Close coupled catalyst vs underfloor

[azmio3]

Hey guys,

I am wondering on whether anyone here has any ballpark figure in terms of HC+CO emissions or catalyst lightoff difference between close coupled catalyst and underfloor one?

 

[Lou Scannon]

Your best bet is probably to search SAE technical papers. This goes back around 20 years, so the most useful information may be from the 90's.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[BrianPetersen]

Modern engines normally use close-coupled catalysts. The under-floor catalyst is, for the most part, old-skool.

The modern engines emit an order of magnitude less HC/CO/NOx than what they did prior, but for sure, not all of this is from simply relocating the catalyst. I don't know of an engine family that came with both arrangements in which the repositioning of the catalyst didn't also come along with a whole bunch of other changes that make direct comparisons futile.

Powertrain engineers are applying every trick in the book to reduce emissions, particularly cold-start emissions, and bringing the catalyst close to the engine is one of them.

Mazda's Skyactiv system is a notable modern-day exception in which they had to move the catalyst out in order to accommodate a long-tube header design. But that engine uses a lot of other emission-control trickery.

Perhaps it will help to know why you are asking the question.

 

[azmio3]

Hi Brian,

Nowadays, the use of turbocharger will push away the catalytic converter from the exhaust port similar to the old days naturally aspirated engines. In my view, I am questioning on how much works have been done in the past to make underfloor catalyst to still achieve the Federal -7C NMOG. Furthermore, the amount of fuel enrichment at WOT to protect the closed coupled cat is just ridiculous, I dont recall having this much of fuel enrichment in the old days.

 

[BrianPetersen]

The current trend is to build the exhaust manifold integral to the cylinder head, which allows coolant passages around it, to control the peak thermal load situations at least to some extent.

As for the fuel enrichment ... the official EPA test doesn't require full throttle operation, so it never happens during the EPA test procedure, so it doesn't affect the EPA numbers!

Maybe that's why various production cars with turbo gasoline engines aren't living up to their EPA estimates ...

 

[azmio3]

The integrated manifold does not come free as well. The heat that should be rejected to the exhaust tailpipe will now have to be cooled by the coolant and rejected to a bigger coolant pump and radiator. There are also patent royalties that automakers have to pay.

This goes back to my earlier question, have we exhausted the solutions to make underfloor catalyst to work for the current legislations?

 

[MikeHalloran]

Nobody is worrying about the current regulations.

They must be going out of their minds trying to deal with the next generation of legislative scientific extrapolation.


Mike Halloran
Pembroke Pines, FL, USA

 

[azmio3]

HC and CO limits for EURO 5 and 6 are the same thus the aftertreatment solution now for HC and CO will still be valid few months from now. However, that is still not my question, I am more interested on whether there is still potential of using underfloor nowadays?

 

[MikeHalloran]

If I had to guess, I'd expect the catalyst to move in the other direction, i.e. into the combustion chamber or the exhaust manifold.


Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

I.e., if you just bought an underfloor converter factory, your investment is at risk.


Mike Halloran
Pembroke Pines, FL, USA

 

[azmio3]

Mike,

There are so many turbo engines and the catalyst is placed right after the turbine. This is where I am wondering on whether underfloor can still be used for naturally aspirated

 

[GregLocock]

Not by preference because of the first bag emissions.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[tbuelna]

Depends on the configuration of the engine. A V8 engine would need to locate the catalyst downstream of where the two exhaust systems merge. For the NOx function of the converter to be effective, the catalyst bed needs to be at a certain minimum temperature. The further away the catalyst bed is from the exhaust ports, the less effective it performs with NOx reduction. For HC and CO reduction, the catalyst temperatures required are not as high.

 

[GregLocock]

I think on V engines they use one cat per bank

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[SomptingGuy]

Whatever happened to SAAB's bag idea for storing startup emissions? Nice bit of lateral thinking, even if it didn't get out of the lab. Or maybe it did?

- Steve

 

[BrianPetersen]

A lot of production V-configuration engines have a separate close-coupled catalyst per bank complete with its own oxygen sensors and its own separate air/fuel ratio control logic for each bank. Minor poking around on Google Images reveals that the GM LS7 is like that, and the Chrysler Pentastar is like that.

 

[Lou Scannon]

1 Cat per bank goes back at least to the early-mid 90s, GM LT1 being a case in point.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[azmio3]

There are supporting technologies like electrically heated catalyst and secondary air, have we tried all these?

 

[BrianPetersen]

Lots of cars have secondary air injection; mine does.

NOT having to electrically heat a catalyst is a good thing. Less to go wrong. If it were a choice between a close-coupled but not electrically heated catalyst, and an under-floor but heated catalyst, I would take the close-coupled catalyst.

 

[azmio3]

how about combining secondary air and electric heater?