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Home > Vol 22, No 2 (2026) > Buntekt

Influence of Engine Oil Viscosity on the Combustion and Performance Characteristics of Gasoline Direct Injection Engines

Nuttapon Buntekt, Punya Promhuad, Mongkol Dangsunthornchai

Abstract


The present study examines the influence of lubricating oil viscosity on the combustion dynamics and overall performance of a gasoline direct injection (GDI) engine. Experimental evaluations were conducted using three multigrade oils (SAE 10W-30, 10W-40, and 20W-50) at a constant engine speed of 2000 rpm across low (10 Nm) and medium (50 Nm) load conditions. The rheological analysis confirmed an exponential decrease in viscosity with rising temperatures, with SAE 20W-50 exhibiting the highest flow resistance. Engine performance data revealed that the lowest viscosity variant (SAE 10W-30) minimized frictional losses, thereby maximizing peak cylinder pressure, indicated mean effective pressure, and fuel economy. Furthermore, SAE 10W-30 recorded the optimal brake thermal efficiency (≈25%). Although SAE 20W-50 offered enhanced film strength, it resulted in marginally reduced efficiency. Under medium-load conditions, the thermal reduction of viscosity diminished the performance variations among the tested oils. The study concludes that strategic selection of lower-viscosity lubricants can substantially improve the mechanical efficiency and fuel economy of GDI engines under partial loads, while maintaining adequate component durability.

Keywords


GDI engine; Lubricant viscosity; Combustion performance; Friction losses; Brake thermal efficiency

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DOI: 10.14416/j.ind.tech.2026.08.004

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