The substrate of a catalytic converter fulfils a vital role in supporting the catalytic material, and at the same time must be capable of withstanding some extremely arduous conditions. Operating temperatures may be in excess of 900 C and the substrate will also be exposed to fast moving, corrosive exhaust gases, rapid changes in temperature and pressure and external factors such as shocks and vibration. Another important attribute of the substrate, apart from durability, is that it must not cause an excessive pressure drop in the exhaust system. Two types of substrate predominate: ceramic extrusions and another type made from ultra-thin metal foil.
Ceramic substrates dominate the car market, primarily because they are mass-produced and therefore cheap. However, metal substrates offer the following advantages:
- As the metal foil is typically only 0.05mm thick, compared with a typical wall thickness of 0.16mm in a ceramic substrate, the pressure drop in the exhaust system is reduced.
- Metal has a lower specific heat capacity than ceramic materials, which allows catalytic converters based on metal substrates to reach their required operating temperature more quickly after a cold start.
- Metal substrates are less brittle than the ceramic alternative, making them less likely to suffer damage as a result of shocks and vibration
- The thermal conductivity of metal is greater than that of ceramics and this fact assists in maintaining an even temperature throughout the substrate and preventing hot-spots which can cause irreversible damage.
- A good quality metal substrate incorporates an outer band of stainless steel which is brazed to the core of the substrate and is intended to be welded into the exhaust system. By contrast, ceramic substrates (and some cheaper metal substrates) are retained in a metal casing using an expandable fibre mat. This ‘canning’ stage introduces a potential durability problem which does not apply with a good quality metal substrate.
(Photo: Metal Substrates)