Heavy duty engines for long-haul trucks are quite difficult to electrify, due to the large amount of energy that should be stored on-board to achieve a range comparable to that of conventional fuels. In particular, this paper considers a stock engine with a displacement of 12.9 L, developed by the manufacturer in two different versions. As a standard diesel, the engine is able to deliver about 420 kW at 1800 rpm, whereas in the compressed natural gas configuration the maximum power output is 330 kW, at the same speed. Three possible alternatives to these fossil fuels are considered in this study: biodiesel (HVOlution by Eni), bio-methane and green hydrogen.
While the replacement of diesel and compressed natura gas with biofuels does not need significant hardware modifications, the implementation of a hydrogen spark ignition combustion system requires a deep revision of the engine concept. For a more straightforward comparison among the alternative fuels, the same engine platform has been considered.
The hydrogen engine has been optimized with the support of CFD-1D simulation (GT-Power), using models calibrated with experimental data, obtained on the diesel and compressed natural gas versions. The numerical tool includes a predictive combustion model (SI-Turb), also calibrated with experimental data on a hydrogen prototype.
The study shows that the implementation of a combustion system running on lean mixtures of hydrogen, permits to cancel the emissions of CO2, while maintaining the same power output of the compressed natural gas / bio-methane engine (but about 20% lower than the biodiesel). Moreover, the concentration of NOx is very low (<20 ppm) at all the operating conditions, enabling a strong simplification of the after-treatment system, at least in comparison to the original diesel/biodiesel version. Finally, the hydrogen solution exhibits an average increase of approximately 9% in efficiency respect to the compressed natural gas configuration, but it remains less efficient if compared to its biodiesel counterpart (-11%).