Ethanol–Hydrogen Reactivity Management for High-Efficiency, Low-Emission Reactivity-Controlled Compression Ignition Engines: A Systematic Review of Combustion, Control, and Life Cycle Impact

The increasing efforts to decarbonise the energy sector have made it possible to reconsider advanced combustion modes that could simultaneously increase engine efficiency and meet stringent emission regulations. Reactivity-controlled compression ignition (RCCI) has emerged as a strong candidate due to its dual-fuel approach, which enables flexible control over in-cylinder reactivity and heat release patterns. Ethanol and hydrogen have recently attracted attention as a complementary low-reactivity and high-reactivity fuel pair within RCCI systems, typically implemented in a tri-fuel configuration using a small diesel pilot for ignition control. Therefore, most practical implementations operate as ethanol–hydrogen–diesel RCCI systems rather than pure dual-fuel ethanol–hydrogen modes. Research published between 2020 and 2025 provides a clearer picture of how these two fuels behave when used together in RCCI engines. Most studies report a noticeable improvement in the brake thermal efficiency of 4–7%. Particulate matter emissions reduce substantially from 20% to 50%. Lower carbon monoxide and hydrocarbon levels are often reported, and usually, a stable ignition is found throughout a wide range of operating conditions. However, if the combustion phasing is not properly controlled, hydrogen’s reactivity can lead to increased nitrogen oxide emissions, thus making it necessary to recirculate exhaust gases. Besides the challenges of combustion, practical aspects still remain as major hurdles. The problems of material compatibility between two fuels, hydrogen storage safety, and the requirement for low-carbon fuel production pathways can play a vital role in deciding commercialisation. To summarise, research findings point to the ethanol–hydrogen RCCI combination as a very promising route for the improvement of cleaner and more efficient engine technologies, provided the technical and logistical barriers can be addressed. Accordingly, this review primarily addresses ethanol–hydrogen–diesel tri-fuel RCCI architectures, while also discussing dual-fuel ethanol–hydrogen concepts where applicable in order to avoid conceptual overlap with spark-ignited ethanol–hydrogen systems.