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Heat Engine - Efficiency

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Last updated date: 25th Jul 2024
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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 = \[\frac{W}{Q_{H}}\]


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.


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. Which heat engines have higher thermal efficiency?

When compared to all other heat engines, real diesel engines have higher efficiency. It is higher than that of the autocycles. This is because the diesel engines have more compression, which increases the temperature and pressure levels. After cooling down, the obtained work level will be medium or low. The difference between the input and output or the amount present in the heat reservoir and the amount present in the cold reservoir is higher than that of any other heat engine.

2. Is it Possible to Build a 100% Efficient Heat Engine?

Of course not. The thermal efficiency can be defined as a fraction of the amount of heat given to the amount of heat acquired from the heat engine. If we provide 100% heat energy, it gets reduced to its lowest level after the process has been done. So, at every point in time, we will observe a certain amount of difference between those two amounts and that difference is nothing but the thermal efficiency, which could be neither 1% nor 100%.

3. Define Stroke and Stroke Length?

Stroke is a particular phase of the piston travelled in the conversion process done in the heat engine. The stroke length is defined as the distance travelled by a piston from top to bottom or bottom to top. Stroke length is considered to be the distance travelled by a single cycle.

4.  What is a heat engine?

A heat engine is a mechanism that turns heat into mechanical energy, which may subsequently be used to perform mechanical work, according to thermodynamics and engineering. It accomplishes this by lowering the temperature of a working substance from a higher state temperature. A heat source produces thermal energy, which raises the temperature of the working substance. The working material is produced in the engine's working body while transferring heat to a colder sink until it reaches a low temperature. Using the qualities of the working substance, some of the thermal energy is turned into work throughout this process. Any system with a non-zero heat capacity can be used as the working substance, however it is most commonly a gas or a liquid. Some heat is generally lost to the environment during this process and is not converted into work. Friction and drag also render some energy ineffective.

An engine transfers energy into mechanical work in general. The efficiency of heat engines is fundamentally constrained by Carnot's theorem, which sets them apart from other forms of engines. Heat engines have a wide range of uses because the heat source that gives thermal energy to the engine can be fueled by nearly any type of energy.

5. What are the everyday examples of a heat engine?

Thermal power plants, internal combustion engines, weapons, freezers, and heat pumps are all examples of heat engines. Heat engines that run in a forward motion, in which heat flows from a hot reservoir to a cool reservoir to create work as the desired product, are examples of heat engines. Refrigerators, air conditioners, and heat pumps are examples of reverse heat engines, which employ effort to take heat energy at a low temperature and elevate it to a higher temperature in a more efficient manner than basic work-to-heat conversion. Refrigerators take heat from a low-temperature environment and vent it into a thermally sealed chamber at a higher temperature, while heat pumps take heat from a low-temperature environment and vent it into a thermally sealed chamber at a higher temperature.

Heat engines, in general, take advantage of the thermal properties associated with gas expansion and compression according to gas laws, as well as the features connected with phase shifts between gas and liquid states.

6. Describe thermal efficiency.

Heat engines convert heat to energy. The thermal efficiency is the percentage of heat that is converted into usable work. The symbol for thermal efficiency is η, and it may be computed using the following equation:

\[\eta\] = \[\frac{W}{Q_{H}}\]

W stands for beneficial work and

The total heat energy input from the hot source is denoted by Q\[_{H}\].

Due to practical constraints, heat engines typically function at roughly 30% to 50% efficiency. According to the Second Law of Thermodynamics, heat engines cannot achieve 100 percent thermal efficiency (=1). This is impossible because in a heat engine, some waste heat is always created. Even if perfect efficiency in a heat engine is impossible, there are a number of ways to improve the overall efficacy of a system.

7. What is meant by carnot efficiency?

Sadi Carnot, a scientist, discovered the greatest achievable efficiency of a heat engine. The equation for this, according to the principles of thermodynamics, is

\[\eta\] \[_{max}\] = 1-\[\frac{T_{L}}{T_{H}}\]

Where the temperature of the cold sink is TL and

The temperature of the heat reservoir is given by TH.

This is the efficiency of an idealistic engine, which is impossible to accomplish in reality.

The lower the sink temperature TL or the higher the source temperature TH, the more work is available from the heat engine, according to this equation. The work energy is derived from a reduction in the total energy of the fluid in the system. As a result, the higher the temperature shift, the larger the fluid drop, and hence, the greater the energy available to conduct work.