JP7725986B2 - Control method and control device for internal combustion engine - Google Patents

Description

The present invention relates to a control method for an internal combustion engine and a control device for an internal combustion engine.

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 engine start is set to the intake stroke, and then the timing of fuel injection is switched to the compression stroke.

This technology improves ignition and combustion during cranking, and also improves startability by smoothing the rise in engine speed and transition to idle after cranking.

JP 10-103117 A

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

However, Patent Document 1 does not take into consideration NOx in the exhaust gas from the time the internal combustion engine is started until it is operating at a lean air-fuel ratio, and there is room for further improvement in this regard.

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 of the air-fuel mixture in the cylinders, 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 starts, 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, the engine repeatedly performs an air-fuel ratio lean process to make the air-fuel ratio leaner while keeping the engine load and engine speed constant, and a load increase process to increase the engine load while keeping the air-fuel ratio constant. Furthermore, during the startup transition period, the internal combustion engine of the present invention performs, in addition to the air-fuel ratio lean process and load increase process, a rotation speed increase process to increase the engine speed while keeping the air-fuel ratio constant, and repeats these processes in the order of air-fuel ratio lean process, rotation speed increase process, and load increase process to ensure combustion stability.

The internal combustion engine of the present invention can significantly reduce NOx emissions during the startup transition by leaning the air-fuel ratio while maintaining a constant engine load during the startup transition.

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

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

Figure 1 is an explanatory diagram that shows 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 to drive the drive wheels 2.

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

The power generation unit 4 has a generator 9 as a first electric motor that generates electricity to supply 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.

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

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

The generator 9 converts the rotational energy generated by the internal combustion engine 10 into electrical energy, for example, to charge 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 used to charge the battery 8, but may instead be supplied directly to the drive motor 5 depending on the operating conditions, for example.

Internal combustion engine 10 is, for example, a direct-injection internal combustion engine capable of injecting fuel directly into the cylinders, and is capable of transmitting the rotation of the crankshaft to the rotor of generator 9. 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 greater than the theoretical air-fuel ratio.

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

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 the crankshaft (not shown) of the internal combustion engine 10, an accelerator opening sensor 16 that detects the amount of depression of the accelerator pedal (not shown) of the vehicle 1, an A/F sensor 17 that detects the air-fuel ratio, and an oxygen sensor 18.

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

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

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

The oxygen sensor 18 is a sensor whose output voltage changes ON/OFF (rich, lean) within a narrow range around the stoichiometric air-fuel ratio, detecting only rich and lean air-fuel ratios. It is located on the outlet side (downstream) of the exhaust catalyst device. In other words, the oxygen sensor 18 is located in the exhaust passage downstream of the exhaust purification catalyst.

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., based on detection signals from various sensors, and also controls the air-fuel ratio of the internal combustion engine 10.

The control unit 13 calculates the required load of the internal combustion engine 10 (the load on the internal combustion engine 10) using the detection value of the accelerator opening sensor 16.

The control unit 13 controls the internal combustion engine 10 to operate at a predetermined power generation operating point. Specifically, when generating electricity using 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 using 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.

The steady-state 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 steady torque.

The internal combustion engine 10 drives the generator 9 and is started as needed while the vehicle 1 is running. Since the internal combustion engine 10 is started by cranking from a stopped state while the vehicle 1 is running, the internal combustion engine 10 is started under operating conditions that are lower than the steady-state rotation speed, lower than the steady-state load, and 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.

Here, the 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.

In other words, the amount of HC in exhaust gas decreases as the engine speed of the internal combustion engine increases and as the engine load on the internal combustion engine increases, but does not change significantly even if the air-fuel ratio is made leaner as long as combustion is stable. However, the amount of HC in exhaust gas from an internal combustion engine increases sharply if the combustion stability of the internal combustion engine deteriorates.

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 amount of 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 changes due to engine speed and engine load.

Therefore, after the internal combustion engine 10 starts, the control unit 13 prioritizes leaning the air-fuel ratio toward the steady-state 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 their steady-state speed, steady-state load (steady-state torque), and steady-state air-fuel ratio, respectively.

In other words, after the internal combustion engine 10 starts, 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 leans the air-fuel ratio 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 while keeping the air-fuel ratio constant and increases the engine load.

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

Air-fuel ratio lean processing is performed so that the air-fuel ratio does not exceed the surge limit, at which combustion becomes unstable. The rotational speed increase processing increases the engine speed to a speed at which stable combustion is possible with an air-fuel ratio leaned up to the surge limit. The load increase processing increases the torque (engine load) until the knock limit is reached at an engine speed at which stable combustion is possible with an air-fuel ratio leaned up to the surge limit and an air-fuel ratio leaned up to the surge limit.

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

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.

When the operating point of the internal combustion engine 10 exceeds the surge limit, combustion becomes unstable, and when the operating point exceeds the knock limit, knocking becomes more likely to occur. For the same air-fuel ratio, the operating point at the knock limit results in 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 knock limit.

In other words, during the startup transition, 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 an 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 at an engine speed at which stable combustion is possible with an air-fuel ratio leaned to the surge limit.

If 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.

This is because, as shown in Figure 2, the surge limit of the air-fuel ratio expands in the lean direction as the engine speed and 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.

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

In other words, the first air-fuel ratio lean processing is to make the air-fuel ratio leaner and move (change) the operating point from A to C.

The first rotation speed increase process moves (changes) the operating point from C to D while keeping the air-fuel ratio constant.

The first engine load increase process involves moving (changing) the operating point from D to E while keeping the air-fuel ratio and engine speed constant.

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 Figure 2, the second lean air-fuel ratio process causes the air-fuel ratio to reach the steady-state air-fuel ratio, which is the target air-fuel ratio for power generation operation.

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-state rotation speed, which is the target rotation speed for power generation operation, during the second rotation speed increase process.

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

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

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

The air-fuel ratio at operating point B is set to be as lean as possible without exceeding the surge limit for homogeneous lean combustion.

The air-fuel ratio at operating point C is set to be as lean as possible without exceeding the surge limit for stratified lean combustion.

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

The engine speed at operating point D is within the range that allows stable combustion at the air-fuel ratio at operating point C.

The torque at operating point E is set so as not to exceed the knock limit at the air-fuel ratio and engine speed at operating point D.

The air-fuel ratio at operating point F is set so as not to exceed the surge limit for 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 for stratified lean combustion, so the air-fuel ratio at operating point F becomes the steady-state air-fuel ratio for power generation operation.

The engine speed at operating point G is within the range where stable combustion is possible at the air-fuel ratio of operating point F, and is the steady-state speed. 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.

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.

Figure 3 is a timing chart showing an example of the behavior of the air-fuel ratio and other parameters during the startup transition.

Time t1 is the timing when fuel injection begins and the internal combustion engine 10 starts.

In the example shown in Figure 3, the internal combustion engine 10 is motored by the generator 9 prior to firing (sustained operation) of the internal combustion engine 10, and the intake volume (amount of intake air) is secured before time t1.

Time t2 is the timing when lean air-fuel ratio processing begins. The lean air-fuel ratio processing that began at time t2 ends at time t4. In the example in Figure 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.

Time t4 is the timing when the rotation speed increase process begins. The rotation speed increase process that began at time t4 ends at time t5.

Time t5 is the timing to start the load increase process.

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

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 in step S4 the operating point is power generation operation, operation at the power generation operating point is initiated. If in step S4 the operating point is not power generation operation, the process returns to step S1. In other words, during the startup transition period, steps S1 to S3 are repeatedly performed until the operating point reaches the power generation operating point.

Figure 5 is a subroutine showing the contents of step S1 in Figure 4, and is a flowchart showing the control flow of the air-fuel ratio lean processing.

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 air-fuel ratio change allowed per air-fuel ratio change, 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.

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. rpm_mid = rpm_act + Δrpm_step.

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

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 of the control unit 13. load_act = f(rpm_act, Qair_act, A/F_act).

In step S15, assuming that the engine load is fixed, rpm_mid and load_act are used to calculate A/F_surge, 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 of the control unit 13. A/F_surge = f(rpm_mid, load_act).

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 during the air-fuel ratio lean processing, changing its value 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.

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

In step S19, fuel injection is performed to achieve the target air-fuel ratio.

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

Figure 6 is a subroutine showing the contents of step S2 in Figure 4, and is a flowchart showing the control flow of the rotation speed increase process.

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

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. rpm_mid = rpm_act + Δrpm_step.

In step S33, various instantaneous parameters (current values) are read. 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 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 of the control unit 13. load_act = f(rpm_act, Qair_act, A/F_act).

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

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 during the engine speed increase process, changing its value so that the engine speed during the engine speed increase process gradually approaches the intermediate target engine speed. rpm_int = rpm_act + Δrpm_inst.

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

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 in step S38 the target air-fuel ratio is greater (leaner) than the rotation speed increase processing target air-fuel ratio, the process proceeds to step S39. If in step S38 the target air-fuel ratio is equal to or less (richer) than the rotation speed increase processing target air-fuel ratio, the process proceeds to step S40.

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

In step S40, the engine speed is increased to the target speed, rpm_int. Steps S36 to S40 correspond to steps in which a preset rotation speed is repeatedly added until the engine speed reaches the target rotation speed for increase processing, and when the engine speed exceeds the target rotation speed for increase processing, the engine speed is adjusted to ensure combustion stability.

In step S41, fuel is injected so that the air-fuel ratio becomes the target air-fuel ratio, A/F_int.

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 ends. 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.

Figure 7 is a subroutine showing the contents of step S3 in Figure 4, and is a flowchart showing the control flow of the load increase processing.

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

In step S52, various instantaneous parameters (current values) are read. 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 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 of the control unit 13. load_act = f(rpm_act, Qair_act, A/F_act).

In step S54, rpm_act and load_act are used to calculate A/F_knock, 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 of the control unit 13. A/F_knock = f(rpm_act, load_act).

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

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

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. In other words, load_int = load_act.

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

Steps S55 to S58 correspond to steps in which a preset load is repeatedly added until the engine load reaches the target load for increased processing, and if the engine load exceeds the target load for increased processing, the engine load is adjusted to ensure combustion stability.

In step S59, fuel is injected so that the air-fuel ratio becomes the target air-fuel ratio, A/F_int.

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.

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

As explained above, the internal combustion engine 10 of the first embodiment can significantly reduce NOx emissions during the startup transition by leaning the air-fuel ratio while keeping the engine load constant during the startup transition.

During the startup transition, the internal combustion engine 10 leans the air-fuel ratio to the lean processing target air-fuel ratio, which ensures combustion stability at the engine load and engine speed at the start of air-fuel ratio lean processing. This suppresses an increase in NOx due to an increase in engine speed and engine load.

When the lean processing target air-fuel ratio is greater (leaner) than the steady-state air-fuel ratio, the internal combustion engine 10 sets the steady-state air-fuel ratio as the lean processing target air-fuel ratio, thereby suppressing deterioration of combustion stability during the startup transition.

When performing 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. This suppresses an increase in engine load and can suppress an increase in NOx due to an increase in engine load.

The internal combustion engine 10 repeatedly adds a preset rotation speed until the engine rotation speed reaches the increase processing target rotation speed, and if the engine rotation speed exceeds the increase processing target rotation speed, it adjusts the engine rotation speed to ensure combustion stability. As a result, the internal combustion engine 10 can suppress deterioration of combustion stability when the rotation speed is increased during the startup transition period.

When the target load increase set in the load increase process is smaller than the steady-state load, the internal combustion engine 10 performs lean air-fuel ratio processing after the load increase process is completed, and then performs lean air-fuel ratio processing after the load increase process is completed. This allows the operating conditions for power generation operation to be reached while avoiding operating conditions that result in high NOx emissions, significantly reducing the cumulative NOx amount during transient operation.

Figure 8 is a flowchart showing the control flow during the startup transition (startup) of an 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 during the initial air-fuel ratio lean processing at startup.

When the internal combustion engine 10 of the second embodiment is started, in step S71, the air-fuel ratio for homogeneous lean combustion in the internal combustion engine 10 is set. In step S72, the air-fuel ratio for stratified lean combustion in the internal combustion engine 10 is set. Steps S71 and S72 are steps for performing air-fuel ratio lean processing, and the details are the same as those in Figure 5 described above.

In step S73, the engine speed of the internal combustion engine 10 is set. Step S73 is a step in which the speed increase process is carried out, and the details are the same as those in Figure 6 described above.

In step S74, the load (torque) of the internal combustion engine 10 is set. Step S74 is a step in which the load increase process is carried out, and the details are the same as those in Figure 7 described above.

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 initiated. 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, the processes of steps S72 to S74 are repeated until the operating point reaches the power generation operating point.

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

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

DESCRIPTION OF SYMBOLS 1...vehicle 2...drive wheels 3...drive unit 4...power generating unit 5...drive motor 6...first 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 (12)

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 . 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. 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. 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. 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. 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. 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. 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. The method for controlling 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. 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. 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 . 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, 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.