Patent Publication Number: US-2021189953-A1

Title: Internal combustion engine

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/316,513 filed Jan. 9, 2019, which is a national stage application pursuant to 35 U.S.C. § 371 of International Application No. PCT/JP2017/024047 filed Jun. 29, 2017, which claims priority under 35 U.S.C. § 119 to JP Application No. 2016-139574 filed Jul. 14, 2016, the disclosures of which are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an internal combustion engine having a fuel reformation cylinder for reforming fuel and an output cylinder for yielding an engine power by combustion of fuel or reformed fuel. 
     BACKGROUND ART 
     For example, Patent Literature 1 (Hereinafter, PTL 1) describes an internal combustion engine having a fuel reformation cylinder for reforming fuel and an output cylinder for obtaining an engine power by combustion of fuel or reformed fuel. 
     Specifically, a fuel such as light oil or heavy oil is supplied to the fuel reformation cylinder, and an air-fuel mixture having a high equivalence ratio is adiabatically compressed within the fuel reformation cylinder. As a result, the fuel is reformed under a high temperature and high pressure environment, and a reformed fuel (fuel with a high octane value) having a high anti-knock property such as hydrogen, carbon monoxide, and methane is generated. This reformed fuel is then supplied to the output cylinder together with the air, and the lean mixture is combusted (uniform lean combustion) in the output cylinder, to yield an engine power. 
     With this type of internal combustion engine, uniform lean combustion is performed in the output cylinder. The NOx emission amount and the soot discharge amount can therefore be reduced. Further, since a fuel with a high anti-knock property is combusted, knocking is suppressed or reduced, and since diesel micro-pilot ignition enables combustion at a suitable timing, the combustion efficiency can be also improved. 
     CITATION LIST 
     PTL 1: Japanese Patent Application Laid-Open No. 2014-136978 
     SUMMARY OF INVENTION 
     The present inventors have found that, when reforming fuel in a fuel reformation cylinder, a light gas concentration in a reformed gas increases proportionally as the temperature of a reaction gas increases (see  FIG. 8 ). In other words, the inventors have found that the higher the temperature of the reaction gas, the higher the reforming efficiency of the fuel becomes, and thus the present invention has been proposed based on this finding. 
     It is an object of the present invention to improve fuel reforming efficiency as much as possible in an internal combustion engine having a fuel reformation cylinder for reforming fuel and an output cylinder for yielding an engine power by combustion of fuel or reformed fuel. 
     An aspect of the present invention is an internal combustion engine including a fuel reformation cylinder for reforming a fuel and an output cylinder for yielding an engine power by combusting fuel or reformed fuel, wherein at least a part of a surface constituting a volume-variable reaction chamber of the fuel reformation cylinder has a highly heat-insulative material. 
     This structure can reduce heat radiation from the reaction chamber to the outside, i.e., reduce the heat loss from the reaction chamber, when the fuel is reformed in the reaction chamber. 
     Accordingly, the temperature during the reforming reaction of fuel in the reaction chamber can be maintained higher as compared to a case where the highly heat-insulative material is not employed. Therefore, the reforming efficiency of the fuel can be improved as compared with the case where the highly heat-insulative material is not employed. 
     Further, the surfaces constituting the reaction chamber are preferably an inner circumferential surface of the fuel reformation cylinder and a top surface of a piston housed in the fuel reformation cylinder in a cylinder block, and a blast surface covering the fuel reformation cylinder in a cylinder head, wherein at least one of these surfaces is made of the highly heat-insulative material. 
     The above specifies that the surface constituting the reaction chamber exists in a plurality of separate members, and specifies that a surface made of the highly heat-insulative material. 
     More specifically, the present invention encompasses a mode of forming a highly heat-insulative material on all the surfaces constituting the reaction chamber; a mode of forming a highly heat-insulative material on one of the inner circumferential surface of the fuel reformation cylinder and the top surface of the piston in the cylinder block, and the blast surface of the cylinder head; and a mode of forming a highly heat-insulative material on any two of the inner circumferential surface of the fuel reformation cylinder and the top surface of the piston hosed in the cylinder block, and the blast surface of the cylinder head covering the fuel reformation cylinder. 
     Further, a flow speed of a stirring flow by a swirl flow, a tumble flow, and squish in the reaction chamber is preferably reduced as compared to that in a combustion chamber of the output cylinder. 
     A conceivable measure for reducing the flow speed of the swirl flow and the tumble flow is defining at least one of the connection position and the inclination angle of an air-intake port relative to the reaction chamber. More specifically, the above measure can be any one of the following modes: reducing an offset amount in a radial direction of a central axis of the air-intake port with respect to a center of the reaction chamber in a plan view; reducing an inclination angle of the air-intake port with respect to a central axis of the reaction chamber in a side view; and increasing a passage area of the air-intake port to suppress or reduce its change. 
     A conceivable measures for lowering the flow speed of the stirring flow by the squish are: reducing unevenness of the top surface of the piston for fuel reformation as small as possible, preferably flattening the top surface of the piston; and increasing a top clearance (an opposing distance between the piston  22  positioned at a top dead point and a blast surface  1   f  of the cylinder head  1   b ). 
     This structure can reduce heat radiation from the surfaces constituting the reaction chamber to the outside, i.e., reduce the heat loss from the reaction chamber, when the fuel is reformed in the reaction chamber. 
     Further, an external reaction chamber having a constant volume is preferably provided outside the reaction chamber and communicated with the reaction chamber through a communication passage, and the fuel to be reformed is preferably supplied to the external reaction chamber. 
     In this structure, the fuel to be reformed is directly supplied to the external reaction chamber, and the fuel to be reformed is not directly supplied to the reaction chamber. 
     Since the fuel hardly adheres to the surfaces constituting the reaction chamber, the risk of adhered fuel being scraped off by the reciprocation of the piston can be reduced. 
     Although the supplied fuel may adhere on the inner surface of the external reaction chamber, the fuel is evaporated by an increase in the pressure and an increase in the temperature with rising of the piston. 
     The inner surface of the external reaction chamber is preferably made of a highly heat-insulative material. 
     This structure can reduce heat radiation from the inner surface of the external reaction chamber to the outside, i.e., reduce the heat loss from the external reaction chamber, when the fuel is reformed in the external reaction chamber. 
     Accordingly, the temperature during the reforming reaction of fuel supplied to the external reaction chamber can be maintained higher as compared to a case where the highly heat-insulative material is not employed. Therefore, the reforming efficiency of the fuel can be improved as compared with the case where the highly heat-insulative material is not employed. 
     The present invention can improve fuel reforming efficiency as much as possible in an internal combustion engine having a fuel reformation cylinder for reforming fuel and an output cylinder for yielding an engine power by combustion of fuel or reformed fuel. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a diagram showing only a fuel reformation cylinder shown in  FIG. 3 . 
         FIG. 2  illustrates an enlarged view of an essential part of  FIG. 1 . 
         FIG. 3  illustrates a diagram showing a schematic structure of one embodiment of an internal combustion engine related to the present invention. 
         FIG. 4  illustrates a diagram showing another embodiment of a highly heat-insulative material shown in  FIG. 2 . 
         FIG. 5  illustrates a diagram showing another embodiment of a fuel reformation cylinder shown in  FIG. 1 . 
         FIG. 6  illustrates a diagram showing yet another embodiment of a fuel reformation cylinder shown in  FIG. 1 . 
         FIG. 7  illustrates a diagram showing yet another embodiment of a fuel reformation cylinder shown in  FIG. 1 . 
         FIG. 8  illustrates a graph showing a relationship between a temperature of a reaction gas and a light gas concentration in a reformed gas at a time of reforming fuel. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes in detail preferred embodiments of the present invention with reference to the attached drawings. 
       FIG. 1  to  FIG. 3  show an embodiment of the present invention. Prior to description of the characteristics of the present invention, a schematic structure of one embodiment of an internal combustion engine according to the present invention will be described with reference to  FIG. 3 . 
     The internal combustion engine  1  according to the present embodiment includes a fuel reformation cylinder  2  and an output cylinder  3 . Further, the internal combustion engine  1  includes, as a piping system for supplying (introducing) gas or discharging (leading out) a gas to and from the fuel reformation cylinder  2  and the output cylinder  3 , an air-intake system  4 , a reformed fuel supply system  5 , an exhaust system  6 , an EGR system  7 , and an output cylinder bypass system  8 . 
     The fuel reformation cylinder  2  and the output cylinder  3  are both structured as a reciprocation type. Specifically, the cylinders  2 ,  3  have, in their cylinder bores  21 ,  31  formed in a cylinder block (not shown), pistons  22 ,  32  in such a manner as to be able to reciprocate, respectively. 
     In the fuel reformation cylinder  2 , a reaction chamber (also referred to as fuel reformation chamber)  23  is formed by the cylinder bore  21 , the piston  22 , and a cylinder head  1   b . In the output cylinder  3 , a combustion chamber  33  is formed by the cylinder bore  31 , the piston  32 , and the cylinder head  1   b.    
     The internal combustion engine  1  of the present embodiment includes four cylinders in the cylinder block, and one of the cylinders is structured as the fuel reformation cylinder  2 , whereas three other cylinders are structured as the output cylinder  3 . The numbers of the cylinders  2 ,  3  are not limited to the above. For example, the cylinder block may include six cylinders, and two of the cylinders are structured as the fuel reformation cylinder  2 , whereas four other cylinders are structured as the output cylinder  3 . 
     The pistons  22 ,  32  of the cylinders  2 ,  3  are connected to a crankshaft  11  through connecting rods  24 ,  34 , respectively. This way, the motion is converted from reciprocation of the pistons  22 ,  32  to rotation of the crankshaft  11 . 
     The crankshaft  11  can be connected to a screw shaft of the ship through a clutch mechanism (not shown). 
     The piston  22  of the fuel reformation cylinder  2  and the piston  32  of the output cylinder  3  are connected to each other through the connecting rods  24 ,  34  and the crankshaft  11 . This enables power transmission between the cylinders  2 ,  3 , transmission of output power from the cylinders  2 ,  3  to the screw shaft, and the like. 
     The fuel reformation cylinder  2  includes an injector  25  configured to supply a pre-reformed fuel such as light oil to the reaction chamber  23 . With supply of fuel from the injector  25 , the reaction chamber  23  adiabatically compresses air-fuel mixture with a high equivalence ratio. As a result, the fuel is reformed under a high temperature and high pressure environment, and a reformed fuel having a high anti-knock property such as hydrogen, carbon monoxide, and methane is generated. 
     The output cylinder  3  includes an injector  35  configured to supply a fuel such as light oil to the combustion chamber  33 . To the combustion chamber  33 , the reformed fuel generated in the fuel reformation cylinder  2  is supplied together with the air. Then, premixed combustion of the lean mixture or propagation flame combustion using a small amount of fuel injected from the injector  35  as an ignition source is performed in the combustion chamber  33 . This way, the crankshaft  11  rotates with reciprocation of the piston  32 , and an engine power is obtained. 
     The air-intake system  4  is configured to introduce air (fresh air) to the reaction chamber  23  of the fuel reformation cylinder  2  and the combustion chamber  33  of the output cylinder  3 . 
     The air-intake system  4  includes a main air-intake passage  41 . This main air-intake passage  41  is branched into two systems: i.e., a fuel reformation cylinder air-intake passage  42  and an output cylinder air-intake passage  43 . The main air-intake passage  41  includes a compressor wheel  12   a  of a turbocharger  12 . 
     The fuel reformation cylinder air-intake passage  42  communicates with the air-intake port of the fuel reformation cylinder  2 . Between this air-intake port and the reaction chamber  23  of the fuel reformation cylinder  2 , an air-intake valve  26  that can open/close is arranged. Further, the fuel reformation cylinder air-intake passage  42  includes an air-intake amount adjust valve  45  whose opening degree is adjustable. 
     The output cylinder air-intake passage  43  communicates with an air-intake port of the output cylinder  3 . Between this air-intake port and the combustion chamber  33  of the output cylinder  3 , an air-intake valve  36  that can open/close is arranged. Further, the output cylinder air-intake passage  43  includes an intake-air cooler (inter cooler)  44 . 
     The reformed fuel supply system  5  supplies reformed fuel generated in the fuel reformation cylinder  2  to the combustion chamber  33  of the output cylinder  3 . 
     The reformed fuel supply system  5  includes a reformed fuel supply passage  51  The reformed fuel supply passage  51  includes a reformed fuel cooler  52 . An upstream end of the reformed fuel supply passage  51  communicates with the exhaust port of the fuel reformation cylinder  2 . Between this exhaust port and the reaction chamber  23  of the fuel reformation cylinder  2 , an exhaust valve  27  that can open/close is arranged. A downstream end of the reformed fuel supply passage  51  communicates with the output cylinder air-intake passage  43 . 
     In a communicating portion between the reformed fuel supply passage  51  and the output cylinder air-intake passage  43 , a mixer  53  is provided. In the mixer  53 , the reformed fuel generated by the fuel reformation cylinder  2  is mixed with the air flowing through the output cylinder air-intake passage  43 , and is supplied to the combustion chamber  33  of the output cylinder  3 . 
     The exhaust system  6  is configured to discharge exhaust gas generated in the output cylinder  3 . The exhaust system  6  includes an exhaust passage  61 . The exhaust passage  61  includes a turbine wheel  12   b  of the turbocharger  12 . The exhaust passage  61  communicates with an exhaust port of the output cylinder  3 . Between this exhaust port and the combustion chamber  33  of the output cylinder  3 , an exhaust valve  37  that can open/close is arranged. 
     An EGR system  7  includes a fuel reformation cylinder EGR system  7 A and an output cylinder EGR system  7 B. 
     The fuel reformation cylinder EGR system  7 A is configured to direct and supply a part of exhaust gas to the reaction chamber  23  of the fuel reformation cylinder  2 , the exhaust gas flowing through the exhaust passage  61 . 
     The fuel reformation cylinder EGR system  7 A includes a fuel reformation cylinder EGR passage  71 . The fuel reformation cylinder EGR passage  71  has its upstream end communicated with the exhaust passage  61 , and has its downstream end communicated with the downstream side of the air-intake amount adjust valve  45  in the fuel reformation cylinder air-intake passage  42 , respectively. The fuel reformation cylinder EGR passage  71  includes an EGR gas cooler  72 . On the downstream side of the EGR gas cooler  72  in the fuel reformation cylinder EGR passage  71  (in a position closer to the fuel reformation cylinder air-intake passage  42 ), an EGR gas amount adjusting valve  73  is provided. 
     Further, the fuel reformation cylinder EGR system  7 A is provided with a cooler bypass passage  74  for letting the EGR gas bypassing the EGR gas cooler  72 . In the cooler bypass passage  74 , a bypass amount adjusting valve  75  is provided. 
     The output cylinder EGR system  7 B is configured to return a part of exhaust gas to the combustion chamber  33  of the output cylinder  3 , the exhaust gas flowing through the exhaust passage  61 . The output cylinder EGR system  7 B includes an output cylinder EGR passage  76 . 
     The output cylinder EGR passage  76  has its upstream end communicated with the exhaust passage  61 , and has its downstream end communicated with the downstream side of a mixer  53  in the output cylinder air-intake passage  43 , respectively. The output cylinder EGR passage  76  includes an EGR gas cooler  77 . On the downstream side of the EGR gas cooler  77  in the output cylinder EGR passage  76  (in a position closer to the output cylinder air-intake passage  43 ), an EGR gas amount adjusting valve  78  is provided. 
     The output cylinder bypass system  8  is used to introduce exhaust gas from the fuel reformation cylinder  2  into the exhaust passage  61  without supplying the gas to the output cylinder  3  (i.e., by bypassing the output cylinder  3 ). 
     The output cylinder bypass system  8  includes an output cylinder bypass passage  81 . The output cylinder bypass passage  81  has its upstream end communicated with the upstream side of a reformed fuel cooler  52  in a reformed fuel supply passage  51 , and has its downstream end communicated with the upstream side of the EGR gas cooler  77  (the side close to the exhaust passage  61 ) in the output cylinder EGR passage  76 . Further, the output cylinder bypass passage  81  includes a bypass amount adjusting valve  82 . 
     For the coolers  44 ,  52 ,  72 ,  77  provided in each of the above-described systems, engine cooling water, seawater, or the like is used as a cooling heat source for cooling the gas. Further, the coolers  44 ,  52 ,  72 ,  77  may be of an air-cooled type. 
     Next, a basic operation of the internal combustion engine  1  configured as described above will be described. 
     The air introduced into the main air-intake passage  41  is pressurized by the compressor wheel  12   a  of the turbocharger  12 . 
     The air is then branched into the fuel reformation cylinder air-intake passage  42  and the output cylinder air-intake passage  43 . At this time, the flow rate of the taken-in air flowing through the fuel reformation cylinder air-intake passage  42  is adjusted by the air-intake amount adjust valve  45 . 
     Further, the EGR gas having flown through the fuel reformation cylinder EGR system  7 A is introduced into the fuel reformation cylinder air-intake passage  42 . At this time, the amount of the EGR gas introduced into the fuel reformation cylinder air-intake passage  42  is adjusted by the EGR gas amount adjusting valve  73 . 
     Further, the temperature of the EGR gas introduced into the fuel reformation cylinder air-intake passage  42  is adjusted by the EGR gas amount bypassing the EGR gas cooler  72  according to the opening degree of the bypass amount adjusting valve  75 . As a result, the air and the EGR gas are introduced into the reaction chamber  23  of the fuel reformation cylinder  2 . At this time, the flow rate of the EGR gas adjusted by the opening degree of the EGR gas amount adjusting valve  73 , and the temperature of the EGR gas adjusted by the opening degree of the bypass amount adjusting valve  75  are adjusted so as to set a high equivalence ratio in the reaction chamber  23 , and to achieve a gas temperature in the reaction chamber  23  that enables favorable fuel reformation. 
     Through the process described above, fuel is supplied from the injector  25  to the reaction chamber  23  while the air and the EGR gas are introduced into the reaction chamber  23  of the fuel reformation cylinder  2 . 
     The fuel supply amount from the injector  25  is basically set according to the required engine power. Specifically, the valve opening period of the injector  25  is set so as to achieve a target fuel supply amount according to the fuel pressure in the injector  25 . 
     The opening timing of the injector  25  in this case is preferably set such that injection of the target fuel supply amount is completed by the time the air-intake stroke of the fuel reformation cylinder  2  is finished. However, the fuel injection period may be continued up to the middle of the compression stroke, if evenly mixed air-fuel mixture is obtainable before the piston  22  approaches the compression top dead point. This generates a homogeneous mixture (air-fuel mixture having a high equivalence ratio) in the reaction chamber  23  before the piston  22  reaches the compression top dead point. 
     While the piston  22  moves toward the compression top dead point, the pressure and the temperature of the reaction chamber  23  increase. In the reaction chamber  23 , the air-fuel mixture having a high equivalence ratio (e.g., air-fuel mixture having an equivalent ratio of 4.0 or more) is adiabatically compressed. As a result, the dehydrogenation reaction of the fuel, a partial oxidation reaction, a steam reforming reaction, and a thermal dissociation reaction take place under a high temperature and high pressure environment, thus reforming the fuel to generate reformed fuel having a high anti-knock property, such as hydrogen, carbon monoxide, and methane. 
     The reformed fuel discharged from the reaction chamber  23  is cooled in the reformed fuel cooler  52  while the reformed fuel flows through the reformed fuel supply passage  51 . With this cooling, preignition of the reformed fuel in the output cylinder air-intake passage  43  and the combustion chamber  33  is suppressed or reduced. 
     The cooled reformed fuel is then mixed with the air flowing in the output cylinder air-intake passage  43  in the mixer  53 , and is supplied to the combustion chamber  33  of the output cylinder  3 . Further, the EGR gas amount adjusting valve  78  is opened as needed to introduce the EGR gas into the combustion chamber  33  of the output cylinder  3  through the output cylinder EGR passage  76 . 
     Through the above process, the air, the reformed fuel, and the EGR gas are introduced into the combustion chamber  33  of the output cylinder  3 , and the equivalence ratio in the combustion chamber  33  is adjusted to approximately 0.1 to 0.8. 
     In the compression stroke, the leaned mixed gas is adiabatically compressed in the output cylinder  3 . When the piston  32  reaches the compression top dead point, a small amount of fuel is injected from the injector  35 . This causes self-ignition of the air-fuel mixture in the combustion chamber  33 , and premixed combustion of the lean mixture is performed. In cases where the air-fuel mixture in the combustion chamber  33  is self-ignited (premixed compression self-ignition) without injection of the fuel from the injector  35 , the injection of the fuel from the injector  35  is not necessarily required. 
     The above combustion reciprocates the piston  32  and rotates the crankshaft  11 , thereby outputting an engine power. This engine power is transmitted to the screw shaft. Also, a part of the engine power is used as a drive source for the reciprocating movement of the piston  22  in the fuel reformation cylinder  2 . 
     As shown in  FIG. 2 , the fuel reformation cylinder  2  has a cylinder liner  21   a  fitted into a cylinder hole (reference numeral omitted) of the cylinder block  1   a . In this case, the inner surface of the cylinder liner  21   a  serves as the cylinder bore  21 . In  FIG. 2 , reference numeral  1   c  denotes a water jacket of the cylinder block  1   a , and reference numeral  1   d  denotes a head gasket. 
     The reaction chamber  23  is a space surrounded by the inner circumferential surface of the cylinder liner  21   a , the top surface  22   a  of the piston  22  housed in the cylinder liner  21   a , and a surface (hereinafter, referred to as a blast surface)  1   f  covering an opening of the cylinder liner  21   a  on the top side (close to the cylinder head  1   b ) in the cylinder head  1   b . It is assumed that the blast surface  1   f  includes inner surfaces (surfaces exposed to the reaction chamber  23 ) of cone-like portions of the air-intake valve  26  and the exhaust valve  27  arranged in the fuel reformation cylinder  2 . 
     Since the reaction chamber  23  has such a structure, it should be clear that the volume thereof varies depending on the reciprocation of the piston  22 . 
     As shown in  FIG. 1  and  FIG. 2 , a cylindrical highly heat-insulative material  10  is fitted and attached to the inner circumferential surface of the cylinder liner  21   a.    
     The highly heat-insulative material  10  is arranged in a predetermined area of the inner circumferential surface of the cylinder liner  21   a  ranging from the top side edge to a predetermined position on the bottom side. 
     Specifically, in the predetermined area of the inner circumferential surface of the cylinder liner  21   a  ranging from the top side edge to the predetermined position on the bottom side, an expanded-diameter portion  21   b  having an expanded inner diameter is arranged. The cylindrical highly heat-insulative material  10  is fitted to the expanded-diameter portion  21   b . During the fitted state, a highly heat-insulative material  10  protrudes radially inward from the inner circumferential surface of the cylinder liner  21   a.    
     To keep the highly heat-insulative material  10  from interfering with the piston  22 , a reduced-diameter portion  22   b  having a reduced outer diameter is arranged in a predetermined area of the outer circumferential surface of the piston  22  ranging from an edge close to the top surface  22   a  to a predetermined position on the bottom side. 
     Further, an axial dimension B of the highly heat-insulative material  10  (see  FIG. 2 ) is suitably set according to an area that is desirably kept at a high temperature during a reforming reaction of the fuel in the reaction chamber  23 . Specifically, the area that is desirably kept at a high temperature refers to a length relative to the axial direction ranging from the top edge of the cylinder liner  21   a  to the top surface  22   a  of the piston  22  at the top dead point. Therefore, the axial dimension B is preferably set to be equal to or larger than the length relative to the axial direction. 
     Examples of the highly heat-insulative material  10  include ceramics of various compositions generally known, iron based metals, and a suitable base material whose surface is coated with a highly heat-insulative resin. 
     As described above, in the embodiment to which the present invention is applied, heat radiation from the reaction chamber  23  to the outside, i.e., heat loss from the reaction chamber  23 , at the time of reforming fuel in the reaction chamber  23  can be reduced by attaching the highly heat-insulative material  10  on the inner circumferential surface of the cylinder liner  21   a.    
     Accordingly, the temperature during the reforming reaction of fuel in the reaction chamber  23  can be maintained higher as compared to a case where the highly heat-insulative material  10  is not employed. Therefore, the reforming efficiency of the fuel can be improved as compared with the case where the highly heat-insulative material  10  is not employed. 
     It is to be noted that the present invention is not limited to the above embodiment, and can be appropriately modified within the scope of the claims and within the scope of the scope of the present invention. 
     As shown in  FIG. 4 , for example, the highly heat-insulative material  10  of the above embodiment may have, in an axially intermediate area of its outer diameter side, an annular groove  10   a  opened radially outward. 
     In this case, while the highly heat-insulative material  10  is fitted and attached to the cylinder liner  21   a , an annular space  10   b  surrounded by the cylinder liner  21   a  and the annular groove  10   a  forms an air layer. Therefore, heat-insulating effect of the reaction chamber  23  can be improved as much as possible. This can further reduce the heat loss from the reaction chamber  23 . 
     The above embodiment deals with a case where the cylindrical highly heat-insulative material  10  is fitted and attached to the inner circumferential surface of the cylinder liner  21   a , but the present invention is not limited to this. 
     For example, although illustration is omitted, coating of a highly heat-insulative material may be provided by thermal-spraying or painting to a predetermined area of the inner circumferential surface of the cylinder liner  21   a.    
     This structure can contribute to cost reduction, because the expanded-diameter portion  21   b  on the cylinder liner  21   a  and a reduced-diameter portion  22   b  on the piston  22  are not necessary. 
     In addition to the above embodiment, the present invention encompasses, for example, a mode of forming a highly heat-insulative material  10  to all the surfaces (the inner circumferential surface of the cylinder liner  21   a , the top surface  22   a  of the piston  22 , and the blast surface  1   f  of the cylinder head  1   b ) constituting the reaction chamber  23 ; a mode of forming the highly heat-insulative material  10  to one of the top surface  22   a  of the piston  22  and the blast surface  1   f  of the cylinder head  1   b ; and a mode of forming a highly heat-insulative material  10  to any two of the inner circumferential surface of the cylinder liner  21   a , the top surface  22   a  of the piston  22 , and the blast surface  1   f  of the cylinder head  1   b , although illustration of these modes are omitted. 
     Further, in a case of forming the highly heat-insulative material  10  on the top surface  22   a  of the piston  22  and the blast surface  1   f  of the cylinder head  1   b , coating of a highly heat-insulative material is preferably provided by thermal-spraying or painting, instead of attaching the highly heat-insulative material  10 . 
     Further, in a case of forming the highly heat-insulative material  10  on the top surface  22   a  of the piston  22 , the piston  22  itself can be formed by an iron based metal which may serve as a highly heat-insulative material. 
     In the above embodiment, the heat generation quantity in the fuel reformation cylinder  2  is lower than the heat generation quantity in the output cylinder  3 . Taking this into account, as shown in  FIG. 1 , a straight distance A from the bottom portion of a water jacket  1   e  arranged in the cylinder head  1   b , nearby the fuel reformation cylinder  2 , to the blast surface  1   f  may be set larger than a straight distance (not shown) corresponding to the straight distance A, on the side of the output cylinder  3  in the cylinder head  1   b . Alternatively, the water jacket  1   e  may be eliminated. 
     In the above embodiment, a flow speed of a stirring flow by a swirl flow, a tumble flow, and squish in the reaction chamber  23  is preferably reduced as compared to that in the combustion chamber  33  of the output cylinder  3 . 
     For example, a conceivable measure for reducing the flow speed of the swirl flow and the tumble flow is defining at least one of the connection position and the inclination angle of an air-intake port (not shown) relative to the reaction chamber  23 . More specifically, the above measure can be any one of the following modes: reducing an offset amount in a radial direction of a central axis of the air-intake port with respect to a center of the reaction chamber  23  in a plan view; reducing an inclination angle of the air-intake port with respect to a central axis of the reaction chamber  23  in a side view; and increasing a passage area of the air-intake port to suppress or reduce its change. 
     A conceivable measures for lowering the flow speed of the stirring flow by the squish are: reducing unevenness of the top surface  22   a  of the piston  22  for fuel reformation as small as possible, preferably flattening the top surface  22   a ; and increasing a top clearance (an opposing distance between the piston  22  positioned at a top dead point and a blast surface  1   f  of the cylinder head  1   b ). 
     With this structure, heat radiation from the surfaces constituting the reaction chamber  23  (the inner circumferential surface of the cylinder liner  21   a , the top surface  22   a  of the piston  22 , and the blast surface  1   f  of the cylinder head  1   b ) to the outside, i.e., heat loss from the reaction chamber  23 , at the time of reforming fuel in the reaction chamber  23  can be reduced. 
     Since the fuel supplied to the reaction chamber  23  hardly adheres to the surfaces constituting the reaction chamber  23 , the risk of adhered fuel being scraped off by the reciprocation of the piston  22  can be reduced. 
       FIG. 5  to  FIG. 7  show other embodiments of the present invention, which will be described in detail hereinbelow. In the embodiment shown in  FIG. 5  to  FIG. 7 , an external reaction chamber  20  is provided outside of the reaction chamber  23  of the fuel reformation cylinder  2 . 
     Specifically, in the embodiment shown in  FIG. 5 , the external reaction chamber  20  is provided nearby the reaction chamber  23  in the cylinder head  1   b . In the embodiment shown in  FIG. 6 , the external reaction chamber  20  is provided in the piston  22  for fuel reformation. In the embodiment shown in  FIG. 7 , the external reaction chamber  20  is provided nearby the reaction chamber  23  in the cylinder block  1   a.    
     The external reaction chamber  20  is formed, for example, in a substantially spherical shape, and its volume is set to be constant. However, the external reaction chamber  20  may be formed in an oval shape or the like, other than the shape described above. The external reaction chamber  20  is communicated with the reaction chamber  23  through a communication passage  20   a , so that fuel is directly supplied from the injector  25 . 
     The communication passage  20   a  is configured so that its axis does not pass through the center of the external reaction chamber  20 . The injector  25  is installed so that the injected fuel does not reach the reaction chamber  23  through the communication passage  20   a.    
     Next, the following describes an operation related to fuel reformation of the above described embodiments. 
     First, during the air-intake stroke of the fuel reforming cylinder  2 , the piston  22  moves from the top dead point to the bottom dead point, and the air-intake valve  26  is opened. This increases the volume of the reaction chamber  23 , and reduces the internal pressure of the reaction chamber  23 , thereby sucking in supplied are (containing outside air and EGR gas) with oxygen concentration suitable for fuel reformation. 
     Then, in the compression stroke of the fuel reformation cylinder  2 , the piston  22  moves from the bottom dead point to the top dead point, which reduces the volume of the reaction chamber  23 . This increases the internal pressure of the reaction chamber  23 , and therefore the supplied air in the reaction chamber  23  is adiabatically compressed. Since the adiabatically compressed supplied air in the reaction chamber  23  flows into the external reaction chamber  20  through the communication passage  20   a  at a high speed, a high speed vortex flow is formed in the external reaction chamber  20 . This brings the inside of the reaction chamber  23  and the inside of the external reaction chamber  20  into a high temperature and a high pressure state. 
     In the compression stroke, fuel of an equivalence ratio suitable for fuel reformation is injected from the injector  25  into the external reaction chamber  20  in the high-temperature and the high pressure state. Therefore, the fuel is rapidly mixed (premixed) with the supplied air and evaporated. When the piston  22  reaches the vicinity of the top dead point, the reforming reaction of this air-fuel mixture is started. Since the internal pressure of the external reaction chamber  20  drops lower than the internal pressure of the reaction chamber  23  as the reforming reaction progresses, the air-fuel mixture does not flow into the reaction chamber  23 . 
     In the expansion stroke of the fuel reformation cylinder  2 , the piston  22  moves from the top dead point to the bottom dead point. This increases the volume of the reaction chamber  23  and reduces the internal pressure. Therefore, the reformed fuel in the external reaction chamber  20  moves into the reaction chamber  23  and adiabatically expanded. The reformed fuel is cooled by the adiabatic expansion, and the pressure is reduced, thereby stopping the reforming reaction. 
     In the subsequent exhaust stroke of the fuel reformation cylinder  2 , the piston  22  moves from the bottom dead point to the top dead point, and the exhaust valve  27  is opened. This way, the reformed fuel is introduced to the output cylinder air-intake passage  43  through the output cylinder bypass passage  81  and the EGR gas cooler  77 . 
     As described, in the embodiments of  FIG. 5  to  FIG. 7 , the fuel to be reformed is directly supplied to the external reaction chamber  20 , and the fuel to be reformed is not directly supplied to the reaction chamber  23 . 
     Since the reforming reaction of the fuel does not take place in the reaction chamber  23 , the fuel supplied to the external reaction chamber  20  hardly adheres to the surfaces of the reaction chamber  23  (the cylinder head  1   b , the cylinder block  1   a , and the piston  22 ). Therefore, the risk of adhered fuel being scraped off by the reciprocation of the piston  22  can be reduced. 
     On the other hand, since the fuel supplied to the external reaction chamber  20  is evaporated while being mixed with the supplied air by the synergic action of the increase in the pressure and temperature associated with the rise of the piston  22  and the high speed vortex flow, the fuel hardly adheres on the inner surface of the external reaction chamber  20 . 
     Further, although illustration is omitted, the highly heat-insulative material  10  is attached or coating of a highly heat-insulative material is provided by thermal-spraying or painting to at least a part of the surfaces constituting the reaction chamber  23  (any one of the inner circumferential surface of the cylinder liner  21   a , the top surface  22   a  of the piston  22 , and the blast surface  1   f  of the cylinder head  1   b ) in the embodiments of  FIG. 5  to  FIG. 7 , for the aim of the present invention. 
     Further, although illustration is omitted, the highly heat-insulative material  10  may be attached or coating of a highly heat-insulative material may be provided by thermal-spraying or painting to the inner surface of the external reaction chamber  20 . 
     These embodiments can reduce heat radiation from the inner surface of the external reaction chamber  20  to the outside, i.e., reduce the heat loss from the external reaction chamber  20 , when the fuel is reformed in the external reaction chamber  20 . 
     Accordingly, the temperature during the reforming reaction of fuel in the external reaction chamber  20  can be maintained higher as compared to a case where the highly heat-insulative material  10  is not employed. Therefore, the reforming efficiency of the fuel can be improved as compared with the case where the highly heat-insulative material  10  is not employed. 
     It should be noted that the present invention may be embodied in various other forms without departing from the spirit or essential characteristics thereof. Therefore, each of the embodiments described above and each of the embodiments described above are merely exemplary, and should not be construed as limiting the scope of the present invention. The scope of the present invention is indicated by the appended claims and is not to be limited in any way by the text of the specification. Further, the scope of the present invention encompasses all changes and modifications falling within the scope of the appended claims. 
     The present invention can be suitably applied to an internal combustion engine having a fuel reformation cylinder for reforming fuel and an output cylinder for yielding an engine power by combustion of fuel or reformed fuel. 
     REFERENCE SIGNS LIST 
     
         
           1 , internal combustion engine 
           1   a , cylinder block 
           1   b , cylinder head 
           1   f , blast surface 
           2 , fuel reformation cylinder 
           21 , cylinder bore 
           21   a , cylinder liner 
           21   b , expanded-diameter portion 
           22 , piston 
           22   a , top surface 
           22   b , reduced-diameter portion 
           23 , reaction chamber 
           3 , output cylinder 
           10 , highly heat-insulative material 
           10   a , annular groove 
           20 , external reaction chamber 
           20   a , communication passage