What's wrong with reversing cats and turbo?

[Torquey]

Dear Gentlemen,
I rarely post to this forum but read what you fellows post... all the time. There is no other forum that quite compares to this one. And Pat Primer, you're the shizzle. Here is a question that's had me thinking for the past year...

If a turbocharger depends on high pressure gas, why wouldn't it make sense to put a catalytic converter before the turbo? Wouldn't a pre-turbo cat make the turbo run harder because of (a) combustion of the unburnt gas (more gas volume) and (b) the resultant higher temperatures of the exhaust? The only draw back might be temperatures too excessive for the turbine bearings but couldn't that be solved by adding a little distance between the cat and the turbo? In particular, I was thinking of running individual mini-cats on each exhaust tube (before a 4-in-1 collector). Any insights? Thanks.

Walter Carter

 

[BrianPetersen]

Turbochargers usually depend rather strongly on the energy in the individual exhaust pulsations - this is why on turbo engines, generally the exhaust manifold is made as small volume as possible. Putting the catalyst before the turbo goes against that. BUT, by no means is it impossible. There is a company that builds aftermarket turbo kits which take the place of the muffler on vehicles where it's hard to find enough under-hood room for a turbo. Obviously that puts the turbo after all of the emission control equipment. Not saying it's the most efficient turbo design out there ... but it can be done.

 

[patprimmer]

The increased volume will cause lag. The heat from the cat can help when it is in fact making significant amounts of heat, but I am not sure if their is enough excess fuel to use the cat in this way under most circumstances.

It might also adversely effect the life of the expensive cat.

Although heat might be picked up in the cat, some will also be lost in the extra length of pipe and there will be extra resistance to exhaust flow imposed by the cat.

I think keeping the exhaust as short and au non-restrictive as possible is the best approach.

Regards

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[TurboXS]

I would suspect that the heat loss due to the extra length would negate any gains from the cat plus the extra length would delay the response. I also suspect that the heat would reduce the life of the cat.

In any case the earlier Subaru MY01 WRX had a small cat prior to the turbo mainly for emissions reasons. One of the modifications for this car was to replace the pre-turbo cat with a straight pipe which gave more power and faster spool up.

regards
Pete

 

[GregLocock]

I really doubt that the gas flow after a cat is more energetic than the gas flow at the flange on the exhaust manifold. Admittedly in some cases people dump a lot of unburnt fuel down the exhaust... but is there any oxygen left to burn it with?

Hey Pat- you da shizzle.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Yes Greg, I wondered about that. I have been called lots of things, but never a shizzle before



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

pat- thats a sign of respect.

Yep on the ('02 in the US) WRX removing the preturbo cat provides full boost ~200-500 rpm sooner. Also if EGT's get too high, or just wear/tear, causing the cat to breakdown -- well what do you think an exhaust turbine will look like after processing solids?

 

[j2bprometheus]

Torquey -

Some years back, Merecedes made some engines with the catalyst before the turbo. The catalyst had a metal substrate that degraded, resulting in metal passing through the turbine.

The turbine didn't like metal mixed with the exhaust gases and Mercedes ended up with big warranty claims.


j2bprometheus

 

[Torquey]

Thanks to all who put this misguided idea to rest for me.