Hydrogen Internal Combustion Engines (H₂ICEs) offer the potential for near-zero carbon emissions. However, while nitrogen oxide (NOₓ) emissions have been extensively studied, particle number (PN) emissions, primarily resulting from lubricant oil pyrolysis due to hydrogen’s short quench distance, remain less well understood. This study investigates exhaust particle characteristics from a spark-ignition, single-cylinder research engine, based on MAHLE Powertrain’s downsizing engine combustion system, at Brunel University, London. It compares gasoline and hydrogen direct-injection strategies (central vs. side injection) across a wide range of operating conditions, including variations in engine speed, load, air-fuel ratio (λ), rail pressure, and spark timing. PN and particle size distribution were measured using Cambustion’s DMS500, paired with a catalytic stripper to isolate solid particles. To assess the impact of engine wear, tests were conducted on both a worn engine configuration