13 March 2008

At the Conexpo show in Las Vegas, MTU Detroit Diesel displayed its Series 900 engine that complied with the nonroad interim Tier 4 (Tier 4i) emission standards using selective catalytic reduction (SCR) technology, without a diesel particulate filter. For engines above 130 kW, the US EPA Tier 4i standards (NOx = 2 g/kWh and PM = 0.02 g/kWh) become effective on January 1, 2011.

Series 900 and Series 500 engine families were the ideal candidates for the use of SCR to meet the Tier 4 interim emission regulations, said MTU. The Tier 4i Series 900 and Series 500 engines will be calibrated for low PM emissions, to meet the PM emission standard without the need for a particulate filter. The increased NOx emissions resulting from the low-PM calibration will be controlled using urea-SCR technology.

An SCR system works by injecting urea into the exhaust stream, where it decomposes to ammonia, which in turn reacts with nitrogen oxides over the SCR catalysts to produce harmless products. Urea consumption varies with the engine load and other factors; it is not expected to exceed 5% of fuel consumption, according to MTU. Since the fuel consumption is reduced by a similar amount compared to MTU Tier 3 engines, the total consumption of fuel + urea solution will remain the same.

The SCR system consists of engine-mounted hardware, urea lines, electrical wiring and a catalyst unit somewhat larger than a muffler. The catalyst unit will replace the standard muffler.

Urea will likely be required for highway diesel engines to meet the US 2010 emission standards, so the delivery infrastructure is being built out rapidly, said MTU. Urea is non-toxic and requires no special handling. Service trucks will refill nonroad equipment with urea at the same time as they refuel that equipment.

The strategy of using SCR with low-PM engine calibration, rather than a particulate filter, was first used on Euro IV/V highway truck engines in early 2005. Just as it happened in the highway engine market—with selective catalytic reduction supported by some manufacturers (Daimler, Volvo, DDC) and avoided by others (MAN, Scania, Cummins, International)—nonroad engine manufacturers are becoming divided on the SCR technology. John Deere has announced that its Tier 4i engines will not use SCR, but will be fitted with diesel particulate filters. However, Deere engines may still need to use urea-SCR to meet the final Tier 4 limits (NOx = 0.4 g/kWh and PM = 0.02 g/kWh) in 2014. It also remains to be seen if the final Tier 4 standards can be met using the SCR technology alone, without a particulate filter.

Inline 4- and 6-cylinder MTU Series 900 engines have ratings from 90 to 240 kW (120 - 322 hp), while V-configuration 6- and 8-cylinder Series 500 engines cover ratings from 260 to 480 kW (349 - 644 hp). More than a half-million Series 900 engines are in service today. The same SCR technology will also be used in the related MTU 460 inline 6-cylinder engine, which covers a power range from 260 to 360 kW (348 - 483 hp).

Source: MTU Detroit Diesel