gas engine

Abstract

To provide a gas engine capable of suppressing NOx emissions in the Non-Road Transient Cycle regarding the EU Stage V for a gas engine that operates through spark ignition combustion of a premixed air-fuel mixture.SOLUTION: A gas engine 2 includes: a fuel gas supply valve 21 that controls a supply amount of gaseous fuel; a gas mixer 29 that generates an air-fuel mixture by mixing the gaseous fuel and air with each other; a throttle valve 22 that controls an amount of the air-fuel mixture; a three-way catalyst 25 that purifies exhaust gas; an oxygen sensor 261 that measures an oxygen concentration of exhaust gas; and a control section 27 that executes control of a valve opening on the basis of a relation between engine speed and intake pressure and executes feedback control of the valve opening. In open loop control immediately after a hot operation of an NRTC is started after a cold operation and shutdown of the NRTC are completed, the control section 27 executes the control of the valve opening on the basis of a value obtained by adding a correction value to a fuel learning value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas engine that runs on gaseous fuel. [Background technology]

[0002] As regulations regarding engine exhaust gas become stricter, there are gas engines equipped with three-way catalysts that purify the CO, HC, and NOx contained in the exhaust gas through oxidation-reduction reactions.

[0003] For example, Patent Document 1 discloses a gas engine equipped with a three-way catalyst and an oxygen sensor installed in the exhaust path. The gas engine described in Patent Document 1 includes a first valve, a second valve, and a control unit. The first valve has lower responsiveness and a larger fuel flow rate adjustment range than the second valve. The second valve has higher responsiveness and a smaller fuel flow rate adjustment range than the first valve. When the average output value obtained from the oxygen sensor installed in the exhaust path of the gas engine during actual operation within a period in which the operating conditions of the gas engine are considered constant deviates from the oxygen sensor output target value set in the control unit under those conditions, the control unit adjusts the opening of the first valve so that the output average value becomes the output target value. Thus, the invention described in Patent Document 1 aims to provide a gas engine capable of controlling the air-fuel ratio in response to changes in the composition of the fuel gas.

[0004] For off-road gas engines equipped with a three-way catalyst and with a rated output of 56 kW or more, there is room for further improvement in compliance with the EU Stage V exhaust gas regulations (EU Stage V exhaust gas regulations for engines rated at 56 kW or more). Specifically, the transient test mode (NRTC: Non-Road Transient Cycle) of the EU Stage V exhaust gas regulations involves 20 minutes of cold operation, a 20-minute shutdown, and 20 minutes of hot operation, in that order. In this transient test mode of the EU Stage V exhaust gas regulations, there is room for improvement in the amount of NOx emitted during open-loop control, which is performed during the predetermined time (e.g., several tens of seconds) from immediately after the start of hot operation until feedback control of the gaseous fuel supply amount is initiated.

[0005] That is, in the transient test mode of the EU Tier 5 exhaust gas regulations, the total NOx emission amount in the transient test mode is the sum of 10% of the NOx emission amount in cold operation and 90% of the NOx emission amount in hot operation. Therefore, the weight of the NOx emission amount in hot operation is heavier than the weight of the NOx emission amount in cold operation. Furthermore, according to the knowledge obtained by the present inventors, the NOx emission amount immediately after the start of hot operation accounts for the majority of the NOx emission amount in the entire transient test mode. Therefore, in the transient test mode of the EU Tier 5 exhaust gas regulations, there is room for improvement in the amount of NOx emitted by open-loop control, which is executed for a predetermined period of time from immediately after the start of hot operation until the start of feedback control.

[0006] Furthermore, compared to gas engines that use an injector to inject gaseous fuel into, for example, an intake port, gas engines that pre-mix gaseous fuel with air to create a mixture can reduce the generation of particulate matter (PM), but can sometimes make it difficult to control the air-fuel ratio of the mixture. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-240615 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in consideration of the above circumstances, and aims to provide a gas engine that can reduce NOx emissions in a transient test mode related to the European Tier 5 exhaust gas regulations for gas engines that operate by spark-igniting and burning a mixture of gaseous fuel and air that has been pre-mixed. [Means for solving the problem]

[0009] A first aspect of the present invention is a gas engine that operates by spark-igniting and burning in a cylinder a mixture of gaseous fuel and air that has been mixed in advance, and includes a fuel gas supply valve that controls the amount of gaseous fuel supplied by adjusting the pressure of the gaseous fuel in accordance with a valve opening degree, a gas mixer that mixes the gaseous fuel that has passed through the fuel gas supply valve with the intake air to generate the mixture, a throttle valve that is provided in an intake system and opens and closes to control the amount of the mixture, a three-way catalyst that is provided in an exhaust system and purifies exhaust gas discharged from the cylinder, and a gas mixer that is provided in the exhaust system between the cylinder and the three-way catalyst and measures the oxygen concentration of the exhaust gas. and a control unit that controls the valve opening based on the relationship between engine speed and intake pressure, calculates an operation value for the valve opening such that the air-fuel ratio of the mixture becomes a target air-fuel ratio based on the oxygen concentration measured by the oxygen sensor, stores the operation value as a fuel learning value, and performs feedback control of the valve opening, wherein the control unit controls the valve opening based on a value obtained by adding a correction value to the stored fuel learning value during open-loop control immediately after cold operation and operation shutdown in a transient test mode related to the European Tier 5 exhaust gas regulations are completed and hot operation is started.

[0010] According to a first aspect of the present invention, a control unit controls the valve aperture of a fuel gas supply valve based on the relationship between engine speed and intake pressure. The relationship between engine speed and intake pressure is pre-stored in a storage unit, for example, as a basic map that sets the amount of gaseous fuel supplied relative to the engine speed. The control unit also calculates an operating value for the valve aperture of the fuel gas supply valve that will result in a target air-fuel ratio (i.e., the stoichiometric air-fuel ratio) based on the oxygen concentration measured by the oxygen sensor, and stores the calculated value as a fuel learning value. The control unit then performs feedback control of the valve aperture of the fuel gas supply valve using the operating value for the valve aperture of the fuel gas supply valve (i.e., the fuel learning value) calculated based on the oxygen concentration measured by the oxygen sensor. Furthermore, during open-loop control immediately after cold operation and shutdown in a transient test mode related to the EU Tier 5 exhaust gas regulations are completed and hot operation is started, the control unit controls the valve aperture of the fuel gas supply valve based on a value obtained by adding a correction value to the stored fuel learning value. This makes it possible to reduce NOx emissions immediately after starting hot operation in the transient test mode of the EU Tier 5 exhaust gas regulations. Because the weight of NOx emissions during hot operation is heavier than that during cold operation, and because NOx emissions immediately after starting hot operation account for the majority of NOx emissions throughout the entire transient test mode, reducing NOx emissions immediately after starting hot operation makes it possible to reduce NOx emissions throughout the entire transient test mode of the EU Tier 5 exhaust gas regulations.

[0011] A second aspect of the present invention is the gas engine of the first aspect of the present invention, characterized in that the control unit sets the correction value to a richer side than the fuel learning value.

[0012] According to the second aspect of the present invention, the control unit sets the correction value to be added to the fuel learning value to be richer than the fuel learning value, so that the air-fuel ratio of the mixture immediately after the start of hot operation becomes richer, thereby more reliably suppressing NOx emissions immediately after the start of hot operation.

[0013] A third aspect of the present invention is a gas engine in which, in the first or second aspect of the present invention, the control unit maintains the correction value constant until a predetermined time has elapsed since the hot operation started, and then executes control to gradually decrease the correction value after the predetermined time has elapsed.

[0014] According to the third aspect of the present invention, it is possible to smoothly transition from open loop control, which is executed immediately after the start of hot operation, to feedback control, which is executed a predetermined time after the start of hot operation.

[0015] A fourth aspect of the present invention is a gas engine according to any one of the first to third aspects of the present invention, characterized in that the control unit sets the correction value according to a component ratio of the gaseous fuel.

[0016] According to the fourth aspect of the present invention, the control unit sets a correction value to be added to the fuel learning value according to the component ratio of the gaseous fuel, thereby making it possible to reduce NOx emissions immediately after starting hot operation regardless of the properties of the gaseous fuel.

[0017] A fifth aspect of the present invention is a gas engine according to any one of the first to fourth aspects of the present invention, characterized in that the control unit sets the correction value within a range in which the CO contained in the exhaust gas is equal to or less than a predetermined value.

[0018] According to the fifth aspect of the present invention, it is possible to suppress the amount of NOx emitted immediately after the start of hot operation, while suppressing the amount of CO emitted immediately after the start of hot operation. [Effects of the Invention]

[0019] According to the present invention, a gas engine can be provided that can reduce NOx emissions in a transient test mode related to the European Tier 5 exhaust gas regulations for gas engines that operate by spark-igniting and burning a mixture of gaseous fuel and air that has been premixed. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram illustrating an overview of a gas engine according to an embodiment of the present invention. [Figure 2] 1 is a flowchart illustrating a transient test mode of the European Tier 5 exhaust gas regulations. [Figure 3] 4 is a flowchart illustrating control of the amount of gaseous fuel supplied that is executed in the NRTC by the ECU of this embodiment. [Figure 4] 4 is a flowchart illustrating control of the amount of gaseous fuel supplied that is executed in the NRTC by the ECU of this embodiment. [Figure 5] 6 is a graph illustrating a correction value that the ECU of the present embodiment adds to the fuel learning value. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The embodiments described below are preferred examples of the present invention, and therefore various technically preferable limitations are applied thereto, but the scope of the present invention is not limited to these aspects unless otherwise specified in the following description to the effect that the present invention is particularly limited. Furthermore, in each drawing, similar components are designated by the same reference numerals, and detailed descriptions thereof will be omitted as appropriate.

[0022] FIG. 1 is a schematic diagram showing an overview of a gas engine according to this embodiment. The gas engine 2 according to this embodiment is an internal combustion engine that runs on gaseous fuel such as hydrogen gas, natural gas, LPG, biogas, etc. The gas engine 2 shown in Fig. 1 is mounted on industrial machinery such as construction machinery or agricultural machinery.

[0023] The gas engine 2 according to this embodiment includes a fuel gas supply valve 21, a gas mixer 29, a throttle valve 22, cylinders 24 formed in a cylinder block (not shown), a first oxygen sensor 261, a second oxygen sensor 262, a three-way catalyst 25, and an ECU 27. The second oxygen sensor 262 is not necessarily provided.

[0024] The fuel gas supply valve 21 is provided in the intake system 281, and controls the supply amount of gaseous fuel by adjusting the pressure of the gaseous fuel according to the valve opening. Specifically, the fuel gas supply valve 21 controls the supply amount of gaseous fuel by adjusting the pressure difference (i.e., differential pressure) of the gaseous fuel between the inlet and outlet of the fuel gas supply valve 21. As indicated by arrow A1 in FIG. 1 , the gaseous fuel supplied to the fuel gas supply valve 21 passes through the fuel gas supply valve 21 and is guided toward the gas mixer 29.

[0025] The gas mixer 29 is provided in the intake system 281, and generates an air-fuel mixture by mixing the gaseous fuel that has passed through the fuel gas supply valve 21 with air that is taken in through an air cleaner (not shown) as indicated by arrow A2 in FIG. 1. The gas mixer 29 mixes the gaseous fuel and air in an appropriate ratio (i.e., a target air-fuel ratio) that makes the gaseous fuel combustible, and guides the mixture containing the gaseous fuel and air toward the throttle valve 22. In this way, the gas engine 2 according to this embodiment is not a gas engine that injects gaseous fuel into, for example, an intake port using an injector, but a gas engine that generates an air-fuel mixture by pre-mixing the gaseous fuel and air.

[0026] The throttle valve 22 is, for example, an electronically controlled throttle valve, and is provided in the intake system 281. The throttle valve 22 controls the amount of the mixture supplied from the gas mixer 29 (i.e., the intake or supply amount of the mixture) by opening and closing based on, for example, a control signal received from the ECU 27. In other words, the throttle valve 22 controls the amount of both the gaseous fuel that passes through the fuel gas supply valve 21 as indicated by arrow A1 in FIG. 1 and the air (i.e., intake air) that is drawn in through an air cleaner (not shown) as indicated by arrow A2 in FIG. 1. The mixture that passes through the throttle valve 22 is guided toward the intake manifold 23 and the intake port as indicated by arrow A3 in FIG. 1. As shown in FIG. 1, for example, a flow rate sensor is provided in the intake manifold 23. The flow rate sensor transmits a signal related to the flow rate of the mixture to the ECU 27.

[0027] As shown in Fig. 1, the air-fuel mixture generated by the gas mixer 29 passes through the throttle valve 22, the intake manifold 23, and the intake valve 241 in this order, and is supplied to the cylinder 24. The air-fuel mixture supplied to the cylinder 24 and compressed in the cylinder 24 is ignited and combusted by a spark generated by the spark plug 243. In this way, the gas engine 2 according to this embodiment is a gas engine that operates by spark-igniting and combusting a premixed air-fuel mixture in the cylinder 24. The combusted air-fuel mixture in the cylinder 24 passes through the exhaust valve 242, as indicated by arrow A4 in Fig. 1, and is discharged as exhaust gas from the cylinder 24 toward the three-way catalyst 25.

[0028] The three-way catalyst 25 is provided in the exhaust system 282 and purifies the exhaust gas emitted from the cylinders 24 through an oxidation-reduction reaction. Specifically, the exhaust gas emitted from the cylinders 24 contains CO, HC, and NOx. The three-way catalyst 25 purifies the CO and HC The three-way catalyst 25 simultaneously purifies CO, HC, and NOx through an oxidation reaction of CO and a reduction reaction of NOx. In order for the three-way catalyst 25 to function effectively, the air-fuel ratio of the mixture must be set to the stoichiometric air-fuel ratio.

[0029] The first oxygen sensor 261 is provided in the exhaust system 282 between the cylinder 24 and the three-way catalyst 25. In other words, the first oxygen sensor 261 is provided at the inlet of the three-way catalyst 25. The first oxygen sensor 261 of this embodiment is an example of the "oxygen sensor" of the present invention. The first oxygen sensor 261 measures the oxygen concentration of the exhaust gas discharged from the cylinder 24 and transmits a signal related to the measured oxygen concentration (for example, a voltage signal) to the ECU 27.

[0030] The second oxygen sensor 262 is provided at the outlet of the three-way catalyst 25. The second oxygen sensor 262 measures the oxygen concentration of the exhaust gas that has passed through the three-way catalyst 25, and transmits a signal relating to the measured oxygen concentration (for example, a voltage signal) to the ECU 27.

[0031] The ECU 27 is an electronic control unit (ECU) and includes an arithmetic processing unit (not shown) and a memory unit (not shown). The ECU 27 controls the operation of the fuel gas supply valve 21. For example, the ECU 27 controls the valve opening of the fuel gas supply valve 21. The ECU 27 of this embodiment is an example of a "control unit" of the present invention. The arithmetic processing unit of the ECU 27 functions as a CPU (Central Processing Unit) and reads out programs stored in the memory unit of the ECU 27 to perform various calculations and processes.

[0032] ECU 27 The storage unit stores (memorizes) various programs, maps or conditional expressions relating to the valve opening degree of the fuel gas supply valve 21. Examples of the storage unit of the ECU 27 include a ROM (Read Only Memory) and a RAM (Random Access Memory).

[0033] The ECU 27 controls the valve opening of the fuel gas supply valve 21 based on the relationship between the engine speed and the intake pressure. The relationship between the engine speed and the intake pressure is stored in advance in a storage unit as, for example, a basic map that sets the supply amount of gaseous fuel relative to the engine speed.

[0034] Furthermore, the ECU 27 calculates an operation value for the valve opening of the fuel gas supply valve 21 such that the air-fuel ratio of the mixture calculated based on the oxygen concentration measured by the first oxygen sensor 261 becomes a target air-fuel ratio (i.e., the stoichiometric air-fuel ratio), and stores the operation value in the storage unit as a fuel learning value. In other words, the ECU 27 updates the fuel learning value. The fuel learning value is stored (updated), for example, as a map related to the fuel learning value. Then, the ECU 27 performs feedback control of the valve opening of the fuel gas supply valve 21 using the operation value (i.e., the fuel learning value) for the valve opening of the fuel gas supply valve 21 calculated based on the oxygen concentration measured by the first oxygen sensor 261.

[0035] Specifically, the ECU 27 compares the air-fuel ratio of the mixture calculated based on the oxygen concentration received from the first oxygen sensor 261 with a target air-fuel ratio (i.e., the stoichiometric air-fuel ratio). If the air-fuel ratio of the mixture is leaner than the target air-fuel ratio, the ECU 27 calculates an operation value for increasing the valve opening of the fuel gas supply valve 21 compared to the current valve opening (i.e., opening the valve), and stores the operation value in a storage unit as a fuel learning value. Then, the ECU 27 executes feedback control of the valve opening of the fuel gas supply valve 21 using the operation value (i.e., the fuel learning value). That is, the ECU 27 executes feedback control to increase the amount of gaseous fuel supplied.

[0036] On the other hand, if the air-fuel ratio of the mixture is richer than the target air-fuel ratio, the ECU 27 calculates an operation value for reducing the valve opening of the fuel gas supply valve 21 below the current valve opening (i.e., closing the valve) and stores the operation value in the storage unit as a fuel learning value. Then, the ECU 27 executes feedback control of the valve opening of the fuel gas supply valve 21 using the operation value (i.e., the fuel learning value). That is, the ECU 27 executes feedback control to reduce the amount of gaseous fuel supplied.

[0037] FIG. 2 is a flowchart illustrating the transient test mode of the European Tier 5 exhaust gas regulations. Currently, the European Tier 5 exhaust gas regulations (EU Stage V exhaust gas regulations for engines rated at 56 kW or more) apply to off-road gas engines with a rated output of 56 kW or more. Specifically, the transient test mode of the European Tier 5 exhaust gas regulations (NRTC: Non-Road Transient Cycle) is applied. Here, the transient test mode of the European Tier 5 exhaust gas regulations (hereinafter referred to as "NRTC" for convenience of explanation) will be explained with reference to the drawings.

[0038] In NRTC, first, in step S1, a 20-minute cold operation is performed. In NRTC cold operation, operation of the gas engine 2 is initiated as a cold air start. Next, in step S2, operation is stopped for 20 minutes. That is, operation of the gas engine 2 is stopped for 20 minutes. Then, finally, in step S3, a 20-minute hot operation is performed. In NRTC hot operation, operation of the gas engine 2 is initiated as a warm air start. When the 20-minute hot operation is completed, NRTC ends.

[0039] Here, the ECU 27 executes open-loop control of the amount of gaseous fuel supplied for a predetermined time (e.g., several tens of seconds) from immediately after the start of the gas engine 2 until the start of feedback control of the amount of gaseous fuel supplied. That is, the ECU 27 does not execute feedback control until the predetermined time (e.g., several tens of seconds) has elapsed from immediately after the start of the gas engine 2, and executes open-loop control of the valve opening of the fuel gas supply valve 21 based on a basic map showing the relationship between the engine speed and the intake pressure and a fuel learning value stored in the memory unit.

[0040] 2, the ECU 27 performs open-loop control of the valve opening of the fuel gas supply valve 21 based on the basic map and the fuel learning value stored (updated) during cold operation of the NRTC. According to the knowledge of the present inventors, there is room for improvement in the amount of NOx emitted during open-loop control performed immediately after the start of hot operation of the NRTC.

[0041] That is, in NRTC, the total NOx emissions of the NRTC are the sum of 10% of the NOx emissions in cold operation and 90% of the NOx emissions in hot operation. Therefore, the weight of the NOx emissions in hot operation is heavier than the weight of the NOx emissions in cold operation. Furthermore, according to the knowledge of the inventors, the NOx emissions immediately after the start of hot operation account for the majority of the total NOx emissions of the NRTC. Therefore, there is room for improvement in the amount of NOx emitted in the open-loop control executed in the NRTC during the predetermined time period from immediately after the start of hot operation until the start of feedback control.

[0042] Specifically, in the open-loop control executed immediately after starting the NRTC hot operation, the air-fuel ratio of the mixture tends to be lean, which tends to increase NOx emissions compared to when the air-fuel ratio of the mixture is stoichiometric or rich.

[0043] In contrast, the ECU 27 of the gas engine 2 according to this embodiment controls the valve opening of the fuel gas supply valve 21 based on the fuel learning value stored in the storage unit (i.e., the updated fuel learning value) plus a correction value during open-loop control executed immediately after starting NRTC hot operation. Specifically, the ECU 27 sets the correction value to a value richer than the fuel learning value stored in the storage unit (i.e., the updated fuel learning value). The control executed by the ECU 27 of the gas engine 2 according to this embodiment will be further described below with reference to the drawings.

[0044] 3 and 4 are flowcharts illustrating the control of the amount of gaseous fuel supplied that is executed by the ECU in the NRTC according to this embodiment. FIG. 5 is a graph illustrating the correction value that the ECU of this embodiment adds to the fuel learning value.

[0045] First, as shown in Fig. 3, in step S11, the cold operation of the NRTC is started. Then, in step S12, the ECU 27 does not execute feedback control until a predetermined time has elapsed from immediately after the start of the cold operation of the NRTC, but executes open-loop control of the valve opening of the fuel gas supply valve 21 based on the basic map and the fuel learning value stored in the memory unit. Here, the fuel learning value stored in the memory unit is the fuel learning value updated during the previous operation of the gas engine 2 (for example, the operation of the gas engine 2 on the previous day).

[0046] Subsequently, when a predetermined time has elapsed since the start of cold operation of the NRTC, the ECU 27 executes feedback control regarding the supply amount of gaseous fuel. That is, in step S13, the first oxygen sensor 261 measures the oxygen concentration of the exhaust gas and transmits a signal regarding the measured oxygen concentration to the ECU 27.

[0047] Next, in step S14, the ECU 27 calculates an operating value for the valve opening of the fuel gas supply valve 21 that makes the air-fuel ratio of the mixture become the target air-fuel ratio (i.e., the stoichiometric air-fuel ratio) based on the deviation between the air-fuel ratio of the mixture calculated based on the oxygen concentration received from the first oxygen sensor 261 and the target air-fuel ratio (i.e., the stoichiometric air-fuel ratio).

[0048] Next, in step S15, the ECU 27 stores the manipulated value of the valve opening of the fuel gas supply valve 21 as a fuel learning value in the storage unit. That is, the ECU 27 updates the fuel learning value. Next, in step S16, the ECU 27 controls the valve opening of the fuel gas supply valve 21 based on the fuel learning value.

[0049] Next, in step S17, if 20 minutes have not yet elapsed since the start of cold operation of the NRTC (step S17: NO), the ECU 27 executes the feedback control regarding the supply amount of gaseous fuel described above in relation to steps S13 to S16. On the other hand, if 20 minutes have elapsed since the start of cold operation of the NRTC in step S17 (step S17: YES), a 20-minute operation shutdown is executed in step S18.

[0050] Next, as shown in FIG. 4, in step S21, NRTC hot operation is started. Then, in step S22, the ECU 27 does not execute feedback control until a predetermined time has elapsed from immediately after the start of NRTC hot operation, and executes open-loop control of the valve opening of the fuel gas supply valve 21 based on the basic map and a value obtained by adding a correction value to the fuel learning value stored in the memory unit. Here, the fuel learning value stored in the memory unit is the fuel learning value updated during NRTC cold operation. The correction value is a value of the same dimension as the fuel learning value and has a function of offsetting or shifting the fuel learning value.

[0051] 5, the amount of NOx emissions in the range A11 where the correction value is near "0" is higher than the amount of NOx emissions in the range A12 where the correction value is near "1" to "3". In addition, the slope of the amount of NOx emissions in the range A11 where the correction value is near "0" is steeper than the slope of the amount of NOx emissions in the range A12 where the correction value is near "1" to "3". In other words, the robustness of the amount of NOx emissions in the range A11 is lower than the robustness of the amount of NOx emissions in the range A12.

[0052] A state in which the correction value is "0" shown in FIG. 5 means that the ECU 27 does not offset or shift the fuel learning value. In this state, similar to the case described above with respect to step S12 shown in FIG. 3, the ECU 27 controls the valve opening of the fuel gas supply valve 21 based on the basic map and the fuel learning value stored in the storage unit. A state in which the correction value is positive shown in FIG. 5 means that the ECU 27 offsets or shifts the fuel learning value to the rich side. A state in which the correction value is negative shown in FIG. 5 means that the ECU 27 offsets or shifts the fuel learning value to the lean side.

[0053] The ECU 27 sets the correction value to a value richer than the fuel learning value in order to suppress the amount of NOx emissions immediately after the start of NRTC hot operation. That is, in step S22 shown in Fig. 4, the ECU 27 executes open-loop control of the valve opening of the fuel gas supply valve 21 based on the basic map and a value obtained by adding a positive correction value to the fuel learning value. As a result, if the ECU 27 sets the correction value to a value richer, for example, in the vicinity of "1" to "3" shown in Fig. 5, the amount of NOx emissions immediately after the start of NRTC hot operation can be suppressed to within the range A12 shown in Fig. 5.

[0054] Furthermore, the ECU 27 sets the correction value within a range in which the amount of CO emissions in open-loop control immediately after the start of NRTC hot operation is equal to or less than a predetermined value. For example, if the NRTC CO regulation value is equal to or less than 5 g / kWh, as shown in Fig. 5, the ECU 27 sets the correction value within a range A12 in the vicinity of "1" to "3," thereby making it possible to suppress the amount of CO emissions in open-loop control immediately after the start of NRTC hot operation to approximately 3 to 4 g / kWh, which is equal to or less than the predetermined value.

[0055] Subsequently, when a predetermined time has elapsed since the start of the hot operation of the NRTC, the ECU 27 executes feedback control regarding the supply amount of gaseous fuel. That is, the first oxygen sensor 261 and the ECU 27 execute the processes described above with respect to steps S13 to S16 shown in FIG.

[0056] In step S27 following step S26, if 20 minutes have not yet elapsed since the start of hot operation of the NRTC (step S27: NO), the ECU 27 executes the feedback control regarding the supply amount of gaseous fuel described above in relation to steps S23 to S26. On the other hand, if 20 minutes have elapsed since the start of hot operation of the NRTC (step S27: YES), the NRTC ends.

[0057] According to the gas engine 2 of this embodiment, in the open loop control immediately after the start of NRTC hot operation, the ECU 27 controls the valve opening of the fuel gas supply valve 21 based on the basic map and the value obtained by adding a correction value to the fuel learning value stored in the memory unit. This makes it possible to reduce NOx emissions immediately after the start of hot operation in the NRTC. Because the weight of NOx emissions during hot operation is heavier than that during cold operation, and because NOx emissions immediately after the start of hot operation account for the majority of the total NOx emissions in the NRTC, reducing NOx emissions immediately after the start of hot operation makes it possible to reduce NOx emissions in the entire NRTC.

[0058] Furthermore, the ECU 27 sets the correction value to be added to the fuel learning value to be richer than the fuel learning value, i.e., sets a positive correction value as the correction value to be added to the fuel learning value, so that the air-fuel ratio of the air-fuel mixture immediately after the start of hot operation becomes richer. This makes it possible to further reliably suppress NOx emissions immediately after the start of hot operation.

[0059] The ECU 27 may maintain the correction value constant until a predetermined time has elapsed since the start of NRTC hot operation, and then execute control to gradually decrease the correction value after the predetermined time has elapsed. This allows for a smooth transition from open-loop control, which is executed immediately after the start of NRTC hot operation, to feedback control, which is executed a predetermined time after the start of NRTC hot operation. Note that the timing at which the ECU 27 gradually decreases the correction value may be before or after the start of feedback control related to the supply amount of gaseous fuel.

[0060] ECU The ECU 27 may set the correction value according to the component ratio of the gaseous fuel. For example, the ECU 27 may set the correction value according to the component ratio of propane and butane contained in LPG. Alternatively, the ECU 27 may set the correction value according to the component ratio of natural gas. This makes it possible to suppress NOx emissions immediately after starting hot operation of the NRTC regardless of the properties of the gaseous fuel.

[0061] If the gas engine 2 is equipped with the second oxygen sensor 262, the second oxygen sensor 262 measures the oxygen concentration of the exhaust gas that has passed through the three-way catalyst 25, and transmits a signal (e.g., a voltage signal) related to the measured oxygen concentration to the ECU 27. As a result, the ECU 27 performs feedback control of the valve opening of the fuel gas supply valve 21 based on the deviation between the air-fuel ratio of the mixture calculated based on the oxygen concentration received from the second oxygen sensor 262 and a target air-fuel ratio (i.e., the stoichiometric air-fuel ratio), thereby allowing the three-way catalyst 25 to purify the exhaust gas. Furthermore, even if the three-way catalyst 25 has deteriorated, the ECU 27 can control the valve opening of the fuel gas supply valve 21 by setting the oxygen concentration of the exhaust gas that has passed through the three-way catalyst 25 as a target. Furthermore, even if there is variation in the performance of the three-way catalyst 25 due to mechanical differences in the three-way catalyst 25 or the amount of precious metal carried by the three-way catalyst 25, the ECU 27 can control the valve opening of the fuel gas supply valve 21 by targeting the oxygen concentration of the exhaust gas that has passed through the three-way catalyst 25.

[0062] The above describes the embodiments of the present invention. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the claims. The configurations of the above embodiments can be partially omitted or arbitrarily combined in a different manner from the above. [Explanation of symbols]

[0063] 2: Gas engine, 21: Fuel gas supply valve, 22: Throttle valve, 23: Intake manifold, 24: Cylinder, 25: Three-way catalyst, 27: ECU, 29: Gas mixer, 241: Intake valve, 242: Exhaust valve, 243: Spark plug, 261: First oxygen sensor, 262: Second oxygen sensor, 281: Intake system, 282: Exhaust system

Claims

1. A gas engine that operates by spark ignition and combustion in a cylinder of a mixture of gaseous fuel and air, a fuel gas supply valve that controls the supply amount of the gaseous fuel by adjusting the pressure of the gaseous fuel in accordance with a valve opening degree; a gas mixer that mixes the gaseous fuel that has passed through the fuel gas supply valve with the intake air to generate the mixture; a throttle valve provided in the intake system for controlling the amount of the air-fuel mixture by opening and closing the throttle valve; a three-way catalyst provided in an exhaust system for purifying exhaust gas discharged from the cylinder; an oxygen sensor provided in the exhaust system between the cylinder and the three-way catalyst for measuring an oxygen concentration in the exhaust gas; a control unit that controls the valve opening based on a relationship between an engine speed and an intake pressure, calculates an operation value for the valve opening that makes the air-fuel ratio of the mixture a target air-fuel ratio based on the oxygen concentration measured by the oxygen sensor, stores the operation value as a fuel learning value, and performs feedback control of the valve opening; Equipped with The control unit controls the valve opening based on a value obtained by adding a correction value to the stored fuel learning value during open-loop control immediately after cold operation and shutdown in a transient test mode for the European Tier 5 exhaust gas regulations are completed and hot operation is started.

2. 2. The gas engine according to claim 1, wherein the control unit sets the correction value so that the air-fuel ratio when the valve opening control is executed based on a value obtained by adding the correction value to the fuel learning value is richer than the air-fuel ratio when the valve opening control is executed based on the fuel learning value.

3. 2. The gas engine according to claim 1, wherein the control unit maintains the correction value constant until a predetermined time has elapsed since the hot operation started, and then executes control to gradually decrease the correction value after the predetermined time has elapsed.

4. 2. The gas engine according to claim 1, wherein the control unit sets the correction value in accordance with a component ratio of the gaseous fuel.

5. 2. The gas engine according to claim 1, wherein the control unit sets the correction value within a range in which the amount of CO emissions in the open loop control immediately after the hot operation is started is equal to or less than a predetermined value.

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