Emissions regulations vary considerably from jurisdiction to
jurisdiction, as do what engines are regulated. In North America any
spark ignition engine of over 19 kW (25 hp) power output built later
than January 1, 2004 probably has a three-way catalytic converter
Diesel engine regulations are similarly varied, with some jurisdictions
focusing on NOx (Nitric Oxide and Nitrogen Dioxide) emissions and others
focusing on particulate (soot) emissions.
For compression ignition (i.e., Diesel) engines, the most commonly used
catalytic converter is the diesel oxidation catalyst. The catalyst uses
excess O2 (oxygen) in the exhaust gas stream to oxidize CO (Carbon
Monoxide) to CO2 (Carbon Dioxide) and HC (hydrocarbons) to H2O (water)
and CO2. These converters often reach 90% effectiveness, virtually
eliminating diesel odor and helping to reduce visible particulates
(soot), however they are incapable of reducing NOx as chemical reactions
always occur in the simplest possible way, and the existing O2 in the
exhaust gas stream would react first.
To reduce NOx on a compression ignition engine it is necessary to change
the exhaust gas - two main technologies are used for this - selective
catalytic reduction (SCR) and NOx (NOx) traps (or NOx Adsorbers).
Another issue for diesel engines is particulate (soot). This can be
controlled by a soot trap or diesel particulate filter (DPF), as
catalytic converters are unable to affect elemental carbon (however they
will remove up to 90% of the soluble organic fraction). A clogging soot
filter creates a lot of back pressure decreasing engine performance.
However, once clogged, the filter goes through a regeneration cycle
where diesel fuel is injected directly into the exhaust stream and the
soot is burned off. After the soot has been burned off the regeneration
cycle stops and injection of diesel fuel stops. This regeneration cycle
will not affect performance of the engine.
All major diesel engine manufacturers in the USA (Ford, Caterpillar,
Cummins, Volvo, MMC) starting January 1, 2007 are required to have a
catalytic converter and a soot filter inline, as per a new DoT