Research Papers: Internal Combustion Engines

Three Way Catalyst-Selective Catalytic Reduction Aftertreatment System Evaluation for a Lean Burn Gasoline Engine Operating in Homogenous Charge Compression Ignition, Spark-Assisted Compression Ignition, and Spark-Ignited Combustion Modes

[+] Author and Article Information
Jordan Easter

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: jeeaster@umich.edu

Stanislav V. Bohac

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: sbohac@umich.edu

1Present address: U.S. EPA, 2565 Plymouth Rd, Ann Arbor, MI 48105.

Contributed by the IC Engine Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 15, 2017; final manuscript received January 15, 2018; published online November 20, 2018. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(12), 122806 (Nov 20, 2018) (7 pages) Paper No: GTP-17-1619; doi: 10.1115/1.4039424 History: Received November 15, 2017; Revised January 15, 2018

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 Environmental Protection Agency (EPA) Tier 3 emissions standards without the use of lean aftertreatment. The three way catalyst (TWC)-SCR lean aftertreatment concept investigated in this work uses periodic-rich operation to produce NH3 over a TWC to be stored on a selective catalytic reduction (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 spark-ignited (SI) combustion to evaluate NOx conversion and fuel efficiency benefits. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system calculations are also presented to allow for comparisons of system performance for different levels of stored NH3. 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 that 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 7–17% for lean/rich HCCI/SACI with zero tailpipe NOx and −1 to 5% for lean/rich SI with zero tailpipe NOx emissions. Although the previous work indicated that the use of HCCI/SACI increases the time for NH3 to start forming over the TWC during rich operation, reduces NH3 production over the TWC per fuel amount, and increases NH3 slip over the SCR catalyst, if NOx conversion efficiency could be enhanced, improvements in fuel efficiency could be realized while meeting stringent tailpipe NOx standards.

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Grahic Jump Location
Fig. 1

NH3 generation over a TWC during rich operation and reduction of NOx over an SCR catalyst during lean operation

Grahic Jump Location
Fig. 2

The NOX conversion efficiency of the SCR catalyst during lean operation after the switch from the respective rich condition held for 80 s

Grahic Jump Location
Fig. 3

(a) Comparisons of the stored, NH3 stored, and the NOX converted for cases where rich condition was held for 80 s prior to the switch into lean operation and (b) engine-out NOx values during lean operation

Grahic Jump Location
Fig. 4

Comparisons of relative species stored and converted over the scr catalyst during lean-rich switches: (a) SI λ = 0.96 → SI λ = 1.25 and (b) SACI λ = 0.96 → HCCI λ = 1.34

Grahic Jump Location
Fig. 5

NOx conversion per unit of NH3 stored for the cases where the rich lambda was held at 0.96

Grahic Jump Location
Fig. 6

Evaluation of the fuel efficiency benefit with respect to tailpipe NOx values for the calculations performed with rich: (a) SI (λ = 0.96) → SI (λ = 1.34) and (b) SACI (λ = 0.96) → HCCI (λ = 1.38)

Grahic Jump Location
Fig. 7

Direct comparison of the SACI/HCCI and SI results for the case study when the SCR catalyst is saturated during the rich operation



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