1st Semester
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Catalyst
A catalytic converter utilizes a given surface to effect the reaction of a specific substance. With the reduction of UHC being our primary concern in the exhaust emissions, the use of an oxidation catalyst should be employed. The most widely used oxidation catalyst is platinum, which is typically applied to a base or substrate. This base, although typically in a honeycomb in the exhaust stream, may also be applied to piston heads or exhaust port walls. The platinum when heated causes the oxidation of UHC’s to water and carbon dioxide.
Catalytic converters are commonly used in industry and are therefore widely available. Typically catalysts are provided from a manufacture based on: expected incoming exhaust components, exhaust flows, and requested exit emissions. Catalyst companies will typically accommodate a variety of geometries to fit the design.
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Thermal Reactor
The other major choice for emission control is a thermal reactor. Based on research of existing patents and SAE/industry papers it was determined that a thermal reactor works on the principal of inducing a secondary combustion in the exhaust. This secondary combustion essentially re-burns what is left of the fuel from the exhaust of the primary combustion.
Finding ample oxidizers in the existing exhaust stream, although present at an almost stoichiometric amount is an almost impossible task due to the amount of diluents in the exhaust stream. This is why the reactants don't combust in an unchanged exhaust pipe. The problem is addressed with the reactors mixing charactoristics. The inlet to the reactor induces the mixing of the different exhaust constituants. The volume and shape of the reactor then allow time for the reaction to take place. In four-stroke automobiles secondary air is also added, but this shouldn't be necissary because of the excess oxygen in two-stroke exhaust.
Reactor Volume, Inlet Geometry, Reactor Insulation, and Exhaust Temperature are the primary inducing factors to the secondary combustion.
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