The U.S. EPA Tier 4 Interim and the EU’s Stage IIIB call for a reduction in the levels of NOx of between 15 and 50%, depending on rating. A more significant aspect of the regulations includes the reduction of particulate matter (PM) of up to 95%.
Optimization of in-cylinder combustion has been the primary way in which Perkins Engines had achieved NOx reduction. “However, with the level of NOx needed for Tier 4, we do not believe this is the only way to achieve NOx removal,” said Ben Reed, Engineering Manager for Perkins’ six-cylinder engine programs.
Others technologies do exist for managing NOx, though two that Perkins had investigated but will not be considering for Tier 4 Interim are HCCI (homogeneous-charge compression ignition) and NOx traps.
“These technologies do still hold some interest for us in the future, but at this stage we do not believe that they are robust enough for off-highway use or as yet perfect technologies,” said Reed. “This really left us with a choice between SCR [selective catalytic reduction] and the use of inert exhaust gases as a dilutant in the combustion process.”
Because NOx is produced at high temperatures within the cylinder, exhaust gas is used to cool the combustion. In Perkins’ exhaust gas recirculation (EGR) system, exhaust gas is taken from the manifold, through a cooler via a control valve, and into the inlet manifold where it is mixed with the air for cooler combustion
”We believe that this system represents something that is simply packaged on engine, that in comparison to SCR offers the lowest cost, and that delivers improved transient response,” said Reed.
In the SCR system that Perkins considered, the urea is dosed into the exhaust manifold in a fine spray and then mixed with the exhaust gas. That urea is first converted into ammonia and then the ammonia reacts with the NOx, turning it into nitrogen and water. One drawback that many cite about SCR systems is their bulk.
“SCR systems include the urea tank, the pump, a control unit, and an injector, plus the catalyst itself,” said Reed. “The systems do provide good fuel economy at engine level, but bring a disadvantage of causing the customer to source and use urea. Also, urea freezes in cold weather, and that must be taken into consideration as well during system design.”
While Perkins ultimately choose EGR for Tier 4 Interim because of the conviction that it provides the best trade-off of package, fuel consumption, and cost, Reed stresses that technology choice is not the overall differentiator; it is the implementation of that technology.
“It’s more important on how you work with the customer,” agreed Alistair Dennis, New Product Introduction (NPI) Product Marketing Manager, Perkins.
In terms of Tier 4 Final, however, everything including SCR is back on the table.
“There is also HCCI, NOx traps, and other types of combustion technologies,” said Maareten Verkiel, NPI Program Manager, Perkins. “What we're doing is making sure we ascertain what all the technologies are, try to develop them to a certain point, and understand very clearly what implications a particular technology may bring to any of our customers, what value would they provide.
“We’re on a stepping stone to Tier 4 Final.”
Interim levels will take effect in January 2011 for engines greater than 130 kW (174 hp) and January 2012 for engines from 56 to 130 kW (75 to 174 hp). In 2014 and 2015, Tier 4 Final/Stage IV will demand a further 80 to 90% reduction in NOx.