torque) of the internal combustion engine 10. Step S74 is a step for carrying out a load increasing process, the details of which are the same as those in FIG.

[0113] In step S75, it is determined whether the operating point is a power generation operating point. If the operating point is power generation operation in step S75, operation at the power generation operating point is started. If the operating point is not power generation operation in step S75, the process returns to step S72. In other words, during the startup transition period, the processes of steps S72 to S74 are repeatedly performed until the operating point reaches the power generation operating point.

[0114] The internal combustion engine 10 of the second embodiment can also achieve substantially the same effects as the internal combustion engine 10 of the first embodiment described above.

[0115] While specific embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and various modifications are possible without departing from the spirit of the present invention. The present invention relates to a method for controlling an internal combustion engine and a control device for an internal combustion engine. [Explanation of symbols]

[0116] 1...Vehicle 2...Drive wheels 3...Drive unit 4...Power generation unit 5...Drive motor 6...1st gear train 7...Differential gear 8...Battery 9...Generator 10...Internal combustion engine 11...Second gear train 13...Control unit 14...Air flow meter 15...Crank angle sensor 16...Accelerator opening sensor 17...A / F sensor 18...Oxygen sensor

Claims

1. A control method for an internal combustion engine that performs power generation operation at a predetermined steady engine speed, a predetermined steady engine load, and a predetermined steady air-fuel ratio of an air-fuel mixture in a cylinder, comprising: starting the internal combustion engine under operating conditions of a lower rotation speed than the steady rotation speed, a lower load than the steady load, and a richer air-fuel ratio than the steady air-fuel ratio; After the internal combustion engine is started, during a startup transition period until the engine speed, engine load, and air-fuel ratio reach the steady speed, steady load, and steady air-fuel ratio, an air-fuel ratio lean process for leaning the air-fuel ratio while keeping the engine load and engine speed constant, and a load increase process for increasing the engine load while keeping the air-fuel ratio constant are repeatedly performed, A control method for an internal combustion engine, which repeats air-fuel ratio lean processing and load increase processing in this order during the starting transition period so as to ensure combustion stability.

2. A control method for an internal combustion engine that performs power generation operation at a predetermined steady engine speed, a predetermined steady engine load, and a predetermined steady air-fuel ratio of an air-fuel mixture in a cylinder, comprising: starting the internal combustion engine under operating conditions of a lower rotation speed than the steady rotation speed, a lower load than the steady load, and a richer air-fuel ratio than the steady air-fuel ratio; After the internal combustion engine is started, during a startup transition period until the engine speed, engine load, and air-fuel ratio reach the steady speed, steady load, and steady air-fuel ratio, an air-fuel ratio lean process for leaning the air-fuel ratio while keeping the engine load and engine speed constant, and a load increase process for increasing the engine load while keeping the air-fuel ratio constant are repeatedly performed, A control method for an internal combustion engine in which, during the above-mentioned startup transition period, in addition to the air-fuel ratio lean processing and load increase processing, a rotation speed increase processing is carried out to increase the engine rotation speed while keeping the air-fuel ratio constant, and these processes are repeated in this order to ensure combustion stability.

3. 3. The control method for an internal combustion engine according to claim 2, wherein in the air-fuel ratio lean processing, the air-fuel ratio is made lean up to a lean processing target air-fuel ratio that can ensure combustion stability at the engine load and engine speed at the start of the air-fuel ratio lean processing.

4. In the case where the air-fuel ratio lean processing is performed multiple times during the startup transition period, the lean processing target air-fuel ratio set for each air-fuel ratio lean processing is changed, 4. The control method for an internal combustion engine according to claim 3, wherein when the lean process target air-fuel ratio is higher than the steady air-fuel ratio, the steady air-fuel ratio is set as the lean process target air-fuel ratio.

5. 5. The control method for an internal combustion engine according to claim 2, wherein the rotation speed increasing process increases the engine rotation speed up to an increasing process target rotation speed at which combustion stability can be ensured at the air-fuel ratio at the start of the rotation speed increasing process.

6. 6. A control method for an internal combustion engine according to claim 5, wherein the rotation speed increase process repeats adding a predetermined rotation speed until the engine rotation speed reaches an increase process target rotation speed, and when the engine rotation speed exceeds the increase process target rotation speed, the engine rotation speed is adjusted so as to ensure combustion stability.

7. 7. The control method for an internal combustion engine according to claim 2, wherein the load increasing process increases the engine load up to an increasing process target load that is limited by knocking.

8. 8. The control method for an internal combustion engine according to claim 7, wherein the load increasing process repeats adding a predetermined load until the engine load reaches an increasing process target load, and when the engine load exceeds the increasing process target load, adjusts the engine load so as to ensure combustion stability.

9. 9. The control method for an internal combustion engine according to claim 7, wherein, in the load increasing process, when the target load for increasing process becomes larger than the steady load, the steady load is set as the target load for increasing process.

10. 10. The control method for an internal combustion engine according to claim 7, wherein, when an increase processing target load set in the load increase processing is smaller than the steady load, an air-fuel ratio lean processing is performed after the load increase processing is completed, and a load increase processing is performed after the air-fuel ratio lean processing is completed.

11. A control device for an internal combustion engine that performs a power generating operation at a predetermined steady engine speed, a predetermined steady engine load, and a predetermined steady air-fuel ratio of an air-fuel mixture in a cylinder, a control unit that starts the internal combustion engine at a speed lower than the steady rotation speed, a load lower than the steady load, and at a richer side than the steady air-fuel ratio, and that repeatedly performs an air-fuel ratio lean process that leans the air-fuel ratio while keeping the engine load and engine rotation speed constant, and a load increasing process that increases the engine load while keeping the air-fuel ratio constant, during a startup transition period after the internal combustion engine has started until the engine rotation speed, engine load, and air-fuel ratio reach the steady rotation speed, the steady load, and the steady air-fuel ratio; A control device for an internal combustion engine that repeats air-fuel ratio lean processing and load increase processing in this order during the startup transition period so as to ensure combustion stability.

12. A control device for an internal combustion engine that performs power generation operation at a predetermined steady engine speed, a predetermined steady engine load, and a predetermined steady air-fuel ratio of the mixture in the cylinder, a control unit that starts the internal combustion engine at a speed lower than the steady rotation speed, a load lower than the steady load, and at a richer side than the steady air-fuel ratio, and that repeatedly performs an air-fuel ratio lean process that leans the air-fuel ratio while keeping the engine load and engine rotation speed constant, and a load increasing process that increases the engine load while keeping the air-fuel ratio constant, during a startup transition period after the internal combustion engine has started until the engine rotation speed, engine load, and air-fuel ratio reach the steady rotation speed, the steady load, and the steady air-fuel ratio; A control device for an internal combustion engine that, during the above-mentioned startup transition period, in addition to the air-fuel ratio lean processing and load increase processing, performs a rotation speed increase processing that increases the engine rotation speed while keeping the air-fuel ratio constant, and repeats each of these processes in the order of air-fuel ratio lean processing, rotation speed increase processing, and load increase processing so as to ensure combustion stability.

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