Control method and control device for internal combustion engine

Abstract

An internal combustion engine (1) includes a turbocharger (2) and a variable valve timing mechanism (18) for changing intake valve closing timing (IVC), and performs an early closing Miller cycle operation in which the IVC is set before bottom dead center during high load operation. When the density of intake air is low in a highland, the IVC is corrected to be retarded closer to the bottom dead center. When the density of intake air is low, the maximum output that can be output by the internal combustion engine (1) is restricted by an intake air amount limit (L33), which is determined by a rotational speed limit of the turbocharger 2, and a knocking limit (L13). When the IVC is corrected to be retarded, the knocking limit decreases as shown by a line (L14), but the intake air amount limit increases as shown by a line (L34), and consequently, the maximum output increases from an intersection point (C13) to an intersection point (C14).

Description

Control method and control device for internal combustion engine

[0001] This invention relates to high altitude control of an internal combustion engine equipped with a turbocharger and a variable valve timing mechanism that changes the intake valve closing timing, and that performs early-closing Miller cycle operation with the intake valve closing timing before bottom dead center during high-load operation.

[0002] In spark ignition internal combustion engines, commonly known as gasoline engines, knocking becomes a problem in the high load range, and the maximum possible output is limited by knocking. At the same time, exhaust temperature rises in the high speed, high load range, so the maximum output of the internal combustion engine is limited by the exhaust temperature limit from the viewpoint of protecting exhaust system components, etc.

[0003] To suppress knocking by lowering the actual compression ratio during the compression stroke, a known method is to use an early-closing Miller cycle, which uses a variable valve timing mechanism to close the intake valve just before bottom dead center. Another known technique combines this early-closing Miller cycle with turbocharging. This configuration can compensate for the decrease in intake efficiency caused by early closing of the intake valve by increasing the boost pressure, ensuring the amount of fresh air required for high power output.

[0004] However, as the altitude increases and the density of the intake air decreases, the intake air volume limit determined by the turbocharger rotation speed limit decreases, resulting in a problem of a decrease in maximum power output.

[0005] Patent Document 1 describes that the lower the atmospheric pressure is during high-load operation of an internal combustion engine, the greater the valve overlap amount is, thereby suppressing a decrease in engine output at high altitudes. However, this document does not take into consideration the limitations on maximum output due to knocking or the limitations on maximum output due to the intake air amount limit determined by the turbocharger rotation speed limit.

[0006] Japanese Patent Application Publication No. 10-141098

[0007] This invention is a control method for an internal combustion engine that is equipped with a turbocharger and a variable valve timing mechanism that changes the intake valve closing timing at least on the intake valve side, and that performs early-closing Miller cycle operation with the intake valve closing timing before bottom dead center during high-load operation.In this method, information on the density of intake air taken in as fresh air is obtained, and when the density of this intake air is low, the intake valve closing timing in the early-closing Miller cycle operation is corrected to approach bottom dead center.

[0008] In an internal combustion engine that combines early-closing Miller cycle operation with a turbocharger, the maximum possible output is determined by the knocking limit and exhaust temperature limit at flat ground, and the maximum output is not limited by the turbocharger speed limit. However, when the intake air density decreases at high altitudes, the effective boost pressure at the turbocharger speed limit decreases, lowering the intake air volume limit. Therefore, the maximum output is determined by the knocking limit and the intake air volume limit, and the maximum output decreases as the intake air density decreases.

[0009] In contrast, in this invention, when the intake air density is low, the intake valve closing timing is corrected to approach bottom dead center during early-closing Miller cycle operation. This relatively lowers the knocking limit, but the effective intake stroke is extended, raising the intake air volume limit and, as a result, increasing the maximum possible output.

[0010] 1 is an explanatory diagram showing a system configuration of an internal combustion engine to which the present invention is applied; a flowchart showing a flow of control of IVC and EGR rate according to intake air density in one embodiment; a characteristic diagram showing IVC characteristics with respect to density; a characteristic diagram showing EGR rate characteristics with respect to density; a flowchart for setting a first density threshold; a characteristic diagram showing the relationship between the ignition timing retard amount and a first density threshold; a valve timing chart showing an example of a general IVC; a characteristic diagram showing output limit characteristics under a general IVC; a valve timing chart showing an example of IVC in early-closing Miller cycle operation; a characteristic diagram showing output limit characteristics under early-closing Miller cycle operation; a characteristic diagram showing output limit characteristics at high altitudes under early-closing Miller cycle operation; a valve timing chart showing an example of IVC with retard correction to move the IVC closer to bottom dead center; a characteristic diagram showing output limit characteristics under IVC after retard correction; a characteristic diagram showing maximum output characteristics with intake air density (altitude) regarding IVC retard correction; a characteristic diagram showing maximum output characteristics with intake air density (altitude) including changes in EGR rate.

[0011] An embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0012] 1 shows the system configuration of an internal combustion engine 1, for example for an automobile, to which the present invention is applied. This internal combustion engine 1 is a four-stroke, spark-ignition internal combustion engine equipped with a turbocharger 2. A pair of intake valves 14 and a pair of exhaust valves 15 are disposed on the ceiling wall of each cylinder 13, and an ignition plug 16 is disposed in the center surrounded by these intake valves 14 and exhaust valves 15. A fuel injection valve 17 is provided below the intake valve 14 to supply fuel into the cylinder 13. The ignition timing of the spark plug 16 and the injection timing and injection amount of fuel by the fuel injection valve 17 are controlled by an engine controller 8. Note that the internal combustion engine 1 may be a port injection type in which the fuel injection valve injects fuel into an intake port.

[0013] The intake valve 14 and the exhaust valve 15 are equipped with variable valve timing mechanisms 18, 19 that can change their opening timing (referred to as IVO and EVO, respectively) and closing timing (referred to as IVC and EVC, respectively). These variable valve timing mechanisms 18, 19 may be of any type, but for example, a mechanism that retards the phase of the camshaft relative to the phase of the crankshaft can be used.

[0014] The intake passage 21 has an intake collector 21a, and upstream of this intake collector 21a is provided an electronically controlled throttle valve 22 whose opening is controlled by a control signal from the engine controller 8. The compressor 2a of the turbocharger 2 is located upstream of the throttle valve 22, and upstream of the compressor 2a are provided an air flow meter 24, for example of a hot wire type, for detecting the intake air amount, and an air cleaner 25. A water-cooled intercooler 26, for example, is provided between the compressor 2a and the throttle valve 22 to cool the high-temperature, high-pressure intake air. A recirculation valve 27 is also provided to communicate the discharge side and intake side of the compressor 2a.

[0015] A turbine 2b of the turbocharger 2 is located in the exhaust passage 30, and a pre-catalyst device 31 and a main catalyst device 32, each of which is made up of a three-way catalyst, are disposed downstream of the turbine 2b. The main catalyst device 32 is disposed under the floor of the vehicle.

[0016] An air-fuel ratio sensor 33 that detects the air-fuel ratio is disposed upstream of the turbine 2b in the exhaust passage 30. The turbine 2b is provided with a wastegate valve 34 that bypasses part of the exhaust gas in accordance with the boost pressure in order to control the boost pressure. The wastegate valve 34 is an electrically operated valve whose opening is controlled by the engine controller 8.

[0017] The engine is also provided with an exhaust gas recirculation passage 35 that recirculates a portion of the exhaust gas from the exhaust passage 30 to the intake passage 21. The exhaust gas recirculation passage 35 is provided with, for example, a water-cooled EGR gas cooler 37 and an EGR valve 38. The engine controller 8 controls the opening of the EGR valve 38, thereby controlling the EGR rate.

[0018] In addition to the air flow meter 24 and air-fuel ratio sensor 33, the engine controller 8 also receives detection signals from various sensors, such as a crank angle sensor 41 for detecting engine speed, a water temperature sensor 42 for detecting coolant temperature, a boost pressure sensor 43 for detecting boost pressure, and an atmospheric pressure sensor 44 for detecting atmospheric pressure. The atmospheric pressure sensor 44 can determine the density of intake air taken in as fresh air. Based on these detection signals and various signals input via other controllers, the engine controller 8 optimally controls the fuel injection amount and injection timing, ignition timing, the opening of the throttle valve 22, the phase of the variable valve timing mechanisms 18 and 19, the boost pressure (i.e., the opening of the wastegate valve 34), the EGR rate, and the like.

[0019] Next, the maximum output of the internal combustion engine 1, which is limited by the knocking limit, the exhaust temperature limit, and the intake air amount limit, will be described.

[0020] 7 shows typical valve timing characteristics of intake and exhaust valves as a reference example. As shown in the figure, IVO is generally set to be more advanced than the intake top dead center, and IVC is set to be more retarded than the intake bottom dead center. EVC is set to be more retarded than the intake top dead center, with an appropriate overlap between it and IVO.

[0021] FIG. 8 is a characteristic diagram showing the maximum output characteristic under such a general valve timing characteristic together with its limiting factors. In other words, FIG. 8 is an explanatory diagram showing the correlation between several output limiting factors. Note that FIGS. 10, 11, and 13, which will be described later, are similar characteristic diagrams. In these figures, the vertical axis of FIG. 8 etc. is the output (kW) of the internal combustion engine 1, and the horizontal axis is the combustion center of gravity (e.g., MB50 (° ATDC)). The combustion center of gravity correlates with the ignition timing, and here it is assumed to be a parameter that can be arbitrarily controlled by changing the ignition timing. In other words, FIG. 8 etc. shows the maximum output that can be obtained by appropriately controlling the combustion center of gravity (in other words, the ignition timing).

[0022] The output of the internal combustion engine 1, which is a spark ignition internal combustion engine, in the high-speed, high-load range is first limited by the knocking limit indicated by line L11. In Figure 8, etc., operation is not possible in the region above and to the left of the knocking limit L11 due to the occurrence of knocking, and operation is possible only in the region below and to the right of the knocking limit L11. Knocking is more likely to occur the earlier the ignition timing is, and therefore the output is limited, so the knocking limit L11 is represented by a sloped characteristic line as shown in the figure.

[0023] Similarly, in the high-speed, high-load range, the exhaust temperature rises, and therefore, from the viewpoint of protecting exhaust system components, the output of the internal combustion engine 1 is limited by the exhaust temperature limit indicated by line L21. Since the exhaust temperature increases as the combustion center of gravity is further retarded, the exhaust temperature limit L21 appears as a characteristic line sloping downward to the right in Figure 8 and other figures, and operation is possible only in the range below and to the left of this exhaust temperature limit L21.

[0024] The output of the internal combustion engine 1 is further limited by the intake air amount limit imposed by the turbocharger 2. This intake air amount limit is a curve that slopes slightly downward to the right, as shown as line L31 in Figure 8. This is a constraint imposed by the rotational speed limit of the turbocharger 2. The intake air amount limit L31 is determined by the amount of gas that is forced into the cylinder when the turbocharger 2 is rotating at the rotational speed limit, and the maximum output cannot exceed this intake air amount limit L31.

[0025] Therefore, from the viewpoint of the knocking limit and the exhaust temperature limit, the internal combustion engine 1 can only operate in the region surrounded by the characteristic line L11 and the characteristic line L21, and the maximum output of the internal combustion engine 1 can be obtained as being further restricted by the intake air amount limit L31. In the reference example of Figure 8, the maximum output is not restricted by the intake air amount limit L31, and is obtained at the intersection C11 of the knocking limit L11 and the exhaust temperature limit L21.

[0026] Figure 9 shows an example of valve timing characteristics for early-closing Miller cycle operation, which is the premise of the present invention. In one embodiment, early-closing Miller cycle operation is performed during high-load operation. In this early-closing Miller cycle operation, as shown in the figure, the IVC is set to the advanced side of intake bottom dead center. Note that the valve operating angle of the intake valve in this embodiment is smaller than that of the reference example in Figure 7. The IVO is advanced from intake top dead center, and has an appropriate overlap with the EVC.

[0027] FIG. 10 shows the maximum output characteristics during such early-closing Miller cycle operation. As described above, the maximum output of the internal combustion engine 1 is basically limited by the knocking limit L11, the exhaust temperature limit L21, and the intake air volume limit L31. However, by closing the IVC earlier than bottom dead center, the actual compression ratio decreases, and the knocking limit, as shown by line L12, becomes higher than the knocking limit shown by dashed line L11 (the characteristic line shifts upward in the figure). Therefore, the maximum output is at the intersection C12 between the knocking limit L12 and the exhaust temperature limit L21, which is higher than the intersection C11 shown in FIG. 8. Because the intake valve closes before bottom dead center, the intake air volume limit, as shown by line L32, becomes lower than the intake air volume limit shown by dashed line L31 when the IVC is after bottom dead center. However, this intake air volume limit L32 does not limit the maximum output. Conversely, the capacity and rotational speed limits of the turbocharger 2 are generally designed so as not to restrict the maximum output at flat ground (standard atmospheric pressure). Therefore, the maximum possible output is determined by the knocking limit L12, which is determined by the degree of IVC advance, and the exhaust temperature limit L21.

[0028] 11 shows the limit on maximum output when the intake air density is low at high altitude during early-closing Miller cycle operation. As the intake air density decreases, the maximum boost pressure obtainable by the turbocharger 2 at the same engine speed limit decreases, and the intake air amount limit, as shown by line L33, becomes lower than the intake air amount limit at standard atmospheric pressure, as shown by dashed line L32. In other words, at a certain altitude, the intake air amount limit L33 begins to limit the maximum output that can be output. Under such conditions of low intake air density, the maximum output of the internal combustion engine 1 is limited by the knocking limit L13 and the intake air amount limit L33, and the intersection C13 of line L13 and line L33 represents the maximum output that can be output.

[0029] In other words, up to a certain intake air density (in other words, a certain altitude), the maximum output is restricted by the knocking limit (L12) and the exhaust temperature limit (L21) as shown in Figure 10, but under conditions where the intake air density is lower than a certain density (higher than a certain altitude), the maximum output is restricted by the knocking limit (L13) and the intake air amount limit (L33) as shown in Figure 11. Note that Figure 11 shows the knocking limit as not changing with the intake air density, but even if the knocking limit changes with the intake air density, the maximum output is still restricted by the knocking limit (L13) and the intake air amount limit (L33).

[0030] FIG. 12 shows an example of a valve timing characteristic in which the IVC is retarded to approach bottom dead center during early-closing Miller cycle operation under conditions of high altitude and low intake air density. In this example, the IVC is retarded so that it is slightly advanced from bottom dead center. Note that the IVC may be retarded to bottom dead center, or even slightly retarded from bottom dead center, taking into account intake inertia. As long as the charging efficiency is within the range of maximum, it can be considered early-closing Miller cycle operation. In the illustrated example, the EVO and EVC are the same as those in FIG. 9, but the EVO and EVC may also be retarded in the same way to ensure appropriate valve overlap with the IVO.

[0031] Figure 13 shows the maximum output limit for the valve timing characteristics illustrated in Figure 12 under conditions of high altitude and low intake air density. By retarding the IVC, the actual compression ratio becomes relatively high, and the knocking limit, as shown by line L14, becomes lower than the knocking limit without retard correction, as shown by dashed line L13 (the characteristic line shifts downward in the figure). However, as the IVC approaches bottom dead center, the amount of gas flowing into the cylinder increases, and the intake air volume limit, as shown by line L34, becomes higher than the intake air volume limit without retard correction, as shown by dashed line L33. The maximum output that can be output by the internal combustion engine 1 is the intersection C14 of line L14, which represents the knocking limit, and line L34, which represents the intake air volume limit. This is generally higher than the intersection C13 of line L13 without retard correction and line L33.

[0032] In other words, by correcting the retard angle so that the IVC approaches bottom dead center, the decrease in maximum output that accompanies an increase in altitude (a decrease in intake air density) can be further reduced.

[0033] FIG. 14 is a characteristic diagram showing the relationship between intake air density (altitude) and maximum output characteristics resulting from IVC correction as shown in FIGS. 12 and 13 . The vertical axis of the diagram represents the maximum output (kW) of the internal combustion engine 1, and the horizontal axis represents intake air density and altitude. Lines L41 and L42 represent maximum output restricted by the knocking limit and exhaust gas temperature limit (see FIG. 10 ). The solid line L41 represents the characteristics for the basic IVC in early-closing Miller cycle operation as shown in FIG. 9 , while the dashed line L42 represents the characteristics when the IVC is retarded to approach bottom dead center as shown in FIG. 12 . On flat ground where the intake air density is higher than a certain density ρ1, the maximum output is restricted by line L41 or line L42. Due to differences in the knocking limit, line L41 is higher than line L42, so a relatively high maximum output can be obtained by using the basic IVC up to density ρ1.

[0034] Lines L43 and L44 show the maximum output restricted by the knocking limit and intake air volume limit (see FIG. 11 ). Line L43, shown as a solid line, represents the characteristics for the basic IVC in early-closing Miller cycle operation as shown in FIG. 9 , while line L44, shown as a dashed line, represents the characteristics when the IVC is retarded to approach bottom dead center as shown in FIG. 12 . At high altitudes where the intake air density is lower than a certain intake air density ρ1, the maximum output is restricted by line L43 or line L44. Because line L44 is higher than line L43 due to differences in the intake air volume limit, at high altitudes where the density is lower than ρ1, retarding the IVC can achieve a relatively high maximum output. For example, the maximum output is at point C41 at density ρ1, and at density ρ2, which is lower than ρ1, the maximum output is at point C42. If the IVC is not retarded, the maximum output would drop to point C43.

[0035] In one embodiment, as will be described later, when the density of intake air is low, the exhaust gas recirculation rate (EGR rate) is decreased in conjunction with the retardation correction of the IVC. In a preferred embodiment, when the density of intake air is lower than a density ρ2 which is even lower than the density ρ1, the EGR rate is decreased.

[0036] That is, exhaust gas recirculation is sometimes performed primarily for the purpose of suppressing knocking and reducing exhaust gas temperature in high load ranges. However, when exhaust gas recirculation is performed in high load ranges including the maximum power operating point, it can cause a significant decrease in maximum power when the altitude increases and the density of the intake air decreases. Therefore, in a preferred embodiment, in addition to the retard correction of the IVC, the EGR rate is reduced or even set to zero.

[0037] Fig. 15 is a characteristic diagram similar to Fig. 14 when IVC retard correction and EGR rate reduction correction are used in combination. The vertical axis of the diagram represents the maximum output (kW) of the internal combustion engine 1, and the horizontal axis represents the intake air density and altitude. Lines L51 and L52 represent the maximum output restricted by the knocking limit and the exhaust temperature limit when the EGR rate is set to the basic EGR rate. Line L51, shown by the solid line, represents the characteristic in the case of the basic IVC in early-closing Miller cycle operation as shown in Fig. 9, and line L52, shown by the dashed line, represents the characteristic in the case of the IVC retard correction to approach bottom dead center as shown in Fig. 12. Lines L53 and L54 show the maximum output power constrained by the knocking limit and exhaust gas temperature limit when the EGR rate is reduced to zero. The solid line L53 represents the characteristics of the basic IVC in early-closing Miller cycle operation as shown in FIG. 9 , while the dashed line L54 represents the characteristics of the IVC when retarded to approach bottom dead center as shown in FIG. 12 . Comparing lines L51 and L52 with lines L53 and L54 makes it clear that exhaust gas recirculation suppresses knocking and reduces exhaust gas temperature, resulting in higher maximum output power than when the EGR rate is zero. Furthermore, line L51, which does not involve IVC retard correction, represents the highest maximum output power. Therefore, on flat ground where the intake air density is higher than density ρ1, exhaust gas recirculation at the basic EGR rate and basic IVC can achieve the highest maximum output power.

[0038] Lines L55 and L56 show the maximum output restricted by the knocking limit and the intake air amount limit when the EGR rate is set to the basic EGR rate, with line L55 shown by a solid line representing the characteristics for the basic IVC in early-closing Miller cycle operation as shown in Figure 9 and line L56 shown by a dashed line representing the characteristics when the IVC is retarded to approach bottom dead center as shown in Figure 12. Lines L57 and L58 show the maximum output restricted by the knocking limit and the intake air amount limit when the EGR rate is reduced to 0, with line L57 shown by a solid line representing the characteristics for the basic IVC in early-closing Miller cycle operation as shown in Figure 9 and line L58 ​​shown by a dashed line representing the characteristics when the IVC is retarded to approach bottom dead center as shown in Figure 12.

[0039] As is clear from a comparison of these lines L55, L56, L57, and L58, under conditions where the intake air density is low and the maximum output is restricted by the knocking limit and the intake air amount limit, it is advantageous to retard the IVC and decrease the EGR rate (for example, to 0) in order to ensure maximum output. For example, when the intake air density becomes ρ3 in Figure 15, the maximum output is at point C51 on line L58.

[0040] In a preferred embodiment, the IVC retard correction is initiated, for example, at density ρ1, and is gradually retarded as the density decreases. When density ρ2 (which is lower than density ρ1) is reached, the IVC reaches its final most retarded position (the position closest to bottom dead center) and is completed. On the other hand, the EGR rate reduction correction is initiated at density ρ2 when the IVC reaches its most retarded position, and is gradually reduced as the density decreases, until the EGR rate reaches 0 at a certain density (e.g., ρ3). In this way, when the intake density decreases, the IVC retard correction is performed first, and the EGR rate is gradually reduced after the IVC retard correction is completed. This reduces the maximum output at the midpoint between density ρ1 and density ρ3.

[0041] That is, assuming a process in which the density gradually decreases (the altitude gradually increases), when the density is ρ1, the maximum output is at point C52 by setting the EGR rate to the basic EGR rate and the IVC to the basic IVC. If retard correction of the IVC is gradually started at density ρ1 and the retard correction is completed at density ρ2 while maintaining the basic EGR rate, the maximum output is at point C53 on line L56. If the EGR rate is gradually reduced from here and the EGR rate finally reaches 0, the maximum output at density ρ3 is at point C51 on line L58 ​​described above.

[0042] Therefore, as the density gradually decreases from density ρ1 (as the altitude gradually increases), the maximum output changes from C52 to C53 to C51.

[0043] In contrast to this, as a comparative example, in the process of gradually decreasing the density (gradually increasing the altitude), if the EGR rate is reduced first when the density decreases to density ρ1, and the IVC retard correction is started after the EGR rate becomes 0, the maximum output changes from C52 to C54 to C51. C54 is the intersection of line L53 and line L57.

[0044] In the figure, the line passing through "C52 → C53 → C51" is relatively higher (i.e., on the side with higher maximum output) than the line passing through "C52 → C54 → C51." Therefore, even if the maximum output C52 at density ρ1 and the final maximum output C51 are the same regardless of whether the IVC retard correction or the EGR rate reduction correction is prioritized, in the process of changing from intermediate densities, that is, from C52 to C51, the embodiment that prioritizes the IVC retard correction is more advantageous in terms of maximum output than the comparative example.

[0045] The same effect can be obtained even if the timing (or density) at which the IVC retard correction started at density ρ1 reaches the most retarded position does not completely coincide with the timing (or density) at which the EGR rate reduction correction starts. In other words, the same effect can be obtained as long as the start of the IVC retard correction precedes the start of the EGR rate reduction correction.

[0046] 2 is a flowchart showing the flow of the IVC retard correction and EGR rate reduction correction process related to the intake air density (altitude) executed by the engine controller 8. Note that this process is based on the assumption that the operating point determined by the load and rotation speed of the internal combustion engine 1 is in a relatively high load range and that early-closing Miller cycle operation is being performed.

[0047] First, in step 1, intake air information related to the density of the intake air is acquired. For example, atmospheric pressure information detected by the atmospheric pressure sensor 44 is read. Note that altitude information obtained from a car navigation system or temperature information from an outside air temperature sensor may be used instead of the atmospheric pressure sensor 44. Then, in step 2, the density ρ of the intake air is calculated from this information.

[0048] Next, in step 3, the density ρ of the intake air is compared with a first density threshold ρ1. Specifically, it is determined whether the density ρ is less than the first density threshold ρ1. If the density ρ is equal to or greater than the first density threshold ρ1, the process proceeds from step 3 to step 4, where the basic IVC and basic EGR rate are used as they are. In other words, the IVC is not retarded and the EGR rate is not decreased during early-closing Miller cycle operation. The first density threshold ρ1 corresponds to, for example, an atmospheric pressure slightly lower than the standard atmospheric pressure. The basic IVC is determined, for example, from a map in which a target basic IVC value is assigned using the load and rotational speed of the internal combustion engine 1 as parameters. Similarly, the basic EGR rate is determined from a map in which a target basic EGR rate value is assigned using the load and rotational speed of the internal combustion engine 1 as parameters.

[0049] If the density ρ of the intake air is less than the first density threshold ρ1, the process proceeds to step 5, where the IVC is retarded. For example, a correction coefficient corresponding to the density ρ is obtained from a table prepared in advance, and the corrected IVC is obtained by multiplying the basic IVC by this correction coefficient.

[0050] Next, in step 6, the density ρ of the intake air is compared with a second density threshold ρ2. Specifically, it is determined whether the density ρ is less than the second density threshold ρ2. The second density threshold ρ2 is a value lower than the first density threshold ρ1. If the density ρ is equal to or greater than the second density threshold ρ2 (i.e., between the first density threshold ρ1 and the second density threshold ρ2), the process proceeds from step 6 to step 7, where the basic EGR rate is used as the EGR rate. In other words, although the IVC retard correction during early-closing Miller cycle operation is performed in accordance with step 5, the EGR rate is not reduced.

[0051] If the density ρ of the intake air is less than the second density threshold ρ2, the process proceeds from step 6 to step 8, where the EGR rate is reduced. For example, a correction coefficient corresponding to the density ρ is obtained from a table prepared in advance, and the corrected EGR rate is obtained by multiplying the basic EGR rate by this correction coefficient.

[0052] FIG. 3 shows the IVC characteristics after the retard correction in step 5. The horizontal axis represents the density ρ of intake air, and the vertical axis represents IVC. In the region where the density ρ is equal to or greater than the first density threshold ρ1, the IVC remains at the basic IVC. When the density ρ is lower than the first density threshold ρ1, the IVC is retarded according to the density ρ. In the region where the density ρ is lower than the second density threshold ρ2, the IVC is constant at the most retarded position.

[0053] 4 shows the characteristics of the EGR rate after the reduction correction in step 8. The horizontal axis represents the density ρ of the intake air, and the vertical axis represents the EGR rate. In a region where the density ρ is equal to or greater than the second density threshold ρ2, the EGR rate remains at the basic EGR rate. When the density ρ is lower than the second density threshold ρ2, the EGR rate decreases according to the density ρ. In a region where the density ρ is lower than a certain density ρ3, the EGR rate becomes 0.

[0054] As described above, since the first density threshold ρ1 that starts the IVC retard correction and the second density threshold ρ2 that starts the EGR rate reduction correction are different, the IVC retard correction is performed preferentially (i.e., before the EGR rate reduction correction).

[0055] In Figures 14 and 15, for ease of understanding, the same terms "ρ1" and "ρ2" as the density thresholds are used for explanation, but these density values ​​for explaining the interchange of the limiting elements of maximum output do not necessarily have to be completely consistent with the first density threshold ρ1 and second density threshold ρ2 for control shown in Figures 2 to 4.

[0056] Furthermore, the likelihood of knocking, which determines the knocking limit, is also affected by the outside air temperature. Therefore, in a preferred embodiment, the first density threshold ρ1 and the second density threshold ρ2 are variably set according to the outside air temperature. Specifically, the higher the outside air temperature, the lower the first density threshold ρ1 and the second density threshold ρ2 are set.

[0057] In addition to the outside air temperature, other indicators that indicate the degree of knocking or the likelihood of knocking occurring include the fuel octane number, intake air temperature, coolant temperature, etc., and the first density threshold ρ1 and the second density threshold ρ2 may be set in the same manner as above based on one or more of these indicators, including the outside air temperature.

[0058] Furthermore, the amount of ignition timing retard in ignition timing retard control based on the output of a knocking sensor (not shown) can be used as an index indicating the actual degree or likelihood of knocking, regardless of factors such as outside air temperature. Figures 5 and 6 show an example of setting the first density threshold ρ1 based on the amount of ignition timing retard. The flowchart in Figure 5 shows the process for setting the first density threshold ρ1. In step 11, the amount of ignition timing retard due to knocking is read, and in step 12, the first density threshold ρ1 is set using a predetermined table. Figure 6 shows an example of the characteristics of the table used in step 12. The larger the amount of ignition timing retard, the lower the first density threshold ρ1 is set. The same applies to the second density threshold ρ2. The larger the amount of ignition timing retard, the lower the second density threshold ρ2 is set based on the amount of ignition timing retard.

[0059] Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible. For example, in the above embodiment, the variable valve timing mechanism 19 is also provided on the exhaust valve 15 side, but the valve timing of the valve operating mechanism on the exhaust valve 15 side may be configured to be fixed. Note that in an embodiment in which the variable valve timing mechanism 19 is provided on the exhaust valve 15 side, changes to the opening and closing timing of the intake valve 14 are less restricted by the amount of overlap, and the degree of freedom in the opening and closing timing of the intake valve 14 is increased. Furthermore, the variable valve timing mechanism that changes the intake valve closing timing may be of any known configuration.

Claims

1. A control method for an internal combustion engine that is equipped with a turbocharger and a variable valve timing mechanism that changes the intake valve closing timing at least on the intake valve side, and that operates in an early-closing Miller cycle with the intake valve closing timing before bottom dead center during high-load operation, by obtaining information on the density of the intake air taken in as fresh air, and correcting the intake valve closing timing in the early-closing Miller cycle so that it approaches bottom dead center when the density of this intake air is low.

2. A control method for an internal combustion engine according to claim 1, wherein exhaust gas recirculation is performed during high load operation, and the exhaust gas recirculation rate is reduced when the intake air density is low.

3. A control method for an internal combustion engine as described in claim 1, wherein when the density of intake air is equal to or higher than a first density threshold, the intake valve closing timing is set to a basic intake valve closing timing according to the operating conditions, and when the density is lower than the first density threshold, the intake valve closing timing is corrected to be more retarded than the basic intake valve closing timing according to the density of intake air.

4. A control method for an internal combustion engine as described in claim 2, wherein when the density of intake air is equal to or higher than a first density threshold, the intake valve closing timing is set to a basic intake valve closing timing according to the operating conditions, and when the density is lower than the first density threshold, the intake valve closing timing is corrected to be more retarded than the basic intake valve closing timing according to the density of intake air, when the density of intake air is equal to or higher than a second density threshold, the exhaust gas recirculation rate is set to a basic exhaust gas recirculation rate according to the operating conditions, and when the density of intake air is lower than the second density threshold, the exhaust gas recirculation rate is corrected to be less than the basic exhaust gas recirculation rate according to the density of intake air or is set to 0.

5. The control method for an internal combustion engine according to claim 4, wherein the second density threshold is set to a density lower than the first density threshold.

6. The control method for an internal combustion engine according to claim 3, wherein the first density threshold is set corresponding to the density of intake air at which the maximum output of the internal combustion engine is limited by the knocking limit and the intake air amount limit of the turbocharger.

7. A control method for an internal combustion engine as described in claim 6, wherein an index indicating the degree of knocking occurring or the likelihood of knocking occurring is obtained, and based on this index, the first density threshold is set to a lower density side as the conditions become more likely to cause knocking.

8. The control method for an internal combustion engine according to claim 7, wherein the index includes at least one of the octane number of the fuel, the outside air temperature, the intake air temperature, and the coolant temperature.

9. The control method for an internal combustion engine according to claim 7, wherein the index is a retard amount in knocking control that retards ignition timing based on a detection signal from a knocking sensor.

10. A control device for an internal combustion engine that is equipped with a turbocharger and a variable valve timing mechanism that changes the intake valve closing timing at least on the intake valve side, and that performs early-closing Miller cycle operation with the intake valve closing timing before bottom dead center during high-load operation, and that obtains information on the density of intake air taken in as fresh air, and when the density of this intake air is low, corrects the intake valve closing timing in early-closing Miller cycle operation so that it approaches bottom dead center.

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