This study experimentally investigates hydrogen-diesel dual-fuel combustion using full-metal and optical single- cylinder research engines. Premixed hydrogen is ignited by a diesel pilot over a range of engine speeds, lambda (𝜆), combustion phasing, and hydrogen energy shares (HES), at constant total fuel energy input. The full-metal engine achieves stable operation up to 50% HES, with ∼50% CO2 reduction and gross indicated thermal efficiency of ∼44%–47%. At the highest-soot condition, 50% HES reduces soot by ∼80%, while NO𝑥 and H2O increase by up to ∼66%. Exhaust gas temperature decreases by ∼4%–5% with increasing HES. AHRR analysis shows increasing hydrogen influence on the mixing-controlled stage beyond ∼30% HES. Optical measurements show delayed NFL and OH∗ signal appearance and increasing IDT with HES, opposite to the full-metal trend. This difference may be attributed to the combined influence of in-cylinder thermal conditions and injector-dependent mixture formation.