Oxygen Sensor

The exhaust gas oxygen sensor (EGO) also called, as ‘-sensor' or ‘lambda sensor' is used to control air-fuel ratio within about 1% of stoichiometric value for operation of 3-way catalysts. The sensor is fitted in the exhaust pipe just upstream of the catalytic converter. EGO operates on the principle of electro-chemical cell. Lambda-sensor is used to detect the presence or absence of free oxygen in the exhaust gas. Typical construction of an EGO is shown in Fig 5.16. Solid zirconium oxide (ZrO₂ ) stabilized with yttrium oxide (Y₂O₃ ) is used as electrolyte. The outer and inner surfaces of the hollow cylindrical are coated with porous platinum to form inner and outer electrodes. The outer electrode is exposed to the exhaust gas while the inner electrode to air having a fixed oxygen concentration. Due to catalytic effect of platinum electrode the exhaust gas reaches equilibrium composition very rapidly.. The electrochemical reactions at the electrodes produce oxygen ions that carry current through solid electrolyte producing a voltage signal. The e.m.f. voltage, e 0 produced is a function of the ratio of partial pressures of oxygen at the two electrodes and is given by Nernst equation (Eq 5.6) :

where R is the universal gas constant, T is sensor temperature in K, F is Faraday constant equal to 9.649 x 10 7 C/kmol, and are partial pressures of oxygen in the reference gas and the sample, respectively. 
The partial pressure of oxygen in the exhaust gas increases by many orders of magnitude (from about 10 ^-20 to 10 ³ P a at 500 ºC) as the fuel equivalence ratio varies from 1.01 to 0.99 . The effect of fuel –air equivalence ratio on sensor output signal is shown on Fig. 5.17. The sensor output voltage increases rapidly during transition from lean to rich mixture. For rich conditions a voltage of about 800 mV is produced and for lean mixtures about 50 mV is generated. The stoichiometric point is set at about 0.5 V. Voltage signal lower than the set point is taken by the engine control unit as lean mixture and higher voltage signal as the rich operation.