1. Field of Application
The present invention relates to an internal combustion engine having an exhaust gas cleansing device in the exhaust system of the engine and a pressure difference detection apparatus for detecting a pressure difference developed across the exhaust gas cleansing device. In particular, the invention relates to a diagnostic apparatus capable of diagnosing a specific cause of abnormal operation of the pressure difference detection apparatus.
2. Description of Related Art
As described for example in Japanese patent publication No. 2005-3078830 (designated in the following as reference document 1), it is known to detect abnormal operation of a pressure difference detection apparatus used in the exhaust system of a diesel engine, with the detection of abnormal operation being based upon detection results obtained by the pressure difference detection apparatus. Such a pressure difference detection apparatus is used to detect the pressure difference appearing across an exhaust gas cleansing device called a DPF (diesel particulate filter) which passes the exhaust gas flow from the engine. Specifically, the pressure difference detection apparatus detects the pressure difference between the respective interiors of a first tube (i.e., passage) that communicates with the exhaust gas flow upstream from the DPF and a second tube that communicates with the exhaust gas flow downstream from the DPF.
Such a diagnostic apparatus calculates an estimated amount of accumulated particulate matter in the DPF (i.e., which is obstructing the flow of exhaust gas through the DPF), based on the running conditions of the engine, with such an estimated amount of particulate matter being referred to in the following as the operation history-based estimated amount PM1. In addition, the diagnostic apparatus calculates a second estimated value of that accumulated amount of particulate matter, based on the detected pressure difference between upstream and downstream locations with respect to the DPF in the exhaust gas flow, as obtained by the pressure difference detection apparatus, with such an estimated amount of particulate matter being referred to in the following as the pressure difference-based estimated amount PM2. Based on the estimated value of PM2 (more specifically, the most recently derived value of PM2) and a threshold value that is determined based on the operation history-based estimated amount PM1, a decision is made as to whether abnormal operation of the pressure difference detection apparatus is occurring.
Due to the fact that the aforementioned first tube of the pressure difference detection apparatus is located upstream from the DPF, communicating with the exhaust gas flow (i.e., with the interior of a gas flow passage constituted by an exhaust pipe of the engine) particulate matter becomes deposited within that tuber so that blockage can occur not only within the exhaust pipe due to accumulated particulate matter in the DPF, but may also occur within the upstream-side tube of the pressure difference detection apparatus. If such flow obstruction in the upstream-side tube occurs, then when the diagnostic apparatus detects that there is abnormal operation of the pressure difference sensor, it is difficult for the diagnostic apparatus to diagnose that the abnormal operation is being caused specifically by blockage of the upstream-side tube, rather than by some other cause such as an electrical fault in the pressure difference detection apparatus.
Hence, this is an obstacle to taking appropriate countermeasures against the specific cause of the abnormal operation of the pressure difference detection apparatus.