Experimental investigation of wall heat flux characteristics in direct-injection hydrogen internal combustion engines

Direct-injection hydrogen internal combustion engines (DI-HICEs) can achieve high efficiency; however, hydrogen's high adiabatic flame temperature, short quenching distance, and rapid combustion can intensify wall heat transfer, which has been demonstrated to be up to three times higher than that in methane engines. However, experimental measurements of wall heat transfer in DI-HICEs remain limited, although they are important for understanding the heat transfer process. This study experimentally measures local instantaneous wall heat flux in a DI-HICE using a fast-response thin-film sensor synchronized with crank-angle-resolved pressure. Engine speed, excess air ratio (λ), manifold absolute pressure (MAP), start of injection (SOI), and combustion phasing (CA50) are evaluated from heat-flux profiles, peak heat flux, and stage-resolved cumulative heat transfer. The coefficient of variation (CoV) of peak heat flux is 11.78% under motored operation and reaches 26.33% at λ = 3.2 under fired operation, despite Pmax and IMEP CoVs of only 5.75% and 2.26%, respectively, at an engine speed of 2500 rpm. Increasing engine speed from 2000 to 3000 rpm raises peak heat flux from 6.00 to 7.36 MW/m 2 at λ = 1.6, but can reduce cumulative heat transfer because of the shorter physical transfer time. Increasing λ from 1.6 to 3.2 reduces peak heat flux from 6.66 to 1.44 MW/m 2 at 2500 rpm, whereas increasing MAP from 50 to 100 kPa raises it from 3.22 to 6.82 MW/m 2 at λ = 2.0. Stage-resolved cumulative analysis at MAP = 75 kPa shows that the expansion-stage contribution decreases from 60% to 30% as λ increases from 1.6 to 3.2. SOI and CA50 further modify local thermal loading through mixture formation and combustion phasing. At 3000 rpm, decreasing λ from 3.2 to 1.6 shortens BD10–90 by 46.3%, while cumulative heat transfer increases from 4.5 to 13.0 kJ/m 2 , corresponding to an increase of 188.9%. Owing to this sharp increase in wall heat transfer, ITE does not increase monotonically as λ decreases despite the shorter combustion duration. Instead, ITE reaches a maximum of 39.76% at λ = 2.4 and decreases as the mixture is further enriched. These results clarify the distinct thermal effects of operating and control parameters and provide valuable data for the thermal-management-oriented calibration of DI-HICEs.