Controller for hydrogen engine

A controller for a hydrogen engine, the controller being configured to control a hydrogen engine including a selective catalytic reduction device installed in an exhaust passage and a urea addition device that adds urea to exhaust gas flowing through a portion of the exhaust passage upstream of the selective catalytic reduction device, the controller comprising processing circuitry, wherein

• the processing circuitry is configured to execute
• a reduction process that causes a urea addition amount of the urea addition device to be smaller when a urea deposition amount of the selective catalytic reduction device is greater than or equal to a prescribed value than when the urea deposition amount is less than the prescribed value, and
• a lean-burn limiting process that sets an air-fuel ratio of an air-fuel mixture to be burned in the hydrogen engine to a value less than or equal to a prescribed lean-burn limiting value while the reduction process is reducing the urea addition amount, and
• the lean-burn limiting value is richer than an upper limit value of a control range of the air-fuel ratio in a case in which the reduction process is not reducing the urea addition amount.