Mele says in the trucking industry, all but one engine manufacturer — Navistar — has chosen to use SCR technology to meet its 2010 standards. But agriculture is a different application, with different duty cycles. What’s right for on-road may not be right for off-road, offers Alan Hansen, an engineer at the University of Illinois.

Hansen says a key difference between on-road and off-road engine usage is the higher levels of average torque and speed to which off-road engines are subjected. “The so-called duty cycle for an off-road engine is higher than that for on-road because there is a higher average power demand for off-road applications such as agriculture or earthmoving,” Hansen explains. “This creates differences in combustion, exhaust gas temperature and, hence, in emissions.  Maybe higher exhaust gas temperatures from generating more power may facilitate the regeneration of diesel particulate filters making the EGR system viable, whereas for on-road it may not be so.” 

Hansen’s colleague, Xinlei Wang, who has worked extensively with after-treatment systems, says it may be too early to tell which system is better because engine makers will face even stricter standards in 2014, when Tier 4 Final regulations go into effect.

 Wang says, “Technologies to be deployed for meeting 2014 emissions regulations are still somewhat in the experimental stage and may be regarded as not being mature enough to allow definite statements to be made about the relative merits of EGR versus SCR.”