Nel lavoro di tesi è stato preso in esame un'innovativo sistema di iniezione diretta di idrogeno a bassa pressione ideato e sviluppato presso il Dipartimento di Energetica della Facoltà di Ingegneria di Pisa.
Scopo della tesi nello specifico è stato determinare mediante un’analisi numerica CFD condotta in parallelo ad una attività sperimentale le geometrie e le condizioni ottimali di funzionamento del sistema di iniezione. La ricerca è stata condotta su un motore monocilindrico di 650cc a 5 valvole di derivazione motociclistica (Aprilia Pegaso 650i.e. 2002).
Mixture formation strategy is a key factor in hydrogen-fuelled internal combustion engines.
External mixture formation is characterised by excellent homogeneity and low residual pressure requirement for hydrogen storage. However, the high specific volume of hydrogen leads to poor energy density with consequent low engine specific power, especially if far-from-stoichiometric mixtures are employed to avoid pre-ignition and backfire problems. The use of liquid injection, due to its low storage temperature and latent heat of vaporisation, allows increasing mixture density and preventing pre-ignition and backfire, but requires sophisticate and expensive technologies and does not save tank mass and size significantly.
Direct hydrogen injection provides high mixture energy density (even higher than with gasoline) avoids backfire, but pre-ignition and combustion roughness can occur at heavy loads unless a certain amount of the fuel is injected during combustion to accomplish compression stroke with a lean mixture and to positively control the heat release rate. Yet, injection during combustion can cause combustion instability and of course leads to high residual hydrogen storage pressure.
In this work, 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.