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The diversified forms of heat engines necessitate developing energy‐efficient engine oils and coolants. The article proposes a method of thermal diffusivity tuning of the base fluid (ethylene glycol) using multiwalled carbon nanotubes (MWCNTs), annealed at different temperatures, without altering the solid volume fraction. Having studied the annealing‐induced structure modifications of MWCNTs through thermogravimetric and spectroscopic studies, the thermal diffusivity variations are investigated through the sensitive photothermal mode‐mismatched dual‐beam thermal lens technique. The study reveals that the annealing‐induced structure modifications lower the nanofluid's thermal diffusivity and unveils the existence of transition annealing temperature of MWCNTs above which the nanoparticles can make the nanofluid a heat trapper. The study also proposes the importance of making more energy‐efficient engine oils by keeping the temperature of the engine oil below the transition temperature. [ABSTRACT FROM AUTHOR]

Ageing of engine oil is an important issue determining the engine life and performance. The present work attempts to delineate the ageing-induced changes in engine oil through the mode-mismatched dual-beam thermal lens (MMDBTL) technique and other conventional spectroscopic techniques. For the analyses, engine oil samples were collected after every 200 km of runtime. As the thermal diffusivity is related to the nonradiative deexcitation upon optical absorption, comprehensive radiative and nonradiative analyses were carried out. The Ultraviolet-Visible, Fourier transform infrared, and Nuclear magnetic resonance spectroscopic analyses point to the structural modification as a result of the breaking of the long-chain hydrocarbons into ketones, aldehydes, esters, and other compounds. This modifies the absorption pattern, which can also be understood from the nonlinear refractive index study using the Z-scan technique. The compositional variations associated with the degradation upon ageing, the length of the hydrocarbon chain, and the formation of newer molecules account for the enhancement of the thermal diffusivity revealed through the MMBDTL techniques. The complementary nature of the radiative and nonradiative emission is understood from the fluorescence study. Thus, the study reveals the possibility of thermal diffusivity measurement as an effective tool for the quality monitoring of engine oil. [ABSTRACT FROM AUTHOR]