Exhaust purifying device for internal combustion engine and method for controlling same

Abstract

An engine ECU (100) executes a fuel addition process using a fuel addition valve (80) before executing a DPF regeneration process. For the fuel addition process, after implementation of refueling, when an exhaust temperature reaches a temperature (T2) lower than a temperature (T1pset) preset as a fuel addition implementation temperature (YES in S10), the engine ECU (100) implements fuel addition from the fuel addition valve (80) (S20). Then, when the exhaust temperature rises to a threshold temperature (ΔT) or above (YES in S40), the engine ECU (100) changes the fuel addition implementation temperature from the preset temperature (T1pset) to the temperature (T2) lower than the preset temperature (T1pset) (S50).

Description

Exhaust gas purification device for internal combustion engine and control method thereof

[0001] The present disclosure relates to an exhaust gas purification device for an internal combustion engine and a control method thereof.

[0002] Japanese Patent Laid-Open Publication No. 2023-159546 (Patent Document 1) discloses an exhaust gas purification device for an internal combustion engine equipped with a DPF (Diesel Particulate Filter). In this exhaust gas purification device, an oxidation catalyst (DOC: Diesel Oxidation Catalyst) is provided upstream of the DPF, and during DPF regeneration, fuel is added (injected) into the exhaust passage upstream of the DOC to raise the temperature of the DPF and burn particulate matter (PM: Particulate Matter) accumulated in the DPF. Patent Document 1 describes the use of carbon-neutral, environmentally friendly biofuels (see Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2023-159546

[0004] In recent years, biofuels, hydrogen-derived fuels, gas-derived fuels, and the like have been attracting attention as carbon-neutral, environmentally friendly fuels, rather than conventional petroleum-derived fuels. These fuels may be used alone or may be mixed with conventional petroleum-derived fuels (hereinafter, these fuels will also be referred to as "multi-fuels" to distinguish them from conventional petroleum-derived fuels). However, such multi-fuels have significantly different properties from conventional petroleum-derived fuels, and the range of properties varies greatly from fuel to fuel, which can lead to inadequate DPF temperature rise and poor DPF regeneration.

[0005] The present disclosure has been made to solve such problems, and an object of the present disclosure is to provide an exhaust gas purification device for an internal combustion engine and a control method thereof that can perform appropriate DPF regeneration even when multiple fuels are used.

[0006] The exhaust purification device disclosed herein is an exhaust purification device for an internal combustion engine, and includes a DPF, a DOC, a fuel addition means, a temperature sensor, and a control device. The DPF is provided in an exhaust passage of the internal combustion engine and traps PM contained in exhaust gas flowing through the exhaust passage. The DOC is provided upstream of the DPF in the exhaust passage. The fuel addition means adds fuel into the exhaust passage upstream of the DOC. The temperature sensor detects the temperature of the exhaust gas downstream of the DOC. The control device executes a regeneration process to burn the PM deposited in the DPF by adding fuel using the fuel addition means when an execution condition according to the amount of PM accumulation is met and the temperature is higher than a fuel addition execution temperature. The control device further executes a fuel addition process to add fuel before executing the regeneration process. The fuel addition process includes a first process and a second process. The first process is a process for adding fuel when the temperature reaches a second temperature lower than a first temperature preset as a fuel addition temperature after refueling, and the second process is a process for changing the fuel addition temperature from the first temperature to the second temperature when the temperature rises by a threshold temperature or more as a result of the first process.

[0007] The present disclosure also provides a control method for an exhaust gas purification device for an internal combustion engine, the exhaust gas purification device including a DPF, a DOC, a fuel addition means, and a temperature sensor. The DPF is provided in an exhaust passage of the internal combustion engine and traps PM contained in exhaust gas flowing through the exhaust passage. The DOC is provided in the exhaust passage upstream of the DPF. The fuel addition means adds fuel into the exhaust passage upstream of the DOC. The temperature sensor detects the temperature of the exhaust gas downstream of the DOC. The control method further includes the steps of: executing a regeneration process to burn PM accumulated in the DPF by adding fuel using the fuel addition means when an execution condition according to the PM accumulation amount is met and the temperature is higher than a fuel addition execution temperature; and executing a fuel addition process to add fuel before executing the regeneration process. The fuel addition process includes a first process and a second process. The first process is a process for adding fuel when the temperature reaches a second temperature lower than a first temperature preset as a fuel addition temperature after refueling, and the second process is a process for changing the fuel addition temperature from the first temperature to the second temperature when the temperature rises by a threshold temperature or more as a result of the first process.

[0008] In the above-described exhaust gas purification device and control method, a fuel addition process is performed before the regeneration process is performed. In the fuel addition process, when the temperature of the exhaust gas reaches a second temperature lower than a first temperature preset as a fuel addition temperature, fuel is added by the fuel addition means. When the exhaust gas temperature downstream of the DOC rises above a threshold temperature, the fuel addition temperature is changed from the first temperature to the second temperature. This allows the DOC and DPF to be heated under appropriate conditions (temperatures) according to the fuel properties after refueling. Therefore, appropriate DPF regeneration can be performed even when multiple fuels are used.

[0009] The fuel addition process may further include a third process and a fourth process. The third process is a process of adding fuel when the temperature reaches a first temperature if the increase in the temperature due to the first process is smaller than a threshold temperature. The fourth process is a process of changing the fuel addition execution temperature to a third temperature higher than the first temperature if the increase in the temperature due to the third process is smaller than the threshold temperature.

[0010] In this exhaust gas purification device, if the increase in exhaust gas temperature due to the first process is smaller than the threshold temperature, when the exhaust gas temperature reaches the first temperature, the fuel addition means adds fuel again. Then, if the increase in exhaust gas temperature due to the third process is smaller than the threshold temperature, the fuel addition temperature is changed from the first temperature to a third temperature higher than the first temperature. This makes it possible to raise the temperatures of the DOC and DPF under appropriate conditions (temperatures) according to the fuel properties, even for fuels with a high oxidation onset temperature in the DOC.

[0011] The control device may further perform an estimation process to estimate the cetane number of the fuel in the fuel tank, and may perform a fuel addition process when the cetane number estimated by the estimation process is equal to or greater than a threshold value.

[0012] For fuels with high cetane numbers, no correlation was found between the distillation temperature and the oxidation onset temperature (light-off temperature) in the DOC, and it was confirmed that the light-off temperature tended to be << the distillation temperature. According to the above configuration, when the cetane number is equal to or greater than a threshold value, the fuel addition process is executed, so that the DOC and DPF can be heated under appropriate conditions according to the fuel properties, thereby regenerating the DPF.

[0013] The fuel addition process may include a fifth process and a sixth process. The fifth process is a process of adding fuel when the temperature reaches a first temperature if the cetane number is smaller than a threshold value. The sixth process is a process of changing the fuel addition execution temperature to a fourth temperature higher than the first temperature if the increase in temperature due to the fifth process is smaller than the threshold temperature.

[0014] With this configuration, even when using fuel with a low cetane number, the temperatures of the DOC and DPF can be raised under appropriate conditions according to the fuel properties.

[0015] The first process may include a process of adding fuel multiple times when the temperature of the exhaust gas reaches a second temperature after refueling, and the second process may include a process of changing the fuel addition temperature to the second temperature when a temperature increase equal to or greater than the threshold temperature due to the first process is detected multiple times.

[0016] By performing fuel addition and checking the resulting temperature rise multiple times, it is possible to increase the reliability of changing the fuel addition temperature to the second temperature.

[0017] The temperature sensors may include a first temperature sensor that detects the temperature of the exhaust gas between the DOC and the DPF, and a second temperature sensor that detects the temperature of the exhaust gas downstream of the DPF.

[0018] By using a plurality of temperature sensors, the reliability of the temperature detection value can be increased, and as a result, the reliability of changing or maintaining the fuel addition execution temperature can be increased.

[0019] The fuel addition means may include a fuel addition valve provided in the exhaust passage upstream of the DOC.

[0020] In this case, when the fuel addition temperature is changed from a first temperature to a second temperature during the fuel addition process, the control device may increase the amount of fuel added by the fuel addition valve when the regeneration process is performed compared to when the fuel addition temperature is not changed.

[0021] When the exhaust gas temperature becomes high, there is a possibility that the fuel added through the fuel addition valve will pass through the DOC without reacting with it (this phenomenon is also called "slip"). However, by lowering the fuel addition temperature to the second temperature, slippage is suppressed. This allows the amount of fuel added through the fuel addition valve to be increased, and by increasing the amount of fuel added, the DOC temperature is raised more quickly, thereby improving the DPF regeneration effect.

[0022] The fuel addition means may include performing post injection of fuel into the combustion chamber of the internal combustion engine during the expansion stroke of the internal combustion engine.

[0023] In this case, when the fuel addition temperature is changed from a first temperature to a second temperature during the fuel addition process, the control device may increase the amount of post-injection fuel injection when the regeneration process is performed compared to when the fuel addition temperature is not changed.

[0024] As described above, by lowering the fuel addition temperature to the second temperature, slippage is suppressed, so the amount of fuel injected in the post-injection can be increased, and by increasing the amount of post-injection, the DOC is heated up earlier, thereby improving the regeneration effect of the DPF.

[0025] In addition, when the control device changes the fuel addition temperature from a first temperature to a second temperature during the fuel addition process, the control device may delay the injection timing of the post injection when the regeneration process is performed compared to when the fuel addition temperature is not changed.

[0026] Whether or not fuel burns is determined by temperature, pressure, and the amount of oxygen, but if the post-injection timing is retarded, the piston descends and post-injection is performed when the temperature, pressure, and amount of oxygen in the combustion chamber are all lower, suppressing fuel combustion in the combustion chamber and increasing the amount of fuel supplied to the DOC. As a result, the DOC is heated earlier, improving the DPF regeneration effect.

[0027] According to the exhaust gas purification device for an internal combustion engine and the control method thereof disclosed herein, it is possible to perform appropriate DPF regeneration even when multiple fuels are used.

[0028] FIG. 1 is an overall configuration diagram of an internal combustion engine equipped with an exhaust purification device according to a first embodiment. FIG. 2 is a diagram showing an example of the relationship between distillation temperature and light-off temperature. FIG. 3 is a flowchart showing an example of the procedure of a fuel addition process executed by an engine ECU. FIG. 4 is a flowchart showing an example of the procedure of a DPF regeneration process executed by an engine ECU. FIG. 5 is a flowchart showing an example of the procedure of a fuel addition process executed by an engine ECU in a second embodiment. FIG. 6 is a flowchart showing an example of the procedure of a fuel addition process executed by an engine ECU in a third embodiment. FIG. 7 is a flowchart showing an example of the procedure of a fuel addition process executed by an engine ECU in a fourth embodiment.

[0029] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0030] [First Embodiment] Fig. 1 is an overall configuration diagram of an internal combustion engine equipped with an exhaust gas purification device according to a first embodiment of the present disclosure. Referring to Fig. 1, engine 1 is a compression autoignition internal combustion engine (diesel engine). That is, fuel is injected from an injector (fuel injection valve) 14 into a combustion chamber formed in a cylinder 12 of an engine body 10, and the air-fuel mixture in cylinder 12 is compressed to autoignite. Note that, although this first embodiment shows an example in which engine 1 has four cylinders, the number of cylinders is not limited to this.

[0031] The intake passage 20 is provided with an air cleaner 22, an intercooler 24, and a throttle valve (diesel throttle valve) 26. Fresh air, from which foreign matter has been removed by the air cleaner 22, is supercharged (compressed) by a compressor 32 of a turbocharger 30, cooled by the intercooler 24, and then supplied to an intake manifold 28 and supplied to each combustion chamber from an intake port in the engine body 10.

[0032] Exhaust gases discharged from the combustion chambers are collected in an exhaust manifold 50 and released into the outside air through an exhaust passage 52. A portion of the exhaust gases is recirculated to the intake manifold 28 through an EGR (Exhaust Gas Recirculation) passage 60. An EGR cooler 62 and an EGR valve 64 are provided in the EGR passage 60.

[0033] The exhaust passage 52 is provided with, from the upstream side, the turbine 34 of the turbocharger 30, a DOC (oxidation catalyst) 70, and a DPF 72. The DPF 72 is a filter that traps PM (particulate matter) in the exhaust gas and purifies the exhaust gas by appropriately burning and removing the trapped PM. Although not shown, a urea addition valve and a selective reduction catalyst may be provided downstream of the DPF 72. Note that a NOx storage-reduction catalyst (NSR (NOx Storage-Reduction) catalyst) may be provided instead of or in addition to the urea addition valve and the selective reduction catalyst.

[0034] Fuel supplied from the outside is stored in a fuel tank 40. The fuel in the fuel tank 40 is supplied to a high-pressure fuel pump 42 by a feed pump 41, and the high-pressure fuel discharged from the high-pressure fuel pump 42 is pressure-fed to a common rail 44 through a fuel passage 43. The high-pressure fuel stored in the common rail 44 is injected into the combustion chamber (into the cylinder) from the injector 14.

[0035] A fuel addition valve 80 is provided in the exhaust passage 52 upstream of the DOC 70. In the first embodiment, the fuel addition valve 80 is provided upstream of the turbine 34, but it may be provided downstream of the turbine 34. Fuel in the fuel tank 40 is supplied to the fuel addition valve 80 by the feed pump 41 through the fuel passage 45. When the fuel addition valve 80 opens, fuel is added (injected) into the exhaust passage 52.

[0036] The engine ECU 100 (Electronic Control Unit) includes a CPU (Central Processing Unit) 101, a memory 102 including a ROM (Read Only Memory) and a RAM (Random Access Memory), and input / output ports (not shown) for inputting and outputting various signals. The engine ECU 100 executes predetermined arithmetic processing based on information stored in the memory 102 and information from various sensors, and controls the injector 14, the throttle valve 26, the high-pressure fuel pump 42, the fuel addition valve 80, etc. The engine ECU 100 corresponds to the "control device" in this disclosure.

[0037] Various sensors are provided, including an engine rotation speed sensor that detects the rotation speed of the engine, an accelerator pedal sensor that detects the amount of accelerator pedal operation by the user, an air flow meter that detects the amount of intake air into the engine 1 (none of which are shown), as well as temperature sensors 114, 115, a pressure sensor 116, etc.

[0038] The temperature sensor 114 is provided in the exhaust passage 52 between the DOC 70 and the DPF 72, and detects the temperature Tc of the exhaust gas that has passed through the DOC 70. The temperature sensor 114 may also detect the outlet temperature of the DOC 70. The temperature sensor 115 is provided in the exhaust passage 52 downstream of the DPF 72, and detects the temperature Tf of the exhaust gas that has passed through the DPF 72. The temperature sensor 115 may also detect the temperature of the DPF 72 (the bed temperature of the DPF 72). The pressure sensor 116 detects the pressure difference ΔP in the exhaust passage 52 upstream and downstream of the DPF 72.

[0039] In the engine 1 configured as described above, the DPF 72 collects PM contained in the exhaust gas emitted from the engine body 10. When the amount of PM accumulated in the DPF 72 reaches or exceeds a predetermined value, the engine ECU 100 executes a regeneration process to regenerate the DPF 72.

[0040] The DPF 72 is regenerated by raising the temperature of the DPF 72 and burning off the PM that has accumulated in the DPF 72. The temperature of the DPF 72 is raised by adding fuel to the exhaust passage 52 through the fuel addition valve 80. The added fuel is oxidized in the DOC 70 and generates heat (combusts). This raises the temperature of the exhaust gas that passes through the DOC 70 and flows into the DPF 72, raising the temperature of the DPF 72.

[0041] This engine 1 uses multiple fuels, including biofuels, hydrogen-derived fuels, and gas-derived fuels, rather than conventional petroleum-derived fuels. That is, such multiple fuels are supplied to a fuel tank 40, and the fuel in the fuel tank 40 is injected from the injector 14 into the combustion chamber of the engine body 10, and when the DPF is to be regenerated, the fuel is injected from a fuel addition valve 80 into the exhaust passage 52.

[0042] These multi-fuels have properties that are significantly different from conventional petroleum-derived fuels. Furthermore, the properties vary depending on the fuel component ratios contained in the multi-fuels, and the range of properties varies greatly from fuel to fuel. This can lead to various problems, such as poor DPF regeneration. For example, differences in the distillation characteristics of the fuel supplied can cause differences in the degree of fuel oxidation reaction in the DOC, which can prevent the DPF from heating up properly and result in poor DPF regeneration. This can also lead to DOC abnormalities and poor fuel economy. Furthermore, because of the wide range of fuel properties, the above problems can also occur when fuel outside the DOC design range is used.

[0043] Therefore, this disclosure provides a method for performing proper DPF regeneration even when multiple fuels are used, as will be explained in detail below.

[0044] As described above, DPF regeneration is performed by adding fuel to the exhaust passage 52 through the fuel addition valve 80. The added fuel is oxidized in the DOC 70 and generates heat (combustion). Conventionally, the oxidation start temperature (light-off temperature) of the fuel has been thought to be correlated with the distillation characteristics of the fuel. Therefore, a fuel addition temperature corresponding to the distillation temperature of petroleum-derived fuel is stored in advance, and fuel is added to the exhaust passage when the conditions for DPF regeneration are met (large PM accumulation amount) and the exhaust temperature reaches the fuel addition temperature.

[0045] However, as mentioned above, various fuels have significantly different properties from conventional petroleum-derived fuels, and the properties vary widely among fuels, and the distillation temperatures also differ. Furthermore, among various fuels, there are some fuels for which there is no correlation between the distillation temperature and the light-off temperature. Specifically, there are fuels for which the light-off temperature is less than the distillation temperature.

[0046] 2 is a graph showing an example of the relationship between distillation temperature and light-off temperature. In FIG. 2, the horizontal axis represents the exhaust gas temperature before the DOC, and the vertical axis represents the DOC temperature. The dotted line L3 is a line where the DOC temperature is approximately equal to the gas temperature before the DOC.

[0047] 2, line L1 shows an example of a fuel for which a correlation is observed between the distillation temperature and the light-off temperature. That is, the distillation temperature (e.g., T50) of this fuel is TA1, the light-off temperature is TA2, and the temperature difference ΔTA therebetween is small.

[0048] On the other hand, line L2 shows an example of a multi-fuel where no correlation is observed between the distillation temperature and the light-off temperature. That is, the distillation temperature (e.g., T50) of this fuel is TB1, while the light-off temperature is TB2. The temperature difference ΔTB is large, and light-off occurs at a temperature significantly lower than the distillation temperature.

[0049] There are multiple types of fuels represented by line L2, and the other multiple fuels have different fuel properties. Therefore, in the first embodiment, the following fuel addition process is executed before the regeneration process for regenerating the DPF is executed. That is, in this fuel addition process, after refueling the fuel tank 40, fuel is added by the fuel addition valve 80 when the temperature of the exhaust gas reaches a temperature T2 (e.g., several tens of degrees Celsius lower than T1pset), which is lower than a temperature T1pset preset in the engine ECU 100 as a fuel addition execution temperature. Note that this fuel addition is not for the purpose of regenerating the DPF, but for the purpose of confirming light-off in the DOC 70, and may be performed for a short period (a small amount). Furthermore, to increase the reliability of the light-off confirmation, the fuel addition may be executed multiple times.

[0050] If the temperature rise of the exhaust gas due to the fuel addition is equal to or greater than the threshold value, the fuel addition temperature for performing DPF regeneration is changed from T1pset to T2 until the next refueling. This allows light-off in the DOC 70 to be achieved under appropriate conditions (temperatures) according to the fuel properties after refueling, and the temperature of the DPF 72 to be increased. Therefore, even when multiple fuels are used, appropriate DPF regeneration can be performed. The threshold value may be any temperature at which light-off in the DOC 70 can be confirmed.

[0051] 3 is a flowchart showing an example of the procedure of the fuel addition process executed by the engine ECU 100. The series of processes shown in this flowchart is executed when the engine ECU 100 is activated in conjunction with the start of the engine 1 after the fuel tank 40 has been filled with fuel.

[0052] 3, engine ECU 100 acquires exhaust gas temperature Tc detected by temperature sensor 114 and determines whether temperature Tc is equal to or higher than temperature T2 (step S10). As described above, temperature T2 is lower than temperature T1pset, which is preset as the fuel addition temperature for performing DPF regeneration, and is, for example, several tens of degrees Celsius lower than T1pset.

[0053] If it is determined in step S10 that the exhaust gas temperature Tc is equal to or higher than the temperature T2 (YES in step S10), the engine ECU 100 controls the fuel addition valve 80 to add fuel for a short period (small amount) multiple times (step S20). The short period (small amount) of fuel addition means a shorter period (small amount) of fuel addition compared to the fuel addition performed during DPF regeneration. The reason for adding fuel multiple times is to increase the reliability of the light-off confirmation, and it is not necessary to add fuel multiple times.

[0054] When fuel is added by the fuel addition valve 80, the engine ECU 100 determines whether or not the exhaust gas temperature has increased multiple times (step S30). The reason for checking the temperature increase multiple times is to increase the reliability of the light-off confirmation, and the number of times is not necessarily multiple. Furthermore, the number of times the temperature increase has been checked does not necessarily have to be the same as the number of times fuel has been added. If the temperature increase has not been confirmed multiple times (NO in step S30), it is determined that the fuel oxidation reaction has not occurred in the DOC 70, and the process proceeds to the end.

[0055] On the other hand, if multiple temperature increases are confirmed in step S30 (YES in step S30), engine ECU 100 determines whether the temperature increase is equal to or greater than threshold value ΔT (step S40). Threshold value ΔT is a set value for confirming light-off in DOC 70, and is set appropriately based on the results of a prior evaluation experiment, etc.

[0056] If it is determined in step S40 that the temperature rise is equal to or greater than threshold value ΔT (YES in step S40), engine ECU 100 changes temperature T1pset, which is preset as the fuel addition temperature for performing DPF regeneration, to temperature T2 used in step S10 (step S50). For example, engine ECU 100 rewrites the value of T1pset stored in memory 102 with the value of temperature T2. This change in the fuel addition temperature is maintained until the next time fuel is refueled into fuel tank 40.

[0057] 4 is a flowchart showing an example of the procedure of the DPF regeneration process executed by the engine ECU 100. The series of processes shown in this flowchart is repeatedly performed while the engine ECU 100 is running as the engine 1 operates.

[0058] 4, engine ECU 100 determines whether or not a condition for performing DPF regeneration is met (step S110). This condition is a condition corresponding to the amount of PM accumulated in DPF 72. For example, the condition is met when the pressure difference ΔP in exhaust passage 52 upstream and downstream of DPF 72, as detected by pressure sensor 116, exceeds a threshold value, and it is determined that the estimated amount of PM accumulated is large.

[0059] If the execution condition is not met in step S110 (NO in step S110), DPF regeneration is not necessary, and the process proceeds to RETURN.

[0060] On the other hand, if it is determined in step S110 that the execution condition is met (YES in step S110), engine ECU 100 obtains exhaust gas temperature Tc detected by temperature sensor 114 and determines whether temperature Tc is higher than a fuel addition temperature (step S120). This fuel addition temperature is temperature T2 if a change (rewrite) from temperature T1pset to temperature T2 has been made in the fuel addition process shown in FIG. 3, or is the preset temperature T1pset if no change has been made.

[0061] When it is determined in step S120 that the exhaust temperature Tc is equal to or lower than the fuel addition temperature (NO in step S120), DPF regeneration is not performed and the process proceeds to RETURN.

[0062] On the other hand, if it is determined in step S120 that exhaust temperature Tc is higher than the fuel addition temperature (YES in step S120), engine ECU 100 controls fuel addition valve 80 to add fuel to regenerate the DPF (step S130). As a result, the fuel added to exhaust passage 52 is oxidized in DOC 70 and generates heat (combusts). As a result, the exhaust gas heated by DOC 70 flows into DPF 72, raising the temperature of DPF 72 and regenerating DPF 72 (combusting PM).

[0063] As described above, in this first embodiment, the fuel addition process (FIG. 3) is executed before the DPF regeneration process (FIG. 4) is executed. In the fuel addition process, fuel is added by the fuel addition valve 80 when the temperature Tc of the exhaust gas downstream of the DOC reaches a temperature T2 that is lower than a temperature T1pset that is preset as a fuel addition temperature. Then, when the temperature Tc rises by a threshold temperature or more, the fuel addition temperature is changed from T1pset to T2. This allows the DOC 70 and DPF 72 to be heated under appropriate conditions (temperatures) according to the fuel properties after refueling. Therefore, appropriate DPF regeneration can be performed even when multiple fuels are used.

[0064] In addition, in this first embodiment, fuel addition is performed multiple times in the fuel addition process shown in Figure 3. If a temperature rise equal to or greater than the threshold value ΔT is detected multiple times as a result of the multiple fuel additions, the fuel addition temperature is changed to T2. This increases the reliability of changing the fuel addition temperature from T1pset to T2.

[0065] [Embodiment 2] As described above, there are fuels for which there is no correlation between the distillation temperature and the light-off temperature. In the first embodiment, a fuel is assumed whose light-off temperature is lower than the distillation temperature and lower than the temperature T1pset, which is preset as the fuel addition temperature. However, it is also assumed that some fuels have a light-off temperature higher than the temperature T1pset. In the second embodiment, a method for performing appropriate DPF regeneration is shown for such fuels.

[0066] 5 is a flowchart showing an example of the procedure of the fuel addition process executed by engine ECU 100 in embodiment 2. This flowchart corresponds to FIG. 3 described in embodiment 1. The series of processes shown in this flowchart is also executed when engine ECU 100 is activated in conjunction with the start of engine 1 after fueling to fuel tank 40 is completed.

[0067] 5, the processes in steps S210 to S250 are the same as the processes in steps S10 to S50 shown in FIG. 3, respectively.

[0068] In this second embodiment, when multiple temperature increases are not confirmed in step S230 (NO in step S230), or when the temperature increase is smaller than threshold value ΔT in step S240 (NO in step S240), engine ECU 100 determines whether exhaust gas temperature Tc detected by temperature sensor 114 is equal to or higher than temperature T1pset, which is preset as the fuel addition temperature for performing DPF regeneration (step S260).

[0069] When it is determined that the exhaust gas temperature Tc is equal to or higher than the temperature T1pset (YES in step S260), the engine ECU 100 controls the fuel addition valve 80 to add fuel for a short period (small amount) multiple times (step S270). Note that the reason for adding fuel multiple times is to increase the reliability of the light-off confirmation, and it is not necessary to add fuel multiple times.

[0070] When fuel is added by fuel addition valve 80, engine ECU 100 determines whether the increase in temperature Tc due to the fuel addition is smaller than threshold value ΔT (step S280). Threshold value ΔT is assumed to be the same value as threshold value ΔT in step S240, but it does not necessarily have to be the same value.

[0071] When it is determined in step S280 that the temperature rise is equal to or greater than the threshold value ΔT (NO in step S280), it is determined that an oxidation reaction is occurring at the temperature T1pset, and the process proceeds to END without changing the temperature T1pset, which is preset as the fuel addition temperature.

[0072] On the other hand, if it is determined in step S280 that the temperature rise is smaller than threshold value ΔT (YES in step S280), engine ECU 100 changes temperature T1pset, which is preset as the fuel addition temperature for performing DPF regeneration, to temperature T3 (step S290). This temperature T3 is higher than temperature T1pset, which is preset as the fuel addition temperature for performing DPF regeneration, for example, several tens of degrees Celsius higher than T1pset. Engine ECU 100, for example, rewrites the value of T1pset stored in memory 102 to the value of temperature T3. This change in the fuel addition temperature is also maintained until the next time fuel is refueled into fuel tank 40.

[0073] As described above, in the second embodiment, if the temperature rise at temperature T2 lower than temperature T1pset cannot be confirmed during the fuel addition process, or if the temperature rise amount is smaller than threshold value ΔT, and the exhaust temperature Tc reaches temperature T1pset, which is preset as the fuel addition execution temperature, fuel addition is again performed by fuel addition valve 80. Then, even in this case, if the temperature rise amount is smaller than threshold value ΔT, the fuel addition execution temperature is changed from T1pset to temperature T3, which is higher than T1pset. This makes it possible to raise the temperatures of DOC 70 and DPF 72 under appropriate conditions (temperatures) according to the fuel properties, even for fuel with a high light-off temperature.

[0074] In the first and second embodiments described above, the fuel addition process is performed for any fuel. However, the inventors of the present disclosure have found that, as a rule, for fuels with a high cetane number, the light-off temperature is lower than the distillation temperature. Therefore, in the third embodiment, the cetane number of the fuel in the fuel tank 40 is estimated, and the same fuel addition process as in the first embodiment is performed for fuels with a high cetane number.

[0075] 6 is a flowchart showing an example of the procedure of the fuel addition process executed by engine ECU 100 in embodiment 3. This flowchart also corresponds to FIG. 3 described in embodiment 1. The series of processes shown in this flowchart is also executed when engine ECU 100 is activated in conjunction with the start of engine 1 after fueling to fuel tank 40 is completed.

[0076] 6, in this third embodiment, when fueling to fuel tank 40 is completed and engine ECU 100 is activated in conjunction with the start of engine 1, engine ECU 100 estimates the cetane number of the fuel in fuel tank 40 (step S301). Various known methods can be used to estimate the cetane number. For example, a known method is to perform a fuel injection for cetane number estimation when fuel injection from injector 14 is stopped, and estimate the cetane number based on the torque equivalent generated by that fuel injection.

[0077] After the cetane number of the fuel is estimated in step S301, the engine ECU 100 determines whether the cetane number is equal to or greater than a threshold value Cth (step S302). The threshold value Cth is a value for determining whether a fuel has a light-off temperature << its distillation temperature, and is set appropriately based on the results of a prior evaluation experiment or simulation.

[0078] If it is determined in step S302 that the cetane number is equal to or greater than the threshold value Cth (YES in step S302), the process proceeds to step S310. The processes in steps S310 to S350 are the same as the processes in steps S10 to S50 shown in Fig. 3. That is, the same fuel addition process as in the first embodiment is performed on fuel whose cetane number is equal to or greater than the threshold value Cth.

[0079] On the other hand, when it is determined in step S302 that the cetane number is lower than threshold value Cth (NO in step S302), the process proceeds to END without executing the subsequent processes. That is, for fuel having a cetane number lower than threshold value Cth, fuel addition is not performed in the fuel addition process described in the first embodiment, and the fuel addition temperature for performing DPF regeneration is the preset temperature T1pset.

[0080] As described above, according to this third embodiment, the above-mentioned fuel addition process is executed when the cetane number of the fuel is equal to or greater than the threshold value Cth, so that the DOC 70 and DPF 72 can be heated under appropriate conditions according to the fuel properties, thereby regenerating the DPF 72.

[0081] [Fourth Embodiment] In this fourth embodiment, the cetane number of the fuel in the fuel tank 40 is estimated as in the third embodiment, and the fuel addition process similar to that in the second embodiment is carried out for fuel with a high cetane number.

[0082] 7 is a flowchart showing an example of the procedure of the fuel addition process executed by engine ECU 100 in embodiment 4. This flowchart corresponds to FIG. 5 described in embodiment 2. The series of processes shown in this flowchart is also executed when engine ECU 100 is activated in conjunction with the start of engine 1 after fueling to fuel tank 40 is completed.

[0083] 7, the processes in steps S401 and S402 are the same as the processes in steps S301 and S302 shown in Fig. 6. That is, the cetane number of the fuel in fuel tank 40 is estimated, and it is determined whether the cetane number is equal to or greater than a threshold value Cth.

[0084] If it is determined in step S402 that the cetane number is equal to or greater than the threshold value Cth (YES in step S402), the process proceeds to step S410. The processes in steps S410 to S490 are the same as the processes in steps S210 to S290 shown in Fig. 5. That is, the same fuel addition process as in the second embodiment is performed on fuel whose cetane number is equal to or greater than the threshold value Cth.

[0085] On the other hand, in this fourth embodiment, when it is determined in step S402 that the cetane number is lower than threshold value Cth (NO in step S402), the process proceeds to step S460. That is, for fuel having a cetane number lower than threshold value Cth, it is expected that the light-off temperature will be higher than temperature T1pset. Therefore, the process from step S460 onwards, which corresponds to the process from step S260 onwards shown in FIG. 5, is executed.

[0086] As described above, according to the fourth embodiment, the temperatures of the DOC 70 and the DPF 72 can be raised under appropriate conditions according to the fuel properties, and the DPF 72 can be regenerated.

[0087] In the above embodiments, the temperature Tc detected by the temperature sensor 114 is used as the exhaust temperature in the fuel addition process and the DPF regeneration process, but the temperature Tf detected by the temperature sensor 115 may also be used. For example, if the temperature Tf detected by the temperature sensor 115 shows a similar tendency to the temperature Tc detected by the temperature sensor 114, a change in the fuel addition temperature may be permitted in the fuel addition process.

[0088] [Other Embodiments] In each of the above embodiments, when the fuel addition temperature is changed, the amount of fuel added through the fuel addition valve 80 during the DPF regeneration process may be changed. Specifically, when the fuel addition temperature is changed from T1pset to T2, the amount of fuel added during the DPF regeneration process may be increased compared to when the fuel addition temperature is not changed. When the exhaust gas temperature becomes high, there is a possibility that the fuel added through the fuel addition valve 80 passes through the DOC 70 without reacting with the DOC 70 (this phenomenon is also referred to as "slip"). However, by lowering the fuel addition temperature to T2, slippage is suppressed. Therefore, the amount of fuel added through the fuel addition valve 80 can be increased, and by increasing the amount of fuel added, the DOC 70 is heated more quickly, thereby improving the regeneration effect of the DPF 72.

[0089] Furthermore, in each of the above embodiments, fuel addition in the fuel addition process and DPF regeneration process is performed by the fuel addition valve 80. However, instead of fuel addition by the fuel addition valve 80, "post injection" in which fuel is injected into the combustion chamber of the cylinder 12 during the expansion stroke (combustion expansion stroke) of the engine body 10 may be performed. By post injection, fuel that is not burned in the combustion chamber can be supplied (added) to the exhaust passage 52 and supplied to the DOC 70 through the exhaust passage 52. By this post injection, it is possible to achieve the same effects as fuel addition from the fuel addition valve 80.

[0090] When the temperature of the DOC 70 is increased by post-injection, post-injection is performed in the fuel addition process when the exhaust gas temperature reaches a temperature T5 that is lower than a temperature T4pset that is preset as a post-injection temperature. This post-injection in the fuel addition process is not performed to regenerate the DPF 72 but to confirm light-off in the DOC 70, and may be performed for a short period (a small amount). Furthermore, post-injection may be performed multiple times to increase the reliability of the light-off confirmation.

[0091] If the temperature rise of the exhaust gas due to the post-injection is equal to or greater than the threshold value, the post-injection temperature for performing DPF regeneration is changed from T4pset to T5 until the next refueling. This makes it possible to achieve light-off in the DOC 70 and raise the temperature of the DPF 72 under appropriate conditions (temperatures) according to the fuel properties after refueling, even in DPF regeneration using post-injection. The threshold value may also be any temperature at which light-off in the DOC 70 can be confirmed.

[0092] In addition, when post-injection is used for DPF regeneration, if the post-injection temperature is changed, the fuel injection amount (post-injection amount) or fuel injection timing (post-injection timing) by post-injection during DPF regeneration processing may be changed.

[0093] Specifically, when the post-injection temperature is changed from T4pset to T5, the post-injection amount during the DPF regeneration process may be increased compared to when the post-injection temperature is not changed. When post-injection is performed at a high temperature, slippage may occur in the DOC 70, as described above. However, lowering the post-injection temperature to T5 suppresses slippage. Therefore, the post-injection amount can be increased, and by increasing the post-injection amount, the DOC 70 is heated more quickly, thereby enhancing the regeneration effect of the DPF 72.

[0094] Furthermore, when the post-injection temperature is changed from T4pset to T5, the post-injection timing during the DPF regeneration process may be retarded compared to when the post-injection temperature is not changed. Whether or not fuel burns is determined by temperature, pressure, and the amount of oxygen. If the post-injection timing is retarded, the piston descends and post-injection is performed when the temperature, pressure, and amount of oxygen in the combustion chamber are all reduced. This suppresses fuel combustion and increases the amount of fuel supplied to the DOC 70. As a result, the DOC 70 is heated earlier, and the regeneration effect of the DPF 72 can be improved.

[0095] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The technical scope of the present disclosure is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims.

[0096] REFERENCE SIGNS LIST 1 Engine 10 Engine body 12 Cylinder 14 Injector 20 Intake passage 22 Air cleaner 24 Intercooler 26 Throttle valve 28 Intake manifold 30 Turbocharger 32 Compressor 34 Turbine 40 Fuel tank 41 Feed pump 42 High-pressure fuel pump 43, 45 Fuel passage 44 Common rail 50 Exhaust manifold 52 Exhaust passage 60 EGR passage 62 EGR cooler 64 EGR valve 70 DOC 72 DPF 80 Fuel addition valve 100 Engine ECU 101 CPU 102 Memory 114, 115 Temperature sensor 116 Pressure sensor

Claims

1. An exhaust purification device for an internal combustion engine, comprising: a filter provided in an exhaust passage of the internal combustion engine and collecting particulate matter contained in exhaust gas flowing through the exhaust passage; an oxidation catalyst provided in the exhaust passage upstream of the filter; fuel addition means for adding fuel into the exhaust passage upstream of the oxidation catalyst; a temperature sensor for detecting the temperature of the exhaust gas downstream of the oxidation catalyst; and a control device that executes a regeneration process to burn the particulate matter deposited on the filter by adding fuel using the fuel addition means when an execution condition according to the amount of deposited particulate matter is met and the temperature is higher than a fuel addition execution temperature, wherein the control device further executes a fuel addition process that adds fuel before executing the regeneration process, and the fuel addition process includes: a first process that adds fuel when the temperature reaches a second temperature lower than a first temperature that is preset as the fuel addition execution temperature after the fuel is refueled; and a second process of changing the fuel addition temperature from the first temperature to the second temperature when the temperature rises by a threshold temperature or more due to the first process.

2. An exhaust gas purification device for an internal combustion engine as described in claim 1, wherein the fuel addition process further includes: a third process for adding fuel when the temperature reaches the first temperature if the increase in temperature due to the first process is smaller than the threshold temperature; and a fourth process for changing the fuel addition temperature to a third temperature higher than the first temperature if the increase in temperature due to the third process is smaller than the threshold temperature.

3. An exhaust purification device for an internal combustion engine as described in claim 1 or claim 2, wherein the control device further performs an estimation process to estimate the cetane number of the fuel in the fuel tank, and performs the fuel addition process when the cetane number estimated by the estimation process is equal to or greater than a threshold value.

4. An exhaust gas purification device for an internal combustion engine as described in claim 3, wherein the fuel addition process further includes: a fifth process of adding fuel when the temperature reaches the first temperature if the cetane number is smaller than the threshold value; and a sixth process of changing the fuel addition temperature to a fourth temperature higher than the first temperature if the increase in temperature due to the fifth process is smaller than the threshold temperature.

5. An exhaust purification device for an internal combustion engine as described in claim 1, wherein the first process includes a process of performing the fuel addition multiple times when the temperature reaches the second temperature after the refueling is performed, and the second process includes a process of changing the fuel addition temperature to the second temperature when a temperature rise equal to or greater than the threshold temperature due to the first process is detected multiple times.

6. An exhaust gas purification device for an internal combustion engine according to claim 1, wherein the temperature sensors include: a first temperature sensor that detects the temperature of the exhaust gas between the oxidation catalyst and the filter; and a second temperature sensor that detects the temperature of the exhaust gas downstream of the filter.

7. An exhaust gas purification device for an internal combustion engine according to claim 1, wherein the fuel addition means includes a fuel addition valve provided in the exhaust passage upstream of the oxidation catalyst.

8. An exhaust purification device for an internal combustion engine as described in claim 7, wherein when the fuel addition temperature is changed from the first temperature to the second temperature during the fuel addition process, the control device increases the amount of fuel added by the fuel addition valve when the regeneration process is performed compared to when the fuel addition temperature is not changed.

9. An exhaust gas purification device for an internal combustion engine according to claim 1, wherein the fuel addition means includes post-injection of fuel into the combustion chamber of the internal combustion engine during the expansion stroke of the internal combustion engine.

10. An exhaust purification device for an internal combustion engine as described in claim 9, wherein when the fuel addition temperature is changed from the first temperature to the second temperature in the fuel addition process, the control device increases the amount of fuel injected in the post injection when the regeneration process is performed compared to when the fuel addition temperature is not changed.

11. An exhaust purification device for an internal combustion engine as described in claim 9, wherein when the fuel addition temperature is changed from the first temperature to the second temperature during the fuel addition process, the control device retards the fuel injection timing of the post-injection when the regeneration process is performed compared to when the fuel addition temperature is not changed.

12. A control method for an exhaust purification device of an internal combustion engine, wherein the exhaust purification device includes: a filter provided in an exhaust passage of the internal combustion engine and collecting particulate matter contained in exhaust gas flowing through the exhaust passage; an oxidation catalyst provided in the exhaust passage upstream of the filter; fuel addition means for adding fuel into the exhaust passage upstream of the oxidation catalyst; and a temperature sensor for detecting the temperature of the exhaust gas downstream of the oxidation catalyst, the control method including the steps of: executing a regeneration process for burning the particulate matter deposited on the filter by adding fuel using the fuel addition means when an execution condition according to the amount of deposited particulate matter is met and the temperature is higher than a fuel addition execution temperature; and executing a fuel addition process for adding fuel before executing the regeneration process, wherein the fuel addition process includes: a first process for adding fuel when the temperature reaches a second temperature lower than a first temperature preset as the fuel addition execution temperature after the fuel is refueled; a second process for changing the fuel addition temperature from the first temperature to the second temperature when the temperature rises by more than a threshold temperature due to the first process.

13. The control method according to claim 12, wherein the fuel addition means includes a fuel addition valve provided in the exhaust passage upstream of the oxidation catalyst.

14. The control method according to claim 12, wherein the fuel addition means performs post-injection of fuel into the combustion chamber of the internal combustion engine during an expansion stroke of the internal combustion engine.

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