Nanotechnologies are interface technologies that contain many different science and application areas such as in- ternal combustion engines. Nanofluids are the most usable nanotechnology in engines to improve their effi- ciency. Three main types of nanofluids; i.e. nano-fuels, nano-coolant and nano-lubricants are used by researchers in engines as consumable liquids. In this review part, recent studies on the nano-lubricants are gath- ered and summarized in main group applications such as engine oil, gear oil and refrigeration system nano- lubricants. Furthermore, the effects of these nano-lubricants on the wears, cold start damage, energy saving and etc. of engines are investigated. It is tried to find suitable nanoparticle additives to have maximum benefits from tribological and thermal efficiencies aspects. So, the effects of nanoparticles type on tribological, thermal and rheological properties of nanolubricants are performed to introduce the most efficient nanoparticles in these applications. As a main outcome, TiO 2 and Al 2 O 3 -SAE40 nanofluids were introduced as the most efficient nano-lubricants due to maximum 86% reduction in coefficient of friction (COF), 29% wear reduction and 51% re- duction of frictional power losses, while Gr had the best BSFC (17%) and emissions (5.42%) reductions, so using hybrid nanoparticles (such as Al 2 O 3 -Gr) maybe fill this gap which is ignored.

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