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TWC-SCR AFTERTREATMENT SYSTEM EVALUATION FOR A LEAN BURN GASOLINE ENGINE OPERATING IN HCCI, SACI, AND SPARK-IGNITED COMBUSTION MODES

[+] Author and Article Information
Jordan Easter

University of Michigan Ann Arbor, MI, U.S.A.
jeeaster@umich.edu

Stanislav V. Bohac

University of Michigan Ann Arbor, MI, U.S.A.
sbohac@umich.edu

1Corresponding author.

ASME doi:10.1115/1.4039424 History: Received November 15, 2017; Revised January 15, 2018

Abstract

Low temperature and dilute Homogenous Charge Compression Ignition (HCCI) and Spark Assisted Compression Ignition (SACI) can improve fuel efficiency and reduce engine-out NOx emissions, especially during lean operation. However, under lean operation these combustion modes are unable to achieve EPA Tier 3 emissions standards without the use of lean aftertreatment. The TWC-SCR lean aftertreatment concept investigated in this work uses periodic rich operation to produce NH3 over a TWC to be stored on an SCR catalyst for NOx conversion during subsequent lean operation. Experiments were performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation and between lean and rich SI (spark ignited) combustion to evaluate NOx conversion and fuel efficiency benefits. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. With the configuration used in this study, lean/rich HCCI/SACI operation resulted in a maximum NOx conversion efficiency of only 10%, while lean/rich SI operation resulted in a maximum NOx conversion efficiency of 60%. If the low conversion efficiency of HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, calculations indicate TWC-SCR aftertreatment has the potential to provide attractive fuel efficiency benefits and near-zero tailpipe NOx. Calculated potential fuel efficiency improvement relative to stoichiometric SI is 7 to 17% for lean/rich HCCI/SACI with zero tailpipe NOx and -1 to 5% for lean/rich SI with zero tailpipe NOx emissions.

Copyright (c) 2018 by ASME
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