Catalytic Converter: Description and Operation
Catalytic Converter:
CATALYTIC CONVERTER
From the outside it resembles an ordinary exhaust muffler, but the unassuming exterior encases an active inner life. The catalytic converter is the central element in the emissions-control system. The first catalytic converters were "unregulated" units, but virtually all of the converters used in the automotive industry today are installed in systems with oxygen sensors and closed-loop (lamda) control of catalytic operation.
According to the scientific definition, a catalyst is a material that promotes or accelerates a chemical reaction; in some cases the catalyst is required before the reaction can take place at all. The actual catalytic material - in our case this is a noble metal such as platinum, rhodium or palladium - does not participate in the chemical reaction, so it remains intact.
Catalyst Surface Layer:
To ensure efficient converter operation, it is important that the catalytic agent be distributed across the largest Possible surface area. The noble metal is thus vaporized onto a ceramic or metal substrate featuring innumerable lateral channels. To further increase the surface area, a supplementary "wash coat" is also applied to the substrate. The resulting, large-area surface for active catalytic reactions is an essential precondition for high-efficiency emissions reductions.
Lamda Window:
CATALYTIC CONTROL OF EXHAUST EMISSIONS:
The catalytic converter hosts two actually contradictory chemical reactions: Carbon monoxide and hydrocarbons oxidize to form carbon dioxide and water, while nitrous oxides are reduced to nitrogen and oxygen. The reduction process is promoted by a low-oxygen environment, but oxidation is more efficient when large amounts of oxygen are present. It is Possible to regulate the ratio of oxygen in the exhaust-gas mixture to obtain an optimized environment (lamda = 0.99...1) for both reactions within the converter. This range is referred to as the lamda window. The data required for this closed-loop control process are provided by the oxygen sensors.
WHAT DOES OBD II ACTUALLY MONITOR?:
A catalytic converter which has been damaged by aging or exhibits other defects will suffer from a lower oxygen-storage capacity, leading to reduced conversion capability. If the applicable limits for exhaust-gas hydrocarbons are exceeded by a factor of 1.5 in official emissions testing, this condition must be recognized on-line.
DIAGNOSIS OF CATALYTIC CONVERSION:
The Engine Control Module (ECM) performs this diagnosis procedure by comparing the respective voltages of the sensors installed upstream and downstream from the catalytic converter. The result is referred to as the amplitude ratio between the two sensors. The engine-management system responds to any deviations in this ratio extending beyond the specified range by recognizing a malfunction in the catalytic converter. The malfunction conditions are satisfied, and the corresponding DTC code is stored in the DTC memory. The MIL alerts the driver to the presence of a malfunction.
THREATS TO THE CATALYTIC CONVERTER:
Due to the extreme temperatures it encounters, over the course of time the catalytic converter is subject to an aging process affecting its conversion response. In addition to this thermal aging, chemical contamination can also exercise an adverse effect on the converter's conversion characteristics (chemical aging). Other types of damage to the catalytic converter are also possible. One example would be damage to the catalyst's active surface stemming from excess converter temperatures due to ignition miss.
Emissions Limits - Gasoline Engines:
EMISSION LIMITS FOR GASOLINE VEHICLES:
The example is based on the limits for passenger vehicles designed to transport a maximum of 12 persons as applicable in the State of California from 1995 model year onward.