Assessment of Tier 4 Final Aftertreatment Strategies 2012-01-1953
Non-road machineries cover a large variety of platforms
requiring aftertreatment strategies which are different from those
utilized in on-road platforms. The right choice of an
aftertreatment strategy is complex due to the consideration of
engine power rating, duty cycle, durability and regulatory
requirements as well as fuel economy concerns and total operating
Some powertrain systems utilize merely a DOC-SCR aftertreatment
system, with or without EGR, depending upon engine-out versus
targeted tailpipe NOx emission. This strategy
necessitates integrating a urea (DEF/AdBlue®) injection system.
When combined with moderate EGR, the requirements on NOx
conversion efficiency can be reduced below 90%. Without EGR, a
conversion efficiency about 95% is required.
Some other platforms intend to use a DOC-DPF system, along with
a "high PM" approach on the NOx/PM trade-off.
While this strategy avoids utilizing a urea dosing system, it also
adversely influences the engine fuel economy and typically
prohibits NO₂-assisted soot oxidation in the DPF due to low
engine-out NOx. Hence this strategy requires an active
DPF regeneration (using in-cylinder fuel post injection, in-situ
fuel dosing in exhaust gas, burner technology or alike), it further
adversely impacts the fuel economy.
An aftertreatment strategy, somewhere "in-between" the
two preceding ones, utilizes a complete system setup which consists
of DOC, DPF and SCR. This offers the opportunity for greater
flexibility in reducing engine emissions across a large variety of
duty cycles and also for use in unique environments (such as in
tunnel construction requiring PM emission at a near-zero level).
These systems are particularly favorable if a passive DPF
regeneration strategy is possible. For this purpose, certain
platforms such as tractors and crawlers have suitable exhaust gas
temperature, along with sufficient NO₂ from the DOC.
Costs for these in many cases larger aftertreatment systems are
kept as low as possible by the use of a modular approach to system
This paper will discuss pros and cons of various non-road