Carbon nanotubes as biofuel additives enabling advanced combustion modulation strategies

This study investigates the effect of carbon nanotubes (CNTs) as ethanol fuel additives in a new class of bionanofuels for internal combustion and jet engines. In-cylinder pressure and thermodynamic behavior, including the heat release rate (HRR), were analyzed using a rapid compression machine (RCM) for pure ethanol, ethanol enriched with CNTs (2.5 mg/L), ethanol containing spherical gold nanoparticles (AuNPs, 0.01 g/L), and gasoline. CNT-enriched ethanol reduced maximum in-cylinder pressure by up to 13 % compared to pure ethanol and lowered the peak HRR by up to 43 % at low injection doses. The flame brightness, quantified by high-speed imaging, was reduced by over 50 %, indicating suppressed soot formation. AuNPs exhibited an opposite trend: after a short ignition delay, they increased HRR by up to 28 %, attributed to catalytic activity. CNTs, in contrast, acted via physical thermal dissipation, i.e., cooling of hot-spots through axial conductivity, and scavenging reactive free-radicals. Optical analysis also showed a smaller flame area and slower propagation speed for CNT fuels. These findings highlight the dual benefit of CNTs: enabling smoother combustion with reduced NO x and particulate precursors, and oxidizing cleanly in-cylinder, offering a scalable, environmentally favorable approach to combustion control in next-generation bionanofuels. • CNTs added to ethanol produce smother combustion. • CNTs reduce ethanol flame brightness as observed in the rapid compression machine. • High heat transfer along a CNT may suppress in-cylinder hot-spot formation. • Eventual in-cylinder oxidation of CNTs at the late combustion stage may produce CO 2 . • Tailored hierarchical nanocarbons may allow for selective control of the combustion process.