Exhaust gas cleaning apparatus for lean burn internal combustion engine

1. An exhaust gas cleaning apparatus for cleaning exhaust gas from an internal combustion engine, the apparatus comprising:

a Lean NO x Trap (LNT) provided in an exhaust gas passage of the engine through which the exhaust gas flows, the LNT being configured to absorb NO x contained in the exhaust gas when the exhaust gas is lean and reduce the absorbed NO x when the exhaust gas turns to rich;

a fuel adding device located upstream of the LNT to add fuel to the exhaust gas for making the exhaust gas rich;

an exhaust gas returning passage that branches off from the exhaust gas passage at a position downstream of a turbocharger of the engine to return part of the exhaust gas to an intake air passage of the engine; and

a controller configured to determine an operating condition of the engine and control the exhaust gas cleaning apparatus to selectively operate in one of first and second operation modes according to the determined operating condition of the engine, in the first operation mode the fuel adding device adding the fuel to the exhaust gas with the exhaust gas returning passage closed, in the second operation mode the part of the exhaust gas being returned to the intake air passage through the exhaust gas returning passage without the fuel being added by the fuel adding device to the exhaust gas, wherein

the controller is configured to determine whether the engine operates in a first operation region, in which both the rotational speed and load of the engine is low, or in a second operation region in which either or both of the rotational speed and load of the engine are high, and

the controller is configured to control the exhaust gas cleaning apparatus to operate in the first operation mode when the engine operates in the first operation region and in the second operation mode when the engine operates in the second operation region.

2. The exhaust gas cleaning apparatus as set forth in claim 1 , wherein

the internal combustion engine is a diesel engine.

3. An exhaust gas cleaning apparatus for cleaning exhaust gas from an internal combustion engine, the apparatus comprising:

a Lean NO x Trap (LNT) provided in an exhaust gas passage of the engine through which the exhaust gas flows, the LNT being configured to absorb NO x contained in the exhaust gas when the exhaust gas is lean and reduce the absorbed NO x when the exhaust gas turns to rich;

a fuel adding device located upstream of the LNT to add fuel to the exhaust gas for making the exhaust gas rich;

an exhaust gas returning passage that branches off from the exhaust gas passage at a position downstream of the fuel adding device to return part of the exhaust gas to an intake air passage of the engine; and

a controller is configured to determine an operating condition of the engine and controls the exhaust gas cleaning apparatus to selectively operate in one of first and second operation modes according to the determined operating condition of the engine, in the first operation mode the fuel adding device adding the fuel to the exhaust gas with the exhaust gas returning passage closed, in the second operation mode the part of the exhaust gas being returned to the intake air passage through the exhaust gas returning passage without the fuel being added by the fuel adding device to the exhaust gas, wherein

the controller is configured to determine whether the engine operates in a first operation region, in which both the rotational speed and load of the engine is low, or in a second operation region in which either or both of the rotational speed and load of the engine are high,

the controller is configured to control the exhaust gas cleaning apparatus to operate in the first operation mode when the engine operates in the first operation region and in the second operation mode when the engine operates in the second operation region,

the exhaust gas cleaning apparatus further comprises a detector configured to detect the deterioration degree of a NO x catalyst of the LNT, and

as the deterioration degree of the NO x catalyst detected by the detector increases, the controller reduces the first operation region of the engine while accordingly expanding the second operation region of the engine.

4. The exhaust gas cleaning apparatus as set forth in claim 3 , wherein the controller is configured to select a first boundary to separate the first and second operation regions of the engine when the deterioration degree of the NO x catalyst detected by the detector is lower than or equal to a predetermined threshold, and a second boundary to separate the first and second operation regions when the deterioration degree of the NO x catalyst is higher than the predetermined threshold.

5. The exhaust gas cleaning apparatus as set forth in claim 3 , wherein the controller is configured to set an expanding rate of the second operation region of the engine as a function of the deterioration degree of the NO x catalyst detected by the detector, the function being such that the expanding rate of the second operation region set with it increases faster than that set with a liner function.

6. The exhaust gas cleaning apparatus as set forth in claim 3 , wherein the internal combustion engine is a diesel engine.