Cooling, Lubrication and Electrical Systems

The Engine Cooling System

Internal combustion generates massive amounts of thermal energy. The primary purpose of an engine cooling system is to remove excess combustion heat and maintain an optimal operating temperature. Without a functioning cooling system, metal parts expand excessively, lose structural integrity, and seize. To manage this heat, engine coolant circulates throughout the block and cylinder head. This liquid coolant is typically a 50/50 mixture of distilled water and ethylene glycol antifreeze. Pure water transfers heat well, but the addition of ethylene glycol antifreeze raises the boiling point and lowers the freezing point of the mixture, protecting the engine across extreme seasonal temperatures.

A water pump moves coolant through passages and hoses; depending on the vehicle, the pump may be driven by the engine or powered electrically. A water pump circulates coolant through the engine block, cylinder head, and radiator. The water pump is typically driven mechanically by a serpentine belt, a timing belt, or a timing chain. As coolant absorbs heat from the cylinders, flexible radiator hoses carry the hot fluid toward the radiator while allowing for normal engine movement and vibration. The radiator functions as a heat exchanger, releasing heat from the hot coolant into ambient air passing through its exterior fins. Cooling fans draw or force air through the radiator when needed, especially at idle or low speeds; they may cycle on and off while the vehicle is stationary.

Pressure plays a critical role in controlling coolant temperatures. A pressure cap on the radiator or coolant reservoir seals the cooling system and helps raise the coolant's boiling point. Increasing cooling-system pressure raises the coolant's boiling point by a common rule of thumb of about 3°F per pound per square inch (psi), although the exact increase varies. For example, if a pressure cap maintains 15 psi of pressure, you can quickly estimate the boiling point increase in your head without a calculator: 15 psi multiplied by 3°F per psi equals an increase of roughly 45°F above the liquid's normal atmospheric boiling point. In a system equipped with a radiator cap and overflow reservoir, a spring-loaded valve in the cap lets expanding coolant flow into the reservoir when internal pressure rises too high.

Temperature regulation is governed by the thermostat, a temperature-sensitive valve that controls coolant flow between the engine and radiator; its exact location varies by engine. When starting a cold engine, a closed thermostat keeps coolant from flowing through the radiator, helping the engine warm up rapidly to its optimal efficiency. During this warm-up stage, coolant can still circulate through an internal bypass and the heater core. The heater core is a small, radiator-like heat exchanger inside the passenger compartment that provides cabin heat. Once the engine reaches its rated operating temperature, the thermostat opens to allow coolant to enter the radiator for continuous heat dissipation.