Just for a sake of somthing to do I'll waffel on about how cats work!

The process that a catalyst uses is based on 2 opposite chemical processes;

• Oxidation- where a substance combines with oxygen
• Reduction- where a substance releases oxygen.

The use of precious metals; platinum, rhodium and palladium convert the pollutants into non toxic gases.

3 way catalytic converters are the most common and fitted to all new petrol engines.

The gasses effected by the cat are CO, NOx and HC.


You get rid of CO by ADDING O2 (oxidation)
You get rid of HC by ADDING O2 (oxidation)

But of these conditions you achive by running the engine a tinyt bit lean (excess o2 in the exhaust gas).

NOx you get rid of by SUBTRACTING 02 (reduction).

To achieve this the exhuast needs to be low in O2, so you run the engine slightly rich.



This is why is you scope a lambda sensor you get a trace like this....

http://www.picotech.com/auto/wavefor..._zirconia.html