Development of a direct-injection spark ignition ultra-lean hydrogen engine by an experimentally validated 0D/1D model including thermo-diffusive flame instabilities

In this work, combined numerical and experimental analyses are carried out on a single cylinder direct-injection hydrogen spark ignition engine, in order to develop and validate a predictive 0D/1D numerical framework. The model is then adopted to design a multi-cylinder hydrogen powertrain operating under ultra-lean mixtures over the entire engine map. In a first stage, a Diesel unit is converted into a hydrogen spark ignition engine and experiments are performed at different loads, revving speeds and air–fuel equivalence ratios, collecting performance parameters and in-cylinder pressure traces. In a second phase, a 0D/1D engine model is realized in GT-Power™ and integrated with phenomenological 0D in-cylinder sub-models, including turbulence and combustion ones. The turbulence sub-model is tuned against the outcomes of 3D-CFD simulations, while the combustion process is reproduced adopting a modified fractal model including the thermo-diffusive instability effects of freely propagating hydrogen flames, according to Howarth theory. The model of the single cylinder hydrogen engine is validated against the measurements in terms of in-cylinder pressure cycles, burn rates, overall engine performance and NO x emissions, denoting satisfactory agreement. Finally, the validated numerical framework is exploited to propose a multi-cylinder hydrogen spark ignition engine, which is calibrated with a specifically developed rule-based strategy to explore the potentials of a hydrogen engine working with ultra-lean mixtures in the entire operating map. The outcomes reveal that, most importantly, thanks to the proposed calibration strategy, the developed multi-cylinder engine preserves high efficiency and low NO x emissions in wide regions of the operating map, especially at part load conditions, which are frequently explored by engines during the standard driving cycles. Moreover, despite the Diesel-like architecture, the introduction of additional technologies such as water injection or active pre-chamber (both in combination with increased compression ratio) enables the overall brake thermal efficiency to reach up to ∼ 43% at the best efficiency point, while keeping NO x emissions as low as 55 ppm.