Engine system and combustion method for gaseous fuel

Abstract

To provide an engine system capable of realizing at least either of suppression of nitrogen oxide production and suppression of residual uncombusted hydrocarbons.SOLUTION: An engine system 100 having a combustion chamber which air and gaseous fuel are supplied to, makes the gaseous fuel combust. The engine system 100 comprises a liquid fuel injection part 66 and a control part 211. The liquid fuel injection part 66 injects liquid fuel to ignite the gaseous fuel. The control part 211 controls the liquid fuel injection part 66. The control part 211 controls the liquid fuel injection part 66 so as to inject the liquid fuel after flame propagation after ignition of the gaseous fuel is finished, and when the gaseous fuel is combusted, at least either of suppression of nitrogen oxide production and suppression of residual uncombusted hydrocarbons is performed.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an engine system and a method for combusting gaseous fuel. [Background technology]

[0002] In the method of operating an internal combustion engine described in Patent Document 1, a diesel-type dual-fuel internal combustion engine is operated. The internal combustion engine includes a combustion chamber, a first fuel supply device for a first fuel, and a second fuel supply device for a second fuel. The method of operating an internal combustion engine includes first to fourth steps.

[0003] In a first step, a first fuel is premixed in a combustion chamber. In a second step, a charge containing the first fuel is compressed to conditions that allow for autoignition of the second fuel. In a third step, a first injection of the second fuel is performed into the combustion chamber to initiate autoignition of the second fuel, thereby igniting the first fuel, thereby initiating a condition for premixed flame-propagating combustion of the first fuel. In a fourth step, at least one subsequent injection is performed, which provides additional kinetic energy to the combustion process. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2012-532273 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the internal combustion engine operating method described in Patent Document 1, a subsequent injection is performed in order to increase the flame propagation speed. When the combustion speed increases by increasing the flame propagation speed, the maximum temperature during combustion becomes higher. As a result, the amount of nitrogen oxides produced during combustion of the first fuel increases. Furthermore, unburned hydrocarbons may be generated during combustion of the first fuel.

[0006] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide an engine system and a gaseous fuel combustion method that can achieve at least one of suppressing the generation of nitrogen oxides and suppressing the remaining unburned hydrocarbons when combusting gaseous fuel. [Means for solving the problem]

[0007] According to one aspect of the present invention, an engine system has a combustion chamber to which air and gaseous fuel are supplied, and combusts the gaseous fuel. The engine system includes a liquid fuel injection unit and a control unit. The liquid fuel injection unit injects liquid fuel and ignites the gaseous fuel. The control unit controls the liquid fuel injection unit. After flame propagation following ignition of the gaseous fuel is completed, the control unit executes a subsequent injection of the liquid fuel and burns it, and when combusting the gaseous fuel, executes at least one of suppressing the generation of nitrogen oxides and suppressing the remaining unburned hydrocarbons.

[0008] According to another aspect of the present invention, a method for burning gaseous fuel in an engine in which air and gaseous fuel are supplied to a combustion chamber and the gaseous fuel is burned includes a step of injecting liquid fuel and igniting the gaseous fuel, and a step of performing a subsequent injection of the liquid fuel and burning it after flame propagation after ignition of the gaseous fuel is completed, and when the gaseous fuel is burned, at least one of suppressing the generation of nitrogen oxides and suppressing the remaining unburned hydrocarbons is performed. Effect of the Invention

[0009] According to the present invention, it is possible to provide an engine and a gaseous fuel combustion method that can achieve at least one of suppressing the generation of nitrogen oxides and suppressing the remaining unburned hydrocarbons when combusting gaseous fuel. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram showing a configuration of an engine system according to an embodiment of the present invention. [Diagram 2]1 is a block diagram showing an engine system according to an embodiment of the present invention; [Diagram 3] 1A is a diagram showing an example of the injection timing of the liquid fuel according to the embodiment, and FIG. 1B is a diagram showing another example of the injection timing of the liquid fuel according to the embodiment. [Figure 4] FIG. 2 is a diagram showing a schematic diagram of the combustion characteristics of gaseous fuel in the engine according to the present embodiment. [Diagram 5] 1A is a graph showing the amount of unburned hydrocarbons remaining in the engine according to the embodiment, and FIG. 1B is a graph showing the amount of nitrogen oxides produced in the engine according to the embodiment. [Figure 6] 4 is a time chart showing a schematic diagram of a combustion cycle of the engine according to the present embodiment. [Figure 7] 5A and 5B are diagrams illustrating the injection angle of the liquid fuel according to the embodiment. [Figure 8] 3 is a flowchart illustrating a gaseous fuel combustion method according to the present embodiment. [Figure 9] 10 is a flowchart showing a gaseous fuel combustion method according to a modified example of the embodiment. [Figure 10] 1(a) is a graph showing the amount of unburned hydrocarbons remaining in the engines according to Examples 1 to 6 of the present invention, and FIG. 1(b) is a graph showing the amount of nitrogen oxides produced in the engines according to Examples 1 to 6 of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference characters and description thereof will not be repeated.

[0012] An engine system 100 according to an embodiment of the present invention will be described with reference to Figs. 1 to 8. First, the engine system 100 according to the present embodiment will be described with reference to Fig. 1. Fig. 1 is a schematic diagram showing a configuration of the engine system 100. The engine system 100 shown in Fig. 1 combusts gaseous fuel. Specifically, the engine system 100 obtains mechanical work by combusting the gaseous fuel. The gaseous fuel is not particularly limited, but may be, for example, hydrogen, ammonia, or natural gas. The natural gas is, for example, vaporized liquefied natural gas (LNG: Liquefied Natural Gas). The engine system 100 is, for example, mounted on a vehicle, installed in a building, or installed outdoors. The vehicle is, for example, a ship, an automobile, a railroad car, or an airplane.

[0013] In the following, a description will be given taking a boat 200 as an example of a vehicle on which the engine system 100 is mounted. In this specification, the boat 200 can be read as a "vehicle".

[0014] 1, a ship 200 includes an engine system 100. The engine system 100 includes an engine 1, a gaseous fuel supply pipe 3, a turbocharger 5, an intercooler 7, an intake pipe 9, an intake manifold 11, and an exhaust pipe 13.

[0015] The engine 1 combusts gaseous fuel. Specifically, the engine 1 combusts the gaseous fuel to obtain mechanical work. The engine 1 is, for example, a four-stroke engine. The engine 1 repeatedly executes a combustion cycle. The combustion cycle includes an intake stroke, a compression stroke, a combustion stroke, and an exhaust stroke. The engine 1 is, for example, an engine for propelling the boat 200, or an engine for driving a generator.

[0016] The gaseous fuel supply pipe 3 supplies gaseous fuel to the engine 1. The intake pipe 9 supplies air outside the engine 1 to the engine 1 via the turbocharger 5, the intercooler 7, and the intake manifold 11. That is, the intake pipe 9 has an intake passage 91. Air flows through the intake passage 91 and is supplied to the engine 1.

[0017] Specifically, the turbocharger 5 and the intercooler 7 are arranged in this order from upstream to downstream of the intake air. The turbocharger 5 supplies air at a pressure higher than atmospheric pressure to the engine 1. Specifically, the turbocharger 5 compresses air flowing through an intake pipe 9, and causes the air at a pressure higher than atmospheric pressure to flow into the intake pipe 9. Hereinafter, "compressed air" refers to air at a pressure higher than atmospheric pressure.

[0018] The intercooler 7 cools the air compressed by the turbocharger 5 and supplies the cooled air to the intake manifold 11. The intake manifold 11 supplies the compressed and cooled air to the engine 1. That is, the intake manifold 11 has an intake passage 111. The compressed and cooled air is supplied to the engine 1 via the intake passage 111. Specifically, the engine 1 has a plurality of cylinders 1a. In FIG. 1, one cylinder 1a is illustrated for the sake of simplicity. The intake manifold 11 supplies the compressed and cooled air to each cylinder 1a. Note that the engine 1 may have one cylinder 1a. In this case, the intake manifold 11 can be omitted.

[0019] Exhaust gas discharged from the engine 1 flows through the exhaust pipe 13. That is, the exhaust pipe 13 discharges the exhaust gas to the outside of the engine 1. Specifically, the exhaust pipe 13 has an exhaust passage 131. Then, the exhaust gas flows through the exhaust passage 131, and the exhaust gas is discharged from the engine 1.

[0020] The exhaust gas is utilized by the turbocharger 5. Specifically, the turbocharger 5 includes a turbine 51 and a compressor 52. The turbine 51 is disposed in the exhaust pipe 13, and the compressor 52 is disposed in the intake pipe 9. The turbine 51 is rotated by the exhaust gas flowing through the exhaust pipe 13, and transmits the rotational force to the compressor 52. The compressor 52 is driven by the rotational force of the turbine 51 and compresses the air flowing through the intake pipe 9 to generate air with a pressure higher than atmospheric pressure.

[0021] The engine 1 includes a cylinder head 61, a cylinder block 62, an intake valve 63, an exhaust valve 64, a gaseous fuel supply unit 65, a liquid fuel injection unit 66, a liner 67, a piston 68, a connecting rod 69, and a crankshaft 70. The engine 1 also has a combustion chamber 71. The combustion chamber 71 is formed inside the cylinder block 62, and is a space in which the gaseous fuel is burned. Air and gaseous fuel are supplied to the combustion chamber 71. That is, the engine 1 supplies air and gaseous fuel to the combustion chamber 71 to combust the gaseous fuel.

[0022] The cylinder head 61 is fixed to an upper part of the cylinder block 62. The cylinder head 61 has an intake passage 72 and an exhaust passage 73.

[0023] The intake manifold 11 is connected to the inlet of the intake passage 72. Therefore, compressed and cooled air is supplied to the intake passage 72 from the intake passage 111 of the intake manifold 11. The outlet of the intake passage 72 is connected to the combustion chamber 71.

[0024] The gaseous fuel supply unit 65 is disposed in the cylinder head 61. The gaseous fuel supply unit 65 supplies the gaseous fuel, which is supplied from the gaseous fuel supply pipe 3, to the intake passage 72, thereby supplying the gaseous fuel to the combustion chamber 71 via the intake passage 72. For example, the gaseous fuel supply unit 65 supplies the gaseous fuel to the intake passage 72.

[0025] Specifically, the gaseous fuel is mixed with air supplied from the intake passage 111 of the intake manifold 11, and supplied to the combustion chamber 71. That is, a mixture of the gaseous fuel and the air is supplied to the combustion chamber 71. The gaseous fuel supply unit 65 is, for example, a gas admission valve (GAV) or a gas injector. The mixture is preferably a lean mixture. In this case, the engine 1 combusts the gaseous fuel in the lean mixture.

[0026] More specifically, an intake valve 63 is disposed at the outlet of the intake passage 72. The intake valve 63 opens and closes the outlet of the intake passage 72. When the intake valve 63 opens the outlet of the intake passage 72, a mixture of gaseous fuel and air is supplied to the combustion chamber 71. In detail, when the intake valve 63 opens, the gaseous fuel supply unit 65 supplies the gaseous fuel to the combustion chamber 71 via the intake passage 72 by injecting the gaseous fuel into the intake passage 72.

[0027] For example, the gaseous fuel supply unit 65 may be disposed in the intake manifold 11, or may be disposed in the intake pipe 9 downstream of the intercooler .

[0028] The inlet of the exhaust passage 73 is connected to the combustion chamber 71. The outlet of the exhaust passage 73 is connected to the exhaust pipe 13. Thus, exhaust gas from the combustion chamber 71 is discharged through the exhaust passage 73 to the exhaust pipe 13. Specifically, an exhaust valve 64 is disposed at the inlet of the exhaust passage 73. The exhaust valve 64 opens and closes the inlet of the exhaust passage 73. When the exhaust valve 64 opens the inlet of the exhaust passage 73, the exhaust gas is discharged through the exhaust passage 73 to the exhaust pipe 13.

[0029] The liquid fuel injection unit 66 injects liquid fuel, which induces ignition of gaseous fuel, into the combustion chamber 71. That is, the liquid fuel injection unit 66 injects liquid fuel into the combustion chamber 71 to ignite the gaseous fuel supplied to the combustion chamber 71. The amount of liquid fuel injected by the liquid fuel injection unit 66 is small enough to induce ignition of the gaseous fuel. The liquid fuel is, for example, light oil or heavy oil. The liquid fuel injection unit 66 is, for example, an injector.

[0030] The cylinder block 62 constitutes the cylinder 1a. The cylinder block 62 accommodates a piston 68, a connecting rod 69, and a crankshaft 70. The liner 67 is a cylindrical body that is fitted into the cylinder block 62. The piston 68 reciprocates up and down along the liner 67 inside the cylinder block 62. That is, the piston 68 reciprocates within a combustion chamber 71. The connecting rod 69 connects the piston 68 to the crankshaft 70. The connecting rod 69 transmits the reciprocating motion of the piston 68 to the crankshaft 70. The crankshaft 70 converts the reciprocating motion of the piston 68 into rotational motion.

[0031] For example, when the piston 68 descends and the intake valve 63 opens with the exhaust valve 64 closed, a mixture of gaseous fuel and air is supplied from the intake passage 72 to the combustion chamber 71 (intake stroke). That is, the gaseous fuel supply unit 65 injects gaseous fuel at the timing when the intake valve 63 opens. Next, with the exhaust valve 64 and the intake valve 63 closed, the piston 68 ascends (compression stroke). Next, at the top dead center of the piston 68, the liquid fuel injection unit 66 injects liquid fuel, and the gaseous fuel ignites and burns (combustion stroke). As a result, the piston 68 descends due to combustion. Next, the piston 68 ascends and the exhaust valve 64 opens with the intake valve 63 closed (exhaust stroke). As a result, exhaust gas is discharged from the combustion chamber 71 to the exhaust passage 73.

[0032] As described above with reference to FIG. 1, the engine 1 generates power by combusting gaseous fuel mixed with air.

[0033] Next, the engine system 100 will be described with reference to Fig. 2. Fig. 2 is a block diagram showing the engine system 100. As shown in Fig. 3, the engine system 100 further includes an engine control device 21 and an operation control device 23. The measurement unit 25 will be described later as a modified example.

[0034] The operation control device 23 receives an operation from an operator, and outputs an operation signal corresponding to the operation from the operator to the engine control device 21.

[0035] The operation control device 23 includes, for example, an input device, a display device, and a computer. The input device includes, for example, a keyboard, a pointing device, a dial, and a push button. The display device is, for example, a liquid crystal display. The display device may include, for example, a touch panel. The computer includes, for example, a processor and a storage device. The operation control device 23 is, for example, an operation control panel.

[0036] The engine control device 21 controls the engine 1. For example, the engine control device 21 controls the engine 1 in response to an operation signal output by the operation control device 23. Also, for example, the engine control device 21 controls the engine 1 in accordance with a computer program.

[0037] The engine control device 21 is, for example, a computer. The computer is, for example, an ECU (Electronic Control Unit). Specifically, the engine control device 21 includes a control unit 211 and a storage unit 212. The control unit 211 includes a processor such as a CPU (Central Processing Unit). The storage unit 212 includes a storage device and stores data and computer programs. The storage device includes, for example, a main storage device and an auxiliary storage device such as a semiconductor memory. The storage device may include a removable medium.

[0038] The control unit 211 controls the engine 1. Specifically, the control unit 211 controls the gas fuel supply unit 65 and the liquid fuel injection unit 66. More specifically, the processor of the control unit 211 controls the gas fuel supply unit 65 and the liquid fuel injection unit 66 by executing a computer program stored in the storage device of the storage unit 212.

[0039] Particularly in this embodiment, the control unit 211 controls the liquid fuel injection unit 66 to inject liquid fuel into the combustion chamber 71 in order to ignite the gaseous fuel. As a result, the liquid fuel injection unit 66 injects the liquid fuel into the combustion chamber 71. Thus, in the combustion chamber 71, the gaseous fuel is ignited and combusted.

[0040] Furthermore, the control unit 211 controls the liquid fuel injection unit 66 to inject the liquid fuel after the flame propagation after the ignition of the gaseous fuel is completed. As a result, the liquid fuel injection unit 66 injects the liquid fuel after the flame propagation after the ignition of the gaseous fuel is completed. Thus, according to the engine system 100 according to this embodiment, when the gaseous fuel is combusted, at least one of suppression of the generation of nitrogen oxides and suppression of remaining unburned hydrocarbons can be realized. This point is demonstrated in the examples described later. Also, for example, when the mixed gas of the gaseous fuel and the air (i.e., the mixed gas supplied to the combustion chamber 71) is in an effective lean state, the injection of the liquid fuel after the flame propagation after the ignition of the gaseous fuel is completed can suppress the generation of nitrogen oxides while suppressing the remaining unburned hydrocarbons. This point is also demonstrated in the examples described later.

[0041] The effective lean state refers to a state in which the excess air ratio λ of the mixed gas is within a specified range. The specified range of the excess air ratio λ is greater than "1" and is a range that corresponds to the specifications of the engine 1. The specified range of the excess air ratio λ refers to a range that is effective for practical use of the engine 1. For example, the specified range of the excess air ratio λ is greater than or equal to 1.80 and less than or equal to 2.00.

[0042] Also, in the present embodiment, preferably, the control unit 211 controls the liquid fuel injection unit 66 so that the liquid fuel injection after the flame propagation is completed is performed in a crank angle range of 30 degrees or more and less than 60 degrees after the top dead center of the piston 68. As a result, the liquid fuel injection unit 66 performs the injection of the liquid fuel after the flame propagation is completed in a crank angle range of 30 degrees or more and less than 60 degrees after the top dead center of the piston 68. Therefore, according to this preferred example, when the gas fuel is combusted, at least one of further suppression of the generation of nitrogen oxides and further suppression of remaining unburned hydrocarbons can be realized. This point is demonstrated in the examples described later. Also, for example, when the mixed gas is in an effective lean state, the injection of the liquid fuel after the flame propagation is performed in a crank angle range of 30 degrees or more and less than 60 degrees after the top dead center of the piston 68, thereby further suppressing the generation of nitrogen oxides and further suppressing remaining unburned hydrocarbons. This point is also demonstrated in the examples described later. When the crank angle is in the range of 30 degrees or more and less than 60 degrees, the state of the combustion chamber 71 is such that flame propagation after ignition of the gaseous fuel has ended.

[0043] Here, in the following description, "injection of liquid fuel for ignition" may be referred to as "main injection". Furthermore, "injection of liquid fuel after completion of flame propagation" refers to injection of liquid fuel following "main injection". Therefore, "injection of liquid fuel after completion of flame propagation" may be referred to as "subsequent injection of liquid fuel", "subsequent injection", or "subsequent injection". However, "subsequent" is not limited to "next to" the "main injection" as long as it is "after" the "main injection", but also includes, for example, "next to" the "next to" the "main injection". In other words, as long as it is "after" the "main injection", it is "subsequent". Furthermore, in this specification, "subsequent" refers to after the "main injection" and after completion of flame propagation.

[0044] Next, main injection and subsequent injection by the liquid fuel injection unit 66 will be described with reference to Figures 2 and 3. Figure 3(a) is a diagram showing an example of the injection timing of liquid fuel by the liquid fuel injection unit 66. The horizontal axis indicates the crank angle. The crank angle is the phase angle of the piston 68. The horizontal axis can also be considered to indicate time by the crank angle.

[0045] 2 and 3(a), the liquid fuel injection unit 66 performs a main injection during a predetermined main injection period P0, and ends the main injection when the predetermined main injection period P0 expires. In the example of FIG. 3(a), the liquid fuel injection unit 66 performs a main injection J0 before the top dead center of the piston 68.

[0046] Furthermore, the liquid fuel injection unit 66 executes a subsequent injection J1 after the flame propagation is completed, with a time interval P01 from the main injection J0. The liquid fuel injection unit 66 executes the subsequent injection J1 for a predetermined subsequent injection period P1, and ends the subsequent injection when the predetermined subsequent injection period P1 expires.

[0047] FIG. 3(b) is a diagram showing another example of the injection timing of liquid fuel by the liquid fuel injection unit 66. As shown in FIG. 2 and FIG. 3(b), the liquid fuel injection unit 66 may execute multiple subsequent injections Jn at a time interval P. In this specification, the "n" at the end of a reference symbol indicates an integer of 1 or more. When multiple subsequent injections Jn are executed, multiple predetermined subsequent injection periods Pn are set corresponding to the multiple subsequent injections Jn, respectively. In this case, the multiple predetermined subsequent injection periods Pn may be the same or different. Furthermore, when three or more subsequent injections Jn are executed, the time intervals P between temporally adjacent subsequent injections Jn may be the same or different.

[0048] Note that the subsequent injection Jn indicates an injection subsequent to the main injection J0. However, the subsequent injection Jn may be any injection subsequent to the main injection J0, and includes not only the injection J1 immediately after the main injection J0, but also the injections J2, J3, ....

[0049] In this specification, one or more injections of liquid fuel may be performed between the main injection J0 and the subsequent injection J1 after the end of flame propagation.

[0050] Next, the combustion characteristics of gaseous fuel, such as flame propagation, will be described with reference to FIG. 4. FIG. 4 is a diagram that shows a schematic diagram of the combustion characteristics of gaseous fuel in a combustion chamber 71. In FIG. 4, the horizontal axis shows the crank angle (deg. aTDC: After Top Dead Center). In FIG. 4, a crank angle of 0 degrees shows the top dead center of the piston 68. The horizontal axis can also be interpreted as showing time by the crank angle. The left vertical axis shows the in-cylinder pressure (au). The in-cylinder pressure shows the internal pressure of the combustion chamber 71. The unit of the in-cylinder pressure is, for example, "MPa". The right vertical axis shows the heat release rate (au) in the combustion chamber 71. The heat release rate shows the combustion state of the gaseous fuel in the combustion chamber 71. The unit of the heat release rate is, for example, "J / deg.".

[0051] As shown in Fig. 4, the heat release rate curve A1 indicates the heat release rate. The pressure curve A2 indicates the pressure inside the cylinder. Note that in Fig. 4, the relationship between the heat release rate curve A1 and the pressure curve A2 and the crank angle is merely an example. Furthermore, the shapes of the heat release rate curve A1 and the pressure curve A2 are also merely an example. Below, attention is focused on the heat release rate curve A1.

[0052] The main injection starts at a timing before the top dead center of the piston 68. In the example of Fig. 4, the main injection starts when the crank angle is "-20 degrees".

[0053] Next, the start of combustion of the gaseous fuel is the timing when the heat release rate indicated by the heat release rate curve A1 rises from zero (almost zero). In the example of Fig. 4, when the crank angle is "-10 degrees", the gaseous fuel is ignited by the liquid fuel, and the combustion of the gaseous fuel starts. At the same time that the gaseous fuel starts to burn in the combustion chamber 71, the flame starts to propagate in the combustion chamber 71.

[0054] Next, the end of flame propagation is generally the timing when the heat release rate indicated by the heat release rate curve A1 reaches a maximum value. In the example of Fig. 4, the flame propagation ends when the crank angle is "10 degrees". The end of flame propagation indicates the timing when the flame reaches the inner circumferential surface of the combustion chamber 71.

[0055] The flame propagation period T0 indicates the period from the start of flame propagation (eg, crank angle = -10 degrees) to the end of flame propagation (eg, crank angle = 10 degrees).

[0056] The first half of the flame propagation period T0 indicates the period from the start of flame propagation (e.g., crank angle = -10 degrees) to the middle of flame propagation (e.g., crank angle = 0 degrees). The second half of the flame propagation period T0 indicates the period from the middle of flame propagation (e.g., crank angle = 0 degrees) to the end of flame propagation (e.g., crank angle = 10 degrees).

[0057] Next, the end of the gaseous fuel combustion is the timing when the heat release rate indicated by the heat release rate curve A1 becomes zero (approximately zero) from a value greater than zero (approximately zero). In the example of Fig. 4, the gaseous fuel combustion ends when the crank angle is "50 degrees".

[0058] The gaseous fuel combustion period T12 indicates the period from the start of gaseous fuel combustion (e.g., crank angle = -10 degrees) to the end of combustion (e.g., crank angle = 50 degrees). The first half period T1 of the gaseous fuel combustion period T12 indicates the period from the start of gaseous fuel combustion (e.g., crank angle = -10 degrees) to the middle of combustion (e.g., crank angle = 20 degrees). The second half period T2 of the gaseous fuel combustion period T12 indicates the period from the middle of gaseous fuel combustion (e.g., crank angle = 20 degrees) to the end of combustion (e.g., crank angle = 50 degrees).

[0059] In this embodiment, the liquid fuel injection unit 66 (FIG. 1) executes an injection of liquid fuel subsequent to the main injection (subsequent injection) after the flame propagation following ignition of the gaseous fuel is completed. In other words, the liquid fuel injection unit 66 executes an injection of liquid fuel subsequent to the main injection after the end of flame propagation (10 degrees) and before the end of combustion of the gaseous fuel (50 degrees). "After the end" includes "at the end".

[0060] Preferably, the control unit 211 (FIG. 2) controls the liquid fuel injection unit 66 so that the injection of the subsequent liquid fuel (injection of the liquid fuel after the flame propagation is completed) is started within the first half period T3 of the combustion period T12 of the gas fuel. As a result, the liquid fuel injection unit 66 executes the injection of the subsequent liquid fuel within the first half period T3 of the combustion period T12 of the gas fuel after the flame propagation after the ignition of the gas fuel is completed. Therefore, according to this preferred example, the temperature of the combustion chamber 71 becomes higher than that at the time of ignition at the timing when the subsequent liquid fuel reaches the vicinity of the crevice volume. Therefore, it is possible to effectively oxidize the unburned hydrocarbons leaking from the crevice volume when the piston 68 moves from the top dead center to the bottom dead center. As a result, it is possible to effectively reduce the remaining amount of unburned hydrocarbons. Note that the period T3 indicates the period in the state in which the flame propagation after the ignition of the gas fuel is completed.

[0061] Here, the crevice volume refers to the gap or the gap volume between the piston 68 and the liner 67.

[0062] Next, the amount of nitrogen oxides produced and the amount of remaining unburned hydrocarbons in this embodiment will be described with reference to FIG. 5. FIG. 5(a) is a graph that shows a schematic representation of the amount of remaining unburned hydrocarbons in the engine 1. The vertical axis shows the amount of remaining unburned hydrocarbons (ppmC1). FIG. 5(b) is a graph that shows a schematic representation of the amount of nitrogen oxides produced in the engine 1. The vertical axis shows the amount of nitrogen oxides produced (ppm). In FIG. 5(a) and FIG. 5(b), the horizontal axis shows the timing of the subsequent injection in terms of crank angle (deg. aTDC). A crank angle of 0 degrees indicates the top dead center of the piston 68. The horizontal axis can also be interpreted as showing time in terms of crank angle.

[0063] In FIG. 5(a), the dashed line L1 indicates the remaining amount of unburned hydrocarbons when subsequent injection is not performed (main injection only, reference example). The dots D1 indicated by black circles indicate the remaining amount of unburned hydrocarbons when subsequent injection is performed. Hereinafter, the remaining amount when subsequent injection is not performed will be referred to as "L1", and the remaining amount when subsequent injection is performed will be referred to as "D1" corresponding to the dots D1.

[0064] The remaining amount D1 of unburned hydrocarbons when subsequent injection is performed is smaller than the remaining amount L1 of unburned hydrogen when subsequent injection is not performed. In particular, the larger the crank angle at which subsequent injection is performed, the smaller the remaining amount D1 of unburned hydrocarbons. In other words, after the end of flame propagation following ignition of the gaseous fuel, the later the timing of the subsequent injection, the smaller the remaining amount D1 of unburned hydrocarbons.

[0065] In Figure 5(b), dashed line L2 indicates the amount of nitrogen oxides produced when subsequent injection is not performed (main injection only, reference example). Dots D2, indicated by black circles, indicate the amount of nitrogen oxides produced when subsequent injection is performed. Hereinafter, the amount produced when subsequent injection is not performed will be referred to as "L2", and the amount produced when subsequent injection is performed will be referred to as "D2" corresponding to dot D2.

[0066] The amount D2 of nitrogen oxides generated when the subsequent injection is performed is smaller than the amount L2 of nitrogen oxides generated when the subsequent injection is not performed. In particular, the smaller the crank angle at which the subsequent injection is performed, the smaller the amount L2 of nitrogen oxides generated. In other words, the earlier the timing of the subsequent injection after the end of flame propagation following ignition of the gaseous fuel, the smaller the amount L2 of nitrogen oxides generated.

[0067] As described above, as shown in Figures 5(a) and 5(b), the remaining amount of unburned hydrocarbons and the generation amount of nitrogen oxides can be reduced by performing subsequent injection. However, the timing of subsequent injection at which the reduction width (=L1-D1) of the remaining amount of unburned hydrocarbons D1 becomes large (e.g., crank angle = 60 degrees) is the timing of subsequent injection at which the reduction width (=L2-D2) of the generation amount of nitrogen oxides D2 becomes small (e.g., crank angle = 60 degrees). On the other hand, the timing of subsequent injection at which the reduction width of the generation amount of nitrogen oxides D2 becomes large (e.g., crank angle = 30 degrees) is the timing of subsequent injection at which the reduction width of the remaining amount of unburned hydrocarbons D1 becomes small (e.g., crank angle = 30 degrees). Therefore, by adjusting the timing of subsequent injection, the remaining amount of unburned hydrocarbons D1 and the generation amount of nitrogen oxides D2 can be adjusted.

[0068] Unburned hydrocarbons and nitrogen oxides are examples of exhaust emissions from engine 1.

[0069] In particular, in this embodiment, it is preferable that the control unit 211 (FIG. 2) adjusts the amount of exhaust substances (unburned hydrocarbons and nitrogen oxides) from the engine 1 by changing at least one of the number of injections of the subsequent liquid fuel (number of injections of liquid fuel after flame propagation is completed), the injection amount of the subsequent liquid fuel (amount of liquid fuel injected after flame propagation is completed), and the injection timing of the subsequent liquid fuel (timing of injection of liquid fuel after flame propagation is completed). According to this preferable example, it is possible to easily adjust the balance between the remaining amount of unburned hydrocarbons and the amount of nitrogen oxides generated, while reducing the remaining amount of unburned hydrocarbons and the amount of nitrogen oxides generated, compared to the case where the subsequent injection is not performed.

[0070] Specifically, the control unit 211 adjusts the remaining amount of unburned hydrocarbons and the generation amount of nitrogen oxides by changing at least one of the number of subsequent injections, the injection amount of subsequent injection, and the timing of subsequent injection based on the relationship between the remaining amount D1 of unburned hydrocarbons and the timing of subsequent injection, and the relationship between the generation amount D2 of nitrogen oxides and the timing of subsequent injection. In this case, the control unit 211 can change the number of subsequent injections, the injection amount of subsequent injection, and the timing of subsequent injection by controlling the liquid fuel injection unit 66.

[0071] As an example, the control unit 211 executes subsequent injections at both the timing of subsequent injections (e.g., crank angle = 30 degrees) where the reduction width (= L2 - D2) of the nitrogen oxide generation amount D2 becomes large, and the timing of subsequent injections (e.g., crank angle = 60 degrees) where the reduction width (= L1 - D1) of the remaining amount of unburned hydrocarbons becomes large. In this example, it is possible to increase both the reduction width of the nitrogen oxide generation amount D2 and the reduction width of the remaining amount of unburned hydrocarbons D1.

[0072] Still referring to FIG. 5, the relationship between unburned hydrocarbons and nitrogen oxides, and the excess air factor λ and the air-fuel ratio will be described.

[0073] The excess air ratio λ is a value (Ma / Mb) obtained by dividing the mass Ma of air actually supplied to the combustion chamber 71 by the theoretically required minimum mass Mb of air, and is an index showing the degree of excess air in the mixed gas. The excess air ratio λ is also equal to a value obtained by dividing the actual air-fuel ratio by the theoretical air-fuel ratio. For example, when the excess air ratio λ is smaller than "1", the mixed gas of gaseous fuel and air is a rich mixture (a mixture with a rich gaseous fuel). A state in which the excess air ratio λ of the mixed gas is smaller than "1" may be referred to as a "rich state". On the other hand, for example, when the excess air ratio λ is larger than "1", the mixed gas of gaseous fuel and air is a lean mixture (a mixture with a lean gaseous fuel). A state in which the excess air ratio λ of the mixed gas is larger than "1" may be referred to as a "lean state".

[0074] The air-fuel ratio is the mass of air Mx divided by the mass of gaseous fuel My (Mx / My).

[0075] In Fig. 5(a), dots D10 indicated by white circles indicate the amount of unburned hydrocarbons remaining when subsequent injection is performed. Hereinafter, the amount of unburned hydrocarbons remaining when subsequent injection is performed may be referred to as "D10" in correspondence with dots D10.

[0076] The excess air ratio λ when the remaining amount of unburned hydrocarbons indicates the remaining amount D10 (e.g., a lean state richer than the effective lean state) is smaller than the excess air ratio λ when the remaining amount of unburned hydrocarbons indicates the remaining amount D1 (e.g., an effective lean state). Therefore, when the timing of the subsequent injection is the same, the smaller the excess air ratio λ (air-fuel ratio), the smaller the remaining amount of unburned hydrocarbons. Therefore, by controlling the excess air ratio λ (air-fuel ratio), the remaining amount of unburned hydrocarbons can be adjusted even when the timing of the subsequent injection is the same.

[0077] In Fig. 5(b), the dot D20 indicated by a white circle indicates the amount of nitrogen oxides generated when the subsequent injection is performed. Hereinafter, the amount of nitrogen oxides generated when the subsequent injection is performed may be referred to as "D20" in correspondence with the dot D20.

[0078] The excess air factor λ when the amount of nitrogen oxides generated indicates the amount D20 (e.g., a lean state richer than the effective lean state) is smaller than the excess air factor λ when the amount of nitrogen oxides generated indicates the amount D2 (e.g., an effective lean state). Therefore, when the timing of the subsequent injection is the same, the smaller the excess air factor λ (air-fuel ratio), the greater the amount of nitrogen oxides generated. Therefore, by controlling the excess air factor λ (air-fuel ratio), the amount of nitrogen oxides generated can be adjusted even when the timing of the subsequent injection is the same.

[0079] Particularly in this embodiment, the control unit 211 (FIG. 2) controls the excess air ratio λ in the combustion chamber 71 according to the conditions when the subsequent liquid fuel is injected (subsequent injection conditions). In other words, the control unit 211 controls the excess air ratio λ in the combustion chamber 71 according to the conditions when the liquid fuel is injected after the end of flame propagation. As a result, it is possible to adjust the balance between the effect of reducing the remaining amount of unburned hydrocarbons and the effect of reducing the amount of nitrogen oxides produced according to the purpose of exhaust emission reduction.

[0080] The condition for the subsequent injection is, for example, the timing of the subsequent injection. As can be seen from Figures 5(a) and 5(b), the contribution of the subsequent injection to either the reduction effect of unburned hydrocarbons or the reduction effect of nitrogen oxides changes depending on the timing of the subsequent injection. Therefore, for example, when the subsequent injection is performed at a crank angle (for example, a timing of 30 degrees) at which the reduction amount of nitrogen oxides is large, the control unit 211 shifts the excess air ratio λ to the rich side while maintaining it lean. As a result, the amount of nitrogen oxides generated, which had easily cleared the legal regulation level, can be returned to the legal regulation level, and the effect of reducing the remaining amount of unburned hydrocarbons can be increased.

[0081] Here, examples of the control amount for controlling the excess air ratio λ include the mixed gas flow rate, the gas fuel flow rate, and the air flow rate. Therefore, the control unit 211 can control the excess air ratio λ by controlling the control amount.

[0082] Next, the combustion cycle BCY of the engine 1 will be described with reference to Fig. 2 and Fig. 6. Fig. 6 is a time chart that shows a schematic representation of the combustion cycle BCY of the engine 1. The horizontal axis indicates time. The time is indicated by, for example, a crank angle. The pulse shape C1 shows a schematic representation of a period during which gaseous fuel is supplied. The curve A1 corresponds to the heat release rate curve A1 in Fig. 4, and shows a schematic representation of a heat generation state. Moreover, the period T0 corresponds to the flame propagation period T0 in Fig. 4, and the period T12 corresponds to the combustion period T12 in Fig. 4.

[0083] As shown in FIG. 2 and FIG. 6, the engine 1 repeats the combustion cycle BCY. Focus on the predetermined period T. The engine 1 executes the combustion cycle BCY multiple times in the predetermined period T. The liquid fuel injection unit 66 executes the ignition of the gas fuel (main injection) and the subsequent injection of the liquid fuel (injection of the liquid fuel after the flame propagation is completed) for each combustion cycle BCY. The control unit 211 prohibits an increase in the injection amount of the liquid fuel when igniting the gas fuel (injection amount of the main injection) within the predetermined period T. "Increase in the injection amount of the liquid fuel when igniting the gas fuel" refers to an increase in the injection amount of the liquid fuel according to an increase in the load on the engine 1. An increase in the load indicates, for example, that the load on the engine 1 exceeds a threshold value. The cause of the load increase is, for example, an increase in the ship speed, a change in the rudder angle, or strong waves and winds that the ship receives. The predetermined period T indicates 10 seconds.

[0084] That is, in this embodiment, an increase in the injection amount of liquid fuel (main injection amount) when igniting the gaseous fuel due to an increase in load is prohibited within 10 seconds. In other words, a short-term increase in the injection amount due to an increase in the load on the engine 1 is prohibited.

[0085] Therefore, in the engine system 100 having the turbocharger 5, injection of liquid fuel to such an extent that it would cause a sudden increase in boost pressure is prohibited. As a result, it is possible to suppress a large fluctuation in the excess air ratio λ (air-fuel ratio) relative to the target value, and to prevent a deterioration in exhaust emissions. A deterioration in exhaust emissions indicates, for example, an increase in the amount of nitrogen oxides produced or an increase in the amount of remaining unburned hydrocarbons.

[0086] 6, the flame propagation period T0 is, for example, about 4.6 milliseconds (=crank angle of about 20 degrees). The gas fuel combustion period T12 is, for example, about 13.4 milliseconds (=crank angle of about 60 degrees). The combustion cycle BCY is, for example, 1 / 6 seconds (=crank angle of 720 degrees).

[0087] Next, the injection angle θ of the liquid fuel by the liquid fuel injection unit 66 will be described with reference to Fig. 7. Fig. 7 is a diagram that illustrates the injection angle θ of the liquid fuel according to this embodiment. As shown in Fig. 7, the unburned hydrocarbons THC that have been pushed into the crevice volume VL leak out from the crevice volume VL when the piston 68 moves from the top dead center to the bottom dead center.

[0088] Therefore, in this embodiment, the injection angle θ of the liquid fuel by the liquid fuel injection unit 66, that is, the injection angle θ of the liquid fuel with respect to the moving direction of the piston 68, is 30 degrees or more and 65 degrees or less. Therefore, at the timing when the unburned hydrocarbons THC leak from the crevice volume VL when the piston 68 moves from the top dead center to the bottom dead center, the liquid fuel injected from the liquid fuel injection unit 66 (for example, the liquid fuel by the subsequent injection) reaches the vicinity of the crevice volume VL. As a result, the unburned hydrocarbons THC leaking from the crevice volume VL can be effectively oxidized by the liquid fuel. Therefore, the remaining amount of unburned hydrocarbons THC can be effectively reduced.

[0089] The control unit 211 (FIG. 2) may impose a limit on at least one of the control of the injection amount of liquid fuel in the subsequent injection and the control of the timing of the subsequent injection, depending on the specifications of the engine 1 and / or the specifications of the liquid fuel injection unit 66. This is because, depending on the injection amount of liquid fuel in the subsequent injection and / or the timing of the subsequent injection, the liquid fuel may collide with the liner 67, and the lubricating oil on the surface of the liner 67 may be diluted by the liquid fuel. The specifications of the engine 1 are, for example, the bore or stroke of the engine 1. The specifications of the liquid fuel injection unit 66 are, for example, the nozzle diameter, injection angle θ, or injection amount of the liquid fuel injection unit 66.

[0090] Next, a gaseous fuel combustion method in the engine 1 will be described with reference to Fig. 2 and Fig. 8. Fig. 8 is a flowchart showing a gaseous fuel combustion method according to this embodiment. The gaseous fuel combustion method is executed by the engine system 100 shown in Fig. 2. As shown in Fig. 8, the gaseous fuel combustion method includes steps S1 to S4.

[0091] First, in step S1, the control unit 211 of the engine system 100 determines whether or not the timing for main injection has arrived.

[0092] If it is determined in step S1 that the timing for the main injection has not yet arrived (No), the process waits for step S1.

[0093] On the other hand, if it is determined in step S1 that the timing for the main injection has arrived (Yes), the process proceeds to step S2.

[0094] Next, in step S2, the control unit 211 controls the liquid fuel injection unit 66 to perform main injection. As a result, the liquid fuel injection unit 66 performs main injection. That is, the liquid fuel injection unit 66 injects liquid fuel and ignites the gas fuel. As a result, the gas fuel is burned.

[0095] Next, in step S3, the control unit 211 determines whether or not the timing for the subsequent injection has arrived.

[0096] If it is determined in step S3 that the timing for the subsequent injection has not yet arrived (No), the process waits for step S3.

[0097] On the other hand, if it is determined in step S3 that the timing for the subsequent injection has arrived (Yes), the process proceeds to step S4.

[0098] Next, in step S4, the control unit 211 controls the liquid fuel injection unit 66 to perform a subsequent injection. As a result, the liquid fuel injection unit 66 performs a subsequent injection. Specifically, the liquid fuel injection unit 66 performs injection of liquid fuel after flame propagation after ignition of the gas fuel is completed. Then, the process proceeds to step S1.

[0099] Steps S1 to S4 are executed in one combustion cycle. Therefore, steps S1 to S4 are executed for each combustion cycle that is repeatedly executed. In other words, steps S1 to S4 are repeated.

[0100] As described above with reference to Fig. 8, according to the gaseous fuel combustion method of this embodiment, the injection (subsequent injection) of liquid fuel is executed after the flame propagation following the ignition of the gaseous fuel is completed. As a result, at least one of the suppression of the generation of nitrogen oxides and the suppression of remaining unburned hydrocarbons can be realized. In particular, when the mixed gas supplied to the combustion chamber 71 is in an effective lean state, the generation of nitrogen oxides can be suppressed while the remaining unburned hydrocarbons can be suppressed.

[0101] When the liquid fuel injection unit 66 executes multiple subsequent injections in one combustion cycle, the same processes as steps S3 and S4 are executed multiple times.

[0102] (Modification) An engine system 100 according to a modification of the present embodiment will be described with reference to Fig. 2 and Fig. 9. The modification mainly differs from the present embodiment in that the engine system 100 according to the modification includes a measuring unit 25. Below, the differences between the modification and the present embodiment will be mainly described.

[0103] As shown in FIG. 2, in the modified example, the engine system 100 further includes a measurement unit 25. The measurement unit 25 measures a physical quantity that directly or indirectly indicates exhaust emissions generated due to the combustion of the gaseous fuel. Then, the control unit 211 controls the subsequent injection of liquid fuel by the liquid fuel injection unit 66 (injection of liquid fuel after the end of flame propagation) based on the measurement result of the measurement unit 25. That is, in the modified example, feedback control of the subsequent injection is performed according to the measurement result of the physical quantity that directly or indirectly indicates the exhaust emissions. Therefore, the subsequent injection can be optimized according to the combustion state of the gaseous fuel. As a result, the amount of exhaust emissions can be effectively reduced. That is, the generation of nitrogen oxides can be effectively suppressed while the remaining unburned hydrocarbons can be effectively suppressed.

[0104] The "physical quantity directly indicating the exhaust substance" measured by the measuring unit 25 is, for example, the concentration or mass of the exhaust substance. In this case, the measuring unit 25 is, for example, a NOx sensor that detects the concentration of nitrogen oxides (NOx). The unit of concentration is, for example, %, or ppm. The unit of mass is, for example, g or kg. Note that when the exhaust substance is soot, the measuring unit 25 is, for example, a soot sensor that detects the amount of soot.

[0105] The control unit 211 controls the subsequent injection by the liquid fuel injection unit 66 based on a physical quantity that directly indicates the exhaust emissions. For example, the control unit 211 controls at least one of the injection amount of liquid fuel in the subsequent injection, the timing of the subsequent injection, and the number of subsequent injections based on a physical quantity that directly indicates the exhaust emissions.

[0106] Furthermore, the "physical quantity indirectly indicating the exhaust material" measured by the measuring unit 25 is, for example, the in-cylinder pressure (the internal pressure of the combustion chamber 71), the temperature of the exhaust gas, or the air excess ratio λ. In this case, the measuring unit 25 is, for example, a pressure sensor or a temperature sensor. Alternatively, the measuring unit 25 is an O2 sensor, a λ sensor, or an A / F sensor for detecting the air excess ratio λ.

[0107] The control unit 211 estimates the combustion state of the gas fuel based on a physical quantity indirectly indicating exhaust emissions. Then, the control unit 211 controls the subsequent injection by the liquid fuel injection unit 66 based on the estimation result of the combustion state of the gas fuel. For example, the control unit 211 controls at least one of the injection amount of liquid fuel in the subsequent injection, the timing of the subsequent injection, and the number of subsequent injections based on the estimation result of the combustion state of the gas fuel.

[0108] When the measuring unit 25 measures the temperature of the exhaust gas, the control unit 211 monitors the temperature of the exhaust gas and executes control so that the temperature of the exhaust gas becomes equal to or lower than the threshold temperature. As a result, it is possible to suppress an excessive rise in the temperature of the exhaust gas during the subsequent injection.

[0109] Next, a gaseous fuel combustion method according to a modified example will be described with reference to Fig. 2 and Fig. 9. Fig. 9 is a flowchart showing the gaseous fuel combustion method according to the modified example. The gaseous fuel combustion method is executed by the engine system 100 shown in Fig. 2. As shown in Fig. 9, the gaseous fuel combustion method includes steps S11 to S15.

[0110] First, in step S11, the control unit 211 of the engine system 100 controls the liquid fuel injection unit 66 to perform main injection. As a result, the liquid fuel injection unit 66 performs main injection. As a result, the gas fuel is combusted.

[0111] Next, in step S12, the control unit 211 controls the liquid fuel injection unit 66 to perform a subsequent injection. As a result, the liquid fuel injection unit 66 performs a subsequent injection. Specifically, the liquid fuel injection unit 66 performs injection of the liquid fuel after the flame propagation after the ignition of the gas fuel is completed.

[0112] Next, in step S13, it is determined whether or not a predetermined number of combustion cycles have been executed.

[0113] If it is determined in step S13 that the predetermined number of combustion cycles has not been executed (No), the process proceeds to step S11.

[0114] On the other hand, if it is determined in step S13 that the predetermined number of combustion cycles have been executed (Yes), the process proceeds to step S14.

[0115] Next, in step S14, the control unit 211 acquires measurement data from the measurement unit 25. The measurement data indicates the measurement results of physical quantities that directly or indirectly indicate exhaust substances.

[0116] Next, in step S15, the control unit 211 determines, based on the measurement data, control parameters for the subsequent injection by the liquid fuel injection unit 66. The control parameters include at least one of the number of subsequent injections, the injection amount of liquid fuel in the subsequent injection, and the timing of the subsequent injection.

[0117] Then, after step S15, the process proceeds to step S11. Next, in step S11, the control unit 211 controls the liquid fuel injection unit 66 to execute a main injection. Next, in step S12, the control unit 211 controls the liquid fuel injection unit 66 to execute a subsequent injection according to the control parameters determined in the previous step S15. In this case as well, the injection of liquid fuel (subsequent injection) is executed after the flame propagation after the ignition of the gaseous fuel is completed. Thereafter, steps S13 to S15 are executed. Furthermore, steps S11 to S15 are repeated as the combustion cycle is repeated.

[0118] As described above with reference to FIG. 9, in the modified example, the combustion cycle is repeated, and the main injection and the subsequent injection are performed for each combustion cycle.

[0119] Next, the present invention will be described in detail based on examples, but the present invention is not limited to the following examples. EXAMPLES

[0120] Examples 1 to 6 of the present invention and a comparative example will be described with reference to Fig. 1 and Fig. 10. In Examples 1 to 6, the engine system 100 shown in Fig. 1 was used. In Examples 1 to 6, the subsequent injection was performed after the flame propagation after the ignition of the gaseous fuel was completed. On the other hand, in the comparative example, the subsequent injection was not performed. In Examples 1 to 6 and the comparative example, natural gas obtained by vaporizing liquefied natural gas was used as the gaseous fuel.

[0121] FIG. 10(a) is a graph showing the remaining amount of unburned hydrocarbons according to Examples 1 to 6 of the present invention. The vertical axis shows the remaining amount of unburned hydrocarbons (ppmC1). FIG. 10(b) is a graph showing the production amount of nitrogen oxides according to Examples 1 to 6 of the present invention. The vertical axis shows the production amount of nitrogen oxides (ppm). In FIG. 10(a) and FIG. 10(b), the horizontal axis shows the timing of the subsequent injection by crank angle (deg. aTDC). A crank angle of 0 degrees indicates the top dead center of the piston 68.

[0122] In FIG. 10(a), the dashed line L1 indicates the remaining amount of unburned hydrocarbons (hereinafter, L1) when subsequent injection is not performed (main injection only, comparative example). The dots Da connected by a solid line indicate the remaining amount of unburned hydrocarbons (hereinafter, Da) when subsequent injection is performed in Example 1. The dots Db connected by a dashed line indicate the remaining amount of unburned hydrocarbons (hereinafter, Db) when subsequent injection is performed in Example 2. The dots Dc connected by a dashed line indicate the remaining amount of unburned hydrocarbons (hereinafter, Dc) when subsequent injection is performed in Example 3.

[0123] In Figure 10(b), dashed line L2 indicates the amount of nitrogen oxides generated when no subsequent injection is performed (main injection only, comparative example) (hereinafter, L2). Dots DA connected by a solid line indicate the amount of nitrogen oxides generated when subsequent injection is performed in Example 1 (hereinafter, DA). Dots DB connected by a dashed line indicate the amount of nitrogen oxides generated when subsequent injection is performed in Example 2 (hereinafter, DB). Dots DC connected by a dashed line indicate the amount of nitrogen oxides generated when subsequent injection is performed in Example 3 (hereinafter, DC).

[0124] As shown in FIG. 10(a) and FIG. 10(b), in each of Examples 1 to 3, the subsequent injection was performed at a crank angle of 30 degrees, 50 degrees, 70 degrees, and 90 degrees. In Example 1 (residual amount Da, production amount DA), the injection amount of the subsequent injection was "20 mm3". In Example 2 (residual amount Db, production amount DB), the injection amount of the subsequent injection was "40 mm3". In Example 3 (residual amount Dc, production amount DC), the injection amount of the subsequent injection was "60 mm3". In Examples 1 to 3 and the comparative example, the mixed gas supplied to the combustion chamber 71 was in an effective lean state. Specifically, λ was about 1.81. In Examples 1 to 3 and the comparative example, the specified range of the excess air ratio λ that defines the effective lean state indicates a range of 1.80 to 2.00.

[0125] As shown in Fig. 10(a), in Examples 1 to 3, the remaining amounts Da to Dc of unburned hydrocarbons when subsequent injections were performed were smaller than the remaining amount L1 of unburned hydrogen when subsequent injections were not performed (Comparative Example). In particular, in Examples 1 to 3, the remaining amounts Da to Dc of unburned hydrocarbons decreased as the crank angle at which subsequent injections were performed increased. In other words, in Examples 1 to 3, after the end of flame propagation following ignition of the gaseous fuel, the later the timing of the subsequent injection, the smaller the remaining amounts Da to Dc of unburned hydrocarbons. Also, from Examples 1 to 3, the larger the injection amount of subsequent injection, the smaller the remaining amounts Da to Dc of unburned hydrocarbons.

[0126] As shown in Fig. 10(b), in Examples 1 to 3, the amounts of nitrogen oxides generated DA to DC when subsequent injections were performed were smaller than the amount of nitrogen oxides generated L2 (comparative example) when subsequent injections were not performed. In particular, in Examples 1 to 3, the smaller the crank angle at which subsequent injections were performed, the smaller the amounts of nitrogen oxides generated DA to DC. In other words, in Examples 1 to 3, the earlier the timing of the subsequent injection after the end of flame propagation following ignition of the gaseous fuel, the smaller the amounts of nitrogen oxides generated DA to DC. Also, in Examples 1 to 3, the larger the amount of subsequent injection, the smaller the amounts of nitrogen oxides generated DA to DC.

[0127] As shown in Figures 10(a) and 10(b), in Examples 1 to 3, the remaining amounts of unburned hydrocarbons Da to Dc and the amounts of nitrogen oxides generated DA to DC were reduced compared to the comparative example. In Example 1, the greater the reduction in the remaining amount of unburned hydrocarbons Da (= L1 - Da), the smaller the reduction in the amount of nitrogen oxides generated DA (= L2 - DA). Also, in Example 1, the greater the reduction in the amount of nitrogen oxides generated DA, the smaller the reduction in the remaining amount of unburned hydrocarbons Da. The same was true for Examples 2 and 3 in these respects.

[0128] Next, still with reference to FIG. 10, Examples 4 to 6 in which the excess air ratio λ is different from that in Examples 1 to 3 will be described.

[0129] In Fig. 10(a), dots da shown as white circles indicate the remaining amount of unburned hydrocarbons (hereinafter, da) when subsequent injection is performed in Example 4. Dots db shown as white triangles indicate the remaining amount of unburned hydrocarbons (hereinafter, db) when subsequent injection is performed in Example 5. Dots dc shown as white squares indicate the remaining amount of unburned hydrocarbons (hereinafter, dc) when subsequent injection is performed in Example 6.

[0130] In Fig. 10(b), dots dA shown by white circles indicate the amount of nitrogen oxides (hereinafter, dA) generated when subsequent injection is performed in Example 4. Dots dB shown by white triangles indicate the amount of nitrogen oxides (hereinafter, dB) generated when subsequent injection is performed in Example 5. Dots dC shown by white squares indicate the amount of nitrogen oxides (hereinafter, dC) generated when subsequent injection is performed in Example 6.

[0131] As shown in FIG. 10(a) and FIG. 10(b), in Examples 4 to 6 (residual amounts da to dc, production amounts dA to dC), the subsequent injection was performed at a crank angle of 30 degrees. In Examples 4 to 6, the mixture gas supplied to the combustion chamber 71 was in a lean state that was richer than the effective lean state. Specifically, λ=1.76. In Examples 4 to 6, the specified range of the excess air ratio λ that defines the effective lean state is a range of 1.80 to 2.00. The lean state indicates a state in which the excess air ratio λ is greater than "1". The other conditions of Example 4 were the same as those of Example 1, the other conditions of Example 5 were the same as those of Example 2, and the other conditions of Example 6 were the same as those of Example 3.

[0132] 10(a), as a result of comparing Examples 4 to 6 (residual amounts da to dc) with Examples 1 to 3 (residual amounts Da to Dc) at the same crank angle (=30 degrees), the remaining amounts da to dc of unburned hydrocarbons in a "lean state richer than the effective lean state" were smaller than the remaining amounts Da to Dc of unburned hydrocarbons in the effective lean state. In other words, when the excess air factor λ (air-fuel ratio) was reduced, the remaining amounts da to dc of unburned hydrocarbons became smaller.

[0133] 10(b), a comparison of Examples 4-6 (production amounts dA-dC) with Examples 1-3 (production amounts DA-DC) at the same crank angle (=30 degrees) showed that the nitrogen oxide production amounts dA-dC in a "lean state richer than the effective lean state" were greater than the nitrogen oxide production amounts DA-DC in the effective lean state. In other words, when the excess air factor λ (air-fuel ratio) was reduced, the nitrogen oxide production amounts dA-dC increased.

[0134] The embodiments and examples of the present invention have been described above with reference to the drawings. However, the present invention is not limited to the above embodiments, and can be implemented in various aspects without departing from the gist of the present invention. In addition, the components disclosed in the above embodiments can be modified as appropriate. For example, a certain component among all the components shown in one embodiment may be added to a component of another embodiment, or some components among all the components shown in one embodiment may be deleted from the embodiment.

[0135] In addition, the drawings are mainly schematic illustrations of each component to facilitate understanding of the invention, and the thickness, length, number, spacing, etc. of each component shown in the drawings may differ from the actual ones due to the convenience of creating the drawings. In addition, the configurations of each component shown in the above embodiment are merely examples and are not particularly limited, and it goes without saying that various modifications are possible within a range that does not substantially deviate from the effects of the present invention.

[0136] The engine system 100 described with reference to FIG. 1 has only a gas fuel mode. The gas fuel mode is a mode in which mechanical work is obtained by burning gas fuel. However, the engine system 100 may have a gas fuel mode and a liquid fuel mode. The liquid fuel mode is a mode in which mechanical work is obtained by burning liquid fuel. In this case, the engine system 100 includes, in addition to the liquid fuel injection unit 66 that injects liquid fuel for ignition, another liquid fuel injection unit that injects liquid fuel into the combustion chamber 71 to obtain mechanical work by combustion. The engine system 100 may also have a dual-fuel mode. The dual-fuel mode is a mode in which both gas fuel and liquid fuel are burned approximately simultaneously to obtain mechanical work.

[0137] <Notes on the invention> According to one aspect of the present invention, an engine system has a combustion chamber to which air and gaseous fuel are supplied, and burns the gaseous fuel. The engine system includes a liquid fuel injection unit and a control unit. The liquid fuel injection unit injects liquid fuel and ignites the gaseous fuel. The control unit controls the liquid fuel injection unit. The control unit controls the liquid fuel injection unit to inject the liquid fuel after flame propagation after ignition of the gaseous fuel has ended.

[0138] According to another aspect of the present invention, a method for burning gaseous fuel in an engine in which air and gaseous fuel are supplied to a combustion chamber and the gaseous fuel is combusted includes the steps of injecting liquid fuel and igniting the gaseous fuel, and performing injection of the liquid fuel after completion of flame propagation after ignition of the gaseous fuel. [Industrial Applicability]

[0139] The present invention relates to an engine system and a gaseous fuel combustion method, and has industrial applicability. [Explanation of symbols]

[0140] 1 Engine 21 Engine control device 25 Measurement section 65 Gaseous fuel supply unit 66 Liquid fuel injection part 68 Piston 71 Combustion chamber 100 Engine System 211 Control Unit

Claims

1. 1. An engine system having a combustion chamber to which air and a gaseous fuel are supplied, the engine system combusting the gaseous fuel, a liquid fuel injection unit that injects liquid fuel and ignites the gas fuel; a control unit for controlling the liquid fuel injection unit; Equipped with The control unit is After the flame propagation after the ignition of the gaseous fuel is completed, a subsequent injection of the liquid fuel is performed and combusted; An engine system that, when combusting the gaseous fuel, at least one of suppressing the production of nitrogen oxides and suppressing the remaining unburned hydrocarbons.

2. 2. The engine system according to claim 1, wherein the control unit is capable of changing at least one of a number of injections of the liquid fuel after completion of flame propagation, an injection amount of the liquid fuel after completion of flame propagation, and an injection timing of the liquid fuel after completion of flame propagation.

3. A plurality of combustion cycles are performed in a predetermined period of time; the liquid fuel injection unit executes ignition of the gaseous fuel and subsequent injection of the liquid fuel after completion of flame propagation for each combustion cycle; 3. The engine system according to claim 1, wherein the control unit prohibits an increase in the injection amount of the liquid fuel when the gaseous fuel is ignited within the predetermined period.

4. an increase in the injection amount of the liquid fuel when the gas fuel is ignited means an increase in the injection amount of the liquid fuel in response to an increase in load on an engine, 4. The engine system of claim 3, wherein the predetermined period of time is ten seconds.

5. 1. A method for combusting gaseous fuel in an engine, comprising: supplying air and gaseous fuel to a combustion chamber and combusting the gaseous fuel, the method comprising the steps of: injecting a liquid fuel to ignite the gaseous fuel; After the flame propagation after the ignition of the gaseous fuel is completed, a subsequent injection of the liquid fuel is performed and combusted. Including, A method for burning a gaseous fuel, comprising: suppressing at least one of the generation of nitrogen oxides and the remaining unburned hydrocarbons when burning the gaseous fuel.

6. 6. The gas fuel combustion method according to claim 5, wherein at least one of the number of injections of the liquid fuel after completion of flame propagation, the amount of injection of the liquid fuel after completion of flame propagation, and the injection timing of the liquid fuel after completion of flame propagation can be changed.

7. A plurality of combustion cycles are performed in a predetermined period of time; In each combustion cycle, ignition of the gaseous fuel and subsequent injection of the liquid fuel after completion of flame propagation are performed; 7. The method for burning gaseous fuel according to claim 5, wherein an increase in an injection amount of the liquid fuel when igniting the gaseous fuel is prohibited within the predetermined period.

8. an increase in the injection amount of the liquid fuel when the gas fuel is ignited means an increase in the injection amount of the liquid fuel in response to an increase in load on the engine, 8. The method of claim 7, wherein the predetermined period of time is ten seconds.

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