This thesis evaluates the combustion behavior of premixed low-carbon fuels for use in diesel pilot-ignited (DPI) applications to address the increasingly stringent greenhouse gas emissions regulations. Focusing on fuels like methane, hydrogen, and ammonia, the work employs the DPI combustion strategy to investigate the viability of these alternatives.
The first phase examines the influence of premixed methane on combustion of the diesel pilot, highlighting the small role of both the thermophysical and chemical properties of methane in influencing the ignition of the pilot. Subsequent work delves into the role of premixed fuel composition and pilot reactivity, underscoring the dominance of pilot reactivity for ignition and the dominance of premixed fuel composition for main combustion. Furthermore, this investigation discusses the concept of controlled end-gas autoignition (c-EGAI), demonstrating its ability to extend the load-limit of DPI. Using techniques such as short-time Fourier transform (STFT), its similarity to the pilot autoignition is confirmed in the frequency-time domain. Its onset is also successfully characterized and validated using kinetics.
The work then focuses on the use of exhaust gas re-circulation (EGR) with premixed methane/hydrogen mixtures. Use of EGR at constant intake pressure revealed a potential for reduction in UHC, CO and NO emissions as compared to a methane-only baseline. However, UHC emissions comparable to conventional diesel combustion (CDC) remain a distant goal due to crevice effects.
Ammonia as an alternative to methane is also studied kinetically. Comparisons between ammonia/hydrogen and methane/hydrogen blends are drawn to discern differences in ignition delay and laminar flame speed. The work underscores the poor ignition quality of ammonia necessitating higher equivalence ratios and hydrogen energy fractions for performance comparable to methane at fixed thermodynamic conditions. Finally, experimental evaluation of aqueous ammonia and aqueous ammonia/hydrogen blends is conducted. While challenges such as combustion stability and high energy requirements for vaporization exist, blends with hydrogen marginally alleviate some performance issues. Hydrogen addition alone, however, is not an adequate solution to address the challenges associated with using aqueous ammonia as a fuel in internal combustion engines.
Overall, this thesis work sheds light on the complex combustion characteristics of various low-carbon fuels in DPI applications, offering valuable insights for the development of more sustainable combustion technologies. However, more work is needed to successfully implement these fuels under the DPI mode. The development of new methane oxidation catalysts will be key to reducing the UHC emissions as compared to CDC, whereas a controls-based approach is needed to implement the c-EGAI methodology for extending the high-load limit.