Energy Management of Hydrogen Hybrid Electric Vehicles -- A Potential Study

The hydrogen combustion engine (H$_2$ICE) is known to be able to burn H$_2$ under ultra-lean conditions, while producing no CO$_2$ emissions and extremely low engine-out NO$_x^{\mathrm{eo}}$ emissions. Immediate goals, as for instance the upcoming EURO 7 NO$_x$ limitations, can be reached more easily as extremely low engine-out NO$_x^{\mathrm{eo}}$ emissions facilitate the reduction of the overall tailpipe NO$_x^{\mathrm{tp}}$ emissions. In this work, the feasibility of achieving consistent reductions in NO$_x^{\mathrm{eo}}$ emissions through the implementation of electric hybridization of an H$_2$ICE-equipped passenger car (H$_2$-HEV), combined with a dedicated energy management strategy (EMS) is discussed. In particular, the mixed H$_2$-HEV architecture is investigated and compared to a series H$_2$-HEV, a parallel H$_2$-HEV, and a base H$_2$-vehicle, which is only equipped with an H$_2$ICE. For hybrid vehicles, a low H$_2$ consumption and low NO$_x^{\mathrm{eo}}$ emissions are conflicting objectives, the trade-off of which depends on the EMS and can be represented as a Pareto front. Overall, through the utilization of a dedicated energy management calibration, the mixed H$_2$-HEV demonstrates the capability to consistently achieve extremely low engine-out NO$_x^{\mathrm{eo}}$ emissions. For a broad range of driving missions, the mixed H$_2$-HEV is able to decrease the engine-out NO$_\mathrm{x}^\mathrm{eo}$ emissions by more than 90%, while, at the same time, the H$_2$ consumption is decreased by over 16%, compared to a comparable non-hybridized H$_2$-vehicle. These significant emission reductions are possible without having to modify the exhaust-gas aftertreatment system, or the optimization of any of the individual drivetrain components, but solely by setting the EMS calibration accordingly.