Method and apparatus for correcting a gas sensor response for moisture in exhaust gas

A method for correcting a NOx concentration detection signal output from a NOx sensor, said NOx sensor having at least two solid electrolyte oxygen ion pump cells, in which a voltage is applied across a first oxygen ion pump cell for pumping out oxygen in the measurement gas, a current corresponding to the oxygen concentration in the measurement gas is caused to flow in said first oxygen ion pump cell, a second oxygen ion pump cell decomposes NOx in the gas not pumped by the first oxygen ion pump cell to pump out dissociated oxygen, with a current corresponding to the NOx gas concentration flowing in the second oxygen ion pump cell, said method comprising:

• applying voltage across the first oxygen ion pump cell for sufficiently pumping out oxygen in the measurement gas to an extent not to allow all NOx to decompose in the measurement gas;
• correcting a NOx concentration detection signal corresponding to the current flowing in the second oxygen ion pump cell based on current flowing in the first oxygen ion pump cell, to thereby remove or reduce the effect of the amount of moisture in the measurement gas from the NOx gas concentration as found based on said NOx gas concentration detection signal,
• said correcting step comprising:
• estimating the current flowing in said first oxygen ion pump cell to correspond to the amount of the moisture in said measurement gas,
• pre-formulating a map containing correction data of said NOx gas concentration detection signal corresponding to current flowing in the first oxygen ion pump cell based on the current flowing in said first oxygen ion pump cell,
• reading out, from said map, pre-set correction data corresponding to current flowing in the first oxygen ion pump cell, and
• correcting the NOx gas concentration detection signal by the read-out correction data.

An apparatus for detecting the concentration of a specific gas in an engine exhaust gas, comprising:

• a gas sensor for detecting the concentration of a specific gas in an engine exhaust gas and outputting a gas concentration signal;
• means for estimating the amount of moisture in the exhaust gas from driving conditions of an engine;
• means for estimating an air to fuel ratio supplied to said engine to correspond to the amount of moisture in said exhaust gas;
• memory means for storing a map containing correction data for said gas concentration detection signal corresponding to said estimated air to fuel ratio based on said estimated ratio, said gas concentration detection signal and true gas concentration value;
• reading means for reading out, from said map, specific correction data corresponding to said estimated ratio as determined by an engine driving condition;
• and
• correcting means for correcting said gas concentration detection signal by the read-out correction data.

A method for determining the NOx concentration in a measurement gas, under a varying condition of the NOx gas concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting state of oxygen ions, comprising:

• (1) introducing the measurement gas into the flow channel;
• (2) forming in said flow channel a residual gas having a NOx concentration different from that before entrance into said flow channel by extracting an oxygen gas from the measurement gas of step (1) through the ceramic body to outside of the flow channel;
• (3) dissociating NOx of said NOx-concentrated residual gas into nitrogen and oxygen by applying a voltage across electrodes formed on the ceramic body;
• (4) measuring an electric current flowing through the ceramic body between the electrodes, said electric current being generated by the electrochemical action of the oxygen dissociated from NOx in step (3);
• (5) determining a basic NOx concentration of the measurement gas, based on the electric current measured in step (4);
• and
• (6) correcting the basic NOx concentration wherein the amount of moisture in said measurement gas is estimated from a driving condition of an engine, and said NOx concentration of step (5) is corrected depending on an estimated amount of moisture.