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.