Refrigerator in a heat engine: why is it needed and how does it work

When it comes to heat engines, most immediately imagine steam engines, internal combustion engines or turbines of power plants. But few people think about the fact that any heat engine cannot operate without a refrigerator and this is not a whim of engineers, but a fundamental law of thermodynamics. Without this element, the efficiency of the system would tend to zero, and the process of converting heat into work would become impossible.

In everyday life, the word “refrigerator” is associated with kitchen appliances for storing food, but in the context of heat engines this concept has a completely different meaning. Here refrigerator is not a device, but a body or environment with a low temperaturewhich absorbs part of the heat that is not converted into useful work. Why is this necessary? Because the second law of thermodynamics prohibits the creation of a perpetual motion machine of the second kind - a device that would completely convert heat into work without releasing part of the energy to a colder reservoir.

In this article we will understand how the refrigerator is integrated into the heat engine cycle, what physical processes are behind this, and why even in modern internal combustion engines (for example, in cars) the role of a refrigerator is played by the environment. You will also find out what happens if you “disconnect” the refrigerator from the system, and where this principle is used besides classical machines. Toyota or Volkswagen) The environment plays the role of a refrigerator. You will also find out what happens if you “disconnect” the refrigerator from the system, and where this principle is used besides classic machines.

What is a refrigerator in thermodynamics?

In thermodynamics refrigerator (or cold reservoir) is a part of the system that receives heat from the working fluid after it has performed useful work. Unlike the heater (heat source), the refrigerator has a lower temperature and serves removal of excess thermal energywhich cannot be converted into mechanical work.

The simplest example: in a steam engine, the refrigerator is a condenser, where the steam is cooled and turns back into water. In an internal combustion engine, this role is played by the cooling system (radiator) and exhaust gases, and in the turbines of a thermal power plant - cooling towers or reservoirs where heat is discharged after passing through the turbine.

• 🔥 Heater - a heat source (for example, burning fuel, a nuclear reactor). Temperature: T₁ (high).
• ❄️ Refrigerator —receiver of excess heat. Temperature: T₂ (low, but above absolute zero).
• ⚙️ Working fluid - a substance that does work (steam, gas, air).

Without a refrigerator, the cycle would be impossible: the working fluid could not return to its original state, and the heat would not have “where to go.” It's like trying to pour water into a glass that is already full - without draining the liquid, the process will stall.

The second law of thermodynamics: why a refrigerator is a must

The second law of thermodynamics has several formulations, but in the context of heat engines the key one sounds like this: "It is impossible to create cyclically an operating heat engine, the only result of which would be the conversion of heat into work without transferring part of the heat to the refrigerator".

In other words, even in an ideal engine (for example, in the Carnot cycle) you cannot do without a refrigerator. Why? Because:

1. Heat spontaneously transfers only from a hotter body to a colder one.
2. To return the working fluid to its original state (complete the cycle), it is necessary remove part of the heat.
3. Engine efficiency is always less than 100% precisely because of the need to transfer heat to the refrigerator.

The formula for the efficiency of an ideal heat engine (Carnot cycle) clearly demonstrates this:

η = (T₁ – T₂) / T₁

Where:

• η — efficiency factor action,
• T₁ —heater temperature,
• T₂ —refrigerator temperature.

The formula shows: the lower temperature of the refrigerator (T₂the higher the efficiency. That is why engineers strive to cool the refrigerator as much as possible (for example, in cryogenic engines or in thermal power plants with cooling towers).

Examples of refrigerators in real heat engines

The theory is good, but how does a refrigerator work in practice? Let's look at a few real examples:

Please note: in jet engines, the refrigerator actually serves atmosphere, where the hot gases go. In an internal combustion engine, the role of the refrigerator is divided between the radiator (removes ~30% of the heat) and the exhaust system (another ~30–40%). Only ~25–30% of the fuel energy is converted into useful work!

Why don't electric cars have a refrigerator in the traditional sense?

In electric cars (for example, Tesla Model 3 or Nissan Leaf) there is no heat engine, which means there is no classic refrigerator. However, there are still cooling systems there - they serve to remove heat from the batteries and the electric motor, but they work on a different principle (most often using liquid cooling and heat exchangers).

What happens if you remove the refrigerator?

Imagine that the refrigerator suddenly “disappeared” in the heat engine. What will happen?

⚠️ Attention: In real conditions, it is impossible to completely remove the refrigerator - this would violate the laws of physics. But hypothetically, such a scenario would lead to:

• 🔥 Immediate stop of the cycle. The working fluid would not be able to return to its original state, and the engine would jam after the first stroke.
• 💥 Overheating and destruction. Heat, having no where disappear, would accumulate in the system, leading to the melting of parts (in the internal combustion engine - pistons, valves).
• ⚡ Efficiency = 0%. Without a temperature difference (T₁ – T₂), the efficiency formula will give zero - the engine would not do any work.

In practice, “turning off” the refrigerator is equivalent to failure of the cooling system. For example, if a car's pump or radiator fails, the engine will overheat in a matter of minutes. The same thing will happen at a thermal power plant if the cooling towers fail: the turbines will have to be stopped urgently.

Is it possible to increase efficiency by improving the refrigerator?

Yes, but with reservations. From the Carnot formula it is clear that efficiency increases with:

1. Increasing the temperature of the heater (T₁).
2. Decreasing the temperature of the refrigerator (T₂).

In practice:

• 🔥 Increase T₁: Modern internal combustion engines use turbocharging and an increased compression ratio (for example, in diesel engines Mercedes OM617 or gasoline Mazda Skyactiv-Xto increase the combustion temperature.
• ❄️ Reduction T₂: At thermal power plants, cooling towers with forced airflow are used, and in cryogenic engines (for example, in rocket engines), the cooler is liquid nitrogen or helium.

However, there are limits:

• The heater temperature is limited by the strength of the materials (for example, in an internal combustion engine - up to ~2500°C, otherwise the pistons will melt).
• The temperature of the refrigerator cannot be below the ambient temperature (or requires huge amounts of energy for cooling).

⚠️ Attention: In the pursuit of increasing efficiency, engineers often encounter the law of diminishing returnsFor example, cooling a refrigerator to -50°C instead of +20°C will increase efficiency by only a few percent, but will require a complex and expensive cryogenic system.

Unusual applications of the refrigerator principle

The refrigerator principle in heat engines is used not only in classical machines. Here are some non-obvious examples:

• 🌍 Geothermal power plants. Here the heater is hot magma, and the refrigerator is cold water or air on the surface. The efficiency of such stations is low (5–10%), but they are environmentally friendly.
• 🚀 Space engines. In ion or plasma engines (for example, NASA's X3the refrigerator is the vacuum of space, into which excess heat is dumped.
• 💡 Heat pumps. This is a “reverse” heat engine: here the refrigerator (for example, the soil in winter) gives off heat, and the house serves as a heater. The efficiency of such systems can exceed 100% (in the sense of the ratio of energy expended to the heat received).

Interesting fact: in Stirling engines (used, for example, in submarines or solar power plants), the refrigerator and heater are often the same heat exchanger, but with different temperatures in different parts of the cycle. This allows achieving efficiency of up to 40% even at relatively low temperatures. temperatures.

Open the hood and check the radiator temperature (should be ~80–90°C)|Make sure the cooling fan turns on when hot|Check the coolant level in the expansion tank|Pay attention to the color of the exhaust gases (white smoke may indicate coolant entering cylinders)-->

FAQ: Frequently asked questions about the refrigerator in heat engines

Why is a refrigerator required in the Carnot cycle, but in real engines its role is often played by the environment?

The Carnot cycle is idealized modelwhere the refrigerator must have fixed temperature T₂. In real engines (for example, internal combustion engines), the temperature of the refrigerator is not constant: part of the heat is released through the radiator (temperature ~80–90°C), and part leaves with the exhaust gases (temperature ~400–600°C). The environment acts as an infinite reservoir capable of absorbing heat without significantly changing its temperature.

Is it possible to make an engine without a refrigerator if you use superconductors or other technologies?

No, even with superconductors. The second law of thermodynamics is fundamental law of naturewhich does not depend on the materials used. Superconductors can reduce losses due to friction or electrical resistance, but they do not eliminate the need to dump heat to complete the cycle. In the best case, you can increase efficiency, but you cannot do without a refrigerator.

Why do electric cars do not have a refrigerator in the traditional sense, but do they have cooling systems?

Electric cars do not heat engine, which means there is no classic cycle with a heater/refrigerator. However, cooling systems are needed there for:

• Removing heat from the batteries (they heat up when charging/discharging).
• Cooling the electric motor and inverter.
• Maintaining the optimal temperature for efficient operation of lithium-ion batteries (usually ~20–40°C).

This is not a refrigerator in the thermodynamic sense, but a technical solution for thermal management.

How does the temperature of the refrigerator affect on the environmental friendliness of the engine?

The lower the temperature of the refrigerator, the higher the efficiency of the engine, which means less fuel is burned in vain. This directly affects CO₂ emissions:

• In modern thermal power plants with cooling towers, the efficiency reaches 60%, which reduces specific emissions by 30–40% compared to old stations (efficiency ~30%).
• In hybrid cars (for example, Toyota Prius), part of the heat from the refrigerator is recovered to heat the interior, which additionally saves fuel.

Are there engines with a refrigerator not needed?

No, it does not exist. Any cyclic heat engine required requires a refrigerator. The only exception is non-cyclical systems, for example:

• Disposable rocket engines (but even there the heat escapes with the exhaust into space).
• Devices operating on the principle none-equilibrium processes (for example, some nano-sized engines), but they are not classical heat engines.

In all other cases, the refrigerator is an integral part of the system.