Heat Engine - Efficiency in Detail
Vehicles are widely used means of transport to move from one place to another. Nowadays, every family can possess at least two-wheelers. Just imagine how the vehicles are moving? What is the energy used in it? What is the process that has undergone? A heat engine is the only answer to all those questions.
What is the Heat Engine?
A heat engine is a device used to convert heat energy into mechanical work which is useful for people. It uses a simple apparatus to perform the procedure. The heat engine processes several advantages along with few limitations.
Classification of Heat Engine
We have five different types of heat engines. There are two types of well-known and widely used heat engines among the five. The characterization has taken place based on the principle which is used to convert heat energy into mechanical work. So the types of heat engines are as follows:
Internal Combustion Engine.
Stirling Engine.
Diesel engine.
Steam engine.
Reciprocally steam engine.
What is the function of a Heat Engine?
The primary function of any heat engine is to convert the available heat energy into useful mechanical work. It undergoes various procedures to convert the same.
Definition of the efficiency of the Heat Engine
Generally, we know that efficiency is capability. However, here the efficiency of a heat engine is the ratio of difference between the hot source and sink to the temperature of the hot source. It can also be termed as the thermal efficiency of the heat engine. The maximum efficiency of a heat engine is possible if there is a highest difference between hot and cold reservoirs. Efficiency does not have any unit.
The thermal efficiency may vary from one heat engine to another heat engine. To understand more about this, let's take the reliable heat engines and their efficiencies. The efficiencies of various heat engines are as follows:
It is just 3% efficient for ocean thermal energy conservation.
Automotive gasoline engines are nearly 25% efficient.
Similarly, coal-fired power stations have 49% efficiency.
It is around 60% efficient for the combined cycle gas turbine.
The Efficiency of a Heat Engine Formula
As the efficiency of the heat engine is a fraction of heat and the obtained useful work, it can be expressed using a formula and a symbol. The efficiency of heat energy formula is,
η = \[\frac{W}{Q_{H}}\]
Where,
η = Thermal efficiency.
W = Useful work obtained.
QH = Given amount of heat energy.
This is known as the heat engine formula.
According to the second law of thermodynamics, it is impossible to get 100 percent of the thermal efficiency. It always ranges between 30% and 60% of thermal efficiency because of the environmental changes and other factors. We can also consider the work attained to be the difference between the initially absorbed amount of heat and the heat released. It can be expressed as
(η) = \[\frac{\left [ Q_{1} -Q_{2}\right ]}{Q_{1}}\]
The heat engine concept was first introduced and discovered by a French Physicist Carnot in 1824. The Carnot engine is the ideal heat engine. As it is the most efficient heat engine, its efficiency is \[\frac{\left [ T_{1}-T_{2} \right ]}{T_{1}}\]. It can be measured for every Carnot cycle.
From the formula and diagram, we can understand that the efficiency of an ideal heat engine also depends on the difference between the hot and cold reservoirs.
PV Diagram
It is the pressure-volume diagram which helps to study and analyze the efficiency of a heat engine. It acts as a visualization tool for the heat engine. As we know that the working substance will be any gas, the PV diagram explains the visuals from the heat engine by considering the ideal gas law. Even though the temperature may vary continuously, the PV diagram helps to explain the three elements of the state of the variables. It also uses the first law of thermodynamics to explain the variations in heat engines.
(image will be uploaded soon)
(Image will be uploaded soon)
If we observe the figure, we can understand that it is the PV diagram of a single cyclic heat engine process. It appeared as a closed-loop. The area inside the loop represents the amount of work we have done in the process and the amount of useful work we obtained. The pressure-volume diagram is beneficial and an advantageous visualization tool to study and analyze the heat engine.
Conclusion
Hence, the heat engine is a system of converting heat energy into mechanical work. The efficiency of a heat engine is the ratio of difference between the hot source and sink to the temperature of the hot source. The efficiency of the heat engine depends on the difference between a hot reservoir and a cold reservoir. We have delivered the formula to find out the efficiency of a heat engine. Also, we can't get 100% efficiency for any heat engine.
FAQs on Heat Engine - Efficiency
1. What is a heat engine and what is its fundamental purpose in thermodynamics?
A heat engine is a device designed to convert thermal energy (heat) into mechanical work. Its fundamental purpose is to take heat from a high-temperature source (hot reservoir), use a working substance to perform work, and then release the remaining heat to a low-temperature sink (cold reservoir). This process is central to thermodynamics and powers everything from car engines to power plants.
2. What is meant by the thermal efficiency of a heat engine?
The thermal efficiency of a heat engine is a measure of how effectively it converts the heat energy it receives into useful mechanical work. It is defined as the ratio of the net work done by the engine to the amount of heat absorbed from the high-temperature source. A higher efficiency means a larger fraction of the heat is converted into work.
3. How is the thermal efficiency of a heat engine calculated?
The thermal efficiency (η) is calculated using the formula:
η = W / QH
Where:
- W is the useful work output.
- QH is the heat absorbed from the hot reservoir.
Alternatively, since the work done is the difference between the heat absorbed (QH) and the heat rejected (QL), the formula can be expressed as:
η = (QH - QL) / QH = 1 - (QL / QH)
4. What are the main types of heat engines based on where combustion occurs?
Heat engines are primarily classified into two main types based on the location of the heat-generating combustion process:
- Internal Combustion (IC) Engines: In these engines, the fuel combustion occurs inside the engine's working chamber. Examples include the petrol and diesel engines found in most vehicles.
- External Combustion (EC) Engines: In these engines, the combustion process takes place outside the engine. The heat is then transferred to a working fluid. A classic example is the steam engine, where burning coal or another fuel heats water in a separate boiler.
5. What are some common real-world examples of heat engines?
Heat engines are a fundamental part of modern technology. Common examples include:
- The internal combustion engine in cars, motorcycles, and trucks.
- Diesel engines used in trains, ships, and heavy machinery.
- Steam turbines used in thermal and nuclear power plants to generate electricity.
- Jet engines on aircraft, which are a type of gas turbine engine.
6. Why can a heat engine never be 100% efficient?
A heat engine can never achieve 100% efficiency due to the Second Law of Thermodynamics. This law states that it is impossible to construct a device that operates in a cycle and converts all the absorbed heat into work. Some amount of heat must always be rejected to a colder reservoir (the sink). This unavoidable heat loss to the surroundings prevents any real or ideal heat engine from reaching perfect efficiency.
7. How does the efficiency of a real engine compare to the theoretical Carnot efficiency?
The Carnot efficiency represents the maximum possible theoretical efficiency for any heat engine operating between two specific temperatures. In contrast, the efficiency of a real engine (like a car's engine) is always lower than the Carnot efficiency. This is because real engines suffer from practical energy losses due to factors like friction between moving parts, incomplete combustion of fuel, and heat loss from the engine to the environment.
8. What is the role of the P-V (Pressure-Volume) diagram in explaining a heat engine's efficiency?
A P-V diagram is a crucial tool for analysing a heat engine's operation. It graphically represents the changes in pressure and volume of the working substance during a thermodynamic cycle. The most important feature is that the area enclosed by the closed loop on the P-V diagram is numerically equal to the net work done (W) by the engine in one complete cycle. By analysing this area, one can determine the work output and thus help calculate the engine's efficiency.
9. Why is the term 'Coefficient of Performance' (COP) used for refrigerators instead of 'efficiency'?
The term 'efficiency' is used for devices like heat engines where the desired output is work. For a refrigerator or a heat pump (a reverse heat engine), the goal is not to produce work but to transfer heat by using work as an input. The Coefficient of Performance (COP) measures the effectiveness of this heat transfer. It is the ratio of desired heat transferred to the work input. Since this value can be greater than 1, the term 'efficiency,' which is always less than 1 for a heat engine, is not appropriate.
