O n t h e T r a c k o f M o d e r n P h y s i c s
4/1
With full throttle
http://www.mpshq.com/images/products_gasturbines1.jpg
Not only Carnot
In a diesel engine, the air is compressed adiabatically of a factor from 15 to 20. This
compression increases the temperature until the
temperature of ignition of the mixture of fuel which is
formed by injecting fuel once that one compressed the air.
In 1769, the very first vehicle with autonomous propulsion was a military tractor invented by a French engineer and mechanic, Nicolas Joseph
Cugnot (1725 - 1804). Cugnot used a steam engine to propulse its vehicle, built following his
instructions in Arsenale of Paris by the mechanic named Brezin. It was used by the French Army to trail artillery at the exorbitant speed of 4 km/h on just 3 wheels.
The gas turbine is a usual example of a system with open Brayton cycle. The air is aspired in a
compressor, is heated and relaxed through turbine, and is discharged in the atmosphere.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/diesel.html
In the Otto cycle the mixture of air and gasoline is taken
with pressure, then compressed adiabatically and lit (3-4) with constant volume; the course of work is 4-5, and in 5-6 the
fume is discharged.
Sadi-Nicolas Carnot, son of general died of cholera at 36 years. Its only work
Reflexions on the horse-power of fire and the machines suitable to develop this power provided fundations of thermodynamics.
Selection of outputs
peak of efficiency of the action or of tools(%) human, swimming, surface 2 human, swimming, underwater 4
human, shoveling 3
steam engine 17 human, cycling 25 reciprocating gasiline engine 30
natural gas turbine 40
muscle contractions 50
combined cycle (gas-steam) turbine 60
fuel cell 60
electric motor 80
combined heat and power turbine 95
http://hypertextbook.com/physics/thermal/engines/
The only method "to circumvent" the laws of Carnot is to increase the temperature of
combustion (by using new materials for the turbines). At temperatures of 1400°C
efficiency from about 60% can be reached.
The silent Stirling
Sometimes it is important to use small differences in temperature. The invention of pastor Robert Stirling is equipped with two pistons and uses either the reheating or the cooling of a gas. It reaches an efficiency up to 40%.
These two engines on the right need differences in very small temperature differences. The duck which drinks and the thermometer of the love.
Like that, the question if they is necessary heat or cold for us, Carnot answered: either heat, or cold.
The fuel cells are a way "of cheating" on the equation of Carnot. Hydrogen and
oxygen are not flarings, but recombined in a chemical way: initially they dissociate in the respective ions and after filter through a special membrane and recombine by
giving water and energy. They would be a real alternative to the combustion engines, if the membranes were not expensive and not reliable and it did not have problems of storage for hydrogen.
Fuel cells
S. -N. Carnot (1976-1832)
Efficiency of Carnot
Entering heat Isothermal expansion
with TH
Isotermal compression with TC Outgoing
heat Adiabat
Adiabat
Volume
Pressure
%
100
H C H
T T T
Injection and combustion of the fuel
Discharge e a
d
b c
Pressure
Volume V1 V2 V3
Q1
Q2
P0
The cycle of Carnot in graphical form was
introduced by Emile Clapeyron in 1834.
An important conclusion of the analysis of the cycle of Carnot it is that the maximum theoretical yield of the engines with heat is directly related to: efficiency = (TH-TC)/TH
The cycle of Carnot is a
idealization, since no process of a real engine is reversible and any physical process implies a small "loss of energy" known as entropy.
The Brayton cycle is not only used in the turbines of plane, but also in the gas turbines used to
produce electricity.
http://thermalmotion.net/Brayton.html
For any irreversible process, the efficiency is smaller than that of the cycle of Carnot. That can be associated a smaller heat flow towards the system or an outgoing larger flow of the system. The inevitable
result is the inequality of Clausius.
dQ T 0
Peltier effect "to carry"
Thanks to the heat pump based on the Peltier effect, one can heat or cool dishes attached to our neck.
The Peltier effect is a thermo-electric principle where an electrical current makes it possible heat to be absorbed on a side when heat is rejected the different one.
Volume Aspiration Discharge
Rejection of heat
Power
Compression Process of combustion
Process with constant volume
Adiabatic process
1 2
3 4
5 6
Pressure
