Influence of Hydrogen Enrichment on Particulate Matter Formation Pathways in Dual-Fuel Compression Ignition Engines

This study evaluates the effects of hydrogen enrichment of compressed natural gas (HCNG, CNG) on particle number emissions and particle size distribution in a dual-fuel compression-ignition (CI) engine. The experiments were conducted using a Fiat 1.3 MultiJet engine operated with conventional diesel fuel, diesel–CNG, and diesel–hydrogen-enriched CNG. Measurements were performed over a range of engine speeds, loads, and energy-substitution conditions. The hydrogen volume fraction in the gaseous fuel ranged from 5–10 vol.% to 53 vol.%. Particle size distributions were measured using an ELPI+ impactor. The use of CNG and hydrogen generally reduced soot-particle emissions in the accumulation mode, particularly within the particle-diameter range of approximately 0.03–0.2 µm. However, under some operating conditions, the number of ultrafine particles increased, particularly in the sub-23 nm range. This finding indicates a transition from a particle-formation mechanism dominated by soot formation and agglomeration to one dominated by nucleation and condensation. This phenomenon is associated with suppressed soot-precursor formation, enhanced soot oxidation by OH radicals, and a reduction in the surface area available for the condensation of volatile components. The effect of hydrogen depended on the engine operating point (EOP) and did not always scale linearly with hydrogen concentration. The results confirm that evaluating alternative fuels solely on the basis of particulate mass is insufficient. Particle number, particle size distribution, and the sub-23 nm fraction must also be considered.