Geng, Sikai
(2024)
Experimental study of ammonia as an alternative fuel for modern spark Ignited internal combustion engines.
PhD thesis, University of Nottingham.
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Abstract
Innovations in spark-ignited internal combustion engines have been driven by soaring demands for increased efficiency and improved urban air quality. A critical step towards decarbonisation is the use of future low-carbon (neutral) fuels produced via renewable energy. Carbon-neutral energy carriers have attracted considerable attention as a means of controlling global greenhouse gas emissions. In spite of hydrogen being a highly attractive sustainable fuel, it continues to face fundamental challenges with storage and transportation processes which impede mass adoption. In very recent years, there has therefore been increasing interest in ammonia as an energy carrier, which is also a more effective carrier of hydrogen than liquid hydrogen on a volumetric basis. Key benefits include the ability to produce ammonia in a cost-effective manner at very large scale and relative ease and low cost of long-term storage. However, due to relative toxicity, future usage seems likely to be restricted to applications where industrial health and safety protocols can always be maintained.
Despite growing interest, fundamental studies on the thermodynamics and emissions of ammonia engine combustion are insufficient. A conventional spark ignition engine platform may encounter an inherent problem due to the low reactivity of ammonia air mixtures. Ammonia has a high minimum ignition energy, low laminar burning velocity and a notably high flame quenching distance, which have historically made it difficult to initiate and sustain combustion. Unless combustion promoters such as hydrogen or other technologies are in place, pure ammonia combustion cannot easily replace hydrocarbon fuels, even under stoichiometric conditions.
The aim of the currently reported work was to improve the understanding of ammonia combustion in modern spark ignition engines. Practical work involved the design and commissioning of a state-of-the-art thermodynamic research engine test rig, fully instrumented to industry standards to allow assessment of the impact and limitations of ammonia upon combustion, performance, fuel consumption and emissions. Engine testing was conducted using gasoline or hydrogen co-fuelling (where necessary), with the work involving a direct comparison of spark ignition and passive turbulent jet ignition operation.
This study presents a preliminary demonstration of positive ignition for near-stoichiometric ammonia combustion and quantifies the current constraints associated with pure ammonia operation. Using gasoline or highly reactive hydrogen, it was possible to generate maximum ammonia substitution maps for both spark ignition and passive jet ignition systems while maintaining acceptable combustion stability. The indicated thermal efficiency improvement compared to gasoline reached up to 5% at higher loads, nearing the 40% mark. Significant improvements over the gasoline spark ignition baseline were also observed in the emission profiles for hydrogen-ammonia co-fuelling operation. This included a substantial decrease in NOx emissions by nearly 40%, as well as a significant reduction in ammonia slip (decreasing exponentially with increasing hydrogen). Compared to spark ignition, the jet ignition system demonstrated faster burning traits with lower NOx. Still, it was apparent that significant chamber redesign is ideally required in the future for optimum ammonia operation, potentially extending to active jet ignition. Overall, this research has demonstrated the plausibility of ammonia operation with co-fuelling as a first step towards appropriate future adoption, identifying critical areas for future research to maximise the impact of future spark and jet ignition engine technologies.
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