Turbine inlet temperature, air-fuel ratio and catalytic converters 2

[aorangi]

On the BorgWarner Turbo systems, which seems to provide outstanding and up-to-date info, we find this :

http://www.3k-warner.de/en/products/t_temp.asp

"The air ratio lambda at the rated output point is currently about lambda=0.75–0.85 since a portion of the fuel is used to cool the inside of the engine. If the air ratio is increased to a value between lambda=0.9-1.0, then a potential fuel savings of up to 20% can be attained. However, this leads to an increase in the exhaust temperature of up to 1050°C and places new demands on the turbocharger, among other things. "

So, I can't figure out how this fits together with the lambda=1 requirement of catalytic converters.

Help, please !
Aorangi

 

[SBBlue]

First, a definition. . . .

Lambda is the ratio of air provided to the amount of air required for stoichometric combustion. If the Lambda ratio is 1, then the air supplied will be enough to completely burn the fuel -- with no oxygen left over.

If the Lambda is 2, then there is enough air to completely burn the fuel, and one-half of the original oxygen will be left. This would be the same as an air/fuel ratio of 29:1 (assuming 14.5:1 ratio for stoichometric combustion).

With a Lambda of 0.5, only one-half of the air necessary to burn the fuel will be present. If combustion occurs, we would expect all the oxygen to be consumed, but fuel to be left over.

Any Lambda less than one will result in no oxygen being left in the exhaust -- theoretically.

The catalytic converter requirements are for a low oxygen or oxygen free environment. That allows for other oxidants -- such as nitrous oxides -- to be reduced.

So therefore, for a catalytic converter, all you need is to keep the Lambda less than one.

Hope this helps!!

 

[aorangi]

Thanks a lot, SBBlue, it definitely helps. That's what I suspected at first but my knowledge about catalytic converters is really poor and I was assured it cannot by so be a knowledgable friend.
Also page 522 of the Bosch Automotive Handbook states that with a 3-way catalytic converter "The engine must be operated whithin a very narrow range in which lambda = 1 + - 0.005 (catalytic converter window). Such precision can only be achieved with precise closed-loop control of the A/F ratio mixture (...)"
So, is that wrong or other types of catalytic converters are used with turbocharged spark ignition engines ?

Cheers,
Aorangi

 

[GregLocock]

You need a bit more detail as to why a cat has to be kept in that tight range. The chart in

http://www.autoshop101.com/forms/h56.pdf

explains all

Basically HC minimum occurs in a fairly broad around lambda of 1.1, NOx reaches a fairly peaky MAXIMUM at about 1.02, and CO falls linearly with lambda up to about 1.02 and then flat lines for all lean mixtures.

Bear in mind that emissions are not measured at full throttle, so for performance work we can do anything we like to lambda so long as it does not damage the cat or the engine.

Even naturally aspirated engines use excess fuel to cool the exhaust valve and the cat, at full throttle, in some cars.





Cheers

Greg Locock

 

[aorangi]

Thanks a lot to you also, Greg, I really appreciate.

Cheers
Aorangi

 

[Chumley]

Some emission control systems on engines that aren't so clean burning or known to be operated regularly at richer mixtures pump extra air (O2) directly into the oxidizing section of three way catalytic converters to help minimize HC and CO tailpipe output during "spirited" driving.

That allows the engine to run rich of stoichiometric and keep the EGT low but also keep the HC/CO emissions down.

Chumley

 

[EngJW]

Along with what Greg and Chumley have said, it could be that the engine is programmed to run close to 1.0 under part load, which is most of the time, but at wide open throttle it goes out of closed loop operation and into open loop where the richer mixture is used.

John Woodward