Ohrnberger, T., Becker, C., and Doehring, C., "Assessment of Tier 4 Final Aftertreatment Strategies," SAE Technical Paper 2012-01-1953, 2012, doi:10.4271/2012-01-1953.
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 costs.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 design.This paper will discuss pros and cons of various non-road aftertreatment strategies.