Daimler: Fueling the debate over SCR for diesel trucks

Implications: EPA regulations in 2010 will require significant reductions in NOx emissions from diesel trucks.  SCR catalysts are the likely solution for most OEMs but Cummins and International have announced plans to proceed without SCR.   Significant fuel cost and performance tradeoffs are being made in the process.

Analysis: Eliminating NOx emissions from diesel exhaust is a major technical challenge since the lean air-fuel ratios of diesels make conventional catalysts useless.  EPA regulations will require major reductions in NOx emissions from diesel trucks in 2010.  Selective Catalytic Reduction [SCR] has been proven effective for NOx over years of industrial use in smokestack applications and is a leading candidate for diesel trucks in 2010.  Drawbacks to SCR are the need for injection of Urea into the exhaust and the lack of effective sensors to provide feedback control through the engine's ECU.  Alternatives to SCR are limited:  Exhaust Gas Recirculation [EGR] is effective but incurs significant fuel economy penalties, NOx adsorbers are being used in some light duty engines but have not been proven for Heavy Duty applications.  The NOx adsorbers in use also rely on expensive Precious Metals; Platinum is near $2000 per TOZ and Rhodium at $9000.

Cummins and International have stated that they will use aggressive EGR rates to control engine out NOx and eliminate SCR [Cummins will rely in part on credits earned through sales of the Dodge Ram pickup with a NOx adsorber catalyst since 2007.]  EGR works by diluting the air charge in the cylinder which in turn reduces peak temperatures but also reduces the efficiency of the engine.  According to SCR proponent Daimler the EGR heavy engines will suffer a 3% fuel consumption penalty vs. SCR [while Urea should only add pennies per gallon.] 

SCR systems are large, heavy, and expensive but they allow engines to be tuned for higher performance and fuel economy.  With diesel at record prices per gallon the fuel advantage may ultimately trump the Urea issue.  Once Urea infrastructure is established and fleet users have confidence in reliability and durability of SCR systems they may see the fuel penalty of EGR as an unsustainable cost.