Combustion Stabilization for Enriched D-EGR Applications via Air-Assisted Pre-Chambers 2021-01-0481

The dedicated exhaust gas recirculation (D-EGR®) concept developed by Southwest Research Institute (SwRI) has demonstrated a thermal efficiency increase on several spark-ignited engines at both low and high-load conditions. Syngas (H₂+CO) is produced by the dedicated cylinder (D-cyl) which operates at a rich air-fuel ratio. The syngas helps to stabilize combustion under highly dilute conditions at low loads as well as mitigating knock at high loads. The D-cyl produces all the EGR for the engine at a fixed rate of approximately 25% EGR for a four-cylinder engine and 33% EGR for a six-cylinder engine. The D-cyl typically runs up to an equivalence ratio of 1.4 for gasoline-fueled engines, beyond which the combustion becomes unstable due to the decreasing laminar burning velocity caused by rich conditions.

Conventional active-fueled and passive pre-chambers have benefits of inducing multi-site ignition and enhancing turbulence in the main chamber. In this study a novel active-air pre-chamber is investigated. By introducing fresh air into the pre-chamber installed in the D-cyl, a locally leaner or near-stoichiometric mixture can be achieved. The leaner mixture permits ignition of the mixture and stabilizes combustion. Therefore, the conventional benefits of the pre-chamber can be utilized for D-EGR applications. With optimized conditions in the pre-chamber for ignition, a richer global mixture can be introduced into the main chamber to generate higher concentrations of syngas. This in turn can further improve thermal efficiency.

This study investigated the rich limit extension by using an air-assisted pre-chamber in a natural gas fueled six-cylinder D-EGR engine. A spark plug was modified to include an air passage which allowed air injection into the pre-chamber. Combustion stability was investigated under different enrichment levels and air injection durations. The results demonstrated that the air-assisted pre-chamber was highly effective in stabilizing the combustion and extended enrichment limits by 11.7 %.