Two-Step Low Pressure Direct Hydrogen Injection

A low-pressure hydrogen direct-injection system has been developed that is characterised by low storage residual pressure (~12 bar) and simple mechanical solutions. The injection is split in two steps. Firstly hydrogen is metered and admitted into a small intermediate chamber. by an electro-injector (a conventional one for CNG application). Next, a mechanically-actuated poppet valve times hydrogen injection from the intermediate chamber to the cylinder, allowing higher flow rate than any electro-injector. Injection must finish early enough to permit achieving good charge homogeneity, anyhow before in-cylinder pressure growth stops hydrogen entry, and must not start before intake valve closing to avoid backfire. A prototype has been realised modifying a single-cylinder 650 cm3 production engine with three intake valves of which the central one has been replaced by the valve for hydrogen injection. Prototype design complies with the results of a previous CFD activity during which hydrogen injection in a quiescent, constant volume through poppet valves of different shapes was simulated in order to investigate how valve and seat-valve geometries act upon fuel-air mixing. Additional predictions were conducted to determine injection settings leading to acceptable fuel distribution in the combustion chamber at spark time (i.e. air-fuel ratios within the flammability range in the spark region) for all engine operating conditions.