Control method of engine system, and engine system

A control method of an engine system, the engine system comprising:

• an engine with a combustion chamber;
• a fuel injection valve configured to supply fuel to the engine;
• and
• an air-fuel ratio sensor provided in a flow path of exhaust gas from the engine,
• the control method of the engine system performing feedback control such that that an air-fuel ratio in the combustion chamber becomes a target value by controlling the fuel injection valve, using an air-fuel ratio measured value obtained by the air-fuel ratio sensor,
• wherein in the feedback control, a transfer function indicated by the following formula (1) is used:
• G
• ⁡
• (
• z
• )
• =
• b
• 1
• ⁢
• z
• -
• 1
• +
• b
• 0
• ⁢
• z
• -
• 2
• 1
• +
• a
• 1
• ⁢
• z
• -
• 1
• +
• a
• 0
• ⁢
• z
• -
• 2
• (
• 1
• )
• wherein in Formula (1), G(z) is the transfer function, and a0, a1, b0, and b1 are parameters defined by system identification of a plant, the transfer function being obtained by the system identification of the plant in which the air-fuel ratio in the combustion chamber serves as an input and the air-fuel ratio measured value obtained by the air-fuel ratio sensor serves as an output, and
• wherein a filtering process is performed to an air-fuel ratio measured value used in the feedback control using a low pass filter obtained by discretizing the transfer function.