Adopting biodiesel as an indirect way to reduce the NOx emission of a hydrogen fumigated dual-fuel engine

Hydrogen has recently attracted extensive attention as an internal combustion engine fuel to tackle the global carbon emissions problem. In a compression ignition engine, hydrogen needs to be combined with a more volatile fuel to avoid misfiring and ensure smooth combustion. Petroleum-derived diesel is often the secondary fuel used in most of the studies found in the literature. Although hydrogen-diesel operation can provide significant benefits in thermal efficiency and harmful emissions, the combination of the two fuels often leads to increased NOx emissions. The present research paper attempts to reduce the NOx emissions in a hydrogen fumigated dual-fuel engine by implementing a low-sulphur and aromatics fuel. The work investigates the effects of replacing diesel fuel with waste cooking oil biodiesel and operating the engine at higher EGR rates. The paper compares the diesel and biodiesel-hydrogen operation for different hydrogen energy share ratios, EGR rates and combustion timings at low and medium engine loads. The comparison study revealed that biodiesel-hydrogen combustion allowed operation with higher EGR rates without worsening of soot emissions. The operation at higher EGR rates led to significant NOx emission benefits of up to 64% while carbon monoxide and total hydrocarbons were also reduced. A drop in the brake thermal efficiency due to the lower energy content of biodiesel could also be eliminated at low and medium engine loads as the increased EGR rates resulted in closer to stoichiometric combustion. Advancing the combustion phasing could contribute to efficiency improvement for the biodiesel-hydrogen engine and comes with further reductions in soot, CO and THC emissions but moderates the NOx improvement levels.