With the ever-increasing demand for cleaner combustion, researchers have turned their attention to innovative solutions, and carbon nano additives (CNAs) have emerged as a promising candidate. The physicochemical properties of CNAs such as high thermal conductivity, catalytic reactivities, and high specific surface area, make them an attractive choice for improving fuel combustion in diesel engines. This review critically and comprehensively examines the utilization of various tested CNAs for the first time, including single wall and multi wall carbon nanotubes (CNTs and MWCNTs), graphene oxide (GO), graphite, carbon nano particle (CNP), carbon quantum dot (CQD), and graphene quantum dot (GCDs) as fuel additives in diesel engines. It explores their effects on fuel properties, combustion characteristics, engine performance, and emissions. These additives have been tested to analyze the combustion behaviors of several fuel blends, including biodiesel/diesel blended with alcohols, water, diethyl ether (DEE). The pros and cons of using CNAs in diesel engines are thoroughly scrutinized. Among fuel properties, the cetane number and viscosity were most improved. CNAs have the potential to enhance overall performance and emissions concurrently. However, GO and MWCNTs demonstrated the greatest simultaneous improvement in both engine performance and emissions. Most CNTs and MWCNTs showed improvements in BSFC and BTE. All studies unanimously indicated that GO could improve BSFC and BTE parameters. Nonetheless, the most troublesome parameter was the NOx pollutant, which most additives were unable to improve. The characteristics of fuel combustion were significantly enhanced using hybrid fuel of CNAs and metal-based additives. Future studies should focus on unsteady condition, optimum combustion condition, the stability of the blends, and identifying the best base fuel for the additives.
