MR2 L4-132 2164cc 2.2L DOHC (5S-FE) (1993)
Three Way Catalytic Converter: Description and Operation
California Models
Exhaust System Layout
PURPOSE
Two three way catalytic converters simultaneously remove up to 90% of all three major pollutants, (HC, CO, and oxides of nitrogen). A complete
catalytic reaction depends on the fuel mixture staying within a very narrow range + 1% of 14.7:1 which can only be achieved with a properly
functioning oxygen sensor system. Two converters are used, a warm-up converter (WU TWC) and a main converter (TWC).
CONSTRUCTION
Each catalytic converter consists of a metal housing, a ceramic grid substrate, and a catalytic coating of platinum and rhodium. The active metal
content is about 2 grams of platinum/rhodium.
OPERATION
As the exhaust gasses containing HC and CO pass through the converter in the presence of oxygen, the platinum catalyst starts the oxidation
(burning) process. The HC and CO then unite with oxygen to form water vapor and carbon dioxide. This oxidation process has no effect on the
NOx emissions.
To reduce the oxides of nitrogen (NOx), a separate reaction is necessary called "reduction." A reduction reaction is the removal of oxygen from a
material. In Three Way Catalyst type converters, rhodium is used as a catalyst to break down oxides of nitrogen into nitrogen and oxygen. The
effective conversion of pollutants begins at about 250°C (480°F).
Due to distance from the combustion chamber, the WU TWC reaches operating temperature much faster than the main TWC. The early operation
of the WU TWC causes faster operation of the O2 sensors and the main TWC.