What happens when a car's engine gets too hot? Well, lots of things, and none of 'em good. Thermal breakdown is among the most damaging effects, and occurs when a car's internal heat causes a chemical reaction in the motor oil, which causes the oil's viscosity to change. Basically, if (or more accurately, when) the engine heats up beyond a certain temperature threshold, the motor oil will start to degrade.
The viscosity (also referred to as the oil's "weight") is crucial to ensuring the oil flows smoothly while properly coating the metal engine components to prevent metal-on-metal contact. The viscosity change caused by thermal breakdown results in decreased oil flow, which can eventually lead to increased oil consumption, deposit buildup and damage to the engine's metal surfaces. (Although the condition is most commonly discussed with regards to motor oil, other oils, such as transmission fluid, are also susceptible to thermal breakdown.)
Thermal breakdown also causes the oil's additives to dissipate or wear out more quickly. It results in the formation and release of gases, acids and other harmful insoluble compounds that can cause damage as they circulate through the engine. The oil's surface tension will also degrade, though at a slower rate than viscosity. A lower surface tension negatively affects the oil's ability to coat and lubricate metal surfaces.
Petroleum-based motor oils are extremely sensitive to temperature changes, and the less refined the oil is, the more quickly it will succumb to thermal breakdown. The naturally-occurring compounds that increase vulnerability to thermal breakdown are the same ones that make an oil susceptible to oxidization (when ingredients in motor oil come into contact with oxygen in the engine, causing a chemical reaction that results in harmful acid buildup). The more an oil can be refined, the more it can resist these weaknesses, but that comes at a cost. In the refinery, the crude oil is boiled to separate it into different parts, and then filtered and processed. Each time an oil is distilled, the price rises, but although the oil gets cleaner with each process, there is no way to remove all the harmful compounds that naturally occur in crude oil. They will still exist, but in very small quantities. So even though a highly refined petroleum-based oil will stay stable longer, it will still eventually succumb to a reaction that causes a breakdown. Synthetic oils are engineered to withstand a broader temperature range and are much more resistant to thermal breakdown than their crude-based counterparts.
Standard passenger cars should be reasonably protected against thermal breakdown as long as the oil is changed regularly. Some engines run hotter, though, and have a lower tolerance for temperature-induced damage. High-performance engines should always be lubed up with a high-quality oil to prevent thermal breakdown. The hotter the engine's temperatures, the more drastically and quickly an oil will degrade, but higher quality oils have better resistance to temperature-induced changes. However, the best step to prevent thermal breakdown is following the car manufacturer's recommended oil change interval.
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