Exhaust emission purifying apparatus and liquid level measuring device

An exhaust emission purifying apparatus comprising:

• a reducing agent tank which stores therein a liquid reducing agent or its precursor;
• a reduction catalytic converter configured to reduce and purify nitrogen oxide in exhaust gas using the liquid reducing agent or its precursor;
• a reducing agent dosing device comprising an injection nozzle, a pump module, and a dosing module, the reducing agent dosing device configured to injection-supply the liquid reducing agent or its precursor, stored in the reducing agent tank, to the exhaust upstream side of the reduction catalytic converter at a dosing flow rate corresponding to an engine operation state;
• a sensor which is attached to the reducing agent tank so as to hang from a top wall of the reducing agent tank and has a liquid level indicator measuring a liquid level of the liquid reducing agent or its precursor based on a capacitance change and a concentration meter measuring a concentration of the liquid reducing agent or its precursor based on heat discharge characteristics, the liquid level indicator and the concentration meter being unified;
• and
• a control unit with a built-in computer, wherein
• when the liquid level measured by the liquid level indicator of the sensor is equal to or greater than a predetermined measurable lower limit, the control unit is configured to output a measured liquid level,
• when the liquid level measured by the liquid level indicator of the sensor is less than the measurable lower limit and the control unit determines that liquid reducing agent or its precursor is left based on the concentration measured by the concentration meter of the sensor, the control unit is configured to output the liquid level estimated based on an integrated value obtained by sequentially integrating the dosing flow rate of the liquid reducing agent or its precursor injection-supplied by the reducing agent dosing device and a tank cross-sectional area, and
• when the liquid level measured by the liquid level indicator of the sensor is less than the measurable lower limit and the control unit determines that no liquid reducing agent or its precursor is left based on the concentration, measured by the concentration meter of the sensor, the control unit is configured to output a liquid level 0.