A low-pressure hydrogen direct-injection solution is presented that allows some typical benefits of direct injection, such as high specific power and backfire prevention, plus low residual storage pressure, that improves vehicle range and is a typical advantage of external mixture formation. Since the injection must end early enough to allow good charge homogeneity and, in any case, before in-cylinder pressure rise constraints hydrogen admission, especially at heavy loads hydrogen flow to the cylinder is higher than present electro-injectors allow. The injection is realised in two steps: hydrogen flow rate is simply controlled by a conventional CNG electro-injector that feeds a small intermediate chamber. From this chamber hydrogen next enters the cylinder in a short crank angle period by means of a mechanically-actuated valve that opens at the intake valve closure to avoid backfire. The prototype is under construction modifying a production single-cylinder motorcycle engine with three intake valves. The central one has been modified and properly timed to admit hydrogen from the intermediate chamber. The paper shows CFD analysis at the basis of the prototype design.
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