Thermal machine: cycle, operation, efficiency. Ecological problems of thermal machines. What is the ideal thermal machine?

The need for the use of mechanical energy in production led to the appearance of thermal machines.

The device of thermal machines

A thermal machine (thermal engine) is a device for converting internal energy into mechanical energy.

Any thermal machine has a heater, a working body (gas or steam), which as a result of heating performs work (drives the turbine shaft, moves the piston and so on) and the refrigerator. The diagram below shows the scheme of the heat engine.

The fundamentals of the action of thermal engines

Each thermal machine is operated by the engine. To do the job, he needs to have a pressure difference on either side of the engine piston or turbine blades. This difference is achieved in all thermal engines as follows: the temperature of the working fluid rises by hundreds or thousands of degrees in comparison with the ambient temperature. In gas turbines and in internal combustion engines (ICE), the temperature rises due to the fact that the fuel burns inside the engine itself. The refrigerator can be an atmosphere or a special purpose device for condensing and cooling the used steam.

The Carnot cycle

Cycle (circular process) - a set of changes in the state of the gas, as a result of which it returns to its original state (it can do work). In 1824, the French physicist Sadi Carnot showed that the cycle of a heat engine (the Carnot cycle), which consists of two processes - isothermal and adiabatic, is beneficial. The figure below shows a graph of the Carnot cycle: 1-2 and 3-4 - isotherms, 2-3 and 4-1 - adiabats.

In accordance with the law of conservation of energy, the work of thermal machines performed by the engine is:

A = Q ₁ - Q ₂ ,

Where Q ₁ is the amount of heat that is received from the heater, and Q ₂ is the amount of heat that is delivered to the refrigerator.
The efficiency of a heat engine is the ratio of the work A, performed by the engine, to the amount of heat that is received from the heater:

Η = A / Q = (Q ₁ - Q ₂ ) / Q ₁ = 1 - Q ₂ / Q ₁ .

In his work "Thoughts on the driving force of fire and on machines that can develop this force" (1824) Carnot described a thermal machine called "an ideal thermal machine with an ideal gas that is a working fluid." Thanks to the laws of thermodynamics, it is possible to calculate the efficiency (the highest possible) of a heat engine with a heater that has a temperature T ₁ and a refrigerator with a temperature T ₂ . Thermal machine Carnot has an efficiency:

Η _max = (T ₁ - T ₂ ) / T ₁ = 1 - T ₂ / T _1.

Sadi Carnot proved that any thermal machine is real, which works with a heater with temperature T ₁ and a refrigerator with temperature T _{2 is} not capable of having an efficiency that would exceed the efficiency of the thermal machine (ideal).

The internal combustion engine (ICE)

The four-stroke ICE consists of one or several cylinders, a piston, a crank mechanism, an intake and exhaust valve, and a candle.

The duty cycle consists of four cycles:

1) suction - the combustible mixture enters the cylinder through the valve;
2) compression - both valves are closed;
3) working stroke - explosive combustion of a combustible mixture;
4) exhaust - exhaust of exhaust gases into the atmosphere.

Steam turbine

In a steam turbine, energy conversion occurs due to the difference in pressure of water vapor at the inlet and outlet.
The capacities of modern steam turbines reach 1300 MW.

Some technical parameters of a 1200 MW steam turbine

• The vapor pressure (fresh) is 23.5 MPa.
• The temperature of the steam is 540 ° C.
• The steam flow rate of the turbine is 3600 t / h.
• The rotor speed is 3000 rpm.
• The vapor pressure in the condenser is 3.6 kPa.
• The length of the turbine is 47.9 m.
• The mass of the turbine is 1900 tons.

The thermal machine consists of an air compressor, a combustion chamber and a gas turbine. Principle of operation: air is adiabatically sucked into the compressor, so its temperature rises to 200 ° C or more. Further compressed air enters the combustion chamber, where simultaneously, under high pressure, liquid fuel enters - kerosene, photogen, fuel oil. When the fuel burns, the air heats up to a temperature of 1500-2000 ° C, expands, and the speed of its movement increases. The air moves at a high speed, and the combustion products are sent to the turbine. After transitioning from the stage to the stage, the combustion products give their turbine blades their kinetic energy. Part of the energy received by the turbine goes to the rotation of the compressor; The remaining part is spent to rotate the rotor of an electric generator, an airplane screw or a sea ship, car wheels.

A gas turbine can be used, in addition to the rotation of the car's wheels and propeller or boat screws, as a jet engine. Air and combustion products are discharged at high speed from the gas turbine, therefore the reactive thrust that arises in this process can be used for the travel of air (aircraft) and water (ship) vessels, railway transport. For example, turboprop engines have An-24, An-124 (Ruslan), An-225 (Dream). Thus, the "Dream" at a flight speed of 700-850 km / h is capable of transporting 250 tons of cargo at a distance of almost 15,000 km. It is the largest transport aircraft in the world.

Environmental problems of thermal machines

A great influence on the climate is the state of the atmosphere, in particular the presence of carbon dioxide and water vapor. Thus, a change in the content of carbon dioxide leads to an intensification or weakening of the greenhouse effect, in which carbon dioxide partially absorbs the heat that the Earth emits into space, delays it in the atmosphere and thereby increases the temperature of the surface and the lower layers of the atmosphere. The phenomenon of the greenhouse effect plays a decisive role in climate mitigation. In its absence, the average temperature of the planet would not be + 15 ° C, but lower by 30-40 ° C.

Now in the world there are more than 300 million different kinds of cars, which create more than half of all atmospheric pollution.

For 1 year, 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 phonon are released to the atmosphere from thermal power plants as a result of fuel combustion.

The composition of the atmosphere includes ozone, which protects all life on earth from the harmful effects of ultraviolet rays. In 1982, J. Farman, an English researcher, discovered an ozone hole over Antarctica - a temporary decrease in ozone in the atmosphere. At the time of the maximum development of the ozone hole on October 7, 1987 the amount of ozone in it decreased by a factor of 2. The ozone hole probably originated as a result of anthropogenic factors, including the use of chlorine-containing chladones (freons) in the industry, which destroy the ozone layer. However, the studies of the 1990s. Did not confirm this point of view. Most likely, the appearance of an ozone hole is not related to human activity and is a natural process. In 1992, an ozone hole was discovered over the Arctic.

If all of the atmospheric ozone is collected in a layer near the surface of the Earth and thickened to a density of air at normal atmospheric pressure and 0 ° C, the thickness of the ozone shield will be only 2-3 mm! That's the whole shield.

A bit of history ...

• July of 1769. In the Meudon park in Paris, the military engineer N.J. Cunyo on a "fiery cart", which was equipped with a two-cylinder steam engine, drove several tens of meters.
• 1885 year. Karl Benz, a German engineer, built the first gasoline four-stroke three-wheel motor car of Motorwagen with a capacity of 0.66 kW, which on January 29, 1886 received a patent. The speed of the car reached 15-18 km / h.
• 1891 year. Gottlieb Daimler, the German inventor, produced a freight trolley with a 2.9 kW (4 horsepower) engine from the car. The maximum speed of the car reached 10 km / h, the carrying capacity in various models was from 2 to 5 tons.
• 1899 year. Belgian K. Zhenatzi in his car "Jame Contant" ("Always dissatisfied") for the first time overcame the 100-kilometer speed limit.

Examples of problem solving

Problem 1. The temperature of the heater is an ideal heat machine equal to 2000 K, and the temperature of the refrigerator is 100 ° C. Determine the efficiency.

Solution :
The formula, which determines the efficiency of the thermal machine (maximum):

Ŋ = T ₁ -T ₂ / T _1.
Ŋ = (2000К-373К) / 2000 К = 0.81.

Answer: Engine efficiency is 81%.

Task 2. In a heat engine, when fuel was burned, 200 kJ of heat was received, and 120 kJ of heat was transferred to the refrigerator. What is the efficiency of the engine?

Decision:
The formula for determining the efficiency is as follows:

Ŋ = Q1 - Q2 / Q1.
Ŋ = (2 · 10 ⁵ J - 1.2 · 10 ⁵ J) / 2 · 10 ⁵ J = 0.4.

Answer: The efficiency of the heat engine is 40%.

Task 3. What is the efficiency of the heat engine, if the working body after the heat quantity of the heater has received 1.6 MJ performed 400 kJ? How much heat was transferred to the refrigerator?

Decision:
Efficiency can be determined by the formula

Ŋ = A / Q _1.

Ŋ = 0.4 · 10 ⁶ J / 1.6 · 10 ⁶ J = 0.25.

The amount of heat transferred to the refrigerator can be determined from formula

Q ₁ - A = Q _2.
Q ₂ = 1.6 · 10 ⁶ J-0.4 · 10 ⁶ J = 1.2 · 10 ⁶ J.
Answer: The heat machine has an efficiency of 25%; The amount of heat transferred to the refrigerator is 1.2 × 10 ⁶ Joules.