A chemical kinetics-based approach to predict uncontrolled self-ignitions in 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. Such an approach avoids also the limitations associated with experimental procedures. 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. Furthermore lubricant oil as a contaminant seeping through the compression rings of a piston in an internal combustion engine, is modelled to evaluate the distribution of mass and temperature inside a droplet of n-hexadecane using 0D- simulations to evaluate the variation of ignition delay time within the droplet in gas-phase and its effect on the local concentration diluting the pure hydrogen in the vicinity and hence increasing the reactivity causing an early source of self- ignition. A final study concerning developing detonations from hot spots is carried out to understand the effect of a temperature gradient other than ‘linear’ within the hot spot that could change the detonation response diagrams and subsequently the modes of reacting front propagation. Such detonations with high peak pressures are detrimental to components inside an internal combustion engine and therefore the need to study any possibility of its occurrence is crucial to a better understanding of the design.