Heat Engine - Efficiency

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Heat Engine - Efficiency in Detail

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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.

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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.

Define the Efficiency of the Heat Engine?

Generally, we know that efficiency is capability. However, here the efficiency of a heat engine is the difference between the hot source and sink. That is the difference between the heat reservoir and the cold reservoir. It can also be termed as the thermal efficiency of the heat engine. The maximum efficiency of a heat engine is the highest difference between hot and cold reservoirs. Kelvin is the measuring unit for this efficiency.

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% efficiency 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.

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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 = W/QH


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 to 60 % of thermal efficiency because of the environmental changes and other factors. We can also consider the work attained is the difference between the initially sent amount of heat and the obtained heat. It can be expressed as,

(η) = [Q1 - Q2]/Q1

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, it's efficiency is [T1 - T2]/T1. It can be measured for every Carnot cycle.

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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.

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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 the heat engine is 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.

FAQ (Frequently Asked Questions)

1. Which Heat Engines have Higher Thermal Efficiency?

Ans. 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 we can say that the amount present in the heat reservoir and the amount present in the cold reservoir is very high than that of any other heat engine.

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

Ans. 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?

Ans. 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 as the distance travelled by a single cycle.