Patent Publication Number: US-2023160352-A1

Title: Control unit for internal combustion engine system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese patent application serial number 2021- 190079, filed Nov. 24, 2021, the contents of which is incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND 
     The present disclosure relates to control units for internal combustion engine systems. For example, the present disclosure relates to a control unit for preventing deterioration of an exhaust gas purifier of an internal combustion engine. 
     Exhaust gases from internal combustion engines contain hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), etc. Vehicles equipped with an internal combustion engine may be equipped with various exhaust gas purifiers for purifying exhaust gases. Some vehicles are equipped with a diesel engine as an internal combustion engine. Such vehicles may be equipped, for example, with a first oxidation catalyst, a particle trap filter (DPF), an urea selective catalytic reducer (SCR), a second oxidation catalyst, and/or the like from an upstream side to a downstream side of exhaustion. In addition, a three-way catalyst, an NSR (NOx Storage-Reduction catalyst), and/or the like may be used as an exhaust gas purifier. 
     Hydrocarbons (HC) are often converted to water (H 2 O) and carbon dioxide (CO 2 ). For example, a first oxidation catalyst with an oxidation function, a particle trap filter, a three-way catalyst, and an NSR may serve to purify hydrocarbons from the exhaust gas using an oxidation reaction. Carbon monoxide (CO) is often converted to carbon dioxide (CO 2 ). For example, the first oxidation catalyst with the oxidation function, the particle trap filter, the three-way catalyst, and the NSR serve to purify carbon monoxide from the exhaust gas using an oxidation reaction. Nitrogen oxides (NOx) are often converted to nitrogen (N 2 ). For example, nitrogen oxides are purified from the exhaust gas by a reduction reaction between ammonia generated from added urea water, and an urea SCR using a reduction function, or by a reduction reaction by an NSR using a reduction function. If excess ammonia is generated, a second oxidation catalyst purifies nitrogen oxides using an oxidation reaction. Particulate matter (PM) is trapped by the particle trap filter and is not released into the atmosphere. 
     The exhaust gas purifier purifies hydrocarbons (HC) through an oxidation reaction. In this case, the exhaust gas purifier adsorbs the hydrocarbons (HC). The adsorbed hydrocarbons (HC) are subjected to an oxidation reaction with the oxygen surrounding the exhaust gas purifier. A shortage of oxygen will not typically occur since fresh exhaust gas containing oxygen continuously flows through the exhaust gas purifier while an internal combustion engine is in operation. On the other hand, if the operation of the internal combustion engine stops, the fresh exhaust gas will no longer flow through the exhaust gas purifier. In this case, the surrounding oxygen supply may run out. If the surrounding oxygen runs out when the temperature of the exhaust gas purifier is higher than or equal to an activation temperature, the oxidation reaction will stop proceeding. As a result, hydrogen (H) is desorbed from the adsorbed hydrocarbons (HC). Carbon (C) thus accumulates as a deposit (so-called coking occurs (adhered by polymerization reaction)), which may lead to the deterioration of the exhaust gas purifier. In order to prevent coking from occurring after the stopping operation of the internal combustion engine, it is necessary to lower the temperature of the exhaust gas purifier or to ensure that the oxygen surrounding the exhaust gas purifier does not run out. 
     A first conventional system for an exhaust gas purifier for an internal combustion engine may shift the air/fuel ratio of the exhaust gas from the operating internal combustion engine from a lean state to a stoichiometric state or rich state. In this case, air or water is used from outside to lower the temperature of the exhaust gas. This lowers the temperature of the exhaust gas purifier and prevents deterioration in NOx purification performance. 
     A second conventional system for an exhaust gas purifier for an engine increases an amount of ammonia adsorption by an SCR after the engine has stopped. This ammonia absorption amount is increased to an amount greater than or equal to a standard amount to ensure sufficient NOx purification performance when the engine is subsequently started. More specifically, an EGR passage is opened while driving an electric turbocharger after the engine has stopped so as to feed fresh air to the SCR. Urea is then supplied after the SCR temperature has lowered. 
     The first conventional exhaust gas purifier for an internal combustion engine is not configured to prevent an occurrence of coking after the internal combustion engine has stopped. The temperature within an exhaust pipe and the temperature of the exhaust gas purifier are lowered using water while the internal combustion engine is in operation. In this case, water drops come directly in contact with the exhaust gas purifier, which may lead to a damage of the exhaust gas purifier. Further, it is necessary to add, for example, a tank for storing water or a water injection apparatus. This makes the system more complex and requires more space for mounting. 
     The second conventional exhaust gas purifier serves to lower the temperature of the SCR after the internal combustion engine has stopped so as to increase an adsorbed ammonia amount. In other words, this apparatus is not intended to allow the hydrocarbons to undergo an oxidation reaction, which would prevent the occurrence of coking after the internal combustion engine has stopped. With this exhaust gas purifier, fresh air is blown using an electric turbocharger and an EGR pipe after the internal combustion engine has stopped. Therefore, in order to lower the temperature of the SCR, an electric turbocharger needs to be operated continuously for a relatively long time. As a result, power consumption will increase. 
     Accordingly, there has conventionally been a need for an exhaust gas purifier having a function to purify specific components in exhaust gas by oxidation reaction. For example, there has conventionally been a need for a structure to appropriately prevent the occurrence of coking so as to prevent deterioration of the exhaust gas purifier while minimizing the amount of power consumption. 
     SUMMARY 
     One aspect of the present disclosure relates to a control unit for an internal combustion engine system. The internal combustion engine system includes an internal combustion engine. An intake pipe is connected to the internal combustion engine. An electric turbocharger used to supercharge an intake air to the internal combustion engine is provided at the intake pipe. An exhaust pipe is connected to the internal combustion engine. An EGR pipe is configured to return a portion of exhaust gas flowing through the exhaust pipe to the intake pipe on an outlet side of the electric turbocharger. An EGR valve adjusts an opening degree of the EGR pipe. An exhaust gas purifier is provided at the exhaust pipe on a side downstream of a connection between the EGR pipe and the exhaust pipe. The exhaust gas purifier adsorbs specific components contained in the exhaust gas. The adsorbed specific components are subjected to an oxidation reaction using surrounding oxygen so as to purify the specific components from the exhaust gas. The control unit detects operation states of the internal combustion engine to control an actuator including the electric turbocharger and the EGR valve. An exhaust gas purifier temperature acquiring section of the control unit serves to acquire the temperature of the exhaust gas purifier. An operation stop detecting section detects that the internal combustion engine in operation has stopped. When the operation stop detecting section detects that the internal combustion engine in operation has stopped, a deterioration prevention controlling part is implemented. The exhaust gas purifier temperature acquiring section acquires the temperature of the exhaust gas purifier while the internal combustion engine is stopped. An oxygen-free period estimating section of the deterioration prevention controlling part estimates an oxygen-free period based on the temperature of the exhaust gas purifier. The oxygen-free period is a period during which the oxygen surrounding the exhaust gas purifier used for the oxidation reaction of the specific components is expected to run out. A fresh air replacement controlling section of the deterioration prevention controlling part allows the EGR valve to open before entering the estimated oxygen-free period and drives the electric turbocharger to replace the air surrounding the exhaust gas purifier with fresh air. After the replacement has been completed, the electric turbocharger is stopped from running. 
     Therefore, the oxidation reaction continues to prevent the occurrence of coking. Further, since the electric turbocharger is stopped after the replacement with fresh air has been completed, the power consumption amount can be reduced. 
     According to another aspect of the present disclosure, the control unit acquires an oxidation reaction rate of the exhaust gas purifier based on the acquired temperature of the exhaust gas purifier that was acquired at an exhaust gas purifier temperature acquiring section when estimating the oxygen-free period at the oxygen-free period estimating section. The control unit estimates the oxygen-free period based on the acquired oxidation reaction rate. Therefore, a more accurate oxygen-free period can be estimated. This allows the power consumption amount of the electric turbocharger to be reduced more appropriately. 
     According to another aspect of the present disclosure, an operation stop detecting section serves to detect whether the internal combustion engine in operation has stopped. While the oxidation reaction in the exhaust gas purifier continues when the internal combustion engine is stopped, the fresh air replacement controlling section replaces the fresh air. The oxygen-free period estimating section estimates a new oxygen-free period after the fresh air has been replaced. The replacement of the fresh air and the estimation of the oxygen-free period can be repeated. 
     The surrounding oxygen may eventually run out after just a single fresh air replacement. Even in such a case, the replacement of the fresh air and the oxygen-free period can be repeated. This prevents the surrounding oxygen from running out. 
     According to another aspect of the present disclosure, one of the specific components may be hydrocarbons. An adsorbed hydrocarbon amount acquiring section of the control unit estimates an adsorbed hydrocarbon amount. The adsorbed hydrocarbon amount is an amount of hydrocarbons adsorbed to the exhaust gas purifier while the internal combustion engine was in operation and/or once it has stopped. An operation stop detecting section detects that the operating internal combustion engine has stopped. The oxygen-free period estimating section estimates an oxygen-free period. At this time, the control unit estimates the oxygen-free period based on the temperature of the exhaust gas purifier and the adsorbed hydrocarbon amount. 
     This allows for a more accurate estimation of the oxygen-free period. The power consumption amount of the electric turbocharger can then be reduced more appropriately. 
     According to one aspect of the present disclosure, the operation stop detecting section detects that the operating internal combustion engine has stopped. A fresh air replacement controlling section replaces the fresh air. In this case, the control unit ends the implementation of the deterioration prevention controlling part if the control unit determined that the hydrocarbons, based on the adsorbed hydrocarbon amount, have been sufficiently eliminated due to the oxidation reaction. Therefore, the deterioration prevention controlling part can be ended at an appropriate timing. As a result, unnecessary power consumption can be properly avoided. 
     According to another aspect of the present disclosure, the operation stop detecting section detects that the operating internal combustion engine has stopped. The fresh air replacement controlling section replaces the fresh air. In this case, the control unit ends the implementation of the deterioration prevention controlling part if the temperature of the exhaust gas purifier, as acquired at the exhaust gas purifier temperature acquiring section, becomes lower than or equal to an end determining temperature. Therefore, the deterioration prevention controlling part can be ended at an appropriate timing. As a result, unnecessary power consumption can be avoided more appropriately. 
     According to another aspect of the present disclosure, a load adjusting section of the control unit can adjust the load to the internal combustion engine. Before the operating internal combustion engine has stopped, the load adjusting section adjusts the load existing immediately before the internal combustion engine stops so as to stop the internal combustion engine such that the crank angle is within the range where both the intake and exhaust valves of at least one of cylinders of the internal combustion engine are open. 
     Therefore, in addition to the EGR pipe, any of the cylinders with both the intake and exhaust valves open may also be used as fresh air passages when the electric turbocharger is driven to replace the fresh air surrounding the exhaust gas purifier. Thus, a pressure loss when blowing the fresh air can be reduced. As a result, the fresh air can be replaced more efficiently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a view illustrating an example of an overall structure of an internal combustion engine. 
         FIG.  2    is a flowchart illustrating an example of an “Entire Processes for Deterioration Prevention Control” for preventing (reducing) occurrence of coking after an operating internal combustion engine has stopped. 
         FIG.  3    is a flowchart illustrating details of the process of “Adjusting Load of Internal Combustion Engine” shown in the flowchart of  FIG.  2   . 
         FIG.  4    is a flowchart illustrating details of the process of “Detecting Stop of Operating Internal Combustion Engine” shown in the flowchart of  FIG.  2   . 
         FIG.  5    is a flowchart illustrating details of the process of “Controlling Replacement of Fresh Air” shown in the flowchart of  FIG.  2   . 
         FIG.  6    is a flowchart illustrating processes for “Controlling EGR Valve”. 
         FIG.  7    is a flowchart illustrating processes for “Controlling Electric Turbocharger”. 
         FIG.  8    is a flowchart illustrating details of the process of “Determining End of Deterioration Prevention Control” shown in the flowchart of  FIG.  2   . 
         FIG.  9    is an example of operation waves of the deterioration prevention control. 
         FIG.  10    is a view illustrating an example of temperature/oxidation reaction rate characteristics of the exhaust gas purifier. 
     
    
    
     DETAILED DESCRIPTION 
     Overall Structure of Internal Combustion Engine System  1  (FIG.  1 ) 
     Hereinafter, a control unit  50  for an internal combustion engine system  1  of the present embodiment will be described with reference to drawings. First,  FIG.  1    is used to describe an example of an overall structure of the internal combustion engine system  1  according to the present embodiment. An internal combustion engine  10  of the internal combustion engine system  1  of an example shown in  FIG.  1    is a diesel engine. Hereinafter, the structure, etc. of the internal combustion engine system  1  will be described in order from an intake side to an exhaust side. 
     An intake pipe  11 A is provided with an air flow rate detector  31 . The air flow rate detector  31  may, for example, be an intake air flow rate sensor that is configured to output detected signals to the control unit  50  in accordance with the flow rate of the intake air into the internal combustion engine  10 . Further, the air flow rate detector  31  is provided with an intake air temperature detector  32 A and an atmospheric air pressure detector  33 A. The intake air temperature detector  32 A may, for example, be an intake air temperature sensor that is configured to output detected signals to the control unit  50  in accordance with the temperature of the intake air (in this case, ambient air). The atmospheric air pressure detector  33 A, may, for example, be a pressure sensor that is configured to output detected signals to the control unit  50  in accordance with the atmospheric pressure. Further, the intake pipe A is connected to a compressor  82  of a turbocharger  80 . 
     Further, a branched intake pipe  11 B is connected to the intake pipe  11 A. The branched intake pipe  11 B is provided with an electric turbocharger  83 . The intake pipe  11 A is provided with a switching valve  83 A, while the branched intake pipe  11 B is provided with a different switching valve  83 B. The control unit  50  closes the switching valve  83 A of the intake pipe  11 A and opens the switching valve  83 B of the branched intake pipe  11 B when driving the electric turbocharger  83 . The switching valve  83 A of the intake pipe  11 A is opened and the switching valve  83 B of the branched intake pipe  11 B is closed when the electric turbocharger  83  is stopped. When the electric turbocharger  83  is driven, the electric turbocharger  83  feeds supercharged air under pressure toward the compressor  82  of the turbocharger  80 . 
     The intake pipe  11 A is connected to an inlet side of the compressor  82  of the turbocharger  80 . Another intake pipe  11 C is connected to an outlet side of the compressor  82 . The compressor  82  is rotated by a turbine  81 , which is driven by the exhaust gas. The compressor  82  feeds the intake air incoming from the inlet side intake pipe  11 A under pressure to the outlet side intake pipe  11 C. A pressure detector  33 B is provided to the intake pipe  11 A located at an upstream side of the compressor  82 . The pressure detector  33 B outputs detected signals to the control unit  50  in accordance with the pressure of the air before being compressed by the compressor  82 . 
     A downstream side of the intake pipe  11 C is connected to an intake manifold  11 D. A pressure detector  33 C, an intercooler  84 , a throttle device  64 , and an intake air temperature detector  32 B are provided at the intake pipe  11 C. The pressure detector 33c may, for example, be a pressure sensor, that is configured to output detected signals to the control unit  50  in accordance with the pressure of the intake air fed under pressure by the compressor  82 . Further, the intercooler  84  lowers the temperature of the intake air fed from the compressor  82  under pressure to increase its oxygen density. The throttle device  64  adjusts an opening degree of a throttle valve to a target throttle opening degree based on control signals from the control unit  50 . The intake air temperature detector  32 B may, for example, be an intake air temperature sensor that outputs detected signals to the control unit  50  in accordance with the temperature of the intake air lowered by the intercooler  84 . 
     A downstream side of the intake air manifold  11 D is connected to an intake port to guide the intake air to respective cylinders of the internal combustion engine  10 . The intake air guided to the intake manifold  11 D is sucked into the respective cylinders of the internal combustion engine  10  and is used for combustion with the fuel injected from an injector. 
     The internal combustion engine  10  is provided with a rotation detector  34 A and a cylinder detector  34 B. The rotation detector  34  may, for example, be a rotation sensor of a crankshaft that is configured to output detected signals to the control unit  50  in accordance with a rotation angle of the crankshaft of the internal combustion engine  10 . The cylinder detector  34 B may, for example, be a rotation sensor of the camshaft that is configured to output detected signals to the control unit  50  at the timing when a piston for a first cylinder reaches a compression top dead center. Further, the internal combustion engine  10  is provided with a load apparatus  63  capable of adjusting the load of the internal combustion engine  10 . The load apparatus  63  may, for example, be an alternator configured to change the load of the internal combustion engine  10  based on load control signals (power generation control signals) from the control unit  50 . 
     An accelerator pedal depression amount detector  38  may, for example, be an accelerator pedal depression amount sensor that is configured to output detected signals to the control unit  50  in accordance with the depression amount of the accelerator pedal operated by a driver. An ignition switch  39  is an input device for a user’s instruction to start or stop the internal combustion engine. A user operates the ignition switch  39  when starting a stopped internal combustion engine, or when stopping an operating internal combustion engine. 
     The control unit  50  calculates a required load based on the rotation speed of the internal combustion engine in accordance with detected signals from the rotation detector  34 A, and based on the depression amount of the accelerator pedal in accordance with detected signals from the accelerator pedal depression amount detector  38 . These signals are used to calculate a fuel amount corresponding to the required load. The control unit  50  then controls an injector at a predetermined timing in accordance with detected signals from the rotation detector  34 A and the cylinder detector  34 B, and injects a fuel amount corresponding to the required load. 
     An exhaust manifold  12 A is connected to an exhaust port of the internal combustion engine  10 . The exhaust gas from the internal combustion engine  10  is guided to the exhaust manifold  12 A, an exhaust pipe  12 B, and a turbine  81  of the turbocharger  80 . The exhaust gas drives a turbine  81  to rotate as it is exhausted to the exhaust pipe  12 C. The exhaust gas from the internal combustion engine  10  (in this case, a diesel engine) contains carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM), and nitrogen oxides (NOx). 
     An inflow side of an EGR pipe  13 , which is for returning some portions of the exhaust gas to the intake air, is connected to the exhaust manifold  12 A or to the exhaust pipe  12 B. An outflow side of the EGR pipe  13  is connected to the intake pipe  11 C or the intake manifold  11 D. An EGR valve  13 A for adjusting an opening degree of the EGR pipe  13  is provided at the EGR pipe  13 . The control unit  50  can adjust a flow rate of the EGR gas by adjusting the opening degree of the EGR valve  13 A while the internal combustion engine is in operation. Further, the control unit  50  opens the EGR valve  13 A while the internal combustion engine  10  is stopped. This allows fresh air fed from the electric turbocharger  83  under pressure to flow through the intake pipe  11 A, the branched pipe  11 B, the intake pipe  11 C, the EGR pipe  13 , and the exhaust pipes  12 B,  12 C, and to an exhaust gas purifier  40 . 
     The exhaust pipe  12 B is connected to an outflow side of the exhaust manifold  12 A. An inflow side of the turbine  81  of the turbocharger  80  is connected to a downstream side of the exhaust pipe  12 B. Another exhaust pipe  12 C is connected to an outflow side of the turbine  81 , and the exhaust gas purifier  40  is connected to a downstream side of this exhaust pipe  12 C. 
     The exhaust gas purifier  40  is provided at the exhaust pipe on a downstream side (in this case, the downstream side of the exhaust pipe  12 B) of a connection between the EGR pipe  13  and the exhaust pipe  12 B (or the exhaust manifold  12 A). The exhaust gas purifier  40  includes an upstream exhaust gas purifier  41  and a downstream exhaust gas purifier  45  disposed at a downstream side of the upstream exhaust gas purifier  41 . From the upstream side, a first oxidation catalyst  42  (DOC: Diesel Oxidation Catalyst) and a particle trap filter  43  (DPF: Diesel Particulate Filter) are provided in an interior of the upstream exhaust gas purifier  41 . 
     The first oxidation catalyst  42  serves to remove carbon monoxide (CO), hydrocarbons (HC), etc. contained in the exhaust gas using oxidation reactions. The particle trap filter  43  (hereinafter, referred to as a “DPF”) serves to trap the particulate matter (PM) contained in the exhaust gas. The exhaust gas flows to the downstream side through the particle trap filter  43 . In addition, the particle trap filter  43  has a function to remove carbon monoxide (CO) and hydrocarbons (HC) using oxidation reactions. 
     The exhaust pipe  12 C on an upstream side of the first oxidation catalyst  42  (upstream side of the upstream exhaust gas purifier  41 ) is provided with an addition valve  61 , an exhaust gas temperature detector  36 A (e.g., an exhaust gas temperature sensor), and the like. The addition valve  61  injects fuel (liquid additive) into the exhaust pipe  12 C. The fuel is subjected to an oxidation reaction within the first oxidation catalyst  42  to raise the temperature of the exhaust gas. The hotter exhaust gas burns and incinerates particulate matter trapped by and deposited in the DPF  43 , thereby regenerating the DPF  43 . Fuel is supplied to the addition valve  61  from a fuel tank (not shown). Further, the exhaust gas temperature detector  36 B (e.g., an exhaust gas temperature sensor) is provided on a downstream side of the first oxidation catalyst  42  and at an upper side of the DPF  43 ). 
     The exhaust gas temperature detector  36 C (e.g., an exhaust gas temperature sensor) is provided on a downstream side of the DPF  43 . Further, a differential pressure sensor  35 , which is used for detecting the differential pressure (e.g., a difference in pressure) of the exhaust gas pressure between the downstream side of the first oxidation catalyst  42  and the upstream side of the DPF  43  and the exhaust pressure on the downstream side of the DPF  43 , is provided within the upstream exhaust gas purifier  41 . 
     The control unit  50  is configured to detect the differential pressure between the upstream side of the DPF  43  and the downstream side of the DPF  43  based on detected signals from the differential pressure sensor  35 . An amount of the particulate matter trapped within the DPF  43  can be estimated in accordance with the detected difference in pressure. The control unit  50  then injects fuel (liquid additive) from the addition valve  61   if the estimated amount of deposit exceeds a threshold value. The injected fuel raises the exhaust gas temperature, which in turn burns and incinerates the particulate matter deposited in the DPF  43  to regenerate the DPF  43 . At this time, the control unit  50  detects the exhaust gas temperature at each position based on the detected signals from the exhaust gas temperature detectors  36 A,  36 B,  36 C, and allows the fuel (liquid additive) to be injected from the addition valve  61  so as to maintain the desired temperature. 
     Further, the downstream exhaust gas purifier  45  is provided from the upstream side with an addition valve  62 , a selective reduction catalyst  46  (SCR: Selective Catalytic Reduction), a second oxidation catalyst  47 , etc. The selective reduction catalyst  46  (hereinafter, referred to as “SCR”) is connected to the downstream side of the DPF  43  via an exhaust pipe  12 D. The addition valve  62  is disposed in the exhaust pipe  12 D, which is on the downstream side of the DPF  43  and on the upstream side of the SCR  46 . The addition valve  62  injects urea water (liquid additive) during exhausting gas at predetermined timings. The injected urea water (liquid additive) is scattered, atomized, and diffused within the exhaust pipe  12 D and reaches the SCR  46 . Further, the urea water is supplied to the addition valve  62  from a urea water tank (not shown). The SCR  46  serves to reduce and purify the nitrogen oxides (NOx) contained in the exhaust gas using ammonia gas generate from the added urea water. 
     Further, a NOx detector  37 A (e.g., a NOx sensor) is provided at the upstream exhaust pipe  12 D of the SCR  46 . Another NOx detector  37 B (e.g. NOx sensor) and an exhaust gas temperature detector  36 D (e.g., an exhaust gas temperature sensor) are provided at an exhaust pipe  12 E downstream of the SCR  46 . The NOx detectors  37 A,  37 B serve to output detected signals to the control unit  50  in accordance with the NOx concentration in the exhaust gas. The exhaust gas temperature detector  36 D serves to output detected signals to the control unit  50  in accordance with the temperature of the exhaust gas. The control unit  50  calculates a NOx purification rate of the SCR  46  based on the detected signals from the NOx detectors  37 A,  37 B, and the exhaust gas temperature detector  36 D. The control unit  50  controls the addition valve  62  based on the calculated NOx purification rate. 
     The second oxidation catalyst  47  is connected to the downstream side of the SCR  46  via the exhaust gas pipe  12 E. The second oxidation catalyst  47  oxidizes and purifies residual ammonia gas from the exhaust gas. The second oxidation catalyst  47  also includes a function to remove carbon monoxide (CO) and hydrocarbons (HC) through oxidation reactions. 
     The control unit  50  may be a known one, and may include a CPU  51 , a RAM  52 , a ROM  53 , a timer  54 , an EEPROM  55 , and the like. The CPU  51  may be configured to implement various calculation processes based on various programs or maps stored in the ROM  53 . Further, the RAM  52  may be configured to temporarily store calculation results calculated by the CPU and store data input from each of the detectors. The EEPROM  55  may be a non-volatile storage device configured, for example, to store data concerning the internal combustion engine  10  while the internal combustion engine  10  is stopped. 
     Further, the control unit  50  is capable of detecting operation states of the internal combustion engine  10  based on the input detected signals. The control unit  50  also obtains requests from an operator based on the detected operation states of the internal combustion engine  10 , detected signals from the accelerator pedal depression amount detector  38 , etc. The control unit  50  outputs control signals to control various actuators, such as an injector for injecting fuel into cylinders, the addition valves  61 ,  62  for injecting fuel or urea water, the electric turbocharger  83 , the EGR valve  13 A, etc. The control unit  50  (CPU  51 ) may include a deterioration prevention controlling part  51 A, an exhaust gas purifier temperature acquiring section  51 B, a load adjusting section  51 C, an operation stop detecting section  51 D, an adsorbed hydrocarbon amount acquiring section  51 E, an oxygen-free period estimating section  51 F, a replacement of fresh air controlling section  51 G, etc., details of these will be described later. These sections may be implemented by circuitry or any other suitable structure. 
     Here, the hydrocarbons (HC) contained in the exhaust gas are adsorbed to the first oxidation catalyst  42  (and the DPF  43 , the second oxidation catalyst  47 ). The temperature of the first oxidation catalyst  42  (and the DPF  43 , second oxidation catalyst  47 , etc.) may be raised higher than or equal to an activation temperature as the internal combustion engine  10  is warmed up. The hydrocarbons could be subjected to an oxidation reaction with oxygen contained in the exhaust gas. Since the exhaust gas contains oxygen and continuously flows while the internal combustion engine  10  is in operation, the oxygen will typically not run short for running the oxidation reaction. However, the internal combustion engine  10  may be stopped while hydrocarbons (HC) are still adsorbed to the first oxidation catalyst  42  (and the DPF  43 , the second oxidation catalyst  47 , etc.). In this case, if the temperature is higher than or equal to the activation temperature, the oxidation reaction of the hydrocarbons will proceed using the surrounding oxygen. However, the surrounding oxygen may eventually run out. If the surrounding oxygen runs out, the oxidation reaction of the adsorbed hydrocarbons (HC) will not proceed, such that the hydrogen (H) will instead be desorbed and that carbon will accumulate as a deposit. This accumulation of carbon may lead to an occurrence of so-called coking. As a result, deterioration of the first oxidation catalyst  42  (and the DPF  43 , the second oxidation catalyst  47 , etc.) will progress further. The control unit  50  described in the present embodiments serves to prevent the occurrence of the coking by executing processes as will be described below. The control unit  50  also serves to prevent deterioration of the exhaust gas purifier  40  (which may include the first oxidation catalyst  42 , the DPF  43 , the second oxidation catalyst  47 , etc.). 
     Process Procedures of Control Unit  50  (FIG.  2  to FIG.  8 ) and Example of Operation Waves (FIG.  9 ) 
     Hereinafter, process procedures of the control unit  50  will be described with reference to the flowcharts shown in  FIG.  2    to  FIG.  8   . An example of operation waves will also be described with reference to  FIG.  9   . 
     Entire Processes for Deterioration Prevention Control (FIG.  2 ) 
     The control unit  50  (CPU  51 ) initiates “Entire Processes of Deterioration Prevention Control” shown in  FIG.  2   , for example, at predetermined time intervals (several milliseconds to several tens of milliseconds). Upon initiation, the control unit  50  proceeds the process to Step S 010 . In the following description, an example will be described in which the “first oxidation catalyst” is deemed as the “exhaust gas purifier”. 
     In Step S 010 , the control unit  50  implements the process of “Adjusting Load of Internal Combustion Engine”, and proceeds the process to Step S 015 . In the process of “Adjusting Load of Internal Combustion Engine”, the load existing immediately before stopping the internal combustion engine  10  will be adjusted. More specifically, the internal combustion engine  10  is stopped such that the crank angle is within a range where both the intake and exhaust valves of at least one of the cylinders is open. Details of an embodiment of this process will be described later. 
     In Step S 015 , the control unit  50  implements a process of “Detecting Stop of Operating Internal Combustion Engine”, and proceeds the process to Step S 020 . The process of “Detecting Stop of Operating Internal Combustion Engine” is a process to detect that a previously operating internal combustion engine  10  has stopped. The details of an embodiment of this will be described later. In the process of “Detecting Stop of Operating Internal Combustion Engine”, an operation flag is set to ON or OFF and a deterioration prevention control flag is set to ON or OFF. The deterioration prevention control flag is the flag which is set to ON when the deterioration prevention control for preventing an occurrence of the above-described coking is started. 
     In Step S 020 , the control unit  50  determines whether or not the deterioration prevention control flag is ON. If the deterioration prevention control flag is ON (Yes), the process proceeds to Step S 025 . If not (No), the process proceeds to Step S 070 . 
     When the process proceeds to Step S 025 , the operating internal combustion engine  10  comes to a full stop The control unit  50  then updates an exhaust gas purifier temperature Ta once the internal combustion engine  10  has stopped and proceeds the process to Step S 030 . For example, the control unit  50  determines a lowered temperature ΔTb after a certain lapse of time. The previous exhaust gas purifier temperature Ta is updated to the current exhaust gas purifier temperature Ta. The current exhaust gas purifier temperature Ta is obtained by subtracting the lowered temperature ΔTb from the previous exhaust gas purifier temperature Ta. In an example of the operation waves in  FIG.  9   , the “Deterioration Prevention Control Flag” is set to ON during a period of time from Time T3 to Time T7. The “Exhaust Gas Purifier Temperature Ta” during the time from Time T3 to Time T7 is determined using the process described in Step S 025 . A method for acquiring exhaust gas purifier temperature Ta while the internal combustion engine  10  is stopped shall not be limited to this method. 
     In Step S 030 , the control unit  50  acquires an oxidation reaction rate Vx of the exhaust gas purifier based, for example, on the exhaust gas purifier temperature Ta. The control unit  50  then proceeds the process to Step S 035 . For example, “Temperature/Oxidation Reaction Rate Characteristics” corresponding to the exhaust gas purifier, an embodiment of which shown in an example of  FIG.  10   , are stored in a storage device of the control unit  50 . The “Temperature/Oxidation Reaction Rate Characteristics” represents the oxidation reaction rate according to the temperature of the target exhaust gas purifier. The example shown in  FIG.  10    indicates that the oxidation reaction rate is almost zero below the activation temperature. The control unit  50  acquires the current oxidation reaction rate Vx of the exhaust gas purifier temperature Ta based on the temperature/oxidation reaction rate characteristics and the exhaust gas purifier temperature Ta. In the example of the operation waves in  FIG.  9   , the “Oxidation Reaction Rate Vx” during the period of time from Time T3 to Time T7, which is when the “Deterioration Prevention Control Flag” is set to ON, is continuously determined using this Step S 030 . 
     In Step S 035 , the control unit  50  updates an amount of surrounding oxygen Oa, which corresponds to the amount of oxygen surround the exhaust gas purifier, based on the oxygen reaction rate Vx. The control unit  50  then proceeds the process to Step S 040 . For example, the control unit  50  determines a decreased oxygen amount ΔOb in accordance with a lapse of time. The decreased oxygen amount ΔOb is subtracted from the previous amount of surrounding oxygen Oa and the resulting value is used as the current amount of surrounding oxygen Oa. In the example of the operation waves in  FIG.  9   , the “Deterioration Prevention Control Flag” is set to ON during the period of time from Time T3 to Time T7. During this period of time, a “Fresh Air Replacement Flag” is set to OFF during certain periods of time, from Time T3 to Time T4a, from Time T4c to Time T5a, from Time T5c to Time T6a, and from Time T6c to Time T7. The “Amount of Surrounding Oxygen Oa” during each of these periods of time are determined using Step S 035 . However, the method for acquiring the amount of surrounding oxygen Oa while the internal combustion engine  10  is stopped shall not be limited to this method. 
     In Step S 040 , the control unit  50  updates an adsorbed hydrocarbon amount Ma based on the oxidation reaction rate Vx, and proceeds the process to Step S 045 . For example, the control unit  50  determines a decreased hydrocarbon amount ΔMb in accordance with a lapse of time. The decreased hydrocarbon amount ΔMb is subtracted from the previous adsorbed hydrocarbon amount Ma and the resulting values is used as a current adsorbed hydrocarbon amount Ma. In the example of the operation waves in  FIG.  9   , the “Deterioration Prevention Control Flag” is set to ON during a period of time from Time T3 to Time T7. The “Adsorbed Hydrocarbon Amount Ma” during this period of time is determined using Step S 040 . However, the method for acquiring the adsorbed hydrocarbon amount Ma while the internal combustion engine  10  is stopped shall not be limited to this method. 
     In Step S 045 , the control unit  50  estimates when there will be an oxygen-free period Tn. In this embodiment, the oxygen-free period Tn is a period of time in which the surrounding oxygen (amount of surrounding oxygen Oa) will run out. The control unit  50  may estimate when the oxygen-free period Tn will occur based on the current oxidation reaction rate Vx, the current amount of surrounding oxygen Oa, the current adsorbed hydrocarbon amount Ma, etc. The control unit  50  then proceeds the process to Step S 050 . In the example of the operation waves in  FIG.  9   , an oxygen-free period T4b will be estimated to occur when, for example, the current time has passed Time T3 before Time T4a. 
     In Step S 050 , the control unit  50  determines whether or not the fresh air replacement flag is ON. If the fresh air replacement flag is ON (Yes), the process proceeds to Step S 060 , and if not (No), the process proceeds to Step S 055 . The fresh air replacement flag is a flag which is set to ON or OFF during Step S 060 . When the fresh air replacement is implemented in Step S 060 , the fresh air replacement flag is a flag which is set to ON. 
     In Step S 055 , the control unit  50  determines whether or not the current time is within Tα before the start of the oxygen-free period Tn (Time T4b, T5b, T6b). If the current time is within Tα before the oxygen-free period Tn (Time T4b, T5b, T6b) (Yes), the process proceeds to Step S 060 , and if not (No), the process proceeds to Step S 065 . In the example of the operation waves shown in  FIG.  9   , the control unit  50  determines that the current time is within Tα before the oxygen-free period T4b when, for example, the current time is between Time T4a and Time 4b. If the current time is between Time T3 and Time T4a, it will be determined that the system is not within Tα before the oxygen-free period T4b. Values of Ta are set to appropriate values, for example based on various experiments, etc. 
     When the process proceeds to Step S 060 , the control unit  50  implements a process of “Controlling Replacement of Fresh Air” and proceeds the process to Step S 065 . The process of “Controlling Replacement of Fresh Air” allows the EGR valve to be open for a certain period of time while the internal combustion engine  10  is stopped. The electric turbocharger is driven during this period of time to replace the air surrounding the exhaust gas purifier with fresh air. This process will be described in detail later. 
     When the process proceeds to Step S 065 , the control unit  50  implements a process of “Determining End of Deterioration Prevention Control”, and ends the process shown in  FIG.  2   . The process of “Determining End of Deterioration Prevention Control” is a process that may be used to set the deterioration prevention control flag to OFF. More specifically, it is a process to be implemented when states where the deterioration prevention control flag, which was set to ON in Step S 015 , are satisfied. If the states are satisfied, the control unit  50  will set the flag to OFF. This process will be described in detail later. 
     If the process proceeds to Step S 070  (see,  FIG.  2   ), the control unit  50  determines whether or not an in-operation flag is ON. If the in-operation flag is ON (Yes), the process proceeds to Step S 075 . If not (No), the process proceed to Step S 090 . The in-operation flag is set to ON in Step S 015  when the internal combustion engine is in operation, and is set to OFF when the internal combustion engine has stopped (see “In-Operation Flag” in  FIG.  9   ). Details of the ON/OFF of the in-operation flag will be described later. 
     When the process proceeds to Step S 075 , it has been determined that the internal combustion engine  10  is not stopped, but is instead still operating. While the internal combustion engine  10  is operating, the control unit  50  acquires the exhaust gas purifier temperature Ta based on operation states of the internal combustion engine  10 . The process then proceeds to Step S 077 . For example, the control unit  50  may acquire (estimate) the exhaust gas purifier temperature Ta based on the temperature of the exhaust gas detected by the exhaust gas temperature detector  36 A, the exhaust gas flow rate estimated from an intake air volume, rotation speed, etc. In the example of the operation waves in  FIG.  9   , the “In-Operation Flag” is set to ON during the period of time from Time T2 to Time T3. The “Exhaust Gas Purifier Temperature Ta” during this period of time is determined using Step S 075 , although other methods may instead be used. 
     In Step S 077 , the control unit  50  acquires the oxidation reaction rate Vx based on the exhaust gas purifier temperature Ta. The process then proceeds to Step S 080 . For example, the control unit  50  may acquire the oxidation reaction rate Vx based on the exhaust gas purifier temperature Ta and the “Temperature/Oxidation Reaction Rate Characteristics” shown in  FIG.  10   , similar to the process of Step S 030 . In the example of the operation waves in  FIG.  9   , the “In-Operation Flag” is set to ON during the period of time from Time T2 to Time T3. The “oxidation reaction rate Vx” during this period of time is determined using Step S 077 , although other methods may instead by used. 
     In Step S 080 , the control unit  50  estimates an amount of surrounding oxygen Oa, which is an amount of oxygen surrounding the exhaust gas purifier during operation, based on the operation states of the internal combustion engine. The process then proceeds to Step S 085 . For example, the control unit  50  may estimate the amount of surrounding oxygen Oa based on the intake air volume, the rotation speed, a fuel injection volume, etc. In the example of the operation waves in  FIG.  9   , the “In-Operation Flag” is set to ON during the period of time from Time T2 to Time T3. The “Amount of Surrounding Oxygen Oa” during this period of time is determined using Step S 080 , although other methods may instead be used. 
     In Step S 085 , the control unit  50  estimates the adsorbed hydrocarbon amount Ma, which is an amount of hydrocarbons adsorbed to the exhaust gas purifier during operation, based on the operation states of the internal combustion engine. The control unit  50  then ends the process shown in  FIG.  2   . For example, the control unit  50  estimates the adsorbed hydrocarbon amount Ma based on the intake air volume, the fuel injection volume, the rotation speed, the exhaust gas purifier temperature Ta, etc. In the example of the operation waves in  FIG.  9   , the “In-Operation Flag” is set to ON during the period of time from Time T2 to Time T3. The “Adsorbed Hydrocarbon Amount Ma” during this period of time is determined using Step S 085 , although other methods may instead be used. 
     If the process proceeds to Step S 090  (see,  FIG.  2   ), the control unit  50  acquires (estimates) the exhaust gas purifier temperature Ta based on the operation states of the internal combustion engine (in this case, while the engine is stopped). The control unit  50  then ends the process shown in  FIG.  2   . For example, the control unit  50  determines an ambient air temperature (intake air temperature detected by the intake air temperature detector 32A) and uses this temperature as the exhaust gas purifier temperature Ta. 
     Adjusting Load of Internal Combustion Engine (FIG.  3 ) 
     A process of “Adjusting Load of Internal Combustion Engine” of Step S 010  of  FIG.  2    will be described in detail with reference to  FIG.  3   . When implementing the process of Step S 010  of the flowchart shown in  FIG.  2   , the control unit  50  proceeds the process to Step S 110 , an embodiment of which is shown in  FIG.  3   . 
     In Step S 110 , the control unit  50  determines whether or not the internal combustion engine has stopped. This may be done by determining whether a stop request (operation of an ignition switch) of the internal combustion engine has been issued by a user. If the engine has been determined to have stopped due to the stop request (Yes), the process proceeds to Step S 115 . If not (No), the process proceeds to Step S 150 . 
     If the process proceeds to Step S 115 , the control unit  50  determines whether or not the rotation speed of the combustion engine is lower than or equal to an adjusted rotation speed (e.g., a speed lower than or equal to the rotation speed immediately before the internal combustion engine comes to a full stop). If the rotation speed is lower than or equal to the adjusted rotation speed (Yes), the process proceeds to Step S 120 . If not (No), the process proceeds to Step S 150 . 
     If the process proceeds to Step S 120 , the control unit  50  determines whether or not the crank angle is greater than or equal to a first rotation angle θ1 and less than or equal to a second rotation angle θ2. If the crank angle is greater than or equal to the first rotation angle θ1 and less than or equal to the second rotation angle θ2 (Yes), the process proceeds to Step S 125 . If not (No), the process proceeds to Step S 150 . For example, a crank angle of greater than or equal to the first rotation angle θ1 and less than or equal to the second rotation angle θ2 is a crank angle where both the intake and exhaust valves of at least one of the cylinders (e.g., the first cylinder) are open. This allows for a passage from an intake port to an exhaust port through at least one of the cylinders, in addition to the passage of the EGR pipe when fresh air is replacing the oxygen deficient air by using the electric turbocharger. As a result, a pressure loss during the replacement of fresh air can be reduced, which in turn improves the efficiency during the replacement of the fresh air. 
     If the process proceeds to Step S 125 , the control unit  50  increases the amount of load to the internal combustion engine to immediately stop the internal combustion engine. For example, a “Load Adjustment Amount” is increased immediately before Time T3 in the example of the operation waves in  FIG.  9   . More specifically, the control unit  50  outputs control signals (signals of increasing amount of power generation) for increasing the amount of load to the load apparatus  63  (alternator) so as to stop the internal combustion engine immediately. The control unit  50  then ends the process shown in  FIG.  3   , and returns the process to Step S 015  shown in  FIG.  2   . 
     If the process proceeds to Step S 150 , the control unit  50  implements the existing control of the load apparatus  63 . Since this control is an existing control, details will be omitted. The control unit  50  then ends the process shown in  FIG.  3   , and returns the process to Step S 015  shown in  FIG.  2   . 
     Detecting Stop of Operating Internal Combustion Engine (FIG.  4 ) 
     A process of “Detecting Stop of Operating Internal Combustion Engine” in Step S 015  of  FIG.  2    will be described in detail with reference to  FIG.  4   . When implementing the process of Step S 015  of the flowchart shown in  FIG.  2   , the control unit  50  proceeds the process to Step S 210  shown in  FIG.  4   . 
     In Step S 210 , the control unit  50  determines whether or not the internal combustion engine has stopped. If the internal combustion engine has stopped (Yes), the control unit  50  proceeds the process to Step S 215 . If not (No), the process proceeds to Step S 220 B. 
     If the process proceeds to Step S 215 , the control unit  50  determines whether or not the in-operation flag was previously set to ON. If the in-operation flag was set to ON (Yes), the process proceeds to Step S 220 A. If not (No), the control unit  50  ends the process shown in  FIG.  4   , and returns the process to Step S 020  shown in  FIG.  2   . 
     If the process proceeds to Step S 220 A, the control unit  50  sets the deterioration prevention control flag to ON and the in-operation flag to OFF. The control unit  50  then ends the process shown in  FIG.  4   , and returns the process to Step S 020  shown in  FIG.  2   . 
     If the process proceeds to Step S 220 B, the control unit  50  sets the deterioration prevention control flag to OFF, and the in-operation flag to ON. The control unit  50  then ends the process shown in  FIG.  4   , and returns the process to Step S 020  shown in  FIG.  2   . 
     Through these processes, as shown in the example of the operation waves in  FIG.  9   , the “In-Operation Flag” is set to ON while the internal combustion engine  10  is in operation. When the operating internal combustion engine has stopped, the “In-Operation Flag” is switched from ON to OFF. In this case, the “Deterioration Prevention Control Flag” is set to ON. 
     Controlling Replacement of Fresh Air (FIG.  5 ) 
     Next, a process of “Controlling Replacement of Fresh Air” of Step S 060  of  FIG.  2    will be described in detail with reference to  FIG.  5   . When implementing the process of Step S 060  of the flowchart shown in  FIG.  2   , the control unit  50  proceeds the process to Step S 310  shown in  FIG.  5   . In the example of the operation waves in  FIG.  9   , the “Controlling Replacement of Fresh Air” process is implemented for a period of time starting from Time T4a (or Time T5a, or Time T6a) until the “Fresh Air Replacement Flag” is set to OFF. The fresh air replacement flag is a flag that is set to ON or OFF during the “Controlling Replacement of Fresh Air” process shown in  FIG.  4   . As will be described below, the fresh air replacement flag is set to ON while the air surrounding the exhaust gas purifier is replaced with fresh air due to the electric turbocharger being driven (which may be done after the operating internal combustion engine has stopped). 
     In Step S 310 , the control unit  50  determines whether or not the fresh air replacement flag has been set to ON. If the fresh air replacement flag has been set to ON (Yes), the control unit  50  proceeds the process to Step S 325 , and if not (No), the control unit  50  proceeds the process to Step S 315 . 
     If the process proceeds to Step S 315 , the control unit  50  initializes and starts a fresh air replacement timer, and then proceeds the process to Step S 320 . 
     In Step S 320 , the control unit  50  sets the fresh air replacement flag to ON and proceeds the process to Step S 325 . 
     The “Fresh Air Replacement Flag” is set from OFF to ON in the processes of above Steps S 310  to S 320 , as shown in the example of the operation waves in  FIG.  9   . In this case, the “Fresh Air Replacement Timer” is initialized and started such that a running time of the electric turbocharger is started to be counted. 
     When the process proceeds to Step S 325 , the control unit  50  determines whether or not the time counted by the fresh air replacement timer is longer than or equal to a target replacement time. If the time counted by the fresh air replacement timer is longer than or equal to the target replacement time (Yes), the control unit  50  proceeds the process to Step S 360 . If not (No), the control unit  50  proceeds the process to Step S 340 . The “Target Replacement Time” is a time in which the air surrounding the exhaust gas purifier is to be replaced with fresh air while the rotation speed of the electric turbocharger is driven at a “Target Rotation Speed”, as will be described later. The “Target Replacement Time” is set to an appropriate value, which may be based on various experiments, etc. 
     If the process proceeds to Step S 340 , the control unit  50  controls the EGR valve to be in a fully opened state. Further, the control unit  50  drives the electric turbocharger such that its rotation speed approaches the target rotation speed. The control unit  50  then proceeds the process to Step S 345 . The “Target Rotation Speed” may be set at a rotation speed of the electric turbocharger with the highest power efficiency. 
     In Step S 345 , the control unit  50  determines a volume of the replaced fresh air based on the rotation speed and the running time of the electric turbocharger (for example the time based on the time counted by the fresh air replacement timer). Further, the control unit  50  calculates an amount by which the surrounding oxygen ΔOd increased based on the volume of the replaced fresh air. The control unit  50  then adds this increased amount of oxygen ΔOd to the previous determined amount of surrounding oxygen Oa and then sets this amount as the current amount of surrounding oxygen Oa. The control unit  50  then ends the process shown in  FIG.  5   , and returns the process to Step S 065  shown in  FIG.  2   . As shown in the example of the operation waves in  FIG.  9   , the “Fresh Air Replacement Flag” is ON for a period of time from Time T4a to Time T4c, from Time T5a to Time 5c, and from Time T6a to Time T6c. During these periods of time (which correspond to when the electric turbocharger is running), the “Amount of Surrounding Oxygen Oa” is gradually increased due to the increased amount of oxygen ΔOd. 
     If the process proceeds to Step S 360 , the control unit  50  stops and initializes the fresh air replacement timer, and proceeds the process to Step S 365 . 
     In Step S 365 , the control unit  50  stops the running of the electric turbocharger, and proceeds the process to Step S 370 . At this time, the EGR valve does not particularly need to be fully closed, nor does it need to be fully open. Therefore, the EGR valve may not be controlled so as to reduce power consumption. 
     In Step S 370 , the control unit  50  sets the fresh air replacement flag to OFF. The control unit  50  then ends the process shown in  FIG.  5   , and returns the process to Step S 065  shown in  FIG.  2   . In the example of the operation waves in  FIG.  9   , the “Adsorbed hydrocarbon amount Ma” becomes 0 (zero) at Time T7. Therefore, the “Deterioration Prevention Control Flag” is set to OFF at Time T7. 
     As described above, and as shown in the example of the operation waves in  FIG.  9   , in the “Controlling Replacement of Fresh Air” process of  FIG.  5   , the “Amount of Surrounding Oxygen Oa” increases since the electric turbocharger is driven from Time T4a (or Time T5a, Time T6a). Time T4a is a time prior to the occurrence of the oxygen-free period T4b (or oxygen-free periods T5b, T6b). As a result, oxygen can be added to the system before it is determined that there was be an insufficient amount of oxygen, thereby preventing the occurrence of coking. 
     In the example of the operation waves in  FIG.  9   , Time T4a to Time T4c (or Time T5a to Time T5c, or Time T6a to Time T6c), which is a running period of time of the electric turbocharger, may be, for example, about several seconds (e.g., about 1 or 2 second(s)). Time T4c to Time T5a (or from Time T5c to Time 6a), which is the period of time the electric turbocharger is not being driven, may be, for example, about several tens of seconds (e.g., 20 or 30 seconds). Therefore, a power consumption amount can be significantly reduced compared with the case where the electric turbocharger is continuously driven. Since the power consumption amount can be reduced, there is no need to install a large battery. A small battery may be sufficient, which can help reduce the vehicle weight. It is also possible to reduce the amount of power required to drive the internal combustion engine for use for generating power using an alternator. This helps contribute to an improvement in fuel consumption. 
     As described above and illustrated in the example of the operation waves in  FIG.  9   , the control unit  50  replaces the fresh air by the fresh air replacement section (by the process of “Controlling Replacement of Fresh Air”). This is done while the oxidation reaction continues in the exhaust gas purifier. In addition, the oxygen-free period estimating section (see Step S 045  in  FIG.  2   ) estimates a new oxygen-free period based on the replaced fresh air. These operations can be repeated. 
     Controlling EGR Valve (FIG.  6 ) 
     Hereinafter, a process of “Controlling (Existing) EGR Valve” will be described in detail with reference to  FIG.  6   . If the “Fresh Air Replacement Flag” is set to ON by “Controlling Replacement of Fresh Air” shown in  FIG.  5   , this will cause the EGR valve to operate. In this case, the EGR valve is prohibited from operating due to another process, the process for “Controlling (Existing) EGR Valve”. The control unit  50  starts the process shown in  FIG.  6    at a timing that would typically implement the “Controlling (Existing) EGR Valve”, and proceeds the process to Step SA 010  shown in  FIG.  6   . 
     In Step SA 010 , the control unit  50  determines whether or not the fresh air replacement flag has been set to ON. If the fresh air replacement flag has been set to ON (Yes), the control unit  50  does not control (drive) the EGR valve, and ends the process shown in  FIG.  6   . If the fresh air replacement flag has not been set to ON (No), the process proceeds to Step SA 020 . 
     When the process proceeds to Step SA 020 , the control unit  50  controls (drives) the EGR valve based on the previously existing EGR valve control process, and ends the process shown in  FIG.  6   . 
     Controlling Electric Turbocharger (FIG.  7 ) 
     Hereinafter, a process of “Controlling (Existing) Electric Turbocharger” will be described in detail with reference to  FIG.  7   . If the “Fresh Air Replacement Flag” is set to ON by “Controlling Replacement of Fresh Air” shown in  FIG.  5   , the electric turbocharger will be instructed to operate. In this case, the electric turbocharger is prohibited from being operated by other processes, for instance the process for “Controlling (Existing) Electric Turbocharger”. The control unit  50  starts the process shown in  FIG.  7    at a timing that would typically implement the “Controlling (Existing) Electric Turbocharger”, and proceeds the process to Step SB 010  shown in  FIG.  7   . 
     In Step SB 010 , the control unit  50  determines whether or not the fresh air replacement flag has been set to ON. If the fresh air replacement flag has been set to ON (Yes), the control unit  50  ends the process shown in  FIG.  7   , without controlling (driving) the electric turbocharger based on the other process. If the fresh air replacement flag has not been set to ON (No), the process proceeds to Step SB 020 . 
     When the process proceeds to Step SB 020 , the control unit  50  controls (drives) the electric turbocharger based on the other (existing) process, and ends the process shown in  FIG.  7   . 
     Determining End of Deterioration Prevention Control (FIG.  8 ) 
     Hereinafter, a process of “Determining End of Deterioration Prevention Control” of Step S 065  of  FIG.  2    will be described in detail with reference to  FIG.  8   . When implementing the process of Step S 065 , the control unit  50  proceeds the process to Step S 410  shown in  FIG.  8   . The “Determining End of Deterioration Prevention Control” is a process that may set the deterioration prevention control flag to OFF, which was set to ON in Step S 015  shown in  FIG.  2   . 
     In Step S 410 , the control unit  50  determines whether or not the adsorbed hydrocarbon amount Ma is 0 (zero). If the adsorbed hydrocarbon amount Ma is 0 (zero) (Yes), the process proceeds to Step S 420  (because the coking would not typically be able to occur). If not (No), the process proceeds to Step S 415 . Instead of determining whether or not the adsorbed hydrocarbon amount Ma is 0 (zero), the control unit  50  may instead determine whether or not the adsorbed hydrocarbon amount is less than or equal to an acceptable small amount. 
     If the process proceeds to Step S 415 , the control unit  50  determines whether or not the exhaust gas purifier temperature Ta is lower or equal to an end determining temperature. If the exhaust gas purifier temperature Ta is lower than or equal to the end determining temperature (Yes), the process proceeds to Step S 420  (since the oxidation reaction of the exhaust gas purifier will not progress). If not (No), the control unit  50  ends the process shown in  FIG.  8   , and returns the process under Step S 065  shown in  FIG.  2   . The “End Determining Temperature” may be, for example, a temperature based on the activation temperature of the exhaust gas purifier, and an appropriate temperature is set. 
     If the process proceeds to Step S 420 , the control unit  50  sets the deterioration prevention control flag to OFF. The process for “Deterioration Prevention Control” for preventing the occurrence of the coking after stopping the internal combustion engine ends, and the process proceeds to Step S 425 . 
     In Step S 425 , the process for controlling deterioration prevention after stopping the operating internal combustion engine ends. Therefore, the control unit  50  may stop supplying power to itself. The power supply stop command to the control unit  50  may be implemented by other processes, which may be implemented after the internal combustion engine stops. 
     In the example of the operation waves in  FIG.  9   , the control unit  50  determines that the adsorbed hydrocarbon amount=0 (zero) at Time T7, and sets the deterioration prevention control flag to OFF. 
     The control unit  50  (CPU  51 ) implementing the processes of Steps S 025 , S 075 , and S 090  shown in  FIG.  2    corresponds to an embodiment of an exhaust gas purifier temperature acquiring section  51 B (see  FIG.  1   ) configured to acquire the temperature of the exhaust gas purifier. 
     The control unit  50  (CPU  51 ) implementing the process for “Adjusting Load of Internal Combustion Engine” shown in  FIG.  3    corresponds to an embodiment of a load adjusting section  51   c  (see  FIG.  1   ). The load adjusting section  51 C is configured to adjust the load existing immediately before the internal combustion engine stops. The internal combustion engine may be stopped such that the crank angle is within a range where both the intake and exhaust valves of at least one of the cylinders are open. 
     The control unit  50  (CPU  51 ) configured to implement the process for “Detecting Stop of Operating Internal Combustion Engine” shown in  FIG.  4    corresponds to an embodiment of an operation stop detecting section  51 D (see  FIG.  1   ). The operation stop detecting section  51 D serves to detect whether the operating internal combustion engine has stopped. 
     The control unit  50  (CPU  51 ) implementing the processes of Steps S 040  and S 085  shown in  FIG.  2    corresponds to an embodiment of an adsorbed hydrocarbon amount acquiring section  51 E (see  FIG.  1   ). The adsorbed hydrocarbon acquiring section  51 E estimates the adsorbed hydrocarbon amount Ma, which is an amount of hydrocarbons adsorbed to the exhaust gas purifier while the internal combustion engine is operating or stopped. 
     The control unit  50  (CPU  51 ) implementing the process of Step S 045  shown in  FIG.  2    corresponds to an embodiment of an oxygen-free period estimating section  51 F (see  FIG.  1   ). The operation stop detecting section  51 D (see  FIG.  1   ) detects that the operating internal combustion engine has stopped. The exhaust gas purifier temperature acquiring section  51 B (see  FIG.  1   ) then acquires the exhaust gas purifier temperature Ta once the internal combustion engine has stopped. The oxygen-free period estimating section  51 F estimates an oxygen-free period, which is a period during which the oxygen surrounding the exhaust gas purifier for use in the oxidation reaction of the specific components (in this case, hydrocarbons) runs out, based on the exhaust gas purifier temperature Ta. 
     The control unit  50  (CPU  51 ) implementing the process for “Controlling Replacement of Fresh Air” corresponds to an embodiment of a replacement of fresh air controlling section  51 G (see  FIG.  1   ). The replacement of fresh air controlling section  51 G allows the EGR valve to open before the estimated oxygen-free period occurs and drives the electric turbocharger to replace the air surrounding the exhaust gas purifier with fresh air. After the replacement with fresh air has been completed, the electric turbocharges will be instructed to stop running. 
     As shown in  FIG.  1   , the deterioration prevention controlling part  51 A (see  FIG.  1   ) includes the exhaust gas purifier temperature acquiring section  51 B, the load adjusting section  51 C, the operation stop detecting section  51 D, the adsorbed hydrocarbon amount acquiring section  51 E, the oxygen-free period estimating section  51 F, and the replacement of fresh air controlling section  51 G. 
     The control unit  50  of the internal combustion engine system  1  shall not be limited to the structures, shapes, configurations, and process steps that are described in the present embodiments, and various modifications, additions, and deletions are possible without departing from the subject matter of the present invention. 
     The present embodiments include a (first) oxidation catalyst as an exhaust gas purifier. The oxidation catalyst adsorbs specific components (e.g., hydrocarbons) in the exhaust gas. The adsorbed specific components are subjected to oxidation reactions using surrounding oxygen, which can in turn purify these specific components. The exhaust gas purifier shall not be limited to the above-described oxidation catalyst. For example, the exhaust gas purifier may be a DPF (particle trap filter), an NSR (NOx Storage-Reduction Catalyst), a three-way catalyst, a (second) oxidation catalyst, or the like. The exhaust gas purifier may have a function to purify predetermined adsorbed components (e.g., hydrocarbons) by oxidation reactions using surrounding oxygen. Further, the present embodiments shall not be limited to diesel engines, but may be applied to various other internal combustion engines having an exhaust gas purifier. For example, it may be applied to gasoline engines or natural gas engines. The exhaust gas purifier may also have a function to purify adsorbed specific components (e.g., hydrocarbons) by oxidation reactions using surrounding oxygen. 
     In the above-described embodiments, with regard to the “Adjusting Load of Internal Combustion Engine” process, a process is performed to stop the internal combustion engine at a crank angle where both the intake and exhaust valves of at least one of the cylinders are open. This process may be omitted. 
     In the above-described embodiments, with regard to the “Determining End of Deterioration Prevention Control” process, an end of deterioration prevention control was determined based on the adsorbed hydrocarbon amount Ma and the exhaust gas purifier temperature Ta. Alternatively, an end of deterioration prevention control may be determined based on a lapse of a certain period of time since the deterioration prevention control flag was set to ON, or based on the number of times the electric turbocharger was driven while the deterioration prevention control flag is set to ON. 
     In the above-described embodiments, in Step S 085  shown in  FIG.  2   , the adsorbed hydrocarbon amount Ma was estimated while the internal combustion engine was in operation. Alternatively, it may be assumed that the maximum about of hydrocarbons were adsorbed in the exhaust gas purifier while the internal combustion engine was in operation. 
     In the above-described embodiments, in addition to the EGR valve being opened when the electric turbocharger is driven for the replacement of fresh air, both the intake and exhaust valves of at least one of the cylinders are open. Alternatively, the process for “Adjusting Load of Internal Combustion Engine” may be omitted to allow only the EGR valve to be opened regardless of the open/closed state of the cylinders. Further, in a case where the internal combustion engine uses hydraulic pressure to drive the intake and exhaust valves, instead of using a cam to drive the intake and exhaust valves, both the intake and exhaust valves may be open utilizing hydraulic pressure when the electric turbocharger is driven for the replacement of fresh air. 
     When greater than or equal to (≥), less than or equal to (≤), greater than (&gt;), less than (&lt;), etc. are mentioned, they may or may not include an equal sign. The numerical values that were used for describing the above embodiments are only some examples, and the scope shall not be limited to these numerical values. 
     The control unit  50  may include at least one programmed electronic processor. The control unit  50  may include at least one memory configured to store instructions or software to be executed by the electronic processor to carry out at least one of the functions of the control unit  50  described herein. For example, in some embodiments, the control unit  50  may be implemented as a microprocessor with a separate memory. 
     The data stores of the control unit  50  may include a volatile and/or a non-volatile memory. Examples of suitable data stores include RAM (Random Access Memory), flash memory, ROM (Read Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), EEPROM (Electrically Erasable Programmable Read-Only Memory), registers, magnetic disks, optical disks, hard drives, or any other suitable storage medium, or any combination thereof. 
     Where the term “processor” or “central processing unit” or “CPU” is used for identifying a unit performing specific functions, it should be understood that, unless otherwise explicitly stated, those functions can be carried out by a single processor or multiple processors arranged in any form, including parallel processors, serial processors, tandem processors, or cloud processing/cloud computing configurations. The software may include, for example, firmware, one or more applications, program data, filters, rules, one or more program modules, and/or other executable instructions. The software includes, for example, firmware, one or more applications, program data, filters, rules, one or more program modules, and other executable instructions.