Control method and control device for an internal combustion engine

Abstract

An internal combustion engine (1) is a spark-ignition type internal combustion engine having a turbocharger (2) and an exhaust gas recirculation device, exhaust gas recirculation is performed also in a high load region including a maximum output operating point, thereby performing operation based on a stoichiometric air-fuel ratio. In the case where the maximum output operating point is requested (step 1), intake air density (p) is calculated based on intake air information such as atmospheric pressure, outside air temperature, etc. (step 2) (step 3), and compared with a prescribed density threshold value (p#) (step 4). When the intake air density (p) is less than or equal to the density threshold value (p#) due to high altitude, etc., the exhaust gas recirculation rate is set to 0 (step 6). Thereby, a higher maximum output can be obtained compared with the case where exhaust gas recirculation is performed.

Description

Technical Field

[0001] This invention relates to the control of exhaust gas recirculation in internal combustion engines, particularly the control of exhaust gas recirculation in high-load areas. Background Technology

[0002] Exhaust gas recirculation technology has been developed primarily to reduce NOx in medium-load areas. In recent years, as disclosed in Patent Document 1, attempts have been made to implement exhaust gas recirculation with the main purpose of suppressing knocking and reducing exhaust gas temperature in high-load areas.

[0003] However, when exhaust recirculation is performed in high-load areas, including the point of maximum output operation, there is a problem of a significant reduction in maximum output if the altitude increases and the intake air density decreases accordingly.

[0004] Patent Document 1: Japanese Patent Application Publication No. 2010-242630 Summary of the Invention

[0005] In the control of the internal combustion engine involved in this invention, density information of the intake air taken in as fresh gas is obtained, and when operating under high load, the exhaust gas recirculation rate is changed based on the density information of the intake air in a manner that reduces the exhaust gas recirculation rate when the density is low.

[0006] This control method reduces the exhaust recirculation rate when the intake air density is low, thereby suppressing the output reduction. Attached Figure Description

[0007] Figure 1 This is a structural diagram illustrating the system structure of an internal combustion engine using the present invention.

[0008] Figure 2 This is a flowchart representing the control of the exhaust recirculation rate during maximum output operation.

[0009] Figure 3 This is a characteristic graph illustrating the characteristics of output variation relative to changes in external air pressure in one embodiment.

[0010] Figure 4 This is a characteristic diagram representing the output limitation characteristics based on the presence or absence of exhaust recirculation at lower altitude locations.

[0011] Figure 5 This is a characteristic diagram representing the output limitation characteristics based on the presence or absence of exhaust recirculation at high altitude locations.

[0012] Figure 6 This is a characteristic graph representing the output characteristics based on a higher density threshold setting under conditions of low external air temperature.

[0013] Figure 7This is a characteristic graph representing the output characteristics based on a lower density threshold setting under conditions of higher external air temperature.

[0014] Figure 8 This is a flowchart used to set the density threshold.

[0015] Figure 9 It is a characteristic graph showing the relationship between ignition timing lag and density threshold. Detailed Implementation

[0016] An embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0017] Figure 1 The diagram illustrates the system structure of, for example, an internal combustion engine 1 for automobiles to which the present invention is applied. This internal combustion engine 1 is, for example, a four-stroke spark-ignition internal combustion engine with a turbocharger 2. A catalyst device 3, consisting of a three-way catalyst, is provided, for example, in the exhaust passage 4 of the internal combustion engine 1. The turbine 2B of the turbocharger 2 is configured upstream of the catalyst device 3. Furthermore, the turbine 2B has a known electrically operated wastegate valve (not shown) for boost pressure control.

[0018] The intake passage 12 of the internal combustion engine 1 has a throttle valve 9 in the middle of the passage, and a compressor 2A of a turbocharger 2 is arranged upstream of the throttle valve 9. Additionally, a negative pressure valve 8 for generating negative pressure is provided upstream of the compressor 2A. Furthermore, an intercooler 11, such as a water-cooled one, is provided between the compressor 2A and the internal combustion engine 1 to cool the high-temperature, high-pressure intake air. In the illustrated example, the intercooler 11 is located directly upstream of the intake manifold.

[0019] As an exhaust recirculation device, the internal combustion engine 1 has an exhaust recirculation path 5 that allows a portion of the exhaust gas to flow back from the exhaust passage 4 to the intake passage 12. For example, one end of the exhaust recirculation path 5 is connected to the downstream side of the catalyst device 3 in the exhaust passage 4, and the other end of the exhaust recirculation path 5 is connected between the negative pressure valve 8 and the compressor 2A in the intake passage 12. The exhaust recirculation path 5 includes, for example, a water-cooled EGR gas cooler 6 that cools the recirculated exhaust gas; and an EGR valve 7 that controls the exhaust recirculation flow rate. The opening degree of the EGR valve 7 is controlled by the engine controller 13, which performs overall control of the internal combustion engine 1.

[0020] A knock sensor 10 for detecting knock-based vibrations is disposed in the main body of the internal combustion engine 1. The output of the knock sensor 10 is used to perform known ignition timing lag control using the engine controller 13.

[0021] In addition to the aforementioned knock sensor 10, the engine controller 13 receives signals from various sensors, including an air flow meter 15 for detecting the intake air volume, a crankshaft angle sensor 16 for detecting the internal combustion engine rotation speed, an air-fuel ratio sensor 17 for detecting the exhaust air-fuel ratio for air-fuel ratio feedback control, a coolant temperature sensor 18 for detecting the coolant temperature, an accelerator pedal opening sensor 19 for detecting the amount of pressure applied by the driver, a vehicle speed sensor 20 for directly or indirectly detecting the vehicle speed, an atmospheric pressure sensor 21 for detecting atmospheric pressure (external air pressure), an external air temperature sensor 22 for detecting external air temperature, and a boost pressure sensor 23 for detecting boost pressure. Based on these detection signals, the engine controller 13 optimizes the fuel injection quantity, injection timing, ignition timing, throttle valve 9 opening, EGR valve 7 opening, and boost pressure.

[0022] In the above structure, exhaust gas recirculation is also performed in the high-load region, including the highest output operating point, via the exhaust gas recirculation device. Operation is based on the stoichiometric air-fuel ratio (λ=1) by suppressing knocking and exhaust temperature based on exhaust gas recirculation. The basic exhaust gas recirculation rate is preset by setting the load and rotational speed of the internal combustion engine 1 as parameters, and the opening of the EGR valve 7 is controlled in a manner that conforms to the basic exhaust gas recirculation rate.

[0023] On the other hand, if the altitude is high and the density of the intake air taken in as new gas is reduced, the maximum output of the internal combustion engine 1 will be reduced accordingly. Figure 3 The vertical axis is set to the output (kW) of internal combustion engine 1, and the horizontal axis is set to the external air pressure (kPa) (equivalent to altitude) to represent the reduction in maximum output. However, especially when exhaust recirculation is present, the output is reduced in a manner proportional to the external air pressure. Without exhaust recirculation, the output reduction is greater at higher altitudes. Furthermore, the term "EGR" below indicates exhaust recirculation. Figure 3 In the above, "with EGR" refers to the characteristic when exhaust gas recirculation occurs, while "without EGR" refers to the characteristic when exhaust gas recirculation does not occur.

[0024] In order to suppress the reduction in maximum output at higher altitudes, in this embodiment, the target exhaust recirculation rate is modified in a way that reduces the exhaust recirculation rate when the intake air density is low.

[0025] Figure 2This is a flowchart illustrating the process of changing the exhaust recirculation rate at the highest output operating point, executed by the engine controller 13. First, in step 1, it is determined whether a condition for requesting maximum output is met. For example, maximum output is requested when the accelerator pedal depressor detected by the accelerator opening sensor 19 is equivalent to being fully open. If the condition is NO in step 1, the process ends. In this case, as previously described, exhaust recirculation is performed according to the basic exhaust recirculation rate corresponding to each operating point.

[0026] If the answer in step 1 is YES, proceed to step 2 to obtain intake information. For example, the atmospheric pressure detected by atmospheric pressure sensor 21 and the outside air temperature detected by outside air temperature sensor 22 are read in as intake information. Alternatively, altitude information obtained from the vehicle navigation system can be used instead of atmospheric pressure sensor 21.

[0027] Next, in step 3, the intake air density ρ is calculated based on the aforementioned intake air information. In step 4, it is determined whether the intake air density ρ is less than or equal to the density threshold ρ#. If the intake air density ρ is greater than the density threshold ρ#, then proceed from step 4 to step 5, directly using the basic exhaust recirculation rate as the target exhaust recirculation rate for exhaust recirculation. If the intake air density ρ is less than or equal to the density threshold ρ#, then proceed from step 4 to step 6, setting the exhaust recirculation rate to 0. Alternatively, it can be left unset to 0 and slightly perform exhaust recirculation.

[0028] By implementing this control, thus... Figure 3 As shown, exhaust recirculation is not performed at an altitude higher than the point equivalent to the density threshold ρ#, as indicated by the "no EGR" line, which results in higher output compared to the "with EGR" characteristic.

[0029] Figure 4 and Figure 5 This is an explanatory diagram illustrating the relationship between several output limitations to achieve the aforementioned effects. Figure 4 , Figure 5 The vertical axis represents the output (kW) of internal combustion engine 1, and the horizontal axis represents the combustion center of gravity, MB50 (°ATDC). MB50 is related to the ignition timing, and here it is assumed to be a parameter that can be arbitrarily controlled by changing the ignition timing. In addition, "no EGR" in the figure represents the characteristics when the exhaust gas recirculation rate is 0, and "with EGR" represents the characteristics under the basic exhaust gas recirculation rate.

[0030] Figure 4 This example illustrates the request for maximum output at an altitude of 0m. The output of internal combustion engine 1, a spark-ignition internal combustion engine, in its high-speed, high-load region is primarily limited by the knock limits shown by lines L11 (with EGR) and L12 (without EGR). Figure 4In the region further to the upper left of the knock limits L11 and L12, operation is impossible due to knocking; operation is only possible in the region further to the lower right of the knock limits L11 and L12. The earlier the ignition timing, the easier it is to generate knocking, thus limiting the output. Therefore, the knock limits L11 and L12 become sloping characteristic lines as shown in the figure. Since EGR suppresses knocking, the line L11, which represents the knock limit with EGR, is located higher than the line L12, which represents the knock limit without EGR (i.e., on the high-output side).

[0031] Similarly, exhaust temperature increases in the high-speed, high-load region. Therefore, from the perspective of protecting exhaust system components, the output of internal combustion engine 1 is limited by the exhaust temperature limits shown by line L21 (with EGR) and line L22 (without EGR). The closer MB50 is to the lag angle side, the higher the exhaust temperature; therefore, the exhaust temperature limits L21 and L22 are... Figure 4 As shown in the diagram, the characteristic line slopes downwards to the right, allowing operation only in the region further to the left and below the exhaust temperature limits L21 and L22. Since the exhaust temperature decreases due to EGR, line L21, representing the exhaust temperature limit with EGR, is located above (i.e., on the high-output side) the line L22, representing the exhaust temperature limit without EGR.

[0032] Furthermore, the output of the internal combustion engine 1 is limited by the air volume limit based on the turbocharger 2. This air volume limit is... Figure 4 The curves shown in the diagram, such as line L31 (with EGR) and line L32 (without EGR), slope downwards to the right. This is based on the speed limit of turbocharger 2. Since the amount of gas forced into the cylinders by turbocharger 2 is constant, the greater the exhaust return flow, the less air (oxygen) is available. Therefore, the air volume limit L31 with EGR is lower (i.e., the lower output side) than the air volume limit L32 without EGR.

[0033] Therefore, based on the viewpoints of knock limit and exhaust temperature limit, operation is only valid within the region enclosed by characteristic lines L11, L12 and L21, L22, and the maximum output of internal combustion engine 1 can be obtained as a parameter limited by air volume limits L31, L32. Figure 4 At an altitude of 0m, without EGR, it is not limited by the air volume limit L32, and the intersection C2 of the knock limit L12 and the exhaust temperature limit L22 is the highest output. With EGR, the intersection C1 of the knock limit L11 and the air volume limit L31 is limited by the air volume limit L31 and becomes the highest output.

[0034] Here, based on the respective height positions of intersections C1 and C2, it is clear that intersection C1 with EGR results in high output. That is, generally speaking, the greater the exhaust recirculation, the better it suppresses knocking and the lower the exhaust temperature, thus achieving a larger maximum output. For example, exhaust recirculation can increase the output by ΔP1.

[0035] Figure 5 The characteristics at higher altitudes are as described above. The knock limit (shown by lines L13 (with EGR) and L14 (without EGR), the exhaust temperature limit (shown by lines L23 (with EGR) and L24 (without EGR), and the air volume limit (shown by lines L33 (with EGR) and L34 (without EGR)) restrict the maximum output. If the intake air density ρ decreases with increasing altitude, the basic maximum output point obtained as the intersection of the knock limit and exhaust temperature limit is slightly lower than at 0m altitude. The air volume limits L33 and L34 also decrease, especially the air volume limit L33 with EGR, which decreases significantly. Therefore, without EGR, the limitation of air volume limit L34 makes the intersection point C4 of knock limit L14 and air volume limit L34 the maximum output. With EGR, the significantly reduced air volume limit L33 makes the intersection point C3 of knock limit L13 and air volume limit L33 the maximum output.

[0036] Here, based on the respective altitude positions of intersection points C3 and C4, it is clear that at higher altitudes, intersection point C4 without EGR results in a higher output. That is, the relationship between the maximum output and the presence or absence of exhaust recirculation is the opposite of the case at 0m altitude. Therefore, stopping exhaust recirculation here can increase the output of ΔP2.

[0037] The ease with which a knock can occur, which specifies the knock limit, is also affected by the external air temperature. Therefore, in a preferred embodiment, the density threshold ρ# is set according to the external air temperature. Specifically, the higher the external air temperature, the lower the density threshold ρ# is set to. Figure 6 as well as Figure 7 Therefore, with Figure 3 Characteristic plots with the same output and external air pressure as parameters. Figure 6 This indicates the characteristics when the outside air temperature is low. Figure 7 This indicates the characteristics when the outside air temperature is high. The exhaust recirculation rate becomes 0 in regions with outside air pressure lower than the density threshold ρ#. (See figure.) Figure 6 The medium density threshold ρ# is set to relatively high. Figure 7The medium density threshold ρ# is set to a relatively low value. This allows for setting the density threshold ρ# on the low density side when the external air temperature is high, which is prone to knocking. This enables exhaust recirculation even at high altitudes, achieving knock suppression based on exhaust recirculation, which in turn advances the ignition timing angle and suppresses exhaust temperature, thus achieving a balance between ensuring maximum output.

[0038] In addition, as an indicator of the ease with which knocking occurs, besides the aforementioned external air temperature, other indicators include the fuel octane number, intake air temperature, and cooling water temperature. The density threshold ρ# can also be set as described above based on one or more of the aforementioned indicators, including external air temperature.

[0039] In addition, as an indicator that represents the actual degree of knocking or the ease of its occurrence without relying on external air temperature, the ignition timing lag amount based on the output of the knock sensor 10 for ignition timing lag control can also be used. Figure 8 as well as Figure 9 This illustrates an embodiment where the density threshold ρ# is set based on the ignition timing lag. Figure 8 The flowchart shows the process for setting the density threshold ρ#. In step 11, the ignition timing lag based on detonation is read in, and in step 12, the density threshold ρ# is set using a specified table. Figure 9 The table used in step 12 represents a characteristic example: the greater the ignition timing lag, the lower the density threshold ρ# is set.

[0040] The present invention has been described above as an embodiment, but the present invention is not limited to the above embodiment and various modifications can be made. For example, in the above embodiment, a so-called low-pressure EGR system in which the recirculated exhaust flows into the intake system at a position upstream of the compressor 2A of the turbocharger 2, or a so-called high-pressure EGR system in which the recirculated exhaust flows into the intake system at a position downstream of the compressor 2A, the present invention can also be applied.

[0041] Furthermore, this invention can also be applied to naturally aspirated internal combustion engines without turbochargers. Even in naturally aspirated internal combustion engines, as the intake air density ρ decreases, the air volume limit shifts towards the lower output side.

Claims

1. A control method for an internal combustion engine, wherein the internal combustion engine performs exhaust gas recirculation during high-load operation, and comprises a spark-ignition internal combustion mechanism with a turbocharger, wherein... The density information of the intake air, which is taken in as fresh gas, is obtained. Under high load operation, based on this intake air density information, exhaust backflow is stopped when the density falls below a density threshold. Obtain an index representing the degree of detonation or the ease with which detonation occurs. Based on this index, the more likely the conditions are to cause detonation, the lower the density threshold should be set.

2. The control method for an internal combustion engine according to claim 1, wherein, The above indicators include at least one of the following: fuel octane number, outside air temperature, intake air temperature, and cooling water temperature.

3. The control method for an internal combustion engine according to claim 1, wherein, The above indicators are the hysteresis amount of knock control based on the detection signal of the knock sensor, which is used to delay the ignition timing.

4. The control method for an internal combustion engine according to claim 1, wherein, The high-load operation during which the exhaust recirculation based on the above-mentioned intake density is stopped is when the turbocharger reaches its maximum output speed limit.

5. The control method for an internal combustion engine according to claim 1, wherein, Under standard intake density, at maximum output operation, exhaust recirculation occurs and combustion is performed at the stoichiometric air-fuel ratio.

6. A control device for an internal combustion engine, the internal combustion engine having an exhaust gas recirculation device and comprising a spark-ignition internal combustion mechanism with a turbocharger, wherein, In operating areas including high-load regions, exhaust gas recirculation based on the aforementioned exhaust gas recirculation device is performed, and the density information of the intake gas taken in as fresh gas is acquired. During high-load operation, based on this intake gas density information, the exhaust gas recirculation is stopped when the density falls below a density threshold. Obtain an index representing the degree of detonation or the ease with which detonation occurs. Based on this index, the more likely the conditions are to cause detonation, the lower the density threshold should be set.

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