Hydrogen directly injected at 350 bar into the cylinder of high compression ratio diesel engine has been investigated in both optically accessible engine and single-cylinder metal engine of ~1 litre/cylinder displacement volume. On a conventional diesel engine equipped with a centrally mounted multi-hole common-rail injector, a separate hydrogen direct injection was realised using a side-mounted single-hole injector. The energy share of hydrogen was 95% with only 5% of the total energy provided by diesel pilot flames achieving significant CO2 reduction. Under the influence of swirl flow, the hydrogen natural combustion luminosity signals obtained from the optical engine were observed to be deformed; however, it was clearly observed that fragments of diesel flames from the pilot injection cause hydrogen jet combustion in a form of flame propagation back towards the nozzle and simultaneously downstream. It was followed by a hydrogen diffusion flame as the injection continued, which also showed a strong influence of in-cylinder swirl flow. The optical diagnostic results led the performance testing of diesel pilot flame ignited hydrogen direct injection combustion in a single-cylinder metal engine connected to the engine dynamometer. A parametric study was conducted for varied hydrogen energy fraction up to 95% and injection timings as well as split injection strategies. It was found that early hydrogen injection timings lead to premixed burn while late injection timings produce diffusion burning, with an intermediate point reached at ~60 °CA bTDC hydrogen injection timing. The test results demonstrated that precisely controlling the hydrogen injection timing in the entire compression stroke range offers a significant benefit of hydrogen jet development decoupled from pilot diesel flames used for the ignition of hydrogen-air mixtures. The diesel pilot flame ignited hydrogen direct injection (DPI-H2DI) combustion was found to achieve up to 93% CO2 reduction compared to the diesel baseline. Notably, this was achieved without compromising engine power output/efficiency and NOx emissions.