Exhaust emission purifying apparatus for and exhaust emission purifying method of internal combustion engine

1. An exhaust emission purifying apparatus of an internal combustion engine, comprising:

a particulate filter disposed in an exhaust passage of the engine for collecting particulate matters in the exhaust gas of the engine;

a reduction catalyst that accelerates the reductive purification of nitrogen oxides in the exhaust gas, the reduction catalyst being disposed downstream of the particulate filter in the exhaust passage;

a bypass passage arranged to diverge from the exhaust passage downstream of the particulate filter thereby extending to bypass the reduction catalyst and to merge with the exhaust passage downstream of the reduction catalyst;

a flow path switching valve disposed for switching a flow path that the exhaust gas having passed through the particulate filter flows between the exhaust passage and the bypass passage;

a temperature sensor disposed upstream of the reduction catalyst in the exhaust passage for detecting a temperature of the exhaust gas having passed through the particulate filter;

a control unit configured to control the switching valve in association with the temperature of the exhaust gas detected by the temperature sensor; and

a filter regenerator configured to forcibly burn particulate matters collected by the particulate filter to regenerate the particulate filter,

wherein, in a regenerating period of the particulate filter when the particulate filter is regenerated by the filter regenerator, the control unit switches the flow path for the exhaust gas having passed through the particulate filter to the bypass passage, and

wherein, after the regeneration period of the particulate filter is completed, the control unit switches the flow path for the exhaust gas to the exhaust passage when the temperature of the exhaust gas detected by the temperature sensor falls and reaches a predetermined temperature.

2. The apparatus according to claim 1 , wherein, after a predetermined time is elapsed after the regeneration of the particulate filter by the filter regenerator is completed, the control unit switches the flow path for the exhaust gas to the exhaust passage.

3. The apparatus according to claim 1 further comprising a flow path shut-off valve disposed at the merging portion of the exhaust passage where the bypass passage merges with the exhaust passage, for selectively shutting off the flow of the exhaust gas upstream of the merging portion in a closed state

wherein the control unit closes the shut-off valve in synchronization with the switching of the flow path to the bypass passage.

4. The apparatus according to claim 1 further comprising a reducing agent addition device disposed downstream of the particulate filter for adding a reducing agent for nitrogen oxides to the exhaust gas flowing into the reduction catalyst.

5. The apparatus according to claim 4 , wherein, in the regenerating period of the particulate filter, the control unit stops the adding operation of the reducing agent by the addition device.

6. An exhaust emission purifying apparatus of an internal combustion engine, comprising:

collecting means for collecting particulate matters in the exhaust gas of the engine, the collecting means being disposed in an exhaust passage of the engine;

purifying means for purifying nitrogen oxides in the exhaust gas with a catalyst, the purifying means being disposed in the exhaust passage downstream of the collecting means;

bypassing means for allowing the exhaust gas to bypass the purifying means downstream of the collecting means;

switching means for selectively allowing the exhaust gas having passed through the collecting means to flow through the bypassing means;

detecting means for detecting a temperature of the exhaust gas having passed through the collecting means, the detecting means being disposed in the exhaust passage upstream of the purifying means;

controlling means for controlling the flow of the exhaust gas to the bypassing means by the switching means in association with the temperature of the exhaust gas detected by the detecting means; and

regenerating means for forcibly burning particulate matters collected by the collecting means to regenerate the collecting means,

wherein, in a regenerating period of the collecting means conducted by the regenerating means, the controlling means urges the exhaust gas having passed through the collecting means to flow through the bypassing means and thereby bypass the purifying means, and

wherein, after the regenerating period of the collecting means is completed, the controlling means controls the flow of the exhaust gas to the exhaust passage when the temperature of the exhaust gas detected by the detecting means falls and reaches a predetermined temperature.

7. An exhaust emission purifying method of an internal combustion engine, in which a particulate filter for collecting particulate matters in the exhaust gas of the engine and a reduction catalyst for accelerating the reduction of nitrogen oxides in the exhaust gas are disposed in an exhaust passage of the engine in this order from the upstream of the exhaust passage, and a bypass passage for allowing the exhaust gas to bypass the reduction catalyst is provided downstream of the particulate filter, the method comprising the steps of:

allowing the exhaust gas having passed through the particulate filter to flow into the reduction catalyst in a normal operating period, so that, in the normal operating period, the particulate matters in the exhaust gas are removed by the particulate filter and the nitrogen oxides in the exhaust gas are purified by the reduction catalyst;

allowing the exhaust gas having passed through the particulate filter to flow into the bypass passage in a filter regenerating period other than the normal operating period in which the particulate matters removed by the particulate filter are forced to burn, so that, in the filter regenerating period, the exhaust gas is urged to bypass the reduction catalyst; and

allowing the exhaust gas having passed through the particulate filter to flow into the reduction catalyst after the filter regenerating period is completed and a detected temperature of the exhaust gas having passed through the particulate filter falls and reaches a predetermined temperature.