Experimental Activity on a Hydrogen Fuelled S.I. Engine with Two-Step D.I. 2011-24-0099

An innovative hydrogen DI system was conceived, realized and tested that requires only 12 bar rail pressure, typical value of PFI systems, and does not need special injectors. The purpose is to combine the well-known benefits of DI with the ones of PFI. The injection is accomplished in two steps: at first hydrogen, metered by an electroinjector (a conventional one for CNG application), enters a small intermediate chamber; then it is injected into the cylinder by means of a mechanically actuated valve that allows very high flow rate (compared with the one of electroinjectors). In-cylinder injection starts at intake valve closing (an earlier injection start could lead to backfire) and stops early enough to allow proper charge homogeneity and, in any case, before cylinder pressure rise constrains hydrogen admission.

The prototype engine was realized modifying a production single-cylinder 650 cm₃ engine with three intake valves. The central one was modified and properly timed to inject hydrogen into the cylinder from the intermediate chamber. The experimental results satisfied the expectations. The prototype engine ran properly at full load, without pre-ignition, knocking or roughness even with stoichiometric or slightly rich mixtures, providing higher maximum power than with gasoline. Also at part load the engine proved to work correctly even with very lean mixtures (till λ ~ 4), reaching its maximum brake thermal efficiency with λ ~ 2.4.