Patent Publication Number: US-10316715-B2

Title: Burner

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a burner for raising the temperature of exhaust gas and, in particular, to a premixing type burner in which a mixture of fuel and air is supplied to a combustion chamber. 
     Conventional diesel engines include, in the exhaust passage, a diesel particulate filter (DPF), which captures particulate matter (PM) contained in exhaust gas. In such a DPF, in order to maintain the function of capturing particulate matter, a regeneration process, in which particulate matter captured by the DPF is burnt using exhaust gas, is performed. 
     For example, Japanese Laid-Open Patent Publication 2011-185493 discloses an exhaust purification device in which combusted gas is generated by combusting a mixture of fuel and air in a combustion chamber of a burner arranged upstream of the DPF. Supply of combustion gas to exhaust gas in the exhaust passage raises the temperature of the exhaust gas that flows into the DPF. 
     As such a burner, a premixing type burner is known that supplies a mixture of fuel and air to the combustion chamber without separately supplying fuel and air to the combustion chamber to improve the ignitability or the combustibility of the air-fuel mixture, thereby reducing unburned fuel contained in the combustion gas. 
     Combustion gas generated by the aforementioned premixing type burner contains more than a little unburned fuel. Since the unburned fuel is not used to generate the power of the engine, it is preferable to reduce the fuel used for increasing the temperature of the exhaust gas to reduce the amount of fuel consumption in the vehicle including the engine. Thus, it is desired to reduce unburned fuel on combustion so that fuel required for obtaining a predetermined heat amount is reduced. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a burner that reduces unburned fuel in combustion gas. 
     A burner according to one embodiment of the present disclosure includes: a first tube portion having a tube end including an ejection port, wherein combustion gas generated by combusting air-fuel mixture is ejected from the ejection port; a second tube portion having an open end and a closed end, the closed end being located closer to the ejection port than the open end, the second tube portion extending in the first tube portion toward the ejection port; a burner head that connects an inner circumferential surface of the first tube portion with an outer circumferential surface of the second tube portion, wherein the burner head and the second tube portion partition a space inside the first tube portion into a premixing chamber including a space inside the second tube portion and a combustion chamber located outside the second tube portion, the combustion chamber leading to the ejection port; a communication passage provided in the burner head, the communication passage allowing air-fuel mixture in the premixing chamber to pass to the combustion chamber; a heat exchanging portion arranged in the second tube portion, wherein the heat exchanging portion vaporizes liquid fuel with combustion heat of the combustion chamber and supplies the vaporized fuel to the premixing chamber, and an outer surface of the second tube portion functions as a heat receiving surface of the heat exchanging portion; an ignition portion arranged in the combustion chamber; and a fuel supply section configured to vaporize liquid fuel and supply the vaporized fuel to the premixing chamber, and supply liquid fuel to the heat exchanging portion. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a diagram showing a schematic general arrangement of a burner according to a first embodiment of the present disclosure; 
         FIG. 2  is a perspective view of a heat receiving tube and a cover of the burner in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1 ; 
         FIG. 4  is a timing diagram showing one example of a manner in which the burner in  FIG. 1  operates; 
         FIG. 5  is a cross-sectional view of a modification of the heat exchanging portion, showing an example in which vaporized fuel flows out from the closed portion of the heat exchanging portion; and 
         FIG. 6  is a cross sectional view of a modification of the heat exchanging portion, showing an example in which fins are arranged on a cover. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A burner according to a first embodiment of the present disclosure will now be described with reference to  FIG. 1 . 
     As shown in  FIG. 1 , a diesel engine  10  (hereinafter, referred to simply as the engine  10 ) includes a diesel particulate filter  12  (hereinafter, referred to as the DPF  12 ) mounted in an exhaust passage  11 , and the diesel particulate filter  12  adsorbs particulate matter contained in exhaust gas. 
     The DPF  12 , which is a component of an exhaust purification device, has a honeycomb structure made of, e.g., a porous silicon carbide and captures particulate matter in exhaust gas at the inner wall surfaces of columnar bodies that define the honeycomb structure. A burner  20  is mounted upstream of the DPF  12 , and carries out a regeneration process of the DPF  12  by raising the temperature of the exhaust gas flowing into the DPF  12 . 
     A cylindrical inner tube  30 , which is one example of a first tube portion, is fixed to a basal plate  21  of the burner  20 . The basal plate  21  closes the basal end of the inner tube  30 . An annular ejection plate  31  is fixed to the distal end of the inner tube  30 . The inner edge of the ejection plate  31  defines an ejection port  32 . 
     A cylindrical tube portion  40  is located inside the inner tube  30 . An annular joint wall portion  41 , which is integrated with the tube portion  40 , couples the inner surface of the inner tube  30  to the outer surface of the tube portion  40 . The outer circumferential edge of the joint wall portion  41  is fixed to the inner tube  30  at a location closer to the basal plate  21  of the inner tube  30 . Thus, the joint wall portion  41  closes the gap between the inner surface of the inner tube  30  and the outer surface of the tube portion  40 . The joint wall portion  41  is shaped such that the joint wall portion  41  approaches the ejection port  32  as the distance from the tube portion  40  decreases. The tube portion  40  extends toward the ejection port  32  from a portion coupled to the joint wall portion  41 , and the tube end that is located closer to the ejection port  32  is open. The inner diameter of the tube portion  40  gradually increases toward the ejection port  32  to allow fuel adhered to the inner surface of the tube portion  40  to be readily discharged toward the ejection port  32 . 
     The inner tube  30  includes an extension  33 , which extends from a portion of the inner tube  30  that is coupled to the joint wall portion  41  toward the basal plate  21 . The extension  33  includes first air introducing ports  34 , which are spaced at predetermined intervals in the circumferential direction. The first air introducing ports  34  introduce air for combustion into a mixing chamber  71 , which is a space surrounded by the extension  33 . The extension  33  includes raised portions  35 . Each of the raised portions  35  is formed by cutting and raising a part of the circumferential wall of the extension  33  from the open edge of the corresponding first air introducing port  34 . The air introduced into the mixing chamber  71  flows into a mixing chamber  72 , which corresponds to a space in the tube portion  40  located closer to the ejection port  32  than the mixing chamber  71 . The inner tube  30  includes a plurality of second air introducing ports  36  for introducing air for combustion inside the inner tube  30 . The second air introducing ports  36  are located between an ignition portion  62  and the ejection port  32 . 
     A heat receiving tube  50 , which is a component of a second tube portion, is located inside the inner tube  30 . The heat receiving tube  50  includes an open end, which is an opened tube end. The tube portion  40  is inserted into the heat receiving tube  50  through the open end. The heat receiving tube  50  includes a closing portion  51 . One of two tube ends of the heat receiving tube  50  that is located closer to the ejection port  32  than the tube portion  40  is a closed end, which is closed by the closing portion  51 . In other words, the closing portion  51  is a component of the closed end of the second tube portion. The inside of the heat receiving tube  50  corresponds to mixing chambers  72 ,  73 , and  74 . The mixing chambers  72 ,  73 , and  74  are continuous with the mixing chamber  71 , which is surrounded by the extension  33  and located between the joint wall portion  41  and the basal plate  21 . The open end of the heat receiving tube  50  is fixed to an annular burner head  55 , which connects the inner circumferential surface of the inner tube  30  with the outer circumferential surface of the heat receiving tube  50 . 
     As shown in  FIG. 2 , the outer circumferential surface of the heat receiving tube  50  is covered by a cover  52 , which is a component of the second tube portion. The second tube portion includes the heat receiving tube  50  and the cover  52 . The heat receiving tube  50  and the cover  52  are excellent in heat resistance to function as a burner, and are made of a metal material such as SUS310, which has excellent thermal conductivity. The outer circumferential surface of the heat receiving tube  50  and the cover  52  are components of a heat exchanging portion  53 , which converts combustion heat of a first combustion chamber  78  into heat for vaporization of liquid fuel. 
     A plurality of grooves  53   a  is formed on an outer circumferential surface  50   a  of the heat receiving tube  50 . The grooves  53   a  are parallel to each other and extend in the circumferential direction. The parallel grooves  53   a  join with joint grooves  53   b . The grooves  53   a  and the joint grooves  53   b  function as a groove portion continuous from the closed end to the open end of the second tube portion. 
     The closing portion  51  of the heat receiving tube  50  includes an inlet groove  53   c , which extends from the center of the heat receiving tube  50  in the radial direction and is connected to the grooves  53   a . One end of the inlet groove  53   c  is connected to a liquid fuel supply pipe  84 , which extends in the axial direction of the heat receiving tube  50  in the mixing chamber  72 , which is an inner gap of the heat receiving tube  50 . The other end of the inlet groove  53   c  is connected to the groove  53   a  that is located at the most distal end of the grooves  53   a.    
     In addition, among the grooves  53   a , the groove  53   a  that is located the closest to the open end of the heat receiving tube  50  includes outlet ports  53   d  arranged on the outer circumferential surface of the heat receiving tube  50 . The outlet ports  53   d  extend through the outer circumferential wall of the heat receiving tube  50  in the thickness direction. In other words, the open end of the heat receiving tube  50  is one example of an outflow end of the second tube portion. With the outlet ports  53   d , the inside of the heat exchanging portion  53  is in communication with the mixing chamber  74 , which is an inner gap of the heat receiving tube  50 . The outlet ports  53   d  are arranged, e.g., at equal intervals in the circumferential direction of the outer circumferential surface  50   a  so that vaporized fuel flows out evenly in the circumferential direction of the mixing chamber  74 . The intervals and the number of the outlet ports  53   d  are not limited to the above arrangement. 
     The tubular cover  52  with a closed end is fitted onto the heat receiving tube  50  configured as above. The cylindrical circumferential wall of the cover  52  covers the outer circumferential surface  50   a  of the heat receiving tube  50 . The bottom wall  52   a , which is a distal end wall of the cover  52 , is a component of the closed end of the second tube portion, and covers the closing portion  51  of the heat receiving tube  50 . When the cover  52  is fitted on the heat receiving tube  50 , the inner surface of the cylindrical circumferential wall  52   b  of the cover  52  is in contact with the outside ends of groove walls  53   e  that are located between the respective adjacent grooves  53   a . The cover  52  defines a flow passage of liquid fuel between the heat receiving tube  50  and the cover  52 . The bottom wall  52   a  of the cover  52  closes the inlet groove  53   c  to define a flow passage toward the grooves  53   a . In other words, the closed end (the closing portion  51 ) of the heat receiving tube  50  and the distal end wall (the bottom wall  52   a ) of the cover  52  are one example of an inflow end of the second tube portion. A closed flow passage is defined by the grooves  53   a  and the joint grooves  53   b  between the inlet groove  53   c  and the outlet ports  53   d.    
     In the heat exchanging portion  53  configured as above, when a flame is generated in the first combustion chamber  78 , the outer surface of the cover  52  functions as a heat receiving surface and the heat by the combustion in the first combustion chamber  78  heats the cover  52  and the heat receiving tube  50 . Liquid fuel is supplied to the grooves  53   a  from the liquid fuel supply pipe  84  through the inlet groove  53   c . The liquid fuel flows, as shown with arrows of  FIG. 2 , from the inlet groove  53   c  to the outlet ports  53   d  along the grooves  53   a  and the joint grooves  53   b  in turn. In this process, the heat exchanging portion  53  converts combustion heat of the first combustion chamber  78  into heat for vaporizing liquid fuel so that the liquid fuel changes to vaporized fuel. Thus, the vaporized fuel flows out from the outlet ports  53   d  into the mixing chamber  74  inside the heat receiving tube  50 . The liquid fuel flowing in the grooves  53   a  directly contacts a surface that defines grooves  53   a  and the inner surface of the cover  52 . This improves heat exchange efficiency. 
     As shown in  FIG. 1 , the inner circumferential edge of the burner head  55  joins to the heat receiving tube  50  over the entire circumference of the outer circumferential surface  50   a  of the heat receiving tube  50 . The outer circumferential edge of the burner head  55  joins to the inner tube  30  over the entire circumference of the inside surface of the inner tube  30 . The burner head  55  and the heat receiving tube  50  partition the inside space of the inner tube  30  into two spaces. One of the two spaces is the combustion chamber  77 , which is a space closer to the ejection port  32  than a boundary defined by the burner head  55  and the heat receiving tube  50 . The other space is the premixing chamber  70 , which is a space closer to the basal plate  21  than the boundary defined by the burner head  55  and the heat receiving tube  50 . The burner head  55  includes a plurality of communication passages  56 , with which the combustion chamber  77  is in communication with the premixing chamber  70 . A wire mesh  57  is attached to the surface of the burner head  55  that is closer to the mixing chamber  75  and covers the communication passages  56 . 
     An ignition portion  62  of a spark plug  61  is located between the burner head  55  and the ejection port  32 . The spark plug  61  is fixed to a cylindrical outer tube  60 , into which the heat receiving tube  50  is inserted. The ignition portion  62  is located in the inner tube  30  through through-holes formed in the outer tube  60  and the inner tube  30 . 
     The burner  20  includes the mixing chamber  73  that is located closer to the ejection port  32  than the tube portion  40 . The mixing chamber  73  is a space surrounded by the heat receiving tube  50  and the closing portion  51  and is in communication with the mixing chamber  72 . The burner  20  further includes the mixing chamber  74  in a space between the tube portion  40  and the heat receiving tube  50 . The mixing chamber  74  is in communication with the mixing chamber  73 . The vaporized fuel, which is vaporized in the heat exchanging portion  53 , flows out from the outlet ports  53   d  to the mixing chamber  74 . The burner  20  further includes the mixing chamber  75  between the joint wall portion  41  and the burner head  55 . The mixing chamber  75  is continuous with the mixing chamber  74 . Air for combustion is supplied to the first combustion chamber  78  through the mixing chambers  71 ,  72 ,  73 ,  74 , and  75  in this order. Hereinafter, the mixing chambers  71 ,  72 ,  73 ,  74 , and  75  are collectively referred to simply as the premixing chamber  70 . 
     The burner  20  includes the first combustion chamber  78 , which is a gap between the inner tube  30  and the heat receiving tube  50 , and a second combustion chamber  79  located between the closing portion  51  and the ejection port  32  in the space surrounded by the inner tube  30 . The first combustion chamber  78  and the second combustion chamber  79  form the combustion chamber  77 . 
     The fuel supply section  80 , which supplies fuel to the premixing chamber  70 , includes a vaporized fuel supply section  81  and a liquid fuel supply section  82 . The vaporized fuel supply section  81  supplies vaporized fuel to the premixing chamber  70 , and the liquid fuel supply section  82  supplies liquid fuel to the premixing chamber  70 . 
     The vaporized fuel supply section  81  includes a vaporized fuel supply pipe  83 , and the liquid fuel supply section  82  includes a liquid fuel supply pipe  84 . The vaporized fuel supply pipe  83  and the liquid fuel supply pipe  84  are fixed to the basal plate  21  at the central portion. The vaporized fuel supply pipe  83  has a distal end located in the mixing chamber  71 . The liquid fuel supply pipe  84  passes through the mixing chambers  72  and  73  in the heat receiving tube  50 , extends to the central portion of the closing portion  51 , and is connected to the inlet groove  53   c.    
     The fuel supply section  80  has a common flow pipe  86  for supplying fuel in a fuel tank  85  to the vaporized fuel supply pipe  83  and the liquid fuel supply pipe  84 . A mechanical fuel pump  87  driven by the engine  10 , a fuel pressure sensor  88  for detecting the pressure of fuel, and a fuel temperature sensor  89  for detecting the temperature of fuel, are arranged along the common flow pipe  86 . The common flow pipe  86 , the fuel pump  87 , the fuel pressure sensor  88 , and the fuel temperature sensor  89  function for both of the vaporized fuel supply section  81  and the liquid fuel supply section  82 . 
     The common flow pipe  86  is divided into a vaporized fuel branch pipe  90  and a liquid fuel branch pipe  91  downstream of the fuel temperature sensor  89 . The vaporized fuel branch pipe  90  connects the common flow pipe  86  to the vaporized fuel supply pipe  83 . The vaporized fuel branch pipe  90  includes a vaporized fuel on-off valve  92  and an electric heater  93 . The vaporized fuel on-off valve  92  is a normally closed solenoid valve, which opens or closes the vaporized fuel branch pipe  90  by duty control. The electric heater  93  is located downstream of the vaporized fuel on-off valve  92 . The electric heater  93  in the on-state is supplied with a predetermined amount of electric power by a power source (not shown) to generate heat, which vaporizes liquid fuel that passes through the electric heater  93 . The vaporized fuel supply pipe  83  supplies vaporized fuel delivered from the electric heater  93  to the mixing chambers  71  and  72 . The vaporized fuel branch pipe  90 , the vaporized fuel on-off valve  92 , and the electric heater  93  are components of the vaporized fuel supply section  81 . 
     The liquid fuel branch pipe  91  connects the common flow pipe  86  to the liquid fuel supply pipe  84 . The liquid fuel branch pipe  91  includes the liquid fuel on-off valve  94  for opening and closing the liquid fuel branch pipe  91 . The liquid fuel on-off valve  94  is a normally closed solenoid valve, which opens or closes the liquid fuel branch pipe  91  by duty control. The liquid fuel supply pipe  84  supplies fuel that has passed through the liquid fuel on-off valve  94  to the liquid fuel supply pipe  84 . The liquid fuel branch pipe  91  and the liquid fuel on-off valve  94  are components of the liquid fuel supply section  82 . 
     The vaporized fuel supply pipe  83 , which includes a distal end located in the mixing chamber  71 , supplies fuel to the mixing chambers  71  and  72 . The fuel is mixed with air for combustion from the mixing chamber  71  to generate air-fuel mixture. After flowing in the mixing chamber  72  toward the ejection port  32 , the air-fuel mixture turns around in the mixing chamber  73  and flows in the mixing chamber  74  in the direction opposite to the flow in the mixing chamber  72 . After that, the air-fuel mixture again turns around in the mixing chamber  75 , and flows into the combustion chamber  77  through the communication passages  56  of the burner head  55 . 
     Liquid fuel is supplied from the liquid fuel supply pipe  84  to the heat exchanging portion  53 . The liquid fuel is also heated in a section upstream of the inlet groove  53   c  since the liquid fuel supply pipe  84  extends in the axial direction of the heat receiving tube  50 . The liquid fuel travels from the inlet groove  53   c , which is located at the distal end of the heat exchanging portion  53 , along the grooves  53   a  and the joint grooves  53   b  and flows to the outlet ports  53   d , which are located at the basal end of the heat exchanging portion  53 . At this moment, when the heat exchanging portion  53  has been heated by a flame in the first combustion chamber  78 , the liquid fuel is vaporized with vaporization heat between the inlet groove  53   c  and the outlet ports  53   d  to become vaporized fuel. The vaporized fuel generated in the heat exchanging portion  53  flows out from the outlet ports  53   d  to the mixing chamber  74 . Thus, the vaporized fuel is mixed with air for combustion in the mixing chambers  74  and  75  to generate air-fuel mixture. After that, the air-fuel mixture again turns around in the mixing chamber  75  and flows into the combustion chamber  77  through the communication passages  56  of the burner head  55 . 
     The ignition portion  62  ignites air-fuel mixture that has flowed into the combustion chamber  77  to generate combustion reaction gas in the combustion chamber  77 . The combustion reaction gas contains a flame, which is air-fuel mixture during combustion, and combustion gas, which is air-fuel mixture after the combustion. The heat receiving tube  50  and the cover  52  are heated by the combustion reaction gas, which flows toward the ejection port  32 , to heat air-fuel mixture in the mixing chambers  73  and  74  and heat the cover  52  and the heat receiving tube  50 , which are components of the heat exchanging portion  53 . 
     The outer tube  60 , into which the inner tube  30  is inserted, is fixed to the basal plate  21 . The outer tube  60  has two tube ends. The tube end at the basal end side is closed by the basal plate  21 . The tube end at the distal end side of the outer tube  60  is closed by an annular closing plate  63  between the outer tube  60  and the heat receiving tube  50 . 
     An air supply pipe  64  has the downstream end connected to the outer tube  60  in an area closer to the ejection port  32 . The air supply pipe  64  has the upstream end connected to an intake passage  13  of the engine  10  at the downstream of the compressor  15 , which is rotated with a turbine  14  arranged in exhaust passage  11 . 
     The air supply pipe  64  includes an air valve  65 . When the air valve  65  is open, some of the intake air flowing in the intake passage  13  flows into an air flow chamber  67  between the inner tube  30  and the outer tube  60 , through the air supply pipe  64  as air for combustion. The air for combustion is supplied to the combustion chamber  77  through the second air introducing ports  36  and introduced to the mixing chamber  71  through the first air introducing ports  34 . 
     A controller  95  controls the opening/closing of the aforementioned vaporized fuel on-off valve  92 , the opening/closing of the liquid fuel on-off valve  94 , the on/off of the electric heater  93 , the opening/closing of the air valve  65 , and the driving of the spark plug  61 . The controller  95  controls the opening/closing of the vaporized fuel on-off valve  92  and the on/off of the electric heater  93  to control the driving of the vaporized fuel supply section  81 . The controller  95  controls the opening/closing of the liquid fuel on-off valve  94  to control the driving of the liquid fuel supply section  82 . For example, the controller  95  is embodied by one or more dedicated hardware circuits and/or one or more processors (control circuitry) operating according to computing programs (software). The processor includes a CPU and a memory such as RAM and ROM. The memory stores program codes and commands configured to execute processes, e.g., shown in  FIG. 4 , on the processor. The memory, i.e., a computer readable medium, may include any available media that an all-purpose or exclusive computer can access. 
     The controller  95  computes the accumulation amount of particulate matter in DPF  12  in a predetermined control cycle, e.g., based on the exhaust flow amount and the pressure loss of the exhaust gas in the DPF  12 . The controller  95  starts a regeneration process of the DPF  12  by the burner  20  when the accumulation amount exceeds a threshold and the regeneration process is required. The controller  95  finishes the regeneration process when the accumulation amount decreases to a level below a threshold at which it is determined that sufficient particulate matter has been burnt. 
     In the regeneration process, the controller  95  determines a fuel supply amount Qf that is supplied to the premixing chamber  70  based on the temperature of the DPF  12  and the target temperature of the DPF  12 , the exhaust temperature and the exhaust flow amount in the exhaust passage  11 , and a state temperature T of the heat receiving tube  50 . The controller  95  determines an air-to-be-burned amount Qa based on the fuel supply amount Qf. The air-to-be-burned amount Qa is an amount of air necessary for combusting fuel of the fuel supply amount Qf. The controller  95  controls the driving of the fuel supply section  80  based on the pressure and the temperature of fuel in the common flow pipe  86  such that fuel of the fuel supply amount Qf is supplied to the premixing chamber  70 . In addition, the controller  95  controls the opening/closing of the air valve  65  such that air of the air-to-be-burned amount Qa is supplied to the air flow chamber  67 . 
     The controller  95  has a first driving state, a second driving state, and a third driving state as driving states of the fuel supply section  80 . In the first driving state, fuel supply to the premixing chamber  70  is performed only by the vaporized fuel supply section  81 . In the second driving state, fuel supply to the premixing chamber  70  is performed by both the vaporized fuel supply section  81  and the liquid fuel supply section  82 . In the third driving state, fuel supply to the premixing chamber  70  is performed only by the liquid fuel supply section  82 . 
     The controller  95  obtains the state temperature T, which is the temperature of the heat receiving tube  50 , as an index for controlling the driving of the fuel supply section  80 . The state temperature T may be a temperature based on a detection signal from a temperature sensor that directly detects the temperature of the heat receiving tube  50  or a temperature estimated based on information of various kinds of sensors, a fuel injection amount, or other information. 
     When the state temperature T is less than or equal to a first threshold T 1 , the controller  95  drives the fuel supply section  80  in the first driving state. When the state temperature T is higher than the first threshold T 1  and less than or equal to a second threshold T 2 , the controller  95  drives the fuel supply section  80  in the second driving state. When the state temperature T is higher than the second threshold T 2 , the controller  95  drives the fuel supply section  80  in the third driving state. 
     In the first driving state, the controller  95  starts the opening/closing control of the vaporized fuel on-off valve  92  after the electric heater  93  reaches a predetermined temperature. The controller  95  controls the opening/closing of the vaporized fuel on-off valve  92  such that fuel of the fuel amount Qf 1 , which can be continuously vaporized by the electric heater  93 , flows into the electric heater  93 . In other words, the fuel supply amount Qf in the first driving state is the fuel amount Qf 1 . 
     The first threshold T 1  is a temperature at which heat by the heat receiving tube  50  and the cover  52  vaporizes liquid fuel even if liquid fuel of a fuel amount Qf 2  is supplied to the heat exchanging portion  53  in addition to the vaporized fuel from the vaporized fuel supply section  81 . In the second driving state, the controller  95  controls the opening/closing of the vaporized fuel on-off valve  92  of the vaporized fuel supply section  81  such that fuel of the fuel amount Qf 1  flows into the electric heater  93 . The controller  95  controls the opening/closing of the liquid fuel on-off valve  94  of the liquid fuel supply section  82  such that liquid fuel of the fuel amount Qf 2  is supplied to the heat exchanging portion  53 . In other words, the fuel supply amount Qf in the second driving state is a fuel amount Qf 3  (Qf 3 =Qf 1 +Qf 2 ). 
     The second threshold T 2  (a target temperature) is a temperature at which liquid fuel is vaporized with vaporization heat from the heat exchanging portion  53  even if fuel of the fuel amount Qf 2  is supplied to the heat exchanging portion  53  as liquid fuel. In the third driving state, the controller  95  controls the vaporized fuel on-off valve  92  in the closed state and stops power supply to the electric heater  93  of the vaporized fuel supply section  81 . The controller  95  controls the opening/closing of the liquid fuel on-off valve  94  of the liquid fuel supply section  82  such that liquid fuel of the fuel supply amount Qf is supplied to the premixing chamber  70  while discretely increasing the fuel supply amount Qf, i.e., fuel amounts Qf 3 , Qf 4 , Qf 5 , Qf 6 , along with the increase of the state temperature T. 
       FIG. 3  is a cross-sectional view of the burner taken along line  3 - 3  of  FIG. 1 , showing a cross-sectional structure. The arrows of  FIG. 2  show general flows of air for combustion. As shown in  FIG. 2 , the raised portions  35  formed in the extension  33  of the inner tube  30  are located to cover the respective first air introducing ports  34 . Each of the raised portions  35  directs air for combustion that flows into the mixing chamber  71  through the first air introducing ports  34  to flow in the circumferential direction of the inner tube  30 . This generates swirling flows of air for combustion in the mixing chamber  71 . The swirls are maintained even after the air has flowed into the first combustion chamber  78 . The combustion reaction gas flows toward the ejection port  32  while swirling around the heat receiving tube  50  in the first combustion chamber  78 . 
     With reference to  FIG. 4 , a manner in which the fuel supply section  80  operates will now be described. In an initial state of the burner  20 , the vaporized fuel on-off valve  92 , the liquid fuel on-off valve  94 , and the air valve  65  are controlled to be in the closed states, and the electric heater  93  is controlled to be in the off-state. 
     As shown in  FIG. 4 , when a regeneration process is started at a time point t 1 , the controller  95  turns on the electric heater  93 . The controller  95  starts the opening/closing control of the air valve  65  and the vaporized fuel on-off valve  92  when determining that the electric heater  93  has reached a predetermined temperature at a time point t 2  when a predetermined amount of time has passed after the time point t 1 . This supplies fuel of the fuel amount Qf 1  that has been vaporized by the electric heater  93  to the premixing chamber  70  from the vaporized fuel supply pipe  83 . After passing through the mixing chambers  72  to  75 , the air-fuel mixture containing the vaporized fuel flows into the first combustion chamber  78  through the communication passages  56  of the burner head  55  and is ignited by the spark plug  61 . The controller  95  drives the fuel supply section  80  in the first driving state during the period from the time point t 2  to the next time point t 3 . The burner  20  has an output P 1  in accordance with the fuel amount Qf 1 . 
     When determining that the state temperature T reaches the first threshold T 1  at the time point t 3 , the controller  95  starts the opening/closing control of the liquid fuel on-off valve  94 . This supplies vaporized fuel of the fuel amount Qf 1  from the vaporized fuel supply pipe  83  and supplies liquid fuel of the fuel amount Qf 2  from the liquid fuel supply pipe  84  to the heat exchanging portion  53 . In other words, the controller  95  drives the fuel supply section  80  in the second driving state during the period from the time point t 3  to the next time point t 4 . The burner  20  has an output P 3  in accordance with the fuel amount Qf 3  (Qf 3 =Qf 1 +Qf 2 ). 
     When determining that the state temperature T reaches the second threshold T 2  (the target temperature) at the time point t 4 , the controller  95  controls the vaporized fuel on-off valve  92  in the closed state and controls the opening/closing of the liquid fuel on-off valve  94  such that liquid fuel of the fuel amount Qf 3  is supplied from the liquid fuel supply pipe  84  to the heat exchanging portion  53 . In other words, the controller  95  drives the fuel supply section  80  in the third driving state after the time point t 4 . The controller  95  discretely increases the fuel supply amount Qf along with increase of the state temperature T such that the fuel supply amount Qf becomes fuel amounts Qf 4 , Qf 5 , and Qf 6  at time points t 5 , t 6 , and t 7 , respectively. The controller  95  controls the opening/closing of the liquid fuel on-off valve  94  such that liquid fuel of fuel amounts Qf 4 , Qf 5 , and Qf 6  is supplied to the premixing chamber  70  at time points t 5 , t 6 , and t 7 , respectively. In accordance with the fuel supply amount Qf, the burner  20  has an output P 3  from the time point t 4  to the time point t 5 , an output P 4  from the time point t 5  to the time point t 6 , an output P 5  from the time point t 6  to the time point t 7 , and an output P 6  from the time point t 7  to a time point t 8 . 
     When the accumulation amount becomes lower than a threshold with which it is determined that particulate matter has been sufficiently burnt at the time point t 8 , the controller  95  controls the air valve  65  and the liquid fuel on-off valve  94  in the closed states to finish the regeneration process. 
     The first embodiment has the following advantages. 
     (1) In the burner  20 , the heat exchanging portion  53  includes the outer circumferential surface  50   a  of the heat receiving tube  50  and the cover  52 . Thus, the cover  52  and the heat receiving tube  50  are heated by a flame so that the heat exchanging portion  53  efficiently changes liquid fuel to vaporized fuel. In addition, adhesion of unburned fuel on the inner circumferential of the heat receiving tube  50  is limited. As a result, combustion quality of air-fuel mixture is improved to reduce unburned fuel. 
     (2) In a system of vaporizing fuel with heat from the electric heater  93 , the electric heater  93  requires driving power whenever the burner  20  is driven. Thus, it is desired to reduce the amount of electric power required for driving of the electric heater  93 . According to the above-illustrated configuration, combustion reaction gas efficiently heats the heat receiving tube  50  of the heat exchanging portion  53 . This reduces the amount of time of driving the electric heater  93  and limits the amount of electric power required for driving of the electric heater  93 . 
     (3) Liquid fuel flowing through the flow passage (grooves  53   a  and  53   b ) between the heat receiving tube  50  and the cover  52  is heated by a flame of the combustion chamber  77 . This efficiently changes the liquid fuel to vaporized fuel. 
     (4) The outer circumferential surface  50   a  of the heat receiving tube  50  includes grooves ( 53   a  and  53   b ), which are continuous from the closed end to the open end. Thus, the whole of the liquid fuel between the closed end and the open end is heated so that the liquid fuel is efficiently changed to vaporized fuel. 
     The first embodiment may be modified in the following forms. 
     The shape or the pattern of the heat exchanging portion  53  may be modified as long as the flow passage to the outlet ports  53   d  has a length enough to heat liquid fuel for vaporization. For example, the number of the grooves  53   a  of the heat exchanging portion  53 , which are parallel with each other, is not particularly limited as long as it is possible to vaporize liquid fuel. 
     It is not necessary to arrange the grooves  53   a  in parallel on the outer circumferential surface  50   a  of the heat receiving tube  50  of the heat exchanging portion  53 . The groove  53   a  may be formed to be a spiral groove, in which one flow passage extends from the inlet groove  53   c  to the outlet ports  53   d.    
     It is not necessary to arrange the grooves  53   a  over the outer circumferential surface  50   a  from the distal end to the basal end. For example, the outlet ports  53   d  may be arranged in the middle between the distal end and the basal end of the outer circumferential surface  50   a  as long as it is possible to vaporize liquid fuel. Furthermore, the outlet ports  53   d  may be located at the distal end of the outer circumferential surface  50   a  as long as it is possible to vaporize liquid fuel. In other words, the heat exchanging portion  53  may be configured such that liquid fuel is supplied from the distal end of the heat receiving tube  50  and vaporized fuel flows out from the distal end of the heat receiving tube  50 . 
     The number of inlet grooves  53   c  at the closing portion  51  does not necessarily need to be one. A plurality of inlet grooves  53   c  may be provided. For example, the inlet grooves  53   c  may be arranged radially from the center of the closing portion  51 , to which the liquid fuel supply pipe  84  connects. 
     Second Embodiment 
       FIG. 5  shows a modification of the heat exchanging portion  53 . In the heat exchanging portion  100  shown in  FIG. 5 , liquid fuel is supplied from the basal end of the heat exchanging portion  100 , and vaporized fuel flows out from the closing portion  51 , which is the distal end of the heat receiving tube  50 . In other words, the open end of the heat receiving tube  50  is one example of an inflow end of the second tube portion. The closed end of the heat receiving tube  50  and the distal end of the cover  52  are one example of an outflow end of the second tube portion. 
     In particular, the liquid fuel supply pipe  84  is connected to the basal end of the cover  52 , which is fitted onto the heat receiving tube  50 , and an inflow portion  101  of liquid fuel is provided at the basal end of the cover  52 . The grooves  53   a  are provided on the outer circumferential surface  50   a  of the heat receiving tube  50 , and the grooves  53   a  are parallel to each other in the circumferential direction. The parallel grooves  53   a  are connected by the joint grooves  53   b . An ejection groove  102  is provided on a surface closer to the ejection port  32  of the closing portion  51 . The ejection groove  102  radially extends from the center and is connected to the groove  53   a . One end of the ejection groove  102  is connected to the groove  53   a  next to the ejection groove  102  on the outer circumferential surface. The other end is connected to an outlet port  103 , which is formed in the central portion of the closing portion  51 . 
     In the heat exchanging portion  100  configured as above, liquid fuel is supplied from the liquid fuel supply pipe  84  to the inflow portion  101 . As shown with the arrows of  FIG. 5 , the liquid fuel flows along the grooves  53   a  and the joint grooves  53   b  in turn from the side close to the inflow portion  101  toward the ejection groove  102  and the outlet port  103 . In this process, the heat exchanging portion  100  converts combustion heat of the first combustion chamber  78  into vaporization heat and changes the flowing liquid fuel to vaporized fuel. Thus, the vaporized fuel flows out from the outlet port  103  to the mixing chamber  73  inside the heat receiving tube  50 . 
     The second embodiment has the following advantages. 
     (5) In the second embodiment, fuel vaporized by the heat exchanging portion  100  flows out to the mixing chamber  73 . The mixing chamber  73  is located upstream of the mixing chamber  74  in the flow of air for combustion. Thus, the air for combustion is steadily mixed with vaporized fuel. 
     (6) Since the liquid fuel supply pipe  84  is not provided inside the heat receiving tube  50 , no obstacles exist in swirling flows of air for combustion from the mixing chamber  71  toward the first combustion chamber  78 . Thus, the swirling flows are not easily disturbed. 
     The second embodiment may be modified in the following form. 
     As long as it is possible to vaporize liquid fuel, the liquid fuel supply pipe  84  may be connected to the outer circumferential surface in the middle between the distal end and the basal end of the cover  52 . 
     Third Embodiment 
       FIG. 6  shows a modification of the heat exchanging portion  53 . A heat exchanging portion  110  shown in  FIG. 6  includes a plurality of fins  111  on the outer circumferential surface of the cover  52 . Each of the fins  111  has the longitudinal direction in the axial direction of the heat receiving tube  50 . The fins  111  are substantially shaped the same and arranged at equal intervals over the outer circumferential surface. 
     The fins  111  increase an area receiving heat, which is the surface area of the heat receiving tube  50  in the combustion chamber  77 . This increases the area where combustion reaction gas contacts the heat receiving tube  50 . The combustion reaction gas efficiently heats the heat receiving tube  50 , and the heat receiving tube  50  efficiently heats air-fuel mixture in the mixing chambers  73  and  74 . The fins  111  may be provided on the outer circumferential surface of the cover  52  according to the first embodiment. The fins  111  may be provided on the outer circumferential surface of the cover  52  according to the second embodiment. 
     By providing the fins  111 , the heat receiving tube  50  of the heat exchanging portion  53 , and the cover  52  are efficiently heated with combustion reaction gas. Thus, compared to the first and second embodiments, the amount of time required for the second tube portion to reach the target temperature, i.e., the amount of time required for the state temperature T of the heat receiving tube  50  to reach the second threshold, is further reduced. This further accelerates the timing to stop the driving of the vaporized fuel supply section  81 . Thus, the driving time of the electric heater  93  is further reduced. 
     The third embodiment may be modified in the following form. 
     The shape of each fin  111  is not limited to the shape in which its longitudinal direction is the axial direction of the heat receiving tube  50  and the fins  111  are parallel to each other. For example, the fin  111  may have a spiral shape. Furthermore, adjacent fins  111  may have different heights, or each fin  111  may have uneven heights. 
     The first to third embodiments may be modified in the following form. 
     In the heat exchanging portions  53 ,  100 , and  110 , a flow passage through which liquid fuel is supplied to the outlet ports  53   d  is configured such that the grooves  53   a  and the inner surface of the cover  52  close the circumferential surface of the flow passage. However, a flow passage through which liquid fuel is supplied may be configured with a pipe. In this case, the pipe is arranged, e.g., in a spiral around the outer circumference of the heat receiving tube  50 . In this case, the cover  52  may be omitted. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.