Exhaust System Knowledge Base

Converter Basics

The purpose of any catalytic convertor is to reduct harmful emissions from the exhaust of a "properly tuned" combustion engine. It accomplishes this through a combination of heat and a precious metal catalysts that causes the harmful emission to either oxidize or reduct to safe elements in the exhausts flow. If the engine is out of tune and not calibrated to OEM specs, the catalytic convertor's efficiency is greatly diminished  and could lead to failure.

The precious metal catalyst is bound to an extruded ceramic honeycomb substrate. The ceramic has hundreds of flow channels that allow the exhausts gasses to come in contact with a maximum amount of surface area where the catalysts reaction takes place. The catalyst must come in direct contact with the exhaust gasses for the reaction to take place.

If the ceramic inside your converter becomes clogged or coated with carbon, lead, or oil, then the converter's efficiency is greatly reduced.

There are three basic types of automotive catalytic converters: Two-Way, Three-Way and Three-Way+Air. Each type uses a slightly different method and chemistry to reduce the harmful elements in exhaust emissions. Early model converters used a pelletized catalysts, but most modern converters are now designed with a free-flowing honeycomb ceramic catalyst. The type of converter required on a particular vehicle varies with model year, engine size and  vehicle weight. Some vehicles use more than one type of converter to meet emission reduction standards.

Pre-Converter and Main Converter

Each of the three types of converters mentioned above have a common need in order to function properly. Each needs to reach a minimum operating temperature before any emission reduction or oxidation takes place. this warm-up period immediately after a vehicle is started is when the catalytic converter is least efficient and the vehicle expels the most pollutants. Some vehicles employ a pre-converter in the exhaust system immediately after the manifold to help during this warm-up period. The pre-converter's small size and proximity to the engine allow it to heat up and start functioning in less time than the main converter. It also pre-heats the exhausts gasses and helps the main converter reach operating temperature sooner.

Two-Way Oxidation Converter

Two-Way converter, used on cars between 1975-1980, oxidizes unburned harmful hydrocarbons and carbon monoxide along with secondary oxygen into water and carbon dioxide.
HC, CO, O2 Oxidize to H2O and CO2

Three-Way Reduction Oxidation
A Three-Way converter is a triple purpose converter. It reduces nitrous oxides into nitrogen and, like the two-way conveyer, it oxidizes unburned harmful hydrocarbons and turns carbon monoxide into water and carbon dioxide.

NOx Reduces to N2 - HC, CO, O2 Oxidizes to H2O and CO2

Three-Way+Air

Reduction / Oxidation Converter

A Three-Way+Air converter performs the same function as the Three-Way converter: It oxidizes and reduces. The difference is the addition of secondary air between the two internal catalysts substrates that improves the oxidation capabilities of the converter. The secondary air is pumped into the middle of the converter between the two separate catalyst substrates. The front performs the reduction and the back ceramic performs the oxidation. It's like having two converters in one.

NOx Reduces to N2 - HC, CO, O2 Oxidizes to H2O and CO2


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