Effect of Hydrogen-Enriched Gas as a Reductant on the Performance of a Lean NO x Trap Catalyst for a Light-Duty Diesel Engine

Abstract

Direct-injection diesel engines have become prime candidates for future transportation needs owing to their high thermal efficiency. However, the increase in nitrogen oxides (NO _x ) within local high-temperature regions and the increase in particulate matter within the diffusion flame region during diesel combustion are problematic and must be resolved. The utilization of NO _x -absorbing catalysts, based on the concept of NO _x storage and release, is one of the most promising techniques able to reduce NO _x emissions within net oxidizing gas conditions. This type of NO _x removal system, called the lean NO _x trap (LNT) catalyst, absorbs NO _x under lean exhaust gas conditions and releases NO _x under rich conditions. This technology provides the advantage of high NO _x conversion efficiency; however, the correct amount of reducing agent must be supplied within the catalytic converter under appropriate conditions in order to guarantee a high NO _x reduction efficiency.

In this research, the performance characteristics of an LNT, using a hydrogen-enriched gas as a reductant, were examined via various injection strategies and rich exhaust gas conditions. Although the hydrogen concentration of hydrogen-enriched gas also affects the LNT performance, the composition of hydrogen-enriched gas was fixed as a similar composition to that for the ideal re-forming of diesel fuel. The NO _x reduction efficiency is closely connected to the injection timing and duration of reductant flow. When the injection was introduced 1.5 s after throttling, the LNT NO _x conversion was maximized. However, the LNT NO _x conversion decreased when the injection timing was earlier or later than 1.5 s. Because the actual rich duration of the exhaust gas is limited at a specific point during the rich operation for 3 s, the LNT NO _x conversion was maximized when the hydrogen-enriched gas was injected at the minimum air-to-fuel ratio observed at the LNT inlet. By optimizing the control of this flow, the system encounters only a 1.9 per cent fuel penalty while maintaining a 90 per cent NO _x reduction efficiency.