3.16.3 Oxygen Sensors in Automobile Exhaust

• Modern automobiles demands good fuel efficiencies and pollution control.

• Exhaust gases are NO_x gases, CO, Hydrocarbons.

• One alternative is to use the catalytic converters, which however do not regulate the fuel consumption.

• The above air/ fuel ratio of ~ 14.5, hydrocarbons and CO are decomposed efficiently but not NO_x, while below 14.5, the reverse is true.

• When there are deviations in the air /fuel ratio from the stoichiometric value of 14.5, pO₂ in the exhaust changes dramatically from ~ 10^-9 to 10^-26 atm. This differential provides a way to use the activity of oxygen, i.e. to measure the combustion efficiency.

• The process is to first measure the pO₂ of the exhaust by using an oxygen sensor. The difference between the pO₂ of the exhaust and air can be translated into an electrical signal which is fed as an electrical input to the fuel injection system to optimize the air/fuel ratio.

• Oxygen sensors are typically conducting ceramics operating on two principles:

• First type is based on Nernst effect where an electrical potential is created when there is a pO₂ gradient across the sensor.

• Second type is where one can measure the variation in the conductivity of the ceramic vs pO₂ .

• Defect concentration varies by orders of magnitude upon changes in background pO₂and temperature. This also affects the electrical conductivity of the ceramic, which is generally proportional to the number of current carriers, dependent upon T and pO₂ .

• Figure 3.11 shows the schematic of an exhaust gas sensor using Y₂O₃ stabilized ZrO₂ which gives rise to sufficient voltages under a gradient of oxygen partial pressures encountered in automobiles which can be used to regulate the fuel/air supply.