Exhaust emission control system of internal combustion engine

1. An exhaust emission control system of an internal combustion engine, the exhaust emission control system comprising:

a filter provided in an exhaust passage of the internal combustion engine, the filter being configured to trap particulate matter in exhaust gas, the filter including a first region as a part of the filter, and a second region as another part of the filter;

a heater configured to raise a temperature of the filter from an upstream side of the filter;

a differential pressure sensor configured to detect an exhaust pressure difference between the exhaust passage upstream of the filter and the exhaust passage downstream of the filter; and

an electronic control unit configured to:

perform a prescribed temperature raising process by activating the heater to raise the temperature of the filter such that at least some of the particulate matter deposited in the first region and in the second region of the filter is oxidized,

calculate a first differential pressure reduction in the exhaust pressure difference in a first oxidation period, wherein the first oxidation period includes at least a part of a period from a time at which a temperature of the first region exceeds a predetermined oxidation start temperature to a time at which a temperature of the second region exceeds the predetermined oxidation start temperature, the predetermined oxidation start temperature being a temperature at which oxidation of the particulate matter begins, and

execute a filter regeneration process to oxidize the particulate matter in the filter based at least in part on the calculated first differential pressure reduction.

2. The exhaust emission control system according to claim 1 ,

wherein, while performing the prescribed temperature raising process, the electronic control unit is further configured to calculate a first deposition amount of the particulate matter deposited in the first region based on the first oxidation period and the first differential pressure reduction.

3. The exhaust emission control system according to claim 2 ,

wherein the calculated first deposition amount increases with an increase in a ratio of the first differential pressure reduction to the first oxidation period.

4. The exhaust emission control system according to claim 3 ,

wherein, during execution of the prescribed temperature raising process, the electronic control unit is further configured to calculate a second differential pressure reduction in the exhaust pressure difference in a second oxidation period after the temperature of the second region exceeds the predetermined oxidation start temperature.

5. The exhaust emission control system according to claim 4 ,

wherein the electronic control unit is further configured to calculate a second deposition amount of the particulate matter based on the second oxidation period and the second differential pressure reduction.

6. The exhaust emission control system according to claim 5 ,

wherein the calculated second deposition amount increases with an increase in a ratio of a second region partial reduction amount to the second oxidation period,

wherein the second region partial reduction amount includes a portion of the second differential pressure reduction corresponding to a differential pressure reduction for the second region.

7. The exhaust emission control system according to claim 6 ,

wherein when the first oxidation period is set to a fixed length of time, the calculated first deposition amount increases with an increase in the first differential pressure reduction, and

wherein when the second oxidation period is set to a fixed length of time, the calculated second deposition amount increases with an increase in the second region partial reduction amount.

8. The exhaust emission control system according to claim 6 ,

wherein the electronic control unit is configured to set the second oxidation period such that the first oxidation period and the second oxidation period are about equal, and

wherein the electronic control unit is configured to calculate the second region partial reduction amount based on a difference between the second differential pressure reduction and the first differential pressure reduction.

9. The exhaust emission control system according to claim 6 , wherein the electronic control unit is configured to control the heater such that an amount of heat supplied to the filter per unit time in the first oxidation period is equal to an amount of heat supplied to the filter per unit time in the second oxidation period.

10. The exhaust emission control system according to claim 6 , wherein the electronic control unit is configured to:

estimate an amount of the particulate matter deposited in the filter as a whole, based on operating conditions of the internal combustion engine,

execute the filter regeneration process, when the amount of the particulate matter deposited in the filter as a whole exceeds a regeneration reference amount,

execute the prescribed temperature raising process when the amount of the particulate matter deposited in the filter as a whole exceeds a partial calculation reference amount that is smaller than the regeneration reference amount, and

execute the filter regeneration process even if the amount of the particulate matter deposited in the filter as a whole does not exceed the regeneration reference amount, when the first deposition amount exceeds a first reference deposition amount, or when the second deposition amount exceeds a second reference deposition amount.

11. The exhaust emission control system according to claim 6 , wherein the electronic control unit is configured to:

estimate an amount of the particulate matter deposited in the filter as a whole, based on operating conditions of the internal combustion engine,

execute the prescribed temperature raising process, when the amount of the particulate matter deposited in the filter as a whole exceeds a regeneration reference amount, and

execute the filter regeneration process, following execution of the prescribed temperature raising process, when the first deposition amount does not exceed a third reference deposition amount, and the second deposition amount does not exceed a fourth reference deposition amount.

12. The exhaust emission control system according to claim 11 ,

wherein the electronic control unit is configured to control the heater during a slow filter regeneration process, when at least the first deposition amount exceeds the third reference deposition amount, or the second deposition amount exceeds the fourth reference deposition amount,

wherein in the slow filter regeneration process, an amount of heat supplied to the filter is smaller than that of the filter regeneration process.

13. The exhaust emission control system according to claim 6 , wherein the electronic control unit is configured to:

estimate an estimated first deposition amount of the particulate matter deposited in the first region, and an estimated second deposition amount of the particulate matter deposited in the second region, based on operating conditions of the internal combustion engine,

estimate an amount of the particulate matter deposited in the filter as a whole, based on operating conditions of the internal combustion engine,

execute the filter regeneration process when the amount of the particulate matter deposited in the filter as a whole exceeds a regeneration reference amount,

execute the prescribed temperature raising process when a predetermined time elapses from completion of the filter regeneration process, and

correct the estimated first deposition amount and the estimated second deposition amount, based on the first deposition amount and the second deposition amount.

14. The exhaust emission control system according to claim 6 ,

wherein the filter further includes a third region as a part of the filter located downstream of the second region,

wherein, during execution of the prescribed temperature raising process, the electronic control unit is configured to:

set the second oxidation period such that the second oxidation period includes at least a part of a period from a time at which the temperature of the second region exceeds the predetermined oxidation start temperature, to a time at which a temperature of the third region exceeds the predetermined oxidation start temperature,

calculate a third differential pressure reduction in the exhaust pressure difference in a third oxidation period after the temperature of the third region exceeds the predetermined oxidation start temperature, and

calculate a third deposition amount of the particulate matter based on the third oxidation period and the third differential pressure reduction,

wherein the calculated third deposition amount increases with an increase in a ratio of a third region partial reduction amount to the third oxidation period, and

wherein the third region partial reduction amount includes a portion of the third differential pressure reduction corresponding to a differential pressure reduction for the third region.