Experimental investigation on the optimal injection and combustion phasing for a direct injection hydrogen-fuelled internal combustion engine for heavy-duty applications

In the current context of increasing demand for clean transportation, hydrogen usage in internal combustion engines (ICEs) represents a viable solution to abate all engine-out criteria pollutants and almost zeroing CO2 tailpipe emissions. Indeed, the wider flammability limits thanks to the higher flame propagation speed and the lower minimum ignition energy compared with conventional fuels, extend the stable combustion regime to leaner mixtures thus allowing high thermal efficiency keeping under control the NOX emissions. To fully exploit the potential of hydrogen as a fuel and to avoid undesired abnormal combustion processes, a deep characterization of the combustion process is needed. With this aim, a 6-cylinder, 12.9-L heavy-duty engine was converted from a port-fuel injected compressed natural gas to a direct injected hydrogen spark ignition one. A wide experimental campaign was carried out, consisting of several sweeps of relative air-fuel ratios, spark advances, and injection timings at different engine speeds and loads, aiming to define a preliminary engine map. The effect of each calibration parameter at different engine load and speed has been analyzed through the combination of relevant combustion parameters, as well as NOX emissions. The results have demonstrated the critical influence of the mixture inhomogeneity when the injection is retarded through the top dead center firing, as indicated by the increase in NOX emissions and combustion variability. The analysis of the combustion timing has indicated the dependence of the optimal MFB50 on the relative air-fuel ratio. Lastly, the analysis of 200 consecutive cycles for each operating condition has allowed the evaluation of the influence of the main calibration parameters on the cyclic variability, thus providing further insights about the lean limit of hydrogen in ICE.