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Catalytic ConvertersWHAT IS A CATALYTIC CONVERTER
A Catalytic Converter (or Cat) is an Exhaust Emissions Device located underneath a vehicle. It uses a combination of heat and precious metals to promote a chemical reaction, which breaks down harmful gases, making the emissions less detrimental to the environment. HOW DOES A CAT WORK?
The operating temperature of a cat is between 350 to 400 degrees Celsius. Once the cat has reached this temperature a chemical reaction occurs as the exhaust gases flow through the coated monolith. The harmful gases are oxidised and converted as shown in the diagram opposite. COMPONENTS OF A CAT
A honeycomb monolith coated with precious metals (usually Platinum, Palladium and Rhodium) is encased in a steel can. Between the monolith and the can is matting. Upon running a cat in, this matting expands when subjected to heat. Once expanded, the matting holds the monolith in place.
Diesel Particulate Filters (DPF)
A DPF is NOT a “flow-through” device, the gasses are forced through. Unlike a catalytic converter, the channels of the filter are blocked at alternate ends, forcing the gasses to flow through the cell walls in order to exit the filter. As the cell walls are porous, the clean gasses can pass through, but the holes are not large enough to let particulate matter pass through. Instead this is deposited on the cell walls and trapped in the filter.
The Engine Management System (ECU) constantly monitors the filter and will carry out a regeneration to stop it blocking.
As a DPF is a “soot trap” it has to be able to clean itself to prevent it becoming blocked and affecting the running of the vehicle. This process is known as regeneration. There are three different types of regeneration:
Passive Regeneration occurs during normal driving conditions when the DPF becomes hot enough to burn off some of the trapped particulates naturally at 550°C.
Active Regeneration is an ECU led process. When the level of soot in the filter reaches around 45%, the ECU will make small adjustments to the fuel injection timing and increases the exhaust gas temperature. The optimum temperature required for particle combustion is 600°C. Active regeneration normally occurs around every 450 miles, but it does depend on how the vehicle is driven. Vehicles that are primarily driven on short urban journeys will regenerate more often than those primarily driven on motorways. This is due to a greater build up of particles at lower speeds. The regeneration cycle will generally be triggered by the back pressure. Failing this, mileage is used as a back up.
During active regeneration it is common to see smoke being emitted from the exhaust as the particulates are burned off. The fuel economy of the vehicle will also decrease during an active regeneration phase.
Forced Regeneration is carried out by garages with Diagnostic equipment.