Viscosity

Viscosity is often referred to as the thickness of a fluid. You can think of water (low viscosity) and honey (high viscosity). In many cases, this might be a correct way of thinking; however, it is clearly oversimplified. At a molecular level, viscosity is a result of the interaction between the different molecules in a fluid. A large viscosity means that a liquid will flow very slowly whereas a small viscosity means that the liquid will flow quickly. It is a property of materials and as such varies between different fluids and gasses.

The importance of viscosity for a lubricant is a given and the classification of the different lubricants is based on this crucial parameter. Lubricants are used to separate moving parts in a system. They reduce friction, surface fatigue, heat, noise and vibrations. Lubricants are able to do this by forming a physical barrier between moving parts. Corresponding the viscosity is an important indicator for this property.

There are two types of viscosity, dynamic or absolute viscosity and kinematic viscosity.

• Dynamic is defined as a quantity describing a fluids resistance to flow and mathematically as the ration of the shearing stress to the velocity gradient in a fluid. Its SI unit is the Pascal second [Pa.s]. And it is represented by the Greek letter η (eta);

• Kinematic is defined as a measure of the restrictive flow of a liquid under the force of gravity and mathematically as the ration of the viscosity of a fluid to its density. Its SI unit is the square metre per second [m2/s]. It is represented by the Greek letter ν (nu)

There are multiple factors that affect viscosity. The main two are pressure and temperature, with temperature causing the most noticeable and dramatic changes to viscosity. The viscosity of simple liquids (Newtonian fluids) decreases with increasing temperature and increases under very high pressures. Non-Newtonian fluids can become thicker or thinner when stress is applied. Some examples of those none-Newtonian fluids are Ketchup or cream as they behave different depending the force added to them.

For coolants, viscosity has also its importance and this in multiple respects. On one hand, the viscosity will be important to lubricate the water pump seal and on the other hand, it will have a direct impact on the heat transfer characteristics of our coolants. Surely, the untrained person underestimates the importance of viscosity on heat transfer in cooling systems.

A faster moving fluid (lower viscosity) will be capable to take away the heat easier. In addition, it will go faster in its turbulent regime and hence the heat transfer characteristics increase with reducing viscosities. So low viscosity of a fluid helps in improving the heat transfer characteristics.

In applications where cold is transported and the liquid operates at lower temperatures, engineers have a strong preference for low viscosity fluids as next to the heat transfer improvements the energy cost to pump around the liquid increases with increasing viscosity.

In on-road applications and high temperature applications, the added value of having a lower viscous fluid is less pronounced as at the operating temperatures most of our fluids like MEG and MPG dilutions show similar viscosities. Still, the selected base fluid affects the energy losses needed to pump the fluid around.

Despite the fact that the engineers want a maximum heat transfer during operations, a lower heat transfer is preferred during cold start. This reduced heat transfer can also be accommodated by increasing the viscosity. During the cold start up cycle of operation, a fast heating of the engine is preferred. Removed heat will result in a longer time to get the engine at optimum operating temperature conditions. The faster the engine warms up the better the fuel economy and the les polluting emission are generated. A similar effect can be achieved by isolating the engine during start-up phase and circulate the cooling when the engine is at the appropriate temperature.

One can understand that the viscosity (at different temperatures) is important also for a coolant and that during product design attention need to be paid to understand the detailed viscosity needs of the customer’s application.
 

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