Jumat, 06 Januari 2012

Chapter 22 Heat Engine, Entropy and the second law of thermodynamic Part 2


RESUME

Chapter 22
Heat Engine, Entropy and the second law of thermodynamic
Part 2






oleh :

Reizsky Reynaldy
3415115829





FAKULTAS MATEMATIKA dan ILMU PENGETAHUAN ALAM
PENDIDIKAN BIOLOGI BILINGUAL
UNIVERSITAS NEGERI JAKARTA
2011


Chapter 22
Heat Engine, Entropy and the second law of thermodynamic
Part 2
  1.      Gasoline & Diesel Engine
  2.      Entropy
  3.      Entropy Changes in Irreversible Process
  4.      Entropy on a Microscopic Scale
Gasoline and Diesel Engine
              
         Gasoline Engine                                                            Diesel Engine

Gasoline Engine

A petrol engine (known as a gasoline engine) is an internal combustion engine with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels.
It differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites. In a petrol engine, the fuel and air are usually pre-mixed before compression (although some modern petrol engines now use cylinder-direct petrol injection).
The pre-mixing was formerly done in a carburetor, but now (except in the smallest engines) it is done by electronically controlled fuel injection. Petrol engines run at higher speeds than diesels, partially due to their lighter pistons, con rods and crankshaft (as a result of lower compression ratios) and due to petrol burning faster than diesel. However the lower compression ratios of a petrol engine give a lower efficiency than a diesel engine.


Working cycles : 4-Stroke Petrol engine
Petrol engines may run on the four-stroke cycle or the two-stroke cycle. For details of working cycles see:
 Cylinder arrangement

Common cylinder arrangements are from 1 to 6 cylinders in-line or from 2 to 16 cylinders in V-formation. Flat engines – like a V design flattened out – are common in small airplanes and motorcycles and were a hallmark of Volkswagen automobiles into the 1990s. Flat 6s are still used in many modern Porsches, as well as Subarus. Many flat engines are air-cooled. Less common, but notable in vehicles designed for high speeds is the W formation, similar to having 2 V engines side by side. Alternatives include rotary and radial engines the latter typically have 7 or 9 cylinders in a single ring, or 10 or 14 cylinders in two rings.

Cooling

Petrol engines may be air-cooled, with fins (to increase the surface area on the cylinders and cylinder head); or liquid-cooled, by a water jacket and radiator. The coolant was formerly water, but is now usually a mixture of water and either ethylene glycol or propylene glycol. These mixtures have lower freezing points and higher boiling points than pure water and also prevent corrosion, with modern antifreezes also containing lubricants and other additives to protect water pump seals and bearings. The cooling system is usually slightly pressurized to further raise the boiling point of the coolant.

Compression ratio

The compression ratio is the ratio between the total volumes of the cylinder AND the combustion chambers – at the beginning, and end of the compression stroke. Broadly speaking, the higher the compression ratio, the higher the efficiency of the engine. However, compression ratio has to be limited to avoid pre-ignition of the fuel-air mixture which would cause engine knocking and damage to the engine. Modern motor-car engine overall have compression ratios of between 9:1 and 10:1, but this can go up to 11 or 12:1 for high-performance engines that run on higher octane fuel

Ignition

Petrol engines use spark ignition and high voltage current for the spark may be provided by a magneto or an ignition coil. In modern car engines the ignition timing is managed by an electronic Engine Control Unit.

Diesel Engine

A diesel engine (also known as a compression-ignition engine) is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber. This is in contrast to spark-ignition engines such as a petrol engine (gasoline engine) or gas engine (using a gaseous fuel as opposed to gasoline), which uses a spark plug to ignite an air-fuel mixture. The engine was developed by Rudolf Diesel in 1893.
The diesel engine has the highest thermal efficiency of any regular internal or external combustion engine due to its very high compression ratio. Low-speed Diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) often have a thermal efficiency which exceeds 50 percent. Diesel engines are manufactured in two-stroke and four-stroke versions. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships. Use in locomotives, trucks, heavy equipment and electric generating plants followed later. In the 1930s, they slowly began to be used in a few automobiles. Since the 1970s, the use of diesel engines in larger on-road and off-road vehicles in the USA increased. As of 2007, about 50 percent of all new car sales in Europe are diesel.



In the sucking motion of the closed environment of diesel engines, air is sucked into the cylinder, then the motion is adiabatic compression compressed to a temperature high enough. So that the fuel is sprayed into the cylinder at the end of this movement on fire without a spark is needed. not as fast burning combustion in engines that use gasoline as fuel.
The first part of the labor movement was largely due to constant pressure. The next part of the labor movement is due to adiabatic development. Then terminated by throwing motion so that the circle becomes closed.


Figure 1.5 shows a closed loop air diesel engines are ideal. 1 starting point, the air is adiabatically compressed to the point 2, then with constant pressure is heated to the point 3. The next, adiabatic expanded to the point of 4 and with a constant volume is cooled to the point 1.


because during the motion of the compression in the cylinder diesel engine there is no fuel, then the light will not occur prematurely. and comparison kompersi v1/v2 can be much larger than heat engines.

Entropy

Entropy is a thermodynamic property that can be used to determine the energy not available for work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when converting energy to work. During this work, entropy accumulates in the system, which then dissipates in the form of waste heat.
In classical thermodynamics, the concept of entropy is defined phenomenologically by the second law of thermodynamics, which states that the entropy of an isolated system always increases or remains constant. Thus, entropy is also a measure of the tendency of a process, such as a chemical reaction, to be entropically favored, or to proceed in a particular direction. It determines that thermal energy always flows spontaneously from regions of higher temperature to regions of lower temperature, in the form of heat. These processes reduce the state of order of the initial systems, and therefore entropy is an expression of disorder or randomness. This picture is the basis of the modern microscopic interpretation of entropy in statistical mechanics, where entropy is defined as the amount of additional information needed to specify the exact physical state of a system, given its thermodynamic specification. The second law is then a consequence of this definition and the fundamental postulate of statistical mechanics.
Thermodynamic entropy has the dimension of energy divided by temperature, and a unit of joules per kelvin (J/K) in the International System of Units.
The term entropy was coined in 1865 by Rudolf Clausius based on the Greek εντροπία [entropía], a turning toward, from εν- [en-] (in) and τροπή [tropē] (turn, conversion).

Answer :
(b). Because the process is reversible and adiabatic, Qr  = 0; therefore, ΔS = 0.

On the Law of Thermodynamics I related to the concept of energy in the U. While the second law of thermodynamics is concerned with variables of thermodynamics is called entropy S. In the discussion of the Carnot cycle in which the obtained relationship between temperature and heat flow in Carnot cycle is :
Qp / Qd = Tp / Td
In this case, Qp is the amount of heat flowing into the cycle and Qd is the amount of heat flowing out of the system, therefore, between the two have opposite signs. So for the carnot cycle (which is the irreversible process) can be written as :
(Qp / Tp) = - (Qd / Td) or (Q1 / T1) + (Q2 / T2) = 0
Thus, the algebraic sum of the amount of Carnot cycle is Q / T = 0.
Now, note any irreversible process (reversible) as shown by the closed curve in the following figure