A chemical-kinetics-based approach for the preliminary design of hydrogen internal combustion engines

The primary objective of the present work is to correlate the hydrogen explosive characteristics with internal combustion engine design parameters particularly the engine compression ratio. An approach, that couples the knowledge about H2 chemical behaviour, and the in-cylinder charge thermodynamic state, has been conceptualized in the form of a unified plot to visually inspect the likelihood of an auto-ignition event. The plot cautions the possible occurrence of autoignition if the state of the charge inside the engine cylinder reaches thermodynamic conditions beyond the explosion limit curve. Having at hand such a tool enables one to cautiously design future experiments to prevent possible damage because of extreme stresses due to an undesired autoignition event. The results of the analyses in the present work have translated into defining a maximum limit on the compression ratio that can be proposed at pre-defined intake thermodynamic state, mixture composition, engine geometry and engine speed. Predictions based on recently developed chemical mechanisms were employed for the analyses, exploiting the well-established knowledge about the chemical kinetics of hydrogen oxidation. Thus, zero-dimensional numerical simulations were performed with Ansys CHEMKIN Pro software in order to emulate hydrogen reactivity within the operating ranges of pressure 1bar≤p≤100bar, temperature K≤T≤1500K and equivalence ratios 0.1≤ϕ≤1. Such an approach avoids also the limitations associated with experimental procedures. The ignition delay times are automatically calculated at any thermodynamic state through a user defined MATLAB script utilizing the data set acquired from simulations. To evaluate the maximum safe compression ratio, both a static and a time-based approach have been employed to study the vicinity of a thermodynamic state to the autoignition limit i.e., the explosion limit of hydrogen. Three possible criteria for the definition of a maximum safe geometrical compression ratio were developed and analysed. The present work has then been finally ensembled in the form of an empirical correlation involving intake pressure, intake temperature and equivalence ratio as the variables