Introducing hydrogen (H2) into the intake air of diesel engines provides a near-term approach to reducing tailpipe CO2 emissions from heavy-duty commercial vehicles. The premixed hydrogen results in a complex H2-Diesel dual fuel (H2DF) combustion process, where H2 can both participate in the non-premixed diesel combustion and result in a propagating H2/air combustion. These interactions influence engine combustion characteristics, including in-cylinder pressure and heat release rate (HRR), as well as emissions. The nature and extent of the impact depends on the amount of H2 introduced as a function of the total fuel energy (H2 energy share ratio - HES), the trapped air mass, and engine operating conditions. To optimize the HES ratio under different conditions, it is crucial to understand how H2DF combustion differs from diesel combustion and how this limits engine operation and impacts emissions. To investigate these effects, a heavy-duty class 8 truck fitted with an H2DF system developed by Hydra Energy Corp. was tested on a chassis dynamometer. The engine was fitted with a suite of instrumentation, including in-cylinder pressure, air system pressure and temperature, exhaust flow rate, and emissions measurement equipment. Tests were conducted over three loads and speeds at fixed HES ratios, and detailed HES ratio studies were conducted at low- and mid-load cases at 1200 RPM. The results show that H2 introduction significantly impacts combustion characteristics and emissions, primarily influenced by the H2 equivalence ratio, with the engine control unit’s adjustments to boost pressure and diesel injection timing playing a critical role in combustion characteristics and engine-out emissions. At higher H2 equivalence ratios than 0.1, an H2/air premixed flame forms, advancing combustion phasing, which increases the maximum rate of pressure rise and reduces PM while raising NOx emissions. The Pcyl and HRR data are used to develop a semi-predictive combustion model imposing the net HRR profile using a multi-Wiebe function. A four-curve Wiebe function model can accurately capture the HRR and combustion characteristics across engine operating points, providing a reliable predictive tool at a given speed/load for various HES ratios. The developed understanding and combustion model provides valuable insight and techniques for future studies to further improve H2 utilization strategies tailored for the retrofit of heavy-duty H2DF truck applications.