chapter 8 Gas Power Cycle

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chapter 8 Gas Power Cycle
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8-1 The Analysis of a Cycle
8-1-1 The average temperature of a process
We define
That is
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8-1-2 The Analysis of a Cycle
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8-2 Otto Cycle
Nicolaus August Otto the inventor of the
four-stroke cycle was born on 14th June 1831 in
Germany. In 1862 he began first experiments with
four-strokes engines. The first four-stroke
engines is shown. they correspond to the today's
engines. He died on 26th January 1891 in Cologne
8-2-1 N. A. Otto
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8-2-2 The Cycle - The Four Strokes
Intake stroke
The piston moves down the cylinder and the
pressure will drop (negative pressure). The
intake valve is opend. Because of the low
pressure the air/fuel mixtures is sucked into the
cylinder.
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Compression stroke
At Bottom Dead Center (BDC) the cylinder is at
its maximum volume and the intake valve is
closed. Now the piston moves backward the Top
Dead Center (TDC) and compresses the air/fuel
mixtures.
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Near the end of the compression stroke, the
ignition starts the combustion and the mixture
burns very rapidly. The expanding gas creates a
high pressures against the top of the piston.
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Power stroke
The force drives the piston downward to crank
shaft (the valves are closed). The volume is
increased and the pressure is decreased. No more
energy is added and because of this the internal
energy of the gas is decreased as so as the
temperature.
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Exhaust stroke
At BDC the exhaust valve is opened and the piston
moves up the cylinder. The pressure drops near
the pressure outside the cylinder because of the
opened exhaust valve. Exhaust gas leaves the
cylinder. The volume is decreased.
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8-2-3 The Cycle - The Four Strokes
The theory cycle
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Theory efficiency of Otto cycle
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e compression ratio
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8-3 Diesel Cycle
8-2-1 Rudolf Diesel
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8-3-2 The Diesel Cycle
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8-3-3 The Efficiency of Diesel Cycle
The theory cycle
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Theory efficiency of Diesel cycle
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Since process 1-4 has a constant volume
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To increase efficiency
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Other internal combustion engine
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8-4 Brayton Cycle
8-4-1 The Equipments of Brayton Cycle
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Advantages
Gas turbine engines have a great power-to-weight
ratio compared to reciprocating engines. That is,
the amount of power you get out of the engine
compared to the weight of the engine itself is
very good.
Gas turbine engines are smaller than their
reciprocating counterparts of the same power
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8-4-2 Brayton Cycle
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8-4-3 Efficiency of Brayton Cycle
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8-4-3 The Optimum Compression Ratio
If T3 is limited
the compression ratio will be increased to get
high efficiency But the power ratio will decrease

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We have to compromise between high efficiency and
high power ratio.
Usually in aerospace field the power ratio is
more important
Obviously there must be an optimum compression
ratio which makes the cycle has maximum power
ratio
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This ratio is denoted as emax
The efficiency depends on T3 basically
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8-4-4 The methods to increase the efficiency
(1) Regenerative Brayton Cycle
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Combustion chamber
oil
regenerator
Air in
gas turbine
compressor
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(2) Isothermal compression and regenerative cycle
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8-5 Jet Engine
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3
4
5
6
1
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The methods to increase the power ratio of jet
engine
(1) After burning
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4
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5
3
2
1
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(2) Increase T4
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8-5 The Stirling Cycle
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The End Of This Chapter Thank You