Numerical investigation of hydrogen fuelled internal combustion engines

Abstract

Hydrogen is a clean and carbon free alternative fuel which has the potential to drastically reduce harmful emissions while maintaining or even improving the energy efficiency of internal combustion engines traditionally fuelled by fossil fuels. The focus of this thesis is to, through numerical modelling and simulation, enhance the fundamental understanding of optimal hydrogen utilisation in compression ignition (CI) engines operating with diesel-hydrogen dual-fuel combustion technologies where a high percentage of the original diesel fuel is replaced by hydrogen. The first part of this thesis investigates high hydrogen energy share (HES) diesel-hydrogen dual-fuel combustion in a CI engine when operated with intake manifold inducted hydrogen. For this, a range of numerical simulations were carried out to investigate the performance and emissions characteristics of the engine when operating under a novel combustion strategy, currently in development by industrial collaborator Covaxe, in which the diesel injection and the majority of the hydrogen combustion occur at a constant volume. Initially, a parametric study on pure diesel operation was performed to investigate the effects of the duration and timing of the constant volume combustion phase (CVCP) on the engine’s performance and emissions outputs, with comparisons to the conventional engine. The results demonstrate that the CVCP strategy is capable of yielding reduced gross indicated specific fuel consumption by up to 20% and far lower carbon based emissions. The results also found that a combination of CVCP strategy, exhaust gas recirculation (EGR) and improved fuel injection methods can counter the increased NO_x emissions somewhat, while still maintaining the improved engine performance and low carbon-based emissions. Following this, a study was performed to investigate the effects of the CVCP strategy on combustion, performance and emissions of the engine operating in diesel-hydrogen dual-fuel mode with hydrogen intake induction at up to 90% HES. The results demonstrate that the CVCP strategy can improve thermal efficiency at all HESs and load conditions with far lower carbon-based emissions. Conventional diesel-hydrogen dual-fuel engines struggle at low load high HESs due to the reduced diesel injection failing to ignite the leaner premixed charge. Through use of a CVCP thermal efficiency at low load 90% HES increased from 11% to 38% with considerably reduced hydrogen emission due to the increased temperatures and pressures allowing for the wholesale ignition of the hydrogen-air mix. It was also found that increasing the time allowed for combustion within the CVCP, by advancing the diesel injection, can lead to even further thermal efficiency gains while not negatively impacting emissions. Direct gaseous fuel injection in internal combustion engines is a potential strategy for improving in-cylinder combustion processes and performance while reducing emissions outputs and increasing HES when compared to intake induction operation. The second part of this thesis studied combustion in a direct hydrogen injection diesel hydrogen dual-fuel CI engine at up to 99% HES. To facilitate this study, the gaseous sphere injection (GSI) model, which utilises the Lagrangian discrete phase model to represent the injected gas jet, is improved upon to accurately predict the high pressure hydrogen direct gas injection strategy. The improved GSI model is then validated against experimental hydrogen and methane underexpanded freestream jet studies, mixing in a direct injection hydrogen spark ignition engine and combustion in a dual direct injection diesel-methane compression ignition engine. The improved GSI model performs well across all cases examined which cover various pressure ratios, injector diameters, injection conditions and disparate gases (hydrogen and methane) while also allowing for relatively coarse meshes, with no remeshing requirement on injector parameter change, to be used when compared to those needed for fully resolved modelling of the gaseous injection process. The model is shown to be accurate, easy to implement and computationally inexpensive. The improved GSI model, coupled with computational fluid dynamics and other necessary numerical models, was applied to simulate the mixing and combustion process in a dual direct injection diesel-hydrogen dual-fuel CI engine targeting non-premixed operation. The combustion process of hydrogen in this type of engine was mapped out and compared to that of the the same engine using methane direct injection rather than hydrogen. This was followed up with a comprehensive parametric study aimed at identify how various key fuel injection and engine operational variables can be optimised to improve high hydrogen energy share (99%) performance/operation and reduce emissions at high and low load conditions. Four distinct phases of combustion were found which differ from that of pure diesel operation. Interaction of the injected gas jets with the chamber walls is discovered to be by far the most important factor in diesel-hydrogen dual direct injection operation. When combined, the likes of nozzle diameter reduction, split injection strategies, gaseous injection included angle, timing between diesel pilot and gaseous injection, start time of gaseous injection, inlet pre-heating/cooling, turbocharging and EGR were all found to be effective ways to improve performance and combustion stability/consistency while increasing hydrogen energy share and reducing NO_x emissions at both high and low load. The novel insights gained from this numerical study can be used to guide any further investigations on high HES dual direct injection diesel-hydrogen dual-fuel CI engines.

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ORCID for Janaka Ranga Dinesh Kahanda Koralage: orcid.org/0000-0001-9176-6834

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