Patent ID: 8061125
Filing Date: 2011-11-22
Classification: F01N,F02B,F02D,Y02T

Abstract:
1. An exhaust gas purification system for an internal combustion engine, comprising: an adsorption-reduction type NOx catalyst that is provided in an exhaust passage of the internal combustion engine and adsorbs NOx and reduces the adsorbed NOx using a reducing agent; an engine air-fuel ratio control device that controls an engine-out exhaust gas air-fuel ratio, which is the ratio-between an air contained in an exhaust gas discharged from the internal combustion engine and a fuel element contained in the same exhaust gas and acting as a reducing agent at the adsorption-reduction type NOx catalyst, by controlling the air-fuel ratio of a gas to be combusted in the internal combustion engine; a fuel addition device that is provided upstream of the adsorption-reduction type NOx catalyst in the exhaust passage and adds fuel into an exhaust gas; a SOx poisoning recovery control device that executes a SOx poisoning recovery control for reducing SOx adsorbed in the adsorption-reduction type NOx catalyst, in which the engine-out exhaust gas air-fuel ratio is controlled by the engine air-fuel ratio control device and fuel is added into the exhaust gas from the fuel addition device such that a catalyst-entering exhaust gas air-fuel ratio, which is the ratio between an air contained in an exhaust gas entering the adsorption-reduction type NOx catalyst and a fuel element contained in the same exhaust gas and acting as a reducing agent at the adsorption-reduction type NOx catalyst, decreases to a target exhaust gas air-fuel ratio enabling SOx reduction reactions and such that the temperature of the adsorption-reduction type NOx catalyst increases to a target temperature enabling SOx reduction reactions, the exhaust gas purification system being characterized in that: when reducing the catalyst-entering exhaust gas air-fuel ratio to the target exhaust gas air-fuel ratio and increasing the temperature of the adsorption-reduction type NOx catalyst to the target temperature, the SOx poisoning recovery control device controls the engine-out exhaust gas air-fuel ratio using the engine air-fuel ratio control device and controls the amount of fuel added from the fuel addition device so as to minimize the sum of the amount of fuel injected in the internal combustion engine and the amount of fuel added from the fuel addition device; when reducing the catalyst-entering exhaust gas air-fuel ratio to the target exhaust gas air-fuel ratio, the SOx poisoning recovery control device reduces the engine-out exhaust gas air-fuel ratio to a level at which the amount of a fuel element contained in the exhaust gas discharged from the internal combustion engine and not acting as a reducing agent at the adsorption-reduction type NOx catalyst reaches an upper limit value, and the SOx poisoning recovery control device controls the fuel addition device to add, into the exhaust gas, fuel of an amount needed to compensate for the shortfall in the fuel amount required to reduce the catalyst-entering exhaust gas air-fuel ratio to the target exhaust gas air-fuel ratio; wherein the upper limit value of the non-reductive fuel element is a threshold used to determine whether the fuel used for further reducing the catalyst-entering exhaust gas air-fuel ratio can be saved by accomplishing the reduction by adding fuel into the exhaust gas from the fuel addition device or by accomplishing the reduction by reducing the engine-out exhaust gas air-fuel ratio by reducing the air-fuel ratio of the gas to be combusted in the internal combustion engine; and wherein, if the amount of the non-reductive fuel element is larger than the upper limit value, the fuel amount needed to bring about a given reduction of the catalyst-entering exhaust gas air-fuel ratio is considered to be smaller when the same reduction of the catalyst-entering exhaust gas air-fuel ratio is accomplished by adding fuel into the exhaust gas from the fuel addition device than when the same reduction of the catalyst-entering exhaust gas air-fuel ratio is accomplished by reducing the engine-out exhaust gas air-fuel ratio by reducing the air-fuel ratio of the gas to be combusted in the internal combustion engine.