Measurement and analysis of transient hydrogen injection rates from pintle-type injector in choked flow conditions

The transient variation of fuel injection rate is a critical factor affecting mixture formation and combustion phase as well as the energy supply rate in direct-injection engines. Despite its great importance, transient injection rates of hydrogen direct-injection injectors have rarely been characterized previously under various injector operating conditions considering injector dynamics. The current study measures and analyzes the transient injection rates of a pintle-type hydrogen injector under various injection and ambient pressures in choked flow conditions and examines the predictability of transient injection rates based on the compressible flow theory of converging-diverging nozzles and pintle dynamics. The transient injection rate profile is obtained by measuring the jet force at the nozzle exit and calibrating it to the time-averaged mass flow rate data. An increase in injection pressure caused a steeper rising slope of injection rate and a longer total injection duration with a linear increase in quasi-steady injection rate. As the ambient pressure increased, the rising slope of the injection rate and the total injection duration decreased, while the quasi-steady injection rate remained nearly the same. These transient and quasi-steady injection rate characteristics were able to be predicted based on the mass flow rate equation of converging-diverging nozzles in choked flow conditions and the transient pintle stroke as the input parameter.