What is the function of a thermostat in a liquid-cooled engine?

• A. To increase coolant temperature rapidly.
• B. To regulate coolant flow based on temperature to maintain operating range.
• C. To cool the coolant before it enters the engine.
• D. To filter the coolant.

Answer: (B)

The main idea is that a thermostat acts as a temperature-sensitive valve that regulates coolant flow to keep the engine within its optimal operating temperature range. When the engine is cold, the valve stays mostly closed so the coolant stays circulating through the engine to warm up quickly. Once the coolant reaches the set temperature, the valve opens to allow coolant to flow to the radiator to be cooled and then return to the engine. This keeps engine temperature within a narrow band for efficient operation and longevity. It doesn’t actively heat the coolant, it doesn’t cool it before it enters the engine, and it doesn’t filter the coolant.