Learn Engine Cooling Systems Explained – Coolant, Thermostat, Water Pump, Radiator, Transmission Cooler, Pressure Cap, Expansion Tank, Hoses, Water Jackets, Heater System, Cooling Fans, Temperature Sensors and How the Entire System Works

Introduction:

Today's internal combustion engines generate an extraordinary amount of heat during operation. This heat is primarily produced during the combustion process, with metal temperatures around the combustion chamber reaching as high as 537.7°C. Without proper management, this intense heat would quickly destroy the engine. The cooling system serves a dual purpose: it must remove excess heat to prevent engine damage while simultaneously allowing the engine to warm up quickly to its desired operating temperature and maintain that temperature regardless of operating conditions.

The cooling system accomplishes this through a carefully engineered series of components that work together to regulate engine temperature. Heat is removed from around the combustion chambers by a heat absorbing liquid called coolant that circulates inside the engine. This coolant then flows to the radiator where the absorbed heat is transferred to the outside air. A pump moves the coolant through the engine block and then through the cylinder head, while a thermostat controls the operating temperature by regulating coolant flow. When the coolant temperature is below the desired operating temperature, the thermostat remains closed, allowing the coolant to recycle through the engine. Once normal operating temperatures are reached, the thermostat opens to permit coolant flow to the radiator. The coolant flows to the top of the radiator and loses heat as it moves downward through the radiator core. Ram air and airflow from the cooling fan pass through the radiator to cool the coolant. The cooled coolant then leaves the radiator, enters the water pump, and is sent back through the engine to repeat the cycle.

The engine cooling system is a sophisticated network of components designed to manage the intense heat generated during internal combustion. Understanding how these components function and interact is fundamental to properly maintaining and diagnosing cooling system issues, ultimately ensuring reliable engine operation and longevity.

Coolant:

Engine coolant is a mixture of water and antifreeze/coolant that serves as the primary heat transfer fluid in the cooling system. This mixture has a higher boiling temperature and a lower freezing point than plain water, making it suitable for year-round operation. The exact boiling and freezing temperatures depend on the specific mixture ratio, with the typical recommended mixture being a 50/50 solution of water and antifreeze/coolant.

Thermostat:

The thermostat controls the minimum operating temperature of the engine, while the maximum operating temperature is determined by the amount of heat produced by the engine and the cooling system's ability to dissipate that heat. Essentially, the thermostat is a temperature responsive coolant flow control valve that regulates both the temperature and amount of coolant entering the radiator. When the engine is cold, the thermostat remains closed, allowing coolant to circulate only within the engine. This permits the engine to warm up uniformly. As the coolant reaches a specified temperature, the thermostat begins to open and allows coolant to flow to the radiator. The hotter the coolant gets, the more the thermostat opens, sending more coolant to the radiator. Once the coolant moves through the radiator, it reenters the water pump and is pushed through the passages in the engine, starting the cycle again.

The thermostat serves several critical functions. It permits fast engine warmup, which is important because slow warmup causes condensation in the crankcase that can lead to sludge formation. The thermostat also keeps the coolant above a specific minimum temperature to ensure efficient engine performance. Thermostats must start to open at a specified temperature, normally within 1.6°C above or below their temperature rating. They must be fully opened at about –7°C above the start to open temperature. Modern thermostats contain a specially formulated wax and powdered metal pellet contained in a heat conducting copper cup with a piston inside a rubber boot. When heat causes the wax to expand, it forces the piston outward, opening the thermostat's valve. The pellet responds to temperature changes and opens and closes the valve to control coolant temperature and flow.

Today's thermostats are also designed to slow the flow of coolant when they are open. This prevents the coolant from moving too quickly through the engine, which could result in overheating because fast moving coolant may not have enough time to absorb heat. Most thermostats are located on the top and front of an engine. The heat element fits into a recess in the block where it is exposed to hot coolant. The top of the thermostat is then covered by the water outlet housing, which holds it in place and provides a connection to the upper radiator hose.

The Water Pump:

The water pump serves as the heart of the cooling system, with its primary job being to move coolant through the entire system. Typically, the water pump is driven by the crankshaft through pulleys and a drive belt. On some engines, however, the pump may be driven by the camshaft, timing belt or chain, or an electric motor. Regardless of the drive method, these units all work essentially the same way. Water pumps are centrifugal type pumps that use a rotating impeller to move the coolant. The shaft is mounted in the water pump housing and rotates on bearings, while a seal prevents coolant from passing through the shaft area. The inlet of the pump connects to the lower radiator hose, and its outlet connects to the engine block, establishing the flow path for coolant circulation.

The Radiator:

The radiator functions as a heat exchanger, transferring heat from the engine to the air passing through it. The radiator itself consists of a series of tubes and fins, collectively called the core, that expose the coolant's heat to as much surface area as possible. Attached to the sides or top and bottom of the core are plastic or aluminum tanks. One tank holds hot coolant entering the radiator, while the other holds the cooled coolant leaving it. Radiator cores are normally comprised of flattened aluminum tubes surrounded by thin aluminum fins. The fins conduct the heat from the tubes to the air flowing through the radiator. Most radiators have drain petcocks or plugs located near the bottom. Coolant is added either at the radiator cap or through the recovery tank.

The efficiency of a radiator depends on several factors: its basic design, the area and thickness of the core, the amount of coolant going through the radiator, and the temperature of the cooling air. Today's radiators are designed to limit the amount of heat dissipated, keeping the coolant somewhat hot at all times. This is necessary because keeping engines operating at a high temperature is essential for maintaining low emission levels. Radiators are normally based on one of two designs: cross flow or down flow. In a cross-flow radiator, coolant enters on one side, travels through tubes horizontally, and collects on the opposite side. In a down-flow radiator, coolant enters the top and is drawn downward by gravity. Cross-flow radiators are more common on late-model cars because all the coolant flows through the fan's airstream, and the design accommodates body styles with lower hood profiles.

Transmission Cooler:

Radiators used in vehicles with automatic transmissions include a sealed heat exchanger, or a form of oil cooler, located in the coolant outlet tank of the engine's radiator. Metal or rubber hoses carry hot automatic transmission fluid to this oil cooler. The coolant passing over the sealed oil cooler cools the fluid, which is then returned to the transmission. Cooling the transmission fluid is essential for the efficiency and durability of an automatic transmission.

The Radiator Pressure Cap:

Radiator caps serve two important functions. They keep the coolant from splashing out of the radiator and maintain the coolant at a specified pressure level. This pressure raises the boiling point of the coolant. For every pound of pressure put on the coolant, the boiling point is raised about 1.8°C. Today's caps are normally designed to hold between 14 and 18 psi (93 and 123 kPa). This pressurization allows the coolant to reach higher than normal temperatures without boiling, enabling the coolant to absorb more heat from the engine and allowing more heat to transfer from the radiator core to the outside air. This is due to a basic law of nature stating that the greater the temperature difference between two objects, the faster the heat of the hotter object will move to the cooler object.

The pressure in the system is regulated by a pressure relief or vent valve in the radiator cap. When the cap is tightened on the radiator's filler neck, it seals against the upper and lower sealing surfaces of the neck. The pressure relief valve is compressed against the lower seal. Coolant pressure builds up as the temperature of the coolant rises. When the pressure reaches the pressure rating of the cap, it pushes up on the spring in the pressure relief valve, opening the valve and allowing excess pressure to exit the radiator through a bore between the upper and lower seals. This bore is connected by a tube to the expansion or recovery tank. When enough pressure has been released to drop system pressure below the cap's rating, the spring closes the pressure relief valve.

When the coolant cools, its pressure drops. This low pressure opens the vacuum relief valve, which then draws coolant from the expansion tank to refill the radiator. All radiator caps are designed to meet SAE standards for safety. This standard specifies that there shall be a detent or safety stop position allowing pressure to escape from the system without allowing hot coolant to blow out of the radiator's neck. Only after all pressure has been relieved should the cap be removed from the filler neck.

Radiator pressure cap specifications require that the cap must not leak below the low limit of the pressure range and must open above the high limit. Caps are labeled by the amount of pressure they should hold. For domestic vehicles, pressure is stated in psi or kPa. Normally, kPa ratings are expressed as a number times 100 kPa. For example, a rating of 1.3 - 100 kPa indicates a 130 kPa cap.

Radiator caps for some imported vehicles may have different markings. Some may be marked "0.9 Bar," indicating that the pressure rating is 0.9 times normal atmospheric pressure. Since atmospheric pressure is 14.7 psi, a 0.9 bar cap has a pressure rating of about 13.2 psi (14.7 × 0.9). Another common marking is "100," which indicates that the pressure rating is 100% of atmospheric pressure, or 14.7 psi.

The Expansion Tank:

All late-model cooling systems have an expansion or recovery tank. Cooling systems with expansion tanks are called closed-cooling systems. They are designed to catch and hold any coolant that passes through the pressure cap. As the engine warms up, the coolant expands, eventually causing the pressure cap to release. The coolant then passes to an expansion tank. When the engine is shut down, the coolant begins to shrink. Eventually, the vacuum spring inside the pressure cap opens, and the coolant in the expansion tank is drawn back into the cooling system.

Hoses and Connections:

Coolant flows from the engine to the radiator and from the radiator back to the engine through radiator hoses. These hoses are usually made of butyl or neoprene rubber to cushion engine vibrations and prevent damage to the radiator. A hose typically consists of three parts: an inner rubber tube, reinforcement material, and an outer rubber cover. Different covers and reinforcements are used depending on the application, location, and the temperature and pressure the hose will face. Cooling system hoses must be able to endure heavy vibrations and be resistant to oil, heat, abrasion, weathering, and pressure. Most vehicles have at least four hoses in the cooling system, with some having five or more. Two small diameter hoses send hot coolant from the water pump to the heater core and back. Two larger diameter hoses move the coolant from the water pump to the radiator and back into the engine block. The fifth hose is a small diameter bypass hose that allows coolant to circulate within the engine when the thermostat is closed. This hose is not required on all engines because the bypass feature is built into the engine block or cylinder head.

Hoses are sized according to their inside diameter. Common heater hoses are 5/8 or 3/4 inch in diameter. Radiator hoses are larger and have reinforcements that allow them to withstand about six times the normal operating pressure of the cooling system. Radiator hoses are seldom straight tubes because they typically must bend or curve around parts to make proper connections without kinking. Straight hoses are not used because bending would cause them to collapse at the bend, creating a restriction. Most original equipment radiator hoses are molded to specific shapes to fit particular applications. Lower radiator hoses are normally reinforced with wire to prevent them from collapsing due to the suction of the water pump. Heater hoses are made with reinforcements to help maintain their shape. Some applications require a molded shape due to complex routing or curves, while others can use straight pieces of hose, though most have bends.

Water Outlet:

The water outlet is the connection between the engine and the upper radiator hose through which hot coolant from the engine is pumped into the radiator. This component has been called a gooseneck, elbow, inlet, outlet, or thermostat housing. Generally, it covers and seals the thermostat and, in some cases, includes the thermostat bypass. Most water outlets are made of cast iron, cast aluminum, or stamped steel.

Water Jackets:

Hollow passages in the block and cylinder heads, known as water jackets, surround the areas closest to the cylinders and combustion chambers. Included in the water jackets are soft (core) plugs and a block drain plug. Some engines are equipped with plastic liners that direct coolant flow around critical areas. The soft plugs and drain plugs are usually removed during engine teardown, with new ones installed during reassembly. Core plugs are prone to rust and corrosion and, therefore, may weep coolant or rust through completely. When this happens, the core plugs should be replaced.

Hose Clamps:

Hoses are attached to the engine and radiator with clamps designed to apply clamping pressure around the outside of the hose at connection points to the radiator, engine block, water pump, or heater core. The pressure exerted on these connections is important to making and maintaining a proper seal.

Belt Drives:

Belt drives are used to power the water pump and cooling fan on many engines. The belts must be in good condition and properly tensioned to drive these components at the correct speed.

The Heater System:

A hot liquid passenger compartment heater is part of the engine's cooling system. Heated coolant flows from the engine through heater hoses and a heater control valve to a heater core located on either side of the firewall. Air is directed or blown over the heater core, and the heated air then flows into the passenger compartment. Movable doors can be controlled to blend cool air with heated air for more or less heat.

Cooling Fans:

The efficiency of the cooling system depends on the amount of heat that can be removed from the system and transferred to the air. At highway speeds, ram air through the radiator should be sufficient to maintain proper cooling. At low speeds and idle, however, the system needs additional air, which is delivered by a fan. The fan may be driven by the engine via a belt or by an electric motor.

Belt Driven Fans:

A belt-driven fan is bolted to a pulley on the water pump and turns constantly with the engine, always drawing air through the radiator from the rear. The fan has several blades made of steel, nylon, or fiberglass, which are attached to a metal hub. A fan placed more than 3 inches from the radiator is ineffective because it merely recirculates hot air around the fan blades. This is why most radiators are equipped with shrouds. A shroud is a large, circular piece of plastic, metal, or cardboard-like material that extends outward from the radiator to enclose the fan and increase its effectiveness. These shrouds should always be kept intact and not modified. Because fan air is usually only necessary at idle and low-speed operation, various design concepts are used to limit fan operation at higher speeds. Horsepower is required to turn the fan, so the operation of a cooling fan reduces the available horsepower to the drive wheels as well as fuel economy. Fans are also very noisy at high speeds, adding to driver fatigue and total vehicle noise.

To eliminate this power drain when fan operation is not needed, many of today's belt-driven fans operate only when the engine and radiator heat up. This is accomplished by a fan clutch located between the water pump pulley and the fan. When the engine and fan clutch are cold, the fan moves independently from the clutch and moves little air. The clutch locks the fan to its hub when the temperature of the air around the fan reaches a particular point. In most cases, the clutch slips at high speeds, so it is not turning at full engine speed. The clutch assemblies rely on a thermostatic spring or silicone fluid. Some engines have flexible blades or flex-blades that bend or change pitch based on engine speed. At slower speeds, the blade pitch is at maximum. As engine speed increases, the blade pitch decreases, reducing horsepower losses and noise levels.

Electric Cooling Fans:

In most late-model applications, to save power and reduce noise levels, the conventional belt-driven, water pump mounted engine cooling fan has been replaced with an electrically driven fan. This fan and motor are mounted to a shroud. The 12 volt, motor driven fan is electrically controlled by an engine coolant temperature switch and/or the air conditioner switch. The cooling fan motor is connected to the battery through a normally open set of contacts in the cooling fan relay. During normal operation, with the air conditioner off and the engine coolant below a predetermined temperature of approximately 101.6°C, the relay contacts are open and the fan motor does not operate.

When coolant temperatures exceed approximately 110°C, the coolant temperature switch closes. This energizes the fan relay coil, which in turn closes the relay contacts. The contacts provide 12 volts to the fan motor. When the air conditioner select switch is set to any cool position, regardless of engine temperature, a circuit is completed through the relay coil to ground. This closes the relay contacts to provide 12 volts to the fan motor which then operates as long as the air conditioner and ignition switches are on.

There are many types of electric cooling fans. Some provide a cool down period, meaning the fan continues to run after the engine has been stopped and the ignition switch is turned off. These systems have a second temperature sensor, and the fan stops only when the engine coolant falls to a predetermined temperature, usually about 98.8°C. In some systems, the fan does not start when the air conditioner is turned on unless the high side of the A/C system is above a predetermined temperature and/or pressure. Some late model cars control the cooling fan by completing the ground through the engine control computer. It is important to check the service manual to understand how a specific electric cooling fan is controlled before working with it.

Temperature Sensors and Indicators:

Proper electric cooling fan operation depends on the operation of a temperature sensor. A temperature sensor responds to changes in temperature. Some vehicles use more than one sensor to control the fans and to send engine temperature readings to the PCM. Based on this information, the PCM will adjust the fuel injection and ignition systems to provide efficient engine operation. Coolant temperature indicators alert the driver of an overheating condition. These indicators are typically a temperature gauge and/or a warning light. A temperature sensor is threaded into a bore in a water jacket. Besides displaying coolant temperatures, temperature sensors supply important information to the computers in engine control systems.

How the Entire Cooling System Works?

The process begins when the engine starts cold. With the coolant temperature below the desired operating point, the thermostat remains closed. This forces the coolant to circulate only within the engine block and cylinder head through the water jackets, allowing the engine to warm up uniformly and quickly, which prevents crankcase condensation and sludge formation. As the engine runs and combustion heats the metal around the cylinders to as high as 537.7°C, the circulating coolant absorbs this extreme heat. Once the coolant reaches a specified temperature, the thermostat's wax pellet expands, pushing the piston outward to open the valve. The hotter the coolant gets, the more the thermostat opens, allowing a greater volume of hot coolant to flow out of the engine through the water outlet and into the upper radiator hose.

The hot coolant enters the inlet tank of the radiator and travels across the core, which consists of flattened aluminum tubes surrounded by thin aluminum fins. As the coolant flows downward or sideways in cross-flow designs, the fins conduct the heat from the tubes to the outside air. At highway speeds, ram air provides enough airflow to cool the radiator; at low speeds or idle, the cooling fan (belt-driven or electric) pulls additional air through the core. This combination of airflow effectively removes the heat from the coolant. The now cooled coolant exits the bottom of the radiator and passes through the lower radiator hose into the water pump inlet. The water pump's rotating impeller then pushes this cooled coolant directly back into the engine block, where it re-enters the water jackets to absorb more heat from the combustion chambers. The cycle repeats continuously.

Throughout this process, the radiator pressure cap maintains the system between 14 and 18 psi (93 and 123 kPa). This pressurization raises the coolant's boiling point, allowing it to absorb more heat without boiling. As the coolant expands when hot, excess pressure opens the cap's relief valve, sending fluid into the expansion tank. When the engine cools and the coolant contracts, the vacuum valve opens, drawing that stored coolant back into the radiator to keep the system full. This entire closed loop cycle from cold start to full operating temperature ensures the engine quickly reaches its ideal heat range, stays there regardless of driving conditions, and maintains low emission levels.