• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

How a Catalytic converter actually works!

Ecomike

NAXJA# 2091
NAXJA Member
Location
MilkyWay Galaxy
This is the best, most complete write up I have ever seen. Some great info there for troubleshooting (understanding) emission test failures and cures I think, by understanding the details of how they actually work!!!


http://www.bearriverconverters.com/data/CatOpp.pdf
 
From page 6!!!!!!!!!!!!!!!!!!!

The main purpose of the O2 sensor is to keep the oxygen level in the exhaust constantly
changing.

If the oxygen level in the exhaust stabilizes, the catalytic converter efficiency will drop
considerably. In order for a catalyst to best clean up NOx the A/F ratio must be richer than
14.7:1 however, NOx emissions from the engine are highest when the engine is lean. For the
catalyst to best clean up CO & HC the A/F ratio must be lean, but CO and most HC is created
when the engine is rich! Three way catalysts (TWC s) overcome this problem by using cerium
for oxygen storage. This oxygen storage trick will greatly increase the efficiency of the TWC if
the oxygen level is cycling slightly rich and slightly lean.
The only way that proper exhaust oxygen level and oxygen cycling can be maintained is with O2
sensor feedback. This is called closed loop. After the computer has determined the proper fuel
metering (injector on time), the O2 sensor is used for feedback to fine tune fuel metering and to
make the oxygen content in the exhaust fluctuate slightly.
 
Very good info.
 
Last edited by a moderator:
From page 6!!!!!!!!!!!!!!!!!!!

Ah. That seems to explain why the NHEGO sensor is still in use, even though the WHEGO would do a better job overall of maintaining either a stoichiometric or an ideal combustion ratio (as we've discussed, Mike, the two values really are different. "Chemically balanced" combustion isn't necessarily the most efficient combustion value, when dealing with real-world engine output...)
 
Back
Top