Dynamic study of porous coating layers of exhaust gas oxygen sensors

We investigate the dynamics of the porous coating layers and gas-diffusion-limiting apertures in exhaust gas oxygen sensors and wide range air to fuel ratio sensors. We measure the dynamic responses of the sensors with and without the porous layers to a small perturbation of the air to fuel ratios of the exhaust gas (transfer function measurement). By comparison of the data, we conclude that the coating layer dynamic mechanism can be described by the one-dimension-mass-transport equation. Only one parameter is involved in the model, L/√D where L represents the effective diffusion length, and D is the gas diffusion constant. Although the effective average diameter of the pores of the porous coating layers and gas-diffusion-limiting apertures is not involved in the above parameter directly, it dictates the types of gas-diffusion mechanism (gas molecular versus Knudsen's), which in turns determines the temperature and pressure effects on the gas diffusion constant D.