SCR on gasoline lean burn engine?

[H-Man]

I was thinking about how some of the newer diesel cars have SCR and urea injection to reduce NOx, why don't auto makers use the same thing on lean burn engines so that the NOx output is acceptable even in California?

[mechman600]

Because people are freaked out about anything abnormal.

GM was messing around with ultra lean burn engines that used a NOx sponge (a la VW 2.0L TDI) that required a NOx regen once in a while. Apparently ammonia, created by briefly running rich through a catalyst (you know when you smell ammonia behind a car going up a long hill?) goes into the NOx sponge and converts the NOx into happier gasses once the NOx sponge is full. No DEF (water/urea) required.

Remember that DEF costs money too, so the increased fuel mileage must offset the added cost. Diesels typically burn 3-5% DEF in relation to diesel fuel, and DEF costs about $4/gallon.

[H-Man]

Gasoline costs $3.979 a gallon where I live for regular so DEF wouldn't be too costly.

[cRiPpLe_rOoStEr]

DEF is not really so good, and the SCR is not so reliable. Basically one more thing to go wrong. Might eventually work better than EGR, however.

[mechman600]

Originally Posted by cRiPpLe_rOoStEr

DEF is not really so good, and the SCR is not so reliable. Basically one more thing to go wrong. Might eventually work better than EGR, however.

There is nothing wrong with DEF. SCR is a very effective way of scrubbing NOx - over 90% efficient. It's also quite reliable because it is very simple: catalyst, decomposition tube, injector, pump and DEF tank. Far more reliable than efforts to achieve the same NOx levels (in diesels) with EGR alone. As a matter of fact, other than a few teething problems the SCR systems are proving to be nearly trouble free.

[mwebb]

in VW
the systems that use AD Blu also use 2 EGR systems , hi and low pressure
and
there are heaters for the AD Blu / DEF

because
Ad Blu / DEF freezes like water freezes at near the same temperature

but
if you run low or run out of AD Blu / DEF the system will become angry
if you do not heed the warnings
eventually
on all OEMs that use AD blu / DEF
the engine will not start or run
until you get the system to the dealer to be repaired

[cRiPpLe_rOoStEr]

Originally Posted by mwebb

in VW
the systems that use AD Blu also use 2 EGR systems , hi and low pressure
and
there are heaters for the AD Blu / DEF

because
Ad Blu / DEF freezes like water freezes at near the same temperature

but
if you run low or run out of AD Blu / DEF the system will become angry
if you do not heed the warnings
eventually
on all OEMs that use AD blu / DEF
the engine will not start or run
until you get the system to the dealer to be repaired

Some have an extremely limited limp-home mode, very restrictive to power and torque, but others require towing to the nearest stealer.. err.. dealership

[mechman600]

DEF is a 32.5% urea/water solution that freezes at -11F. This ratio allows it to freeze and thaw without degradation.
The 2007-2012 Jetta/Golf TDI do not use DEF. They employ a NOx sponge made of copper zeolite that absorbs NOx. When it is full, the engine is run at a stoichiometric AF ratio which allows the NOx sponge to regenerate.
The VW high pressure EGR is conventional for diesels. The low pressure EGR system pulls clean (soot free) exhaust post-DPF into the turbo compressor inlet.
DEF equipped engines only require one EGR system because SCR is so effective at annihilating NOx. NOx sponge equipped engines require much higher EGR levels because NOx sponges are less effective than SCR and need more in-cylinder emission reduction help.

[cRiPpLe_rOoStEr]

Originally Posted by mechman600

NOx sponge equipped engines require much higher EGR levels because NOx sponges are less effective than SCR and need more in-cylinder emission reduction help.

This, and the subsequently frequent regen cycles for both the NOx trap and the DPF end up killing some of the MPG advantage from a Diesel engine

[mechman600]

Originally Posted by cRiPpLe_rOoStEr

Some have an extremely limited limp-home mode, very restrictive to power and torque, but others require towing to the nearest stealer.. err.. dealership

All HD diesels are limited to 5 mph in the most severe SCR driver inducement mode. The most severe driver inducement mode occurs after 20 hours after the DEF pump loses prime OR if the DEF pump loses prime and the operator cycles the keyswitch, so there is plenty of time to get more DEF. 5 mph is basically enough to get off the road and call a tow truck. All it requires for repair is to fill the DEF tank and the fault codes to be cleared. Basically, the EPA really really want the operators to keep DEF tanks from going empty.