The Effect of Adding Coolant to Engine Oil on Its Dynamic Viscosity

The genesis of the conducted research was the intention to determine how the rheological properties of engine oil change upon the addition of a coolant. The aim of the study was to assess the effect of coolant addition to engine oil on its dynamic viscosity. The experiments were carried out for six different fresh engine oils and six of the same oils sampled during routine oil replacement after a vehicle mileage of 10,000 km. Two oils were selected from each of the three primary categories: synthetic, semi-synthetic, and mineral oils. Then, mixtures of engine oils with the addition of coolant were prepared at 0, 5, 10, 20, 30, 40 and 50% (v/v), respectively. The study involved measuring the dynamic viscosity of the samples at 1000 s−1, covering temperatures from 0 to 50 °C. The tests were conducted using a measurement setup equipped with a RHEOLABQC rotational rheometer manufactured by Anton Paar GmbH (Ostfildern, Germany). To investigate the temperature dependence of dynamic viscosity, a GRANT thermostatic bath was coupled with the rheometer. The studies demonstrated that dynamic viscosity strongly depends on temperature, as well as on the type and condition of the engine oil. At 0 °C, the dynamic viscosity of fresh oils ranged from approximately 500 to 700 mPa·s, whereas at 50 °C it decreased to approximately 100 mPa·s. The addition of 5% (v/v) coolant to engine oil resulted in only a slight change in dynamic viscosity. In contrast, a substantial decrease in dynamic viscosity was observed when 50% (v/v) coolant was added to the tested engine oils. The results indicate that coolant, which may enter the engine oil in the event of an engine failure, can significantly deteriorate the rheological properties of this lubricant.