The transition to low-carbon fuels in heavy-duty compression ignition (CI) engines is a promising pathway for reducing greenhouse gas emissions while retaining the high thermal efficiency associated with mixing-controlled combustion. However, many sustainable fuels, including methanol, ethanol, and propane, possess significantly lower cetane numbers than conventional diesel, making reliable autoignition and stable low load operation major challenges. Addressing these challenges requires a fundamental understanding of fuel reactivity, together with effective ignition-assistance strategies that extend compression ignition to low-carbon fuels.
This dissertation systematically investigates multiple ignition-assistance strategies for enabling stable low-load compression ignition of low-cetane and low-carbon fuels. Building on knowledge developed through one of most widely market available low cetane fuels Gasoline in Compression Ignition (GCI), the work first establishes the thermodynamic requirements for compression ignition by evaluating the effects of intake pressure and temperature on fuels with varying reactivities. The study then investigates two advanced ignition-assistance technologies: active prechamber ignition and exhaust rebreathing, to enhance combustion stability. Active prechamber combustion is evaluated using gasoline and propane as main-chamber fuels and methane, propane, and hydrogen as prechamber fuels to examine the influence of fuel chemistry on ignition performance. Exhaust rebreathing is subsequently investigated using gasoline, propane, methanol, and ethanol to determine its ability to promote stable compression ignition across fuels with widely varying ignition characteristics. Finally, exhaust rebreathing is integrated with methanol–diesel dual-fuel combustion to increase methanol substitution while maintaining stable low-load operation.
The results demonstrate that both active prechamber ignition and exhaust rebreathing effectively overcome the poor autoignition characteristics of low-cetane fuels, enabling stable low-load compression ignition with indicated thermal efficiencies comparable to conventional diesel operation. Exhaust rebreathing successfully enables stable compression ignition of methanol and ethanol, while providing a practical pathway for fuel-flexible CI combustion. Furthermore, rebreathing-assisted dual-fuel combustion increases methanol utilization from 52% to 84%, reduces unburned emissions, and maintains comparable engine efficiency. Collectively, these investigations establish practical ignition-assistance strategies that can extend the operating range of future low-carbon compression-ignition engines.
Beyond the experimental investigations, this dissertation introduces a unified framework for quantitatively evaluating ignition-assistance technologies. The proposed Ignition Assistance Number (IAN) relates ignition enhancement to an equivalent cetane number, enabling direct comparison of different ignition-assistance methods on a common reactivity scale. An accompanying Ignition Assistance Effectiveness (IAE) metric further accounts for the energy required to provide ignition assistance, enabling objective assessment of both ignition performance and energetic efficiency. Together, these methodologies provide an easily interpretable framework for benchmarking ignition-assistance technologies across fuels, engine platforms, and operating conditions.
Overall, this research advances the fundamental understanding of ignition-assisted mixing-controlled combustion and provides practical pathways to enable efficient, low-emission operation of heavy-duty CI engines on a diverse range of sustainable, low-carbon fuels. The experimental findings and quantitative methodologies developed in this work provide valuable guidance for the design and optimization of next-generation fuel-flexible compression ignition engines.