Acids are chemical compounds that play a major role in automotive systems. The most common automotive acid is sulfuric acid, which is mixed with water to make the electrolyte fluid in lead-acid car batteries. This acid reacts with lead plates to store and release electrical energy, allowing the starter motor to turn the engine.
Acids also form inside engines as a byproduct of combustion. When fuel burns, it creates sulfur and nitrogen oxides. If these gases blow past the piston rings, they mix with moisture in the oil to form acids. Left unchecked, these acids will eat away at brass bushings and steel crankshaft journals.
Engine oils contain detergents called Total Base Number (TBN) to neutralize these acids. As you drive, these detergents get used up. Regular oil changes are needed to refresh the TBN levels, preventing acid corrosion from damaging internal engine parts.