G420E/G424E
Tier
LP Engine
System Operational Overview
26
Catalytic Muffler
All exhaust gases pass through a catalyst that is
mounted in the catalytic muffler. It filters the harmful
gases through a dense honeycomb structure coated
with precious metals such as platinum, palladium,
and rhodium. Chemical reactions occur on these
surfaces to convert the pollutants into less harmful
gases. Catalysts store oxygen on lean mixtures (less
than optimal amount of fuel) and release oxygen on
rich mixtures (more than optimal amount of fuel). The
primary pollutant produced on the lean swing is
nitrous oxide. Oxygen is removed from nitrous oxide
by the converter, resulting in nitrogen gas, a harmless
emission. On the rich cycle, the primary pollutant is
carbon monoxide. By adding the oxygen that was
stored on the lean cycle to the carbon monoxide,
carbon dioxide is produced.
Inside the catalytic muffler is a three-way catalyst as
well as sound dampening and spark arresting
features. The three-way catalyst section consists of a
honeycomb coated with a mixture of platinum,
palladium and rhodium.
As engine exhaust gases
flow through the converter passageways, they contact
the coated surface, which initiate the catalytic process.
The
reduction catalyst
is the first stage of the
catalytic converter.
It uses platinum and rhodium to
help reduce the NOx emissions. The
oxidation
catalyst
is the second stage of the catalytic converter.
It reduces the unburned hydrocarbons and carbon
monoxide by burning (oxidizing) them over a platinum
and palladium catalyst. Cerium is also used to
promote oxygen storage and improve oxidation
efficiency.
As exhaust and catalyst temperatures rise the
following reaction occurs:
z
Oxides of nitrogen (NOx) are reduced into simple
nitrogen (N2) and carbon dioxide (CO2).
z
Hydrocarbons (HC) and carbon monoxide (CO)
are oxidized to create water (H2O) and carbon
dioxide (CO2).
The third stage is the MI-04 control system that
monitors the exhaust stream, and uses this
information to control the air/fuel mixture. By using
the signal from the HEGO the SECM can increase or
decrease the amount of oxygen in the exhaust by
modulating the FTV and adjusting the air/fuel ratio.
This control scheme allows the SECM to make sure
that the engine is running at close to the
stoichiometric point, and also to make sure that there
is enough oxygen in the exhaust to allow the
oxidization catalyst to burn the unburned
hydrocarbons and CO.
Reduction:
NOx, CO > N2 + CO2
Oxidation:
HC, CO, O2 > H2O + CO2
3-Way Catalytic
Muffler
Oxidation and Reduction Process
Figure 25
Catalyst
Efficiency
Air Fuel Ratio
Richer
Leaner
Operational
Range of
Converter
