JP7764359B2 - gas engine - Google Patents

Description

The present invention relates to a gas engine that runs on gaseous fuel.

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 exhaust gas through oxidation-reduction reactions.

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 is equipped with 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. If, during actual operation within a period in which the gas engine's operating conditions are considered constant, the average output obtained from the oxygen sensor installed in the gas engine's exhaust path deviates from the oxygen sensor's 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. The invention described in Patent Document 1 thus aims to provide a gas engine that can perform air-fuel ratio control in response to changes in the fuel gas composition.

Here, 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 European Tier 5 exhaust gas regulations (EU Stage V exhaust gas regulations for vehicles 56 kW or more). Specifically, the transient test mode (NRTC: Non-Road Transient Cycle) of the European Tier 5 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 European Tier 5 exhaust gas regulations, there is room for improvement in the amount of NOx emitted during open-loop control, which is performed during the predetermined period (e.g., several tens of seconds) from immediately after the start of hot operation until feedback control of the gaseous fuel supply amount begins.

In other words, in the transient test mode of the EU Tier 5 exhaust gas regulations, the total NOx emissions in the transient test mode is the sum of 10% of the NOx emissions in cold operation and 90% of the NOx emissions in hot operation. Therefore, the weight of NOx emissions in hot operation is heavier than the weight of NOx emissions in cold operation. Furthermore, according to the inventor's findings, the NOx emissions immediately after the start of hot operation account for the majority of the NOx emissions 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 during the predetermined time from immediately after the start of hot operation until feedback control begins.

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 an air-fuel mixture can reduce the generation of particulate matter (PM), but can make it difficult to control the air-fuel ratio of the mixture.

JP 2014-240615 A

The present invention was 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 pre-mixed mixture of gaseous fuel and air.

A first aspect of the present invention is a gas engine that operates by spark-igniting and burning a premixed mixture of gaseous fuel and air in a cylinder, and includes a fuel gas supply valve that controls the amount of gaseous fuel supplied by adjusting the pressure of the gaseous fuel according to the valve opening, 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 the intake system and opens and closes to control the amount of the mixture, a three-way catalyst that is provided in the 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 operating value for the valve opening that will bring the air-fuel ratio of the mixture to a target air-fuel ratio based on the oxygen concentration measured by the oxygen sensor, stores the operating value as a fuel learning value, and performs feedback control of the valve opening. 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 related to the European Tier 5 exhaust gas regulations are completed and hot operation is started.

According to a first aspect of the present invention, the control unit controls the valve opening of the 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 the memory 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 opening of the fuel gas supply valve that will result in the air-fuel ratio of the mixture becoming a target air-fuel ratio (i.e., the stoichiometric air-fuel ratio) based on the oxygen concentration measured by the oxygen sensor, and stores this value as a fuel learning value. The control unit then performs feedback control of the valve opening of the fuel gas supply valve using the operating value for the valve opening 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 the cold operation and shutdown of the transient test mode for the EU Tier 5 exhaust gas regulations are completed and hot operation is started, the control unit controls the valve opening 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. Since the weight of NOx emissions during hot operation is greater than the weight of NOx emissions during cold operation, and 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.

A second aspect of the present invention is a gas engine according to 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.

According to the second aspect of the present invention, the control unit sets the correction value to be added to the fuel learning value richer than the fuel learning value, so the air-fuel ratio of the mixture immediately after starting hot operation becomes richer. This makes it possible to more reliably reduce NOx emissions immediately after starting hot operation.

A third aspect of the present invention is a gas engine according to the first or second aspect of the present invention, 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.

The third aspect of the present invention allows for a smooth transition from open-loop control, which is executed immediately after hot operation starts, to feedback control, which is executed a predetermined time after hot operation starts.

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 the component ratio of the gaseous fuel.

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 reducing NOx emissions immediately after starting hot operation regardless of the properties of the gaseous fuel.

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.

According to the fifth aspect of the present invention, it is possible to reduce the amount of NOx emissions immediately after the start of hot operation while also reducing the amount of CO emissions immediately after the start of hot operation.

The present invention provides a gas engine that can reduce NOx emissions in a transient test mode for the European Tier 5 exhaust gas regulations for gas engines that operate by spark-igniting a pre-mixed mixture of gaseous fuel and air.

1 is a schematic diagram illustrating an overview of a gas engine according to an embodiment of the present invention. 10 is a flowchart illustrating a transient test mode of the European Tier 5 exhaust gas regulations. 4 is a flowchart illustrating control of the amount of gaseous fuel supplied that is executed by the ECU in the NRTC according to the embodiment. 4 is a flowchart illustrating control of the amount of gaseous fuel supplied that is executed by the ECU in the NRTC according to the embodiment. 6 is a graph illustrating a correction value that the ECU of the present embodiment adds to the fuel learning value.

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.

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 and agricultural machinery.

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.

The fuel gas supply valve 21 is provided in the intake system 281 and controls the amount of gaseous fuel supplied by adjusting the pressure of the gaseous fuel according to the valve opening. Specifically, the fuel gas supply valve 21 controls the amount of gaseous fuel supplied 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 directed toward the gas mixer 29.

The gas mixer 29 is provided in the intake system 281 and generates an air-fuel mixture by mixing gaseous fuel that has passed through the fuel gas supply valve 21 with air that is drawn 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., the target air-fuel ratio) that makes the gaseous fuel combustible, and directs 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 rather a gas engine that generates an air-fuel mixture by pre-mixing gaseous fuel and air.

The throttle valve 22, for example, an electronically controlled throttle valve, is provided in the intake system 281. The throttle valve 22 controls the amount of mixture supplied from the gas mixer 29 (i.e., the intake or supply amount of the mixture) by opening and closing based on a control signal received from, for example, the ECU 27. In other words, the throttle valve 22 controls the amount of both the gaseous fuel passing through the fuel gas supply valve 21 (as indicated by arrow A1 in FIG. 1) and the air (i.e., intake air) 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 directed toward the intake manifold 23 and the intake port (as indicated by arrow A3 in FIG. 1). As shown in FIG. 1, for example, the intake manifold 23 is provided with a flow sensor. The flow sensor sends a signal related to the flow rate of the mixture to the ECU 27.

As shown in FIG. 1, the mixture generated by the gas mixer 29 passes through the throttle valve 22, intake manifold 23, and intake valve 241 in that order before being supplied to the cylinder 24. The mixture supplied to 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 mixture of gaseous fuel and air premixed in the cylinder 24. The mixture combusted 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.

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 simultaneously purifies CO, HC, and NOx through an oxidation reaction of CO and HC and a reduction reaction of NOx. For the three-way catalyst 25 to function effectively, the air-fuel ratio of the mixture needs to be set to the stoichiometric air-fuel ratio.

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 (e.g., a voltage signal) to the ECU 27.

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 (e.g., a voltage signal) related to the measured oxygen concentration to the ECU 27.

The ECU 27 is an electronic control unit (ECU) and includes a calculation 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 degree 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 calculation processing unit of the ECU 27 functions as a CPU (Central Processing Unit) and reads programs stored in the memory unit of the ECU 27 to perform various calculations and processes.

The storage unit of the ECU 27 stores various programs, maps or conditional expressions relating to the valve opening degree of the fuel gas supply valve 21, etc. Examples of the storage unit of the ECU 27 include a read only memory (ROM) and a random access memory (RAM).

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

The ECU 27 also calculates an operating 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 the target air-fuel ratio (i.e., the stoichiometric air-fuel ratio), and stores this operating value in a memory 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. The ECU 27 then performs feedback control of the valve opening of the fuel gas supply valve 21 using the operating 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.

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 above its current valve opening (i.e., opening the valve), and stores this operation value in a memory unit as a fuel learning value. The ECU 27 then performs feedback control of the valve opening of the fuel gas supply valve 21 using this operation value (i.e., the fuel learning value). In other words, the ECU 27 performs feedback control to increase the amount of gaseous fuel supplied.

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 this operation value in the memory as a fuel learning value. The ECU 27 then performs feedback control of the valve opening of the fuel gas supply valve 21 using this operation value (i.e., the fuel learning value). In other words, the ECU 27 performs feedback control to reduce the amount of gaseous fuel supplied.

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 cycle (NRTC: Non-Road Transient Cycle) of the European Tier 5 exhaust gas regulations applies. Here, the transient test cycle (hereinafter referred to as "NRTC" for convenience of explanation) of the European Tier 5 exhaust gas regulations will be described with reference to the drawings.

In NRTC, first, in step S1, cold operation is performed for 20 minutes. 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. In other words, operation of the gas engine 2 is stopped for 20 minutes. Then, finally, in step S3, hot operation is performed for 20 minutes. In NRTC hot operation, operation of the gas engine 2 is initiated as a warm air start. When the 20 minutes of hot operation are completed, NRTC ends.

Here, for a predetermined time (e.g., several tens of seconds) from immediately after the gas engine 2 is started until feedback control of the gas fuel supply amount is initiated, the ECU 27 executes open-loop control of the gas fuel supply amount. In other words, for a predetermined time (e.g., several tens of seconds) from immediately after the gas engine 2 is started until the elapse of the predetermined time, the ECU 27 does not execute feedback control, 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 engine speed and intake pressure and a fuel learning value stored in the memory unit.

Therefore, in the open-loop control executed immediately after the start of NRTC hot operation in step S3 shown in Figure 2, the ECU 27 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 (updated) during NRTC cold operation. According to the inventor's findings, there is room for improvement in the amount of NOx emitted in the open-loop control executed immediately after the start of NRTC hot operation.

In other words, in NRTC, the total NOx emissions for the NRTC are the sum of 10% of the NOx emissions during cold operation and 90% of the NOx emissions during hot operation. Therefore, the weight of NOx emissions during hot operation is heavier than the weight of NOx emissions during cold operation. Furthermore, according to the inventor's findings, NOx emissions immediately after the start of hot operation account for the majority of the total NOx emissions for the NRTC. Therefore, there is room for improvement in the amount of NOx emitted during open-loop control, which is executed during the predetermined time from immediately after the start of hot operation until feedback control begins in NRTC.

Specifically, in the open-loop control that is executed immediately after starting NRTC hot operation, the air-fuel ratio of the mixture tends to become lean. As a result, NOx emissions tend to increase compared to when the air-fuel ratio of the mixture is stoichiometric or rich.

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.

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.

First, as shown in FIG. 3, in step S11, NRTC cold operation is initiated. Then, in step S12, the ECU 27 does not execute feedback control until a predetermined time has elapsed from immediately after the initiation of NRTC cold operation, and 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 (e.g., operation of the gas engine 2 on the previous day).

Next, after a predetermined time has elapsed since the start of NRTC cold operation, the ECU 27 executes feedback control of the amount of gaseous fuel supplied. That is, in step S13, the first oxygen sensor 261 measures the oxygen concentration in the exhaust gas and transmits a signal related to the measured oxygen concentration to the ECU 27.

Next, in step S14, the ECU 27 calculates an operating value for the valve opening of the fuel gas supply valve 21 that will bring the air-fuel ratio of the mixture to 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).

Next, in step S15, the ECU 27 stores the operation value of the valve opening of the fuel gas supply valve 21 in the memory unit as a fuel learning value. In other words, 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.

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

Next, as shown in FIG. 4, in step S21, NRTC hot operation is initiated. Then, in step S22, the ECU 27 does not execute feedback control until a predetermined time has elapsed from immediately after the initiation 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 the fuel learning value stored in the memory plus a correction value. Here, the fuel learning value stored in the memory 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 the function of offsetting or shifting the fuel learning value.

As shown in Figure 5, the NOx emission amount in range A11, where the correction value is near "0", is higher than the NOx emission amount in range A12, where the correction value is near "1" to "3". Furthermore, the slope of the NOx emission amount in range A11, where the correction value is near "0", is steeper than the slope of the NOx emission amount in range A12, where the correction value is near "1" to "3". In other words, the robustness of the NOx emission amount in range A11 is lower than the robustness of the NOx emission amount in range A12.

When the correction value shown in FIG. 5 is "0," 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 memory unit. When the correction value shown in FIG. 5 is positive, the ECU 27 offsets or shifts the fuel learning value to the rich side. When the correction value shown in FIG. 5 is negative, the ECU 27 offsets or shifts the fuel learning value to the lean side.

The ECU 27 sets the correction value richer than the fuel learning value in order to suppress NOx emissions immediately after starting NRTC hot operation. That is, in step S22 shown in FIG. 4, 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 value obtained by adding a positive correction value to the fuel learning value. As a result, if the ECU 27 sets the correction value, for example, on the rich side, near "1" to "3" shown in FIG. 5, the NOx emissions immediately after starting NRTC hot operation can be suppressed to within the range A12 shown in FIG. 5.

The ECU 27 also sets the correction value within a range that keeps CO emissions during open loop control below a predetermined value immediately after NRTC hot operation begins. For example, if the NRTC CO regulation value is 5 g/kWh or less, as shown in Figure 5, the ECU 27 can set the correction value within range A12, which is between "1" and "3," to keep CO emissions during open loop control immediately after NRTC hot operation begins to approximately 3 to 4 g/kWh, which is below the predetermined value.

Next, after a predetermined time has elapsed since the start of NRTC hot operation, the ECU 27 executes feedback control of the amount of gaseous fuel supplied. That is, the first oxygen sensor 261 and the ECU 27 execute the processing described above with respect to steps S13 to S16 shown in FIG. 3.

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

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 the amount of NOx emissions in the NRTC immediately after the start of hot operation. Because the weight of NOx emissions in hot operation is greater than the weight of NOx emissions in cold operation, and because the amount of NOx emissions immediately after the start of hot operation accounts for the majority of the total NOx emissions in the NRTC, reducing the amount of NOx emissions immediately after the start of hot operation makes it possible to reduce the total NOx emissions in the NRTC.

In addition, 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 the air-fuel ratio of the mixture immediately after starting hot operation becomes richer. This makes it possible to more reliably reduce NOx emissions immediately after starting hot operation.

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 feedback control regarding the amount of gaseous fuel supplied is started, or after feedback control regarding the amount of gaseous fuel supplied is started.

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 reduce NOx emissions immediately after starting NRTC hot operation regardless of the properties of the gaseous fuel.

If the gas engine 2 is equipped with a 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. This allows the ECU 27 to perform 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 the 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 with the oxygen concentration of the exhaust gas that has passed through the three-way catalyst 25 as the target. Furthermore, even if there are variations in the performance of the three-way catalyst 25 due to differences in the three-way catalyst 25 itself 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.

The above describes an embodiment of the present invention. However, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the claims. The configurations of the above embodiment can be partially omitted or arbitrarily combined in a different manner than described above.

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 (5)

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. 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 . The gas engine described in claim 1, characterized in that 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. The gas engine described in claim 1, characterized in that the control unit sets the correction value according to the component ratio of the gaseous fuel. The gas engine described in claim 1, characterized in that the control unit sets the correction value within a range in which the CO emissions in the open loop control immediately after the hot operation starts are below a predetermined value.