1. Field of the Invention
The present invention relates to an exhaust gas cleaning system having a particulate filter for collecting particulate matters included in exhaust gas of an internal combustion engine.
2. Description of Related Art
Particulate matters discharged from a diesel engine have a great effect on the environment. As a countermeasure to it, conventionally, a diesel particulate filter (a DPF, hereafter) formed of a ceramic porous body is employed, for instance. The DPF is disposed in an exhaust pipe in order to collect the particulate matters at its porous partition walls. The DPF is regenerated by eliminating the collected particulate matters through combustion regularly.
In the regeneration of the DPF, a quantity of the accumulated particulate matters (a PM accumulation quantity m, hereafter) is calculated based on a pressure difference across the DPF. If the PM accumulation quantity m exceeds a predetermined quantity, temperature increasing means is operated to heat the DPF above a certain temperature, at which the particulate matters can be combusted, so the DPF is regenerated. Under some operating conditions of the engine, the temperature of the exhaust gas increases to a high temperature, at which spontaneous combustion of the particulate matters is possible. In order to regenerate the DPF efficiently, the temperature increasing means should be preferably operated in accordance with the operating condition of the engine. A technology of such a kind aiming at regenerating the DPF efficiently is disclosed in Japanese Patent Unexamined Publication No. 2000-170521, for instance.
The above patent document discloses a method for selecting temperature increasing means in accordance with an operating condition of an engine and for regenerating the DPF by increasing the temperature of the DPF with the selected temperature increasing means when the PM accumulation quantity m reaches a predetermined quantity. The operating condition (a load condition) of the engine is classified into a plurality of areas based on engine rotation speed and output torque, for instance. Different kinds of regenerating operations are performed in the respective areas. In an area where the spontaneous combustion of the accumulated particulate matters is possible, no special operation is performed. Thus, the regeneration of the DPF can be performed appropriately while inhibiting an increase in fuel consumption.
However, the method disclosed in the above patent document does not perform the temperature increasing operation in an area where the engine rotation speed is low and a load is light even if the PM accumulation quantity m reaches a quantity at which the regeneration of the DPF is necessary. It is because the temperature increase of the DPF to the temperature enabling the combustion of the particulate matters is difficult in the low speed and light load area. More specifically, in the technology disclosed in the above patent document, the regenerating operation is not performed if the operating condition of the engine is in the low rotation speed and low load area in the case where the PM accumulation quantity m reaches the quantity at which the regeneration is necessary. If the operating condition of the engine enters the low rotation speed and low load area during the regeneration, the regenerating operation is stopped.
However, if the operation of the engine in the low rotation speed and light load area such as an idling operation or an operation in a traffic congestion continues for a long time, a large amount of the particulate matters will be accumulated in the DPF beyond a permissible quantity.
If the PM accumulation quantity m increases, exhaust gas pressure will be increased and an engine output will be degraded. Moreover, reaction heat generated when the large amount of the accumulated particulate matters is combusted rapidly can degrade or damage the DPF and a catalyst. In order to prevent these problems, the permissible value of the PM accumulation quantity m is determined.
Therefore, in the case where the particulate matters greater than the permissible quantity are accumulated, there is a possibility that the engine output may be degraded in the technology disclosed in the above patent document. Moreover, if the operating condition of the engine is changed to a middle load operating condition or a heavy load operating condition afterward, there is a possibility that the large amount of the accumulated particulate matters may be combusted rapidly, and the DPF and the catalyst may be degraded or damaged.