Three-Way Catalysts (TWC)
Three-Way Catalysts (TWC) are the main technology used to control emissions from petrol engines. The catalyst uses a ceramic or metallic substrate with an active coating incorporating alumina, ceria and other oxides and combinations of the precious metals - platinum, palladium and rhodium. Three-way catalysts operate in a closed-loop system including a lambda or oxygen sensor to regulate the air:fuel ratio on petrol engines. The catalyst can then simultaneously oxidise CO and HC to CO2 and water while reducing NOx to nitrogen.
Fast light-off catalysts allow the catalytic converter to work sooner by decreasing the exhaust temperature required for operation. Untreated exhaust emitted at the start of the legislated emissions test and on short journeys in the real world is curtailed. Changes to the thermal capacity of substrates and type and composition of the active precious metal catalyst have together resulted in big improvements.
More thermally durable catalysts with increased stability at high temperature allow the catalytic converter to be mounted closer to the engine and increase the life of the catalyst, particularly during demanding driving conditions. Precious metal catalysts with stabilised crystallites and washcoat materials that maintain high surface area at temperatures around 1000°C are needed. Improved oxygen storage components stabilise the surface area of the washcoat, maximise the air:fuel ‘window’ for three-way operation and indicate the ‘health’ of the catalyst for On-Board Diagnostics (OBD) systems.
Diesel engines and lean-burn direct injection gasoline engines offer the benefit of lower carbon dioxide emissions and reduced fuel consumption. The conventional three-way catalyst technology used on petrol engines needs a ‘richer’ environment with less oxygen in the exhaust than is available on these engines to be able to reduce NOx, so new approaches are required. Selective Catalytic Reduction, Lean De-NOx catalysts, and NOx adsorbers are technologies that can be used in these lean applications.
Three-way catalytic converter
Close-coupled catalytic converters, close to the engine for faster light-off, with underfloor catalyst and oxygen sensor.
Optimised systems incorporating these new technologies are in production. The use of catalytic converters close to the exhaust manifold (close-coupled position) reduces the time to light-off in the cold start and, therefore, the total emissions. Light-off times have been reduced from as long as one to two minutes to a few seconds. Improved substrate technology, combined with highly thermally stable catalysts and oxygen storage components, allows the close-coupled catalyst approach to meet the Euro 6 standards as well as the California Low Emission Vehicle (LEV), Ultra Low Emission Vehicle (ULEV) and Super Ultra Low Emission Vehicle (SULEV) regulations.
Electrically heated catalysts work in seconds.
Electrically heated catalyst systems
Electrically heated catalyst systems use a small catalyst ahead of the main catalyst. The substrate, onto which the catalyst is deposited, is made from metal so that, when an electric current is passed, it will heat up quickly. This brings the catalyst to its full operating temperature in a few seconds.
Oxidation catalysts are the original type of catalysts and were used from the mid-1970’s for petrol-engined cars until superseded by three-way catalysts. They look much the same as three-way catalysts and their construction and composition is similar but slightly less complex. Oxidation catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO2) and water but have little effect on nitrogen oxides (NOx). They are now rarely used on petrol cars in Europe because of the advantages of three-way catalysts, but they are still used in some parts of the world where emissions legislation is less stringent. They may also be used on some buses running on Compressed Natural gas (CNG), motorcycles and for applications such as small petrol engines for hand-held equipment such as strimmers.
Diesel Oxidation Catalysts (DOC) remain a key technology for diesel engines where the high oxygen content of the exhaust precludes the use of three-way catalysts. These Diesel Oxidation Catalysts (DOC) convert CO and HC but also decrease the mass of diesel particulate emissions by oxidising some of the hydrocarbons that are adsorbed onto the carbon particles.
Selective Catalytic Reduction (SCR)
Selective Catalytic Reduction (SCR) was originally introduced on stationary power plants and stationary engines and then large engines such as those on ships, but it is now fitted to most new heavy-duty (i.e. truck and bus) diesel engines in Europe and most new trucks in the US. Systems are also being introduced on diesel passenger cars and light-duty diesel vehicles.
The efficiency of SCR for NOx reduction also offers a benefit for fuel consumption. It allows diesel engine developers to take advantage of the trade-off between NOx, PM and fuel consumption and calibrate the engine in a lower area of fuel consumption than if they had to reduce NOx by engine measures alone. Particulate emissions are also lowered and SCR catalytic converters can be used alone or in combination with a particulate filter.
Selective Catalytic Reduction
SCR catalyst showing urea injector and mixing device
In the SCR system, ammonia is used as a selective reductant, in the presence of excess oxygen, to convert over 70% (up to 95%) of NO and NO2 to nitrogen over a special catalyst system. Different precursors of ammonia can be used; one of the most common option is a solution of urea in water (e.g. AdBlue®) carefully metered from a separate tank and sprayed into the exhaust system where it hydrolyses into ammonia ahead of the SCR catalyst.
AdBlue® is a stable, non-flammable, colourless fluid containing 32.5% urea which is not classified as hazardous to health and does not require any special handling precautions. It is made to internationally-recognised standards. Urea is used as an artificial fertiliser and is found in products such as cosmetics. The consumption of AdBlue® is typically 3-4% of fuel consumption for a Euro IV engine, 5-7% for a Euro V engine, and 3-5% for a Euro VI engine, depending on driving, load and road conditions. A truck can have an AdBlue® tank which will hold enough urea solution to last for up to 10 000 km.
A special website www.findadblue.com is available to show facilities where AdBlue® is available.
With the introduction of Real-Driving Emissions (RDE), SCR technology is often used to control NOx emissions for diesel passenger cars and light commercial vehicles. A compromise has to be found by the vehicle manufacturer between the size of the urea tank and the refill intervals. On a light-duty vehicle, typical AdBlue® consumption of early Euro 6 vehicles was about 1l/1000 km. Vehicle manufacturers expect it to rise to 2-3.5 l/1000 km when a NOx Conformity Factor of 1.5 enters force. Typical urea tank size for cars and vans is 10-17 litres. On-board systems alert the driver when it is time to fill up the urea tank.
® = registered Trademark of the Verband der Automobilindustrie
Urea dosing system