1. Field of the Invention
The present invention relates to a failure diagnosing technique for determining whether an exhaust flow control apparatus, which suppresses the flow of exhaust, has failed or not.
2. Description of the Related Art
Conventionally, an exhaust emission control technique utilizing the reaction on a catalyst has been known as a technique for reducing harmful substances (e.g., unburned substances such as HC, CO, and H2 as well as smoke, NOx, and so forth) in exhaust.
This exhaust emission control technique, however, has the problem that unburned substances such as HC are emitted into the atmosphere during the period until the catalyst is activated. This is a serious problem because the amount of harmful substances emitted into the atmosphere during the period until the catalyst is activated reaches about 90% of the total emission in a cold mode.
To address this problem, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 3-117611 and Japanese Laid-Open Patent Publication (Kokai) No. 4-183921, a technique where the exhaust pressure is increased so as to promptly activate a catalyst in a cold mode has been developed.
According to this technique, a sealed type variable exhaust valve, for example, serving as an exhaust flow control device is disposed downstream in an exhaust passage. With this arrangement, when the sectional area of an exhaust pipe is varied by regulating the sealed type variable exhaust valve, the exhaust passage resistance and the exhaust concentration are increased or the exhaust flow rate is lowered to increase the exhaust pressure as well as the exhaust gas temperature.
The increase in the exhaust pressure as mentioned above promotes the reaction of unburned substances in an exhaust system (from a combustion chamber to an exhaust passage), and increases the exhaust gas temperature, thus promptly activating the catalyst and improving the exhaust purifying efficiency.
Incidentally, since the above-mentioned sealed type variable exhaust valve is subjected to high temperatures and high pressures, it may break down if used for a long period of time, causing malfunction of its valve.
The malfunction of the valve of the sealed type variable exhaust causes the problem that the valve cannot close and therefore the exhaust pressure cannot be increased even when the exhaust pressure is required to be increased, or the valve is kept closed and therefore the exhaust pressure remains high even when the exhaust pressure is not required to be increased.
If the exhaust pressure cannot be increased, the harmful substances emitted until the activation of the catalyst may not be satisfactorily purified because the purification of exhaust emission is not promoted. Also, if the exhaust pressure is kept at a high pressure, the exhaust efficiency is lowered to decrease the output of the internal combustion engine, which is not preferable.
To address this problem, it has been proposed that an exhaust pressure sensor is provided in an exhaust passage so that a failure of the sealed type variable exhaust valve can be detected based on exhaust pressure information supplied from the exhaust pressure sensor.
In the case where the exhaust pressure sensor is used, however, there is the necessity of additionally providing the exhaust pressure sensor. The exhaust pressure sensor is usually expensive because it must be configured to withstand high temperatures and high pressures, thus increasing the cost of parts. Moreover, the exhaust pressure sensor itself may break down.
Further, in consideration of the fact that the diffusion speed varies according to the exhaust pressure, it has been proposed that an exhaust gas sensor (such as an O2 sensor, an LAFS, or the like) is used to detect a failure of the sealed type variable exhaust valve.
In this case, however, since the failure detecting accuracy of the sealed variable exhaust valve is greatly affected by the accuracy of the exhaust gas sensor, the failure detecting accuracy is lowered as the exhaust gas sensor deteriorates with age. Moreover, as is the case with the exhaust pressure sensor, the exhaust gas sensor itself may break down.