[Diy_efi] What is stoichometric....really?

Phil Lamovie phil at injec.com
Fri Oct 18 16:36:30 GMT 2002



Thom wrote...

> But... isn't there a problem with increased NOx levels at higher
> combustion temps?

Yes there is.

At Lambda 1.1 NOX is at it's maximum as the temperatures are
also at a maximum. But then so is the efficiency.

The real problem lies in the legislation adopted for testing vehicles
and their emission compliance.

The std drive cycle test such as the NY27 or the Australian ADR27
set down a max. speed of 92 km/hr.

Up to that speed all run closed loop so as to minimize emissions.

Over that speed the ECU knows that it's not in a test so it's free
to implement lean burn and save the fuel.

The Catalytic converter can not be damaged by these exhaust temps.
Lean cruise exhaust temps are in the 350 to 400 deg C range.
The std 3 way cat can handle temperatures of 1100 degrees C.

Most of the high temp damage occurs during oxidization, this is
simply combustion that takes place within the cat's substrate and
thus releases a lot of heat.

These instances of damage occur when a condition of rich offset
occurs in the  fueling regime. A rich offset is a condition whereby
the ECU cycles around Lambda 0.9 something instead of
Lamda 1.0.  Melt down of the ceramic will often follow.

All it takes is a dirty connection with a 0.05 volt drop to achieve
this offset. If combined with a moderate load such as a small
caravan and sustained 85 to 90 km/hr cruise it won't be long
before there is a catastrophic failure.

Very small Japanese cars with 600 cc engines are also susceptible
to this type of failure as they are at half throttle when they cruise.

As there is no excess fuel during lean burn there is no reduction of
NOx with out resorting to a storage type cat. These cats store the
NOX and release it again during suburban driving or period of reduced
speed where the reduction can again be achieved with very little loss
of economy.

Phil



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