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The paper describes the CFD analysis, the arrangement and the first experimental results of a single-cylinder engine that employs an innovative low-pressure hydrogen direct-injection system, characterized by low fuel rail pressure (12 bar) and consequent low residual storage pressure. The injection is split in two steps: at first hydrogen is metered and admitted into a small intermediate chamber by an electroinjector (a conventional one usually employed for CNG), next a mechanically actuated poppet valve, that allows high volumetric flow rates, times hydrogen injection from the intermediate chamber to the cylinder within a short time, despite the high hydrogen volume due to the low injection pressure. Injection must be properly timed to maintain pressure below 6 bar (or little more) in the intermediate chamber and thus keep sonic flow through the electroinjector, to maximize volumetric efficiency and to avoid backfire in the intake pipe.

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.

Storing hydrogen is one of the major problems concerning its utilization on board vehicles. Today hydrogen can be compressed and stored at 200 or 350 bar (it is foreseen that in a near future storage pressure will reach 700 bar, according to new expected regulations and using tanks in composite materials) or cryogenically liquefied. An alternative solution is storing hydrogen in the form of ammonia that is liquid at roughly 9 bar at environmental temperature and therefore involves relatively small masses and volumes and requires light and low-cost tanks. Moreover, ammonia contains almost 18% hydrogen by mass and, by volume, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen. It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support combustion especially in the case of high-speed S.I. engines.

Storing hydrogen is one of the major issues concerning its utilization on board vehicles. A promising solution is storing hydrogen in the form of ammonia that contains almost 18% hydrogen by mass and is liquid at roughly 9 bar at environmental temperature. As a matter of fact, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen itself, thus involving relatively small volumes and light and low-cost tanks. It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support and speed up combustion especially in the case of high-speed S.I. engines. The best promoter is hydrogen, due to its opposed and complementary characteristics to those of ammonia, Hydrogen has high combustion velocity, low ignition energy and wide flammability range, whereas ammonia has low flame speed, narrow flammability range, high ignition energy and high self-ignition temperature.