Engine cooling — purpose of a thermostat in the coolant circuit In an automotive liquid-cooling system, the thermostat's main function is to:

Introduction / Context:
Thermostats improve engine efficiency, emissions, and durability by regulating coolant flow. They help achieve and maintain the optimal operating temperature.

Given Data / Assumptions:

• Wax-pellet thermostat with a set opening temperature (e.g., 85–95°C).
• Pressurised cooling system with a separate radiator cap.
• Coolant temperature sensor and gauge are independent components.

Concept / Approach:
When cold, the thermostat stays closed, restricting flow to the radiator and recirculating coolant within the engine. This reduces warm-up time. As temperature rises to the set point, the thermostat opens progressively, allowing heat rejection through the radiator and stabilising temperature.

Step-by-Step Solution:
On cold start: thermostat closed → quick warm-up.At operating temp: thermostat modulates flow to maintain setpoint.Therefore, its primary purpose is to allow the engine to warm up quickly and then regulate temperature.

Verification / Alternative check:
Cooling systems without thermostats suffer from excessive warm-up time and unstable temperatures, harming fuel economy and wear.

Why Other Options Are Wrong:

• Prevent boiling/pressurise system: handled mainly by radiator cap and system design.
• Indicate temperature: that is the sensor/gauge function.

Common Pitfalls:
Assuming thermostat controls pressure; it only controls flow using temperature-sensitive actuation.

Final Answer:
allow the engine to warm up quickly