There are three ways to lower NOx emissions in diesels: The first is exhaust gas recirculation. EGR recirculates a portion of the engine’s exhaust back into the engine at a lower temperature. The cooled gases have a higher heat capacity and contain less oxygen than air, lowering combustion temperatures and reducing the formation of NOx. EGR is prevalent in today’s clean diesel engines to reduce NOx, but itâ??s not efficient enough in current form to meet 2010 emissions levels.
Navistar is the only diesel engine manufacturer that says it will use in-cylinder EGR only to reduce NOx next year, but it will be limited to large over-the-road truck applications and not HD pickups and chassis cabs.
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The second way is the use of EGR plus a special “absorber” catalyst material to soak up and break down remaining NOx molecules before they leave the tailpipe. Chrysler is the only heavy-duty pickup manufacturer in the segment to use this approach in its Dodge Ram 2500 and 3500 trucks. The current 2007-09 6.7-liter Cummins six-cylinder diesel powertrain reached 2010 NOx emissions requirements three years early; it will carryover into 2010 and beyond without change in these models while Ford and GM are expected to update their next-generation diesel engines for 2010 using the last technology, below.
The third, and newest, approach is selective catalytic reduction using DEF. The urea-based solution (32.5 percent industrial urea and 67.5 percent deionized water) is held in a separate storage tank and injected as a fine mist into the hot exhaust gases. The heat turns the urea into ammonia that – when combined with a special catalytic converter – breaks down the NOx into harmless nitrogen gas and water vapor.
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Like Ford and GM, Chrysler will use diesel exhaust fluid to scrub NOx from the exhaust but only in its new 2 Dodge Ram 3500, 4500 and 5500 commercial Chassis Cabs.
You might be wondering why Chrysler is using a NOx adsorber on its HD pickups and urea SCR on its Chassis Cabs. It’s because the NOx adsorber depends on rare earth metals. Until recently, the prices of these metals had been sky high. DEF is much cheaper than rhodium or palladium. The drawback against urea though is it requires periodic maintenance and driver action.
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