ASTRACT
In the starting, the combuntion
engine and gas cycles were identified, and to facilitate the study of the
cycles, assuming for air and motor and review the otto cycle
and his strokes and the laws to calculate the work out and the heat addition and review the valves of the otto cycle and
compare with the perfect and real sluice moton and review the different between the real and perfect
otto cycle . in short the cycles is developed to reduce the loses
in the heat and we applied gasoline and
diessel cycle with assumation to study cycles like no fraction in the fluid and no heat transfer from the atm to cycle .in the summary of it the Otto
cycle is applied in car and other application and it has two kind :2or 4 stokes
and it has a two
reversible adiabatic & two reversible isochoric process and the different
between perfect and real cycle is the loses by fraction and the time delay
between the unlocked and close valves.
Ø The apparatus or frameworks applied to make a net power output are often called engines, and the thermodynamic cycles they run on are called power cycles.
Ø A thermodynamic cycle is a sequence or
series of operation performed on a framework, that eventually back up the framework
to its original state. Gas power cycles are thermodynamic cycles, which custom
air, as the runing fluid. A gas power cycle may contain heat transfer, work
transfer, pressure differences, temperature differences, volume differences and
entropy differences. In a gas power cycle, the source of heat source and the
sink for heat rejection are considered to be out source to the runing fluid.
Ø Four different kinds of gas power
cycles are commonly applied. They are as follows:
1.
Otto Cycle
2.
Diesel Cycle
3.
Dual Cycle
4.
Brayton Cycle
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Burning Kind | Cycle/Process | Compression | Heat Addition | Expansion | Heat Rejection |
externel Burning (Locked Cycle) | isentropic | isothermal | isentropic | isothermal | |
isothermal | isometric | isothermal | isometric | ||
isothermal | isobaric | isothermal | isobaric | ||
adiabatic | isobaric | adiabatic | isobaric | ||
adiabatic | isobaric | adiabatic | isobaric | ||
Internal Burning (Unlocked Cycle) | none | isometric | isentropic | isobaric | |
adiabatic | isometric | adiabatic | isometric | ||
adiabatic | isometric | adiabatic | isometric | ||
adiabatic | isometric | adiabatic | isometric | ||
adiabatic | isobaric | adiabatic | isometric | ||
adiabatic | isobaric | adiabatic | isobaric |
Heat engine analsisThe heat cycle contants
three or more basic thermodynamic basic operation, typically four, to
transform the state of the runing fluid and return it to its original state.
These are; compression, heat addition, expansion and heat rejection and each
of these operation can be carried out under one or more of the following
conditions:
(8)
·
Isothermal - At fixed temperature, maintained
with heat added or removed from a heat source or sink ·
Isobaric - At fixed pressure ·
Isometric / Isochoric /
Iso-volumetric -
At fixed volume ·
Adiabatic - At fixed entropy. No heat
is added or removed from the framework. No work done. ·
Isentropic At fixed entropy. Reversible
adiabatic conditions No heat added or lost. No work done. 
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ü The perfectizations and
simplifications in the dissection of power cycles:
1. The cycle does not contant any attrition.
2.Therefore,
the runing fluid does not test any pressure loses as it flows in tubes or apparatus
such as heat exchangers.
3. All expansion and compression operationtake
place in a quasi-balance manner.
4. The tubes connecting the various components
of a framework are well
insulated, and heat transfer through
them is negligible. (1)
ü Air-standard assumption
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Ø Internal burning motor runs on an unlocked
cycle since its runing fluid is thrown out of the motor at some point instead
of being returned to its initial state. That means the runing fluid does not
undergo a complete thermodynamic cycle. A detailed study of the performance
of an real gas power cycle is rather complex and accurate modeling of
internal burning motors normally contants computer simulation. To conduct
elementary thermodynamic analyses of
internal burning motors, considerable simplification is required. To simplify
the dissection, air-standard assumptions are made: (1). (11) ·
Gas
and air mixture are modeled as air and an perfect gas, which continuously
circulates in a locked cycle. Thus, there are no intake and exhaust operation. ·
All
the operation making up the cycle are internally reversible. ·
The
burning process is replaced by a heat-addition process from an out source. ·
The
exhaust process is replaced by a heat-rejection process and the gas back up
to its initial state. Ø In addition, if specific heats are
assumed fixeds at their ambient temperature, this assumption is called a cold
air-standard assumption. (1) |
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OTTO CYCLE Ø  Ø Nicholas-A-otto,a german motorer developed the first
cycle which is first successful motor runing on this cycle. So it is called
as otto cycle(4). This
cycle consists of the two reversible adiabatic & two reversible isochoric
process(3)
Ø  The
Otto cycle customs
the following operation
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Ø
In this motor, the inlet sluice unlocked during the
suction stroke and the exhaust sluice unlocked during the exhaust stroke. The
exact movement at which each of the sluices unlocked and locked with reference
to the position of piston and crank can be shown graphically in a diagram. This
diagram is known as a sluice timing diagram.
Ø Theoretically, the inlet sluice unlocked exactly at the startning of the suction stroke and locked at the end of the stroke. Both the sluices scrap locked during compression and power strokes. The exhaust sluice unlocked exactly at the startning of exhaust stroke and locked at the end of the stroke. the theoretical sluice timing diagram for a four-stroke Otto cycle motor.
Ø
The unlocking and locking of sluice with reference to
the position of piston and crankshaft during the four strokes are described as
follows:
1.
Suction Stroke: The inlet sluice unlocked. The piston
starts to move downward from top dead centre (TDC) position and reaches to
bottom dead centre (BDC) position. A fresh charge of air-fuel mixture enters
the cylinder. The exhaust sluice scraps locked.
2.
Compression Stroke: Both the sluices scrap locked. The piston
starts to move upward from B.D.C, thus compressing the charge until it reaches
the T.D.C.
3.
Runing Stroke: Both the sluices locked. Sparking takes
place from the spark plug with ignites the compressed charge. The piston moves
downward from T.D.C and reaches to B.D.C.
In real practice, the above cycle is slightly modified. The exact moments of unlocking and locking the sluices with reference to the piston and crankshaft are shown in the figure. This diagram is known as the real sluice timing diagram.
Inlet
valves and Exhaust Sluices
Inlet Sluice
Ø
The inlet sluice starts unlocking 10° to 30° before
T.D.C. as measured in degrees of crankshaft rotation. It scraps unlocked during
180° of the normal suction stroke and, in addition, 30° to 40° or even 60°
after B.D.C., at the startning of the compression stroke.
Ø The reason for unlocking
the inlet sluice before the starts of suction stroke is that the sluice is made
to unlocked and close very slowly, and this timing of unlocking the sluice is
permited to unlocked sufficiently during the suction stroke.
Ø
The sluices are arranged to unlocked and close slowly
to provide silent operation under high-speed conditions. The column of charge
in the inlet tubes requires to be accelerated before the suction stroke starts,
so that sufficient charge may enter the cylinder during the suction stroke.
Runing
of Inlet valves
Ø
As the piston moves downward during the suction
stroke, the pressure decreases inside the cylinder which cacustoms the gases to
rush in and fill up space above the piston. The pistons reach at the end of the
stroke before a complete charge has time to enter through the small inlet sluice
unlocking.
Ø
Therefore pressure in the burning space will still be
below atmospheric, and the gases will be moving in the direction of the motion
of the piston with high velocity. If the inlet sluice is locked at this point so
that no more charge enters, less charge will scrap in the cylinder.
Ø Thus, the inlet sluice
is made to scrap unlocked until the piston reaches a point in its next stroke
at which the pressure in the cylinder equals the pressure outside. Also, the real
locking point of the sluice coincides with the point when the motion of the
rushing charge would reverse the direction. (10)
Timing Data of The Inlet valves
Ø It scraps unlocked
during the 180° of the normal suction stroke, and addition, during 44° of the startning
of the compression stroke. This gives a total inlet sluice unlocking of 229°of crankshaft
rotation.
Exhaust valves(10)
Ø The exhaust sluice
starts unlocking 30° to 60° before B.D.C., scraps unlocked during 180° of the
normal exhaust stroke, and in addition, 8° to 10° or even 25° after T.D.C. at
the startning of the suction stroke. The reason for unlocking the exhaust sluice
before the start of the exhaust stroke is that the gases have an outlet for
expansion to start to rush of their own pressure.
Ø This removes the
greater part of the gases reducing the amount of the work to be done by the
piston on its return stroke. This reduction compensates the waste due to the
early release of gases. During the next outward stroke, the scraping gases are
forced out through the unlocked exhaust sluice. This cacustoms a slight
compression of the gases ahead of the piston.
Runing of Exhaust valves
Ø When it reaches to
T.D.C. positions, there will be a certain amount of compressed exhaust gases in
the clearance space. If the exhaust sluice is locked at this point, this amount
of the exhaust gases will scrap in the cylinder.
1.
Thus, the exhaust sluice is locked a little after,
the end of the exhaust stroke. It may result in drawing the exhaust gases back
into the cylinder. But this drawing back is prevented by two conditions,
2.
Gases under compression exceed the pressure in the manifold
and will continue to flow out becacustom of this difference in pressure.
3.
This piston, while at the top of the stroke, passageways
but very little for 10° to 15° movements of the crankshaft. This does not
materially rise the burning space as shown in fig.
The
crankarm is in a place as at A, for a certain quantity of degrees say 15°
movement of the crankshaft, the piston will move rising for a considerable
distance. When the crankarms are as at B, for the same 15° rotating of the crankshaft,
the distance moved by the piston will be less.
When
the crank arms are as at C, for the same 15° turning, there is very little rising
movement of the piston. It can be seen that between certain points there is
practically no motion of the piston travel in the region is called the rock of
the piston. Within the region usually, the exhaust sluice is locked after top
dead centre.
Timing Data of The Exhaust valves
Ø
It scraps unlocked during the 180° of the normal
exhaust stroke, and in addition, during 12° of the startning of the suction
stroke. This gives a total exhaust sluice unlocking of 239° of crankshaft
rotation. (10)
· where, r = Expansion ratio
·
Where r = Compression ratio
4-From
equation (2) and (3)
Since the right side is the same,
Interchanging
terms
Substituting
the value of T2/T3 in equation 1(7)
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Relation
between Thermal Efficiency and Compression Ratio with k =1.4 |
Note (4)
Ø It
is to be noted that in the theoretical p-v diagram, every corner is
sharp which represents the unlocking and locking of the sluice instantaneously.
Also, the suction and exhaust take place at atmospheric pressure.
Ø
The unlocking and locking of the sluice
cannot take place instantaneously but take some time, by which every corner in
the p-v diagram will be round, as shown in the real p-v diagram.
Also, the suction takes place at a pressure slightly lower than the atmospheric
pressure due to the resistance of the inlet sluice of the entering charge. (5)
Ø The exhaust takes place at a pressure slightly higher than the atmospheric pressure due to the resistance of the exhaust sluice to the exhaust gases. This gives an area, in the form of a small loop, and indicates what is called the pumping loss of the motor. This area is treated as negative and hence subtracted from the area of the larger loop which is treated as positive, giving us the network done during the cycle. (5)
Application of Otto cycle
Ø The Otto cycle is the thermodynimic model of
the spark- excited reciprocating piston internal combustion engine—the type of motor
utilized in most petroleum-powered motor vehicles automobiles, motorcycles and small gas engines like lawn
mowers.
v Application to
increase Efficiency
of Otto cycle :-
Ø Some limitations of maximizing the Otto cycle
are just being overcome with the introduction of electronic intake and exhaust
valve controls. This removes the dependencies of the valve timing from the
camshaft/crankshaft and the mechanical/physical connection between the two so
valve timing can be modified for rpm, power, load, etc.. and valve opening and
closing can be much faster.
Conclusions
In the
end .the gas power cycle is benefit for
life and its developed many time to the high efficience , otto cycle is a gasoline cycle with assumption to make it perfect
and must know this assumption and the different between the real and perfect cycle .
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