Maiello, Armando (2024) Bridging the transition to clean energy: hydrogen-natural gas blends for short-term applications and hydrogen free piston engines for long-term sustainability. [Tesi di dottorato]

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Abstract

To limit the impact of global warming, the European Union introduced the European Green Deal, a strategy aimed at reducing greenhouse gas emissions and achieving carbon neutrality by 2050. In line with this, many alternative solutions are being researched and developed by institutes and industries to transform the current transportation sector, which is still largely dominated by internal combustion engines powered vehicles. However, internal combustion engines are expected to remain a key component of the transportation systems in both the short and long term. The environmental impact of internal combustion engines can largely be attributed to the fuels used to convert chemical energy into mechanical energy. Therefore, the first step toward their decarbonization involves analysing the fuels that could reduce pollutant emissions, particularly CO2, while maintaining adequate engine performance and efficiency. The adoption of biofuels, e-fuels (especially hydrogen, as outlined in the European Union’s Hydrogen Roadmap) presents a promising opportunity to reduce emissions from internal combustion engines. These fuels can leverage existing technical expertise and industrial structure with minimal disruption to society and industry. This Ph.D. thesis focuses on the usage of hydrogen in internal combustion engines over both short and long term, analysing the behaviour of engines when hydrogen is used either as a blend with compressed natural gas or as the sole fuel. Initially, hydrogen and compressed natural gas blends are studied to understand how the engine responds to the substitution of hydrogen to compressed natural gas in small fractions. Subsequently, after optimizing the engine for each fuel to maximize efficiency, the use of hydrogen as a standalone fuel is analysed and compared to compressed natural gas. The findings indicate that substituting hydrogen to compressed natural gas in existing internal combustion engines is virtually feasible, offering immediate environmental benefits with minimal cost or technical drawbacks. However, new infrastructures will be necessary to support the widespread use of hydrogen-powered engines. Despite this, concerns remain about abnormal combustion phenomena in engines running on pure hydrogen. Specifically, pre-ignition, a random and difficult-to-study phenomenon, appear as the most critical abnormal combustion event in hydrogen-powered engines. Concerning such issue, part of the Ph.D. program has been dedicated to a comprehensive study of the existing scientific literature. Hence, a part of the thesis explores the phenomenon and the other abnormal combustion events, providing insights into key factors that could mitigate them and paving the way for future research. Looking to the long term power technologies that use hydrogen, part of the Ph.D. activity has been addressed to explore its use in combination with an innovative engine architecture known as the free piston engine linear generator. In this system, the crankshaft is eliminated, and the movement depends on a force balance accurately controlled by the engine control system. During operation, the piston movement generates electrical energy through a linear electric machine that acts as a generator. This design can be used to power one or more electric motors that drive the wheels or charge a battery pack. The absence of a crankshaft is a novel solution with considerable potential, as it reduces friction and enhances overall engine efficiency. Moreover, the rise of pressure deriving from the combustion increases the expansion speed of the piston with respect to conventional engines. That reduces the average temperature of the gases and components thanks to the faster expansion, further improving the system efficiency by reducing heat transfer losses and also reducing the probability of abnormal combustion. The free piston linear generator architecture can be implemented in various configurations. After a comprehensive review of the existing literature and establishing a simulation methodology for this system, the thesis compares two configurations (boxer and opposed piston) through numerical simulations. In conclusion, hydrogen can effectively decarbonize the transportation sector, allowing internal combustion engines to remain a viable powertrain option. However, abnormal combustion events raise concerns about engine lifespan and safety when hydrogen is employed as a pure fuel. The use of lean mixtures and free piston engines can help reduce the occurrence and intensity of abnormal combustion events while simultaneously minimizing heat losses. That is due to the lower in-cylinder temperature deriving from the excess air in the combustion chamber and the rapid expansion of the gases, respectively.

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