Modern realities require extensive operation of heat engines.Numerous attempts to replace them with electric motors is undergoing failure.The problems associated with the accumulation of electric power in autonomous systems are solved with great difficulty.
still relevant issues of electricity production technology batteries based on their long-term use.Speed characteristics of electric vehicles are far from those of the cars internal combustion engines.
first steps in the creation of hybrid engines can significantly reduce harmful emissions in metropolitan areas, solving environmental problems.
little history
possibility of converting steam energy into kinetic energy has been known since ancient times.130 BC: Philosopher Hero of Alexandria presented to spectators steam toy - eolipil.Sphere filled with steam, came in rotation under the action of the jets emanating from it.This prototype of the modern steam turbine at the time did not find the application.
many years and centuries of development of the philosopher considered a fun toy.In 1629 the Italian D. Branca created a proactive turbine.Steam set in motion the disc provided with blades.
Since then, the rapid development of steam engines.
heat engine
transformation of inner energy into the energy of motion of machine parts and mechanisms used in heat engines.
Main parts: a heater (system of obtaining energy from the outside), working body (makes a beneficial effect), refrigerator.
heater is designed for the working fluid to accumulate enough stock of the internal energy for useful work.Refrigerator removes excess energy.
main characteristic efficiency called the efficiency of heat engines.This value shows how much energy spent on heating is spent on useful work.The higher the efficiency, the more profitable the operation of the machine, but this value can not exceed 100%.
calculation efficiency
Let the heater purchased from outside energy equal to Q1.Working the body do the work A, while the energy Devoted refrigerator was Q2.
Based on the definition, calculate the amount of efficiency:
η = A / Q1.We assume that A = Q1 - Q2.
Hence the efficiency of a heat engine, the formula has the form η = (Q1 - Q2) / Q1 = 1 - Q2 / Q1, the following conclusions:
- efficiency can not exceed 1 (or 100%);
- to maximize this value, you must either increase the energy received from the heater, or a decrease in the energy, gave the refrigerator;
- increase energy heater work to change the quality of fuels;
- reduce energy given the refrigerator, allow to achieve the design features of engines.
ideal heat engine
possible to create such an engine, the efficiency of which would be the maximum (in an ideal - equal to 100%)?Find the answer to that question tried to French physicist and a talented engineer Sadi Carnot.In 1824 his theoretical calculations of processes in gases were released.
basic idea inherent in the ideal machine can be considered as carrying out reversible processes with ideal gas.Start with isothermal expansion of the gas at a temperature T1.The amount of heat required for this purpose, - Q1.When the gas expands without heat (adiabatic process).Reaching the temperature T2, the gas is compressed isothermally, passing the refrigerator energy Q2.The return of the gas to its original state is performed adiabatically.
efficiency of an ideal Carnot heat engine with precise calculation is the ratio of the temperature difference between the heating and cooling devices for temperature, which has a heater.It looks like this: η = (T1 - T2) / T1.
possible efficiency of a heat engine, the formula has the form: η = 1 - T2 / T1, depends only on the temperatures of the heater and cooler and can not be more than 100%.
Moreover, this relationship allows us to prove that the efficiency of heat engines can be equal to unity only when the refrigerator absolute zero temperature.As is known, this value is unattainable.
theoretical calculations allow to determine the maximum Carnot efficiency of a heat engine of any design.
Proven Carnot's theorem is as follows.An arbitrary heat engine under any circumstances are not able to have an efficiency of more than the same efficiencies ideal heat engine.
Example solving
Example 1. What is the efficiency of an ideal heat engine, when the heater temperature is 800 ° C, and the temperature below 500 ° C in the refrigerator?
T1 = 800 ° C = 1073 K, ΔT = 500 ° C = 500 K, η -?
Solution:
By definition: η = (T1 - T2) / T1.
We were not given the temperature of the refrigerator, but ΔT = (T1 - T2), hence:
η = ΔT / T1 = 500 K / 1073 K = 0.46.
Answer: efficiency = 46%.
Example 2. Determine the efficiency of an ideal heat engine, if due to the acquisition of one kilojoule of energy heater is done useful work 650 J. What is the temperature of the heater heat engine when the coolant temperature - 400 K?
Q1 = 1 kJ = 1000 J, A = 650 J, T2 = 400 K, η -?, T1 =?
Solution:
In this problem we are talking about the heat setting, the efficiency of which can be calculated by the formula:
η = A / Q1.
to determine the temperature of the heater use formula ideal heat engine efficiency:
η = (T1 - T2) / T1 = 1 - T2 / T1.
completing mathematical transformations, we obtain:
T1 = T2 / (1- η).
T1 = T2 / (1- A / Q1).
We compute:
η = 650 J / 1000 J = 0.65.
T1 = 400 K / (1- 650 J / 1000 J) = 1142.8 K.
Answer: η = 65% T1 = 1142.8 K.
actual conditions
ideal heat engine designed to meetideal processes.The work is done only in the isothermal processes, its value is defined as the area bounded by the graph of the Carnot cycle.
In fact, to create the conditions for changing the state of the process gas without accompanying changes in temperature is not possible.No such materials, which would prevent the heat exchange with the surrounding objects.Adiabatic process becomes impossible to carry out.In the case of heat transfer temperature of the gas have to be changed.
efficiency of heat engines, created under real conditions differ significantly from the ideal efficiency of the engines.Note that the process in real engines is so rapid that the variation of the internal thermal energy of working substance in the process of changing its volume can not be compensated by influx of heat amount from the heater and return the refrigerator.
Other heat engines
real engines operate on different cycles:
- Otto cycle: the process at constant volume adiabatic changes, creating a vicious cycle;
- cycle Diesel: isobar, adiabatic, isochor, adiabatic;
- gas turbine: a process that occurs at constant pressure, is replaced by adiabatic closes the loop.
Create equilibrium processes in real engines (in order to bring them closer to the ideal) in the conditions of modern technology is not possible.Efficiency of heat engines is significantly lower, even with the same temperature conditions as in the ideal heat setting.
But do not reduce the role of the calculation formula of the Carnot cycle efficiency, as it is becoming a reference point in the process to improve the efficiency of real engines.
ways to change the efficiency
Comparing ideal and real heat engines, it is worth noting that the temperature of the refrigerator can not be the last either.Usually considered a refrigerator atmosphere.Take the temperature of the atmosphere can only approximate calculations.Experience indicates that the coolant temperature equal to the temperature of exhaust gases in the engine, as is the case in internal combustion engines (abbreviated as DVS).
ICE - the most common in our world of heat engine.Efficiency heat engine in this case depends on the temperature created by burning fuel.The essential difference from the steam engine is a fusion machine functions and working fluid heater device in an air-fuel mixture.Burning, the mixture creates pressure on the moving parts of the engine.
Increasing the operating temperature of gases is achieved by changing the fuel properties significantly.Unfortunately, it is impossible to do it indefinitely.Any material which is made from a combustion chamber of the engine, has its melting point.Heat resistance of materials - the main characteristics of the engine, and the ability to significantly affect the efficiency.
Values engine efficiency
If we consider the steam turbine, the temperature of the working steam inlet which is 800 K, and the exhaust gas - 300 K, the efficiency of this machine is 62%.In reality, however, this value does not exceed 40%.This reduction is caused by heat loss upon heating the turbine housing.
greatest value efficiency of internal combustion engines does not exceed 44%.Increasing this value - the question of the near future.Changing the properties of materials, fuel - this is a problem over which employs the best minds of humanity.
