Combustion Optimization in a Hydrogen-Enhanced Lean-Burn SI Engine

As part of ongoing research on hydrogen-enhanced lean burn SI engines, this paper details an experimental combustion system optimization program. Experiments focused on three key areas: the ignition system, in-cylinder charge motion produced by changes in the inlet ports, and uniformity of fuel-air mixture preparation. Hydrogen enhancement is obtained with a H₂, CO, N₂ mixture produced by a fuel reformer such as the plasmatron. The ignition system tests compared a standard inductive coil scheme against high-energy discharge systems. Charge motion experiments focused on the impact of different flow and turbulence patterns generated within the cylinder by restrictor plates at the intake port entrance, as well as novel inlet flow modification cones. The in-cylinder fluid motion generated by each configuration was characterized using swirl and tumble flow benches. Mixture preparation tests compared a standard single-hole pintle port fuel injector against a fine atomizing 12-hole injector.

Results indicate that optimizations of the combustion system in conjunction with hydrogen-enhancement can extend the relative air/fuel ratio λ at the lean limit of operation by roughly 25% compared against the baseline configuration. Nearly half of this improvement may be attributed to improvements in the combustion system. Furthermore, hydrogen-enhancement produces a nearly constant lean misfire limit improvement of ∼ 0.20 - 0.25 λ values, regardless of baseline combustion behavior. In contrast, the improvement of the amount of dilution with excess air at the point of peak engine efficiency decreases as engine operation becomes leaner, due to the inherently lengthening burn duration as λ increses.