Estimates of engine technology benefits are given in table 3-5, assuming that a lean-NOx catalyst is available for lean-bum and DISC engines. The mean for all manufacturers over the long term suggests that use of a DISC engine coupled with available friction reduction technologies can yield a 17 to 18 percent fuel consumption reduction, while an optimistic view suggests that as much as 25 percent may be available.
These reductions can be achieved with no tradeoff in performance although cost and complexity will increase. Lean-NOx Catalysts The potential for conventional lean-bum and DISC engines to meet future emissions standards is critically dependent on lean-NOx catalysts. Traditional three-way catalysts do not reduce Nox in the lean air-fuel ratio region, since the reduction reaction does not take place in the presence of oxygen.
The new zeolite catalysts being developed have shown the ability to reduce NOX in lean exhaust, providing some hydrocarbon is present. First generation zeolite catalysts, however, had very poor durability. New zeolite catalysts have shown NOX conversion rates of over 60 percent at 500° C in laboratoy tests, but this rate falls to 40 percent or less at higher temperatures of 700° C–temperatures characteristic of high load conditions.
Relatively new zeolite catalysts have been tested in cars and provided NOX conversion efficiency of close to 60 percent, while maintaining HC conversion efficiencies over 90 percent. If such conversion efficiencies are maintained over the useful life of a vehicle, it makes lean-bum engines viable even at California low emission vehicle (LEV) and ultralow emission vehicle (ULEV) standards.
However, the catalysts available thus far are very bulk.54 The pace of development in lean NOX catalysts has been remarkable. Several manufacturers are working with nonzeolite catalysts that have been more resistant to thermal degradation and have displayed high NOX conversion efficiencies. At least two manufacturers stated that they were optimistic that lean-NOx catalysts could be ready for production by 2005.
Considerable research into catalysts is continuing at all major manufacturers; Japan is finding these developments at national laboratories, and materials such as Ag/Al2O3 have shown NOX conversion efficiencies as high 90 percent in the laboratory.
Hence, both the conversion efficiency and the thermal durability of such catalysts could be equivalent to current three-way catalysts by 2005 (current three-way catalysts maintain NOX conversion efficiencies of more than 70 percent throughout a useful life of 100,000 miles).