Measuring apparatus for monitoring residual oxygen in an exhaust gas

Abstract

A lambda probe ( 1 ) is used with the measuring apparatus for monitoring residual oxygen in an exhaust gas, in which a measuring point for oxygen in a sensor ( 2 ) is connected via a diffusion gap ( 22 ) with a reaction chamber ( 24 ). During operation of the probe the reaction chamber drives a stream of oxygen I O2 along the diffusion gap by means of a controllably adjustable oxygen partial pressure p i . By means of an electro-chemical, oxygen ion pump driven by an electrical pump current I p , an oxygen partial pressure p i predetermined as a desired value is set in the reaction chamber. In this arrangement the pump current, the strength of which is proportional to the strength of the stream of oxygen driven along the diffusion gap, can be used as a measurement parameter for the partial pressure p m of the residual oxygen in the exhaust gas or its concentration. The residual oxygen can be monitored during a normal operating phase, the phase N. The lambda probe can be operated for test purposes, at times, particularly intermittently in a phase H or a phase L. In these operating phases H and L the oxygen partial pressure pi in the reaction chamber ( 24 ) adopts a largely minimum value or a largely maximum value. By means of changing between the named operating phases, by registering the pump current I p and by comparing the registered pump currents I p with empirical values, conclusions with regard to the ability of the probe to function ( 1 ) can be derived, so that if necessary, in the case of a lacking or faulty ability to function, measures can be introduced to remedy the deficiencies or to exchange the sensor ( 2 ).

1 . A measuring apparatus for monitoring residual oxygen in an exhaust gas using a lambda probe ( 1 ) in which a measuring point for oxygen at a sensor ( 2 ) is connected with a reaction chamber ( 24 ) via a diffusion gap ( 22 ), which during operation of the probe drives a stream of oxygen I O2  along the diffusion gap by means of a controllably adjustable oxygen partial pressure p i , wherein by means of an electrochemical oxygen ion pump driven by an electrical pump current I p  a predetermined oxygen partial pressure p i  can be set in the reaction chamber as a desired value and thus the pump current, the strength of which is proportional to the strength of the oxygen stream driven along the diffusion gap, can be used as a measurement parameter for the partial pressure p m  of the residual oxygen in the exhaust gas, characterised in that the residual oxygen can be monitored during a normal operating phase the phase N, in that the lambda probe can be operated at times for test purposes, in particular intermittently in a phase H or a phase L, in which operating phases the oxygen partial pressure p i  in the reaction chamber ( 24 ) adopts a largely minimum value or a largely maximum value and in that by mean of changing between the named operating phases, by registering the pump current I p  and by comparing the pump currents I p  with empirical values, conclusions with regard to the ability of the probe ( 1 ) to function can be derived so that if necessary, in the case of a lacking or faulty ability to function, measures can be introduced to remedy the deficiencies or to exchange the sensor ( 2 ).  
     
     
         2 . A measuring apparatus in accordance with  claim 1  characterised in that the oxygen partial pressure p i  in the reaction chamber ( 25 ) can be determined as an actual value electrochemically by means of a Nernst cell, in that the desired value can be set by comparison with a desired value determining reference voltage and a regulation of the pump current  1   p  and in that a high reference voltage, a medium reference voltage and a low reference voltage ( 31 ) can be respectively preset for the selection of the phases H, N and L.  
     
     
         3 . A method for operating a measuring apparatus in accordance with  claim 2  characterised in that except in the phase N, the measuring apparatus is intermittently operated in the phase L and/or alternatingly in the phase H by gradually changing the reference voltage determining the desired value in steps between medium, high and/or low.  
     
     
         4 . A method in accordance with  claim 3  characterised in that the reference voltage determining the desired value is continuously varied stepwise from low to medium to high and periodically in this manner.  
     
     
         5 . A method in accordance with  claim 3  characterised in that the reference voltage in left unchanged at every step during a time interval, at least until a settled state has been set wherein the operating phases N, L and/or H can last for differing lengths of time.  
     
     
         6 . A method in accordance with  claim 3 , characterised in that an ohmic resistance heater ( 27 ) for a solid electrolyte ( 20   a ,  20   b ) which is integrated in the sensor ( 2 ) is checked periodically with the heater current switched off during the checking of the heater resistance.  
     
     
         7 . A method in accordance with  claim 6  characterised in that when registering a change of the heater resistance a corresponding correction of the strength of the heater current is carried out by means of a regulating current in order to maintain a predetermined operating temperature of the sensor ( 2 ).  
     
     
         8 . The use of the method in accordance with  claim 3  in an apparatus in which exhaust gases of a combustion process or of an electrochemical conversion are monitored with the apparatus being a vehicle, a heating device or a fuel cell system with which not only thermal energy but also electrical energy can be produced simultaneously from a fuel.