To understand the principles of operation of any heat engine, be it a car engine, a steam turbine or a jet engine, it is necessary to understand the fundamental laws of thermodynamics. The key element here is not just the presence of fuel, but the presence of a temperature difference, which allows the conversion of thermal energy into mechanical work. Without creating an artificial temperature difference, not a single internal combustion engine can perform a useful action.
Many people mistakenly believe that the engine works only by burning fuel, forgetting about the second, no less important part of the cycle - heat removal. It is the heater i refrigerator (or cooler) that form the very necessary system in which heat flows from a hot body to a cold one, and part of this flow turns into the movement of a piston or the rotation of a shaft. If you remove one of these elements, the energy conversion process will stop, and the engine will turn into a useless piece of metal.
In this article, we will look in detail at the physics that requires two heat reservoirs, and explain why it is impossible to create an engine that runs on only one heat source. You will learn exactly how the Carnot cycle works in real devices and why Efficiency any unit is always less than one hundred percent due to the physical limitations of nature.
Fundamental principle: why temperature alone is not enough
The basis of the operation of any heat engine is the second law of thermodynamics, which states that heat spontaneously transfers only from a more heated body to a less heated one. To obtain work, we need to force this flow of energy to “turn the wheels” or move the pistons as it passes from point A to point B. If we have only one body with a certain temperature, then there is nowhere for heat to flow, and, therefore, there is nothing to do work.
The heater, the temperature of which is designated as T1, transfers energy to the working body (gas or steam). At this moment, the working fluid expands and pushes the piston. However, if the gas simply expands and remains hot, the cycle will not complete. In order for the piston to return to its original position and the cycle to repeat, the gas must be compressed. It is much more difficult to compress a hot gas than a cold one, so it must be cooled first.
This is where the refrigerator (cooler) with temperature T2comes into play. It accepts waste heat, allowing the working fluid to compress with minimal energy expenditure. The difference between the energy received from the heater and the heat transferred to the refrigerator is the very useful work that the engine does. Without this “lower” temperature, the cycle would not be possible.
Role of the heater: source of energy and expansion gas
The heater in a heat engine performs the function of an energy supplier. In internal combustion engines (ICE), the role of a heater is performed by the combustion chamber, where the fuel-air mixture is ignited. In steam turbines, the heater is a boiler in which coal, gas or fuel oil burns, heating water to steam. The temperature in this zone must be as high as possible to ensure high gas pressure.
When the working fluid receives heat from the heater, its internal energy increases sharply. Gas molecules begin to move more chaotically and at higher speeds, which leads to an increase in pressure. It is this pressure that creates the force acting on the piston or turbine blades. This stage is called the power stroke, and it is this stage that generates useful power.
It is important to understand that Efficiency (efficiency) directly depends on the temperature of the heater. The higher the temperature T1, the greater the theoretically possible efficiency of the engine. However, the materials from which the engine is made have a limit of heat resistance. Engineers have to find a balance between the desire to increase the combustion temperature and the physical capabilities of metals and alloys.
- 🔥 In an internal combustion engine, the heater is the ignition zone of the mixture, where the temperature reaches 2000°C and higher.
- 🚀 In jet engines, heating occurs in the combustion chamber in front of the turbine, creating thrust.
- ⚡ In nuclear power plants, the role of a heater is performed by a nuclear reactor that heats the coolant.
Do not forget that not all the energy released by the heater is converted into work. A significant part is spent on heating the engine parts themselves and the exhaust gases. That is why cooling systems and piston group materials are critical design elements.
Function of the refrigerator: closing the cycle and preparing for compression
If the heater provides energy, the refrigerator (cooler) makes it possible to repeat the process. After the gas has expanded and done work, it is still under high pressure and at high temperature. In order for the piston to return to its starting point (exhaust stroke or special intake stroke), the pressure inside the cylinder must drop.
The cooler removes excess heat from the exhaust gas. In automobile engines, this role is played by the atmosphere into which the exhaust gases are emitted, and the cooling system (radiator), which removes heat from the cylinder walls. The temperature T2 should be as low as possible so that the temperature difference T1 - T2 is maximum, which increases efficiency.
⚠️ Attention: In real engines, the “cooler” is often the environment. Trying to completely isolate the exhaust system without a heat exchanger will result in the engine overheating and stalling due to the inability to compress the hot gas.
The process of transferring heat to the refrigerator is called isothermal compression or cooling, depending on the cycle. The gas gives up some of its internal energy, its pressure drops, and an external force (or the inertia of the flywheel) can easily compress it, preparando to a new combustion cycle. Without this step, the engine would simply "lock up" in a high-pressure state.
In some complex systems, such as refrigeration plants (operating in a reverse cycle), the roles of the refrigerator and heater are reversed depending on the task, but the principle of two temperatures remains the same. For a heat engine, a refrigerator is an “output” for thermal energy that is useless at this stage.
Carnot cycle and energy conversion efficiency
An ideal example of the operation of a heat engine is the Carnot cycle. This is a theoretical process consisting of two isotherms and two adiabats, which demonstrates the highest possible efficiency for any two temperatures. Carnot's formula says: η = (T1 - T2) / T1. From this formula it can be seen that efficiency depends only on the temperatures of the heater and refrigerator.
To get closer to the ideal, engineers strive to increase the combustion temperature (T1) and lower the exhaust temperature (T2). However, in reality, achieving 100% efficiency is impossible, since this would require either an infinitely high heater temperature or absolute zero in the refrigerator, which is unattainable.
Why is the efficiency always less than 1?
The efficiency of a heat engine is always less than 100%, because some of the heat is inevitably transferred to the refrigerator. According to the second law of thermodynamics, it is impossible to convert all the heat into work without leaving a remainder.
Modern engines use complex heat recovery systems, turbocharging and multi-stage expansion to make the most of temperature differences. However, even the most advanced diesel engines have an efficiency of about 40-50%, the rest is lost in the form of heat lost to the refrigerator (atmosphere).
Comparison of temperature conditions in different engines
Different types of heat engines operate in vastly different temperature ranges, which determines their design and application. Below is a table illustrating typical temperature values for common units.
| Engine type | Heater temperature (T1) | Refrigerator temperature (T2) | Average efficiency |
|---|---|---|---|
| Steam engine | ~200°C | ~100°C | 10-15% |
| Gasoline internal combustion engine | ~2000°C | ~100°C | 25-30% |
| Diesel internal combustion engine | ~2200°C | ~100°C | 35-45% |
| Gas turbine | ~1300°C | ~500°C | 30-40% |
As can be seen from the table, increasing the temperature of the heater and decreasing the temperature of the refrigerator directly affects efficiency. Diesel engines are more efficient than gasoline engines, largely due to a higher compression ratio and, accordingly, higher temperatures in the cycle, as well as better use of expansion energy.
Practical limitations and heat losses
In the real world, creating an ideal heater and refrigerator is impossible. Engine materials are subject to thermal expansion, oxidation and melting. Therefore, engineers are forced to use forced cooling systems, which, in essence, are an artificial amplifier of the operation of the “refrigerator.”
Radiator, antifreeze, oil pan - all these are elements that help maintain the temperature of parts within acceptable limits, actually acting as an intermediate refrigerator before releasing heat into the atmosphere. If the cooling system fails, the engine overheats, the clearances decrease, and seizure occurs.
On the other hand, too low an engine temperature is also harmful. A cold engine operates with low efficiency, since the temperature difference has not yet reached operating mode, and the fuel does not burn completely. That is why warming up the engine before driving (at least briefly) is important for the resource of the unit.
- 🛑 Overheating leads to deformation of the cylinder head and burnout of the pistons.
- ❄️ Hypothermia increases the viscosity of the oil and wear of parts during startup.
- ⚙️ Optimal thermal conditions provide maximum Efficiency and minimum wear.
⚠️ Attention: Operating an engine with a faulty thermostat or a clogged radiator upsets the temperature balance. This can lead to the engine not reaching the designed temperature range, which will sharply reduce its power and service life.
☑️ Diagnostics of the cooling system
FAQ: Frequently asked questions
Is it possible to create an engine that does not need a refrigerator?
No, this is impossible according to the second law thermodynamics. Without a refrigerator (a body with a lower temperature), it is impossible to close the thermodynamic cycle and return the working fluid to its original state to repeat the process. An engine operating from one heat source is called a perpetual motion machine of the second kind, and its creation is impossible.
Why can’t the efficiency of the engine be 100%?
The efficiency cannot be 100%, because part of the heat must be given to the refrigerator to complete the cycle. Even in an ideal Carnot cycle, efficiency depends on the temperature difference and is always less than unity if the temperature of the refrigerator is above absolute zero.
What is a refrigerator in a jet aircraft?
In a jet engine, the role of a refrigerator is played by atmospheric air, which flows around the engine and mixes with exhaust gases, as well as heat exchangers that cool the oil and fuel. The final heat release occurs into the atmosphere.
How does ambient temperature affect engine operation?
The lower the ambient (refrigerator) temperature, the higher the theoretical efficiency of the engine. In winter, engines often operate more efficiently and consume less fuel at the same load than in hot summers, due to the greater temperature difference.