A tremendous amount of heat is generated in engines used in todays vehicles. A gasoline engine’s combustion chamber can reach temperatures above 2500°F. Approximately a third of the combustion heat is actually used to power the vehicle; another third passes through the exhaust; and the last third is the responsibility of the cooling system. The heat has to be reduced as high temperatures can damage parts of the engine. The cooling system’s job is to maintain consistent and even temperature throughout the vehicle’s operating range. It does this by means of water jackets, a water pump, thermostat, coolant, fan, hoses and finally a radiator, which exchanges surplus heat with ambient air.
Cooling System Flow
The water jackets are cast as part of the engine block and heads and consist of passages to allow the coolant to flow around each individual cylinder in the block and head. The water pump is used to pressurize and circulate coolant through the water jackets and is usually located on the front of the engine block. Water pumps are generally belt driven and some may be driven by the timing belt or timing chain. Radiator hoses connect the engine to the radiator. The radiator, which is usually made of aluminum, is a series of tubes connected to end tanks. The tubes have fins attached to them. As coolant flows through the radiator, airflow through the radiator removes some of the heat from the coolant. The coolant then returns to the engine to start again. While airflow over the radiator is usually adequate while the car is in motion, idling or driving at low speed reduces airflow, so fans are used. There are a few different types of fans. Some are mounted on the water pump shaft. These either turn at water pump speed or have thermostatically controlled clutches that allow thermostatic control of the fan. For increased efficiency, a shroud, a large duct that directs the airflow through the radiator, often surrounds the fan. On most newer vehicles, and all cars where the engine is mounted transversely, electric fans are mounted directly on the radiator and controlled by electric thermoswitches or by the Powertrain Control Module (PCM) or Engine Control Module (ECM).
A thermostat is installed where the coolant leaves the engine. The thermostat’s function is to allow the engine to come to operating temperature and to maintain a minimum temperature. It remains closed until a specific temperature is reached. This allows the engine to warm up as fast as possible. The thermostat then opens and allows coolant to circulate to the radiator and cool the engine. When the thermostat is closed, coolant may be recirculated through a bypass system made up of hoses or coolant passages. Thermostats have different heat ranges, so it is important to use the correct thermostat for the application. Correct operating temperatures are critical in computer controlled vehicles to reduce emissions, improve gas mileage and prolong engine life. One last note on thermostats would be that unless a thermostat has failed to open, it will never be the cause of an engine overheating problem.
Cooling System Operating Temperature
The maximum operating temperature of an engine is designed into the cooling system itself. The boiling point of the coolant rises when the system is under pressure, so modern engines use pressurized cooling systems. These systems are at normal atmospheric pressure when cool, but because they are sealed, the pressure increases as the temperature rises. The system is not completely sealed, however. The radiator cap is vented with a pressure relief valve that keeps the system from exploding if the pressure builds too high. When the relief valve opens it will allow coolant to flow into an overflow container. As the engine cools, the coolant is drawn back into the radiator from the overflow by vacuum created within the radiator. Typical release pressure is 16 psi. The cap should not be removed when the engine is hot, to avoid getting burned by coolant that is under pressure. Unless the system has been opened for service, or a large amount of coolant has been lost, the fluid level is usually checked and adjusted in the overflow container. Overheating essentially just means that the cooling system is taking on more heat than it can dissipate. When coolant takes on too much heat, it boils. This means that the water jacket surfaces will be covered with more steam than coolant. Steam can’t absorb heat like coolant can, and this exponentially exacerbates the problem: more heat results in more steam, which leads to less effective coolant, and, in turn, more boiling of coolant. Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport and dissipate heat. Some causes of overheating are:
- Low coolant level, loss of coolant, or insufficient coolant capacity
- Buildup of deposits that cause poor conduction of heat into the cooling system
- A thermostat that won’t open
- Poor airflow through the radiator
- Damaged or worn fan clutch, or broken fan
- Collapsed radiator hose
- Loose or defective water pump impeller
- Defective radiator cap
- Late ignition timing
Coolant & Cooling System Service
The coolant is usually a 50/50 mixture of water and ethylene or propylene glycol. The vehicle manufacturers designate the type of antifreeze required by each engine. Preventive maintenance on the cooling system should be done on a regular basis. Following the maintenance schedule on the vehicle offers natural add-on jobs that protect the customer’s vehicle and offers the service consultant sales opportunities. The extended change times that have been part of normal service maintenance schedules have been found to be detrimental to many engines, requiring all service technicians to test coolant on a regular basis after the first 3 years in service.