The present invention relates to an engine exhaust purification device provided with a catalyst.
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein an oxygen amount stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is estimated based on an engine intake air amount and an air fuel ratio of an exhaust flowing into the catalyst, and engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is constant.
To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of the three-way catalyst at a maximum, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. If the oxygen storage amount of the catalyst is maintained constant, oxygen in the exhaust is stored in the catalyst even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes lean, and conversely, oxygen stored in the catalyst is released even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes rich, so the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.
Therefore, in an exhaust purification device performing this type of control, it is required to calculate the oxygen storage amount precisely to maintain the conversion efficiency of the catalyst at a high level, and various methods of computing the oxygen storage amount have been proposed.
However, non-oxygen components such as HC, CO are stored by the catalyst apart from oxygen, and an error occurs in the real oxygen storage amount due to their influence. In the prior art, the reason for this error was not considered, so there was a risk of decreasing the precision of air-fuel ratio control of the catalyst atmosphere and impairing exhaust emissions.
It is therefore an object of this invention to resolve the above problems, and to provide an exhaust purification device for an engine which eliminates the effect of non-oxygen components and permits precise control of the oxygen storage amount.
In order to achieve above object, this invention provides an engine exhaust purification device which comprises a catalyst provided in an engine exhaust passage, a first sensor which detects an exhaust characteristic flowing into the catalyst, a second sensor which detects an exhaust characteristic flowing out of the catalyst, and a microprocessor programmed to compute an oxygen storage amount of the catalyst using the detected exhaust characteristic, to compute the storage amount of a non-oxygen component of the catalyst, to compute the air fuel ratio of the engine so that the oxygen storage amount of the catalyst is a target amount based on the computed oxygen storage amount, and to control the computed air fuel ratio based on the computed non-oxygen component storage amount.