aftertreatment technologies must be reassessed from
time to time.
Also, the effects of traffic induced CO2 emissions
must be taken into account in the evolution of
emissions reduction technologies. In order to meet
the aim of decreasing anthropogenic CO2 emissions
by 25% by the year 2005 compared to 1990, which
was required by the German government during the
1st World Climate Conference in Berlin, a significant
drop in traffic CO2 emissions needs to be effected.
The properly working standard closed-loop TWC
reduces emissions of HC, CO and NOx of a spark-
ignition engine by 80-90%. The main potential to
further reduce emissions is to reduce the time needed
for the catalyst to reach light-off temperature.
Currently, about 60-80% of total HC emissions
over the New European Driving Cycle (NEDC) and
the FTP 75 cycle are emitted within the first 200 see of
the cold start phase. As a large part of inner-urban
driving is done over small distances (less than 3 km),
the reduction of cold start emissions can significantly
reduce the overall traffic induced immission burden.
Legislation in the U.S., especially California,
new + alternative
already requires much lower emission standards
than can be achieved with the conventional TWC
technology. Yet for Europe more stringent emission
regulations are also scheduled, including a more
realistic test procedure (e.g. omitting the 40 sec idle
period at the beginning of the driving cycle), and new
requirements regarding evaporative emissions and
on-board diagnostics.
The basis for the new U.S. regulations are the 1990
Clean Air Act Amendments (CAAA), with the
objective to improve air quality, particularly in large
cities where in summer high ozone levels and in
winter high CO levels are encountered. Besides tighter
tail-pipe emission standards, the CAAA require the
establishment of improved inspection-maintenance
programmes, a reformulated gasoline program,
legislation covering standards for vehicle fleet
operators and for 'clean fuels-clean vehicles', which
could lead to the introduction of alternative fuels.
Exhaust emissions standards defined in the CAAA
are listed in Table 3. The current standard is Tier I
which was phased in between 1994 and 1996, when
certification up to 100,000 miles will also be required,