need for use of mechanical energy in the production led to thermal machines.
device heat engines
heat engine (heat engine) - a device for converting internal energy into mechanical energy.
Any heat engine has a heater, the working fluid (gas or vapor), which performs work by heating (rotates a turbine shaft, the piston moves, and so on) and a refrigerator.The figure below shows a diagram of the heat engine.
basis of the current heat engines
Each heat engine functioning due to the engine.To perform the work he needs to on either side of the piston engine or turbine blades was pressure difference.This difference is achieved in all the heat engines as follows: the temperature of the working body rises hundreds or thousands of degrees in comparison with the ambient temperature.In gas turbines and internal combustion engines (ICE), the temperature rises due to the fact that the fuel is burned inside the engine.Refrigerator can act atmosphere or special-purpose devices for condensing and cooling waste steam.
Carnot cycle
cycle (cyclic process) - a set of changes in the state of gas, as a result of which it returns to its original state (can do the job).In 1824, French physicist Sadi Carnot showed that it is advantageous to heat engine cycle (Carnot cycle), which consists of two processes - isothermal and adiabatic.The figure below shows a graph of the Carnot cycle: 1-2 and 3-4 - isotherm, 2-3 and 4-1 - adiabatic.
In accordance with the law of conservation of energy, the work of heat engines, which carries the engine, is:
A = Q1- Q2,
where Q1 - the amount of heat that is produced from the heater, and Q2 - the amount of heat which isconsigned to the refrigerator.
thermal efficiency is the ratio of the work machine A which executes the engine to the quantity of heat which is obtained from the heater:
η = A / Q = (Q1- Q2) / Q1 = 1 - Q2 / Q1.
In "Thoughts on the motive power of fire and on machines that are able to develop this force" (1824) described the Carnot heat engine called "ideal heat engine with an ideal gas, which is a working body."Due to the laws of thermodynamics can calculate the efficiency (the maximum possible) with the heat engine heater which has a temperature T1, and a refrigerator with temperature T2.Carnot heat engine efficiency is:
ηmax = (T1 - T2) / T1 = 1 - T2 / T1.
Sadi Carnot proved that what you want is a real heat engine that operates with a heater with a temperature T1 and a fridge with a temperature T2 is not able to have a COP which would be higher than the efficiency of a heat engine (ideal).
internal combustion engine (ICE)
four-stroke internal combustion engine consists of one or more cylinders, piston, crank mechanism, intake and exhaust valves, spark.
working cycle consists of four measures:
1) suction - combustible mixture flows through the valve in the cylinder;
2) compression - both valves closed;
3) stroke - the explosive combustion of the fuel mixture;
4) exhaust - exhaust emissions.
steam turbine in the steam turbine energy conversion occurs due to the pressure difference of water vapor on the inlet and outlet.
power of modern steam turbines reach 1,300 MW.
Some technical parameters of steam turbine capacity of 1200 MW
- Vapor Pressure (fresh) - 23.5 MPa.
- steam temperature - 540 ° C.
- consumption steam turbine - 3600 t / h.
- Rotor speed - 3000 rpm / min.
- steam pressure in the condenser - 3.6 kPa.
- length of Turbine - 47.9 m.
- Mass turbine - 1900 m.
heat engine consists of an air compressor, a combustion chamber and the gas turbine.Principle: air is sucked into the compressor adiabatically so that its temperature rises to 200 ° C or more.Further, the compressed air enters the combustion chamber, where both high pressure liquid fuel is supplied - kerosene, Photog, oil.When combustion air is heated to a temperature of 1500-2000 ° C, it expands and increases its velocity.The air moving at high speed, and the products of combustion are directed into the turbine.After the transition from one stage to the combustion turbine blades give its kinetic energy.Part of the energy received by the turbine is to rotate the compressor;the remainder is spent on the rotation of the rotor generator, propeller aircraft or ship, the wheels of the car.
gas turbines can be used in addition to the rotation of the wheels of the car and aircraft propellers or ship, as the jet engine.Air and combustion products are ejected at high velocity from the gas turbine, so the jet thrust, which occurs during this process, can be used to move aircraft (airplane) and water (boat) ships, railway transport.For example, turboprops have aircraft AN-24, AN-124 ("Ruslan"), AN-225 ("Dream").Thus, the "Dream" at a flight speed of 700-850 km / h is capable of carrying 250 tons of cargo over a distance of almost 15 000 km.It is the largest transport aircraft in the world.
Environmental problems of thermal machines
great influence on the climate has a state of the atmosphere, in particular the presence of carbon dioxide and water vapor.Thus, the change in carbon dioxide content leads to an increase or decrease in the greenhouse effect, in which carbon dioxide absorbs some of the heat that radiates into the earth space, delays in the atmosphere and thereby increases the surface temperature and the lower atmosphere.The phenomenon of the greenhouse effect plays a crucial role in mitigating climate change.In its absence, the average temperature of the atmosphere would not be +15 ° C and lower by 30-40 ° C.
Now there are more than 300 million of various types of vehicles, which create more than half of all air pollution.
1 year into the atmosphere from thermal power plants from fuel combustion released 150 million tons of sulfur oxides, 50 million tons of nitrogen oxide, 50 million tons of ash, 200 million tons of carbon monoxide, 3 million tons of Theon.
The composition of the atmosphere includes ozone, which protects all life on earth from the harmful effects of ultraviolet rays.In 1982, George. Farman, the British explorer, over the Antarctic ozone hole was discovered - a temporary reduction of ozone in the atmosphere.At the peak of the ozone hole 7 October 1987 the amount of ozone in it has decreased by 2 times.The ozone hole is probably arose as a result of anthropogenic factors, including industrial use of chlorine-containing Freon (CFCs) that deplete the ozone layer.However, the 1990 study.not confirmed this view.Most likely, the emergence of the ozone hole is not related to human activities, and is a natural process.In 1992, the Arctic ozone hole was discovered.If all
collect atmospheric ozone layer in the earth's surface and thicken it to the density of air at atmospheric pressure and a temperature of 0 ° C, the thickness of the ozone shield is only 2-3 mm!That's the whole board.
bit of history ...
- July 1769.In the Paris park Meudon military engineer NZ Kyunyo to "fire wagon", which was equipped with a two-cylinder steam engine, drove a few tens of meters.
- 1885.Karl Benz, a German engineer, built the first four-wheeled automobile gasoline Motorwagen power of 0.66 kW, which 29 January 1886 received a patent.Machine speed reaches 15-18 km / h.
- 1891.Gottlieb Daimler, German inventor, made the pallet truck with an engine capacity of 2.9 kW (4 horsepower) from the car.Its top speed was 10 km / h, capacity in various models ranging from 2 to 5 tons.
- 1899.Belgian K. Zhenattsi in his car, "Jamet Contant" ("never satisfied") for the first time crossed the 100-kilometer line speed.
Examples of solving problems
Task 1. heater temperature ideal heat engine is equal to 2000 K, and refrigerator temperature - 100 ° C.Determine efficiency.
decision :
formula that determines the efficiency of a heat engine (maximum):
ŋ = T1-T2 / T1.
ŋ = (2000K - 373 K) / 2000 K = 0.81.
Answer: engine efficiency - 81%.
Task 2. In the heat engine during the combustion of fuel was obtained 200 kJ of heat transferred to the refrigerator and 120 kJ of heat.What is the efficiency of the engine?
Solution:
formula for determining the efficiency of this type is:
ŋ = Q1 - Q2 / Q1.
ŋ = (2 · 105 J - 1.2 x 105 J) / 2 · 105 J = 0.4.
Answer: heat engine efficiency - 40%.
Task 3. What is the efficiency of a heat engine, if the working body after receiving the amount of heat from the heater to perform the work of 1.6 MJ 400 kJ?How much heat was transferred to the refrigerator?
Solution:
efficiency can be determined by the formula
ŋ = A / Q1.
ŋ = 0,4 · 106 J / 1,6 · 106 J = 0.25.
Transmitted fridge amount of heat can be determined by the formula
Q1 - A = Q2.
Q2 = 1,6 · 106 J - 0.4 · 106 J = 1.2 × 106 J.
Answer: heat engine has an efficiency of 25%;the amount of heat transferred to the refrigerator - 1.2 x 106 J.
