Understanding the operating principles of heat engines is fundamental to the study of thermodynamics and modern energy. These devices, which convert the internal energy of fuel into mechanical work, surround us everywhere: from car engines to giant turbines of power plants. To understand exactly how this transformation occurs, it is necessary to examine in detail three key elements of the system: the heater, the working fluid and the refrigerator.
Each of these parts performs a strictly defined function, without which the continuous cycle of engine operation would be impossible. Violation of the interaction between these components leads to a stop of the mechanism or a sharp drop in its efficiency. In this article, we will take a deep dive into the physics of the processes occurring inside the engine and find out why the presence of all three elements is critical.
It is worth noting that although we often talk about internal combustion engines, the principles described are universal for all heat engines. Whether it's a 19th century steam engine or a modern aircraft jet turbine, the basic design remains the same. It is the cyclical nature of the process that dictates the need for a heat source, an energy converter and a cooler.
Heater: source of internal energy
The first and main link in the energy conversion chain is heater. This is a body or system that has a higher temperature compared to other engine elements. It is from the heater that the working fluid receives the necessary portion of heat, which will subsequently partially turn into useful mechanical work.
In different types of engines, the role of a heater can be performed by completely different objects. In internal combustion engines, these are combustion products of the fuel-air mixture formed directly in the cylinder. In steam turbines, the heater is steam superheated in the boiler due to the combustion of coal, gas or a nuclear reaction.
⚠️ Attention: The heater temperature is the limiting factor for the efficiency of the engine. The higher the temperature of the heater, the greater the theoretically possible efficiency, but the engine materials must withstand these loads without destruction.
The function of the heater is not limited to simple heat transfer. It creates the necessary temperature gradientwhich causes the working fluid to expand and move. Without a constant supply of energy from the heater, the pressure in the fast system would drop, and the movement of the piston or turbine would stop.
- 🔥 Provides heat ($Q_1$) to the working fluid.
- 🌡️ Maintains the high temperature necessary for expansion gas.
- ⚙️ Compensates for energy losses, keeping the work cycle continuous.
Working fluid: converter energy
The central element of any heat engine is working fluid. This is a substance (usually gas or steam) that directly participates in the thermodynamic cycle. It is the working fluid that receives heat from the heater, expands, performing mechanical work, and then gives off part of the energy to the refrigerator.
Gases are most often used as a working fluid, since they have a high compressibility ability for significant expansion when heated. In steam engines this is water vapor, in Stirling engines helium or hydrogen can be used, and in internal combustion engines - a mixture of air and combustion products.
The process of operation of the working fluid can be described through a change in its state. When heated, gas molecules begin to move faster, increasing pressure on the cylinder walls or turbine blades. This pressure is transferred to the mechanical parts of the engine, causing them to move. Then, having given up part of the energy, the gas is compressed and returned to its original state to repeat the cycle.
It is important to understand that the working fluid does not disperse during operation (in a closed cycle), but only changes its parameters: pressure, volume and temperature. However, in internal combustion engines, the composition of the working fluid changes after each stroke, since exhaust gases are ejected and fresh gases are re-entered.
Refrigerator: waste heat remover
The third mandatory component of the system is refrigerator. Contrary to the everyday understanding of this word, in thermodynamics a refrigerator is any body with a temperature lower than that of the working fluid at the end of the expansion cycle. Its main function is to remove heat ($Q_2$), which the working fluid could not convert into work.
The existence of a refrigerator is dictated by the second law of thermodynamics. It is impossible to create an engine that would completely convert heat into work without releasing part of the energy into the environment. Therefore, a refrigerator is necessary to closing the cycle and return the working fluid to its original state with minimal energy consumption.
| Engine type | Role of the refrigerator | Temperature conditions |
|---|---|---|
| Steam turbine | Condenser with running water | About 20-30°C |
| Car internal combustion engine | Atmospheric air (through the exhaust) | About 800-1000°C (exhaust) |
| Jet engine | Ambient atmosphere | Depends on flight altitude |
| Stirling engine | Finned cooling radiator | 40-60°C |
In real conditions, the role of a refrigerator is often played by atmospheric air or water from a nearby reservoir. The efficiency of the refrigerator directly affects the efficiency: the lower the temperature of the refrigerator, the greater the temperature difference with the heater and the higher the efficiency of the engine.
Interaction of parts in the thermal cycle
All three components - heater, working fluid and refrigerator - work in strict sequence, forming thermodynamic cycle. First, the working fluid comes into contact with the heater, receiving energy. It then expands and pushes the piston. After this, it connects to the refrigerator, releasing residual heat, and is compressed for a new cycle.
This process can be represented as a continuous flow of energy. Heat flows from a hot body to a cold one, and along this path part of the energy is “removed” in the form of mechanical work. If you remove any of the elements, the flow will stop. For example, without a refrigerator, the gas will not be compressed back, and the piston will not be able to make a new stroke.
For a better understanding, consider the classical Carnot cycle, which is an idealized example of the operation of a heat engine. In this cycle, the stages of isothermal expansion (contact with the heater) and isothermal compression (contact with the refrigerator) are clearly visible.
- 🔄 Heating and expansion: energy absorption.
- 💨 Mechanical stroke: performing useful work.
- ❄️ Cooling and compression: heat release and preparation for repetition.
⚠️ Attention: In real engines, cycles differ from ideal ones due to friction, heat loss through the cylinder walls and incomplete combustion of fuel. This reduces the actual efficiency compared to the theoretical one.
Why can't all the heat be used?
According to the second law of thermodynamics, it is impossible to convert 100% of thermal energy into mechanical work. Part of the energy must be dissipated in the environment through the refrigerator, otherwise the entropy of the system will not allow the cycle to repeat.
The influence of the design on the efficiency of the parts
The design of the heater, working fluid and refrigerator determines the class of the engine. In external combustion enginessuch as steam engines, the heater is separated from the working fluid by the walls of the boiler. This allows the use of any fuel, but reduces the rate of heat transfer.
B internal combustion engines the heater is the working fluid itself (combustion gases). This ensures high efficiency and compactness, but imposes stringent requirements on the heat resistance of the materials of the cylinders and pistons. The refrigerator is the exhaust gas exhaust system.
Modern engineering solutions are aimed at optimizing each element. Improving the shape of the combustion chamber, using turbocharging to increase the mass of the working fluid and using more efficient cooling radiators are all ways to increase power and efficiency.
☑️ Factors for increasing engine efficiency
Problems and limitations of real engines
Despite the well-established theory, in practice engineers are faced with a number of limitations. Heater materials may melt at too high temperatures, limiting the upper limit of efficiency. The refrigerator cannot be colder than the environment without spending additional energy, which sets the lower limit.
The working fluid also makes its own adjustments. Gases can become corrosive at high temperatures, causing corrosion. In addition, at high pressures, the properties of gases deviate from ideal, which requires complex calculations and adjustments in the design.
The environmental aspect also becomes critical. The products of interaction between the working fluid and fuel are often toxic. Therefore, modern systems include complex filters and catalytic converters, which, in fact, are additional elements of the cleaning system before being released into the “refrigerator” (atmosphere).
Can an engine operate without a refrigerator?
No, it cannot. The absence of a refrigerator means the inability to remove heat. The working fluid will not return to its original state, and the cycle will be interrupted after the first expansion stroke. This contradicts the second law of thermodynamics.
Why is the engine efficiency always less than 100%?
The efficiency is always less than one, because part of the heat must be given to the refrigerator. It is impossible to convert all thermal energy into mechanical work in a cyclic process. The maximum possible efficiency is determined by the Carnot formula.
What happens if the heater overheats?
Overheating of the heater (combustion chamber or boiler) can lead to burnout of the walls, melting of the pistons, or even an explosion due to exceeding the design pressure. The cooling system and thermostats are designed to prevent such situations.