An automotive cooling system protects an engine from catastrophic failure (e.g., blown head gaskets, and warped or cracked cylinder heads or cylinder blocks ) due to overheating. Basically, it consists of the following:
- A series of galleries cast into the engine block and cylinder head, surrounding the combustion chambers with circulating coolant to carry away heat.
- A mixture of water and antifreeze in proportions appropriate to the climate. Antifreeze itself is usually ethylene glycol or propylene glycol (with a small amount of corrosion inhibitor).
- A radiator, consisting of many small tubes equipped with a honeycomb of fins to dissipate heat rapidly, that receives and cools hot liquid from the engine.
- A water pump, usually of the centrifugal type, to circulate the coolant through the system.
- A thermostat to control temperature by varying the amount of coolant going to the radiator.
- A fan to draw cool air through the radiator.
The radiator transfers the heat from the fluid inside to the air outside, thereby cooling the fluid, which in turn cools the engine. Radiators are also often used to cool automatic transmission fluids, air conditioner refrigerant, intake air, and sometimes to cool motor oil or power steering fluid. Radiators are typically mounted in a position where they receive airflow from the forward movement of the vehicle, such as behind a front grill. Where engines are mid- or rear-mounted, it is common to mount the radiator behind a front grill to achieve sufficient airflow, even though this requires long coolant pipes.
Automobile radiators are constructed of a pair of metal or plastic header tanks, linked by a core with many narrow passageways, giving a high surface area relative to volume. This core is usually made of stacked layers of metal sheet, pressed to form channels and soldered or brazed together. For many years radiators were made from brass or copper cores soldered to brass headers. Modern radiators have aluminum cores, and often save money and weight by using plastic headers with gaskets. This construction is more prone to failure and less easily repaired than traditional materials. An earlier design used honeycomb construction.
Honeycomb Radiator Tubes
Radiators first used downward vertical flow, driven solely by a thermo-syphon effect.
Thermo-syphon cooling system of 1937, without circulating pump
Coolant is heated in the engine, becomes less dense, and so rises. As the radiator cools the fluid, the coolant becomes denser and falls. This effect is sufficient for low-power stationary engines, but inadequate for all but the earliest automobiles. All automobiles for many years have used centrifugal pumps to circulate the engine coolant because natural circulation has very low flow rates.
2009 Challenger Water Pump
A heater is, basically a small radiator, consisting of valves or baffles. This small radiator, and the associated blower fan, is called the heater core, and serves to warm the cabin interior. Like the radiator, the heater core acts by removing heat from the engine. (Note- For this reason, automotive technicians often advise operators to turn on the heater and set it to high if the engine is overheating, to assist the main radiator).
2018 Challenger Heater
The engine temperature on modern cars is primarily controlled by wax-pellet type of thermostat, a valve which opens once the engine has reached its optimum operating temperature.
When the engine is cold, the thermostat is closed except for a small bypass flow so that the thermostat experiences changes to the coolant temperature as the engine warms up. Engine coolant is directed by the thermostat to the inlet of the circulating pump and is returned directly to the engine, bypassing the radiator. Directing water to circulate only through the engine allows the engine to reach optimum operating temperature as quickly as possible while avoiding localized "hot spots." Once the coolant reaches the thermostat's activation temperature, it opens, allowing water to flow through the radiator to prevent the temperature rising higher.
Once at optimum temperature, the thermostat controls the flow of engine coolant to the radiator so that the engine continues to operate at optimum temperature. Under peak load conditions, such as driving slowly up a steep hill whilst heavily laden on a hot day, the thermostat will be approaching fully open because the engine will be producing near to maximum power while the velocity of air flow across the radiator is low. (The velocity of air flow across the radiator has a
major effect on its ability to dissipate heat.) Conversely, when cruising fast downhill on a motorway on a cold night on a light throttle, the thermostat will be nearly closed because the engine is producing little power, and the radiator is able to dissipate much more heat than the engine is producing.
The thermostat is therefore constantly moving throughout its range, responding to changes in vehicle operating load, speed and external temperature, to keep the engine at its optimum operating temperature.
Other factors influence the temperature of the engine, including radiator size and the type of radiator fan. The size of the radiator (and thus its cooling capacity) is chosen such that it can keep the engine at the design temperature under the most extreme conditions a vehicle is likely to encounter (such as climbing a mountain whilst fully loaded on a hot day).
Airflow speed through a radiator is a major influence on the heat it dissipates. Vehicle speed affects this, in rough proportion to the engine effort, thus giving crude self-regulatory feedback. Where an additional cooling fan is driven by the engine, this also tracks engine speed similarly.
Engine-driven fans are often regulated by a fan clutch from the drive belt, which slips and reduces the fan speed at low temperatures. This improves fuel efficiency by not wasting power on driving the fan unnecessarily. On modern vehicles, further regulation of cooling rate is provided by either variable speed or cycling radiator fans. Electric fans are controlled by a thermostatic switch or the engine control unit. Electric fans also have the advantage of giving good airflow and cooling at low engine revs or when stationary, such as in slow-moving traffic.
2017 Challenger Radiator Fan
Because the efficiency of internal combustion engines increases with internal temperature, the coolant is kept at higher-than-atmospheric pressure to increase its boiling point. A calibrated pressure-relief valve is usually incorporated in the radiator's fill cap. This pressure varies between models, but typically ranges from 4 to 30 psi.
Clear Plastic Overflow Container and Pressure Cap on 6.1 Hemi (top right)
As the coolant expands with increasing temperature, its pressure in the closed system must increase. Ultimately, the pressure relief valve opens, and excess fluid is dumped into an overflow container. Fluid overflow ceases when the thermostat modulates the rate of cooling to keep the temperature of the coolant at optimum. When the engine coolant cools and contracts (as conditions change or when the engine is switched off), the fluid is returned to the radiator through additional valving in the cap.
Different vehicles require different coolants. There are varieties for every type of vehicle, from diesel engines to American, Asian and European vehicles. Although the ethylene glycol base is the same, each one is specifically formulated to keep its designated engine type running in extreme temperatures. It is important to use the correct antifreeze to prevent possible engine damage.
There are six types of coolants (antifreeze):
IAT (Inorganic Additive Technology)
Application- Older Vehicles
OAT (Organic Acid Technology)- GM Dexcool- 10 yrs. or 100,000 miles, whichever comes first; Mopar- 10 years and 150,000 miles, whichever comes first.
Application- Chrysler (2013-2019), GM, Saab and VW
Composition- No silicates, nitrates or phosphates
HOAT (Hybrid OAT)- 5 years or 102,000 miles, whichever comes first.
Application- Chrysler (2002-2012), Ford and European
Composition- Silicates and organic acids
Hybrid HOAT (Phosphate free)
Application- BMW, Volvo, Tesla, Mini and others.
Composition- NAP free
P-HOAT (Phosphated HOAT)
Application- Toyota, Nissan, Honda, Hyundai, KIA, and other Asian vehicles
Composition- Phosphates and organic acids
Si-OAT (Silicated HOAT)
Application- Mercedes, Audi, VW and Porsche)
Composition- Silicates and organic acids
Here is an informative video by Pat Goss of Motorweek, explaining why coolants need to be changed according to the manufacturer's maintenance schedule.