WHAT IS A CAT­ALYTIC CON­VERTER?

Car Mechanics (UK) - - Catalytic Converters & Dpfs -

Put sim­ply, these are large steel cans that are fit­ted to the front ex­haust pipe. Crammed within them is a ‘brick’ that pro­vides a very large sur­face area – most com­monly, this tends to be a hon­ey­comb ce­ramic. What makes them es­pe­cially at­trac­tive to scrap metal thieves is the thin layers of rare and pre­cious met­als (typ­i­cally plat­inum, pal­la­dium and rhodium) that coat the brick. When these el­e­ments reach around 300°C (ex­perts use the term ‘light off tem­per­a­ture’), they pro­mote a chem­i­cal re­ac­tion with the spent emis­sions, while re­main­ing un­changed them­selves.

This is why cat­alytic con­vert­ers have a very long life. Two-way catal­y­sers com­bine oxy­gen with car­bon monox­ide and un­burned hy­dro­car­bons to cre­ate the less harm­ful car­bon diox­ide and water. More ad­vanced three-way cat­alytic con­vert­ers are more pop­u­lar in mod­ern petrol cars and deal with ni­trous ox­ide (NOX) emis­sions as well, by turn­ing them into the in­ert ni­tro­gen, al­though ex­haust gas re­cir­cu­la­tion (EGR) valves and cool­ers have helped to re­duce NOX even fur­ther to com­ply with ever-tight­en­ing reg­u­la­tions. These types tend to be reg­u­lated with one, or more, oxy­gen sen­sors.

Pop­u­lar pro­duc­tion diesel cars have used un­reg­u­lated cat­alytic con­vert­ers since the 1990s, the main pur­pose of which is to ox­i­dise un­burned hy­dro­car­bons and the tiny lev­els of car­bon monox­ide to car­bon diox­ide and water. Later cars, fit­ted with DPFS, are also equipped with cat­alytic con­vert­ers and, on many models, they are in­cor­po­rated within the same can­is­ter and can­not be re­moved sep­a­rately. Some cat­a­lysts are in­tended to produce ni­tro­gen diox­ide, which, de­spite be­ing a prominent air pol­lu­tant, is de­sir­able be­cause it pro­duces an exother­mic re­ac­tion within the DPF, thus rais­ing the tem­per­a­ture. How­ever, ex­haust-mounted diesel catal­y­sers are far less able to deal with NOX (con­sist­ing of NO and NO2) emis­sions, which is why some cars are fit­ted with NOX traps (which are main­te­nance-free and are con­trolled by the en­gine man­age­ment sys­tems) or Ad­blue/se­lec­tive Cat­a­lyst Re­duc­tion (SCR) tech­nol­ogy.

The Lambda Sond (oxy­gen sen­sor) was a vi­tal in­ven­tion to per­mit cor­rect func­tion­ing of the three-way cat­alytic con­ver­tor on petrol en­gines, by main­tain­ing the nar­row op­er­at­ing mix­ture of 14.3:1 air/fuel. Mod­ern cars can have sev­eral of these sen­sors...

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