Gas engine

1. A fuel gas engine comprising:

a first valve and a second valve provided in parallel on a supply path of fuel gas, the first valve having a responsiveness being lower and a fuel flow rate adjustment range being wider than the second valve, the second valve having a responsiveness being higher and a fuel flow rate adjustment range being narrower than the first valve; and

an electronic control unit coupled to a memory and to an oxygen sensor, the first valve and the second valve, the electronic control unit configured to perform processing operations comprising perturbation control using the second valve at a time of stoichiometric operation by moving the second valve from a predetermined opening degree to a lean side where the opening degree becomes smaller and a rich side where the opening degree becomes larger alternatively at a predetermined pitch in a state where the first valve is opened at a predetermined opening degree;

wherein, in a period in which an operation condition of the fuel gas engine is deemed to be stable under the perturbation control of the second valve at a predetermined engine rotational speed with a predetermined load, when an average value of outputs obtained from the oxygen sensor disposed on an exhaust passage of the fuel gas engine deviates from an output target value of the oxygen sensor, the output target value being set in the electronic control unit for the operation condition, the electronic control unit adjusts an opening degree of the first valve in such a manner that the output average value equals the output target value; and

wherein the electronic control unit is configured to perform processing operations further comprising calculating the output average value by extracting a maximum output and a minimum output of the oxygen sensor respectively corresponding a maximum opening degree and a minimum opening degree in the perturbation control by the second valve in the period.

2. The fuel gas engine according to claim 1 , wherein the electronic control unit is configured to perform processing operations further comprising adjusting the opening degree of the first valve so that the output average value converges into the output target value with a certain range.

3. The fuel gas engine according to claim 1 , wherein the electronic control unit is configured to perform processing operations further comprising adjusting the opening degree of the first valve so that the output average value converges into the output target value with a certain range.

4. The fuel gas engine according to claim 1 , wherein the first valve and the second valve are disposed with respect to each cylinder head or with respect to a group of cylinder heads.

5. The fuel gas engine according to claim 1 , wherein the first valve comprises a plurality of first valves and the second valve comprises a plurality of second valves.

6. The fuel gas engine according to claim 1 , wherein the oxygen sensor comprises an upstream oxygen sensor disposed on an upstream side of the exhaust passage relative to a catalyst.

7. The fuel gas engine according to claim 1 , wherein the oxygen sensor comprises a downstream oxygen sensor disposed on a downstream side of the exhaust passage relative to a catalyst.

8. A gas heat pump system including the fuel gas engine according to claim 1 as a drive source.

9. A cogeneration system including the fuel gas engine according to claim 1 as a drive source.

10. The fuel gas engine according to claim 1 , further comprising:

a mixer downstream of the first valve and the second valve;

a throttle valve positioned downstream of the mixer; and

an air intake passage of the fuel gas engine, the throttle valve positioned between the mixer and the air intake passage.

11. The fuel gas engine according to claim 1 , wherein the electronic control unit is configured to obtain a dead range.

12. The fuel gas engine according to claim 11 , wherein, in the period in which the operation condition of the fuel gas engine is deemed to be stable under the perturbation control of the second valve at the predetermined engine rotational speed with the predetermined load, the electronic control unit is configured to:

determine a difference between the average value of outputs and the output target value; and

determine whether the difference is included in the dead range.

13. The fuel gas engine according to claim 11 , wherein, in the period in which the operation condition of the fuel gas engine is deemed to be stable under the perturbation control of the second valve at the predetermined engine rotational speed with the predetermined load, the electronic control unit is configured to:

determine a difference between the average value of outputs and the output target value; and

determine whether the difference is not included in the dead range.

14. The fuel gas engine according to claim 11 , wherein the electronic control unit is configured to adjust the opening degree of the first valve based on a comparison between the dead range and a difference between the average value of outputs and the output target value.

15. The fuel gas engine according to claim 14 , wherein the electronic control unit is configured to increase the opening degree of the first valve based on an absolute value of the difference between the average value of outputs and the output target value being greater than the dead range.