Maintenance Tips, Tricks And Hacks For Your Vehicle

How Coolant Flows Through An Engine – Cooling System Explained




As the engine produces power by combustion, a lot of heat is generated in the process. It is up to the coolant and the cooling system to take the heat away from the engine so it won’t overheat. But how does coolant actually flows through an engine?

In this blog post, I will do my best to break down how the cooling system works and how exactly coolant flows through an engine.

Key Takeaway

  • Coolant flows from the lower radiator tank into the coolant passages around the engine. When it gets hot, the coolant then goes to the expansion tank to cool down before going on another cycle.
  • The water pump is in charge of circulating coolant around the engine.
  • The radiator stores the coolant when the vehicle is turned off.
  • A thermostat determines when the coolant is too hot and opens up a passage to an expansion tank where the coolant cools down.
  • A coolant temperature sensor measures the temperature of the coolant that flows around the engine and determines the current temperature of the engine.
  • The heater core receives hot coolant and releases this heat into the cabin.

How Coolant Flows Through An Engine

When the engine is started, the water pump begins to pump the cold coolant from the lower radiator tank into the coolant passages around the engine cylinder. The coolant flows from the lower radiator tank to the engine block, then to the cylinder head, and towards the outlet of the radiator. As the coolant absorbs the heat from the engine cylinders, it heads toward the radiator where with the help of a thermostat, it gets redirected toward an overflow tank where it gets cooled.

The thermostat is actually a valve that regulates the flow of the coolant. The thermostat is always soaked in coolant and that is how it actually measures the temperature. When you first start your engine, both the engine and the coolant are cold. While the coolant is cold, the bypass valve on the thermostat stays open and the coolant is recirculated to the engine.

The coolant actually bypasses the radiator and recirculates through the engine. This way, the engine can get up to a working temperature faster. Eventually, the temperature of the coolant will increase by absorbing the heat from the engine. Once the coolant reaches a temperature between 160 to 190 degrees Fahrenheit, the bypass valve of the thermostat will close and the main valve will open.

When the main valve on the thermostat opens, the cold coolant from the radiator starts flowing to the engine side, and in return, the hot coolant from the engine side flows to the radiator. If the temperature of the coolant is way too high on the radiator side, the coolant temperature sensor will trigger the cooling fan. This is the end of one cooling cycle.

Cooling System Main Parts

The engine runs hot due to the constant combustion occurring in the engine cylinders. The ideal operating temperature for an engine is about 200 degrees Fahrenheit or about 90 degrees Celsius. However, without a proper cooling system in place, the ideal engine temperature can easily be surpassed. This is where the cooling system comes into place. Below, I will present to you the main parts of a car cooling system and how they work.

1. Water pump

How Coolant Flows Through An Engine - Cooling System Explained

When the engine is first started, the water pump pulls cold coolant from the bottom of the radiator and sends it to the front of the engine block. The coolant then travels around the cylinders, then goes up to the head where it cools the valves, and finally comes back out of the cylinder head towards the radiator to get cooled down.

Basically, the purpose of the water pump in a car is to keep the coolant circulating. If the engine is sitting in one place, it will start to boil next to the hottest parts of the engine (engine cylinders). The car water pumps are so powerful that they circulate all of the coolant around the engine about twenty times in one minute.

The water pump is located on the front of the engine and has a pulley that is driven by a belt from the crankshaft. It is the same belt that drives the alternator. This is where the water pump gets its power from. Being driven by the crankshaft belt means that as soon as the engine is started, the water pump starts running.

A water pump consists of a housing made from cast aluminum with a couple of mounting points for the timing system, a shaft that runs through the body, an impeller, and a pulley attached to a flange. The water pump sits on the engine block with the impeller inside. Coolant actually comes from the bottom of the radiator and into the center of the impeller.

The impeller has blades that spin the coolant around and creates a lower-pressure area that pulls in more coolant from the radiator.

2. Radiator

How Coolant Flows Through An Engine - Cooling System Explained

Radiators are an essential part of a car’s cooling system, helping to ensure optimal engine performance and efficiency. Radiators consist of a collection of metal tubes filled with fins in which liquid coolant flows in order to dissipate heat that is produced by the engine. These tubes and fins are usually pressed together into an aluminum or plastic shell, which contains all the components within it.

The engine produces heat and coolant flows through it to cool it down. The coolant absorbs the heat from the engine and flows from the engine to the radiator where it cools down and flows back into the engine.

When the hot coolant reaches the radiator, it cools down because of the air flowing across the radiator. That is why the radiator is always placed on the front of the vehicle. This design works very well if you are moving at highway speeds.

3. Thermostat

How Coolant Flows Through An Engine - Cooling System Explained

The thermostat is a temperature-controlled valve that controls the flow of the coolant to the radiator. When the engine is cold, the thermostat is closed. This way the coolant stays inside the engine and helps it to get warmed up quicker. Once the coolant is warmed up from the engine, it needs to be cooled down.

In this case, the thermostat opens up and allows the coolant to flow around the radiator where it gets cooled down by the airflow, radiator, or overflow tank.

Car thermostats are complicated yet crucial components of any vehicle’s engine. Essentially, the thermostat works by controlling the temperature of the cooling system and then regulating when to open or close the circuit that circulates coolant from the radiator.

At its core, a car thermostat consists of three main parts: a casing, a wax-filled container, and an actuator; all crafted from durable metals for longevity. The casing holds the wax-filled container which is designed to expand and contract based on coolant temperature fluctuations. Once the coolant reaches a certain temperature, the wax pushes against the actuator which opens or closes the coolant circuit.

4. Coolant Temperature Sensor (CTS)

How Coolant Flows Through An Engine - Cooling System Explained

The coolant temperature sensor is a critical component of a car’s engine. It monitors the temperature of the engine and its surrounding components, sending electric signals back to the engine control unit in order to adjust its performance accordingly.

In cold weather, for example, the sensor will tell the ECU to make adjustments so that the car’s engine runs optimally. It works constantly to ensure that the internal temperature remains consistent and does not get too unbearably hot or too overwhelmingly cold.

These sensors are constructed from very durable materials that can withstand high temperatures and pressure. They feature a ceramic core surrounded by a steel shell that isolates the electrical connections inside and can keep fluids away from the interior components.

On one end of the sensor is a heat-resistant plug that senses the temperature of the coolant as it runs through the system, while on the other side there is a connector for an electrical harness.

5. Coolant (Antifreeze + Water)

Coolant is a liquid used to absorb the heat from engines and prevent overheating. It is made up of mostly water, combined with antifreeze and other chemical components that can reduce corrosion, improve freezing protection and ultimately make machines more efficient.

The most common coolants are ethylene glycol-based, though there are other varieties of coolants made from propane glycol-based formulations or even organic materials like vegetable oil. It helps prevent overheating by transferring heat away from critical components like pistons, allowing them to function at their optimum temperatures without damaging the surrounding components.

With its ability to draw heat away, coolant safely eliminates heat buildup for increased reliability and performance in any engine.

6. Overflow tank

How Coolant Flows Through An Engine - Cooling System Explained

An overflow tank is a significant component of a car’s cooling system. It stores the excess hot coolant from the radiator and releases the excess pressure to avoid damage to the cooling system. Generally, the overflow tank is made out of plastic and shaped like a rectangular box with two small hoses connected to either side. Inside its enclosed walls is a channel for coolant to flow in and out when necessary.

Furthermore, there is a fill line within the enclosure that indicates when the level should be refilled if ever it gets too low over time. The overflow tank has proven to be an essential part of the cooling system as it plays an important role in maintaining engine temperature, reducing air pockets, and cooling down hot coolant.

The overflow tank in a car is typically connected to the radiator. During the normal operation of a car, coolant circulates through the vehicle’s cooling system creating pressure and heat that needed to be relieved.

The overflow tank is activated when this pressure reaches a certain threshold by automatically opening vents that allow excess fluids to flow out of the vehicle’s cooling system into and out of the overflow tank.

Once temperatures within the system regulate and dip below the set threshold, these vents are closed again, preventing more coolant from flowing out and maintaining a safe level of fluid within the cooling system.

7. Radiator pressure cap

The radiator pressure cap has three purposes. First, it acts as a cap that seals off the cooling system and prevents coolant spills. The second purpose of the radiator cap is to act as a pressure release valve rated at about 15 PSI. Last but not least, when we turn off the engine and the coolant cools off and shrinks, the radiator cap allows the coolant to get sucked from the overflow tank back into the engine.

The radiator pressure cap consists of a spring-loaded valve mechanism that reacts to high pressure. When the pressure inside the cooling system reaches above 15 PSI, the spring mechanism gets pushed up and the hot coolant will flow to the expansion tank until the pressure and temperature of the coolant go to an operational level.

8. Heater core

The car heater core is an integral component of a vehicle’s climate control system. This modern marvel is engineered with precision and durability to ensure it will provide years of service in all types of climate conditions. It works by using coolant that is heated by the engine and distributed throughout the car, providing warmth to passengers on particularly cold days.

The heater core is basically a radiator that is used to transfer the heat from the hot coolant to the air that flows inside the passenger compartment.

The heater core itself is typically composed of an aluminum or copper tube which allows the hot coolant to run through its passages, releasing heat into the interior air through carefully crafted vents in the dashboard.

A series of other components work together to complete this cycle, such as pumps and electronic parts that are essential for controlling temperatures inside a vehicle. All these pieces come together to create one sophisticated automotive feature that has been keeping drivers comfortable for decades.



Vide Polowenski, Senior Mechanic

The information in this article is current and up-to-date in accordance with the latest mechanic SOPs.

Please take the time to leave a comment if this article has helped you in any way, you need additional help, or you have a suggestion.

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