Assessment of Knock Tendency in a Hydrogen-Fuelled High-Performance Internal Combustion Engine: a Chemistry-Based Numerical Study

Hydrogen is a viable option to power high-performance internal combustion engines while reducing pollutant emissions thanks to its high lower heating value (LHV) and fast combustion rate. Furthermore, if compared to gasoline, hydrogen is characterized by a higher ignition delay time, which makes it more knock-resistant under the same thermodynamic conditions. In this paper, hydrogen potential as a fuel in a high-performance PFI naturally aspirated engine under stoichiometric conditions and high load regimes is investigated through zero and three-dimensional simulations. The analyses show that a stoichiometric hydrogen mixture reaches higher pressure and temperature values during compression than iso-octane at the same operating conditions, hence limiting the maximum engine compression ratio to avoid undesired ignitions throughout the combustion process. Additionally, hydrogen low density causes a reduction in terms of trapped energy inside the cylinder. Thus, despite its LHV is almost three times higher than conventional gasoline, a 20% reduction in terms of power output is noted. Finally, a hot-spot sensitivity is carried out: with respect to conventional gasoline, hydrogen exhibits a lower quenching distance, which increases the wall heat transfer. Furthermore, its lower ignition energy makes this fuel more prone to surface ignition. Indeed, it is found that this phenomenon may occur into the high surface-to-volume ratio zones, such as the exhaust valve crevices and the spark plug, if a certain temperature threshold is met.