Your engine is a controlled explosion chamber. It gets hot. This is normal. However, when heat exceeds design limits, things can quickly deteriorate. The quietest and most expensive failure is not a blown gasket or a cracked cylinder block. This is the thermal degradation of engine oil.
Thermal degradation occurs when internal heat causes a chemical reaction in the lubricant. This reaction changes the viscosity of the oil. Viscosity is just a fancy word for thickness or “weight”. This determines how well the oil covers the metal parts and how well it protects them from wear. When the oil deteriorates due to heat, its viscosity changes. Flow drops. The protection disappears.
It’s not just about diluting the oil. This is a series of failures. When the flow rate decreases, oil consumption increases. This causes sludge and deposit to build up. metal-on-metal wear, leaving scratches on the surface of the engine. We usually talk about engine oil, but transmission fluid also suffers the same fate.
Chemistry of Failure
Heat doesn’t just thin the oil; breaks the chemistry down. Additives wear out faster. Gases, acids and insoluble compounds are released into the sump. These contaminants circulate inside the engine, causing corrosion and wear.
The surface tension also decreases. It happens more slowly than the drop in viscosity, but is more important. Low surface tension means that the oil does not adhere effectively to metal surfaces. Lubrication failed. Friction spikes. The heat increases even more. This is a vicious cycle.
Oil vs. Synthetic Oil: Who Wins?
Petroleum-based oils are sensitive. Very sensitive. The less refined the crude oil, the faster it succumbs to heat. The same natural compounds that make base oils more susceptible to thermal breakdown also make them more susceptible to oxidation. Oxidation occurs when oil components react with oxygen in the engine. This produces harmful acids.
Refining helps. Crude oil distillation removes impurities. Each step refines the oil and increases its price. But not all bad things can be removed. Even highly refined oil eventually breaks down. It’s just a matter of time.
Synthetic oils are different. They are engineered. Withstands a wider temperature range. They are more resistant to thermal breakdown than crude oil. If you are going to abuse your engine, synthetics will extend its life.
Prevention and Limits
If you change the oil on time, your passenger cars is well protected. However, some engines operate at higher temperatures. High-performance blocks withstand less thermal damage. A high-quality lubricant is required. The higher the engine temperature, the faster the oil degrades. Better oils can withstand this change.
But what is the best preventative measure? Follow the oil change intervals recommended by the manufacturer. Don’t guess. Don’t stretch.
Temperature Thresholds
How hot is too hot? Conventional motor oil can typically withstand temperatures of up to 250 degrees Fahrenheit. When it reaches 275 degrees Fahrenheit, the oil begins to break down. This is the critical point where chemical stability breaks down.
Frequently asked questions
What exactly is thermal breakdown?
The oil deteriorates due to excessive internal heat. The heat causes a chemical reaction that changes the viscosity of the oil and breaks down the additives.
What is viscosity breakdown?
It’s the same phenomenon. As the temperature rises, the viscosity of the oil changes. This reduces flow and causes poor lubrication throughout the engine.
At what temperature does oil break down?
Conventional oil begins to break down at about 275 degrees Fahrenheit. It can withstand temperatures of up to 250 degrees before it begins to deteriorate significantly.
Are there different types of thermal breakdown?
No. The mechanism is the same regardless of the type of oil. Heat can cause damage and changes in viscosity. Speed and severity depend on the composition and degree of refining of the oil.
The engine doesn’t care about your schedule. It cares about heat. If the oil is not changed, the heat will take over.
