Patent Publication Number: US-11661912-B2

Title: Engine

Description:
CROSS-REFERENCE 
     This is a continuation of US patent application, application Ser. No. 17/347,464, filed Jun. 14, 2021, which is a continuation of application Ser. No. 16/892,224, filed Jun. 3, 2020, which is a continuation of application Ser. No. 16/316,791, with the § 371 (1), (2) date of Jan. 10, 2019, which is a national phase entry of PCT/JP2017/022728 filed on Jun. 20, 2017, which claims priority to Japanese Patent Application, JP2016-139446, fled on Jul. 14, 2016. The documents cited herein are all incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an engine employing multiple types of fuel. 
     BACKGROUND ART 
     A known so-called dual fuel engine is capable of being driven while selecting one of a premixed combustion system in which fuel gas such as natural gas is mixed with air for combustion and a diffusion combustion system in which liquid fuel (fuel oil) such as heavy oil is diffused for combustion. Patent Literature 1 (PTL 1) discloses a dual fuel engine. 
     The dual fuel engine of PTL 1 includes a fuel injection valve that supplies fuel to combustion chambers during combustion in the diffusion combustion system and supplies fuel to the combustion chambers in order to ignite gaseous fuel during combustion in the premixed combustion system. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Application Laid-Open No. 2002-4899 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In PTL 1, fuel is injected from the same fuel injection valve in both the diffusion combustion system and the premixed combustion system. However, the diffusion combustion system and the premixed combustion system have different purposes of fuel injection, and thus, cannot achieve optimum fuel injection in some cases when employing the same fuel injection valve. 
     In view of this, a configuration using different fuel injection valves between the diffusion combustion system and the premixed combustion system has been proposed. An engine of this type uses two types of fuel injection valves, and thus, needs two types of fuel supply paths. For the engine of this type, however, no specific fuel paths designed in consideration of fuel purification have been disclosed to date. 
     Some aspects of the present invention have been made in view of the foregoing circumstances, and have an object of providing a fuel path designed in consideration of fuel purification in an engine using different fuel injection devices between a diffusion combustion system and a premixed combustion system. 
     Solution to Problem and Advantages 
     Problems to be solved by the invention are as described above, and next, means for solving the problems and advantages thereof will be described. 
     In an aspect of the invention, an engine having the following configuration is provided. 
     That is, the engine is operable in a premixed combustion system in which gaseous fuel mixed with air is caused to flow into a combustion chamber and a diffusion combustion system that injects liquid fuel into the combustion chamber for combustion. The engine includes a main fuel injection valve, a pilot fuel injection valve, a liquid fuel tank, a main fuel supply path, a pilot fuel supply path, a pilot fuel filter, a pilot fuel high-pressure pump, a pilot fuel tank, and a pilot fuel supply pump. The main fuel injection valve supplies liquid fuel to the combustion chamber during combustion in the diffusion combustion system. The pilot fuel injection valve supplies liquid fuel as pilot fuel to the combustion chamber in order to ignite gaseous fuel during combustion in the premixed combustion system. The liquid fuel tank stores liquid fuel. The main fuel supply path supplies liquid fuel stored in the liquid fuel tank to the main fuel injection valve. The pilot fuel supply path supplies liquid fuel stored in the liquid fuel tank as pilot fuel to the pilot fuel injection valve. The pilot fuel filter is disposed in an intermediate portion of the pilot fuel supply path. The pilot fuel high-pressure pump is disposed in an intermediate portion of the pilot fuel supply path and sends, to the pilot fuel injection valve, pilot fuel that has passed through the pilot fuel filter. The pilot fuel tank is disposed in an intermediate portion of the pilot fuel supply path and stores pilot fuel sent from the pilot fuel high-pressure pump and not injected by the pilot fuel injection valve. The pilot fuel supply pump is disposed in an intermediate portion of the pilot fuel supply path and sends, to the pilot fuel filter, pilot fuel stored in the pilot fuel tank. 
     Accordingly, uninjected pilot fuel does not return to the liquid fuel tank and is supplied to the pilot fuel injection valve again. Thus, pilot fuel purified by the pilot fuel filter is not easily mixed with liquid fuel in the liquid fuel tank. Consequently, even with a low cleanliness of liquid fuel stored in the liquid fuel tank, the maintenance frequency of the pilot fuel filter can be reduced. 
     The engine preferably has the configuration as follows. Specifically, the engine includes a fuel purification path. The fuel purification path is a path for circulation through a purification filter for purifying pilot fuel and the pilot fuel tank. 
     Accordingly, pilot fuel is also purified by the purification filter, and thus, the maintenance frequency of the pilot fuel filter can be further reduced. 
     In the engine, the pilot fuel supply path and the fuel purification path preferably partially overlap each other. 
     Accordingly, since the two paths overlap, the length of fuel pipes can be reduced. In addition, a pressure loss occurring when fuel flows through the fuel pipes can be reduced. 
     In the engine, the purification filter is preferably an automatic backwash filter having a filtration efficiency lower than that of the pilot fuel filter. 
     Accordingly, the pilot fuel filter only needs to remove foreign substances not removed by the purification filter, and thus, the maintenance frequency of the pilot fuel filter can be further reduced. 
     The engine preferably has the configuration as follows. Specifically, the engine further includes a pilot fuel supply common rail pipe and a main fuel filter. The pilot fuel supply common rail pipe is disposed in an intermediate portion of the pilot fuel supply path, and supplies pilot fuel to the pilot fuel injection valve at a pressure higher than that a pressure at which liquid fuel is supplied to the main fuel injection valve. The main fuel filter is disposed in an intermediate portion of the main fuel supply path. The pilot fuel filter has a filtration efficiency higher than that of the main fuel filter. 
     Accordingly, clogging with pilot fuel injected at high pressure can be more efficiently prevented or reduced, and gaseous fuel can be ignited with reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    A view schematically illustrating an engine according to one embodiment of the present invention. 
         FIG.  2    A rear view of an engine body 
         FIG.  3    A partial rear cross-sectional view illustrating a configuration around a combustion chamber in detail. 
         FIG.  4    A front view of the engine body. 
         FIG.  5    A plan view of the engine body. 
         FIG.  6    A right side view of the engine body. 
         FIG.  7    A schematic front perspective view illustrating a liquid fuel supply path. 
         FIG.  8    A left side surface of the engine body. 
         FIG.  9    A perspective view of the engine body illustrating a state where a part of a side cover and a heat shielding cover are detached. 
         FIG.  10    A schematic view for describing a fuel supply path that supplies fuel oil to the engine body 
         FIG.  11    A schematic view for describing a fuel supply path that supplies fuel oil to an engine body according to a variation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described with reference to the drawings.  FIG.  1    is a view schematically illustrating an engine  100  according to one embodiment of the present invention.  FIG.  2    is a rear view of an engine body  21 .  FIG.  3    is a partial rear cross-sectional view illustrating a configuration around a combustion chamber  110  in detail.  FIG.  4    is a front view of the engine body  21 .  FIG.  5    is a plan view of the engine body  21 .  FIG.  6    is a right side view of the engine body  21   FIG.  7    is a schematic front perspective view illustrating a liquid fuel supply path.  FIG.  8    is a left side view of the engine body  21 .  FIG.  9    is a perspective view of the engine body  21  illustrating a state where a part of a side cover  43  and a heat shielding cover  45  are detached. 
     The engine  100  (engine with a fuel supply system) according to this embodiment illustrated in  FIG.  1    is a so-called dual fuel engine operable in both a premixed combustion system in which gaseous fuel mixed with air is caused to flow into combustion chambers and a diffusion combustion system that injects liquid fuel into combustion chambers for combustion. The engine  100  according to this embodiment includes an engine body  21  that generates a driving force using supplied fuel. The engine body  21  serves as a driving source of a propulsive and power generating mechanism of an unillustrated ship and is mounted to an inner bottom plate of an engine room of the ship with a base interposed therebetween. 
     A crank shaft  24  serving as an engine output shaft projects rearward from a rear end of the engine body  21 . An unillustrated speed-reducer is coupled to one end of the crank shaft  24  to enable power transfer. The speed reducer is sandwiched between the crank shaft  24  and an unillustrated propeller shaft of the ship, and the propeller shaft is disposed coaxially with the crank shaft  24 . A propeller for generating propulsive power of the ship is attached to an end of the propeller shaft. The speed-reducer includes a PTO shaft, and an unillustrated shaft-driving power generator is coupled to the PTO shaft to enable power transfer. 
     This configuration enables a driving force of the engine body  21  to be branched into the propeller shaft and the shaft-driving power generator and transferred through the speed-reducer. Accordingly, propulsive power of the ship is generated, and electric power generated by driving the shaft-driving power generator is supplied to electric circuits in the ship. 
     Next, the engine  100  will be described in detail with reference to the drawings. The engine  100  is a dual fuel engine as described above, and can be driven while selecting one of a premixed combustion system in which fuel gas such as natural gas is mixed with the air for combustion and a diffusion combustion system in which liquid fuel (fuel oil) such as heavy oil is diffused for combustion. 
     Positional relationship will be described below with a side connected to the speed-reducer (a side where a flywheel is disposed) being defined as rear, the opposite side being defined as front of the engine body  21 . In this positional relationship, left and right toward the front surface (front) of the engine body  21  are respectively defined as left and right of the engine body  21 . Accordingly, the front-rear direction (longitudinal direction) can be a direction parallel to the axis of the crank shaft  24 , and the left-right direction (lateral direction) can be a direction perpendicular to the axis of the crank shaft  24 . It should be noted that this description is not intended to limit the orientation of the engine body  21 , and the engine body  21  can be placed in various orientations in accordance with application and others. 
     As illustrated in  FIG.  1   , a gaseous fuel supply path  30  and a liquid fuel supply path  31  are connected to the engine body  21 . A gaseous fuel tank  32  for storing liquefied natural gas (LNG) is connected to the gaseous fuel supply path  30 , whereas a liquid fuel tank  33  for storing marine diesel oil (MDO) is connected to the liquid fuel supply path  31 . In this configuration, the gaseous fuel supply path  30  supplies fuel gas to the engine body  21 , and the liquid fuel supply path  31  supplies fuel oil to the engine body  21 . 
     In the gaseous fuel supply path  30 , a gaseous fuel tank  32  that stores gaseous fuel in a liquefied state, a vaporizing device  34  that vaporizes the liquefied fuel in the gaseous fuel tank  32 , and a gas valve unit  35  that adjusts the supply rate of fuel gas from the vaporizing device  34  to the engine body  21 , are arranged in this order from the upstream side. 
     As illustrated in  FIGS.  2  through  4   , the engine body  21  is an in-line multi-cylinder engine configured by mounting cylinder heads  26  onto a cylinder block  25 . The crank shaft  24  is rotatably supported on a lower portion of the cylinder block  25  with an axis  24   c  oriented in the front-rear direction as illustrated in  FIGS.  2  and  4   . 
     In the cylinder block  25 , a plurality of (six in this embodiment) cylinders are arranged in a line (in series) along the axis of the crank shaft  24 . As illustrated in  FIG.  3   , each cylinder houses a piston  78  such that the pistons  78  are slidable in the top-bottom direction. This piston  78  is coupled to the crank shaft  24  through an unillustrated rod. 
     As illustrated in  FIGS.  5  and  6   , the plurality of (six in this embodiment) cylinder heads  26  are attached to the cylinder block  25  to cover the cylinders individually from above. The cylinder heads  26  are provided to the individual cylinders, and are fixed to the cylinder block  25  using head bolts  99 . As illustrated in  FIG.  3   , in each cylinder, the combustion chamber  110  is defined in space surrounded by the upper surface of the piston  78  and the cylinder head  26 . 
     As illustrated in  FIG.  5   , a plurality of head covers  40  correspond to the individual cylinders and are arranged on the cylinder heads  26  in a line along the direction of the axis  24   c  of the crank shaft  24  (front-rear direction). As illustrated in  FIG.  3   , each of the head covers  40  houses a valve mechanism constituted by a push rod, a rocker arm, and so forth in order to operate an intake valve and an exhaust valve. In a state where the intake valves are open, intake air from an intake manifold  67  can be taken in the combustion chambers  110 . In a state where the exhaust valves are open, exhaust air from the combustion chambers  110  can be emitted to an exhaust manifold  44 . 
     As illustrated in  FIG.  3   , the upper end of a pilot fuel injection valve  82  described later is disposed near the left of each head cover  40 . With respect to an imaginary vertical plane P 1  (see  FIG.  7   ) including the axis  24   c  of the crank shaft  24 , the pilot fuel injection valves  82  are inserted in the cylinder heads  26  from one side (from the left in this embodiment) of the imaginary vertical plane P 1  and extend obliquely downward toward the combustion chambers  110 . 
     In the following description, a location at one side/the other side of the imaginary vertical plane P 1  including the axis  24   c  of the crank shaft  24  will be sometimes referred to as a location at one side/the other side of the crank shaft  24 . The imaginary vertical plane P 1  can be an infinite plane in both the direction of the axis  24   c  of the crank shaft  24  and the top-bottom direction, but  FIG.  7    shows only a part of the imaginary vertical plane P 1  near the engine body  21  for convenience of expression in the perspective view. 
     As illustrated in  FIGS.  2 ,  3 ,  8   , a gas manifold  41  for distributing and supplying gaseous fuel to the combustion chambers  110  of the cylinders during combustion in the premixed combustion system is provided at the left of the cylinder heads  26 . The gas manifold  41  extends in the front-rear direction along the left side surfaces of the cylinder heads  26 . The plurality of (six in this embodiment) gas branch pipes  41   a  corresponding to the combustion chambers  110  of the cylinders are connected to the gas manifold  41 , and as illustrated in  FIG.  3   , gas injectors  98  for injecting gaseous fuel are provided at the front ends of the gas branch pipes  41   a . The front ends of the gas injectors  98  face intake branch pipes  67   a  corresponding to the cylinders and formed inside the cylinder heads  26 . By injecting gaseous fuel from the gas injectors  98 , the gaseous fuel can be supplied to the intake branch pipes  67   a  of the intake manifold  67 . 
     As illustrated in  FIGS.  3 ,  7 , and  9   , a liquid fuel supply rail pipe  42  for distributing and supplying liquid fuel to the combustion chambers  110  of the cylinders during combustion in the diffusion combustion system is disposed at the right of the cylinder block  25 . The liquid fuel supply rail pipe  42  extends in the front-rear direction along the right side surface of the cylinder block  25 . Liquid fuel supplied to the liquid fuel supply rail pipe  42  is distributed and supplied to main fuel injection pumps  89  corresponding to the cylinders. As illustrated in  FIG.  3   , each cylinder is provided with a main fuel injection valve  79  that injects liquid fuel supplied from the main fuel injection pump  89 . The main fuel injection valves  79  are inserted in the cylinder heads  26  vertically from above the cylinder heads  26 , the upper ends of the main fuel injection valves  79  are disposed inside the head covers  40 , and the lower ends of the main fuel injection valves  79  face the combustion chambers  110  of the cylinders. The main fuel injection pumps  89  and the main fuel injection valves  79  are connected to each other through liquid fuel supply paths  106  formed in the cylinder heads  26 . 
     A liquid fuel return aggregate pipe  48  for collecting redundant fuel returned from the main fuel injection pumps  89  is disposed near the bottom of the liquid fuel supply rail pipe  42 . The liquid fuel return aggregate pipe  48  is disposed in parallel with the liquid fuel supply rail pipe  42 , and connected to the main fuel injection pumps  89 . A fuel return pipe  115  for returning liquid fuel to the liquid fuel tank  33  is connected to an end of the liquid fuel return aggregate pipe  48 . 
     As illustrated in  FIGS.  3 ,  7  and  9   , a pilot fuel supply rail pipe (pilot fuel supply common rail pipe)  47  for distributing and supplying pilot fuel to the combustion chambers  110  of the cylinders in order to ignite gaseous fuel during combustion in the premixed combustion system is disposed at the right of the cylinder block  25  and above the liquid fuel supply rail pipe  42 . The pilot fuel supply rail pipe  47  extends in the front-rear direction along the right side surface of the cylinder block  25 . As illustrated in  FIGS.  3  and  7   , the cylinders are provided with the pilot fuel injection valves  82  for injecting liquid fuel (pilot fuel) supplied from the pilot fuel supply rail pipe  47 . The pilot fuel injection valves  82  are inserted in the cylinder heads  26  vertically from above the cylinder heads  26 , the upper ends of the pilot fuel injection valves  82  are disposed immediately at the left side of the head covers  40 , and the lower ends of the pilot fuel injection valves  82  face the combustion chambers  110  of the cylinders. As illustrated in  FIG.  7   , pilot fuel branch pipes  109  corresponding to the cylinders branch off from the pilot fuel supply rail pipe  47 . The pilot fuel branch pipes  109  pass between the head covers  40  arranged side by side, and are connected to the upper ends of the pilot fuel injection valves  82 . The pilot fuel branch pipes  109  are covered with a branch pipe cover  105  for preventing leaked fuel from scattering. 
     As illustrated in  FIGS.  3 ,  7 , and  9   , a step is formed on an upper portion of the right side surface of the engine body  21  constituted by the cylinder block  25  and the cylinder heads  26 . The pilot fuel supply rail pipe  47 , the liquid fuel supply rail pipe  42 , and the main fuel injection pumps  89  are disposed on this step. A side cover  43  is attached to the cylinder block  25  and the cylinder heads  26  to cover the step. The pilot fuel supply rail pipe  47 , the liquid fuel supply rail pipe  42 , and the main fuel injection pumps  89  are covered with the side cover  43   FIG.  9    illustrates a state where a part of the side cover  43  is detached. 
     As illustrated in  FIGS.  2 ,  3 , and  9   , the exhaust manifold  44  for collecting exhaust air generated by combustion in the combustion chambers  110  of the cylinders and emitting the exhaust air to the outside is disposed in parallel with the gas manifold  41  above the left of cylinder heads  26  and above the gas manifold  41 . The outer periphery of the exhaust manifold  44  is covered with a heat shielding cover  45  (where the heat shielding cover  45  is detached in  FIG.  9   ). As illustrated in  FIG.  3   , exhaust branch pipes  44   a  corresponding to the cylinders are connected to the exhaust manifold  44 . The exhaust branch pipes  44   a  communicate with the combustion chambers  110  of the cylinders. 
     The intake manifold  67  for distributing and supplying outside air (intake air) to the combustion chambers  110  of the cylinders is disposed in parallel with the gas manifold  41  inside the cylinder block  25  and near the left of the cylinder block  25 . As illustrated in  FIG.  3   , the six intake branch pipes  67   a  branching off from the intake manifold  67  are formed inside the cylinder heads  26  and communicate with the individual combustion chambers  110 . 
     With this configuration, in combustion in the diffusion combustion system, an appropriate amount of liquid fuel is injected from the main fuel injection valves  79  into the combustion chambers  110  at an appropriate timing when air supplied to the cylinders from the intake manifold  67  is compressed by sliding of the pistons  78 . Injection of liquid fuel into the combustion chambers  110  causes the pistons  78  to reciprocate in the cylinders with propulsive power obtained by combustion in the combustion chambers  110 , and the reciprocating movement of the pistons  78  is converted to rotation movement of the crank shaft  24  through a rod, thereby obtaining a driving force. 
     On the other hand, in combustion in the premixed combustion system, gaseous fuel from the gas manifold  41  is injected from the gas injectors  98  into the intake branch pipes  67   a  so that air supplied from the intake manifold  67  and the gaseous fuel are mixed. At an appropriate timing when the air mixture of the air introduced into the cylinders and the gaseous fuel is compressed by sliding of the pistons  78 , a small amount of pilot fuel is injected from the pilot fuel injection valves  82  into the combustion chambers  110  so that the gaseous fuel is ignited. The pistons  78  reciprocates in the cylinders with propulsive power obtained by combustion in the combustion chambers  110 , and the reciprocating movement of the pistons  78  is converted to rotation movement of the crank shaft  24  through the rod, thereby obtaining a driving force. 
     In either case of combustion in the diffusion combustion system and combustion in the premixed combustion system, exhaust air generated by combustion is pushed out from the cylinders by movement of the pistons  78 , and collected in the exhaust manifold  44 , and then emitted to the outside. 
     As illustrated in  FIG.  4   , a cooling water pump  53 , a lubricating oil pump  55 , and a pilot fuel high-pressure pump  56  are disposed at the front end surface (front surface) of the engine body  21  and surround a front end portion of the crank shaft  24 . The pilot fuel high-pressure pump  56  is disposed near the right of the crank shaft  24 . A front end portion of the engine body  21  is provided with an unillustrated rotation transfer mechanism that transfers a rotative force of the crank shaft  24 . Accordingly, a rotative force of the crank shaft  24  is transferred through the rotation transfer mechanism so that the cooling water pump  53 , the lubricating oil pump  55 , and the pilot fuel high-pressure pump  56  provided at the outer periphery of the crank shaft  24  are thereby driven. 
     As illustrated in  FIG.  8   , a lubricating oil cooler  58  and a lubricating oil strainer  59  are attached to the left side surface of the cylinder block  25 . Lubricating oil supplied from the lubricating oil pump  55  is cooled by the lubricating oil cooler  58 , then purified by the lubricating oil strainer  59 , and supplied to components of the engine body  21 . 
     Cooling water sent from the cooling water pump  53  illustrated in  FIG.  4    cools cylinders of the engine body  21 , and then is collected to an over-cylinder head cooling water pipe  46  illustrated in, for example,  FIG.  5   . 
     The intercooler  51  is disposed along the front end of the engine body  21 , and cools air compressed by a compressor of the turbocharger  49 . As illustrated in  FIG.  8   , a cylinder block left cooling water pipe  60  extends rearward from ahead of the cylinder block  25  along the gas manifold  41  to a position between the lubricating oil cooler  58  and the lubricating oil strainer  59 , and supplies cooling water to the lubricating oil cooler  58 . 
     As illustrated in  FIGS.  5  and  9   , the over cylinder head cooling water pipe  46  is disposed in parallel with the exhaust manifold  44  and located between a plurality of head covers  40  above the cylinder heads  26  and the exhaust manifold  44 . The over cylinder head cooling water pipe  46  is coupled to cooling water branch pipes corresponding to the cylinders, and is connected to cooling water channel of the cylinders (cooling water channels formed in the cylinder heads  26 ) through the cooling water branch pipes. 
     The pilot fuel high-pressure pump  56  illustrated in  FIG.  4    is driven to rotate to thereby increase the pressure of fuel oil (liquid fuel) supplied from the liquid fuel tank  33  illustrated in  FIG.  1    by an unillustrated fuel feed pump, and sends the fuel oil to the pilot fuel supply rail pipe  47  by way of the pilot fuel supply main pipe  107  illustrated in, for example,  FIG.  7   . Pilot fuel filters  141  for filtering fuel oil are provided in an intermediate portion of a fuel path from the liquid fuel tank  33  to the pilot fuel high-pressure pump  56 . 
     The unillustrated fuel feed pump (different from the pilot fuel high-pressure pump  56 ) is driven by a motor so that the fuel feed pump sucks fuel oil from the liquid fuel tank  33  and sends the oil to the liquid fuel supply rail pipe  42  by way of the liquid fuel supply main pipe  108  illustrated in, for example,  FIG.  7   . Main fuel filters  131  for filtering fuel oil are disposed in an intermediate portion of a supply path of fuel oil from the liquid fuel tank  33  to the liquid fuel supply rail pipe  42 . 
     As illustrated in  FIG.  7   , the pilot fuel supply main pipe  107 , the liquid fuel supply main pipe  108 , and the fuel return pipe  115  are disposed immediately ahead of the cylinder block  25  and extend along the right side surface of the cylinder block  25 . The pilot fuel supply main pipe  107 , the liquid fuel supply main pipe  108 , and the fuel return pipe  115  extend in the top-bottom direction along the right side surface of the cylinder block  25  through a plurality of clamp members  11 I projecting rightward from the front end surface of the cylinder block  25 . 
     An engine-side operation control device  71  for performing control such as start and stopping of the engine body  21  is disposed toward the right and ahead of the cylinder heads  26 , more specifically, on the right side surface of the intercooler  51  with a stay interposed therebetween (see  FIGS.  6  and  9   ). The engine-side operation control device  71  includes an operation section such as a switch that accepts start and stopping of the engine body  21  by an operator, and a display that displays an operating state of the engine body  21 . The operator operates the engine-side operation control device  71  to thereby drive the engine body  21  in one of the premixed combustion system and the diffusion combustion system. 
     Next, configurations of the main fuel filters  131  and the pilot fuel filters  141  and a configuration around these filters will be more specifically described with reference to  FIG.  10   .  FIG.  10    is a schematic view for describing the liquid fuel supply path  31  that supplies fuel oil to the engine body  21 . 
     As illustrated in  FIG.  10   , in this embodiment, the liquid fuel supply path  31  that supplies fuel oil can be divided into two paths (specifically circulation paths) a main fuel supply path  121  that supplies fuel to the main fuel injection valves  79  and a pilot fuel supply path  122  that supplies fuel to the pilot fuel injection valves  82 . The engine  100  includes a fuel purification path  123  for purifying fuel oil to be supplied to the pilot fuel injection valves  82 . 
     In the main fuel supply path  121 , a main fuel pump  161 , the main fuel filters  131 , main fuel injection pumps  89 , main fuel injection valves  79 , a main pressure retention valve  162 , and an air separator  163  are arranged in this order from an upstream side in the flow direction of fuel oil. 
     Fuel oil (liquid fuel) in the liquid fuel tank  33  is sucked by the main fuel pump  161  disposed in the engine body  21 . Fuel oil emitted from the main fuel pump  161  is supplied to the main fuel injection valves  79  through the main fuel supply path  121 . The main fuel supply path  121  includes, for example, the liquid fuel supply main pipe  108 , the liquid fuel supply rail pipe  42 , the liquid fuel supply paths  106 , the liquid fuel return aggregate pipe  48 , and the fuel return pipe  115  described above. As illustrated in  FIG.  10   , redundant fuel oil returned from the main fuel injection pumps  89  is returned to the air separator  163  by way of the liquid fuel return aggregate pipe  48  and the fuel return pipe  115 . The air separator  163  removes air included in liquid fuel circulating in the main fuel supply path  121 . The air separator  163  is disposed on the main fuel supply path  121  and at an intersection of a path toward the main fuel injection pumps  89  and a path from the main fuel injection pumps  89 . Thus, liquid fuel returned from the main fuel injection pumps  89  and liquid fuel newly supplied from the liquid fuel tank  33  are combined together inside the air separator  163 . Accordingly, the combined liquid fuel circulates in the main fuel supply path  121 . The main pressure retention valve  162  for retaining the pressure of the returned fuel is disposed between the fuel return pipe  115  and the liquid fuel tank  33 . 
     The main fuel filters  131  are disposed in an intermediate portion of the main fuel supply path  121  in order to remove foreign substances contained in liquid fuel. Specifically, the main fuel filters  131  filter liquid fuel yet to be supplied to the main fuel injection valves  79  and capture foreign substances, dirt, and the like contained in the liquid fuel on switching wash filters. As illustrated in  FIG.  10   , the main fuel filters  131  are provided as a pair such that while one of the main fuel filters is under maintenance, the other filter can be used. Accordingly, during driving of the engine body  21 , the main fuel filters  131  can be consecutively used. 
     As illustrated in  FIG.  10   , in the pilot fuel supply path  122 , a solenoid valve  172 , a pilot fuel tank  171 , a pilot fuel supply pump  173 , maintenance valves  181  through  183 , an automatic backwash filter (purification filter)  174 , the pilot fuel filters  141 , the pilot fuel high-pressure pump  56 , and the pilot fuel injection valves  82  are arranged in this order from an upstream side in the flow of fuel oil. 
     The pilot fuel tank  171  stores liquid fuel (pilot fuel) to be supplied to the pilot fuel injection valves  82 . The pilot fuel tank  171  is disposed separately from the liquid fuel tank  33 . 
     The pilot fuel tank  171  and the liquid fuel tank  33  are connected to each other by an appropriate pipe. The pipe is provided with the solenoid valve  172 . Fuel oil is supplied from the liquid fuel tank  33  to the pilot fuel tank  171  appropriately by opening and closing the solenoid valve  172  so that a constant amount or more of fuel oil in the pilot fuel tank  171  can be maintained. 
     The pilot fuel supply pump  173  is disposed downstream of the pilot fuel tank  171 . The pilot fuel supply pump  173  is driven by an unillustrated electric motor to suck fuel oil in the pilot fuel tank  171  and emit the oil toward the automatic backwash filter  174 . Fuel oil from which foreign substances have been removed by the automatic backwash filter  174  returns to the pilot fuel tank  171  by way of a pilot pressure retention valve  184  so that fuel oil can be continuously cleaned (the fuel purification path  123  described later). On the other hand, the pilot fuel supply path  122  includes a path that supplies fuel oil subjected to pressure control and cleaning to the pilot fuel high-pressure pump  56 . In this configuration, fuel oil is continuously cleaned before supply to the pilot fuel high-pressure pump  56 , and can be supplied at a pressure necessary for the pilot fuel high-pressure pump  56 . The pilot fuel supply path  122  includes the pilot fuel supply main pipe  107 , the pilot fuel supply rail pipe  47 , and the pilot fuel branch pipes  109  described above. The pilot fuel high-pressure pump  56  described above is disposed in an intermediate portion of the pilot fuel supply path  122 . 
     Each of the pilot fuel supply main pipe  107 , the pilot fuel supply rail pipe  47 , and the pilot fuel branch pipes  109  has a double-pipe structure, can supply fuel oil from the pilot fuel high-pressure pump  56  to the pilot fuel injection valves  82  and can also return fuel oil leaked from the pilot fuel injection valves  82  to the pilot fuel high-pressure pump  56 . Fuel oil returned from the pilot fuel injection valves  82  to the pilot fuel high-pressure pump  56  and fuel oil redundant in the pilot fuel high-pressure pump  56  are returned to the pilot fuel tank  171  through appropriate pipes. The fuel oil returned to the pilot fuel tank  171  is not returned to the liquid fuel tank  33 , and is sent to the automatic backwash filter  174  and the pilot fuel filters  141  by the pilot fuel supply pump  173 . Thus, the pilot fuel tank  171  serves as a return tank. The fuel oil returned to the pilot fuel tank  171  may be partially returned to the liquid fuel tank  33 . 
     Here, a path passing through the automatic backwash filter  174  and a path bypassing the automatic backwash filter  174  are formed between the pilot fuel supply pump  173  and the pilot fuel filters  141 . In the path passing through the automatic backwash filter  174 , the maintenance valve  181  and  183  are disposed upstream and downstream of the automatic backwash filter  174 . In the path bypassing the automatic backwash filter  174 , the maintenance valve  182  is disposed. With this configuration, when the maintenance valve  181  and the maintenance valve  183  are closed and the maintenance valve  182  is opened, no fuel flows in the automatic backwash filter  174 , and thus, maintenance can be performed on the automatic backwash filter  174 . 
     The pilot fuel filters  141  is disposed in an intermediate portion of the pilot fuel supply path  122  in order to remove foreign substances contained in liquid fuel supplied as pilot fuel. Specifically, the pilot fuel filters  141  filter liquid fuel yet to be supplied to the pilot fuel injection valves  82  and capture foreign substances, dirt, and the like contained in the liquid fuel on interchangeable paper filters. The pilot fuel filters  141  have a filtration efficiency higher than that of the automatic backwash filter  174  (e.g., has a smaller filter clearance, a smaller mesh; the same hereinafter). In this manner, foreign substances or the like not filtered by the automatic backwash filter  174  can be filtered by the pilot fuel filters  141 . The filtration efficiency of the pilot fuel filters  141  may be equal to that of the automatic backwash filter  174 , or may be lower than that of the automatic backwash filter  174 . The pilot fuel filters  141  are provided as a pair such that while one of the pilot fuel filters is under maintenance, the other filter can be used. Accordingly, during driving of the engine body  21 , the pilot fuel filters  141  can be consecutively used. As the paper filters of the pilot fuel filters  141 , filters having a higher filtration efficiency than that of the switching wash filters of the main fuel filters  131  are used. 
     As described above, fuel oil to be supplied to the pilot fuel injection valves  82  circulates in a looped path including a looped path (fuel purification path  123 ) including the pilot fuel tank  171 , the pilot fuel supply pump  173 , the automatic backwash filter  174 , and the pilot pressure retention valve  184 , the pilot fuel filters  141 , and the pilot fuel high-pressure pump  56 . This path is substantially independent of a path for supplying fuel oil to the main fuel injection valves  79  of the main fuel supply path  121 . Thus, while sharing the liquid fuel tank  33  as a source, filtration performance of the main fuel filters  131  and the pilot fuel filters  141  can be obtained independently of each other. With the circulation in the looped path described above, pilot fuel oil passes through the fine-mesh automatic backwash filter  174  and the fine-mesh pilot fuel filters  141  multiple times. Accordingly, considerably high cleanliness can be achieved for pilot fuel oil. 
     The engine  100  according to this embodiment includes the fuel purification path  123 . The fuel purification path  123  is a circulation path for purifying fuel oil. Specifically, the fuel purification path  123  is a path connecting the pilot fuel tank  171 , the pilot fuel supply pump  173 , and the automatic backwash filter  174  together in a loop. Fuel oil sent from the pilot fuel tank  171  by the pilot fuel supply pump  173  is purified by the automatic backwash filter  174 , and is partially returned to the pilot fuel tank  171  through the pilot pressure retention valve  184 . This configuration can obtain higher cleanliness of pilot fuel oil. 
     The fuel purification path  123  overlaps the pilot fuel supply path  122  in a section indicated by bold lines in  FIG.  10   . Thus, as compared to a configuration in which the fuel purification path  123  is disposed not to overlap the main fuel supply path  121 , the volumes of the pilot fuel tank  171 , the pilot fuel supply pump  173 , and the automatic backwash filter  174 , for example, can be reduced. 
     In the engine  100  according to this embodiment, the main fuel filters  131  and the pilot fuel filters  141  are individually disposed. Thus, maintenance can be performed on the main fuel filters  131  or the pilot fuel filters  141  in accordance with frequency of use in each of the diffusion combustion system and the premixed combustion system. Consequently, the frequency of maintenance of the (fuel) filters can be reduced as a whole. 
     In addition, as described above, as the paper filters of the pilot fuel filters  141 , filters having a filtration efficiency higher than that of the switching wash filters of the main fuel filters  131  are used. Pilot fuel is supplied to the pilot fuel supply rail pipe (common rail pipe)  47 , and supplied to the pilot fuel injection valves  82  at high pressure to be injected in a small amount to the combustion chambers  110 . Thus, the pilot fuel generally passes through pipes having small diameters. Thus, in a conventional configuration, pipes for supplying pilot fuel, clogging especially easily occurs as compared to pipes for supplying main fuel. In this regard, in this embodiment, the paper filters of the pilot fuel filters  141  have a filtration efficiency higher than that of the switching wash filters of the main fuel filters  131 . Thus, clogging of the pilot fuel can be effectively prevented or reduced, and gaseous fuel can be ignited with reliability. The filtration efficiency of the pilot fuel filters  141  may be equal to or lower than that of the main fuel filters  131 . 
     As the filtration efficiency of a filter increases, the costs for the filter increases in general. In this embodiment, the pilot fuel filters  141  having high filtration efficiency are disposed in the pilot fuel supply path  122  that is a path substantially independent of the main fuel supply path  121 . Thus, a flow of fuel oil from the main fuel supply path  121  into the pilot fuel filters  141  can be prevented or reduced, and thus, the frequency of replacement of the pilot fuel filters  141  can be reduced. In addition, clogging is less likely to occur in the expensive pilot fuel filters  141 , costs for maintenance can be reduced. 
     As described above, the engine  100  according to this embodiment is operable in the premixed combustion system in which gaseous fuel mixed with air is caused to flow into the combustion chambers and the diffusion combustion system that injects liquid fuel into the combustion chambers for combustion. The engine  100  includes the main fuel injection valves  79 , the pilot fuel injection valves  82 , the liquid fuel tank  33 , the main fuel supply path  121 , the pilot fuel supply path  122 , the pilot fuel filters  141 , the pilot fuel high-pressure pump  56 , the pilot fuel tank  171 , and the pilot fuel supply pump  173 . The main fuel injection valves  79  supply liquid fuel to the combustion chambers during combustion in the diffusion combustion system. The pilot fuel injection valves  82  supply liquid fuel as pilot fuel in order to ignite gaseous fuel during combustion in the premixed combustion system. The liquid fuel tank  33  stores liquid fuel. The main fuel supply path  121  supplies liquid fuel stored in the liquid fuel tank  33  to the main fuel injection valves  79 . The pilot fuel supply path  122  supplies liquid fuel stored in the liquid fuel tank  33  as pilot fuel to the pilot fuel injection valves  82 . The pilot fuel filters  141  are disposed in an intermediate portion of the pilot fuel supply path  122 . The pilot fuel high-pressure pump  56  is disposed in an intermediate portion of the pilot fuel supply path  122 , and sends, to the pilot fuel injection valves  82 , pilot fuel that has passed through the pilot fuel filters  141 . The pilot fuel tank  171  is disposed in an intermediate portion of the pilot fuel supply path  122 , and stores pilot fuel sent from the pilot fuel high-pressure pump  56  and not injected by the pilot fuel injection valves  82 . The pilot fuel supply pump  173  is disposed in an intermediate portion of the pilot fuel supply path  122 , and sends pilot fuel stored in the pilot fuel tank  171  to (the automatic backwash filter  174  and) the pilot fuel filters  141 . 
     Accordingly, uninjected pilot fuel does not return to the liquid fuel tank  33  and is supplied to the pilot fuel injection valves  82  again. Thus, pilot fuel purified by the pilot fuel filters  141  is not easily mixed with liquid fuel in the liquid fuel tank  33 . Consequently, even with a low cleanliness of liquid fuel stored in the liquid fuel tank  33 , the maintenance frequency of the pilot fuel filters  141  can be reduced. 
     The engine  100  according to this embodiment also includes the fuel purification path  123 . The fuel purification path  123  is a path for circulation through the automatic backwash filter  174  for purifying pilot fuel and the pilot fuel tank  171 . 
     Accordingly, pilot fuel is purified by the automatic backwash filter  174  in addition to the pilot fuel filters  141 , and thus, the maintenance frequency of the pilot fuel filters  141  can be further reduced. 
     In the engine  100  according to this embodiment, the pilot fuel supply path  122  and the fuel purification path partially overlap each other. 
     Accordingly, since the two paths overlap, the length of fuel pipes can be reduced. In addition, a pressure loss occurring when fuel flows through the fuel pipes can be reduced. 
     In the engine  100  according to this embodiment, the automatic backwash filter  174  has a filtration efficiency lower than that of the pilot fuel filters  141 . 
     Accordingly, the pilot fuel filters  141  only need to remove foreign substances not removed by the automatic backwash filter  174 , and thus, the maintenance frequency of the pilot fuel filters  141  can be further reduced. 
     The engine  100  according to this embodiment further includes the pilot fuel supply rail pipe  47  and the main fuel filters  131 . The pilot fuel supply rail pipe  47  is disposed in an intermediate portion of the pilot fuel supply path  122 , and supplies pilot fuel to the pilot fuel injection valves  82  at a pressure higher than that of liquid fuel supplied to the main fuel injection valves  79 . The main fuel filters  131  are disposed in an intermediate portion of the main fuel supply path  121 . The pilot fuel filters  141  have a filtration efficiency higher than that of the main fuel filters  131 . 
     Accordingly, clogging with pilot fuel injected at high pressure can be more efficiently prevented or reduced, and gaseous fuel can be ignited with reliability. 
     Next, the following describes a variation of the embodiment with reference to  FIG.  11   .  FIG.  11    is a schematic view for describing a fuel supply path that supplies fuel oil to the engine body  21  according to a variation. In a manner similar to the embodiment, the engine  100  according to the variation includes a pilot fuel tank  171 , and supplies at least a part of fuel oil returned to the pilot fuel tank  171  to the pilot fuel injection valves  82  again without returning the oil to the liquid fuel tank  33 . 
     In this variation, second pilot fuel filters  142  that are switching wash filters are disposed instead of the automatic backwash filter  174 . The second pilot fuel filters  142  may have any filtration efficiency, but the filtration efficiency of the second pilot fuel filters  142  is preferably lower than that of the pilot fuel filters  141 . Common fuel filters  190  are disposed on a path that sends fuel from the liquid fuel tank  33  and a path shared by the main fuel supply path  121  and the pilot fuel supply path  122  (a path before being branched). The common fuel filters  190  may have any filtration efficiency, but the filtration efficiency of the common fuel filters  190  is preferably lower than that of the main fuel filters  131 . As described above, a plurality of types of filters having different filtration efficiencies are disposed so that the maintenance frequency of each filter can be reduced. The number of filters disposed in the main fuel supply path  121  and the number of filters disposed in the pilot fuel supply path  122  are not limited to the example described above, and may be changed. 
     In this embodiment, the pilot pressure retention valve  185  is disposed in parallel with the pilot fuel supply pump  173 , and no fuel purification path  123  is formed. The fuel purification path  123  may be omitted in this manner. 
     The foregoing description is directed to the preferred embodiment of the present invention, and the configuration described above may be changed, for example, as follows. 
     In the embodiment described above, the engine  100  is used as a driving source of a propulsive and power generating mechanism of a ship, but this is not restrictive, and the engine  100  may be used only for propulsion of the ship and may be used only for power generation in the ship. The engine body  21  is not limited to ships, and may be placed in other moving objects or buildings. 
     The main fuel filters  131 , the pilot fuel filters  141 , and the automatic backwash filter  174  may have different configurations from those described above as long as foreign substances or the like contained in fuel can be removed. That is, the main fuel filters  131  and the pilot fuel filters  141  may be made of paper, metal, or nylon. The automatic backwash filter  174  may be replaced by a paper filter, for example. 
     In the main fuel supply path  121 , the main fuel pump  161  for pumping liquid fuel (fuel oil) may be disposed downstream of the main fuel filters  131  or upstream of the main fuel filters  131 . 
     In the embodiment described above, the pilot fuel high-pressure pump  56  is disposed downstream of the pilot fuel filters  141 , but this is not restrictive. For example, in a case where a low fuel cleanliness is required for the pilot fuel high-pressure pump  56 , the pilot fuel high-pressure pump  56  may be disposed upstream of the pilot fuel filters  141 . 
     In the embodiment described above, the fuel purification path  123  overlaps the main fuel supply path  121 , but may not overlap the main fuel supply path  121 . 
     REFERENCE SIGNS LIST 
     
         
           21  engine body 
           79  main fuel injection valve 
           82  pilot fuel injection valve 
           100  engine 
           121  main fuel supply path 
           122  pilot fuel supply path 
           123  fuel purification path 
           131  main fuel filter 
           141  pilot fuel filter 
           171  pilot fuel tank 
           174  automatic backwash filter (purification filter)