In order to ensure the longevity of an engine, and to avoid catastrophic damage during periods of heavy load, its oil must be kept at a reasonable temperature.
This is achieved, in the most simple of fashions, by ensuring that adequate airflow passes over the engine's sump while the car is being driven; this cooling air maintains the temperature of the oil at the desired point, aided by the engine's own cooling system if it has one.
When the vehicle is driven particularly hard - such as on a track - then this cooling may not be sufficient, resulting in excessive oil temperatures. Other factors may also increase the oil temperature to dangerous levels, such as high ambient temperatures, increased maximum engine speed or aftermarket modifications that cause hotter running.
As the temperature of the engine oil increases, its protective qualities and viscosity begin to fall. This causes premature wear and a drop in oil pressure, ultimately leading to a terminal engine failure. Consequently, oil-to-air coolers or oil-to-water coolers may be used to keep the temperature of the engine oil in the optimum range.
The term 'oil cooler' is primarily used when discussing the cooling of engine oil, circulated by the engine's own oil pump. Similar systems, which use their own pumps, are also used to cool the fluid of other key assemblies. These set-ups include transmission coolers, power steering coolers, differential coolers and fuel coolers.
How do oil-to-air coolers work?
This type of oil cooler is a straightforward, effective way to regulate oil temperature. In such a configuration, the engine's oil pump circulates oil through a cooling loop which contains a heat exchanger. Ambient air passes through the cooler, lowering the temperature of the oil, and the cooled oil returns to the engine.
This set-up is often employed in aftermarket installations, as it's comparatively easy to fit - typically requiring just a mounting point for the heat exchanger and the use of an adaptor that fits onto the engine's oil filter housing. The adaptor creates an inlet and outlet, which can be plumbed into the heat exchanger, for the necessary circulation of oil.
There are a few downsides to using an oil-to-air cooler outside of the additional cost and packaging challenges. Most prominently, the heat exchanger itself can easily be damaged by debris. This can lead to a loss of oil and pressure, ultimately resulting in engine damage unless noticed. Similarly, because there are more connections and pipework in an oil cooling system, the chance of leaks also increases.
Additionally, if the cooler and associated oil lines are improperly specified and reduce the flow of oil, the fitment of an oil cooler may cause a reduction in oil pressure. If suitable fittings and heat exchangers are chosen, however, there should be a negligible change in oil pressure.
What about oil-to-water coolers?
These heat exchangers function in much the same way as water-to-air intercoolers, which are often referred to as 'charge coolers'. Oil is pumped into the core of the cooler, while coolant flows through passageways surrounding the core - removing heat from the lubricant and transferring it to a radiator. Typically, it is engine coolant that is pumped through the heat exchanger.
These systems are more complicated and expensive but they are also more compact, making them easier to install, and they aren't prone to damage. Attention has to be paid to ensuring that the coolant temperature is lower than the oil temperature, however, for it to have the desired effect - and, should the cooling system be at its limits, the additional thermal load may cause the water temperature itself to rise to excessive levels.
Some engines may also feature a small oil-to-water heat exchanger on the block. As the coolant warms up more quickly than the oil, it can be circulated through the heat exchanger to help bring the oil up to temperature more quickly. It can then also help maintain the temperature of the oil but, should the oil start heating up excessively, it will then usually be redirected through a larger external cooler.
How is the desired temperature maintained?
Both configurations can feature an 'oilstat', which is a thermostatic valve that opens at a specified temperature to begin circulating oil around the cooling loop. This thermostatic control primarily helps avoid overcooling of the oil, which can cause gradual degradation of its lubrication qualities - by allowing for fuel and other contaminants, such as water, to build up in it.
Using thermostatic control also helps the engine oil to warm up more quickly, as it avoids it being unnecessarily circulated through the cooler when cold.
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