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Exhaust purifying apparatus and exhaust purifying method for internal combustion engineRelated Patent Categories: Power Plants, Internal Combustion Engine With Treatment Or Handling Of Exhaust Gas, Methods, Anti-pollutionExhaust purifying apparatus and exhaust purifying method for internal combustion engine description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060168939, Exhaust purifying apparatus and exhaust purifying method for internal combustion engine. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to an exhaust purifying apparatus and an exhaust purifying method for an internal combustion engine. BACKGROUND ART [0002] As disclosed in Japanese Laid-Open Patent Publication No. 5-44434, a typical exhaust purifying apparatus applied to an internal combustion engine such as a vehicle diesel engine includes a PM filter that is located in the exhaust system. The PM filter traps particulate matter, which is predominantly composed of soot in exhaust gas. [0003] In an internal combustion engine provided with such an exhaust purifying apparatus, PM elimination control is performed to prevent the PM filter from being clogged with particulate matter (PM) when the accumulation amount of particulate matter in the PM filter estimated from, for example, the operating condition of the engine is more than or equal to a permissible value. In the PM elimination control, fuel is added to exhaust gas in a section upstream of the PM filter so that oxidation of unburned fuel component on the catalyst of the PM filter generates heat to increase the temperature of the catalyst. Accordingly, particulate matter on the PM filter is burned. When it is determined that particulate matter deposited on the PM filter is completely burned, the estimated accumulation amount of particulate matter on the PM filter is set to zero, and the PM elimination control is completed. [0004] The PM elimination control is sometimes suspended due to stopping of the engine during the execution. When the engine is started again and execution of the PM elimination control becomes possible, the PM elimination control is not resumed if the accumulation amount of particulate matter at the time of suspension is less than the permissible value. However, if incomplete execution of the PM elimination control due to suspension is repeated several times, the following drawbacks are likely to occur in relation to the estimated accumulation amount of particulate matter. [0005] The estimated accumulation amount of particulate matter may contain an error in relation to the actual accumulation amount. Such an error is eliminated by setting the estimated amount of particulate matter to zero when the PM elimination control is completed so that particulate matter deposited on the PM filter is completely burned. However, after execution and suspension of the PM elimination control are repeated a few times, accumulation of particulate matter on the PM filter in normal operation of the engine and burning of the particulate matter in the PM elimination control up to its suspension are repeated without the estimated particulate matter accumulation amount being set to zero. While the accumulation amount of particulate matter is repeatedly increased and reduced, the estimated accumulation amount can be greatly deviated from the actual accumulation amount. [0006] When the estimated accumulation amount of particulate matter is significantly less than the actual accumulation amount, execution of control for richening the exhaust air-fuel ratio based on the estimated accumulation amount of particulate matter can excessively increase the catalyst bed temperature of the PM filter. Such an excessive catalyst bed temperature is caused in the following manner. When unburned fuel component is supplied to the PM filter based on the control for richening the exhaust air-fuel ratio, oxidation of the unburned fuel component causes particulate matter deposited on the PM filter to burn. At this time, a greater amount of particulate matter than estimated has been accumulated, and the heat generated when the particulate matter is burned is increased accordingly. SUMMARY OF THE INVENTION [0007] Accordingly, it is an objective of the present invention to provide an exhaust purifying apparatus of an internal combustion engine that prevents an estimated accumulation amount of particulate matter about a catalyst from being significantly deviated from the actual accumulation amount due to suspension of PM elimination control. The present invention further provides an exhaust purifying method. [0008] To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides an exhaust purifying apparatus for an internal combustion engine. The apparatus estimates an accumulation amount of particulate matter trapped about a catalyst in an exhaust system. When the estimated accumulation amount is equal to or more than a permissible value, the apparatus executes PM elimination control for supplying unburned fuel component to the catalyst to increase the temperature of the catalyst and burning the trapped particulate matter. The apparatus sets the estimated accumulation amount to zero at the completion of the PM elimination control. When execution of the PM elimination control becomes possible after suspension of the control, the apparatus resumes the PM elimination control even if the accumulation amount of particulate matter about the catalyst is less than the permissible value. [0009] The present invention further provides an exhaust purifying method for an internal combustion engine. The method includes estimating an accumulation amount of particulate matter trapped about a catalyst in an exhaust system of the internal combustion engine. The method further includes executing PM elimination control when the estimated accumulation amount is equal to or more than a permissible value. In which control, unburned fuel component is supplied to the catalyst to increase the temperature of the catalyst and the trapped particulate matter is burned. The method further includes setting the estimated accumulation amount to zero at the completion of the PM elimination control. The method further includes resuming the PM elimination control when execution of the PM elimination control becomes possible after suspension of the control, even if the accumulation amount of particulate matter about the catalyst is less than the permissible value. [0010] Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS [0011] The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: [0012] FIG. 1 is a diagrammatic view illustrating the overall configuration of an internal combustion engine to which an exhaust purifying apparatus according to the present invention is applied; [0013] FIG. 2 is a graph showing changes in combustion rate of unburned fuel (HC) and PM about a catalyst relative to changes in the catalyst bed temperature; [0014] FIGS. 3(a) and 3(b) are time charts illustrating changes in PM accumulation amount and catalyst bed temperature during PM elimination control; [0015] FIG. 4 is a graph showing the relationship between a PM accumulation amount (determination value) Dc and holding period t2; [0016] FIG. 5 is a graph showing the relationship between a PM accumulation amount (determination value) Db and holding period t3; [0017] FIGS. 6(a) and 6(b) are time charts illustrating the manner of adding fuel and changes in the exhaust air-fuel ratio due to the fuel addition during burn-up control; [0018] FIGS. 7(a) and 7(b) are time charts illustrating changes in the PM accumulation amount and the catalyst bed temperature when the PM elimination control is executed and completed with the estimated PM accumulation amount being deviated from the actual accumulation amount; [0019] FIGS. 8(a) and 8(b) are time charts illustrating changes in the PM accumulation amount and the catalyst bed temperature when the PM elimination control is repeatedly executed and suspended; [0020] FIGS. 9(a) and 9(b) are time charts illustrating changes in the PM accumulation amount and the catalyst bed temperature when the PM elimination control is first suspended and then resumed and completed; and Continue reading about Exhaust purifying apparatus and exhaust purifying method for internal combustion engine... 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