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It is not instead, it is in addition too. The DPF captures soot, this needs extremely high temperature to burn the soot off, which still requires a lot of fuel. Urea injection reacts in a special catalyst to reduce NOx emissions, and thats it.
Current engines can get by with a NOx absorber and lots of EGR. EGR really hurts combustion effeciency on Diesels. So the urea injection will give you a slight mileage gain in the fact that you can use less EGR. But it will still never be as good as the older pre 07 egines because you still need to burn off the soot in the DPF.
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- Ian
1987 Ford Ranger XLT 4x4 2.9
Needs a lot of work, including some rot on the body, but shes getting there.
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