Exhaust purification system of internal combustion engine

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

a hydrocarbon feed valve for feeding hydrocarbons arranged inside an engine exhaust passage;

an exhaust purification catalyst for causing a reaction between the hydrocarbons injected from the hydrocarbon feed valve and NOx contained in exhaust gas arranged in the engine exhaust passage downstream of the hydrocarbon feed valve;

a precious metal catalyst carried on an exhaust gas flow surface of the exhaust purification catalyst;

a basic exhaust gas flow surface part formed around the precious metal catalyst; and

an electronic control unit,

wherein the electronic control unit is configured to control the injection of the hydrocarbons from the hydrocarbon feed valve at predetermined intervals by a first hydrocarbon feed method and a second hydrocarbon feed method, so that an air-fuel ratio of the exhaust gas falls to a predetermined air-fuel ratio, and is configured to control the feed intervals of the hydrocarbons longer than the predetermined feed intervals, wherein

when the electronic control unit is configured to control the injection of hydrocarbons from the hydrocarbon feed valve at the predetermined intervals, the exhaust purification catalyst has a property of chemically reducing the NOx that is contained in the exhaust gas,

in the first hydrocarbon feed method, the electronic control unit is configured to control the injection of hydrocarbons from the hydrocarbon feed valve at the predetermined feed intervals so that the air-fuel ratio of the exhaust gas falls to the predetermined air-fuel ratio, thereby chemically reducing the NOx contained in the exhaust gas,

in the second hydrocarbon feed method, the electronic control unit is configured to control the feed amount of the hydrocarbons from the hydrocarbon feed valve at the predetermined feed intervals in a feed amount that is lower than that of the first hydrocarbon feed method, and the electronic control unit is configured to simultaneously control the feeding of a fuel required for lowering the air-fuel ratio of the exhaust gas to the required air-fuel ratio into a combustion chamber during a second half of an expansion stroke or an exhaust stroke, thereby chemically reducing the NOx contained in the exhaust gas, and

the electronic control unit is configured to selectively perform the first or the second hydrocarbon feed method in accordance with an operating state of the engine, and

when the electronic control unit controls the injections of the hydrocarbons from the hydrocarbon feed valve longer than the predetermined intervals, the exhaust purification catalyst has a property of being increased in a storage amount of NOx that is contained in the exhaust gas.

2. The exhaust purification system of the internal combustion engine as claimed in claim 1 , wherein the second hydrocarbon feed method is used when raising a temperature of an exhaust treatment device.

3. The exhaust purification system of the internal combustion engine as claimed in claim 2 , wherein the exhaust treatment device is comprised of the exhaust purification catalyst and wherein, when the temperature of the exhaust purification catalyst falls below a predetermined activation temperature, the first hydrocarbon feed method is switched to the second hydrocarbon feed method, and a temperature elevation action of the exhaust purification catalyst is performed.

4. The exhaust purification system of the internal combustion engine as claimed in claim 2 , wherein the exhaust treatment device is comprised of a particulate filter that is arranged inside of the engine exhaust passage and wherein when the particulate filter should be regenerated, the first hydrocarbon feed method is switched to the second hydrocarbon feed method, and a temperature elevation action of the particulate filter is performed.

5. The exhaust purification system of the internal combustion engine as claimed in claim 1 , wherein the NOx contained in the exhaust gas and reformed hydrocarbons are reacted inside the exhaust purification catalyst whereby a reducing intermediate containing nitrogen and hydrocarbons is produced and wherein the predetermined feed period of the hydrocarbons is a feed period required for continued production of the reducing intermediate.

6. The exhaust purification system of the internal combustion engine as claimed in claim 5 , wherein the feed period of the hydrocarbons is between 0.3 second to 5 seconds.

7. The exhaust purification system of the internal combustion engine as claimed in claim 1 , wherein the precious metal catalyst is comprised of platinum Pt and at least one of rhodium Rh and palladium Pd.

8. The exhaust purification system of the internal combustion engine as claimed in claim 1 , wherein a basic layer containing an alkali metal, an alkali earth metal, a rare earth, or a metal that can donate electrons to NOx is formed on the exhaust gas flow surface of the exhaust purification catalyst and wherein the surface of the basic layer forms the basic exhaust gas flow surface part.

9. The exhaust purification system of the internal combustion engine as claimed in claim 1 , wherein when the second hydrocarbon feed method is used, the feed amount of the hydrocarbons that are fed from the hydrocarbon feed valve is determined in accordance with an amount of NOx in the exhaust gas flowing into the exhaust purification catalyst.