Quote:
Originally Posted by tjts1
The SCR makes the engine more fuel efficient compared to periodically dumping fuel into the exhaust to regenerate the DPF. Thats why the US market Passat TDI with SCR is more fuel efficient than the Jetta TDI without.
DEF can be purchesed at any truck stop or Walmart for cheap. Its a non issue.
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Not necessarily. SCR is for NOx emissions, DPF is for particualtes (soot). The Cruze has both devices. The SCR will use DEF and the DPF will still need periodic regens (burning extra fuel).
That being said, SCR can lead to fewer DPF regenerations indirectly. Engine-out NOx & particulates are often inversely related--more NOx = less soot, less NOx = more soot. Engines with SCR can afford to put out more engine-out NOx because the SCR will clean it up. As a result, typically there's less engine-out soot, and therefore the DPF won't require regens as frequently.
Although this wasn't done with the Cruze, I have seen some designs where manufactures have used SCR to eliminate the need for a DPF completely by running super-high engine-out NOx (& super-low engine-out PM). They let the SCR take care of the NOx and don't need a DPF. I know some European dieseln trucks met Euro 5 emissions this way. In Euro 6 the PM standard is a lot lower (pretty similar to what the US is now) so I'm not sure if there will be trucks without DPFs.
Now, as far as SCR increasing fuel efficiency, that can be partly because of the fewer DPF regens. Also, engine-out NOx and fuel efficiency are often directly related (more NOx = more efficiency). For that reason, engines with SCR can often run more efficiently than non-SCR engines--they're tuned primarily for efficiency (rather than low NOx) and the SCR cleans up the NOx after the fact. Look at the semi trucks with Navistar engines the last few years. All the other manufacturers went with SCR systems, while Navistar chose to avoid SCR and run low engine-out NOx by running lots of EGR. That decision has nearly bankrupted Navistar because their engine had awful efficiency and effentually they couldn't meet the emissions standards at all without SCR.
As far as the cost of DEF goes, I wouldn't call it "cheap". It's typically less than diesel, but still a couple bucks a gallon.