Summary of Engine Technology Benefits.
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/Al 2 O 3 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).
It should also be mentioned that Toyota and Mazda have introduced catalysts with lean-bum
engines in Japan in their 1995 models.^55 The Toyota catalysts are apparently not true lean-NOx
catalysts, but are “NOx storage” catalysts. NOx is stored when the engine is operating lean, but
released to the catalytic material during periods of rich operation (for example, during