The operation of many types of machines is characterized by such an important indicator as the efficiency of a heat engine. Every year, engineers are striving to create more advanced equipment that, with less fuel, would give the maximum result from its use.
Heat engine device
Before you understand what efficiency is (efficiency), you need to understand how this mechanism works. Without knowledge of the principles of its action, the essence of this indicator cannot be clarified. A heat engine is a device that does work by using internal energy. Any heat engine that converts thermal energy into mechanical energy uses the thermal expansion of substances with increasing temperature. In solid-state motors, not only a change in the volume of a substance is possible, but also a body shape. The action of such an engine is subject to the laws of thermodynamics.
Principle of operation
In order to understand how a heat engine works, it is necessary to consider the basics of its design. For the device to function, two bodies are needed: hot (heater) and cold (refrigerator, cooler). The principle of operation of heat engines (efficiency of heat engines) depends on their type. Often the steam condenser acts as the refrigerator, and the heater is any type of fuel that burns in the furnace. The efficiency of an ideal heat engine is found by the following formula:
Efficiency = (Tnagrev. - Tkholod.) / Tnagrev. x 100%.
In this case, the efficiency of a real engine can never exceed the values ββobtained according to this formula. Also, this indicator will never exceed the above value. To increase efficiency, most often increase the temperature of the heater and reduce the temperature of the refrigerator. Both of these processes will be limited by the actual operating conditions of the equipment.
Efficiency of a heat engine (formula)
During the operation of the heat engine, work is performed, during which the gas begins to lose energy and cools 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 intended for cooling followed by condensation of the exhaust steam. Where there are capacitors, the temperature of the refrigerator is sometimes lower than the ambient temperature.
In a heat engine, the body, when heated and expanded, is not able to give all its internal energy to work. Some of the heat will be transferred to the refrigerator along with exhaust fumes or steam. This part of the thermal internal energy is inevitably lost. The working fluid receives a certain amount of heat Q 1 from the heater when it burns fuel. Moreover, it still does work A, during which it transfers part of the thermal energy to the refrigerator: Q 2 <Q 1 .
Efficiency characterizes the efficiency of the engine in the field of energy conversion and transmission. This indicator is often measured as a percentage. Efficiency formula:
Ξ· * A / Qx100%, where Q - energy expended, A - useful work.
Based on the law of conservation of energy, we can conclude that the efficiency will always be less than unity. In other words, there will never be more useful work than the energy expended on it.
Engine efficiency is the ratio of useful work to energy reported by the heater. It can be represented as such a formula:
Ξ· = (Q 1 -Q 2 ) / Q 1 , where Q 1 is the heat received from the heater, and Q 2 is given to the refrigerator.
Heat engine operation
The work performed by the heat engine is calculated by the following formula:
A = | Q H | - | Q X |, where A is the work, Q H is the amount of heat received from the heater, Q X is the amount of heat given to the cooler.
Efficiency of a heat engine (formula):
| Q H | - | Q X |) / | Q H | = 1 - | Q X | / | Q H |
It equals the ratio of the work that the engine does to the amount of heat received. Part of the thermal energy in this transfer 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 (T) and cooler (Tx). The efficiency of a heat engine operating according to the Carnot cycle is determined by the following formula:
(Tn - Tx) / Tn = - Tx - Tn.
The laws of thermodynamics allowed us to calculate the maximum efficiency that is possible. For the first time, this figure was calculated by the French scientist and engineer Sadie Carnot. He came up with a heat engine that worked on perfect gas. It works on a cycle of 2 isotherms and 2 adiabats. The principle of its operation is quite simple: the contact of the heater is brought to the vessel with gas, as a result of which the working fluid expands isothermally. At the same time, it functions and receives a certain amount of heat. After the vessel is thermally insulated. Despite this, the gas continues to expand, but already adiabatically (without heat exchange with the environment). At this time, its temperature drops to the indicators of the refrigerator. At this point, the gas is in contact with the refrigerator, as a result of which it gives off a certain amount of heat under isometric compression. Then the vessel is again insulated. In this case, the gas is adiabatically compressed to its original volume and state.
Varieties
Nowadays, there are many types of heat engines that operate on different principles and on different fuels. They all have their own efficiency. These include the following:
β’ An internal combustion engine (piston), which is a mechanism where part of the chemical energy of the burning fuel is converted into mechanical energy. Such devices can be gas and liquid. There are 2- and 4-stroke engines. They may have a continuous duty cycle. By the method of preparing a fuel mixture, such engines are carbureted (with external mixture formation) and diesel (with internal). By type of energy converter, they are divided into piston, jet, turbine, combined. The efficiency of such machines does not exceed the indicator of 0.5.
β’ Stirling engine - a device in which the working fluid is in a confined space. It is a type of external combustion engine. The principle of its action is based on periodic cooling / heating of the body with energy due to changes in its volume. This is one of the most efficient engines.
β’ Turbine (rotary) engine with external combustion. Such installations are most often found in thermal power plants.
β’ Turbine (rotary) ICE is used at thermal power plants in peak mode. Not as common as others.
β’ The turboprop due to the propeller creates some of the thrust. He receives the rest due to exhaust gases. Its design is a rotary engine (gas turbine), on the shaft of which a propeller is mounted.
Other types of heat engines
β’ Missile, turbojet and jet engines that receive traction due to the return of exhaust gases.
β’ Solid state motors use solid as fuel. When working, it is not its volume that changes, but the form. When operating the equipment, an extremely small temperature difference is used.
How to increase efficiency
Is it possible to increase the efficiency of a heat engine? The answer must be sought in thermodynamics. She studies the mutual transformations of different types of energy. It was established that it is impossible to convert all available thermal energy into electrical, mechanical, etc. At the same time, their conversion to thermal occurs 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 stronger the body warms up, the faster its component molecules will move. Particle movement will become even more erratic. Along with this, everyone knows that order can easily be turned into chaos, which is very difficult to streamline.