Characterization of an Integrated Three-Way Catalyst/Lean NOx Trap System for Lean Burn SI Engines

Abstract

<div class="section abstract"><div class="htmlview paragraph">The push for environmental protection and sustainability has led to strict emission regulations for automotive manufacturers as evident in EURO VII and 2026 EPA requirements. The challenge lies in maintaining fuel efficiency and simultaneously reducing the carbon footprint while meeting future emission regulations. Alcohol (primarily methanol, ethanol, and butanol) and ether (dimethyl ether) fuels, owing to their comparable energy density to existing fuels, the comparative ease of handling, renewable production, and suitable emission characteristics may present an attractive drop-in replacement, fully or in part as an additive, to the gasoline/diesel fuels, without extensive modifications to the engine geometry. Additionally, lean and diluted combustion are well-researched pathways for efficiency improvement and reduction of engine-out emissions of modern engines. Modern spark ignition (SI) engines typically employ various in-cylinder emission reduction techniques along with a three-way catalyst (TWC) based exhaust after-treatment system to comply with emission standards. However, the periodic lean-rich oscillations for this TWC system necessitate the SI engine to operate at near stoichiometric mixture conditions, which limits the viability of lean burn for SI engines. Lean NOx trap (LNT) system can reduce the engine out NOx under lean conditions at a cost of fuel efficiency penalty due to regeneration. In the present study, the feasibility of using a coupled TWC-LNT system with extensive dilution to achieve ultra-low tailpipe emissions is investigated. Relevant engine-out exhaust conditions from an SI engine, including flow, temperature, and exhaust species, operating at different dilution conditions were replicated on a heated aftertreatment flow bench. A comprehensive analysis of species before and after the catalyst sections was performed using Fourier-transformed infrared (FTIR) and mass spectrometers to study and quantify the conversion and formation of species, including ammonia, methane, and hydrogen, under different engine-out conditions. The results the integration of LNT to a TWC catalyst improves the conversion efficiency of reducing species during the lean operation period. TWC and LNT catalyst simultaneously achieve high conversion efficiency at ~350°C. The LNT regeneration behavior is noticeably affected by the presence of preceding TWC catalyst. The temperature rise because of the oxidation reactions on TWC can deteriorate the LNT regeneration efficiency beyond 400°C.</div></div>