Abstract:
An internal combustion engine, wherein a hydrocarbon supply valve ( 15 ) and an exhaust gas purification catalyst ( 13 ) are disposed inside an engine exhaust gas passage. When an increase in the temperature of the exhaust gas purification catalyst ( 13 ) caused by hydrocarbons supplied from the hydrocarbon supply valve ( 15 ) is smaller than a predetermined increase amount, and a decrease in the pressure of fuel supplied to the hydrocarbon supply valve ( 15 ) when the hydrocarbons have been injected from the hydrocarbon supply valve ( 15 ) is larger than a predetermined decrease amount, the present invention determines that a blockage is occurring in a hydrocarbon injection channel ( 69 ) when the hydrocarbons have been injected from the hydrocarbon supply valve ( 15 ).

Description:
TECHNICAL FIELD 
     The present invention relates to a control system of an internal combustion engine. 
     BACKGROUND ART 
     Known in the art is an internal combustion engine which is comprised of a particulate filter which is arranged inside of an engine exhaust passage, a fuel addition valve which is arranged in the engine exhaust passage upstream of the particulate filter, and a temperature sensor for detecting a temperature of the particulate filter and which, when the particulate filter should be regenerated, injects fuel from the fuel addition valve and uses the heat of oxidation reaction of the injected fuel to make the temperature of the particulate filter rise to a 600° C. or so regeneration temperature. In this regard, if the fuel addition valve is clogged, even if the fuel addition valve injects fuel, the temperature of the particulate filter will no longer rise to the regeneration temperature. Therefore, in this internal combustion engine, when the temperature of the particulate filter does not rise to the regeneration temperature even if injecting fuel from the fuel addition valve, it is judged that the fuel addition valve is clogged (for example, see PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Publication No. 2008-267178A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In this regard, sometimes when the fuel addition valve injects fuel along the injection path, the injected fuel deposits on the inside wall surfaces of the exhaust passage around the injection path, exhaust particulate which is contained in the exhaust gas builds up on this deposited fuel, and as a result the injection path is clogged. In this case, for example even if the fuel addition valve injects fuel to raise the temperature of the particulate filter, the injected fuel deposits on the built up exhaust particulate and, as a result, a sufficient injected fuel is no longer fed to the particulate filter. Therefore, in this case, even if the fuel addition valve is not clogged, the temperature of the particulate filter no longer rises to the regeneration temperature. However, in the above internal combustion engine, even if the fuel addition valve is not clogged, it is judged that the fuel addition valve is clogged. 
     An object of the present invention is to provide a control system of an internal combustion engine which enables accurate judgment of an injection path clogging or a fuel addition valve clogging. 
     Solution to Problem 
     According to the present invention, there is provided a control system of an internal combustion engine comprising:
         an exhaust purification catalyst arranged in an engine exhaust passage,   a hydrocarbon feed valve arranged in the engine exhaust passage upstream of the exhaust purification catalyst, and   a fuel feed device for feeding fuel to the hydrocarbon feed valve, hydrocarbons being injected from the hydrocarbon feed valve into an exhaust gas along a predetermined injection path, fuel pressure of fuel which is fed to the hydrocarbon feed valve falling when hydrocarbons are injected from the hydrocarbon feed valve,
 
wherein when a temperature rise of the exhaust purification catalyst due to the hydrocarbons fed from the hydrocarbon feed valve is smaller than a predetermined rise and a drop of the fuel pressure of fuel fed to the hydrocarbon feed valve is larger than a predetermined drop, it is judged that the injection path is clogged.
       

     Further, according to the present invention, there is provided, a control system of an internal combustion engine comprising:
         an exhaust purification catalyst arranged in an engine exhaust passage,   a hydrocarbon feed valve arranged in the engine exhaust passage upstream of the exhaust purification catalyst, and   a fuel feed device for feeding fuel to the hydrocarbon feed valve, hydrocarbons being injected from the hydrocarbon feed valve into an exhaust gas along a predetermined injection path, fuel pressure of fuel which is fed to the hydrocarbon feed valve falling when hydrocarbons are injected from the hydrocarbon feed valve,
 
wherein when a temperature rise of the exhaust purification catalyst due to the hydrocarbons fed from the hydrocarbon feed valve is smaller than a predetermined rise and a drop of the fuel pressure of fuel fed to the hydrocarbon feed valve is smaller than a predetermined drop, it is judged that the hydrocarbon feed valve is clogged.
       

     Advantageous Effects of Invention 
     In a first aspect of the present invention, it is possible to accurately judge that the injection path of hydrocarbons from the hydrocarbon feed valve is clogged, while in a second aspect of the present invention, it is possible to accurately judge that the hydrocarbon feed valve is clogged. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall view of a compression ignition type internal combustion engine. 
         FIG. 2  is a view which schematically shows the surface part of a catalyst carrier. 
         FIG. 3  is a view for explaining an oxidation reaction at an exhaust purification catalyst 
         FIG. 4  is a view which shows changes in an air-fuel ratio of exhaust gas which flows into an exhaust purification catalyst. 
         FIG. 5  is a view which shows an NO X  purification rate R 1 . 
         FIGS. 6A and 6B  are views for explaining an oxidation reduction reaction in an exhaust purification catalyst. 
         FIGS. 7A and 7B  are views for explaining an oxidation reduction reaction in an exhaust purification catalyst. 
         FIG. 8  is a view which shows changes in an air-fuel ratio of exhaust gas which flows into an exhaust purification catalyst. 
         FIG. 9  is a view which shows an NO X  purification rate R 2 . 
         FIG. 10  is a view which shows a relationship between a vibration period ΔT of hydrocarbon concentration and an NO X  purification rate R 1 . 
         FIGS. 11A and 11B  are views which show maps of the injection amount of hydrocarbons etc. 
         FIG. 12  is a view which shows an NO X  release control. 
         FIG. 13  is a view which shows a fuel injection timing. 
         FIGS. 14A and 14B  are views for explaining a fuel feed device etc. 
         FIG. 15  is a time chart which shows a change in the fuel pressure PX fed into the hydrocarbon feed valve. 
         FIG. 16  is a time chart which shows a change in the temperature TC of the exhaust purification catalyst bed etc. 
         FIGS. 17A and 17B  are views which show an injection amount of hydrocarbons etc. 
         FIG. 18  is a flow chart for performing an injection control. 
         FIG. 19  is a flow chart for performing an injection control. 
         FIG. 20  is a flow chart for performing an injection control. 
         FIG. 21  is a flow chart for performing a regeneration control. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is an overall view of a compression ignition type internal combustion engine. 
     Referring to  FIG. 1, 1  indicates an engine body,  2  a combustion chamber of each cylinder,  3  an electronically controlled fuel injector for injecting fuel into each combustion chamber  2 ,  4  an intake manifold, and  5  an exhaust manifold. The intake manifold  4  is connected through an intake duct  6  to an outlet of a compressor  7   a  of an exhaust turbocharger  7 , while an inlet of the compressor  7   a  is connected through an intake air amount detector  8  to an air cleaner  9 . Inside the intake duct  6 , a throttle valve  10  which is driven by an actuator is arranged. Around the intake duct  5 , a cooling device  11  is arranged for cooling the intake air which flows through the inside of the intake duct  6 . In the embodiment which is shown in  FIG. 1 , the engine cooling water is guided to the inside of the cooling device  11  where the engine cooling water is used to cool the intake air. 
     On the other hand, the exhaust manifold  5  is connected to an inlet of an exhaust turbine  7   b  of the exhaust turbocharger  7 , and an outlet of the exhaust turbine  7   b  is connected through an exhaust pipe  12  to an inlet of an exhaust purification catalyst  13 . In an embodiment of the present invention, this exhaust purification catalyst  13  is comprised of an NO X  storage catalyst  13 . An outlet of the exhaust purification catalyst  13  is connected to a particulate filter  14  and, upstream of the exhaust purification catalyst  13  inside the exhaust pipe  12 , a hydrocarbon feed valve  15  is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown in  FIG. 1 , diesel oil is used as the hydrocarbons which are fed from the hydrocarbon feed valve  15 . Note that, the present invention can also be applied to a spark ignition type internal combustion engine in which fuel is burned under a lean air-fuel ratio. In this case, from the hydrocarbon feed valve  15 , hydrocarbons comprised of gasoline or other fuel used as fuel of a spark ignition type internal combustion engine are fed. 
     On the other hand, the exhaust manifold  5  and the intake manifold  4  are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage  16 . Inside the EGR passage  16 , an electronically controlled EGR control valve  17  is arranged. Further, around the EGR passage  16 , a cooling device  18  is arranged for cooling the EGR gas which flows through the inside of the EGR passage  16 . In the embodiment which is shown in  FIG. 1 , the engine cooling water is guided to the inside of the cooling device  118  where the engine cooling water is used to cool the EGR gas. On the other hand, each fuel injector  3  is connected through a fuel feed tube  19  to a common rail  20 . This common rail  20  is connected through an electronically controlled variable discharge fuel pump  21  to a fuel tank  22 . The fuel which is stored inside of the fuel tank  22  is fed by the fuel pump  21  to the inside of the common rail  20 . The fuel which is fed to the inside of the common rail  21  is fed through each fuel feed tube  19  to the fuel injector  3 . 
     An electronic control unit  30  is comprised of a digital computer provided with a ROM (read only memory)  32 , a RAM (random access memory)  33 , a CPU (microprocessor)  34 , an input port  35 , and an output port  36 , which are connected with each other by a bidirectional bus  31 . Downstream of the exhaust purification catalyst  13 , a temperature sensor  23  is arranged for detecting the temperature of the exhaust gas flowing out from the exhaust purification catalyst  13 , and a differential pressure sensor  24  for detecting the differential pressure before and after the particulate filter  14  is attached to the particulate filter  14 . The output signals of these temperature sensor  23 , differential pressure sensor  24  and intake air amount detector  8  are input through respectively corresponding AD converters  37  to the input port  35 . Further, an accelerator pedal  40  has a load sensor  41  connected to it which generates an output voltage proportional to the amount of depression L of the accelerator pedal  40 . The output voltage of the load sensor  41  is input through a corresponding AD converter  37  to the input port  35 . Furthermore, at the input port  35 , a crank angle sensor  42  is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°. On the other hand, the output port  36  is connected through corresponding drive circuits  38  to each fuel injector  3 , the actuator for driving the throttle valve  10 , hydrocarbon feed valve  15 , EGR control valve  17 , and fuel pump  21 . 
       FIG. 2  schematically shows a surface part of a catalyst carrier which is carried on a substrate of the exhaust purification catalyst  13  shown in  FIG. 1 . At this exhaust purification catalyst  13 , as shown in  FIG. 2 , for example, there is provided a catalyst carrier  50  made of alumina on which precious metal catalysts  51  comprised of platinum Pt are carried. Furthermore, on this catalyst carrier  50 , a basic layer  53  is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanide or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal able to donate electrons to NO X . In this case, on the catalyst carrier  50  of the exhaust purification catalyst  13 , in addition to platinum Pt, rhodium Rh or palladium Pd may be further carried. Note that the exhaust gas flows along the top of the catalyst carrier  50 , so the precious metal catalysts  51  can be said to be carried on the exhaust gas flow surfaces of the exhaust purification catalyst  13 . Further, the surface of the basic layer  53  exhibits basicity, so the surface of the basic layer  53  is called the “basic exhaust gas flow surface parts  54 ”. 
     If hydrocarbons are injected from the hydrocarbon feed valve  15  into the exhaust gas, the hydrocarbons are reformed by the exhaust purification catalyst  13 . In the present invention, at this time, the reformed hydrocarbons are used to remove the NO X  at the exhaust purification catalyst  13 .  FIG. 3  schematically shows the reformation action performed at the exhaust purification catalyst  13  at this time. As shown in  FIG. 3 , the hydrocarbons HC which are injected from the hydrocarbon feed valve  15  become radical hydrocarbons HC with a small carbon number due to the precious metal catalyst  51 . 
       FIG. 4  shows the feed timing of hydrocarbons from the hydrocarbon feed valve  15  and the change in the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13 . Not that, the change in the air-fuel ratio (A/F) depends on the change in concentration of the hydrocarbons in the exhaust gas which flows into the exhaust purification catalyst  13 , so it can be said that the change in the air-fuel ratio (A/F) in shown in  FIG. 4  expresses the change in concentration of the hydrocarbons. However, if the hydrocarbon concentration becomes higher, the air-fuel ratio (A/F) in becomes smaller, so, in  FIG. 4 , the more to the rich side the air-fuel ratio (A/F) in becomes, the higher the hydrocarbon concentration. 
       FIG. 5  shows the NO X  purification rate R 1  by the exhaust purification catalyst  13  with respect to the catalyst temperatures TC of the exhaust purification catalyst  13  when periodically making the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  change so as to as shown in  FIG. 4 , periodically make the air-fuel ratio (A/F) in of the exhaust gas flowing to the exhaust purification catalyst  13  rich. In this regard, as a result of a research relating to NO X  purification for a long time, it is learned that if making the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  vibrate within a predetermined range of amplitude and within a predetermined range of period, as shown in  FIG. 5 , an extremely high NO X  purification rate R 1  is obtained even in a 350° C. or higher high temperature region. 
     Furthermore, it is learned that at this time, a large amount of reducing intermediates which contain nitrogen and hydrocarbons continues to be held or adsorbed on the surface of the basic layer  53 , that is, on the basic exhaust gas flow surface parts  54  of the exhaust purification catalyst  13 , and the reducing intermediates play a central role in obtaining a high NO X  purification rate R 1 . Next, this will be explained with reference to  FIGS. 6A and 6B . Note that, these  FIGS. 6A and 6B  schematically show the surface part of the catalyst carrier  50  of the exhaust purification catalyst  13 . These  FIGS. 6A and 6B  show the reaction which is presumed to occur when the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is made to vibrate within a predetermined range of amplitude and within a predetermined range of period. 
       FIG. 6A  shows when the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is low, while  FIG. 6B  shows when hydrocarbons are fed from the hydrocarbon feed valve  15  and the air-fuel ratio (A/F) in of the exhaust gas flowing to the exhaust purification catalyst  13  is made rich, that is, the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  becomes higher. 
     Now, as will be understood from  FIG. 4 , the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  is maintained lean except for an instant, so the exhaust gas which flows into the exhaust purification catalyst  13  normally becomes a state of oxygen excess. At this time, part of the NO which is contained in the exhaust gas deposits on the exhaust purification catalyst  13 , while part of the NO which is contained in the exhaust gas, as shown in  FIG. 6A , is oxidized on the platinum  51  and becomes NO 2 . Next, this NO 2  is further oxidized and becomes NO 3 . Further, part of the NO 2  becomes NO 2   − . Therefore, on the platinum Pt  51 , NO 2   −  and NO 3  are produced. The NO which is deposited on the exhaust purification catalyst  13  and the NO 2   −  and NO 3  which are formed on the platinum Pt  51  are strong in activity. Therefore, below, these NO, NO 2   − , and NO 3  will be referred to as the “active NO X *”. 
     On the other hand, if hydrocarbons are fed from the hydrocarbon feed valve  15  and the air-fuel ratio (A/F) in of the exhaust gas flowing to the exhaust purification catalyst  13  is made rich, the hydrocarbons successively deposit over the entire exhaust purification catalyst  13 . The majority of the deposited hydrocarbons successively react with oxygen and are burned. Part of the deposited hydrocarbons are successively reformed and become radicalized inside of the exhaust purification catalyst  13  as shown in  FIG. 3 . Therefore, as shown in  FIG. 6B , the hydrogen concentration around the active NO X * becomes higher. In this regard, if, after the active NO X * is produced, the state of a high oxygen concentration around the active NO X * continues for a constant time or more, the active NO X * is oxidized and is absorbed in the form of nitrate ions NO 3   −  inside the basic layer  53 . However, if, before this constant time elapses, the hydrocarbon concentration around the active NO X * becomes higher, as shown in  FIG. 6B , the active NO X * reacts on the platinum  51  with the radical hydrocarbons HO to thereby form the reducing intermediates. The reducing intermediates are adhered or adsorbed on the surface of the basic layer  53 . 
     Note that, at this time, the first produced reducing intermediate is considered to be a nitro compound R—NO 2 . If this nitro compound R—NO 2  is produced, the result becomes a nitrile compound R—CN, but this nitrile compound R—CN can only survive for an instant in this state, so immediately becomes an isocyanate compound R—NCO. This isocyanate compound R—NCO becomes an amine compound R—NH 2  if hydrolyzed. However, in this case, what is hydrolyzed is considered to be part of the isocyanate compound R—NCO. Therefore, as shown in  FIG. 6B , the majority of the reducing intermediates which are held or adsorbed on the surface of the basic layer  53  is believed to be the isocyanate compound R—NCO and amine compound R—NH 2 . 
     On the other hand, as shown in  FIG. 6B , if the produced reducing intermediates are surrounded by the hydrocarbons HC, the reducing intermediates are blocked by the hydrocarbons HC and the reaction will not proceed any further. In this case, if the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is lowered and then the hydrocarbons which are deposited around the reducing intermediates will be oxidized and consumed, and thereby the concentration of oxygen around the reducing intermediates becomes higher, the reducing intermediates react with the NO X  in the exhaust gas, react with the active NO X *, react with the surrounding oxygen, or break down on their own. Due to this, the reducing intermediates R—NCO and R—NH 2  are converted to N 2 , CO 2 , and H 2 O as shown in  FIG. 6A , therefore the NO X  is removed. 
     In this way, in the exhaust purification catalyst  13 , when the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is made higher, reducing intermediates are produced, and after the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is lowered, when the oxygen concentration is raised, the reducing intermediates react with the NO X  in the exhaust gas or the active NO X * or oxygen or break down on their own whereby the NO X  is removed. That is, in order for the exhaust purification catalyst  13  to remove the No X , the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  has to be periodically changed. 
     Of course, in this case, it is necessary to raise the hydrocarbon concentration to a concentration sufficiently high for producing the reducing intermediates and it is necessary to lower the hydrocarbon concentration to a concentration sufficiently low for making the produced reducing intermediates react with the NO X  in the exhaust gas or the active NO X * or oxygen or break down on their own. That is, it is necessary to make the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  vibrate by within a predetermined range of amplitude. Note that, in this case, it is necessary to hold these reducing intermediates on the basic layer  53 , that is, the basic exhaust gas flow surface parts  54 , until the produced reducing intermediates R—NCO and R—NH 2  react with the NO X  in the exhaust gas or the active NO X * or oxygen or break down themselves. For this reason, the basic exhaust gas flow surface parts  54  are provided. 
     On the other hand, if lengthening the feed period of the hydrocarbons, the time until the oxygen concentration becomes higher becomes longer in the period after the hydrocarbons are fed until the hydrocarbons are next fed. Therefore, the active NO X * is absorbed in the basic layer  53  in the form of nitrates without producing reducing intermediates. To avoid this it is necessary to make the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  vibrate within a predetermined range of period. 
     Therefore, in the embodiment according to the present invention, to react the NO X  contained in the exhaust gas and the reformed hydrocarbons and produce the reducing intermediates R—NCO and R—NH 2  containing nitrogen and hydrocarbons, the precious metal catalysts  51  are carried on the exhaust gas flow surfaces of the exhaust purification catalyst  13 . To hold the produced reducing intermediates R—NCO and R—NH 2  inside the exhaust purification catalyst  13 , the basic exhaust gas flow surface parts  54  are formed around the precious metal catalysts  51 . The reducing intermediates R—NCO and R—NH 2  which are held on the basic exhaust gas flow surface parts  54  are converted to N 2 , CO 2 , and H 2 O. The vibration period of the hydrocarbon concentration is made the vibration period required for continuation of the production of the reducing intermediates R—NCO and R—NH 2 . Incidentally, in the example shown in  FIG. 4 , the injection interval is made 3 seconds. 
     If the vibration period of the hydrocarbon concentration, that is, the injection period of hydrocarbons from the hydrocarbon feed valve  15 , is made longer than the above predetermined range of period, the reducing intermediates R—NCO and R—NH 2  disappear from the surface of the basic layer  53 . At this time, the active NO X * which is produced on the platinum Pt  53 , as shown in  FIG. 7A , diffuses in the basic layer  53  in the form of nitrate ions NO 3   −  and becomes nitrates. That is, at this time, the NO X  in the exhaust gas is absorbed in the form of nitrates inside of the basic layer  53 . 
     On the other hand,  FIG. 7B  shows the case where the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  is made the stoichiometric air-fuel ratio or rich when the NO X  is absorbed in the form of nitrates inside of the basic layer  53 . In this case, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO 3   − →NO 2 ), and consequently the nitrates absorbed in the basic layer  53  successively become nitrate ions NO 3   −  and, as shown in  FIG. 7B , are released from the basic layer  53  in the form of NO 2 . Next, the released NO 2  is reduced by the hydrocarbons HC and CO contained in the exhaust gas. 
       FIG. 8  shows the case of making the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  temporarily rich slightly before the NO X  absorption ability of the basic layer  53  becomes saturated. Note that, in the example shown in  FIG. 8 , the time interval of this rich control is 1 minute or more. In this case, the NO X  which was absorbed in the basic layer  53  when the air-fuel ratio (A/F) in of the exhaust gas was lean is released all at once from the basic layer  53  and reduced when the air-fuel ratio (A/F) in of the exhaust gas is made temporarily rich. Therefore, in this case, the basic layer  53  plays the role of an absorbent for temporarily absorbing NO X . 
     Note that, at this time, sometimes the basic layer  53  temporarily adsorbs the NO X . Therefore, if using term of “storage” as a term including both “absorption” and “adsorption”, at this time, the basic layer  53  performs the role of an NO X  storage agent for temporarily storing the NO X . That is, in this case, if the ratio of the air and fuel (hydrocarbons) which are supplied into the engine intake passage, combustion chambers  2 , and upstream of the exhaust purification catalyst  13  in the exhaust passage is referred to as “the air-fuel ratio of the exhaust gas”, the exhaust purification catalyst  13  functions as an NO X  storage catalyst which stores the NO X  when the air-fuel ratio of the exhaust gas is lean and releases the stored NO X  when the oxygen concentration in the exhaust gas falls. 
     The solid line of  FIG. 9  shows the NO X  purification rate R 2  when making the exhaust purification catalyst  13  function as an NO X  storage catalyst in this way. Note that, the abscissa of the  FIG. 9  shows the catalyst temperature TC of the exhaust purification catalyst  13 . When making the exhaust purification catalyst  13  function as an NO X  storage catalyst, as shown in  FIG. 9 , when the catalyst temperature TC is 250° C. to 300° C., an extremely high NO X  purification rate is obtained, but when the catalyst temperature TC becomes a 350° C. or higher high temperature, the NO X  purification rate R 2  falls. Note that, in  FIG. 9 , the NO X  purification rate R 1  shown in  FIG. 5  is illustrated by the broken line. 
     In this way, when the catalyst temperature TC becomes 350° C. or more, the NO X  purification rate R 2  falls because if the catalyst temperature TC becomes 350° C. or more, NO X  is less easily stored and the nitrates break down by heat and are released in the form of NO 2  from the exhaust purification catalyst  13 . That is, so long as storing NO X  in the form of nitrates, when the catalyst temperature TC is high, it is difficult to obtains a high NO X  purification rate R 2 . However, in the new NO X  purification method shown from  FIG. 4  to  FIGS. 6A and 6B , the amount of NO X  stored in the form of nitrates is small, and consequently, as shown in  FIG. 5 , even when the catalyst temperature TC is high, a high NO X  purification rate R 1  is obtained. 
     In the embodiment according to the present invention, to be able to purify NO X  by using this new NO X  purification method, a hydrocarbon feed valve  15  for feeding hydrocarbons is arranged in the engine exhaust passage, an exhaust purification catalyst  13  is arranged in the engine exhaust passage downstream of the hydrocarbon feed valve  15 , precious metal catalysts  51  are carried on the exhaust gas flow surfaces of the exhaust purification catalyst  13 , the basic exhaust gas flow surface parts  54  are formed around the precious metal catalysts  51 , the exhaust purification catalyst  13  has the property of reducing the NO X  contained in exhaust gas if making a concentration of hydrocarbons flowing into the exhaust purification catalyst  13  vibrate within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of NO X  contained in exhaust gas if making the vibration period of the concentration of hydrocarbons longer than this predetermined range, and, at the time of engine operation, the hydrocarbons are injected from the hydrocarbon feed valve  15  within the predetermined range of period to thereby reduce the NO X  which is contained in the exhaust gas in the exhaust purification catalyst  13 . 
     That is the NO X  purification method which is shown from  FIG. 4  to  FIGS. 6A and 6B  can be said to be a new NO X  purification method designed to remove NO X  without forming so much nitrates in the case of using an exhaust purification catalyst which carries precious metal catalysts and forms a basic layer which can absorb NO X . In actuality, when using this new NO X  purification method, the nitrates which are detected from the basic layer  53  are smaller in amount compared with the case where making the exhaust purification catalyst  13  function as an NO X  storage catalyst. Note that, this new NO X  purification method will be referred to below as the “first NO X  removal method”. 
     Now, as mentioned, before, if the injection period ΔT of the hydrocarbons from the hydrocarbon feed valve  15  becomes longer, the time period in which the oxygen concentration around the active NO X * becomes higher becomes longer in the time period after the hydrocarbons are injected to when the hydrocarbons are next injected. In this case, in the embodiment shown in  FIG. 1 , if the injection period ΔT of the hydrocarbons becomes longer than about 5 seconds, the active NO X * starts to be absorbed in the form of nitrates inside the basic layer  53 . Therefore, as shown in  FIG. 10 , if the vibration period ΔT of the hydrocarbon concentration becomes longer than about 5 seconds, the NO X  purification rate R 1  falls. Therefore, the injection period ΔT of the hydrocarbons has to be made 5 seconds or less. 
     On the other hand, in the embodiment of the present invention, if the injection period ΔT of the hydrocarbons becomes about 0.3 second or less, the injected hydrocarbons start to build, up on the exhaust gas flow surfaces of the exhaust purification catalyst  13 , therefore, as shown in  FIG. 10 , if the injection period ΔT of the hydrocarbons becomes about 0.3 second or less, the NO X  purification rate R 1  falls. Therefore, in the embodiment according to the present invention, the injection period of the hydrocarbons is made from 0.3 second to 5 seconds. 
     Now that, in the embodiment according to the present invention, when the NO X  purification action by the first NO X  purification method is performed, by controlling the injection amount and injection timing of hydrocarbons from the hydrocarbon feed valve  15 , the air-fuel ratio (A/F) in of the exhaust gas flowing into the exhaust purification catalyst  13  and the injection period ΔT of the hydrocarbons are controlled so as to become the optimal values for the engine operating state. In this case, in the embodiment according to the present invention, the optimum hydrocarbon injection amount WT when the NO X  purification action by the first NO X  purification method is performed is stored as a function of the injection amount Q from fuel injectors  3  and the engine speed N in the form of a map such as shown in  FIG. 11A  in advance in the ROM  32 . Further, the optimum injection period ΔT of the hydrocarbons at this time is also stored as a function of the injection amount Q from the fuel injectors  3  and the engine speed N in the form of a map such as shown in  FIG. 11B  in advance in the ROM  32 . 
     Next, referring to  FIG. 12  and  FIG. 13 , an NO X  purification method when making the exhaust purification catalyst  13  function as an NO X  storage catalyst will be explained specifically. The NO X  purification method in the case of making the exhaust purification catalyst  13  function as an NO X  storage catalyst in this way will be referred to below as the “second NO X  removal method”. 
     In this second NO X  removal method, as shown in  FIG. 12 , when the stored NO X  amount ΣNO X  of NO X  which is stored in the basic layer  53  exceeds a predetermined allowable amount MAX, the air-fuel ratio (A/F) in of the exhaust gas flowing into the exhaust purification catalyst  13  is temporarily made rich. If the air-fuel ratio (A/F) in of the exhaust gas is made rich, the NO X  which was stored in the basic layer  53  when the air-fuel ratio (A/F) in of the exhaust gas was lean is released from the basic layer  53  all at one and reduced. Due to this, the NO X  is removed. Note that if the operating state of the engine is determined, the amount of NO X  which is exhausted from the engine is accordingly determined. In the example shown in  FIG. 12 , the stored NO X  amount ΣNO X  is calculated from the amount of NO X  exhausted in accordance with the operating state of the engine. 
     In this second NO X  removal method, as shown in  FIG. 13 , by injecting an additional fuel WR into each combustion chamber  2  from the fuel injector  3  in addition to the combustion-use fuel Q, the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  is made rich. Note that, in  FIG. 13 , the abscissa indicates the crank angle. This additional fuel WR is injected at a timing at which it will burn, but will not appear as engine output, that is, slightly before ATDC90° after compression top dead center. This fuel amount WR is stored as a function of the injection amount Q and engine speed N in the form of a map in advance in the ROM  32 . Note that, in the embodiment according to the present invention, roughly speaking, the second NO X  removal method is used when the catalyst temperature TC is low while the first NO X  removal method is used when the catalyst temperature TC is high. 
       FIGS. 14A and 14B  show enlarged views of the surrounding of the hydrocarbon feed valve  15  shown in  FIG. 1 . Note that,  FIG. 14A  shows a fuel feed device  60  for feeding hydrocarbons, that is, fuel to the hydrocarbon feed valve  15 . As shown in  FIG. 14A , the fuel feed device  60  is comprised of a pump chamber  61  which is filled with pressurized fuel, a pressurizing piston  62  for pressurizing the fuel in the pump chamber  61 , an actuator  63  for driving the pressurizing piston  62 , a pressurized fuel outflow chamber  65  which is connected through the fuel feed pipe  64  to the hydrocarbon feed valve  15 , and a pressure sensor  66  for detecting the fuel pressure in the pressurized fuel outflow chamber  65 . The pump chamber  61  is on the one hand connected to the fuel tank  22  through a check valve  67  which enables flow only from the fuel tank  22  toward the pump chamber  61  and on the other hand connected to the pressurized fuel outflow chamber  65  through a check valve  68  which enables flow only from the pump chamber  61  toward the pressurized fuel outflow chamber  65 . 
     If the actuator  63  causes the pressurizing piston  62  to move to the right in  FIG. 14A , the fuel in the fuel tank  22  is sent through the check valve  67  to the inside of the pump chamber  61 , while if the actuator  63  causes the pressurizing piston  62  to move to the left in  FIG. 14A , the fuel in the pump chamber  61  is pressurized and sent through the check valve  63  to the inside of the pressurized fuel outflow chamber  65 . Next, this fuel is fed to the hydrocarbon feed valve  15 . The fuel which is fed to the hydrocarbon feed valve  15 , that is, the hydrocarbons, is injected from the nozzle port of the hydrocarbon feed valve  15  along the injection path  69  to the inside of the exhaust gas. In the example which is shown in  FIG. 14A , the nozzle port of this hydrocarbon feed valve  15  is arranged in a recessed part  70  which is formed at the inside wall surface of the exhaust pipe  12 . At the inside of this recessed part  70 , the injection path  69  is formed. 
       FIG. 15  shows an injection signal of hydrocarbons from the hydrocarbon feed valve  15 , a pump drive signal for driving the pressurizing piston  62  by the actuator  63 , a change of fuel pressure PX of fuel which is fed to the hydrocarbon feed valve  15 , and a change of air-fuel ratio of exhaust gas which flows into the exhaust purification catalyst  13  when an NO X  removal action is performed by the first NO X  removal method. Note that, the fuel pressure PX of fuel which is fed to the hydrocarbon feed valve  15  shows the fuel pressure inside the hydrocarbon feed valve  15 , that is, the fuel pressure inside the fuel feed pipe  64 . If the pump drive signal is generated, the actuator  63  is driven and the fuel in the pump chamber  61  is pressurized by the pressurizing piston  62 . Due to this, as shown in  FIG. 15  by the solid line, the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  is made to rapidly rise just a bit. Next, the fuel pressure PX falls just slightly due to the leakage of fuel to the pump chamber  61  etc. This fuel pressure PX, as shown in  FIG. 15  by the solid line, is made to increase a little at a time until the target fuel pressure PXO each time the pump drive signal is generated. If the fuel pressure PX reaches the target fuel pressure PXO, after that, the pressurizing piston  62  is made to operate when the fuel pressure PX falls lower than the target fuel pressure PXO and the action of increasing the fuel pressure PX is performed. 
     On the other hand, if the hydrocarbon injection signal is issued, the hydrocarbon feed valve  15  is made to open. Due to this, the fuel, that is, hydrocarbons, is injected from the hydrocarbon feed valve  15 . Note that at this time the opening time of the hydrocarbon feed valve  15  is made the injection time WT which is calculated from the map shown in  FIG. 11A . If hydrocarbons are injected from the hydrocarbon feed valve  15 , as shown in  FIG. 15  by the solid line, the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  rapidly falls. If the fuel pressure PX falls, the pressurizing piston  62  is made to operate each time the pump drive signal is generated and the fuel pressure PX is made to increase a little at a time until the target fuel pressure PXO. 
     In this regard, if the hydrocarbon feed valve  15  is clogged, the amount of hydrocarbons which are injected from the hydrocarbon feed valve  15  per unit time decreases. As a result, as shown an  FIG. 15  by the broken line, the drop ΔPX 2  of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  when the hydrocarbon feed valve  15  is made to open becomes smaller. Not that, in  FIG. 15 , ΔPX 1  shows the drop of fuel pressure PX when the hydrocarbon feed valve  15  is not clogged. If the hydrocarbon feed valve  15  is clogged in this way, compared with when the hydrocarbon feed valve  15  is not clogged, the drop ΔPX of the fuel pressure PX becomes smaller. Therefore, when the drop ΔPX of the fuel pressure PX becomes smaller, it can be judged that the hydrocarbon feed valve  15  is clogged. 
     Now, in  FIG. 1 , for example, when the fuel injector  3  is clogged, the drop in the fuel pressure inside the common rail  20  when fuel is injected from the fuel injector  3  decreases. However, in this case, since the volume of the common rail  20  is large, at this time, the drop in fuel pressure inside the common rail  20  is extremely small. Therefore, at this time, it is difficult to detect clogging of the fuel injector  3  from the change in the drop of fuel pressure in the common rail  20 . However, in the fuel feed device  60  which is used in the present invention, the sum of the volumes of the parts which store the fuel which is fed to the hydrocarbon feed valve  15 , that is the sum of the volumes of the inside of the fuel feed pipe  64 , the inside of the hydrocarbon feed valve  15 , and the inside of pressurized fuel outflow chamber  65 , is small. Therefore, when the hydrocarbon feed valve  15  is clogged, the drop ΔPX of the fuel pressure TX of the fuel which is fed to the hydrocarbon feed valve  15  greatly appears. Therefore, in the present invention, it becomes possible to accurately detect from the drop ΔPX of the fuel pressure PX whether the hydrocarbon feed valve  15  is clogged. 
     Note that, as will be understood from  FIG. 15 , when the drop ΔPX of the fuel pressure PX falls from ΔPX 1  to ΔPX 2 , the fuel pressure PX when it falls the most increases from PX 1  to PX 2 , the time tX after the fuel pressure PX falls, then rises to the target pressure PXO is shortened from tX 1  to tX 2 , and the number of times the pump is driven when the fuel pressure PX falls, then rises to the target pressure PXO decreases. In the present invention, at expressed in a representative manner to cover all of these phenomena, a drop ΔPX of the fuel pressure PX is used. Therefore, in the present invention, a small drop ΔPX of the fuel pressure PX includes an increase of the fuel pressure PX when fallen the most, a shorter time tX from when the fuel pressure PX falls, then rises to the target pressure PXO, and a decreased number of times the pump is driven when the fuel pressure PX falls, then rises to the target pressure PXO. 
     Now, if hydrocarbons are injected from the hydrocarbon feed valve  15 , the hydrocarbons are partially oxidized or oxidized on the exhaust purification catalyst  13 . The heat of oxidation reaction which occurs at this time is used to make the exhaust purification catalyst  13  rise in temperature. Regarding the cases where the hydrocarbons which are injected from the hydrocarbon feed valve  15  are used to make the exhaust purification catalyst  13  rise in temperature, these include the case of warming up the exhaust purification catalyst  13 , the case of releasing the SO X  from the exhaust purification catalyst  13 , and other cases, but below, the case of regenerating the particulate filter  14  by using the hydrocarbons which are injected from the hydrocarbon feed valve  15  to make the exhaust purification catalyst  13  rise in temperature will be used as an example to perform control to raise the temperature of the exhaust purification catalyst  13 . To regenerate the particulate filter  14 , it is necessary to make the temperature of the particulate filter  14  rise until the 600° C. or so regeneration temperature. In order to make the temperature of the particulate filter  14  rise until the regeneration temperature, the temperature of the exhaust purification catalyst  13  has to be raised to the target temperature at which the particulate filter  14  can start the regeneration action. Next, the temperature raising control of the exhaust purification catalyst  13  will be explained with reference to  FIG. 16 . 
       FIG. 16  shows the the injection signal of hydrocarbons from the hydrocarbon feed valve  15 , the injection amount of hydrocarbons from the hydrocarbon feed valve  15 , and the change of the catalyst bed temperature TC of the exhaust purification catalyst  13  when performing regeneration control of the particulate filter  14  while performing the NO X  removal action by the first NO X  removal method. Note that, in  FIG. 16 , TCX shows the target temperature at which the particulate filter  14  starts the regeneration action. In the region in  FIG. 16  which is shown by A, the temperature raising action of the exhaust purification catalyst  13  is not performed. At this time, the NO X  removal action by the first NO X  removal method is performed. At this time, the catalyst bed temperature TC of the exhaust purification catalyst  13  is maintained at a relatively low temperature. 
     Next, the temperature raising control of the exhaust purification catalyst  13  is performed while performing the NO X  removal action by the first NO X  removal method. At this time, the injection period of hydrocarbons from the hydrocarbon feed valve  15  is made shorter, and the amount of injection of hydrocarbons from the hydrocarbon feed valve  15  per unit time is increased. In the embodiment according to the present invention, the optimal hydrocarbon injection amount FWT when performing temperature raising control of the exhaust purification catalyst  13  while performing the NO X  removal action by the first NO X  removal method is stored as a function of the injection amount Q from the fuel injector  3  and the engine speed N in the form of a map such as shown in  FIG. 17A  in advance in the ROM  32 . Further, the optimal injection period ΔFT of hydrocarbons at this time is also stored as a function of the injection amount Q from the fuel injector  3  and the engine speed N in the form of a map such as shown in  FIG. 17B  in advance in the ROM  32 . 
     If temperature raising control of the exhaust purification catalyst  13  is performed, usually as shown in  FIG. 16  by the solid line, the catalyst bed temperature TC of the exhaust purification catalyst  13  is raised by exactly ΔTC 1  and reaches the target temperature TCX whereby the action of regeneration of the particulate filter  14  is performed. That is, the amount of injection of hydrocarbons per unit time, corresponding to the operating state of the engine, required for raising the catalyst bed temperature TC of the exhaust purification catalyst  13  by exactly ΔTC 1  is found in advance. Hydrocarbons are injected from the hydrocarbon feed valve  15  by this amount of injection of hydrocarbons per unit time found in advance required for raising the catalyst bed temperature TC of the exhaust purification catalyst  13  by exactly ΔTC 1 . At this time the catalyst bed temperature TC of the exhaust purification catalyst  13  is raised by exactly ΔTC 1  and reaches the target temperature TCX whereby the action of regeneration of the particulate filter  14  is performed. 
     In this regard, in this case, if for example the hydrocarbon feed valve  15  clogs, even if an instruction is issued for injecting the hydrocarbons from the hydrocarbon feed valve  15  by the amount of injection of hydrocarbons found in advance required for raising the catalyst bed temperature TC of the exhaust purification catalyst  13  by exactly ΔTC 1 , the amount of injection of hydrocarbons from the hydrocarbon feed valve  15  is decreased. As a result, for example, as shown in  FIG. 16  by the broken line, the catalyst bed temperature TC of the exhaust purification catalyst  13  only rises by ΔTC 2 . Therefore, in this case, it is necessary to correct the hydrocarbon injection amount per unit time to increase so that the catalyst bed temperature TC of the exhaust purification catalyst  13  reaches the target temperature TCX. However, when in this way using the catalyst bed temperature TC of the exhaust purification catalyst  13  as the basis to correct the injection amount of hydrocarbons, it is necessary to accurately estimate the catalyst bed temperature TC of the exhaust purification catalyst  13 . 
     In this regard, if a large amount of hydrocarbons per injection is injected from the hydrocarbon feed valve  15  such as when the NO X  removal action by the first NO X  removal method is performed, the precision of estimation of the catalyst bed temperature TC of the exhaust purification catalyst  13  ends up falling. That is, even in the past, at the time of regeneration of the particulate filter, sometimes additional fuel is fed into the combustion chamber or exhaust passage, but when, as in the present invention, the regeneration control of the particulate filter  14  is performed while performing the NO X  removal action by the first NO X  removal method, the amount of hydrocarbons per injection from the hydrocarbon feed valve  15  becomes considerably greater compared with the past. If the amount of hydrocarbons per injection becomes greater, the hydrocarbons cannot completely react at just the front end of the exhaust purification catalyst  13  and react at the downstream side to generate the heat of reaction. As a result, the temperature gradient in the exhaust purification catalyst  13  becomes uneven. The catalyst bed temperature TC of the exhaust purification catalyst  13  is obtained by estimation or detection of one part somewhere in the exhaust purification catalyst  13 . Therefore, if the temperature gradient in the exhaust purification catalyst  13  becomes uneven, the estimated or detected temperature no longer represents the be temperature TC of the catalyst as a whole. As a result, the precision of estimation of the catalyst bed temperature TC falls. 
     In this way, when the NO X  removal action by the first NO X  removal method is being performed, the precision of estimation of the catalyst bed temperature it falls. Therefore, for example, regardless of the fact that the hydrocarbon feed valve  15  is not clogged, there is the danger of the hydrocarbon feed valve  15  being mistakenly judged as clogged. To prevent such mistaken judgment, it is necessary to make up for the drop in the precision of estimation of the catalyst bed temperature TC. Therefore, in the present invention, the judgment of the results of detection of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  is jointly used. Due to this, it is possible to judge clogging of the hydrocarbon feed valve  15  with a higher precision compared with judgment from a change of the catalyst bed temperature TC. 
     In this regard, the temperature of the catalyst bed temperature TC of the exhaust purification catalyst  13  was found to be greatly affected not only by clogging of the hydrocarbon feed valve  15 , but also other phenomena. Next, this will be explained with reference to  FIG. 14B . That is, if hydrocarbons are injected from the hydrocarbon feed valve  15  along the injection path  69 , the injected fuel deposits on the inside wall surfaces of the exhaust pipe  12  around the injection path  69 , mainly the inside, wall surfaces of the recessed part  70 , and sometimes the particulates contained in the exhaust gas gradually build up on the deposited injected fuel. In this case, deposits  71  form on the inside wall surfaces of the exhaust pipe  12 . Due to the deposits  71 , the injection path  69  is clogged. 
     If the deposits  71  form on the inside wall surfaces of the exhaust pipe  12  in this way, for example, even if hydrocarbons are injected from the hydrocarbon feed valve  15  to regenerate the particulate filter  14 , the hydrocarbons deposit on the deposits  71  and, as a result, the exhaust purification catalyst  13  is no longer sufficiently fed with hydrocarbons. Therefore, in this case, even if the hydrocarbon feed valve  15  is not clogged, the catalyst bed temperature TC of the exhaust purification catalyst  13  no longer reaches the target temperature TCX. That is, even if the hydrocarbon feed valve  15  is clogged or even if the injection path  69  is clogged by the deposits  71 , the temperature rise of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  becomes smaller than a predetermined rise. In other words, when the temperature rise of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  becomes smaller than the predetermined rise, it can be judged that the hydrocarbon feed valve  15  is clogged or the injection path  69  is clogged by the deposits  71 . 
     In this case, when hydrocarbons are injected from the hydrocarbon feed valve  15 , if the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  becomes smaller, it is judged that the hydrocarbon feed valve  15  is clogged. Therefore, when the temperature rise of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  becomes smaller than a predetermined rise, if the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  becomes larger, it can be judged that the injection path  69  is clogged by the deposits  71 . 
     Therefore, in the present invention, in a control system of an internal combustion engine which comprises an exhaust purification catalyst  13  arranged in an engine exhaust passage, a hydrocarbon feed valve  15  arranged in the engine exhaust passage upstream of the exhaust purification catalyst  13 , and a fuel feed device  60  for feeding fuel to the hydrocarbon feed valve  15 , and in which hydrocarbons is injected from the hydrocarbon feed valve  15  into an exhaust gas along a predetermined injection path, and fuel pressure of fuel which is fed to the hydrocarbon feed valve  15  falls when hydrocarbons are injected from the hydrocarbon feed valve  15 , when a temperature rise of the exhaust purification catalyst  13  due to the hydrocarbon fed from the hydrocarbon feed valve  15  as smaller than a predetermined rise and a drop of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is larger than a predetermined drop, it is judged that the injection path  69  is clogged. 
       FIG. 18  shows the injection control routine for working this invention. This routine is executed by interruption every fixed time period. 
     Referring to  FIG. 18 , first, at step  80 , hydrocarbons are injected from the hydrocarbon feed valve  15  and the NO X  removal action by the first NO X  removal method is performed. Next, at step  81 , the change of the catalyst bed temperature TC of the exhaust purification catalyst  13  is estimated. This catalyst bed temperature TC can be estimated using a model and can be estimated from the output value of the temperature sensor  23 . Next, at step  82 , the change of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  is detected by the fuel pressure sensor  66 . 
     Next, at step  83 , it is judged if the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons fed from the hydrocarbon feed valve  15  is smaller than a predetermined set amount and the drop ΔPX of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is larger than a predetermined set amount. In this case, the predetermined set amount for the temperature rise ΔTC is, for example, made a temperature rise corresponding to 80 percent of the predetermined temperature rise ΔTC 1 , while the predetermined set amount for the drop ΔPX of the feed fuel pressure PX is, for example, made a fuel, pressure drop corresponding to 80 percent of the drop ΔPX 1  of the feed fuel pressure PX when the hydrocarbon feed valve  15  is not clogged. 
     When, at step  83 , it is judged that the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons fed from the hydrocarbon feed valve  15  is smaller than the predetermined set amount and the drop ΔPX of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is larger than the predetermined set amount, the routine proceeds to step  84  where it is judged that the injection path  69  is clogged. 
     On the other hand, when it is judged from the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  that the hydrocarbon feed valve  15  is clogged, if the catalyst bed temperature TC of the exhaust purification catalyst  13  reaches the target temperature TCX, it becomes questionable if the hydrocarbon feed valve  15  is actually clogged. As opposed to this, when it is judged from the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  that the hydrocarbon feed valve  15  is clogged, if the catalyst bed temperature TC of the exhaust purification catalyst  13  does not reach the target temperature TCX, the possibility of the hydrocarbon feed valve  15  being clogged becomes extremely high. 
     That is, when the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  becomes small when hydrocarbons are injected from the hydrocarbon feed valve  15 , if the temperature rise of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  becomes smaller than a predetermined rise, it can be judged that the hydrocarbon feed valve  15  is clogged. 
     Therefore, in the present invention, in a control system of internal combustion engine which comprises an exhaust purification catalyst  13  arranged in an engine exhaust passage, a hydrocarbon feed valve  15  arranged in the engine exhaust passage upstream of the exhaust purification catalyst  13 , and a fuel feed device  60  for feeding fuel to the hydrocarbon feed valve  15 , and in which hydrocarbons is injected from the hydrocarbon feed valve  15  into an exhaust gas along a predetermined injection path, and fuel pressure of fuel which is fed to the hydrocarbon feed valve  15  fails when hydrocarbons are injected from the hydrocarbon feed valve  15 , when a temperature rise of the exhaust purification catalyst  13  due to the hydrocarbons fed from the hydrocarbon feed valve  15  is smaller than a predetermined rise and a drop of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is smaller than a predetermined drop, it is judged that the hydrocarbon feed valve  15  is clogged. 
       FIG. 19  shows the injection control routine for working this invention. This routine is executed by interruption every fixed time period. 
     Referring to  19 , first, at step  90 , hydrocarbons are injected from the hydrocarbon feed valve  15  and the NO X  removal action by the first NO X  removal method is performed. Next, at step  91 , the change of the catalyst bed temperature TC of the exhaust purification catalyst  13  is estimated. This catalyst bed temperature TC can be estimated using a model and can be estimated from the output value of the temperature sensor  23 . Next, at step  92 , the change of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  is detected by the fuel pressure sensor  66 . 
     Next, at step  93 , it is judged if the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons fed from the hydrocarbon feed valve  15  is smaller than a predetermined set amount and the drop ΔPX of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is smaller than a predetermined set amount. In this case as well, in the same way as the injection control routine which is shown in  FIG. 18 , the predetermined set amount for the temperature rise ΔTC is, for example, made a temperature rise corresponding to 80 percent of the predetermined temperature rise ΔTC 1 , while the predetermined set amount for the drop ΔPX of the feed fuel pressure PX is, for example, made a fuel pressure drop corresponding to 80 percent of the drop ΔPX 1  of the feed fuel pressure PX when the hydrocarbon feed valve  15  is not clogged. 
     When, at step  93 , it is judged that the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons fed from the hydrocarbon feed valve  15  is smaller than the predetermined set amount and the drop ΔPX of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is smaller than the predetermined set amount, the routine proceeds to step  94  where it is judged that the hydrocarbon feed valve  15  is clogged. 
       FIG. 20  shows another embodiment of the injection control routine. In this embodiment, when the possibility of the hydrocarbon feed valve  15  clogging is extremely high, an increase correction for increasing the amount of hydrocarbons fed from the hydrocarbon feed valve  15  is performed. Explaining this slightly more specifically, in this embodiment, the injection amount WTO of hydrocarbons from the hydrocarbon feed valve  15  is made a value (=K·WT or K·FWT) which is obtained by multiplying the injection amount WT shown in  FIG. 11A  or the injection amount FT shown in  FIG. 17A  with the correction coefficient K(≧1.0). Furthermore, in this embodiment, when the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  becomes smaller when hydrocarbons are injected from the hydrocarbon feed valve  15 , the correction coefficient K is made greater the smaller the drop ΔPX of the feed fuel pressure PX. For example, when hydrocarbons are injected from the hydrocarbon feed valve  15 , if the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15 , as shown in  FIG. 15 , is decreased from drop ΔPX 1  where the hydrocarbon feed valve  15  is not clogged to the drop ΔPX 2 , the correction coefficient K is made K=ΔPX 1 /ΔPX 2 . 
     On the other hand, in this embodiment, when it is judged that the injection path  69  is clogged by the deposits  71 , an exhaust gas amount increasing action which increases an amount of exhaust gas is performed so that the flow of exhaust gas blows of the deposits  71 . In this case, the amount of exhaust gas which is exhausted from the engine increases the higher the engine load and increases the smaller the opening degree of the FOR control valve  17  is made, that is, the more the amount of recirculation of the exhaust gas is decreased. Therefore, in this embodiment according to the present invention, the amount of exhaust gas is increased by decreasing the amount of recirculation of the exhaust gas. In this case, preferably, at the time of engine high load operation, the EGR control valve  17  is closed to make the recirculation action of the exhaust gas stop so as to increase the amount of exhaust gas. 
       FIG. 20  shows an injection control routine for working this invention. This routine is executed by interruption every fixed time period. 
     Referring to  FIG. 20 , first, at step  100 , the amount of injection WTO of hydrocarbons from the hydrocarbon feed valve  15  (=K·WT or K·FWT) is calculated by multiplying the injection amount NT shown in  FIG. 11A  or the in action amount FWT shown in  FIG. 17A  with the correction coefficient K. That is, when the injection amount WT shown in  FIG. 11A  is used as the injection amount WTO of the hydrocarbons from the hydrocarbon feed valve  15 , the injection amount WT shown in  FIG. 11A  is multiplied with the correction coefficient K (=K·WT) while when the injection amount FWT shown in  FIG. 17A  is used as the injection amount WTO of the hydrocarbons from the hydrocarbon feed valve  15 , the injection amount FWT shown in  FIG. 17A  is multiplied with the correction coefficient K (=K·FWT). 
     At step  101 , hydrocarbons are injected from the hydrocarbon feed valve  15  by the injection amount WTO which is calculated at step  100 , and the NO X  removal action by the first NO X  removal method is performed. Next, at step  102 , the change of the catalyst bed temperature TC of the exhaust purification catalyst  13  is estimated. This catalyst bed temperature TC can be estimated using a model and can be estimated from the output value of the temperature sensor  23 . Next, at step  103 , the change of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  is detected by the fuel pressure sensor  66 . 
     Next, at step  104 , it is judged if the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  is smaller than a predetermined set amount. In this case, the predetermined set amount for the temperature rise ΔTC is, for example, made a temperature rise corresponding to 80 percent of the preset temperature rise ΔTC 1 . When the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  is smaller than the predetermined set amount, the routine proceeds to step  105  where it is judged if the drop ΔPX of the feed fuel pressure PX to the hydrocarbon feed valve  15  when hydrocarbons are injected from the hydrocarbon feed valve  15  is larger than a predetermined set amount. In this case, the predetermined set amount for the drop ΔPX of the feed fuel pressure PX is, for example, made a fuel pressure drop corresponding to 80 percent of the drop ΔPX 1  of the feed fuel pressure PX when the hydrocarbon feed valve  15  is clogged. 
     When at step  105  it is judged that the drop ΔPX of the feed fuel pressure PX to the hydrocarbon feed valve  15  when hydrocarbons are injected from the hydrocarbon feed valve  15  is larger than the predetermined set amount, it is judged that the injection path  69  is clogged, then the routine proceeds to step  106  where the exhaust gas amount increasing action which increases an amount of exhaust gas is performed. As opposed to this, when at step  105  it is judged that the drop ΔPX of the feed fuel pressure PX to the hydrocarbon feed valve  15  when hydrocarbons are injected from the hydrocarbon feed valve  15  is smaller than the predetermined set amount, it is judged that the hydrocarbon feed valve  15  is clogged, then the routine proceeds to step  107  where the correction coefficient K is calculated. That is, the increase correction for increasing the amount of hydrocarbons fed from the hydrocarbon feed valve  15  is performed. 
       FIG. 21  shows an embodiment designed to detect the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  before performing the temperature raising control when an instruction is issued to perform regeneration control of the particulate filter  14 . Note that, when the pressure difference before and after the particulate filter  14  which is detected by the differential pressure sensor  24  is over a predetermined set pressure, an instruction is issued to perform regeneration control of the particulate filter  14 . When an instruction is issued to perform regeneration control of the particulate filter  14 , the regeneration control which is shown in  FIG. 21  is performed. This regeneration control routine is performed by interruption every fixed time period. 
     Referring to  FIG. 21 , first, at step  110 , it is judged if the detection of the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  has been completed. When the detection of the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  has not been completed, the routine proceeds to step  111  where the injection amount WTO (=K·WT) of hydrocarbons from the hydrocarbon feed valve  15  is calculated by multiplying the injection amount WT shown in  FIG. 11A  with the correction coefficient K. Next, at step  112 , hydrocarbons are injected from the hydrocarbon feed valve  15  by the injection amount WTO which is calculated at step  111 , and the NO X  removal action by the first NO X  removal method is performed. Next, at step  113 , it is judged if the steady state of the engine has been continuing for a certain time or more, that is, if the steady state of the engine is stable. When the steady state of the engine is stable, the routine proceeds to step  114 . 
     At step  114 , the chance of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  is detected by the fuel pressure sensor  66 . Next, at step  115 , it is judged if the drop ΔPX of the feed fuel pressure PX to the hydrocarbon feed valve  15  when hydrocarbons are injected from the hydrocarbon feed valve  15  is smaller than a predetermined set amount. In this case, the predetermined set amount for the drop ΔPX of the feed fuel pressure PX is for example made a fuel pressure drop corresponding to 80 percent of the drop ΔPX 1  of the feed fuel pressure PX when the hydrocarbon feed valve  15  is not clogged. When at step  115  it is judged that the drop ΔPX of the feed fuel pressure PX to the hydrocarbon feed valve  15  when hydrocarbons are injected from the hydrocarbon feed valve  15  is smaller than the predetermined set amount, it is judged that the hydrocarbon feed valve  15  is clogged, then the routine proceeds to step  116  where the correction coefficient K is calculated. That is, the increase correction for increasing the amount of hydrocarbons fed from the hydrocarbon feed valve  15  is performed. 
     When the detection of the drop ΔPX of the fuel pressure PX of the fuel which is fed to the hydrocarbon feed valve  15  has been completed, the routine proceeds from step  110  to step  117  where the injection amount WTO (=K·FWT) of hydrocarbons from the hydrocarbon feed valve  15  is calculated by multiplying the injection amount FWT shown in  FIG. 17A  with the correction coefficient K. Next, at step  118 , hydrocarbons are injected from the hydrocarbon feed valve  15  by the injection amount WTO which is calculated at step  117  and the temperature raising control of the exhaust purification catalyst  13  is started. Next, at step  119 , the change in the catalyst bed temperature TC of the exhaust purification catalyst  13  is estimated. This catalyst bed temperature TC can be estimated using a model and can also be estimated from the output value of the temperature sensor  23 . Next, at step  120 , it is judged if the temperature raising action of the exhaust purification catalyst  13  has been completed. When the temperature raising action of the exhaust purification catalyst  13  has been completed, the routine proceeds to step  121 . 
     At step  121 , it is judged if the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  is smaller than a predetermined set amount. In this case, the predetermined set amount for the temperature rise ΔTC is, for example, made a temperature rise corresponding to 80 percent of the temperature rise temperature rise ΔTC 1  found in advance. When the temperature rise ΔTC of the exhaust purification catalyst  13  due to the hydrocarbons which are fed from the hydrocarbon feed valve  15  is smaller than the predetermined set amount, the routine proceeds to step  122  where it is judged if the correction coefficient K is larger than the set value K 0 , that is, if the hydrocarbon feed valve  15  is clogged. When the correction coefficient K is not larger than the set value K 0 , that is, when the hydrocarbon feed valve  15  is not clogged, the routine proceeds to step  123  where the exhaust gas amount increasing action which increases an amount of exhaust gas is performed. 
     In the embodiment which is shown in  FIG. 21 , when in the operating region which is shown in  FIG. 16  by A, the drop of the fuel pressure of fuel fed to the hydrocarbon feed valve  15  is detected, and when the temperature raising control is being performed, the temperature rise of the exhaust purification catalyst  13  is detected. On the other hand, as explained above, when the temperature raising control of the exhaust purification catalyst  13  is performed, compared with the time of the operating region which is shown in  FIG. 16  by A, the amount of hydrocarbons which are injected from the hydrocarbon feed valve  15  per unit time is made to increase. Therefore, the amount of feed of hydrocarbons per unit time when detecting the temperature rise of the exhaust purification catalyst  13  is made larger than the amount of feed of hydrocarbons per unit time when detecting the drop of the fuel pressure of fuel fed to the hydrocarbon feed valve  15 . Further, in the embodiment which is shown in  FIG. 21 , when the amount of feed of hydrocarbons per unit time from the hydrocarbon feed valve  15  is made to increase so as to regenerate the particulate filter  14 , the temperature rise of the exhaust purification catalyst  13  is detected. 
     Note that, as another embodiment, it is also possible to arrange an oxidation catalyst for reforming hydrocarbons upstream of the exhaust purification catalyst  13  in the engine exhaust passage. 
     REFERENCE SIGNS LIST 
     
         
           4  intake manifold 
           5  exhaust manifold 
           7  exhaust turbocharger 
           12  exhaust pipe 
           13  exhaust purification catalyst 
           14  particulate filter 
           15  hydrocarbon feed valve