Car owners are advised to change their engine oil every 3000-5000 miles. But what is actually happening to the oil over time in the engine? We have heard about the benefits of antioxidants in the body. Antioxidants also help engine oil. In use, engine oil is subjected to high temperatures and high shear stresses which can act to break apart the oil molecules, affecting its viscosity and performance. This chemical reaction occurs through the generation of free radicals, or unpaired electrons on the oil molecules. By monitoring the free radical content, it is possible to monitor the change in the oil.
CPG used electron spin resonance spectroscopy to monitor the change in oil in a 6 cylinder gasoline engine, taking samples every 500-1300 miles. Free radical production started around 400 miles, and changed as the free radicals were either stabilized by the antioxidant package in the oil, or reacted to form other radical species. This method is useful for monitoring the chemical changes in the oil, and to see the efficacy of the additives package.
Read the application note for more details.