Controller and control method

A control method executed by a controller configured to control an engine, the engine including a filter disposed in an exhaust pipe and configured to collect particulate matter, the controller storing (i) a first map that outputs an amount of particulate matter emitted from a combustion chamber of the engine when a first fuel is assumed to be combusted in the combustion chamber based on a rotation speed of the engine, a load of the engine, and a temperature of coolant of the engine, and (ii) a second map that outputs an amount of particulate matter emitted from the combustion chamber when a second fuel is assumed to be combusted in the combustion chamber based on the rotation speed of the engine, the load of the engine, and the temperature of the coolant of the engine, the second fuel being less likely to produce particulate matter than the first fuel, the control method, comprising:

• executing a first process routine including
• calculating a first deposition amount using the first map, the first deposition amount being an amount of particulate matter estimated to be deposited on the filter when the first fuel is assumed to be combusted in the combustion chamber, and
• executing a filter protection process that reduces an amount of air supplied to the filter to avoid abnormal burning of the particulate matter deposited on the filter when the first deposition amount is greater than or equal to a first threshold value and a temperature of the filter is higher than or equal to a predetermined temperature;
• and
• executing a second process routine in parallel with the first process routine, the second process routine including
• calculating a second deposition amount using the second map, the second deposition amount being an amount of particulate matter estimated to be deposited on the filter when the second fuel is assumed to be combusted in the combustion chamber, and
• executing a filter regeneration process that increases a fuel injection amount to increase the temperature of the filter and then supplies air to the filter to burn and remove the particulate matter deposited on the filter when the second deposition amount is greater than or equal to a second threshold value that is greater than the first threshold value and an amount of fuel stored in a vehicle on which the engine is mounted is greater than or equal to a predetermined amount.