The cooling circuit includes all thermally-stressed areas of the engine. The coolant circulating in the cooling circuit dissipates the heat produced during combustion and ensures a constant temperature level in the engine. In order to maintain the operating temperature as quickly as possible after starting, the cooling performance during the engine's warm-up phase must remain as low as possible. For this, a small cooling circuit is sufficient. A large cooling circuit is required only after reaching the operating temperature.
The control of the two circuits is provided by a thermostat which switches from the small to the large circuit at 80 to 90°C. It’s important that the cooling circuit is designed to ensure this constant temperature even at peak loads and with extreme ambient temperatures.
Nowadays, a complex mixture of water and various additives is used as a coolant, which not only reduces the freezing point of the cooling liquid, but also increases the boiling point to 120°C. Added inhibitors have the task of preventing corrosion on the various metals which the cooling liquid flows around, e.g.: aluminium cylinder heads.