Kyodo news reports that Mitsubishi Fuso Truck & Bus Corp will license Nissan Diesel’s new emissions reduction system.

Nissan Diesel introduced the Final Low Emission New Diesel System (FLENDS) technology in October 2004. FLENDS combines ultra-high pressure fuel injection for PM reduction with a urea selective catalyst reduction (SCR) system to reduce NO_x emissions. (Diagram at right.) 

The urea SCR catalyst is an approach to nitrogen oxide (NO_x) reduction that major European truck manufacturers—notably DaimlerChrysler with its “BlueTec” systems—have already decided to adopt on their vehicles. DaimlerChrysler owns 85% of Mitsubishi Fuso, so this licensing of FLENDS from Nissan Diesel is interesting.

The FLENDS system conforms to Japan’s 2005 new long-term exhaust emission standards, which mandate particulate matter (PM) emissions of no greater than 0.027 g/kWh and NO_x emissions of no more than 2.0 g/kwh. Nissan Diesel introduced the first production application of FLENDS in a heavy-duty truck last year.

Using ultra-high pressure fuel injection reduces the PM level substantially, however, there is a concomitant large increase in NO_x emissions, due to the higher temperature and more complete combustion attendant to the reduction of PM. The urea SCR catalyst then, in turn, reduces the NO_x emissions to the target level.

Urea SCR systems basically consists of a storage tank for the urea solution, a urea injection system, and a catalyst. The system injects urea into the hot exhaust gas where the urea decomposes into ammonia (NH₃). NO_x reacts with NH₃ on the surface of the catalyst to produce  nitrogen (N₂) and water vapour (H₂O).

The urea is supplied in the form of AdBlue—a standardized, 32.5% liquid urea solution. (Hence DaimlerChrysler’s BlueTec.)

One of the critical factors in a urea SCR system is the dosing.  It is important that neither too much nor too little urea is mixed in with the exhaust gas, and this requires a good sensor/injector system. An underdosage would result in failure to conform to the NO_x emission limits.

The other critical factor with such a system is making sure that the vehicle has a supply of the urea solution. That requires (a) an infrastructure to supply it and (b) drivers that fill up the tank. Not filling the urea tank doesn’t affect the ability of the truck to move; it just torpedoes any compliance with emissions standards.

Accordingly, truck makers in Europe and Japan, where this approach is gaining much momentum, are focusing on building out a reliable supply infrastructure. The trucking companies and the governments will have to handle enforcement of filling procedures.

It’s that human factor that comes up often in objections to urea SCR, especially in the US, as engineers look for other types of solutions: in-fuel reductants, different filters and gas recycling, engine-out reductions.