Picture 1 caption: Close-coupled
annular catalytic converter developed by Continental supports nearcomplete NOx
conversion in turbocharged gasoline engines
company Continental has chosen the 37th Vienna Motor Symposium to present an
innovative emissions control solution for downsized turbocharged gasoline
engines. The introduction of Real Driving Emissions legislation will require
vehicles with this widely used engine concept to meet strict nitrogen oxide
(NOx) limits in all driving situations. This poses a new challenge by demanding
efficient NOx reduction across a very broad spectrum of engine operating
conditions, and not “just” in the current test cycles. However, the existing
close-coupled catalytic converters of today offer little margin for any further
improvements in lambda distribution. On the US front meanwhile, the LEV III
SULEV 30 standards stipulate a 70pc reduction in fleet-average nitrogen oxide
emissions by 2025.
“In order to meet RDE and
SULEV 30 legislation on NOx emissions, a 3-way catalytic converter must achieve
a conversion rate close to 100%. That is only possible, if efficient NOx
reduction is maintained uniformly in all operating situations,” says Dr. Markus
Distelhoff, Executive Vice President Fuel & Exhaust Management Business
Unit in Continental‘s Powertrain Division, adding that “The innovative ring
catalyst in combination with our LS microstructured metal foil will play an
important role in meeting this requirement.”
NOx Reduction: Leaving Nothing
catalytic converters of today already achieve a NOx reduction rate of 99pc. But
that still doesn’t go far enough, and a further efficiency hike must be
targeted. For downsized turbocharged gasoline engines, this presents two
challenges. Firstly, cylinder-to-cylinder variations in exhaust gas composition
mean that the fuel-to-air ratio (lambda value) can deviate from the ideal
value, adversely affecting NOx conversion. The aim must be to avoid such
cylinder influences on exhaust lambda, by flow mixing. However, this is
difficult in the case of a close-coupled catalytic converter, because the down
pipe to the catalytic converter is not long enough. This is why many new
vehicles now feature a second – underfloor – catalyst to convert the remainder
of the nitrogen oxide emissions, a solution which comes at the price of
additional weight and increased exhaust back pressure.
Picture 2 caption:The core of the catalytic converter is wound from
“LS” metal foil, a material developed by Continental. Longitudinal structures
(= LS) in this foil or substrate create more turbulence in the exhaust gas.
This ensures better contact between the nitrogen oxides and the catalytically coated
substrate, resulting in higher conversion efficiency.
Secondly, in some operating
situations, the catalytic converter’s ability to achieve a consistent and
uniform NOx conversion rate can be adversely affected by the turbocharger
system. This is because the opening of the turbine bypass valve (wastegate)
above a certain engine speed affects the flow distribution, resulting in
non-uniform exhaust flow. This can potentially lead to faster local ageing of
the catalytic converter and to a deterioration in NOx reduction performance.
Advantages of the Ring Catalyst
Continental has developed an
innovative solution to address both these challenges: The ring catalyst. An
internal pipe running through the core of the ring catalyst and all the way
along it provides the necessary additional length to allow better exhaust flow
mixing. At the end of this pipe, the gas is redirected through 180°. Only then
does it flow through the catalytically active portion of the catalyst, which
surrounds the inner pipe like a sleeve. “This way the annular catalyst extends
the flow path without extending the overall length of the catalyst. This allows
the catalyst to be placed close to the engine, so there is no increase in the
time required to reach light-off temperature for converting NOx emissions,”
says Rolf Brück, Head of Catalysts & Filters Product Line, Fuel &
Exhaust Management. “With this design, the swirl effect from the turbocharger
wastegate gas actually assists exhaust mixing in the internal pipe.”
The active section of the ring
catalyst is wound from Continental’s innovative LS metal substrate. The
longitudinal structures in this material generate micro-level turbulence in the
exhaust stream, which helps to direct the nitrogen oxides more effectively
towards the catalytically coated catalyst wall, where they are converted.
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