Liquids with stable suspensions of nanoscale materials are defined as nanofluids. As reported in recent scientific literature, a very small amount of suspended nanostructures has the potential to enhance the thermo physical, transport and radiative properties of the base fluid. One of the main applications of this technology is in the field of combustion and fuels. In fact, adding nanomaterials (such as metals, oxides, carbides, nitrides, or carbon-based nanostructures) to liquid fuels is able to enhance ignition and combustion.The focus of this research is to gain a fundamental understanding of the characteristics of a nanofluid fuel prepared using carbon nanoparticles (CNPs) and multi-walled carbon nanotubes (MWCNTs) dispersed in butanol.This study starts with the investigation of the optical properties of the mixtures. The transmission spectra of the nanofluids are measured in a wide wavelength range from UV (250 nm) to near IR (800 nm). Then, the fuel spray delivered using a port fuel injection system was characterised in quiescent conditions by high spatial resolution visualization. The spray macroscopic characteristics were not affected by the nanostructures addition to the fuel.