impossible to keep carbon monoxide and hydrocarbon emissions at acceptable levels by
controlling them in the cylinder without shortening engine life considerably. The most
practical method of controlling these emissions is outside the engine using a catalytic
converter. The catalytic converter is similar in appearance to the muffler and is
positioned in the exhaust system between the engine and muffler. As the engine exhaust
passes through the converter, carbon monoxide and hydrocarbons are oxided (combined with
oxygen), changing them into carbon dioxide and water.
converter contains a material (usually platinum or palladium) that acts as a catalyst. The
catalyst is something that causes a reaction between two substances without actually
getting involved. In the case of the catalytic converter, oxygen is joined chemically with
carbon monoxide and hydrocarbons in the presence of its catalyst. Because platinum and
palladium are both very precious metals and the catalyst must have a tremendous amount of
surface area in order to work properly, it has been found that the following internal
structures work best for catalytic converters:
type (fig. 4-50)
is filled with aluminum oxide pellets that have a very thin coating of catalytic material.
Aluminum oxide has a rough outer surface, giving each pellet a tremendous amount of
surface area. The converter contains baffles to ensure maximum exposure of the exhaust to
uses a one-piece ceramic structure in a honeycomb style form. The structure is coated
thinly with a catalytic material. The honeycomb shape has a tremendous surface area to
ensure maximum exposure of exhaust gases to the catalyst.
amount of oxygen must be present in the exhaust system for the catalytic converter to
operate; therefore, a supporting system, such as an air injection system, usually is
placed on catalytic converter equipped engines to dilute the exhaust stream with fresh