thermodynamics report

8/10/2019 thermodynamics report

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Introduction

Most power-producing devices operate on cycle, and the study of power cycles is an exciting

and important part of thermodynamics. However, existence of various complicated power

cycles make study of these cycles challenging. It is important to have a clear insight and deep

understanding on these cycles as fundamental application in thermodynamic. Ability to

differentiate the cycles is essential in the study of Thermodynamic.

In the following report, we are going to give practical and simple example to differentiate

real heat engine between Sirling cycle, Otto cycle and Diesel Cycle. Then, we will show how

all these heat engines differ to steam engine ( Rankine cycle ).

Instead of merely showing the differences, we will first state the operation of Stirling cycle,

Otto cycle and Diesel cycle. After acquired sufficient understanding and knowledge on the

respective operation, we do the comparison chart for these engine in various aspects. The

comparison chart portrayed the differences in types, efficiency, advantages, disadvantages

and so on. Furthermore, we show the operation of the Rankine cycle and do the comparison

between internal combustion and external combustion engine.


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Stirling Cycle

The stirling cycle is invented by Robert Stirling in 1816. In this cycle involved 4 processes

which is isothermal expansion, constant volume heat removal (regeneration), isothermal

compression and constant volume heat addition (regeneration). The carnot cycles

expansion and compression is replaced by 2 constant volume regeneration process. The

regenerator in the stirling cycle is function as a temporary storage of thermal energy and

usually the main component of the regenerator is wire or a ceramic mesh or any kind of the

porous plug with high thermal mass. One of the real heat engine for stirling cycle is stirling

engine.

Reversible processes:

1-2: T = constant expansion (heat addition from the external source)2-3: v = constant regeneration (internal heat transfer from the working fluid to the

regenerator)

3-4: T = constant compression (heat rejection to the external sink)

4-1: v = constant regeneration (internal heat transfer from the regenerator back to the

working fluid)

The thermal efficiency of Stirling Cycle is:

th, Stirling =1 -

where is temperature of low-temperature reservoirs

is temperature of high-temperature reservoirs

Figure 1: P-V diagram for stirling cycle Figure 2: T-S diagram for stirling cycle


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Stirling Engine

The stirling engine is a kind of real heat engine which is enclosed-cycle regenerative with

gaseous working fluid and the closed-cycle is define as a thermodynamic system in which

the working fluid is permanently contained within the system. Usually this engine is only inused in very specialize application such as used in submarine and auxiliary power generators

for yatchs. Since the stirling engine is using external heat source which could be anything

from gasoline to solar energy means there is no internal explosion needed therefore the

engine is very quiet and suitable for military machines. There are 3 types of stirling engine

which is Alpha, Beta and Gamma. There have similar ways of heating and cooling the

operating gas the only difference is arrangement of the pistons and cylinders to produce

mechanical power.

Figure 3: Alpha stirling engine

Figure 4: Beta stirling engine

Figure 5: Gamma stirling engine


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Process of stirling engine

a) The gas in the left piston is expanded at the constant temperature TH when the heat

energy is transfer to the cylinder from the external. The left piston is started to

move down while the right piston is fixed. In order to maintain a constant

temperature the gas must absorb the heat QH from the reservoir.

b) The left piston is started to move up at the same time the right piston is started to

move down. Both piston are move with the same rate hence the volume of the gas

is constant but the temperature is dropped from TH to TL due to the gas passes

through the regenerator and the heat is store in the regenerator.

c)

The right piston is moved up while the left piston is fixed to compress the gas at the

constant temperature. The heat is transferred to the sink at temperature TL but thepressure is increased.

d) Both piston are moved with the same rate ( left piston move down while right piston

move up) to keep the volume constant. When the gas passes through the

regenerator again the gas will pick up the thermal energy stored there.

Figure 6: Process of stirling engine


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Diesel Cycle

The diesel cycle is invented by Rudolph Diesel in the year 1897. This cycle is the ideal cycle

for compression-ignition reciprocating engines. This cycle is widely uses for diesel engine.

There are 4 processes involved in this cycle which is isentropic compression, constant

pressure heat addition, isentropic expansion and constant volume heat rejection. This cycle

is similar to Otto Cycle but the only difference is has one constant pressure process instead

of a constant volume process.

Process:

1-2: Isentropic compression

2-3: Constant pressure heat addition3-4: Isentropic expansion

4-1: Constant volume heat rejection

The thermal efficiency of the diesel cycle is :

th,Diesel= 1 -

))

rc= cutoff ratio

k= compression ratio

Figure 7: P-V diagram for diesel cycle Figure 8: T-S diagram for diesel cycle


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Diesel Engine

This diesel engine is mainly in the method of initiating combustion which is the air is

compressed to a temperature that is above the autoigition temperature of the fuel, and

combustion starts on the contact as the fuel is injected into this hot air. In diesel engine only

air is compressed during the compression stroke, eliminating the probability of autoignition.

Hence the diesel engine can be designed to operate at much higher compression ratio,

typically between 12 and 24.

Processes of diesel engine

Intake stroke:The piston starts at the top dead center, the intake valve opens, and thepiston moves down to let the engine take in a cylinder-full of air

Compression stroke:The piston moves back up to compress air to a temperature which is

higher than the auto ignition temperature of the fuel.

Combustion stroke (power stroke):When the piston approaches the top of its stroke, fuel

starts to be injected from the fuel injector and the combustion occurs spontaneously, driving

the piston down. Fuel is injected during the first part of the power stroke, resulting in a

longer combustion interval.

Exhaust stroke: Once the piston hits the bottom of its stroke, the exhaust valve opens and

the exhaust leaves the cylinder to go out through the tail pipe.

Figure 9: four stroke diesel engine

Figure 10: Process of diesel engine


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Otto Cycle

Otto cycle is an ideal cycle for spark-ignition recipocating engine. It is proposed by Nikolaus

A. Otto. There are four processes in Otto Cycle which are isentropic compression, constant-

volume heat addition, isentropic expansion and constant-volume heat rejection. In the four

strokes Otto cycle, there are two additional processes which is exhausting of waste heat and

combustion products at constant pressure (isobaric), and one for the intake of cool oxygen-

rich air also at constant pressure.

Process:

1-2: Isentropic compression

2-3: Constant volume heat addition

3-4: Isentropic expansion

4-1: Constant volume heat rejection

Figure 11: P-V diagram of Otto Cycle Figure 12: T-S diagram of Otto Cycle

Figure 13: Actual cycle of P-V diagram of

Otto Cycle


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Thermal efficiency of Otto Cycle is:

th,Otto= 1 -

where k is specific heat ration and r is compression ratio:

r =

Internal Combustion Engine (Otto Engine)

The internal combustion engine is a kind of engine which operate by using spark-ignition.

During combustion, the expansion of the high temperature and pressure gases will apply

direct force to the piston which move over a distance and generating an useful mechanical

energy. There are two types of internal combustion engine which is two strokes engine and

four strokes engine.

Figure 14: Four strokes Otto Engine

Figure 15: Two strokes Otto Engine


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Processes of Four Stroke Engine

1. Intake Stroke:At the beginning of this stage, the intake valve is opened and lets the

air and fuel enter to chamber. This stroke is an adiabatic expansion process hence

there is no heat is added to system.

2. Compression Stroke:The cylinder is sealed off from outside atmosphere then the air

and fuel is compressed hence it will easily and forcefully combust. There is n heat is

added to system since it is adiabatic compression.

3.

Power/Combustion Stroke:Since both valves are closed in this process, when the

temperature increases due to the combustion, the pressure will increase too and

cause a large force pushing the piston down and produce engine torque. The energy

is transformed from heat energy to rotational mechanical energy.

4. Exhaust stroke:During this stroke the exhaust valve will open and the excess heat

and unwanted byproduct such as carbon dioxide and water vapour will remove

thought the valve which is pushing by the piston.

Figure 16: Processes of four strokes

Otto Engine


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Comparison between Otto, Diesel and Striling Cycle

Properties Otto cycle Diesel cycle Stirling cycle

Type of engine Spark-ignition

engine

Compression-ignition

engine

Stirling engine

Heat transfer at Constant volume Constant pressure andvolume

Constant volumeand temperature

Thermal

Efficiencyth,Otto= 1 -

th,Diesel= 1 -

)) th, Stirling =1 -

Efficiency

Increases with

increasing

compression ratio and

specific heat ratio.

Able to operate at much higher

compression ratio (12 to 24).

Efficiency increases with

decreasing compression

ratio and increasing peak

temperature.

Advantages- Good

thermodynamic

efficiency

(conversion of

the heat energy

released when

the fuel burns

into mechanical

work)

- An excellent

power-to-weight

ratio and

reliability due to

relatively simple

operation.

Able to use cheaper fuel, because

less constraint on premature

ignition problem

- Use of an internal

heat exchanger

called a regenerator

which increases the

thermal efficiency

- Combustion can be

done externally

(more choices of

fuel types.)

Disadvantages - Further increasein efficiency is

insignificant.

- Premature

ignition occurs =

engine knock

- It converts heat into energy

rather than sending the heat

out the tailpipe as gas-

powered vehicles do, it

doesnt result in flashy high-

speed performance.

- Harder to start in cold

weather because the mass of

the cylinder block and

cylinder head absorb the

heat of compression,

preventing ignition due to

the higher surface-to-volume

ratio.

Difficult to achieve in

practice:

- involve heat

transfer through

small temperature

difference.

- require very large

heat transfer area

and very long time.

P-V Graph

T-S Graph


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Rankine Cycle

Rankine Cycle is a cycle which commonly used in steam engine which involve continuously

condensation and evaporation of fluid. By using a heat source such as coal, nuclear energy

and other burning fuel to heat up the fluid until it evaporates, by using the vapor to turn the

turbine to generate power. Almost all power plants operating using Rankine cycle concepts.

Rankine Cycle operates in the following steps (Figure 1):

1-2-3 Isobaric Heat Transfer. High pressure water from the pump enter the boiler causing

the water to boil and evaporates to become saturated steam

3-4 Isentropic Expansion. The vapor is expanded in turbine to produce work which can be

converted to kinetic energy or electric energy.

4-5 Isobaric Heat Rejection. The vapor enter the condenser to reduce its temperature and

pressure.

5-1 Isentropic Compression. The water flow into the pump and work is applied to increase

the pressure of the water.

Drop of efficiency of the actual Rankine cycle

Due to some reasons the efficiency of the actual Rankine cycle is lower than the ideal

Rankine cycle.

Pressure drops in the condenser, the boiler and the piping due to the fluid friction.

Heat losses on the whole system.

Irreversibility of the pump and the turbine.

Figure 17: Processes of Steam Engine

Figure 18: Comparison between actual

and ideal Rankine Cycle


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The Differences Between Internal Combustion Engine and External Combustion Engine

Internal Combustion Engine

(Otto and Diesel)

External Combustion Engine

(Stirling and Steam)

- The combustion of fuel and diesel occur

in the engine chamber

-

Power is derived from hot gaseous

product of combustion of fuels.

- Safe

-

This engine need longer time to start

-

High temperature and pressure

- High efficiency

- The steam is produced from the outside

of the cylinder for steam engine while

the heat source is coming from outside

for stirling engine.

- Power is derived from steam under

pressure

-

Due to the high pressure of the steam, it

is relatively unsafe.

-

This engine can be started immediately.- Lower temperature and pressure

compared with internal combustion

engine.

-

Low efficiency


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Conclusion

The process of doing this report has broaden our knowledge in Thermodynamic. This report

shows a clear picture of the differences between the cycles. This report can even be used as

our revision materials for study.

We realized the important of understanding the operation. We manage to see the

relationship between the operations and the properties. Each process plays an important in

altering the cycles. We learn that being an engineer, besides focusing on the formulas and

calculations, we should have look into the operation. It is important to understand the

purpose of each process especially when there is different between one and other.

Last but not least, we would like to thank our lecturer Hj. AMIRRUDDIN BIN ABDUL KADIR

,

the book authors, friends and those who have helped directly or indirectly in our process of

completing the report. All sources of information is being appreciated.


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References:

1. engel oles Thermodynamics: An engineering approach.

Boston: McGraw-Hill.

2. Robert T. Balmer. (2011). Modern Engineering Thermodynamics. London:

Academic Press.

3.

Mike.P.(2011).Thermodynamics For Dummies.Hoboken NJ:Willey

4. Brain, M. (n.d.). Retrieved from

http://science.howstuffworks.com/transport/engines-equipment/steam1.htm

5.

Diesel Cycle - Processes with p-V and T-s Diagrams. Mechteacher.com. (n.d.).

Retrieve fromhttp://mechteacher.com/diesel-cycle/

6.

LTD Stirling Engine - Overview. (n.d.). Retrieved from

http://www.ltdstirling.com/stirling_engines/stirling_engines.php7. Four stroke engine. (n.d.). Retrieved from

http://www.oocities.org/racingworld2002/info/four-stroke-engine.htm

8.

What Is the Otto Cycle? (n.d.). Retrieved fromhttp://www.wisegeek.com/what-is-

the-otto-cycle.htm
http://science.howstuffworks.com/transport/engines-equipment/steam1.htmhttp://science.howstuffworks.com/transport/engines-equipment/steam1.htmhttp://mechteacher.com/diesel-cycle/http://mechteacher.com/diesel-cycle/http://mechteacher.com/diesel-cycle/http://www.ltdstirling.com/stirling_engines/stirling_engines.phphttp://www.ltdstirling.com/stirling_engines/stirling_engines.phphttp://www.oocities.org/racingworld2002/info/four-stroke-engine.htmhttp://www.oocities.org/racingworld2002/info/four-stroke-engine.htmhttp://www.wisegeek.com/what-is-the-otto-cycle.htmhttp://www.wisegeek.com/what-is-the-otto-cycle.htmhttp://www.wisegeek.com/what-is-the-otto-cycle.htmhttp://www.wisegeek.com/what-is-the-otto-cycle.htmhttp://www.wisegeek.com/what-is-the-otto-cycle.htmhttp://www.wisegeek.com/what-is-the-otto-cycle.htmhttp://www.oocities.org/racingworld2002/info/four-stroke-engine.htmhttp://www.ltdstirling.com/stirling_engines/stirling_engines.phphttp://mechteacher.com/diesel-cycle/http://science.howstuffworks.com/transport/engines-equipment/steam1.htm

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