Geometric effects in the design of catalytic converters in car exhaust pipes

We solve the gas dynamics (Euler) equations, augmented by adding a fourth equation governing the fraction of unburnt gas, in a number of cylindrically symmetric configurations of the pipe system. The purpose is to test several duct profiles to see which one favors a higher reduction of the residual noxious gases, at the end of a car’s exhaust pipe. It is found that this purely geometric factor does play a role in the environment’s purification accomplished by the catalytic converter. This is possibly due to the longer time spent by the noxious gases resident inside the device when this has certain profiles, though at the price of a little higher temperature attained. It seems that geometric factors play a role in reducing cars’ noxious gases by means of catalytic converters. A more precise analysis should be formulated as a mathematical inverse problem.