There are numerous viscosities of fluids coursing through a car, be it for cooling, lubrication or both. With internal combustion engines only being around 33 per cent efficient, the other 67 per cent is generally wasted through heat energy and noise, all of which has to dissipate into the surroundings in one way or another.
Oil is without doubt the most important fluid contained within a car. The sheer amount of moving parts inevitably transfers into a tonne of friction, which when spawned from metal-on-metal contact can be extremely wearing on the components. Oil is therefore used to lubricate these moving parts and in turn picks up a lot of heat.
As with the vast majority of heat energy created by an engine, it generally needs to be released to the surroundings through some form of heat exchanger. With the water-cooling system there’s the radiator and with the oil system, you use oil coolers.
Resembling miniature cross-flow heat exchangers, oil coolers can be placed in many interesting positions within a car’s body to maximise their cooling efficiency. With oil coursing through predominantly the engine block, the steering system and a turbocharger in turbocharged vehicles, oil can very quickly gain heat, especially during vigorous driving.
So, before the oil enters the sump or oil reservoir to be distributed around these systems, it needs to be cooled so that the oil doesn’t reach an unusable viscosity. Viscosity is a measure of how easily a fluid flows, and as oils lose and gain heat, their viscosities increase and decrease respectively. So a thick, lumpy oil has a high viscosity and a smooth, thin oil flows more easily and therefore has a lower viscosity.
Automotive oils are specially designed to sit within certain ranges of viscosity. Therefore, if too much heat is transferred to the oil, its viscosity decreases to a point where it would struggle to lubricate the required systems properly. So it becomes a balance; you want your oil to be viscous enough to cling to certain gears and other moving parts to keep them lubricated but you also want them to easily flow throughout the oil system to make it around the car’s mechanicals. And with temperature being an important factor in the change in viscosity of oil, cooling becomes an essential process.
The cooling works just like the water-cooling system, with the thermodynamics based upon the heat energy required to be dissipated to the surroundings. The oil enters the oil cooler at a high temperature, circulates through tubes that are lined with heat-releasing fins and then exits the cooler at a colder temperature, ready to recirculate and start the process all over again. Using the basic principles of thermodynamics, the size of the oil cooler needed can be calculated and can be plumbed into a car using extended oil lines to divert the fluid through the heat exchanger.