Patent Publication Number: US-2015075506-A1

Title: Two-cycle gas engine

Description:
TECHNICAL FIELD 
     The present invention relates to a two-cycle gas engine. 
     BACKGROUND 
     Conventionally, there has been known a gas engine in which a fuel gas such as natural gas is used as a main fuel while a fuel oil such as gas oil having high compression-ignition properties is used as a pilot fuel, and the oil fuel is injected into a combustion chamber of high-temperature atmosphere to cause self-ignition of the oil fuel, thereby combusting the fuel gas being the main fuel. Further, as an example of a gas engine of this type, there has been also known a dual-fuel engine where its operation mode can be arbitrarily switched between a gas operation mode for operating the engine by combusting a fuel gas and a diesel operation mode for operating the engine by combusting a fuel oil. 
     For instance, Patent Document 1 discloses a dual-fuel diesel engine in which the main fuel is a fuel having a low cetane number and low compression-ignition properties such as a fuel gas, while the pilot fuel is a fuel oil having high compression-ignition properties. This engine in Patent Document 1 includes a fuel gas injection valve and a pilot fuel injection valve disposed on a cylinder head. The fuel gas and the pilot fuel are injected into a combustion chamber from the fuel gas injection valve and the pilot fuel injection valve, so as to cause self-ignition of the pilot fuel (fuel oil) in the high-temperature combustion chamber, thereby combusting the main fuel (fuel gas). 
     Furthermore, Patent Document 2 discloses a gas engine in which the main fuel is a fuel gas having low compression-ignition properties, while the pilot fuel is a diesel fuel having high compression-ignition properties such as gas oil or heating oil. This gas engine in Patent Document 2 includes an intake port and a diesel fuel injection unit disposed on a cylinder head, and a fuel gas injection unit disposed on the surrounding wall of a cylinder. 
     Further, during an intake stroke in which a piston descends, air is introduced into a combustion chamber from the intake port, and the fuel gas is injected into the combustion chamber from the fuel gas injection unit at an appropriate timing between a later stage of the intake stroke and a later stage of a compression stroke. Then, when the piston has ascended to reach the vicinity of the top dead center, the diesel fuel is injected into the combustion chamber from the diesel fuel injection unit so as to cause self-ignition of the diesel fuel inside the combustion chamber, thereby combusting the fuel gas being the main fuel. 
     Still further, for instance, Patent Document  3  discloses a dual-fuel engine in which it is possible to employ either of the fuel for premix combustion (gas operation mode) where a fuel gas and the air are mixed before flowing into a combustion chamber or the fuel of diffusion combustion (diesel operation mode) where a fuel oil is directly injected into a combustion chamber to be combusted. Moreover, Patent Document  4  discloses a dual-fuel engine of a direct injection type, capable of switching its operation mode between a diesel operation mode and a gas operation mode of a pilot-injected fuel ignition type. 
     Citation List 
     Patent Literature 
     Patent Document 1: JPS62-45339 
     Patent Document 2: JPH6-137150 
     Patent Document 3: JP2008-202545 
     Patent Document 4:JP2008-51121 
     SUMMARY 
     Technical Problem 
     For the engine in Patent Document 1, the main fuel and the pilot fuel are supplied to the combustion chamber almost at the same time in the vicinity of the top dead center. Thus, the main fuel injected into the combustion chamber is immediately combusted before being stirred. Thus, the combustion of the main fuel takes place as diffusion combustion. In the case of diffusion combustion, uniform combustion is difficult compared to the case of premix combustion, which raises a problem of NOx (nitrogen oxide) being generated easily in a high-temperature combustion range. 
     Further, the above described gas engine in Patent Document 2 is an invention that was made to increase the amount of air taken into a combustion chamber. That is, in the invention disclosed in Patent Document 2, compared to the conventional case in which a mixed air of a fuel gas and air is introduced from an intake port, the air alone is taken in from the intake port while providing the fuel gas injection unit separately. Then, the fuel gas is injected into the combustion chamber at a timing other than the intake stroke by the fuel gas injection unit, so as to increase the amount of air taken into the combustion chamber from the intake port, thereby improving the output of the engine. 
     Patent Document 2 as described above does not disclose the technical idea of promoting premix so as to suppress generation of NOx (nitrogen oxide). 
     Furthermore, the engines in Patent Documents 3 and 4 are both configured to inject a small amount of fuel oil into a high-temperature combustion chamber as a pilot fuel in a gas operation mode so as to cause self-ignition of the injected fuel oil, thereby combusting a fuel gas inside the combustion chamber. This type, where a fuel gas is combusted by using a fuel oil as the pilot oil, has a problem of increased amount of black exhaust or PM generated during the gas operation mode. Also, the above described Patent Documents 3 and 4 both describe an invention related to a four-cycle engine but not a two-cycle gas engine as in the present invention. 
     The present invention is made in view of the above problem of the prior art, and is to provide a two-cycle gas engine capable of suppressing generation of black exhaust, PM, NOx or the like. 
     Solution to Problem 
     The present invention was made in order to achieve the above described object, and a two-cycle gas engine of the present invention comprises: a cylinder; a cylinder head; a piston housed in the cylinder and configured to define a main combustion chamber with a surrounding wall of the cylinder and the cylinder head; a fuel gas injector configured to inject a fuel gas into the main combustion chamber; a scavenging port opened on the surrounding wall of the cylinder and configured to supply air into the main combustion chamber upon the piston being positioned in vicinity of a bottom dead center; a precombustion chamber cap disposed on the cylinder head so as to define a precombustion chamber inside thereof, the precombustion chamber communicating with the main combustion chamber through nozzle holes, and a spark plug being disposed in the precombustion chamber; a fuel injection timing control unit configured to cause the fuel gas injector to inject the fuel gas into the main combustion chamber upon the piston being positioned at 10° to 100° before top dead center in an ascending stroke and to cause the fuel gas injector to inject the fuel gas into the main combustion chamber upon the piston being positioned in the vicinity of the top dead center in a gas operation mode; and an ignition timing control unit configured to operate the spark plug to ignite a mixed air of the fuel gas and the air inside the precombustion chamber upon the piston being positioned in the vicinity of the top dead center. 
     In the two-cycle gas engine of the present invention with the above configuration, a fuel gas is injected by the fuel injection timing control unit upon the piston being positioned at 10° to 100° before top dead center to generate mixed air inside the main combustion chamber. The generated mixed air flows into the precombustion chamber through the nozzle holes on the precombustion chamber cap during further ascension of the piston. Then, upon the piston being positioned in the vicinity of the top dead center, the mixed air inside the precombustion chamber is ignited by the ignition timing control unit so as generate torches inside the precombustion chamber and then the torches are injected into the main combustion chamber through the nozzle holes. As a result, the fuel gas injected upon the piston being positioned in the vicinity of the top dead center and the mixed air inside the main combustion chamber are combusted. 
     According to the present invention, premix of the fuel gas injected upon the piston being positioned at 40° to 100° before top dead center with the air is promoted so as to reduce the proportion of diffusion combustion to the entire combustion, which makes it possible to suppress generation of NOx (nitrogen oxide). 
     Also, since the fuel gas is ignited by a sparkle plug without using a fuel oil, it is possible to suppress generation of black exhaust or particulate matter (PM) as well as to improve fuel economy performance. 
     Further, in the above invention, it is preferable that a fuel gas supply unit for supplying a fuel gas to the fuel gas injector is included, the fuel gas supply unit being configured capable of supplying a fuel gas to the precombustion chamber. 
     As described above, with the fuel gas supply unit being configured to supply a fuel gas to the fuel gas injector and also being capable of supplying a fuel gas to the precombustion chamber, it is possible to generate torches in the precombustion chamber stably regardless of the flow rate, concentration, etc of the mixed air that flows into the precombustion chamber. 
     Advantageous Effects 
     According to the present invention, in a two-cycle gas engine, premix of a fuel gas with air is promoted by injecting the fuel gas upon the piston being positioned at 10° to 100° before top dead center and the mixed air of the fuel gas and the air is ignited by a spark plug without using a fuel oil. As a result, it is possible to provide a two-cycle gas engine where generation of black exhaust, PM, NOx or the like is suppressed and the fuel economy performance is improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top view of a two-cycle gas engine according to the first embodiment of the present invention. 
         FIG. 2A  is a cross-section taken along line A-A of  FIG. 1 . 
         FIG. 2B  is a cross-section taken along line B-B of  FIG. 1 . 
         FIG. 3  is an enlarged cross-sectional view of a precombustion chamber cap according to the first embodiment of the present invention. 
         FIG. 4  is a schematic diagram for describing injection timing of fuel gas and operation timing of spark plugs according to the first embodiment of the present invention. 
         FIGS. 5A to 5C  are schematic diagrams for describing function of the two-cycle gas engine according to the first embodiment of the present invention. 
         FIG. 6  is a schematic cross-sectional view of a two-cycle gas engine according to the second embodiment of the present invention. 
         FIG. 7  is an enlarged cross-sectional view of a precombustion chamber cap according to the second embodiment of the present invention. 
         FIG. 8  is a schematic diagram for describing injection timing of fuel gas and operation timing of spark plugs according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
     First Embodiment 
       FIG. 1  is a top view of a two-cycle gas engine according to the first embodiment.  FIG. 2A  is a cross-section taken along line A-A of  FIG. 1 .  FIG. 2B  is a cross-section taken along line B-B of  FIG. 1 .  FIG. 3  is an enlarged cross-sectional view of a precombustion chamber cap according to the first embodiment.  FIG. 4  is a schematic diagram for describing injection timing of fuel gas and operation timing of spark plugs according to the first embodiment. First, based on  FIGS. 1 ,  2 A,  2 B,  3 , and  4 , the configuration of a two-cycle gas engine according to the first embodiment will be described. 
     As shown in  FIGS. 2A and 2B , a two-cycle gas engine  1  of the present embodiment includes a cylinder  2  of a cylindrical shape, a cylinder head  3  connected to an upper end side of the cylinder  2 , and a piston  4  housed in the cylinder  2  so as to be freely reciprocable. A main combustion chamber c 1  is defined by the surrounding wall  2   a  of the cylinder  2 , the top wall  3   a  of the cylinder head  3 , and the top face  4   a  of the piston  4 . Here, the reference number  5  in the drawing indicates a piston ring. 
     Further, scavenging ports  6  open on the surrounding wall  2   a  at the lower side of the cylinder  2 . The scavenging ports  6  are formed above the top face  4   a  of the piston  4  being positioned in the vicinity of the bottom dead center (the double-dotted chain line indicates such top face  4   a ), so that, when the piston  4  is in the vicinity of the bottom dead center, air is supplied to the main combustion chamber c 1  from the scavenging ports  6 . Also, on the top part of the cylinder head  3 , an exhaust port opens and an exhaust valve  7  for opening and closing the exhaust port is disposed. During a scavenging stroke in which the piston  4  is in the ascending stroke, the exhaust valve  7  is kept open until the piston  4  arrives at the position of approximately 100° before top dead center. Then, the air supplied to the main combustion chamber c 1  from the scavenging ports  6  scavenges the exhaust gas in the main combustion chamber c 1  remaining from the previous stroke. 
     Moreover, the gas engine  1  of the present embodiment is configured as a dual-fuel engine capable of switching its operation mode between a gas operation mode for operating by combusting a fuel gas and a diesel operation mode for operating by combusting a fuel oil. 
     Further, fuel gas injection units  8  (fuel gas injector) for injecting a fuel gas such as natural gas to the main combustion chamber c 1  in the gas operation mode and fuel oil injection units  10  (fuel oil injector) for injecting a fuel oil (not shown) having high compression-ignition properties such as gas oil to the main combustion chamber c 1  similarly are each disposed on the cylinder head  3 . 
     As illustrated in  FIG. 1 , a pair of fuel gas injection unit  8  and fuel oil injection unit  10  is formed on each of the two positions that are distanced by 180° from each other in the circumferential direction around the cylinder center “o” as the rotational center. Also, the fuel gas injection units  8  are configured to inject fuel gas  8   a,    8   b  in the same direction as the direction in which the above described scavenging ports  6  are oriented. 
     Further, as illustrated in  FIGS. 2A and 2B , the fuel gas injection units  8  and the fuel oil injection units  10  are connected to an engine control unit (ECU)  12  through cables  14 . Further, the ECU  12  is connected to a crank angle sensor  15  for detecting a rotational angle of a crank shaft  17  through a cable  16 . Then, the ECU  12  detects a phase of the piston  4  by receiving a signal related to a rotation angle of the crank shaft  17  from the crank angle sensor  15 . The fuel gas injection units  8  and the fuel oil injection units  10  inject the fuel gas  8   a,    8   b  and the fuel oil into the main combustion chamber c 1  at a predetermined timing based on a signal transmitted from the ECU  12 . 
     Specifically, upon the piston  4  being in the ascending stroke and positioned at 10° to 100° before top dead center during the gas operation mode, a signal is transmitted from the ECU  12  to the fuel gas injection units  8 , so that the fuel gas  8   b  is injected into the main combustion chamber c 1  from the fuel gas injection units  8  as illustrated in  FIG. 4 . The injected fuel gas  8   b  is mixed with the air inside the main combustion chamber c 1  in the process in which the piston  4  ascends so as to become mixed air  20 . This mixed air  20  diffuses inside the main combustion chamber c 1 , while also diffusing inside a precombustion chamber c 2  which communicates with the main combustion chamber c 1  through the nozzle holes  9   b.    
     Upon the piston  4  being positioned in the vicinity of the top dead center, a signal is transmitted from the ECU  12  to the fuel gas injection units  8 , so that the fuel gas  8   a  is injected to the main combustion chamber c 1  from the fuel gas injection units  8 . In other words, the ECU  12  corresponds to a fuel injection timing control unit in the present embodiment. Further, as illustrated in  FIG. 4 , upon the piston  4  being positioned in the vicinity of the top dead center, the fuel gas  8   a  and the mixed air  20  are combusted in the main combustion chamber c 1  by torches  9   a  described below injected from the nozzle holes  9   b  of the precombustion chamber cap  9 . 
     Furthermore, in the diesel operation mode, upon the piston  4  being positioned in the vicinity of the top dead center, a signal is transmitted from the ECU  12  to the fuel oil injection units  10 , so that fuel oil is injected into the main combustion chamber c 1  from the fuel oil injection units  10 . Then, the fuel oil having high compression-ignition properties self-ignites inside the main combustion chamber c 1  of a high-temperature atmosphere, which causes the fuel oil to combust in the main combustion chamber c 1 . Herein, “the vicinity of the top dead center” in the present invention means the state in which the piston  4  is positioned in a range of from 10° before top dead center to 20° after top dead center. 
     In the present invention, the number of installed fuel gas injection units  8  and fuel oil injection units  10  is not particularly limited and for instance it may be one for each. However, in the present embodiment where the exhaust valve  7  is disposed on the top part of the cylinder head  3 , it is preferable that a plurality of the fuel gas injection units  8  are arranged at equal intervals in the circumferential direction, and so are a plurality of fuel oil injection units  10 . 
     Further, as illustrated in  FIG. 2A , a fuel gas supply pipe  36  is connected to the fuel gas injection units  8 , so that the fuel gas injection units  8  are connected to a fuel gas cylinder  34  via a regulator  32  that adjusts the supply amount of the fuel gas. By adjusting the opening degree of the regulator  32 , the fuel gas, which is stored in the fuel gas cylinder  34  in the state of being pressurized, is supplied to the fuel injection units  8  through the fuel gas supply pipe  36 . That is, the fuel gas supply unit  30  for supplying fuel gas to the fuel gas injection units  8  includes the regulator  32 , the fuel gas cylinder  34  and the fuel gas supply pipe  36 . 
     Further, as illustrated in  FIG. 2B , a fuel oil supply pipe  48  is connected to the fuel oil injection units  10 , so that the fuel oil injection units  10  are connected to a fuel tank  46  for storing fuel oil via a common rail  42  and a supply pump  44 . The high-pressure fuel oil having been pressurized by the supply pump  44  and accumulated in a common rail  42  is supplied to the fuel oil injection units  10  through the fuel oil supply pipe  48 . That is, the fuel oil supply unit  40  for supplying fuel oil of high-pressure to the fuel oil injection units  10  includes the common rail  42 , the supply pump  44 , the fuel tank  46  and the fuel oil supply pipe  48 . 
     Moreover, as illustrated in  FIGS. 1 and 2A , a precombustion chamber cap  9  is disposed on the cylinder head  3 . The number of the installed precombustion chamber cap  9  is not particularly limited, and two precombustion chamber caps  9  are provided for each of the fuel gas injection units  8  in the present embodiment. Further, as illustrated in  FIG. 3 , each of the precombustion chamber caps  9  has a precombustion chamber c 1  defined inside, and a spark plug  11  is disposed on the upper part of each precombustion chamber c 1 . Further, nozzle holes  9   b  are penetrated on a tip end part of each precombustion chamber cap  9 , so that the precombustion chamber c 2  is in communication with the main combustion chamber c 1  through the nozzle holes  9   b.    
     Further, as illustrated in  FIG. 2A , each precombustion chamber cap  9  and the ECU  12  are connected to each other through a cable  18 . The spark plugs  11  are operated at a predetermined timing based on a signal transmitted from the ECU  12 . Specifically, during the gas operation mode, upon the piston  4  being positioned in the vicinity of the top dead center, a signal is transmitted from the ECU  12  to the spark plugs  11  so that the mixed air  20  inside the precombustion chamber c 2  is ignited to generate torches  9   a  as illustrated in  FIG. 4 . That is, the ECU  12  corresponds to an ignition timing control unit in the present embodiment. 
     Next, the function of the two-cycle gas engine  1  according to the first embodiment with the above configuration will be described based on  FIGS. 5A to 5C , which are schematic diagrams for describing function of the two-cycle gas engine according to the first embodiment of the present invention.  FIGS. 5A to 5C  respectively illustrate (a) a state where the piston  4  is positioned at 10° to 100° before top dead center, (b) a state where the piston  4  is positioned at approximately 5° before top dead center, and (c) a state where the piston  4  is positioned at the top dead center. 
     In the two-cycle gas engine  1  of the present embodiment, fuel gas  8   b  is injected into the main combustion chamber c 1  from the fuel gas injection units  8  based on a signal transmitted from the ECU  12  (fuel injection timing control unit) when the piston  4  is in the ascending stroke and is positioned at 10° to 100° before top dead center (the state illustrated in  FIG. 5A ) as described above. By the fuel gas  8   b  being injected into the main combustion chamber c 1  when the piston  4  is positioned at 10° to 100° before top dead center as described above, the injected fuel gas  8   b  and the air inside the main combustion chamber c 1  are mixed so as to promote premix during further ascension of the piston  4  toward the vicinity of the top dead center. Accordingly, mixed air  20  is generated inside the main combustion chamber c 1  as illustrated in  FIG. 5B . 
     Then, upon the piston  4  arriving at the vicinity of the top dead center (for instance, approximately 5° before top dead center), based on a signal transmitted from the ECU  12  (fuel injection timing control unit), fuel gas  8   a  is injected from the fuel gas injection units  8  as described above. Further, based on a signal transmitted from the ECU  12  (ignition timing control unit) described above, the spark plugs  11  are operated to ignite the mixed air  20  inside the precombustion chambers c 2 . 
     Subsequently, the torches  9   a  generated inside the precombustion chambers c 2  are injected into the main combustion chamber c 1  from the nozzle holes  9   b,  thereby combusting the injected fuel gas  8   a  and the mixed air  20  at the same time. Then, as illustrated in  FIG. 5C , explosive combustion is caused where the main combustion chamber c 1  is entirely filled with flame “f”. 
     As described above, for the two-cycle gas engine  1  of the present embodiment, the fuel gas  8   b  is injected upon the piston  4  being positioned at 10° to 100° before top dead center, and the fuel gas  8   a  is injected upon the piston  4  being positioned in the vicinity of the top dead center while operating the spark plugs  11 . Thus, premix of the fuel gas  8   b,  that has been injected upon the piston  4  being positioned at 10° to 100° before top dead center, with the air is promoted so as to generate the mixed air  20 , causing a part of the combustion to become premix combustion. As a result, compared to the conventional gas engine where the entire combustion is diffusion combustion, it is possible to suppress generation of NOx (nitrogen oxide). 
     Furthermore, since the fuel gas is ignited by the spark plugs  11  without using fuel oil, it is possible to suppress generation of black exhaust, particulate matter (PM), etc as well as to improve fuel economy performance. 
     Second Embodiment 
     Next, a two-cycle gas engine according to the second embodiment will be described based on  FIGS. 6 to 8 .  FIG. 6  is a schematic cross-sectional view of a two-cycle gas engine according to the second embodiment.  FIG. 7  is an enlarged cross-sectional view of a precombustion chamber cap according to the second embodiment.  FIG. 8  is a schematic diagram for describing injection timing of fuel gas and operation timing of spark plugs according to the second embodiment. Here, the two-cycle gas engine  1  of the present embodiment has a basically similar configuration to that of the two-cycle gas engine  1  of the above described embodiment. Thus, the same elements are associated with the same reference signs to omit detailed description. 
     As illustrated in  FIG. 6 , the two-cycle gas engine  1  of the present embodiment is different from the above described embodiment in that, in a fuel gas supply unit  30  including a regulator  32 , a fuel gas cylinder  34 , and a fuel gas supply pipe  36 , branch pipes  36   a  branch from the fuel gas supply pipe  36  and connect to the above described precombustion chamber caps  9 . Further, as illustrated in  FIG. 7 , a control valve  36   b  is disposed in middle of each branch pipe  36   a.  The control valve  36   b  is an electromagnetic control valve connected to the ECU  12 , for instance, and configured to open and close in accordance with a signal transmitted from the ECU  12 . By opening the control valve  36   b,  it is possible to supply fuel gas to the precombustion chamber c 2  defined inside the precombustion chamber cap  9 . 
     As illustrated in  FIG. 8 , in the two-cycle gas engine  1  of the present embodiment with the above configuration, at an appropriate time when the piston  4  is in its ascending stroke, fuel gas  8   c  is supplied to the precombustion chamber c 2  by the above described fuel gas supply unit  30 . Then, upon the piston  4  being positioned in the vicinity of the top dead center, the ECU  12  transmits a signal to the spark plug  11 , and the mixed air of the fuel gas  8   c  and air inside the precombustion chamber c 2  is ignited to generate the torches  9   a.    
     As described above, the fuel gas supply unit  30  for supplying fuel gas to the fuel gas injection units  8  (fuel gas injector) is configured capable of supplying fuel gas to the precombustion chambers c 2  as well, which makes it possible to generate torches  9   a  inside the precombustion chambers c 2  stably regardless of the flow rate, concentration etc of the mixed air  20  that flows into the precombustion chambers c 2 . 
     According to the two-cycle gas engine of the present invention described above, the fuel gas is injected upon the piston being positioned at 10° to 100° before top dead center so as to promote premix of the fuel gas and air, while igniting the mixed air of the fuel gas and the air by the spark plugs without using fuel oil. As a result, it is possible to provide a two-cycle gas engine where generation of black exhaust, PM, NOx, etc is suppressed and fuel economy performance is improved. 
     Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented within a scope that does not depart from the present invention. 
     For instance, in the above embodiments, an example is described where the two-cycle gas engine of the present invention is a dual-fuel engine configured capable of switching its operation mode from a gas operation mode for operating by combusting a fuel gas and a diesel operation mode for operating by combusting fuel oil. However, the two-cycle gas engine  1  of the present invention is not limited to this, and it may be applied to a normal gas engine which has no fuel oil injection unit  10  provided and always operates by combusting fuel gas alone. 
     INDUSTRIAL APPLICABILITY 
     The two-cycle gas engine of the present invention can be suitably used as an engine for a construction machine, for a heavy vehicle, for power generation, etc, and in particular for a ship.