Abstract:
An internal combustion engine in accordance with one aspect of the invention is characterized by comprising a combustion chamber, a reformer, and a control portion. A predetermined fuel component is burnt in the combustion chamber. The reformer has a reforming catalyst, and that produces a reformed gas which contains the fuel component obtained by reforming a mixture of fuel and air and which supplied to the combustion chamber. The control portion sets an air-fuel ratio of the mixture in the reformer such that a reforming efficiency of the reformer is held within a predetermined range, and sets an amount of the mixture supplied to the reformer such that an actual output torque of the internal combustion engine coincides with a target torque.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Applications No. 2003-018261 filed on Jan. 28, 2003 and No. 2003-314368 filed on Sep. 5, 2003 including the specifications, drawings and abstracts are incorporated herein by reference in therein entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an internal combustion engine and a method of operating an internal combustion engine. 
     2. Description of the Related Art 
     There has been known an internal combustion engine that is provided with a reformer including a reforming catalyst and that causes fuel components obtained by reforming various fuels by means of the reformer to burn in combustion chambers with a view to stabilizing combustion and reducing the amounts of HC and NOx contained in exhaust gas (Japanese Patent Application Laid-Open No. 2001-241365). 
     One known reformer for reforming a hydrocarbon fuel such as methanol or the like is designed to produce an intermediate product such as aldehyde or the like in a catalyst on the upstream side with respect to a flow direction of fuel and to produce hydrogen from aldehyde or the like in a catalyst on the downstream side with respect to the flow direction with a view to increasing the content of hydrogen in reformed gas (Japanese Patent Application Laid-Open No. 2000-281307). Furthermore, another known reformer is provided with a heater for heating up a reforming catalyst during a starting process of the reformer so as to produce a reformed gas that is rich in hydrogen within a short period of time (Japanese Patent Application No. 11-130405). There is also known an art wherein the air-fuel ratio in a reformer is set approximately equal to 5 as soon as the temperature of a catalyst reaches a predetermined temperature (600° C.) so as to enhance the yield of a fuel component (H2) in reformed gas (Japanese Patent Application No. 9-21362). 
     On the other hand, when reforming hydrocarbon fuel, suitable management of the temperature of a reforming catalyst (i.e., the temperature of a catalyst bed) is important in preventing the reforming performance from being adversely affected due to a deterioration of a catalyst, suppressing generation of by-products, and reducing the influence of heat on a region around a reformer. As a method of controlling the temperature of a reforming catalyst, there is known an art for suitably setting the temperature of the reforming catalyst by controlling the air-fuel ratio in a reformer using a correlation between the air-fuel ratio of a mixture of air and fuel supplied to the reformer and the temperature reached by the reformer (Japanese Patent Application No. 2002-179405). Documents related to this kind of art include Japanese Patent Application Laid-Open No. 4-58064, Japanese Patent Application Laid-Open No. 2002-154807, Japanese Patent Application Laid-Open No. 2000-323164, and Japanese Patent Application Laid-Open No. 11-92102. 
     Heretofore, as described above, the importance of suitable management of the temperature of a reforming catalyst has been accentuated in a reformer for reforming fuel. However, when an internal combustion engine equipped with a reformer is viewed as a whole, suitable management of the temperature of the reformer alone does not make it easy to produce a reformed gas for causing the internal combustion engine to operate in conformity with a required condition. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to supply a desired reformed gas from a reformer to combustion chambers and to thereby cause an internal combustion engine to operate precisely in conformity with a required condition. 
     An internal combustion engine in accordance with one aspect of the invention is characterized by comprising a combustion chamber, a reformer, and a control portion. A predetermined fuel component is burnt in the combustion chamber. The reformer has a reforming catalyst, and that produces a reformed gas which contains the fuel component obtained by reforming a mixture of fuel and air and which supplied to the combustion chamber. The control portion sets an air-fuel ratio of the mixture in the reformer such that a reforming efficiency of the reformer is held within a predetermined range, and sets an amount of the mixture supplied to the reformer such that an actual torque of the internal combustion engine coincides with a target torque. 
     This internal combustion engine is provided with the reformer that reforms the mixture of air and a fuel such as hydrocarbon fuel or the like and that produces a reformed gas containing fuel components such as CO, H2 and the like, and generates power by causing the fuel components contained in the reformed gas flowing from the reformer to burn in the combustion chamber. In developing the internal combustion engine as described above, the inventors first focused attention on the fact that the reforming efficiency of the reformer is correlated with the air-fuel ratio of fuel and air in the reformer. 
     That is, the reforming efficiency of the reformer changes in accordance with the air-fuel ratio of the mixture in the reformer. However, if the air-fuel ratio of the mixture in the reformer is held within a predetermined range, the reforming efficiency of the reformer is also held within a predetermined period. In view of this, according to this internal combustion engine, the control portion sets the air-fuel ratio of the mixture in the reformer within a predetermined range (preferably as a constant value) such that the reforming efficiency of the reformer is held within a desired range, and sets the amount of the mixture supplied to the reformer such that an actual output torque of the internal combustion engine coincides with a target torque. Thereby, the ratio among the fuel components contained in the reformed gas produced by the reformer can be grasped accurately. This construction makes it possible to supply a desired reformed gas from the reformer to the combustion chamber, and to thereby cause the internal combustion engine to operate precisely in conformity with a required condition. 
     In this case, it is preferable that the reformer produce a reformed gas containing CO and H2 by reforming a mixture of hydrocarbon fuel and air, and that the control portion set a ratio O/C of a number of oxygen atoms in air to a number of carbon atoms in fuel supplied to the reformer approximately within a range of 0.4 to 1.1. It is more preferable that the control portion set a ratio O/C of a number of oxygen atoms in air to a number of carbon atoms in fuel supplied to the reformer approximately within a range of 0.8 to 1.05. This construction makes it possible to hold the reforming efficiency of the reformer within a practically desirable range. 
     In addition, it is preferable that the control portion set an amount of air supplied to the reformer such that an actual output torque of the internal combustion engine coincides with a target torque, and set an amount of fuel supplied to the reformer on the basis of the amount of air and the air-fuel ratio. 
     This construction makes it possible to hold the reforming efficiency of the reformer within a desired range, and to cause the internal combustion engine to generate a desired torque. 
     Further, it is preferable that the internal combustion engine of the aforementioned aspect further comprise an air-supply passage for mixing air with the reformed gas produced by the reformer and an adjustment portion that is provided in the air-supply passage and that is designed to adjust an amount of air mixed with the reformed gas via the air-supply passage, and that the control portion control the adjustment portion such that an air-fuel ratio of the mixture absorbed into the combustion chamber becomes equal to a desired value. 
     This construction makes it possible to set the air-fuel ratio of the mixture in the combustion chamber as a desired value while holding the reforming efficiency of the reformer within a desired range. 
     It is preferable that the control portion substantially simultaneously set amounts of air and fuel supplied to the reformer and an amount of air mixed with the reformed gas in accordance with the target torque. 
     In addition, it is preferable that the internal combustion engine of the aforementioned aspect further comprise a temperature detection portion that detects a temperature of the reforming catalyst, and that the control portion estimate the air-fuel ratio of the mixture in the reformer on the basis of the temperature detected by the temperature detection portion. 
     That is, since the temperature of the reforming catalyst (the temperature of the catalyst bed) is correlated with the air-fuel ratio of the mixture in the reformer, this construction makes it possible to estimate the air-fuel ratio of the mixture in the reformer from the temperature of the reforming catalyst. 
     It is preferable that the control portion be capable of correcting the estimated air-fuel ratio in the reformer in accordance with an amount of fuel supplied to the reformer. 
     As described above, the temperature of the reforming catalyst is correlated with the air-fuel ratio of the mixture in the reformer. However, even if the air-fuel ratio of the mixture in the reformer is constant, the temperature of the reforming catalyst rises in accordance with an increase in amount of fuel supplied to the reformer. Accordingly, if the estimated air-fuel ratio in the reformer is corrected on the basis of an amount of fuel supplied to the reformer as in the case of this construction, an air-fuel ratio of the mixture in the reformer can be calculated with high precision. 
     It is also appropriate that the control portion adjust an amount of fuel supplied to the reformer on the basis of the estimated air-fuel ratio. 
     Furthermore, it is also appropriate that the internal combustion engine of the aforementioned aspect comprise a temperature detection portion that detects a temperature of the reforming catalyst, and that the control portion adjust an amount of fuel supplied to the reformer on the basis of the temperature detected by the temperature detection portion. 
     In addition, it is preferable that the control portion set the air-fuel ratio of the mixture of fuel and air in the reformer larger than the air-fuel ratio corresponding to the reforming efficiency within the predetermined range when the supply of the mixture to the reformer is started. 
     In general, when the supply of the mixture of fuel and air to the reformer is started, the reforming catalyst is often not activated sufficiently. In view of this, this construction is designed to set the air-fuel ratio of the mixture in the reformer larger than the air-fuel ratio corresponding to the reforming efficiency within the aforementioned predetermined range, for example, for a predetermined period since the supply of the mixture of fuel and air to the reformer is started. This makes it possible to promote ignition of fuel in the reforming catalyst, and to raise the temperature of the catalyst to an activation temperature within a short period. 
     In addition, it is also appropriate that an air supply amount be reduced before stoppage of the supply of fuel to the reformer when the supply of the mixture to the reformer is stopped. 
     As described above, when the supply of the mixture to the reformer is stopped, the amount of oxygen in the reformer becomes temporarily excessive, whereby the temperature of the reforming catalyst may rise abruptly. In this case, however, the amount of air to be supplied may be reduced prior to the stoppage of the supply of fuel to the reformer as in the case of this construction. This makes it possible to prevent the amount of oxygen in the reformer from becoming excessive, to suppress the progress of a reaction in the reformer, and to cool the reforming catalyst (the reformer). Accordingly, this construction also makes it possible to reliably inhibit the reforming catalyst from deteriorating due to an abrupt rise in temperature. 
     It is preferable that the internal combustion engine in accordance with the invention further comprise an exhaust gas recirculation portion that causes exhaust gas flowing from the combustion chamber to recirculate to the reformer. 
     In the internal combustion engine in accordance with the invention, the air-fuel ratio of the mixture in the reformer is set such that the reforming efficiency of the reformer is held within a desired range. In this case, however, since fuel and air may not be sufficiently mixed with each other in the reformer, the reforming reaction may not occur at the reforming efficiency within the aforementioned desired range, or the temperature of the catalyst may rise excessively. On the other hand, if the internal combustion engine is provided with the exhaust gas recirculation portion that causes exhaust gas flowing from the combustion chamber to recirculate to the reformer, fuel and air can be suitably mixed with each other in the reformer through recirculation of exhaust gas from the combustion chamber to the reformer. 
     In addition, it is preferable that the internal combustion engine in accordance with the invention further comprise a temperature detection portion that detects a temperature of the reforming catalyst and a control portion that controls the exhaust gas recirculation portion on the basis of the temperature detected by the temperature detection portion. 
     In the internal combustion engine in accordance with the invention, the air-fuel ratio of the mixture in the reformer is set such that the reforming efficiency of the reformer is held within a predetermined range. However, because the temperature of the reforming catalyst abruptly changes in accordance with a change in load or the like, the reforming efficiency within the aforementioned desired range may not be obtained due to an abrupt change in temperature of the reforming catalyst. On the other hand, as in the case of this construction, by constantly monitoring the temperature of the reforming catalyst by means of the temperature detection portion and causing exhaust gas to recirculate from the combustion chamber to the reformer by means of the exhaust gas recirculation portion or increasing an amount of exhaust gas recirculated by the exhaust gas recirculation portion in response to an abrupt change (an abrupt rise) in temperature in the reformer, it becomes possible to inhibit the temperature of the reforming catalyst from changing abruptly and to always hold the reforming efficiency of the reformer within a desired range. 
     It is preferable that the exhaust gas recirculation portion increase an amount of exhaust gas recirculated to the reformer when the supply of the mixture to the reformer is stopped. 
     When the supply of the mixture to the reformer is stopped to stop the reforming reaction in the reformer, the amount of oxygen in the reformer may temporarily become excessive. In this case, the reforming catalyst may deteriorate due to an abrupt rise in the temperature thereof. On the other hand, if the amount of exhaust gas recirculated to the reformer by the exhaust gas recirculation portion is increased in stopping supplying the mixture to the reformer, the amount of oxygen in the reformer can be prevented from becoming excessive, and the reforming catalyst (the reformer) can be cooled. Accordingly, this construction makes it possible to reliably inhibit the reforming catalyst from deteriorating due to an abrupt rise in temperature. 
     Further, it is preferable that the reformer have a plurality of reforming reaction portions disposed along a flow direction of the mixture and an oxygen supply portion that supplies oxygen to an oxygen inflow portion set between the reforming reaction portions, and that the air-fuel ratio of the mixture be set smaller than an air-fuel ratio corresponding to the reforming efficiency within the predetermined range in the reforming reaction portion that is disposed upstream of the oxygen inflow portion with respect to the flow direction. 
     In general, since the reforming reaction progresses within an extremely short period, the temperature in the reformer tends to be high on the upstream side with respect to the flow direction of the mixture. The stronger this tendency becomes, the more the reforming catalyst on the upstream side with respect to the flow direction deteriorates. If air and fuel have not been sufficiently mixed with each other upstream of the reformer (the reforming catalyst), the reforming catalyst is heated up in a range corresponding to an excessive amount of oxygen. As a result, the reforming reaction does not sufficiently progress in a range corresponding to an excessive amount of fuel, so that unreformed fuel may be mixed directly into the reformed gas. 
     On the other hand, according to this construction, the air-fuel ratio of the mixture is set smaller than an air-fuel ratio corresponding to a reforming efficiency within the aforementioned predetermined range (the amount of fuel is made excessive) in the reforming reaction portion located upstream of the oxygen inflow portion with respect to the flow direction of the mixture. Therefore, the reforming catalyst in the reforming reaction portion upstream of the oxygen inflow portion can be inhibited from being excessively heated up. Fuel is gasified in the reforming reaction portion upstream of the oxygen inflow portion, then mixes with oxygen in the oxygen inflow portion, and is excellently reformed in the reforming reaction portion downstream of the oxygen inflow portion. Accordingly, adoption of this construction makes it possible to inhibit the reforming catalyst from locally deteriorating, to reliably reform fuel, and to suitably hold the reforming efficiency of the reformer within a desired range. 
     In addition, it is preferable that the oxygen supply portion supply oxygen to the oxygen inflow portion of the reformer, and that an air-fuel ratio of the mixture be set larger than the air-fuel ratio corresponding to the reforming efficiency within the predetermined range in the reforming reaction portion that is disposed upstream of the oxygen inflow portion with respect to the flow direction, upon fulfillment of a predetermined operational condition on the reformer. 
     As described above, if the air-fuel ratio of the mixture in the reforming reaction portion upstream of the oxygen inflow portion with respect to the flow direction is set as a small value, coking of the reforming catalyst may occur in the reforming reaction portion on the upstream side. On the other hand, even if oxygen is supplied to the oxygen inflow portion of the reformer as in the case of this construction, the occurrence of coking can be suppressed, and the reforming catalyst in which coking has occurred can be regenerated, by setting the air-fuel ratio of the mixture in the reforming reaction portion disposed upstream of the oxygen inflow portion with respect to the flow direction as a large value upon fulfillment of a predetermined operational condition relating to the operating period of the reformer or the temperature of the reforming catalyst. 
     It is preferable that the reformer have a reforming reaction portion in which the reforming catalyst is disposed, that the reforming reaction portion include a catalyst small-amount-carriage region at least either at an upstream end thereof or at a downstream end thereof with respect to the flow direction of the mixture, an amount of the reforming catalyst in the catalyst small-amount-carriage region being smaller than an amount of the reforming catalyst in any other region. 
     As in the case of this construction, if at least one of the upstream and downstream ends of the reforming reaction portion is formed as the catalyst small-amount-carriage region, the heat of reaction generated in a region other than the catalyst small-amount-carriage region of the reforming reaction portion is transferred to the catalyst small-amount-carriage region. Accordingly, adoption of this construction makes it possible to improve the heat radiation performance of the reformer as a whole and to effectively inhibit the reforming catalyst from being heated up. 
     Moreover, if the upstream end of the reformer is formed as the catalyst small-amount-carriage region, the progress of the reforming reaction at the upstream end of the reformer is suppressed. Therefore, an excessive rise in temperature of the reforming catalyst, which generally tends to occur upstream of the reformer, can be suppressed. In this case, the catalyst small-amount-carriage region at the upstream end of the reforming reaction portion receives heat from the downstream region and is suitably heated up. Therefore, fuel supplied to the reformer can be gasified in the catalyst small-amount-carriage region and be suitably mixed with air. 
     According to another aspect of the invention, there is provided a method of operating an internal combustion engine comprising a combustion chamber in which a predetermined fuel component is burnt and a reformer that has a reforming catalyst, that reforms a mixture of fuel and air, that contains the fuel component, and that produces a reformed gas supplied to the combustion chamber. This method comprises the steps of setting an air-fuel ratio of the mixture in the reformer such that a reforming efficiency of the reformer is held within a predetermined range and setting an amount of the mixture supplied to the reformer such that an actual torque of the internal combustion engine coincides with a target torque. 
     The method of operation of the aforementioned aspect makes it possible to supply a desired reformed gas from the reformer to the combustion chamber and to cause the internal combustion engine to operate precisely in conformity with a required condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an internal combustion engine in accordance with a first embodiment of the invention. 
         FIG. 2  is a graph showing a relationship between the air-fuel ratio of a mixture in a reformer and the rate of fuel conversion in the reformer. 
         FIG. 3  is a graph showing a relationship between a ratio O/C of the number of oxygen atoms in air to the number of carbon atoms in fuel supplied to the reformer and the concentration of H2 in a reformed gas obtained from the reformer. 
         FIG. 4  is a graph showing a relationship between the ratio O/C of the number of oxygen atoms in air to the number of carbon atoms in fuel supplied to the reformer and the temperature of a catalyst bed in the reformer. 
         FIG. 5  is a schematic block diagram of an internal combustion engine in accordance with a modification example of the first embodiment of the invention. 
         FIG. 6  is a schematic block diagram of an internal combustion engine in accordance with another modification example of the first embodiment of the invention. 
         FIG. 7  is a schematic block diagram of an internal combustion engine in accordance with still another modification example of the first embodiment of the invention. 
         FIG. 8  is a schematic block diagram of an internal combustion engine in accordance with still another modification example of the first embodiment of the invention. 
         FIG. 9  is a schematic block diagram of an internal combustion engine in accordance with still another modification example of the first embodiment of the invention. 
         FIG. 10  is a schematic block diagram of an internal combustion engine in accordance with a second embodiment of the invention. 
         FIG. 11  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 12  is a pattern diagram showing an exemplary map that is used to set a reformed air supply amount. 
         FIG. 13  is a pattern diagram showing an exemplary map that is used to set a throttle opening. 
         FIG. 14  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 15  is a pattern diagram showing an exemplary map that is used to estimate an air-fuel ratio of a mixture in the reformer. 
         FIG. 16  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 17  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 18  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 19  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 20  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 10 . 
         FIG. 21  is a graph showing how the temperature of the catalyst bed changes when the internal combustion engine shown in  FIG. 10  operates according to the flowchart shown in  FIG. 20 . 
         FIG. 22  is a schematic block diagram of an internal combustion engine in accordance with a third embodiment of the invention. 
         FIG. 23  is a graph for explaining operation of the internal combustion engine shown in  FIG. 22 . 
         FIG. 24  is a schematic block diagram of an internal combustion engine in accordance with a fourth embodiment of the invention. 
         FIG. 25  is a flowchart for explaining operation of the internal combustion engine shown in  FIG. 24 . 
         FIG. 26  is a schematic block diagram of a reformer in accordance with a fifth embodiment of the invention. 
         FIG. 27  is a schematic block diagram of a reformer in accordance with a modification example of the fifth embodiment of the invention. 
         FIG. 28  is a schematic block diagram of a reformer in accordance with another modification example of the fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of an internal combustion engine and a method of operating an internal combustion engine in accordance with the invention will be described in detail with reference to the drawings. 
     [First Embodiment] 
       FIG. 1  is a schematic diagram of an internal combustion engine in accordance with the first embodiment of the invention. An internal combustion engine  1  shown in  FIG. 1  generates power by burning a mixture containing fuel components in a plurality of combustion chambers  3  formed in an engine block  2  and reciprocating pistons  4  in the combustion chambers  3 . An intake pipe  5  and an exhaust pipe  6  are connected to each of the combustion chambers  3 . Each intake port is provided with an intake valve  7 , while each exhaust port is provided with an exhaust valve  8 . The internal combustion engine  1  has an ignition plug  9  for each of the combustion chambers  3 . 
     As shown in  FIG. 1 , the intake pipe  5  is connected to a surge tank  10  to which an air-supply pipe L 1  is connected. The air-supply pipe L 1  is connected to an air intake (not shown) via an air cleaner  11 . The air-supply pipe L 1  extends across an electronic throttle  12  (which is located between the surge tank  10  and the air cleaner  11 ). A bypass pipe L 2  branches off from the air-supply pipe L 1  at a bifurcated portion BP set between the air cleaner  11  and the electronic throttle  12 . The bypass pipe L 2  extends across a flow rate adjusting valve  14  whose leading end (which is not on the side of the bifurcated portion BP) is connected to a reformer  20 . 
     The reformer  20  includes an air-fuel mixing portion  21  to which the bypass pipe L 2  is connected, and a reforming reaction portion  22  adjacent to the air-fuel mixing portion  21 . In addition to the bypass pipe L 2 , a fuel injection device  15  for injecting a hydrocarbon fuel such as gasoline or the like is also connected to the air-fuel mixing portion  21 . A reforming catalyst which is obtained, for example, by having rhodium carried on zirconia is disposed in the reforming reaction portion  22 . An outlet of the reformer  20  is connected to the surge tank  10  via a connecting pipe L 3 . Thus, the reformer  20  is so disposed as to bypass the air-supply pipe L 1 . 
     On the other hand, an air flow meter  16  for detecting a total amount of air introduced from the air intake is installed upstream of the bifurcated portion BP of the air-supply pipe L 1  (more specifically, between the bifurcated portion BP and the air cleaner  11 ). The bypass pipe L 2  is equipped with an air flow meter  17  for detecting an amount of air flowing through the bypass pipe L 2 . The air flow meter  17  is installed between the bifurcated portion BP and the flow rate adjusting valve  14 . In addition, the connecting pipe L 3  is equipped with a temperature sensor  18  for detecting a temperature in the reformer  20  (i.e., a temperature of the reforming catalyst bed). The exhaust pipe  6  connected to each of the combustion chambers  3  is equipped with an exhaust gas A/F sensor  19  for detecting an air-fuel ratio of exhaust gas flowing through the exhaust pipe  6 . 
     The air flow meters  16  and  17 , the temperature sensor  18 , and the exhaust gas A/F sensor  19  are connected to an ECU  50 . The ECU  50  includes a CPU, a ROM, a RAM, input/output ports to which the aforementioned sensors are connected, a storage unit in which various pieces of information are stored, and the like. A signal indicating a depression stroke is delivered to the ECU  50  from an accelerator  51  (an accelerator position sensor), and a signal indicating an actual rotational speed of the internal combustion engine  1  is delivered to the ECU  50  from a rotational speed sensor  52  (a crank angle sensor). On the basis of values detected by the air flow meters  16  and  17 , the temperature sensor  18 , the exhaust gas A/F sensor  19  and the like, signals output from the accelerator  51  and the rotational speed sensor  52 , and the like, the ECU  50  controls openings of the electronic throttle  12  and the flow rate adjusting valve  14 , an amount of fuel injected from the fuel injection device  15 , a timing for igniting the ignition plug  9 , and the like. 
     In operating the internal combustion engine  1  thus constructed, air is introduced into the air-fuel mixing portion  21  of the reformer  20  via the flow rate adjusting valve  14  of the bypass pipe L 2 , and a fuel such as gasoline or the like is injected from the fuel injection device  15  controlled by the ECU  50 . The ECU  50  adjusts an opening of the flow rate adjusting valve  14 . The fuel such as gasoline or the like is gasified in the air-fuel mixing portion  21 , mixes with air flowing from the bypass pipe L 2 , and flows into the reforming reaction portion  22 . In the reforming reaction portion  22 , the reforming catalyst causes hydrocarbon fuel to react with air, and a partial oxidation reaction expressed by a formula (1) shown below progresses, whereby a reformed gas containing CO and H2 is produced. The reformed gas thus produced is introduced into the surge tank  10  from the outlet of the reformer  20  via the connecting pipe L 3 .
 
CmH n +( m /2)O2 →m CO+( n /2)H2  (1)
 
     Air is introduced into the surge tank  10  via the electronic throttle  12  of the air-supply pipe L 1 . The ECU  50  adjusts an opening of the electronic throttle  12 . Accordingly, the reformed gas introduced into the surge tank  10  from the reformer  20  is further mixed with air in the surge tank  10  and then is absorbed into the combustion chambers  3 . If the ignition plug  9  is ignited at a predetermined timing, CO and H2 as fuel components burn in the combustion chambers  3 , and the pistons  4  reciprocate, whereby power can be obtained from the internal combustion engine  1 . In the internal combustion engine  1 , power can also be obtained by stopping supplying the reformer  20  with air and fuel and causing a fuel injection device  15   x  attached to the intake pipe  5  to inject fuel. 
     In the internal combustion engine  1  mentioned above, for example, in view of a correlation between a reforming efficiency of the reformer  20  and an air-fuel ratio of the mixture in the reformer  20 , the ECU  50  sets the air-fuel ratio of the mixture of fuel and air in the reformer  20  as a substantially constant value such that the reforming efficiency of the reformer  20  is held within a desired range that is good for practical purposes. In the internal combustion engine  1 , therefore, the ratio between the fuel components CO and H2 in the reformed gas produced by the reformer  20  is always grasped accurately. Thereby, the internal combustion engine  1  can be so operated as to precisely comply with a required condition by supplying a desired reformed gas from the reformer  20  to the combustion chambers  3 . 
     The air-fuel ratio of the mixture in the reformer  20  and the reforming efficiency of the reformer  20  are correlated with each other as shown in  FIG. 2 .  FIG. 2  is a graph showing a relationship between the ratio O/C of the number of oxygen atoms in air to the number of carbon atoms in fuel supplied to the reformer  20  and the rate of fuel conversion in the reformer  20 . In this case, the ratio O/C is substantially equivalent to the air-fuel ratio of the mixture supplied to the reformer  20 . The rate of fuel conversion is a parameter for evaluating the reforming efficiency. As is apparent from  FIG. 2 , the rate of fuel conversion in the reformer  20  changes in accordance with the ratio O/C of the mixture in the reformer  20 . The rate of fuel conversion increases substantially in proportion to the ratio O/C until the ratio O/C reaches 1. However, once the ratio O/C exceeds 1, there is no recognizable increase in the rate of fuel conversion in the reformer  20 . 
     In this case, from a practical point of view, the reforming efficiency of the reformer  20 , namely, the rate of fuel conversion in the reformer  20  needs to be held equal to at least about 40%. This is because of the following reason. As is apparent from a correlation ( FIG. 3 ) between the ratio O/C of the mixture in the reformer  20  and the concentration of H2 contained in the reformed gas obtained from the reformer  20 , the rate of fuel conversion is approximately 40% when the ratio O/C is approximately 0.4, and the concentration of H2 contained in the reformed gas obtained from the reformer  20  drastically decreases if the ratio O/C drops below 0.4. Moreover, as is apparent from a correlation ( FIG. 4 ) between the ratio O/C of the mixture in the reformer  20  and the temperature of the catalyst bed in the reformer  20 , the temperature of the catalyst bed drops below 700° C. if the ratio O/C drops below 0.4, so that the possibility of coking in the reforming catalyst is enhanced. 
     As is apparent from  FIG. 3 , if the ratio O/C is about 0.8, the concentration of H2 obtained from the reformer  20  is substantially comparable with a peak concentration. Accordingly, it is more preferable from a practical point of view that the reforming efficiency of the reformer  20 , namely, the rate of fuel conversion in the reformer  20  be set as about 80% or more of a value in the case where the ratio O/C is about 0.8. 
     On the other hand, as is apparent from  FIG. 2 , if the ratio O/C exceeds 1, there is no recognizable increase in the rate of fuel conversion in the reformer  20 , and the concentration of H2 decreases correspondingly. As is apparent from  FIG. 4 , the temperature of the catalyst bed starts rising if the ratio O/C exceeds more or less 1.05, and the temperature of the catalyst bed abruptly rises once the ratio O/C exceeds 1.1. Accordingly, the upper limit of the ratio O/C for maintaining practically good reforming efficiency in the reformer  20  can be set as about 1.1, more preferably, about 1.05. 
     In consideration of the foregoing, in the internal combustion engine  1 , the ratio O/C of the number of oxygen atoms to the number of carbon atoms in fuel supplied to the reformer  20  is always set as a constant value (O/C=1 in the first embodiment) that is approximately within a range of 0.4 to 1.1, more preferably, within a range of 0.8 to 1.05. Thereby, the rate of fuel conversion in the reformer  20  can be made substantially constant approximately within a range of 40% to 100%, more preferably, within a range of 80% to 100%. 
     More specifically, the ECU  50  first sets a target torque of the internal combustion engine  1  (a predetermined target rotational speed during idling) corresponding to an accelerator depression stroke indicated by a signal output from the accelerator  51 . In addition, the ECU  50  sets an amount of air to be supplied to the reformer  20  (i.e., a reformed air supply amount) and an amount of fuel injected from the fuel injection device  15 , namely, an amount of the mixture supplied to the reformer  20 , such that the ratio O/C of the mixture in the reformer  20  becomes constant (O/C=1) and that an actual output torque (rotational speed) coincides with the target torque (target rotational speed). At the same time, the ECU  50  sets an amount of air to be supplied into the surge tank  10  from the electronic throttle  12  (i.e., an amount of air to be mixed) so as to equalize an air-fuel ratio of the mixture absorbed into the combustion chambers  3  with a desired value. 
     The ECU  50  then adjusts an opening of the flow rate adjusting valve  14  such that a value detected by the air flow meter  17  for the bypass pipe L 2  coincides with the reformed air supply amount determined previously, and causes the fuel injection device  15  to inject the previously determined amount of fuel. The ECU  50  also adjusts an opening of the electronic throttle  12  such that a difference between a value detected by the air flow meter  16  for the air-supply pipe L 1  and the value detected by the air flow meter  17  for the bypass pipe L 2  coincides with the previously determined amount of air to be mixed. Thereby, the reforming efficiency of the reformer  20  is held within a desired range, and the air-fuel ratio of the mixture in the combustion chambers  3  is set as a desired value. As a result, a desired torque can be generated by the internal combustion engine  1 . 
     In the internal combustion engine  1  mentioned above, it is not absolutely required that the flow rate adjusting valve  14  be disposed upstream of the reformer  20  (between the bifurcated portion BP and the reformer  20 ). That is, as shown in  FIG. 5 , the flow rate adjusting valve  14  may be disposed downstream of the reformer  20 . In other words, the connecting pipe L 3  connecting the reformer  20  to the surge tank  10  may be provided with the flow rate adjusting valve  14 . Adoption of this construction also makes it possible to supply the mixture of air and fuel to the reformer  20  with high precision. In this construction, by closing the flow rate adjusting valve  14 , unreformed HC that have increased in amount in the reforming reaction portion  22  can be prevented from being absorbed into the combustion chambers  3  when the reformer  20  is stopped during operation of the internal combustion engine  1 . 
     Moreover, in the internal combustion engine  1  mentioned above, the bifurcated portion BP as a junction between the air-supply pipe L 1  and the bypass pipe L 2  may be provided with a three-way flow rate adjusting valve  14   x  as shown in  FIG. 6 . In this case, the electronic throttle  12  is disposed upstream of the three-way flow rate adjusting valve  14   x  and downstream of the air flow meter  16 , and the air flow meter for the bypass pipe L 2  is dispensable. An amount of reformed air supplied to the reformer  20  from the bifurcated portion BP through the three-way flow rate adjusting valve  14  and an amount of air to be mixed into the surge tank  10  from the bifurcated portion BP through the three-way flow rate adjusting valve  14  are individually set by the ECU. This construction makes it possible to set an amount of air eventually supplied into the combustion chambers  3  only by the electronic throttle  12 . Therefore, the controllability of the entire engine can be enhanced. 
     As shown in  FIG. 7 , an air pump AP may be disposed between the bifurcated portion BP and the reformer  20 . In this case, the ECU sets an amount of electric power supplied to the air pump AP such that a value detected by the air flow meter  17  for the bypass pipe L 2  coincides with a required supply amount of reformed air. Adoption of this construction also makes it possible to supply the mixture of air and fuel to the reformer  20  with high precision. 
     Furthermore, as shown in  FIG. 8 , the air flow meter  16  for the air-supply pipe L 1  may be disposed downstream of the bifurcated portion BP (more specifically, between the bifurcated portion BP and the surge tank  10 ). In this case, the amount of air eventually supplied to the combustion chambers  3  is the sum of a value detected by the air flow meter  16  and a value detected by the air flow meter  17 . As shown in  FIG. 9 , it is also appropriate that the air flow meter for the air-supply pipe L 1  be eliminated, that a pressure sensor  13  be attached to the surge tank  10 , and that an amount (air-fuel ratio) of the mixture eventually supplied to the combustion chambers  3  be calculated on the basis of a value detected by the pressure sensor  13 . Adoption of these constructions also makes it possible to supply the mixture of air and fuel to the reformer  20  and the combustion chambers  3  with high precision. 
     As will be described in detail in relation to the second embodiment, since the temperature in the reformer  20  (the temperature of the catalyst bed) is correlated with the air-fuel ratio of the mixture in the reformer  20 , an air-fuel ratio (O/C) of the mixture in the reformer  20  can be estimated from a temperature in the reformer  20  (a temperature of the catalyst bed) which is detected by the temperature sensor  18 . Thus, the ECU  50  may also be designed to estimate a ratio O/C (air-fuel ratio) of the mixture in the reformer  20  on the basis of a value detected by the temperature sensor  18 , and to set an amount of the mixture supplied to the reformer  20  such that the estimated ratio O/C becomes equal to a constant value (e.g., O/C=1) within a range of 0.4 to 1.1, more preferably, within a range of 0.8 to 1.05 and corresponds to a target torque. Adoption of this construction makes it possible to dispense with the air flow meter for the bypass pipe L 2 , to lower the cost of the entire engine, and to reduce the volume of a required space. 
     [Second Embodiment] 
     Hereinafter, an internal combustion engine in accordance with the second embodiment of the invention will be described with reference to  FIGS. 10 to 21 . Components identical with those described in relation to the aforementioned first embodiment are denoted by the same reference numerals, and repetition of the same description will be avoided. 
     An internal combustion engine  1 A shown in  FIG. 10  is basically identical in construction with the internal combustion engine  1  in accordance with the aforementioned first embodiment. In the internal combustion engine  1 A in accordance with the second embodiment, however, the air flow meter disposed upstream of the reformer  20  is removed from the bypass pipe L 2  with a view to lowering the cost of the engine as a whole and reducing the volume of a required space. In the internal combustion engine  1 A, a pre-heater  23  such as an electric heater or the like is disposed at an upstream end portion of the reforming reaction portion  22  (i.e., at an end portion on the side of the air-fuel mixing portion  21 ). Hereinafter, it will be described individually and specifically how the internal combustion engine  1 A thus constructed operates when the engine is in an idling-off state, when the engine is in an idling state, when the reformer is started, and when the reformer is stopped. 
     (When Engine is in Idling-Off State) 
     As shown in  FIG. 11 , if a shift from an idling state to an idling-off state is made in response to delivery of a signal indicating an accelerator depression stroke to the ECU  50  from the accelerator  51 , the ECU  50  determines a target torque of the internal combustion engine  1 A corresponding to the signal delivered from the accelerator  51  (S 10 ). After having determined the target torque, the ECU  50  simultaneously sets an amount of air to be supplied to the reformer  20  (a reformed air supply amount), an amount of fuel injected from the fuel injection device  15 , and an opening of the electronic throttle  12  (S 12 ). 
     That is, in S 12 , the ECU  50  calculates a reformed air supply amount corresponding to the target torque determined in S 10  from an exemplary map shown in  FIG. 12 , and calculates an amount of fuel injected from the fuel injection device  15  in relation to the reformed air supply amount thus calculated such that the ratio O/C of the mixture in the reformer  20  becomes equal to 1. At the same time, in S 12 , the ECU  50  calculates an opening of the electronic throttle  12  corresponding to the target torque determined in S 10  from an exemplary map shown in  FIG. 13 . 
     The map for setting a reformed air supply amount as shown in  FIG. 12  is so prepared in advance as to define a relationship between the target torque of the engine and the amount of air to be supplied to the reformer  20  (reformed air supply amount), and is stored in the storage unit of the ECU  50 . The map for setting a throttle opening as shown in  FIG. 13  is so prepared in advance as to define an opening of the electronic throttle  12  for equalizing an air-fuel ratio of the mixture absorbed into the combustion chambers  3  with a desired value in accordance with a target torque, in consideration of the reformed air supply amount calculated from the map shown in  FIG. 12  and the fuel injection amount that is calculated such that the ratio O/C of the mixture in the reformer  20  becomes equal to 1. The map for setting a throttle opening is also stored in the storage unit of the ECU  50 . 
     Thereby, in the processing of S 12 , both an amount of the mixture supplied to the reformer  20  (i.e., reformed air supply amount+fuel injection amount) and an amount of the mixture absorbed into the combustion chambers  3  (i.e., reformed gas+air flowing from the electronic throttle  12 ) are simultaneously set in accordance with a target torque. The air-fuel ratio of the mixture in the reformer  20  is set substantially constant (O/C=1), and the air-fuel ratio of the mixture in the combustion chambers  3  is set, for example, as a desired value such as a stoichiometric air-fuel ratio or the like. 
     Then in S 14 , the ECU  50  sets an opening of the electronic throttle  12  as the opening calculated in S 12 , controls the flow rate adjusting valve  14  such that a value indicated by the air flow meter  16  for the air-supply pipe L 1  becomes equal to the sum of the reformed air supply amount calculated in S 12  and a flow amount corresponding to the opening of the electronic throttle  12  calculated in S 12 , and causes the fuel injection device  15  to inject the amount of fuel calculated in S 12 . In this case, in order to precisely set an air-fuel ratio of the mixture in the reformer  20 , it is preferable that fuel be injected from the fuel injection device  15  as soon as the supply of air from the flow rate adjusting valve  14  stabilizes. 
     After having performed the processing of S 14 , the ECU  50  proceeds to S 16  and executes a fuel injection amount correction routine ( FIG. 14 ) and a throttle opening correction routine ( FIG. 17 ), which will be described below. Thus, the air-fuel ratio of the mixture in the combustion chambers  3  can be set as a desired value while the reforming efficiency of the reformer  20  is held substantially constant, and an actual output torque can be precisely equalized with a target torque. While the engine remains in the idling-off state, the aforementioned processings of S 10  to S 16  are repeated. 
       FIG. 14  is a flowchart for explaining the fuel injection amount correction routine in S 16 . As shown in  FIG. 14 , after having performed the processing in S 14 , the ECU  50  calculates a temperature of the catalyst bed in the reforming reaction portion  22  of the reformer  20  on the basis of a signal received from the temperature sensor  18  attached to the connecting pipe L 3  (S 100 ). After having calculated the temperature of the catalyst bed in the reformer  20 , the ECU  50  estimates an air-fuel ratio (O/C) of the mixture in the reformer  20 , using the calculated temperature of the catalyst bed, the fuel injection amount determined in S 12 , and an exemplary map shown in  FIG. 15  (S 102 ). 
     The temperature of the catalyst bed is correlated with the ratio O/C of the mixture (air-fuel ratio) in the reformer  20 , and the temperature of the catalyst bed in the reforming reaction portion  22  changes in accordance with the ratio O/C of the mixture in the reformer  20 . The temperature of the catalyst bed also changes in accordance with the amount of fuel supplied to the reformer  20 . The temperature of the catalyst bed rises as the amount of fuel supplied to the reformer  20  increases. In view of the foregoing, in the internal combustion engine  1 A, the map ( FIG. 15 ) for defining (correcting) a correlation between the temperature of the catalyst bed and the ratio O/C of the mixture in the reformer  20  in accordance with an amount of fuel injected into the reformer  20  is prepared in advance and stored in the storage unit of the ECU  50 . By using such a map, a ratio O/C of the mixture (air-fuel ratio) in the reformer  20  can be calculated with high precision from a temperature of the catalyst bed in the reforming reaction portion  22  and an amount of fuel supplied to the reformer  20 . 
     After having estimated a ratio O/C of the mixture in the reformer  20 , the ECU  50  determines whether or not an estimated value of the ratio O/C is below a first threshold OCL determined in advance (S 104 ). The first threshold OCL is set, for example, as a value that is smaller than a target value of the ratio O/C determined in S 12  by a predetermined value (a predetermined percentage). If it is determined in S 104  that the estimated value of the ratio O/C is below the threshold OCL, the ECU  50  (slightly) reduces an amount of fuel injected from the fuel injection device  15  by a predetermined amount or an amount corresponding to a difference between the estimated value of the ratio O/C and the threshold OCL (S 106 ). Because the mixture in the reformer  20  is thereby made lean, the ratio O/C can be increased and made close to a target value (1 in the second embodiment). 
     On the other hand, if it is determined in S 104  that the estimated value of the ratio O/C is not below the predetermined value OCL, the ECU  50  further determines whether or not the estimated value of the ratio O/C is above a second threshold OCH determined in advance (S 108 ). The second threshold OCH is set, for example, as a value that is larger than the target value of the ratio O/C determined in S 12  by a predetermined amount (a predetermined percentage). If it is determined in S 108  that the estimated value of the ratio O/C is above the threshold OCH, the ECU  50  (slightly) increases an amount of fuel injected from the fuel injection device  15  by a predetermined amount or an amount corresponding to a difference between the estimated value of the ratio O/C and the threshold OCH (S 110 ). Because the mixture in the reformer  20  is thereby made rich, the ratio O/C can be made smaller than the threshold OCH and close to the target value (1 in the second embodiment). 
     Thus, in the internal combustion engine  1 A, the fuel injection amount correction routine is executed, and the amount of fuel supplied to the reformer  20  is suitably subjected to feedback correction using the estimated ratio O/C of the mixture in the reformer  20 . Therefore, the ratio O/C in the reformer  20  can be held constant, and the reforming efficiency can be excellently held within a desired range. After having terminated the processing of S 106  or S 110 , the ECU  50  performs the basic processings shown in  FIG. 11  again. If it is determined in S 108  that the estimated value of the ratio O/C is not above the second threshold OCH, the ECU  50  performs the basic processings shown in  FIG. 11  again without correcting a fuel injection amount. 
     As described above, it is in the case where the ratio O/C is held approximately within the range of 0.8 to 1.05 that the reforming efficiency of the reformer  20  is good for practical purposes. As is apparent from  FIG. 15 , however, if the ratio O/C is approximately within the range of 0.8 to 1.05, the fluctuation of the temperature of the catalyst bed is also held within a predetermined range. On the other hand, for example, if the ratio O/C is larger than about 1.05, a fluctuation amount ΔT of the temperature of the catalyst bed for each fluctuation of the ratio O/C (i.e., a fluctuation amount of the temperature of the catalyst bed before and after the processing in S 200 ) is larger than a predetermined value ΔT 1 . In view of this, in order to hold the reforming efficiency of the reformer  20  within a desired range, a fuel injection amount may also be corrected on the basis of a fluctuation amount ΔT of the temperature of the catalyst bed as shown in  FIG. 16 . 
     In an example shown in  FIG. 16 , after having performed the processing of S 14 , the ECU  50  temporarily (slightly) reduces an amount of fuel injected from the fuel injection device  15  and thereby makes the mixture in the reformer  20  lean (S 200 ). After having made the mixture in the reformer  20  lean, the ECU  50  calculates a fluctuation amount ΔT of the temperature of the catalyst bed after the processing of S 200  on the basis of a signal output from the temperature sensor  18  (S 202 ), and determines whether or not the calculated fluctuation amount ΔT of the catalyst is above the predetermined value ΔT 1  (S 204 ). If it is determined in S 204  that the fluctuation amount ΔT of the temperature of the catalyst bed is not above the predetermined value ΔT 1 , the ECU  50  repeats the processings of S 200  to S 204 . 
     On the other hand, if it is determined in S 204  that the fluctuation amount ΔT of the temperature of the catalyst bed is above the predetermined value ΔT 1 , the ECU  50  increases an amount of fuel injected from the fuel injection device  15  by a predetermined amount (a predetermined percentage) so as to make the mixture in the reformer  20  rich (S 206 ). After having made the mixture in the reformer  20  rich, the ECU  50  calculates a fluctuation amount ΔT of the temperature of the catalyst bed after the processing of S 206  on the basis of a signal output from the temperature sensor  18  (S 208 ), and determines whether or not the calculated fluctuation amount ΔT of the temperature of the catalyst bed is below the predetermined value ΔT 1  (S 210 ). If it is determined in S 210  that the fluctuation amount ΔT of the temperature of the catalyst bed is not below the predetermined value ΔT 1 , the ECU  50  repeats the processings of S 206  to S 210 . If it is determined in S 210  that the fluctuation amount ΔT of the temperature of the catalyst bed is below the predetermined value ΔT 1 , the ECU  50  returns to the basic processings shown in  FIG. 11 . If the fuel injection amount correction routine is executed according to the procedure described hitherto, the amount of fuel supplied to the reformer  20  is corrected such that the ratio O/C of the mixture in the reformer  20  is held at a substantially constant value that is not above an inflection point in the vicinity of O/C=1. Therefore, the reforming efficiency of the reformer  20  can be excellently held within a desired range. 
       FIG. 17  is a flowchart for explaining the throttle opening correction routine in S 16 . As shown in  FIG. 17 , after having performed the processing of S 14 , the ECU  50  calculates an air-fuel ratio in the combustion chambers  3  on the basis of a signal received from the exhaust gas A/F sensor  19  attached to the exhaust pipe  6  (S 300 ). After having calculated the air-fuel ratio in the combustion chambers  3 , the ECU  50  determines whether or not the calculated air-fuel ratio in the combustion chambers  3  is below a first threshold AF 1  determined in advance (S 302 ). The first threshold AF 1  is determined, for example, as a value that is smaller than a target air-fuel ratio in the combustion chambers  3  by a predetermined amount (a predetermined percentage). The target air-fuel ratio in the combustion chambers  3  is determined by an engine rotational speed, an engine load state, and the like. 
     If it is determined in S 302  that the air-fuel ratio in the combustion chambers  3  is below the threshold AF 1 , the ECU  50  (slightly) increases an opening of the electronic throttle  12  by a predetermined amount or an amount corresponding to a difference between the air-fuel ratio in the combustion chambers  3  and the threshold AF 1 , so as to make the mixture supplied to the combustion chambers  3  lean (S 304 ). On the other hand, if it is determined in S 302  that the air-fuel ratio in the combustion chambers  3  is not below the threshold AF 1 , the ECU  50  further determines whether or not the calculated air-fuel ratio in the combustion chambers  3  is above a second threshold AF 2  (S 306 ). The second threshold AF 2  is determined, for example, as a value that is larger than a target air-fuel ratio in the combustion chambers  3  by a predetermined value (a predetermined percentage). The target air-fuel ratio in the combustion chambers  3  is determined by an engine rotational speed, an engine load state, and the like. If it is determined in S 306  that the air-fuel ratio in the combustion chambers  3  is above the threshold AF 2 , the ECU  50  (slightly) reduces an opening of the electronic throttle  12  by a predetermined amount or an amount corresponding to a difference between the air-fuel ratio in the combustion chambers  3  and the threshold AF 2 , so as to make the mixture supplied to the combustion chambers  3  rich (S 308 ). 
     Thus, in the internal combustion engine  1 A, the throttle opening correction routine is executed, and the opening of the electronic throttle  12  is suitably subjected to feedback correction using an air-fuel ratio of the mixture in the combustion chambers  3 . Therefore, the air-fuel ratio of the mixture in the combustion chambers  3  can be precisely set as a desired value. After having terminated the processing in S 304  or S 308 , the ECU  50  performs the basic processings shown in  FIG. 11  again. If it is determined in S 306  that the air-fuel ratio in the combustion chambers  3  is not above the threshold AF 2 , the ECU  50  performs the basic processings shown in  FIG. 11  again without correcting a throttle opening. 
     (When Engine is in Idling State) 
     As shown in  FIG. 18 , when the internal combustion engine  1 A is in an idling state, the ECU  50  determines a target rotational speed of the internal combustion engine  1 A (S 20 ). After having determined the target rotational speed, the ECU  50  simultaneously sets an amount of air to be supplied to the reformer  20  (reformed air supply amount), an amount of fuel injected from the fuel injection device  15 , and an opening of the electronic throttle  12  (S 22 ). 
     In this case, in S 22 , the ECU  50  calculates a reformed air supply amount corresponding to the target rotational speed from a map (not shown) that is so prepared in advance as to define a relationship between the target rotational speed of the engine and the amount of air to be supplied to the reformer  20  (reformed air supply amount). In S 22 , the ECU  50  calculates an amount of fuel injected from the fuel injection device  15  in relation to the reformed air supply amount thus calculated such that the ratio O/C of the mixture in the reformer  20  becomes equal to 1. Moreover, in S 22 , the ECU  50  calculates an opening of the electronic throttle  12  from a map that is so prepared in advance as to define an opening of the electronic throttle  12  for equalizing an air-fuel ratio of the mixture absorbed into the combustion chambers  3  with a desired value in accordance with a target rotational speed, in consideration of a reformed air supply amount calculated from the map and a fuel injection amount that is calculated such that the ratio O/C of the mixture in the reformer  20  becomes equal to 1. 
     Thereby, in the processing of S 22 , both an amount of the mixture supplied to the reformer  20  (i.e., reformed air supply amount+fuel injection amount) and an amount of the mixture absorbed into the combustion chambers  3  (i.e., reformed gas+air flowing from the electronic throttle  12 ) are simultaneously set in accordance with a target rotational speed. The air-fuel ratio of the mixture in the reformer  20  is set substantially constant (O/C=1), and the air-fuel ratio of the mixture in the combustion chambers  3  is set as a desired value corresponding to an engine rotational speed, an engine load state, or the like. 
     In S 24 , the ECU  50  sets an opening of the electronic throttle  12  as the opening calculated in S 22 , controls the flow rate adjusting valve  14  such that a value indicated by the air flow meter  16  for the air-supply pipe L 1  becomes equal to the sum of the reformed air supply amount calculated in S 22  and a flow amount corresponding to the opening of the electronic throttle  12  calculated in S 22 , and causes the fuel injection device  15  to inject the amount of fuel calculated in S 22 . After having performed the processing of S 24 , the ECU  50  calculates an actual rotational speed of the internal combustion engine  1 A on the basis of a signal output from the rotational speed sensor  52 , and determines whether or not the calculated actual rotational speed is below a threshold Ne 1  that is lower than the target rotational speed determined in S 20  by a predetermined value (a predetermined percentage) (S 26 ). If it is determined in S 26  that the actual rotational speed is below the threshold Ne 1 , the ECU  50  controls the flow rate adjusting valve  14  so as to increase a reformed air supply amount (S 28 ). In this case, the fuel injection amount is calculated in S 22  such that the ratio O/C of the mixture in the reformer  20  becomes equal to 1. Therefore, if the reformed air supply amount increases, the fuel injection amount also increases correspondingly. In order to maintain the air-fuel ratio of the mixture in the combustion chambers  3  at a desired value, the ECU  50  adjusts an opening of the electronic throttle  12  in accordance with an increase in reformed air supply amount and an increase in fuel injection amount. 
     If it is determined in S 26  that the actual rotational speed is not below the threshold Ne 1 , the ECU  50  further determines whether or not the calculated actual rotational speed is above a threshold Ne 2  that is higher than the target rotational speed determined in S 20  by a predetermined value (a predetermined percentage) (S 30 ). If it is determined in S 30  that the actual rotational speed is above the threshold Ne 2 , the ECU  50  controls the flow rate adjusting valve  14  so as to reduce a reformed air supply amount (S 32 ). In this case as well, the fuel injection amount is calculated in S 22  such that the ratio O/C of the mixture in the reformer  20  becomes equal to 1. Therefore, if the reformed air supply amount decreases, the fuel injection amount also decreases correspondingly. In order to maintain the air-fuel ratio of the mixture in the combustion chambers  3  at a desired value, the ECU  50  adjusts an opening of the electronic throttle  12  in accordance with a decrease in reformed air supply amount and a decrease in fuel injection amount. On the other hand, if it is determined in S 30  that the actual rotational speed is not above the threshold Ne 2 , the reformed air supply amount is not corrected. While the engine remains in the idling state, the aforementioned processings of S 20  to S 32  are repeated. Thereby, even when the internal combustion engine  1 A is in the idling state, an actual output torque can be precisely equalized with a target torque while the reforming efficiency of the reformer  20  is held substantially constant. 
     (When the Reformer is Started) 
     When the supply of the mixture to the reformer  20  is started to start the internal combustion engine  1 A, the reforming catalyst in the reforming reaction portion  22  is often not activated sufficiently. Hence, in starting the supply of the mixture to the reformer  20 , the ECU  50  first operates the pre-heater  23  disposed at the upstream end of the reforming reaction portion  22 , as shown in  FIG. 19 . Thus, the temperature of the catalyst bed in the reforming reaction portion  22  of the reformer  20  gradually rises. After the pre-heater  23  has started to operate, the ECU  50  monitors a temperature of the catalyst bed (a temperature in the reformer  20 ) detected by the temperature sensor  18 . 
     If it is determined on the basis of a signal output from the temperature sensor  18  that the temperature of the catalyst bed has reached a predetermined temperature Ta, the ECU  50  starts the supply of air to the reformer  20  from the flow rate adjusting valve  14  and the injection of fuel from the fuel injection device  15 . In this case, as shown in  FIG. 19 , the ECU  50  controls the flow rate adjusting valve  14  and the fuel injection device  15  such that the ratio O/C of the mixture in the reformer  20  becomes equal to a predetermined value that is larger than 1. 
     Thus, in the internal combustion engine  1 A, when the supply of the mixture to the reformer  20  is started, the mixture in the reformer  20  is made leaner (O/C&gt;1) than the mixture during steady operation (O/C=1). Thus, a complete oxidation reaction expressed by a formula (2) shown below is likely to progress in the reforming reaction portion  22 . The complete oxidation reaction generates a larger amount of heat than the partial oxidation reaction expressed by the formula (1) mentioned above.
 
CmH n +( m+n /4)O2 →+m CO2+( n /2)H2O  (2)
 
     As a result, in the internal combustion engine  1 A, ignition of fuel in the reforming catalyst is promoted immediately after the supply of the mixture to the reformer  20  has been started. As shown in  FIG. 19 , the reforming catalyst can be heated up to an activation temperature Tb in a short period by increasing a degree of rise (change amount) in temperature of the catalyst. Adoption of this construction makes it possible to stop preheating (i.e., to shorten a preheating period) or to lower a preheating temperature (i.e., to reduce the amount of electric power supplied to the pre-heater  23 ) before the reforming catalyst reaches the activation temperature Tb as shown in  FIG. 19 . 
     If it is determined on the basis of a signal output from the temperature sensor  18  that the temperature of the catalyst bed has reached the predetermined temperature Tb, the ECU  50  sets the ratio O/C of the mixture in the reformer  20  as 1, which is a value corresponding to steady operation of the engine. Thus, the reforming efficiency of the reformer  20  is held within a desired range, and the reforming catalyst is inhibited from being heated up excessively. 
     (When the Reformer is Stopped) If the supply of the mixture to the reformer  20  is stopped to stop the internal combustion engine  1 A or to operate the internal combustion engine  1 A by causing the fuel injection device  15   x  attached to the intake pipe  5  to inject fuel during so-called cold start, the amount of oxygen in the reforming reaction portion  22  of the reformer  20  may temporarily become excessive, so that the temperature of the catalyst bed may abruptly rise to the extent of deteriorating the reforming catalyst. To prevent this phenomenon, the supply of the mixture to the reformer  20  is stopped in the internal combustion engine  1 A according to a procedure shown in  FIGS. 20 and 21 . 
     In this case, upon receiving a command to stop supplying the mixture to the reformer  20 , the ECU  50  first closes the flow rate adjusting valve  14  completely and stops the supply of air via the bypass pipe  12  completely (S 40 ). After having closed the flow rate adjusting valve  14 , the ECU  50  determines on the basis of a signal output from the temperature sensor  18  whether or not a temperature T of the catalyst bed in the reforming reaction portion  22  is below a predetermined temperature T 1  (S 42 ). If it is determined in S 42  that the temperature T of the catalyst bed is below the predetermined temperature T 1 , the ECU  50  stops the fuel injection device  15  and stops supplying fuel to the reformer  20  (S 44 ). 
     Thus, in the internal combustion engine  1 A, the supply of air is stopped before the supply of fuel to the reformer  20  is stopped. Thereby, the amount of oxygen in the reformer  20  can be prevented from becoming excessive, and the reaction in the reforming reaction portion  22  can be inhibited from progressing. Also, the reforming catalyst (the reformer  20 ) can be cooled and be reliably inhibited from deteriorating as a result of an abrupt rise in temperature. In the second embodiment, the flow rate adjusting valve  14  is completely closed and the supply of air via the bypass pipe L 2  is completely stopped in S 40 . However, the invention is not limited to this construction. Namely, the processing of S 40  has only to reduce the amount of air supplied to the reformer  20 . For example, it is appropriate that the amount of air to be supplied be reduced to about 50% of an amount of previously supplied air. 
     After having stopped supplying fuel to the reformer  20  in S 44 , the ECU determines on the basis of a signal output from the temperature sensor  18  whether or not the temperature T of the catalyst bed in the reforming reaction portion  22  is below a predetermined temperature T 2  (T 2 &lt;T 1 ) (S 46 ). Even if the supply of fuel has been stopped in S 44 , the amount of oxygen in the reformer  20  may become excessive, and the temperature T of the catalyst bed may rise as an unintended effect as indicated by an alternate long and short dash line in  FIG. 21 . Hence, if it is determined in S 46  that the temperature T of the catalyst bed is not below the predetermined temperature T 2 , the ECU  50  further determines whether or not the temperature T of the catalyst bed is above a predetermined temperature T 3  (T 3 &gt;T 1 ) (S 48 ). 
     If it is determined in S 48  that the temperature T of the catalyst bed is above the predetermined temperature T 3 , the ECU  50  again causes the fuel injection device  15  to inject a predetermined amount of fuel (S 50 ). Thus, even after the supply of fuel to the reformer  20  has been stopped temporarily, the amount of oxygen in the reformer  20  is excessive if the temperature T of the catalyst bed has risen. Therefore, by resuming the supply of fuel to the reformer  20  and thus counterbalancing excessiveness of oxygen in the reformer  20 , the temperature T of the catalyst bed can be inhibited from further rising. Consequently, the cooling of the reforming catalyst and thus the entire reformer  20  can be promoted. 
     After having resumed the supply of fuel to the reformer  20  in S 50 , the ECU  50  determines whether or not the temperature T of the catalyst bed is below the predetermined temperature T 1  (S 42 ). If it is determined that the temperature T of the catalyst bed is below the predetermined temperature T 1 , the ECU  50  stops the fuel injection device  15  and stops supplying fuel to the reformer  20  (S 44 ). If it is determined in S 48  that the temperature T of the catalyst bed is not above the predetermined temperature T 3 , the ECU  50  determines again in S 46  whether or not the temperature T of the catalyst bed is below the predetermined temperature T 2 . 
     If it is determined in S 46  that the temperature T of the catalyst bed is below the predetermined temperature T 2 , the ECU  50  sets the opening of the flow rate adjusting valve  14  as an opening determined in advance, and resumes the supply of air to the reformer  20  from the bypass pipe L 2  (S 52 ). Thereby, the mixture remaining in the reformer  20  can be purged. After having opened the flow rate adjusting valve  14  again, the ECU  50  further determines on the basis of a signal output from the temperature sensor  18  whether or not the temperature T of the catalyst bed in the reforming reaction portion  22  is below a predetermined temperature T 5  (T 5 &lt;T 2 ) (S 54 ). 
     Even if the supply of air has been resumed in S 52 , the amount of oxygen in the reformer  20  may become excessive and the temperature T of the catalyst bed may rise as an unintended effect as indicated by an alternate long and two short dashes line in  FIG. 21 . Hence, if it is determined in S 54  that the temperature T of the catalyst bed is not below the predetermined temperature T 5 , the ECU  50  further determines whether or not the temperature T of the catalyst bed is above the predetermined temperature T 4  (T 4 &gt;T 2 ) (S 56 ). 
     If it is determined in S 56  that the temperature T of the catalyst bed is above the predetermined temperature T 4 , the ECU  50  reduces an opening of the flow rate adjusting valve  14  and reduces an amount of air supplied to the reformer  20  by a predetermined amount (S 58 ). Thus, if the temperature T of the catalyst bed has risen again after resumption of the supply of air to the reformer  20 , a further rise in the temperature T of the catalyst bed can be suppressed by reducing an amount of air supplied to the reformer  20 . Consequently, the cooling of the reforming catalyst and thus the entire reformer  20  can be promoted. 
     After having reduced an amount of air supplied to the reformer  20  in S 58 , the ECU  50  determines whether or not the temperature T of the catalyst bed is below the predetermined temperature T 2  (S 46 ). If it is determined that the temperature T of the catalyst bed is below the predetermined temperature T 2 , the ECU  50  sets the opening of the flow rate adjusting valve  14  as an opening determined in advance (S 52 ). If it is determined in S 56  that the temperature T of the catalyst bed is not above the predetermined temperature T 4 , the ECU  50  determines again in S 54  whether or not the temperature T of the catalyst bed is below the predetermined temperature T 5 . If it is determined in S 54  that the temperature T of the catalyst bed is below the predetermined temperature T 5 , the ECU  50  completely closes the flow rate adjusting valve  14  and stops supplying air to the reformer  20  from the bypass line  12  (S 60 ), whereby the reformer  20  is stopped completely. 
     (Third Embodiment) 
     Hereinafter, an internal combustion engine in accordance with the third embodiment of the invention will be described with reference to  FIGS. 22 and 23 . Components identical with those described in relation to the aforementioned first embodiment are denoted by the same reference numerals, and repetition of the same description will be avoided. 
     An internal combustion engine  1 B shown in  FIG. 22  is basically identical in construction with the internal combustion engine  1 A in accordance with the aforementioned second embodiment. The internal combustion engine  1 B in accordance with the third embodiment is constructed such that exhaust gas (inactive gas) flowing from the combustion chambers  3  can be recirculated to the reformer  20 . That is, an exhaust gas recirculation pipe L 4  branches off from the exhaust pipe  6  connected to each of the combustion chambers  3  at a position downstream of the exhaust gas A/F sensor  19 . A flow rate adjusting valve  14 B extends across the exhaust gas recirculation pipe L 4 . The exhaust gas recirculation pipe L 4  is connected at its leading end to the bypass pipe L 2  between the flow rate adjusting valve  14  and the reformer  20 . The ECU  50  controls the flow rate adjusting valve  14 B for the exhaust gas recirculation pipe L 4  as well as the flow rate adjusting valve  14  for the bypass pipe L 2 . The exhaust gas recirculation pipe L 4 , the flow rate adjusting valve  14 B, and the ECU  50  function as an exhaust gas recirculation portion for causing exhaust gas flowing from the combustion chambers  3  to recirculate to the reformer  20 . 
     In the thus-constructed internal combustion engine  1 B of the third embodiment as well, as is the case with the aforementioned internal combustion engines  1  and  1 A, the ECU  50  sets the air-fuel ratio (O/C) of the mixture in the reformer  20  as a constant value (O/C=1) such that the reforming efficiency of the reformer  20  is held within a desired range. However, various factors may bring about a probability that fuel and air will not mix with each other sufficiently in the reformer  20  and that a reforming reaction will not occur at the reforming efficiency within the aforementioned desired range. 
     In view of this, in the internal combustion engine  1 B in accordance with the third embodiment, the flow rate adjusting valve  14 B for the exhaust gas recirculation pipe L 4  is always opened at a predetermined opening, and exhaust gas flowing from the combustion chambers  3  is recirculated to the air-fuel mixing portion  21  of the reformer  20 . Thereby, in the internal combustion engine  1 B, fuel and air can be mixed well with each other, and the interior of the reforming reaction portion  22  can be suitably heated up. Therefore, the reforming efficiency of the reformer  20  can be excellently held within the aforementioned desired range. 
     On the other hand, the temperature T of the catalyst bed in the reformer  20  may abruptly change in response to a change in load or the like. An abrupt change in temperature of the reformer  20  may bring about a probability that the reforming efficiency within the aforementioned desired range will not be achieved or that the temperature T of the catalyst bed will rise excessively. Hence, as shown in  FIG. 23 , the ECU  50  always monitors the temperature T of the catalyst bed in the reforming reaction portion  22  on the basis of a signal output from the temperature sensor  18 . 
     Namely, while the reformer  20  is in operation, the ECU  50  determines on the basis of a signal output from the temperature sensor  18  whether or not the temperature T of the catalyst bed is above a predetermined temperature Tx (S 70 ). If it is determined in S 70  that the temperature T of the catalyst bed is above the predetermined temperature Tx, the ECU  50  increases an opening of the flow rate adjusting valve  14 B for the exhaust gas recirculation pipe L 4  by a predetermined amount, thus increasing an amount of exhaust gas recirculated from the combustion chambers  3  to the reformer  20  (S 72 ). Thus, in the internal combustion engine  1 B, even if the temperature T of the catalyst bed has abruptly risen due to an abrupt change in load or the like, a further rise in the temperature T of the catalyst bed can be suppressed by utilizing exhaust gas (inert gas) recirculated from the combustion chambers  3  to the reformer  20 . As a result, the reforming catalyst and thus the entire reformer  20  can be cooled. 
     If it is determined in S 70  that the temperature T of the catalyst bed is not above the predetermined temperature Tx, the ECU  50  determines whether or not the temperature T of the catalyst bed is below a predetermined temperature Ty (Ty&lt;Tx) (S 74 ). If it is determined in S 74  that the temperature T of the catalyst bed is below the predetermined temperature Ty, the ECU  50  reduces an opening of the flow rate adjusting valve  14 B for the exhaust gas recirculation pipe L 4  by a predetermined amount, thus reducing an amount of exhaust gas recirculated from the combustion chambers  3  to the reformer  20  (S 76 ). Thereby, recirculation of more than a required amount of exhaust gas from the combustion chambers  3  to the reformer  20  can be prevented, and the reforming efficiency of the reformer  20  can always be held within the aforementioned desired range. 
     If it is determined in S 74  that the temperature T of the catalyst bed is not below the predetermined temperature Ty, namely, if the temperature T of the catalyst bed is equal to or higher than Ty and equal to or lower than Tx, the opening of the flow rate adjusting valve  14 B for the exhaust gas recirculation pipe L 4  is not changed but held at an opening corresponding to steady operation of the engine. Thus, the reforming efficiency of the reformer  20  is reliably held within the aforementioned desired range. 
     In the third embodiment, if the supply of the mixture to the reformer  20  is stopped to stop the internal combustion engine  1 B or to operate the internal combustion engine  1 B by causing the fuel injection device  15   x  attached to the intake pipe  5  to inject fuel during so-called cold start, the amount of exhaust gas recirculated from the combustion chambers  3  to the reformer  20  is increased. 
     That is, upon receiving a command to stop supplying the mixture to the reformer  20 , the ECU  50  of the internal combustion engine  1 B increases an opening of the flow rate adjusting valve  14 B for the exhaust gas recirculation pipe L 4  by a predetermined amount from an opening corresponding to steady operation of the engine. Thereby, when the supply of the mixture to the reformer  20  is stopped, the amount of oxygen in the reformer  20  can be prevented from becoming excessive, the occurrence of a reaction in the reforming reaction portion  22  can be suppressed, and the reforming catalyst (the reformer) can be cooled. Thus, the reforming catalyst can be reliably inhibited from deteriorating due to an abrupt rise in temperature. 
     (Fourth Embodiment) 
     Hereinafter, an internal combustion engine in accordance with the fourth embodiment of the invention will be described with reference to  FIGS. 24 and 25 . Components identical with those described in relation to the aforementioned first embodiment are denoted by the same reference numerals, and repetition of the same description will be avoided. 
     An internal combustion engine  1 C shown in  FIG. 24  is basically identical in construction with the internal combustion engine  1 A in accordance with the aforementioned second embodiment. However, the internal combustion engine  1 C in accordance with the fourth embodiment includes a reformer  20 C that is different from the ones provided in the aforementioned internal combustion engines  1 ,  1 A and  1 B. As shown in  FIG. 24 , the reformer  20 C has a plurality of reforming reaction portions  22   a  and  22   b  that are disposed along a flow direction of the mixture, and an air supply portion  24  in which no reforming catalyst is disposed is defined between the reforming reaction portions  22   a  and  22   b.    
     A branch pipe L 5  branches off from the bypass line L 2  at a position between the bifurcated portion BP connected to the air-supply pipe L 1  and the flow rate adjusting valve  14 . A flow rate adjusting valve  14 C extends across the branch pipe L 5 , which is connected at its leading end to the air supply portion  24  between the reforming reaction portions  22   a  and  22   b . The ECU  50  controls the flow rate adjusting valve  14 C for the branch pipe L 5  as well as the flow rate adjusting valve  14  for the bypass pipe L 2 . The branch pipe L 5 , the flow rate adjusting valve  14 C, and the ECU  50  function as means for supplying air (oxygen) to the air supply portion  24  set between the reforming reaction portions  22   a  and  22   b.    
     In the internal combustion engine  1 C thus constructed, an amount of air supplied to the reformer  20 C (reformed air supply amount) via the bypass pipe L 2  and the branch pipe L 5  and an amount of fuel injected from the fuel injection device  15 , namely, an amount of the mixture supplied to the reformer  20 C are set such that the ratio O/C of the mixture in the entire reformer  20 C becomes equal to a constant value (O/C=1) and that an actual output torque (rotational speed) coincides with a target torque (target rotational speed). 
     Because a reforming reaction progresses in an extremely short period, the temperature in the reformer (the temperature of the catalyst bed) tends to be high on the upstream side with respect to a flow direction of the mixture. The stronger this tendency becomes, the more the reforming catalyst on the upstream side with respect to the flow direction deteriorates. If air and fuel have not mixed with each other upstream of the reformer (reforming catalyst), the complete oxidation reaction expressed by the aforementioned formula (2) progresses in a range corresponding to an excessive amount of oxygen, so that the reforming catalyst is heated up. In a range corresponding to an excessive amount of fuel, the reforming reaction does not progress sufficiently, so that unreformed HC may be directly mixed with reformed gas. 
     In view of this, in the internal combustion engine  1 C in accordance with the fourth embodiment, the mixture supplied to the reforming reaction portion  22   a  disposed upstream of the air supply portion  24  with respect to the flow direction is made rich. Namely, in the internal combustion engine  1 C, the ECU  50  suitably adjusts openings of the flow rate adjusting valve  14  for the bypass pipe L 2  and the flow rate adjusting valve  14 C for the branch pipe L 5 , such that the ratio O/C of the mixture in the air-fuel mixing portion  21  of the reformer  20 C becomes smaller than 1. Thereby, the reforming catalyst in the reforming reaction portion  22   a  upstream of the air supply portion  24  can be inhibited from being heated up excessively. 
     In this case, unreformed HC remain in the reforming reaction portion  22   a  upstream of the air supply portion  24 . The unreformed HC are mostly gasified in the reforming reaction portion  22   a  that has been heated up, mix with oxygen in the air supply portion  24 , and are suitably reformed in the reforming reaction portion  22   b  downstream of the air supply portion  24 . Accordingly, the internal combustion engine  1 C makes it possible to suppress a local deterioration of the reforming catalyst, to reliably reform fuel, and to excellently hold the reforming efficiency of the reformer  20 C within a desired range. 
     On the other hand, as described above, if the air-fuel ratio of the mixture in the reforming reaction portion  22   a  upstream of the air supply portion  24  is made rich, the amount of fuel contained in the atmosphere in the reforming reaction portion  22   a  on the upstream side becomes excessive, so that coking of the reforming catalyst may be caused in the reforming reaction portion  22   a . In view of this, in the internal combustion engine  1 C, if a predetermined operational condition is fulfilled even in the case where air is supplied from the branch pipe L 5  to the air supply portion  24  of the reformer  20 C, the mixture is made lean in the reforming reaction portion  22   a  upstream of the air supply portion  24 . 
     That is, as shown in  FIG. 25 , the ECU  50  of the internal combustion engine  1 C always determines whether or not coking may be caused in the reforming reaction portion  22   a  upstream of the air supply portion  24  (S 80 ). In this case, the presence or absence of coking can be determined from an operating period of the reformer  20 C or the like, for example, after a temperature of the catalyst bed, an amount of air supplied to the reforming reaction portion  22   a , an amount of fuel supplied to the reforming reaction portion  22   a , and the like have been grasped. 
     If it is determined in S 80  that coking may be caused in the reforming reaction portion  22   a , the ECU  50  suitably sets openings of the flow rate adjusting valve  14  for the bypass pipe L 2  and the flow rate adjusting valve  14 C for the branch pipe L 5  such that the ratio O/C of the mixture in the air-fuel mixing portion  21  of the reformer  20 C remains above 1 for a predetermined period (S 82 ). Thereby, the amount of oxygen contained in the reforming reaction portion  22   a  is made temporarily excessive. As a result, the occurrence of coking can be suppressed, and the reforming catalyst in which coking has been caused can be recovered. If it is determined in S 80  that the occurrence of coking is unlikely, the ratio O/C of the mixture in the air-fuel mixing portion  21  is set smaller than a usual value of 1 so as to make the mixture in the air-fuel mixing portion  21  of the reformer  20 C rich (S 84 ). 
     In the fourth embodiment, the amount of the catalyst carried in the reforming reaction portion  22   a  on the upstream side may be made smaller than the amount of the catalyst carried in the reforming reaction portion  22   b  on the downstream side. If honeycomb materials on which the reforming catalyst is carried are disposed in the reforming reaction portions  22   a  and  22   b , the number of cells in the honeycomb material disposed in the reforming reaction portion  22   a  on the upstream side may be made smaller than the number of cells in the honeycomb material disposed in the reforming reaction portion  22   b  on the downstream side. Furthermore, the reforming reaction portion  22   a  may be provided with a pre-heater. 
     (Fifth Embodiment) 
     Hereinafter, an internal combustion engine in accordance with the fifth embodiment of the invention will be described with reference to  FIGS. 26 to 28 . Components identical with those described in relation to the aforementioned first embodiment are denoted by the same reference numerals, and repetition of the same description will be avoided. 
       FIG. 26  is a schematic block diagram of a reformer  200  that can be applied to the internal combustion engines  1 ,  1 A and  1 B in accordance with the aforementioned first to third embodiments. In the reformer  200  shown in  FIG. 26 , the reforming reaction portion  22  is divided into a plurality of regions  22   x ,  22   y  and  22   z  in a flow direction of the mixture. A honeycomb material on which a reforming catalyst is carried is disposed on each of the regions  22   x ,  22   y  and  22   z . In the fifth embodiment, the amount of the reforming catalyst carried on the honeycomb material in the region  22   x  on the upstream side with respect to the flow direction of the mixture and the amount of the reforming catalyst carried on the honeycomb material in the region  22   z  on the downstream side with respect to the flow direction of the mixture are smaller than the amount of the reforming catalyst carried on the honeycomb material in the region  22   y  between the regions  22   x  and  22   y.    
     Thus, in the reformer  200  in accordance with the fifth embodiment, the catalyst small-amount-carriage regions  22   x  and  22   y  are included in the reforming reaction portion  22  at its upstream and downstream ends with respect to the flow direction of the mixture respectively, while the amount of the reforming catalyst disposed in each of the catalyst small-amount-carriage regions  22   x  and  22   y  is smaller than the amount of the reforming catalyst disposed in the region  22   y . Thereby, while the reformer  200  is in operation, the progress of a reforming reaction in the region  22   x  on the upstream side of the reforming reaction portion  22  is suppressed. Therefore, an excessive rise in temperature of the reforming catalyst, which generally tends to be caused on the upstream side of the reformer, can be suppressed. 
     In the reformer  200 , the region  22   x  at the upstream end of the reforming reaction portion  22  receives heat from the region  22   y  on the downstream side and is suitably heated up. Thus, fuel supplied to the air-fuel mixing portion  21  of the reformer  200  can be gasified and mixed well with air in the region  22   x  on the upstream side. Moreover, in the reformer  200 , reaction heat generated in the region  22   y  that is not a catalyst small-amount-carriage region is also transferred to the region  22   z  on the downstream side of the reforming reaction portion  22 . Therefore, the heat radiation performance of the reformer  200  can be improved as a whole, and the reforming catalyst can be effectively inhibited from being heated up. 
     In the reformer  200  described above, both the region  22   x  on the upstream side of the reforming reaction portion  22  and the region  22   z  on the downstream side of the reforming reaction portion  22  are designed as catalyst small-amount-carriage regions. However, the invention is not limited to this construction. That is, as in the case of a reformer  200 A shown in  FIG. 27 , it is also appropriate that only the region  22   x  on the upstream side of the reforming reaction portion  22  be designed as a catalyst small-amount-carriage region. As in the case of a reformer  200 B shown in  FIG. 28 , it is also appropriate that only the region  22   z  on the downstream side of the reforming reaction portion  22  be designed as a catalyst small-amount-carriage region. Adoption of these constructions also makes it possible to suppress an excessive rise in temperature of the reforming catalyst which is likely to be caused on the upstream side of the reformer  200 A or  200 B, to improve the heat radiation performance of the reformer  200 A or  200 B as a whole, and to effectively inhibit the reforming catalyst from being heated up. 
     In the reformers  200 ,  200 A and  200 B of the fifth embodiment, the amount of the catalyst carried in the catalyst small-amount-carriage regions may be made zero through a construction wherein honeycomb materials having no catalyst carried thereon are disposed in the regions  22   x  and  22   z  of the reforming reaction portion  22 . Adoption of this construction also makes it possible to suppress an excessive rise in temperature of the reforming catalyst which is likely to be caused on the upstream side of the reformer, to improve the heat radiation performance of the reformer as a whole, and to effectively inhibit the reforming catalyst from being heated up. In addition, if the above-mentioned reformers  200 ,  200 A and  200 B are provided with the air supply portion described in relation to the third embodiment, they can be applied to the above-mentioned internal combustion engine  1 C.