JP 2014-163237 A discloses a conventionally known exhaust apparatus mounted on a vehicle with an engine. The exhaust apparatus disclosed in JP 2014-163237 A includes an aftertreatment device configured to purify exhaust gas emitted from an engine, an exhaust pipe through which the exhaust gas treated by the aftertreatment device is released into the atmosphere, and a nitrogen oxide (NOx) sensor mounted on the exhaust pipe. The NOx sensor detects a NOx concentration in the exhaust gas to be released into the atmosphere. The detection results are utilized for operation control of the aftertreatment device (for example, control of injection quantity of urea water for reduction of NOx) or utilized as information for diagnosing failure or deterioration of the aftertreatment device.
However, NOx sensors are typically weak to water. For example, in a widely used NOx sensor equipped with a sensing portion containing ceramics such as zirconia, adhesion of water to a surface of the ceramics may result in a crack in the worst case, because evaporation of the water on the surface of the ceramics generates latent heat, thereby generating a possibility of large thermal stress in the ceramics. Moreover, since the NOx sensor is typically provided on an exhaust pipe that is communicated with the atmosphere, rain water or high-pressure cleaning water, for example, may enter the exhaust pipe and reach the NOx sensor. In view of the above, it is an important issue to protect the NOx sensor from moisture.
In this regard, the exhaust pipe disclosed in JP 2014-163237 A can effectively protect the NOx sensor from adhesion of external water such as rain water, because the exhaust pipe is formed in a U-shape with an intermediate part between upstream and downstream ends thereof, the intermediate part being the lowest part, and the NOx sensor is provided on the downstream end which is positioned higher than the intermediate part. The exhaust pipe, however, has to have a long passage length and a complicated shape in order to locate the intermediate part at a lower position than the other parts, thus involving significant increase in the size and in the cost of the apparatus.