Efficiency of the heat engine. Thermal engine efficiency - determination formula

The performance of many types of machines is characterized by such an important indicator as the efficiency of a heat engine. Engineers every year strive to create a more advanced technique, which at lower fuel costs would give the maximum result from its use.

The device of the thermal engine

Before you understand what efficiency is (efficiency), you need to understand how this mechanism works. Without knowledge of the principles of its operation, it is impossible to clarify the essence of this indicator. A thermal engine is a device that does work by using internal energy. Any thermal machine that converts thermal energy into mechanical energy uses the thermal expansion of substances as the temperature rises. In solid-state engines, it is possible not only to change the volume of matter, but also the shape of the body. The action of such an engine is subject to the laws of thermodynamics.

Principle of operation

In order to understand how the heat engine works, it is necessary to consider the fundamentals of its design. Two bodies are needed for the functioning of the device: hot (heater) and cold (refrigerator, cooler). The principle of operation of thermal engines (efficiency of thermal engines) depends on their type. Often a condenser is a condenser of steam, and a heater is any kind of fuel that burns in a furnace. The efficiency of an ideal heat engine is given by the following formula:

Efficiency = (Heating - Cooling) / Heating. X 100%.

In this case, the efficiency of a real engine can never exceed the values obtained according to this formula. Also, this indicator will never exceed the above value. To increase the efficiency, most often increase the temperature of the heater and reduce the temperature of the refrigerator. Both these processes will be limited by the actual operating conditions of the equipment.

Thermal engine efficiency (formula)

When the heat engine is functioning, work is performed, as the gas begins to lose energy and is cooled to a certain temperature. The latter, as a rule, is several degrees higher than the surrounding atmosphere. This is the temperature of the refrigerator. Such a special device is designed for cooling with subsequent condensation of the exhaust steam. Where there are condensers, the temperature of the refrigerator is sometimes lower than the ambient temperature.

In a heat engine, the body, when heated and expanding, is unable to give all its internal energy to accomplish the work. Some part of the heat will be transferred to the refrigerator along with the exhaust gases or steam. This part of the thermal internal energy is inevitably lost. The working body, when combusting fuel, receives from the heater a certain amount of heat Q ₁ . At the same time, it still does work A, during which it transfers a part of the thermal energy to the refrigerator: Q ₂ 1 .

Efficiency characterizes the efficiency of the engine in the sphere of energy conversion and transmission. This indicator is often measured in percent. Efficiency formula:

Η * A / Qx100%, where Q - spent energy, A - useful work.

Proceeding from the law of conservation of energy, we can conclude that the efficiency will always be less than unity. In other words, useful work will never be more than energy spent on it.

The efficiency of the engine is the ratio of the useful work to the energy reported by the heater. It can be represented in the form of the following formula:

Η = (Q ₁ -Q ₂ ) / Q ₁ , where Q ₁ is the heat received from the heater, and Q ₂ is the heat given to the refrigerator.

Heat engine operation

The work done by the thermal engine is calculated using the following formula:

A = | Q _H | - | Q _X |, where A is work, Q _H is the amount of heat received from the heater, Q _X is the amount of heat given to the cooler.

Thermal engine efficiency (formula):

| Q _H | - | Q _X |) / | Q _H | = 1 - | Q _X | / | Q _H |

It equals the ratio of work done by the engine to the amount of heat received. Part of the heat energy in this transmission is lost.

Carnot engine

The maximum efficiency of the heat engine is noted in the Carnot device. This is due to the fact that in this system it depends only on the absolute temperature of the heater (Tn) and the cooler (Tx). The efficiency of a heat engine operating in the Carnot cycle is determined by the following formula:

(Тн - Тх) / Тн = - Тх - Тн.

The laws of thermodynamics made it possible to calculate the maximum efficiency that is possible. For the first time this figure was calculated by the French scientist and engineer Sadi Carnot. He came up with a thermal machine that functioned on an ideal gas. It operates on a cycle of 2 isotherms and 2 adiabats. The principle of its operation is quite simple: a contact of the heater with the gas leads to the vessel, so that the working medium expands isothermally. In doing so, it functions and receives a certain amount of heat. After the vessel is insulated. Despite this, gas continues to expand, but already adiabatically (without heat exchange with the environment). At this time, his temperature drops to the refrigerator. At this point, the gas contacts the condenser, which gives it a certain amount of heat under isometric compression. Then the vessel is again insulated. At the same time, the gas is adiabatically compressed to its original volume and state.

Varieties

Nowadays, there are many types of thermal engines that operate on different principles and on different fuels. They all have their own efficiency. These include the following:

• Internal combustion engine (piston), which is a mechanism where part of the chemical energy of the combustion fuel passes into mechanical energy. Such devices can be gas and liquid. Distinguish between 2- and 4-stroke engines. They can have a continuous duty cycle. By the method of preparation of a mixture of fuel, such engines are carburettor (with external mixture formation) and diesel (with internal). By types of the energy converter they are divided into piston, reactive, turbine, combined. The efficiency of such machines does not exceed 0.5.

• The Stirling engine is a device in which the working medium is in a confined space. It is a type of external combustion engine. The principle of its operation is based on periodic cooling / heating of the body with the production of energy due to a change in its volume. This is one of the most efficient engines.

• Turbine (rotary) engine with external combustion of fuel. Such installations are most often found at thermal power plants.

• Turbine (rotary) ICE is used at thermal power plants in peak mode. Not as common as others.

• The turbine engine due to the screw creates some part of the thrust. The rest he receives due to exhaust gases. Its design is a rotary engine (gas turbine), on the shaft of which the air screw is attached.

Other types of thermal engines

• Missile, turbojet and jet engines, which receive traction through the return of exhaust gases.

• Solid-state engines use a solid as fuel. When working, not its volume changes, but its shape. When operating the equipment, an extremely small temperature drop is used.

How to improve efficiency

Is it possible to increase the efficiency of the heat engine? The answer must be sought in thermodynamics. She studies the mutual transformations of different types of energy. It is established that it is impossible to convert all available thermal energy into electric, mechanical, etc. At the same time, their transformation into thermal energy takes place without any restrictions. This is possible due to the fact that the nature of thermal energy is based on the disordered (chaotic) motion of particles.

The more heated the body, the faster the molecules that make up it will move. The movement of particles will become even more erratic. Along with this, everyone knows that order can easily be turned into chaos, which is very difficult to order.