[Physics Class Notes] on Heat Engine – Efficiency Pdf for Exam

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:

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.

Q_H  =  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.

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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.

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.