What is a "rear" or "secondary" catalytic converter for?
 

Hyundai has been fitting "secondary" or "rear" catalytic converters on cars for at least a decade. Probably other automakers do too. Hyundai's rear CAT has no O2 sensors to monitor its function and is further down the exhaust line.

Here is an eBay replacement for a Santa Fe:
https://www.ebay.com/i/253610843152?chn=ps

Here is a salvage from an Equus:
https://www.ebay.com/i/2011-2012-201...664?rmvSB=true

The featured picture I attached is of a cheap aftermarket knock off replacement from autoparts direct because ebay won't left me save the photos of its cheap aftermarket parts to my computer. Hahaha.

Is this thing for NOX or something? Anyone know?

THANKS FOR CITATIONS TO RELIABLE SOURCES! :)

http://ecomodder.com/forum/attachmen...1&d=1528300069

Took an hour to do some further research and am adding some basic info.

Here comes the "engineering explained" smartypants:
https://m.youtube.com/watch?v=HADOcrcMikA

It does not really answer the thread question about secondary catalyst function, but it is a high quality explanation.

And this good article: How Catalytic Converters Work And Why Racers Should Play Clean | Speed Academy ... It also does not really answer the thread question, but it is otherwise interesting and thouhtful about the technology.

Weird. I did not have a problem.

http://ecomodder.com/forum/attachmen...1&d=1528479619

I am not sure what something that small can do, but I am intrigued that they are selling a used cat. I thought they needed to recertify them, although I have read that you can sell them as paperweights or something.

In the 5MT Insight, the rear cat is specifically for NOx, because it has a lean burn motor. I'm unsure if the CVT variant has a rear cat because it doesn't vary from stoich.

You and Ecky might be onto something with the Nox. I also wonder if it's meant for another market with standards they can't meet without it, and it just makes it here as a matter of streamlining the production process?

I would guess the first cat is kept smaller for faster lightoff and the 2nd cat to finish the job. The first cat is actually called a "pre-cat" in that case.

Yes. BMW sometimes calls it a "pre-catalyst" it seems--or at least some of its aftermarket followers are selling items they are calling a pre-cat. But some of the Hyundai literature calls the second catalyst a "secondary" unit and it has no sensors monitoring it. So there could be a variety of ways automakers are doing this.

I have not been successful finding really reliable sources via google (like from the automakers or SAE or something).

Anyone have citations from reliable sources? I am so far failing. Suggestions for databases to search?

Sometimes factory service manuals are good for info like this. The best I can do is a 1998 Chevy Metro service manual. All the '98 Metros had a three-way catalytic converter (TWC). Some (I'm assuming Cali emissions) had a warm-up three-way catalytic converter (WU-TWC).

So, I hunted again and cannot find the 2003-2005 Civic HYBRID free pdf that I trust is a safe download. I find pages and pages of shady or paid webpages, some of which I think will only offer on screen access for a short trial period. I would like to store the info on my device.

So I went to the Honda eStore site and got images of two CATs on the 2003 Civic Hybrid (attached).

2003 Civ Hybrid, first catalyst:
http://ecomodder.com/forum/attachmen...1&d=1528566516

2003 Civ Hybrid, second catalyst (#7, lower left):
http://ecomodder.com/forum/attachmen...1&d=1528566530

By contrast the site shows that the 2012 Civic HF had no second catalyst on the exhaust pipe. [EDIT: 2012 Insight & the CRZ had two CATs, with a sensor on the second one too; & 2017 Honda Fit 6-speed manual base also has a second CAT.]

That market would be, I think, California. And with it so simple, just a device without electronics, they could install it for Cali at minimal cost with simply exhaust production changes. Suggests we could use one and get some emissions benefit.

It could be California. I know most automakers have done away with California-specific cars and just put CARB-approved stuff on for all USDM cars, at least for the most part. My understanding is that from an operations point of view it ultimately streamlines the production thereby lowering costs. I am not really up on what other markets have in terms of emissions standards, though. Anyhow, that's all pure, unadulterated speculation as to the purpose of two cats.