Achieving 90% reduction in NOx emissions from heavy-duty vehicles

By Ameya Joshi·May 20, 2021·3 min read

Achieving 90% reduction in NOx emissions from heavy-duty vehicles

California's Low NOx Omnibus will be phased in through model years 2024 - 2031 and will require >90% reductions in tailpipe NOx emissions from heavy-duty trucks. This is a high-level summary of a multi-year program led by Southwest Research Institute to demonstrate the feasibility of achieving these targets.

Background

Starting back in 2014, California's Air Resources Board (CARB) initiated a program to evaluate the feasibility of reducing tailpipe NOx emissions from heavy-duty vehicles by 90%. Southwest Research Institute (SWRI) has led this comprehensive study in three stages, through implementation of advanced engine and after-treatment solutions. A key objective of the study was to demonstrate meeting the upcoming NOx standards without incurring a significant fuel or CO2 penalty.

For a quick overview of California's low NOx Omnibus rule starting 2024, the reader is referred to this link.

Summary

A combination of modified engine calibration, implementation of new hardware and an advanced after-treatment system with close-coupled catalyst was implemented. After accelerated aging representative of 435,000 miles, the final tailpipe levels were at 0.02 g/bhp-h on the FTP cycle, right at the Low NOx Omnibus target. Even on the challenging low load cycle, NOx emissions are at only 0.029 g/bhp-h. What is remarkable is that CO2 emissions were practically unchanged except for the low load cycle, where the CO2 emissions increased by ~ 1%.

Key Enabling Technologies

Advanced engine calibration and hardware

The baseline engine was a Cummins 15L in-line 6 cylinder engine. The open ECU allowed the team to make several modifications to the engine calibration, such as elevating the idling speed for rapid thermal warm-up. Still, new hardware had to be added to further elevate the exhaust gas temperature (for early SCR activation) - without imposing a fuel penalty. Some of these are shown in the figure and will be covered in detail in other articles. A couple of technologies are highlighted:

Cylinder deactivation (supplied by Eaton) reduces fuel consumption while also raising exhaust temperature through independently deactivating any of the cylinders under reduced load.
SuperTurbo is a driven turbocharger which can mechanically transfer power to and from the turbo shaft enabling compounding of excess turbine energy. For more information on SuperTurbo, read here.

Light-off SCR, model-based controls and aging

An advanced after-treatment was implemented including a "light-off" SCR in the close-coupled position for early heat-up of the catalyst. This and the downstream SCR had separate urea dosing. While a catalyzed soot filter with a zoned DOC coating was implemented and showed promise, a recent rework has also examined the use of a separate DOC + DPF system with even better results.
A critical part of improved after-treatment operation was the implementation of model-based control of the diesel exhaust fluid (DEF) dosing and the associated estimation of ammonia coverage for each SCR unit.
Aging to 435,000 miles is not trivial. This was done using an accelerated protocol - still requiring 1000 hours of engine operation - to capture both hydrothermal and chemical aging.