How does a cat­alytic con­verter work? Q

Motorcycle News (UK) - - Garage -

An­swered by Paul Etheridge, Head of Busi­ness De­vel­op­ment at Ri­cardo Mo­tor­cy­cle, and Dr Franz Berndt, Head of Ther­mo­dy­namic De­vel­op­ment at Ri­cardo Mo­tor­cy­cle Cat­alytic con­vert­ers were in­tro­duced en masse on bikes in the early 2000s. This de­vice is found in­side the mo­tor­cy­cle’s ex­haust and re­duces harm­ful ex­haust emis­sions. It does this by act­ing as the cat­a­lyst for a se­ries of toxin-re­duc­ing re­ac­tions in the ex­haust pipe.

Most modern units are made of stain­less steel and coated with pre­cious met­als in­clud­ing plat­inum, rhodium and pal­la­dium to pro­vide the re­quired cat­alytic re­ac­tions.

There is a re­duc­tion of car­bon monox­ide (CO) to car­bon diox­ide, an ox­i­da­tion of un­burnt fuel (hy­dro­car­bons, HC) to car­bon diox­ide and there is also a re­duc­tion of ni­tric ox­ide to ni­tro­gen, car­bon diox­ide and oxy­gen.

In or­der for these re­ac­tions to take place, the mo­tor­cy­cle must op­er­ate within a very nar­row air-to-fuel ra­tio, which is sup­pos­edly 14.5kg of air to 1kg of fuel. If you don’t do this and you run an en­gine rich, then the cat­a­lyst ef­fi­ciency for re­duc­ing HC and CO will re­duce. If you run a bike lean, then the re­ac­tion for ni­tric ox­ide won’t take place.

To pre­vent this, a Lambda sen­sor mea­sures the amount of oxy­gen in the ex­haust. This then sends mes­sages to the bike’s com­puter to ad­just the air-fuel ra­tio ac­cord­ingly. Fi­nally, the cat­a­lyst must op­er­ate above an ex­haust tem­per­a­ture of about 300 de­grees centi­grade for the re­ac­tions to take place.

Catal­y­sers have evolved over time and the num­ber of holes in them dic­tates the num­ber of cells per square inch. In the early days, this was maybe 100 cells per square inch. The more holes they have, the more back pres­sure they create in the ex­haust. Nowa­days they have around 200-300 cells per square inch for in­creased con­ver­sion ef­fi­ciency. This higher den­sity of cells will then af­fect your bike’s per­for­mance, de­pend­ing on where the cat­a­lyst is in the ex­haust.

Pre­cious met­als help zap dan­ger­ous ex­haust emis­sions

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