[DOCUMENT ~ ~ ~ ~ I S p e c i f i c a t i o n [Title of Invention] EXHAUST GAS PURIFICATION SYSTEM FOR INTERNAL COMBUSTION ENGINE [Field of the Invention] [OOOl] The present invention relates to an exhaust gas purification system for an internal combustion engine provided with an exhaust gas purifier such as a DOC, a DPF and a SCR, and particularly relates to a technique for raising a temperature of the exhaust gas purifier at an early stage during a warm-up operation such as immediately after engine starting. [BACKGROUND OF THE INVENTION] [0002] The exhaust gas purifier such as a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), a selective catalytic reduction catalyst (SCR) fully functions once a certain temperature environment or above is established. For instance, FIG.18 is atable representing relationships among a temperature of a SCR catalyst, NOx concentration at an inlet and an outlet of the SCR and an operation time of the engine in the SCR unit for reducing nitrogen oxides (NOx). As illustrated in FIG.18, the NOx concentration at the outlet of the SCR is relatively high until the catalyst temperature of the SCR unit reaches its activating temperature, and the SCRunitdoes not fully demonstrate the NOx purification function. Therefore it is necessary to raise the temperature of the exhaust gas purifier such as the SCR unit at an early stage during the warm-up operation such as immediately after starting the engine, in order to perform the exhaust gas purification at an early stage. [0003] In order to raise the temperature of the exhaust gas purifier at the early stage, it is known to change a fuel injection timing, or to control (reduce) a flow rate of the air to be supplied to the engine by controlling a variable supercharger and an air supply throttle. For instance, disclosed in Patent Literature 1is an example of controlling the fuel injection timing. Disclosed in Patent Literature 2 is an example of controlling the flow rate of the air to be supplied to the engine. [RELATED ART DOCUMENT] [PATENT DOCUMENT] [0004] [PTL 11 JP 2003-65121 A [PTL 21 JP 3972611 B [OBJECT OF THE PRESENT INVENTION] [OOOS] Among the above-mentioned techniques of raising the temperature of the exhaust gas purifier at the early stage, the air flow control is comparatively most effective in raising the temperature. However, by controlling the flow rate of the air supplied to the engine, the pressure in the cylinder decreases. If the air flow control is performed at an early stage of the warm-up operation, combustion in the engine becomes unstable. [ 0 0 0 6 ] FIG. 1 9 is an illustration of a relationship between timing for starting the air flow control and an engine combustion state according to related art. It is determined whether the combustion state is stable or unstable based on a temperature inside the cylinder ( T ) and a pressure inside the cylinder ( P C ) at the fuel injection timing. The higher the temperature and the pressure are inside the cylinder, the more stable the combustion state is. As illustratedin FIG.19, when the air flow control (ii) is performed immediately in such a state that the engine is still in an unstable combustion state right after the engine starting, the combustion state becomes more unstable. Further, when the injection timing control (i) is performed prior to the air flow control (ii) so as to raise the temperature inside the cylinder, if the temperature is raised insufficiently in the cylinder, the air flow control (ii') causes the pressure in the cylinder to decrease, resulting in unstable combustion state. [ 0 0 0 7 ] Once the engine is in the unstable combustion state, hydrocarbon (HC) emission increases. In the worst scenario, this causes fire extinction and then the engine stops. As the restriction on the exhaust gas emission becomes stricter every year, it is desire to develop an exhaust gas purification system which is capable of raising the temperature of the exhaust gas purifier at an early stage while maintaining the engine in the stable combustion state during the warm-up operation such asimmediatelyafterthe engine starting. [OOOS] In view of the above issues, it is an object of the present invention to provide an exhaust gas purification system for an internal combustion engine, which is capable of suppressing HC emission increase and raising the temperature of the exhaust gas purifier at an early stage while maintaining the stable combustion state during the warm-up operation such as immediately after the engine starting. [MEANS TO ACHIEVE THE OBJECT] [ 0 0 0 9 ] The present invention was made to achieve the object of the present invention in view of the above issues and provides an exhaust gas purification system foraninternalcombustionengineprovidedwithanengine, an exhaust passage where exhaust gas exhausted from the engine passes, and an exhaust gas purifier installed in the exhaust passage. The exhaust gas purification system comprises: an air flow control unit for reducing a flow rate of air to be supplied to the engine so as to raise a temperature ofthe exhaust gas exhausted fromthe engine; and an activation timing control unit for controlling a timing of activating the air flow control unit, wherein the activation timing control unit is configured to control the timing of activating the air flow control unit sothat acombustionstate inthe engine does not become unstable when the air flow control unit is activated and the air to be supplied to the engine is reduced. [OOlO] As described above, even when the air flow control unit is activated and the air to be supplied to the engine is reduced, the timing of activating the air flow control unit is controlled by the activation timing control unit so that the combustion state in the engine does not become unstable. There is a concern that the air flow control possibly makes the engine combustion unstable while being effective in raising the temperature. This concern is addressed by the present invention. According to the present invention, the exhaust gas purification system for the internal combustion engine is capable of suppressing HC emission increase and raising the temperature of the exhaust gas purifier at an early stage while maintaining the stable combustion state during the warm-up operation such as immediately after the engine starting. [ 00111 In the above invention, the activation timing control unit may be configured to activate the air flow control unit after prescribedtime has passed since starting ofthe engine, the prescribedtime being calculatedbased on a rotation speed ofthe engine and a fuel injection amount. In this case, the prescribed time may be corrected based on at least one of an ambient temperature or an ambient pressure in such a state that the engine operates. [0012] According tothis aspect ofthe invention, the temperature rise ofthe exhaust gas purifier just before the engine starting and suppression of the HC exhaust increase can be controlled by a simple method. In this 2 4 SEP 20\4 case, the prescribed time to the activation of the air flow control unit is corrected according to the ambient temperature and/or the ambient pressure and thus the timing for activating the air flow control unit can be determinedwith precision in accordance with the ambient temperature and the ambient pressure. [0013] Further, in the above invention, the activation timing control unit may be configured to activate the air flow control unit when a temperature of cooling water for cooling the engine or a temperature of lubricating oil flowing inside the engine becomes not less than a threshold value. In this case, the threshold value of the cooling water or the lubricating oil may be corrected based on at least one of an ambient temperature or an ambient pressure in such a state that the engine is activated. [0014] According tothis aspect ofthe invention, the timing for activating the air flow control unit can be controlled by knowing the combustion state in the engine from the temperature of the cooling water or the lubricating oil. Thus, it is possible to secure stable combustion in the engine and also to achieve temperature rise of the exhaust gas purifiers at an early stage. In this case, the threshold value of the temperature of the cooling water or the lubricating oil is corrected according to the ambient temperature and/or the ambient pressure to precisely determine the timing for activating the air flow control unit in accordance with the ambient temperature and/or the ambient pressure. [0015] In the above invention, the activation timing control unit may be configured to estimate a temperature and a pressure in a cylinder of the engine and control the timing of activating the air flow control unit based on the estimated temperature and pressure in the cylinder of the engine, the estimated pressure being estimated [0016] According this aspect of the invention, the combustion state in the engine after the air flow control is performed is estimated with precision and then the timing for activating the air flow control unit is controlled. Thus, it is possible to secure stable combustion in the engine and also to achieve temperature rise of the exhaust gas purifier at an early stage. [0017] In the above invention, the activation timing control unit may comprise a pressure detector for detecting a pressure in a cylinder of the engine, and the activation timing control unit may be configured to control the timing of activating the air flow control unit based on the pressure in the cylinder detected by the pressure detector. [0018] According to this aspect of the invention, a cylinder pressure in the cylinder is directly measured by the cylinder pressure measuring unit so as to obtain combustion stability of the engine directly and also to control the timing for activating the air flow control unit at the same time. Therefore, the air flow control can be performed while detecting the combustion state in the engine in real time. As a result, it is possible to secure stable combustion in the engine and also to achieve temperature rise of the exhaust gas purifier at an early stage. [EFFECTS OF THE INVNETION] [0019] According to the present invention, it is possible to provide an exhaust gas purification system for an internal combustion engine, which is capable of securing stable combustion in the engine, suppressing HC emission increase and raising a temperature of an exhaust gas purifier at an early stage during a warm-up operation such as immediately after engine starting. [BRIEF DESCRIPTION OF THE DRAWINGS] [Fig. 11 FIG.1is an overall structure of a diesel engine equipped with an exhaust gas purification system according to embodiments of the present invention. [FIG.2]FIG.2 is a block diagram illustrating an air flow control unit according to embodiments of the present invention. [FIG.3A]FIG.3A is a conceptual drawing illustrating a relationship between a timing of activating an air flow control and an engine combustion state according to a first embodiment. [FIGm3B]FIG.3B is a conceptual drawing illustrating a relationship between an operating time ofthe engine and a temperature of the SCR catalyst support according to the first embodiment. [FIG.4A]FIG.4Ais a standardmap for calculatingatiming (prescribedtime: tl) of activating an air flow control. 2 4 SEP 2014 [FIG.4B]FIG.4B is an ambient temperature correction map for calculating the timing of activating the air flow control. [FIG.4C]FIG.4Cis an ambient pressure correctionmap for calculating the timing of activating the air flow control. [FIG.5] FIG. 5 is a block diagram illustrating calculation of the timing of activating the air flow control according to the first embodiment. [FIG.6]FIG.6 is a control flow chart according to the first embodiment. [FIGe7A]FIG.7A is a table representing a relationship between the timing of activating the air flow control anda combustion state intheengine according to a second embodiment. [FIG.7B]FIG.7B is a table representing a relationship between an engine operating time and temperatures of SCR catalyst support and cooling water according to the second embodiment. [FIG.8A]FIG.8A is a standard map for obtaining a temperature of cooling water according to the second embodiment. [FIG.8B]FIG.8B is an ambient temperature correction map for obtaining a temperature of cooling water according to the second embodiment. [FIG.8C]FIG.8Cis an ambient pressure correctionmap for obtaining a temperature of cooling water according to the second embodiment. [FIG.9]FIG.9 is a block diagram illustrating calculation of a prescribed temperature (twl) the cooling water according to the second embodiment. [FIG.10] FIG. 10 is a control flow chart according to the second embodiment. [FIG.ll]FIG.ll is a conceptual drawing illustrating a third embodiment. [FIG.12]FIG.12 is a block diagram illustrating determination of stable combustion according to the third embodiment. [FIG.13]FIG.13 is a control flow chart according to the third embodiment. [FIG.14A]FIG.l4A is a conceptual drawing according to a fourth embodiment. [FIG.14B]FIG.l4B is another conceptual drawing according to the fourth embodiment. [FIG.15]FIG.15 is a control flow chart according to the fourth embodiment. [FIG.16]FIG.16 is an illustration of other aspect ofthe fourth embodiment. [FIG.17]FIG.17 is a control flow chart according to the other aspect of the fourth embodiment. [FIG.18]FIG.18 is a table representing a relationship among a temperature of a catalyst support of a SCR unit, NOx concentration at an inlet/outlet ofthe SCR unit the engine operating time. [FIG.19]FIG.19 is a table illustrating a relationship between the timing of activating the air flow control and the combustion state in the engine. [Description of Embodiments] [0021] Embodiments ofthe present invention will now be describedindetail with reference tothe accompanying drawings. It is intended, however, that unless particularly specified in these embodiments, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention. [0022] FIG.l is an overall structure of a diesel engine equipped with an exhaust gas purification system according to embodiments of the present invention. Referring to FIG.1, the overall structure ofthe exhaust gas purification system for an internal combustion engine is described. [0023] As illustrated in FIG.1, the diesel engine equipped with the exhaust purification system is formed by: a variety of devices and pipings such as an engine 1, an exhaust passage 3, an air supply passage 13, a variable geometry turbocharger 11, a common fuel injection unit 18, and an EGR piping 23; a variety of exhaust gas purification devices such as a DOC unit 5, an oxidation catalyst (DOC unit) 5, a diesel particulate filter (DPF unit)7, and a selective reduction catalyst (SCR unit) 9; an engine controller (ECU) 19 for controlling these devices; and a variety of sensors. [0024] The exhaust passage 3 is connected to a downstream of the engine 1. The DOC unit 5 and the DPF unit 7 are provided in the exhaust passage 3. The DOC unit 5 is configured to oxidize HC (hydrocarbon) and CO (carbon monoxide) contained in the exhaust gas and also oxidize NO (nitric oxide) contained in the exhaust gas into NO2 (nitrogen oxide). The DPF unit 7 is provided downstream of the DOC unit 5 to capture and remove particulate matter (PM) such as soot contained in the exhaust gas from the exhaust gas. [ 0 0 2 5 ] Further, a urea aqueous injection unit 8 is provided downstream of the DPF unit 7 and immediately downstream of the urea aqueous injection unit 8, a SCR unit 9 is provided. The urea aqueous injection unit 8 injects urea aqueous to the exhaust passage 3 from an injection nozzle 8a in accordance with a control signal from the ECU 19. The urea aqueous is stored in a urea aqueous tank 8b. The urea aqueous injected to the exhaust passage 3 is hydrolyzed by heat of the exhaust gas 27 to ammonia (NH3). The produced ammonia (NH3) becomes reducing agent to perform reduction of NOx contained in the exhaust gas 27 in the SCR unit 9. [ 0 0 2 6 ] To purify the exhaust gas by the DOC unit 5 and the SCR unit 9, the catalyst supported in the DOC unit 5 and the SCR unit 9 needs to be heated to or above the activating temperature. To regenerate the filter by removing the particulate matter collected in the filter, the DPF unit 7 needs to be heated to or above a prescribed temperature. More specifically, it is necessary to heat the exhaust gas purifiers to or above a prescribed temperature for the exhaust gas purifiers to fully function. In the present exhaust gas purification system for an internal combustion engine, an air flow control unit 50 is provided to raise the temperatures ofthe exhaust gas purifiers. The air flow control unit 50 is described later in details. [0027] The air supply passage 13 is connected to an upstream side of the engine 1. The variable geometry turbocharger 11 is provided between the air supply passage 11 and the exhaust passage 3. The variable geometryturbocharger llincludes an exhaust gas turbine llb arranged in the exhaust passage 3 and a compressor lla arranged in the air supply passage 13. The compressor lla is driven coaxially by the exhaust gas turbine Ilb. Further, the variable geometry turbocharger 11 is configured so that an opening degree of a variable nozzle vane (not shown), an opening degree of a wastegate valve (not shown) are adjusted based on a control signal from the ECU 19 to control a flow rate of air 26 exhausted from the compressor lla. [0028] An intercooler 15 and an air supply throttle valve 17 are provided in the air supply passage 13. The air 26 exhausted from the compressor lla is cooled in by the intercooler 15, passes through the air supply throttle valve 17 and then enters a combustion chamber la of each cylinder of the engine 1. Meanwhile, the air supply throttle valve 17 is configured so that the opening is adjusted based on the control signal fromthe ECU 19 to control the flow rate of the air supplied to the engine 1. COO291 In the engine 1, a common rail fuel injection device 18 is provided to inject fuel to the combustion chamber la. The common rail fuel injection device 18 is configured so that the injection timing and the injection amount are controller based on the control signal from the ECU 19. An injection timing control which is described later is performedby controlling the injection timing and the amount of fuel injected to the combustionchamber la fromthe common rail fuel injection timing 18 based on the control signal from the ECU 19 to vary the injection timing and injection amount from those of a normal operation mode. [0030] The EGR piping 23 branches from the exhaust passage 3 on the upstream side ofthe exhaust gas turbine llb and is connected to a downstream side of the air supply throttle valve 17. In the EGR piping 23, an EGR cooler 24 and an EGR valve 25 are arranged. BY controlling opening and closing of the EGR valve 25, the exhaust gas 27 exhausted from the engine 1 partially enters the EGR piping 23 to recirculate to the engine 1. [0031] The exhaust gas 27 exhausted from the engine 1 passes through the exhaust passage 3 to drive the exhaust turbine Ilb, thereby coaxially driving the compressor lla. Then the exhaust gas 27 passes through the exhaust passage 3 and then passes through the DOC unit 5, the DPF unit 7 and the SCR unit 9. In the air supply passage 13, an air flow meter 31 is arranged to detect the flow rate of the air entering the compressor lla. A signal regarding the detected flow rate of air is inputted to the ECU 19. [0032] In the exhaust passage 3, a DOC inlet temperature sensor 35, a DPF inlet pressure sensor 36, a DPF inlet temperature sensor 37, a DPF pressure difference sensor 38 and a DPF outlet temperature sensor 39 are provided. Signals regarding a DOC inlet temperature sensor, a DPF inlet temperature, etc detected by these sensors are inputted to the ECU 19. On a downstream side of the SCR unit 9, a SCR outlet temperature sensor 33 and a NOx sensor 34 are arranged to detect a temperature at the downstream side of the SCR unit 9 and NOx concentration. Signals regarding the detected temperature and the detected NOx concentration are inputted to the ECU 19. [0033] On a downstream side of the air supply throttle valve 17, an air supply temperature sensor 41 and an air supply pressure sensor 43 are arranged to detect an air supply temperature and an air supply pressure respectively. Signals regarding the detected air supply temperature and the detected air supply pressure are inputtedtothe ECU 19. An appropriate EGR amount is calculated based on these air supply temperature and air supply pressure so as to control the opening and closing of the EGR valve 25. [0034] In the ECU 19, an engine rotation speed and the fuel injection amount are calculatedbasedon signals inputtedfromavarietyof sensors suchas a cranksensor, a camsensor, an accelerator sensor andathrottle sensor that are not illustrated. Further, a cooling air passage (not shown) is formed around the engine 1 and a cooling water temperature sensor (not shown) is 2 4 SEP 2014 arranged to detect a temperature of the cooling air flowing in the cooling air passage. [0035] The ECU19is formedbyamicrocomputer having a central processing unit (CPU), a random access memory (RAM), a read-only memory (ROM), 1/0 interface, etc. The signals from the above sensors are inputted to the CPU via the 1/0 interface. The CPU is configured to perform a variety of controls in accordance with a control program stored in the ROM. As illustrated in FIG.1, the air flow control unit 50 and an activation timing control unit 52 are formed by the ECU unit 19. [0036] FIG.2 is a block diagramillustratingthe air flow control unit 50. The air flow control unit 50 of the present embodiment is configured to control the opening of the air supply throttle valve 17 and the v a r i a b l e g e o m e t r y t u r b o c h a r g e r 1 1 b a s e d o n a command from the activation timing control unit 52 so as to control (reduce) the flow rate of the air supplied to the engine 1. Further, the air flow control unit 50 is not necessary configured to control both the air supply throttle valve 17 and the variable geometryturbocharger 11. The air flow control unit 50b may be configured to control one of the air supply throttle valve 17 and the variable geometry turbocharger 11 so as to control (reduce) the flow rate of the air supplied to the engine 1. [0037] The air flow control has a high effect of raising the temperature and is extremely effective in raising the temperature of the exhaust gas purifier in an early stage during the warm-up operation such as immediately after the engine starting. On the other hand, if the flow rate ofthe air is controlled too early after the engine starting, the combustion state in the engine becomes unstable as the pressure in the cylinder decreases once the flow rate of the air is controlled as described in FIG.19. [0038] Therefore, the timing of activating the air flow control unit 50 is controlled by the activating timing control unit52 soas to avoidthe combustion state in the engine being unstable even if the air flow control is performed. Embodiments of the activation timing control unit 52 are described below in details. [0039] (FIRST EMBODIMENT) FIG.3A is a conceptual drawing representing a relationship between a timing of activating an air flow control and an engine combustion state according to a first embodiment. FIG.3B is a conceptual drawing representing a relationship between an operating time of the engine and a temperature of the SCR catalyst support according to the first embodiment. The activation timing control unit 52 of the first embodiment is configuredto performthe injection timing control (i) and the warm-up operation (ii) of the engine lafter the engine starting as illustrated in FIG.3A and FIG.3B. The air flow control (iii) is performed after a prescribed time (tl) since the engine starting. [0040]The prescribed time (tl) is calculated as illustratedin FIG.5 bymultiplyinga standardtime (tlf) by a temperature correction factor and a pressure correction factor (temperature/pressure correction). The standard time is obtained from a standard map 56 by inputting the engine rotation speed (Ne) of the engine 1 and the fuel injection amount (Qf) to the standard map 56. The temperature correction factor is obtained from a temperature correction map 57 by inputting an ambient temperature to the temperature correction map 57. The pressure correction factor is obtained from a pressure correction map 58 by inputting an ambient pressure to the pressure correction map 58. These standard map 56, temperature correction map 57 and pressure correction map 58 are created from experiment or the like and are stored in the ROM of the ECU 19 in advance. [0041]In this embodiment, one or neither of the temperature correction and pressure correction may be performed. In the case of performing neither of the temperature correction and pressure correction, the standard time (tl' ) is the prescribed time (tl) in FIG. 5. [0042]The relationship between the engine rotation speed (Ne) and the standard time (tr 1) represents in FIG.4A that the higher the engine rotation speed (Ne) is, the shorter the standard time (tf 1) becomes, whereas the lower the engine rotation speed (Ne) is, the longer the standard time (tfl) becomes. The relationship between the fuel injection amount (Qf) and the standard time (tlf) representsthat the greaterthe fuel injection amount (Qf) is, the shorter the standard time (tf 1) becomes, whereas the smaller the fuel injection amount (Qf) is, the longer the standard time (tf 1) becomes. Further, the higher the ambient temperature is, the smaller the temperature correction factor is, as illustrated in FIG.4B. Furthermore, the higher the ambient pressure is, the smallerthe pressure correction factor is, as illustrated in FIG.4C. [ 0 0 4 3 ] FIG.6is a control flow chart according tothe first embodiment. As illustrated in FIG.6, the activation timing control unit 52 according to the first embodiment, after the engine starting (Sl), controls the injection timing (S11) to raise the temperature of the engine. As illustrated in FIG.5, the time for activating the air flow control (the prescribed time: tl) is calculated (S12). Then, the calculated prescribed time (tl) is stored (S13) and the stored prescribed time (tl) is compared with an elapsed time (t) (S14). When t2tl (YES in S14), the air flow control is performed (S15). When t