System and control method

A control method for a system comprising an engine operable with a fuel containing hydrogen,

• the system comprising:
• a crankcase and a cylinder head of the engine;
• an engine intake passage through which air is suctioned into the engine;
• a crankcase intake passage branching from the engine intake passage and through which the air is introduced into the crankcase;
• a dilution air valve disposed in the crankcase intake passage and configured to open and close the crankcase intake passage;
• a decompression valve disposed downstream of the dilution air valve in the crankcase intake passage;
• a compressor disposed upstream of a branch portion at which the crankcase intake passage branches from the engine intake passage;
• a hydrogen sensor configured to detect hydrogen concentration inside the crankcase;
• and
• a communication path configured to allow an inside of the crankcase to communicate with an inside of the cylinder head,
• the control method comprising:
• acquiring the hydrogen concentration inside the crankcase;
• determining whether or not the hydrogen concentration is a threshold value or higher;
• opening the dilution air valve when it is determined that the hydrogen concentration is the threshold value or higher;
• and
• closing the dilution air valve when it is determined that the hydrogen concentration is lower than the threshold value.