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 70% 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. “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 to Chance
High-performance 3-way catalytic converters of today already achieve a NOx reduction rate of 99%. 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.
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
Close-coupled annular catalytic converter developed by Continental supports near-complete NOx conversion in turbocharged gasoline engines.
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 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.
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.