In a closed cooling system, what is circulated through the engine?

• A. Fresh water
• B. Sea water
• C. Oil
• D. Antifreeze

Answer: (A)

In a closed cooling system, the liquid that flows through the engine is the coolant mix, typically antifreeze watered to an appropriate ratio. This coolant stays in a sealed loop that runs through the engine jackets, absorbing heat. Antifreeze is needed because it prevents freezing in cold conditions, raises the liquid’s boiling point to avoid overheating, and includes corrosion inhibitors to protect the engine metal. Sea water or fresh water wouldn’t be used inside the engine loop because sea water is highly corrosive and can cause deposits, while plain water can freeze or boil off and lacks the protective additives. Oil isn’t used for cooling the engine. The engine coolant is cooled elsewhere by a heat exchanger that uses a separate cooling medium (often sea water), but what circulates through the engine itself is antifreeze.