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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.
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