Base fluid and the impact on cooling system performance
The performance of a cooling system is to a significant extent dependant on the performance of the coolant used inside of it. Base fluids form the bulk of a coolant composition with typically more then 95% of the coolant being base fluid.
The term base fluid is widely used, but somewhat confusing. Often base fluid refers to the freezing point depressant used, like MEG, MPG, glycerin or a salt. A more scientifically correct interpretation of the term “base fluid” is to call the base fluid the matrix of the coolant which main purpose is to influence the so-called “colligative” properties of a coolant and to keep all other additives in solution or dispersion/emulsion.
Colligative properties are properties of a solution that are ideally only dependant on the molality of the solute and a material property of the solvent. The best known colligative properties are: lowering of freezing point, increase of boiling point and reduction of vapor pressure with increasing solute content. To grasp this concept one needs to consider the freezing point depressant to be the solute and water to be the solvent.
From this definition some critical properties of a base fluid become immediately apparent. Cooling system performance will only be satisfactory if freezing points are sufficiently low and boiling points sufficiently high. A too high vapor pressure will lead to excessive vapor loss in its turn. To assure this sufficient amounts of freezing point depressant will be needed depending the operational conditions and limits of the cooling system.
As the molecules or ions of different freezing point depressants are quite different in size, shape and interactions with water; identical colligative properties (e.g. identical freezing point depression) will still result in significantly different thermophysical properties of the coolant solution. The best known are density, thermal conductivity, heat capacity and viscosity, but less obvious characteristics like surface tension and concentration gradients on superheated surfaces are also affected. As a consequence base fluids tend to significantly impact the thermal management of the cooling system and even the total engine platform. Poor thermal management will affect cooling system performance.
As mentioned above the second important function of the base fluid is to dissolve and stabilize the other additives inside the coolant. Depending the chemical nature of the additive, solubility and stability can be impacted by the choice of the base fluid. By influencing the stability of an additive package, a base fluid can indirectly have a big impact on cooling system performance. Base fluids influence cooling system performance in even more complex ways, mainly by modulating the intensity of the action of certain additives (e.g corrosion inhibitors, stabilizers, sequesterants and defoamers). This implies that a proven performance of a certain additive package in one base fluid is no guarantee for identical performance in another base fluid.
Finally is also worth mentioning that base fluids and dosage of freezing point depressant have a big impact on the electrical properties of a coolant solution, properties that might become more and more important for the performance of e-vehicles cooling architectures.