In this paper, a low-pressure hydrogen direct-injection solution is presented that entails low storage residual pressure (∼12 bar). The injection is realised in two steps. First, hydrogen is simply metered by an electro-injector (a conventional one for Compressed Natural Gas - CNG application) that feeds a small intermediate chamber. Next, hydrogen enters the cylinder by means of a mechanically-actuated valve which allows higher flow than any electro-injector. Injection must end early enough to allow good charge homogeneity and, in any case, before in-cylinder pressure rise constraints hydrogen admission. Backfire is avoided by starting injection at intake valve closing. A prototype has been realised modifying a single-cylinder 650 cc production engine with three intake valves. The central one has been modified and properly timed to in-cylinder inject hydrogen from the intermediate chamber. Hydrogen injection through different-shape poppet valves in a quiescent, constant volume has been simulated in order to investigate the effects of valve and seat-valve geometries in controlling fuel-air mixing in the cylinder. Additional predictions for the actual engine configuration indicate that an acceptable fuel distribution can be obtained in the combustion chamber at the spark timing, with equivalence ratios in the ignition region that are inside the flammability range of the mixture for all the operating conditions (loads and speeds) that have been considered.

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