Gas engine

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.