Excessive pressure in the cooling system of modern cars is not a design error or the result of someone's carelessness. It is a conscious engineering decision to which automakers have been pushed by the evolution of engines. About thirty years ago, the vast majority of engines managed with a much simpler scheme, but since then, the operating conditions of power units have changed radically.
How the Cooling System Worked Before
For decades, cars used an open cooling system. The radiator was directly connected to the expansion tank, equipped with a valve. As the temperature rose, the coolant expanded and was forced into the tank, and after the engine cooled down, it returned back into the circuit. The pressure remained close to atmospheric, and its slight excess only occurred due to the heating of the air remaining in the system.
This scheme was technologically simple and relatively unpretentious, but had a limited margin in terms of temperature and heat dissipation efficiency.
Transition to a Conditionally Closed System
Modern engines operate according to a different logic. Formally, the system cannot be called completely closed, but the principle of its operation has fundamentally changed. Two valves have appeared in the design: one is installed at the inlet to the expansion tank, the second — a shut-off valve — is located directly on the tank itself. This arrangement made it possible to maintain increased pressure inside the system.
The consequence of this approach was a noticeable increase in both the operating temperature of the coolant and the pressure in the circuit.
New Temperature and Pressure Standards
Even 15–20 years ago, the normal operating temperature of the engine was considered to be in the range of 95–98 °C. At the same time, the pressure only slightly exceeded atmospheric pressure, and its increase was not an end in itself — it arose naturally.
In modern engines, the situation is different. The temperature corridor has shifted upwards and today is approximately 110–130 °C. It is generally accepted that the lower limit is characteristic of new engines, while values around +130 °C are more common in engines with a mileage of over 100 thousand km. Along with the increase in temperature, the pressure also increases: the normal range for the cooling system is considered to be 1.1–1.3 bar. As the engine ages, the pressure, as a rule, becomes even higher.
Why Drivers Perceive This as a Problem
Increased pressure is one of the most common causes of dissatisfaction among car owners. Drivers often encounter antifreeze leaks, cracks in hoses, and fluid dripping at joints. These complaints cannot be called far-fetched: increased loads really accelerate the wear of cooling system elements.
However, the increase in pressure itself is not a design error, but a consequence of the more stringent operating conditions of modern engines.
Why Engineers Took This Step
Forcing and Thermal Load
The main reason is the need to force engines. Increasing power is achieved by increasing the speed, pressure, and temperature of gases in the combustion chamber. As a result, the cylinder block and head experience a significantly higher thermal load.
Here, an important technical paradox manifests itself: the higher the pressure in the cooling system, the more efficient the heat removal from the block walls. Increased pressure improves heat transfer and allows you to keep the metal temperature within acceptable limits.
Pressure and Liner Deformation: A Secondary Effect
Sometimes you can find the statement that excessive coolant pressure helps to compensate for the swelling of cylinder liners. This effect is indeed possible, but only under strictly defined conditions — in blocks with wet liners and, as a rule, on heavily worn engines.
Under the action of hot gases, the liner is "expanded" from the inside, and the thickness of its walls in the middle part often decreases to several millimeters. A rupture does not occur, since the pressure in the combustion chamber at the moment the piston is at the bottom dead center is only a couple of atmospheres. In such cases, the back pressure of the antifreeze can partially compensate for the deformation, but for modern engines this factor is not decisive.
The Main Reason is to Combat Cavitation
The key motive for increasing pressure is to prevent cavitation. If the coolant is heated to a temperature close to boiling and passed through the block channels at high speed, the flow begins to boil directly inside the cooling jacket. The reason is the roughness and complex geometry of the internal surfaces.
It would be possible to reduce the flow rate, but then it would be necessary to increase the size of the radiator and the block itself by 20–40 %, which is unacceptable from the point of view of mass, dimensions, and layout. Increasing the pressure turned out to be a much more rational solution.
What is Cavitation Boiling
Cavitation occurs in places where superheated liquid quickly flows around surfaces with sharp edges and irregularities. Microscopic vapor bubbles are formed in the flow, which do not have time to grow into large bubbles and instantly collapse on the metal surface.
At the moment of collapse, a pressure of hundreds, and sometimes thousands, of atmospheres occurs at the point of contact for fractions of a microsecond. In terms of destructive impact, this process is comparable to sandblasting.
The consequences of cavitation are extremely severe: hardened steel with a hard chrome-plated coating is abraded like plastic, and plastic water pump impellers are completely destroyed — only fragments remain at the flange.
Ecological and Corrosive Aspect
A closed system with increased pressure also solves another problem. With a sharp increase in pressure, the valve releases its excess into the expansion tank. After the engine stops and cools down, the pressure in the system returns to atmospheric, and does not go into vacuum, as it did in old designs.
Reduced pressure inside the block would lead to air intake, and therefore to the appearance of oxygen. Under such conditions, aluminum, cast iron, and rubber elements are actively oxidized and corroded. The modern scheme eliminates this process and simultaneously prevents the release of antifreeze into the environment.
Increased pressure in the cooling system is the price to pay for compact, boosted, and thermally loaded engines. It improves heat dissipation, protects the engine from cavitation damage, and reduces corrosion risks. Problems arise not because of the concept itself, but because of component wear and untimely maintenance.