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Quick Look

Phased Air/Fuel Ratio Perturbation - A Fuel Control Technique for NOx Reduction, 08-9104

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Principal Investigator
Cynthia C. Webb

Inclusive Dates: 12/23/98 - 04/23/99

Background - Recent changes in exhaust gas emissions regulations have redirected the emphasis from hydrocarbon (THC) control to NOx (oxides of nitrogen - brown smog) control. Current approaches to improve NOx control involve tuning a vehicle’s exhaust gas recirculation (EGR) system, modifying spark timing, improving air fuel ratio (A/F) control, and increasing catalyst volume and loading. Very tight fuel control is difficult under transient engine operation. Increased EGR and spark retard affect vehicle driveability and fuel economy, and larger catalysts can be expensive.

Under normal control, the exhaust air/fuel ratios of both banks of a dual-bank engine are generally perturbated in-phase (no attempt is made to purposely create a phase shift between the banks). This method delivers a feedgas to the catalyst that is overall rich or lean, thus requiring the catalyst to store O2 (oxygen) during lean excursions for subsequent reaction during rich excursions. Because the O2 storage capacity of the catalyst is limited, the effectiveness of the catalyst during rich or lean conditions is dependent on the duration and the amplitude of the rich or lean excursions. Therefore, typical fuel-control strategies for improving catalyst performance include increasing the frequency of the A/F perturbation and decreasing the amplitude of the lean and rich excursions. This strategy basically attempts to hold the exhaust as close as possible to stoichiometry.

Approach - This quick-look research project examined an emissions control mechanism identified as "phased perturbation." The suggested mechanism of phased perturbation involves independently controlling the fuel delivered to each bank of a dual-bank engine, which allows the two banks to have an adjustable, relative A/F perturbation phase-shift from one another. Exhausts of these two banks can be combined to achieve a near-stoichiometric blend prior to entering a single underbody catalyst. This scenario creates a situation in which both rich exhaust constituents [unburned hydrocarbons (HC) and carbon monoxide (CO)], and lean exhaust constituents [oxygen (O2) and oxides of nitrogen (NOx)], arrive at the catalyst at the same time. HC and CO are oxidized by O2, and NOx is reduced by CO. Since all four chemical species would be present simultaneously, a highly reactive mixture enters the catalyst.

Accomplishments - The study identified a third dimension of A/F control for optimizing exhaust composition as it enters the catalyst -- phasing. Phased perturbation produced a significant improvement of mixed A/F control at the catalyst and catalyst THC, CO, and NOx efficiency. The magnitude of the effect of phase shift on catalyst conversion efficiency was comparable to the effect of either frequency or amplitude alone. Development of this technique could augment or replace EGR for NOx control (particularly at high catalyst space velocity conditions and during cold operation when EGR is not desirable), improve overall CO efficiency, extend catalyst NOx efficiency durability, and reduce cold-start emissions. These improvements should reduce emissions without penalizing fuel economy or power.

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The schematic demonstrates a 180° phase shift in exhaust A/F perturbation

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