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2 Stroke Reed Valve
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Two Stroke Engine Using Reed Valves
CHAPTER 1
INTRODUCTION
In developing countries a large number of private vehicles used two
stroke engines. A large percentage of them employ two stroke engines
because of their simplicity, high power to weight ratio and low cost of
maintenance. Although two stroke engines are less efficient and more
polluting they may continue to popular due to low cost and maintenance.
The most serious drawback of a two stroke S.I. engine is its higher
fuel consumption and higher unburned hydrocarbon emissions when
compared with four stroke engine. Fresh charge loss during the scavenging
process of a two stroke S.I engine is known to be the principal reason for its
high specific fuel consumption and high hydrocarbon emissions. During
scavenging process a part of the fresh charge mixes with the residual exhaust
gas as it scavenges the cylinder while some of it is loss due to short
circuiting. The net effect is that 25%-40% of the charge may be wasted
resulting in high fuel consumptions. Also in two stroke engines oil is used to
lubricate the engines. The excessive percentage of oil in the fresh charge
increases hydrocarbon in the exhaust. It is estimated that up to 10%-15% of
HC may be contributed by the lubricating oil.
Many control techniques to reduce emissions from engines may be
found in the literature. The complexity and cost of the most advanced and
cost of the most advanced or automatic type control system or fuel injection
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Two Stroke Engine Using Reed Valves
type do not justify their use in simpler small two stroke engines. This paper
presents the field test results of technique and its concept used to improve
the fuel efficiency and emissions of a two stroke engine. The estimated cost
of the component is Rs.60 to Rs.90 (approximately).
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Two Stroke Engine Using Reed Valves
CHAPTER 2
PROPOSAL FOR MODIFICATION
In the existing engine, the air fuel mixture is induced in the crank case
when the piston is at the top dead centre. From these the piston moves down,
uncovers the exhaust port first and then the transfer port which contains
slightly compressed charge at a pressure of about 130 kpa.This charge is
transferred to the upper part of the cylinder through the transfer port. The
piston is so shaped that fresh charge of fuel and air will sweep up to the top
of the cylinder and push out the remaining exhaust gas through the exhaust
port by means of a projection on the piston called deflector.
A small amount of unburned gases pass out through the exhaust port,
when both the transfer and exhaust port are open. By the time the piston
covers both these port, a small amount of unburned fuel is lost and also a
small amount of burnt gas is left back in the combustion chamber. The
possibility of introducing a buffer volume of air between the out going and
incoming charges has frequently been advocated. Historically attempts have
been made to minimize the short circuit losses of fuel ever since two stroke
engines were introduced. In recent attempts made at the Indian Institute of
Technology, Madrrass and the Indian Institute of Petroleum, air act as a
buffer gas to separate the fresh charge and the burned gas. Such a volume of
substantially fixed value may be expected to reduce both liability to mixing
and transfer of heat between the fresh charge and the burned gas.
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Two Stroke Engine Using Reed Valves
CHAPTER 3
REED VALVES
A pair of reed valves was employed to introduce the air from out side
to the top of transfer passages that would remain between the crank case
content of live mixture and the closed transfer port to the cylinder. When the
later opened, air would be pumped in a head of mixture forming a buffer
screen of air between the burned gas and the fresh charge.
The reed valve is a unidirectional valve which allows flow in one
direction only. It does not allow the flow in the opposite direction. The fluid
enters through the opening provided at the flange end of the valve. The
flange is provided with holes for admitting a nut so that the valve can be
fitted to the valve casing. The openings are covered by a blade on the outer
surface. Each blade is fitted to the valve body at one end by a pair of rivets,
so that the rivets lie parallel to the flange surface. The blade is made up of
tempered material so that it is flexible when fluid pressure acts on it.The
blades stick to the valve so that the holes below it reclosed. This occurs
when the fluid tries to flow in the direction of arrows as shown in fig.1.1.
When the fluid tries to flow as shown in fig.1.2 one end of the blades move
away from the valve body so that the opening below it are exposed and the
fluid flow in the direction shown by arrows. The position of the valve is
open. Hence it is seen that valve allows fluid only in one direction.
The reed valve is fitted at the transfer port of the engine cylinder. The
valve is fitted by making drilled hole on the walls of the transfer port of the
cylinder. The position of the bore is located such that minimum air
resistance is increased. The bore is threaded internally.
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Two Stroke Engine Using Reed Valves
The reed valve is enclosed in a casing. The casing, reed valve, and the
inlet flange to the reed valve are boiled and held together. The casing has a
connecter and is threaded externally. The connecter is fixed to the engine
cylinder so that the external thread of the connecter and the internal thread of
the engine cylinder fit. The prime purpose of fitting the reed valve at the
inlet of the engine cylinder is to reduce the loss of unburnt charge during
scavenging so that the efficiency of the system can be increased by
admitting the fresh air instead of fuel air mixture for scavenging.
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Two Stroke Engine Using Reed Valves
CHAPTER 4
PRINCIPLE OF OPERATION
In the improved design two reed valves are fitted at the upper end of
transfer duct connecting the crank case and the cylinder as shown in fig.2.
The reed valves are positioned in such a way that air would enter in a
direction perpendicular to the cylinder axis and along its radius. Air from
atmosphere enters the crank case in two different directions,(1) through the
carburetor ,along with the fuel in the conventional way and (2) through the
two reed valves fitted at the transfer ports on each side of the engines. When
the piston moves upward, the pressure inside the crank case is lowered to the
sub atmospheric. This causes the air from the atmosphere to flow into the
transfer passages through the reed valves and the fresh charge enters the
crank case in the normal way through the inlet passage. During the down
ward stroke of the piston, the pressure in the crank case closes the reed
valves. When the piston descends further transfer ports open and air in the
transfer passage enters the cylinder ahead of the fresh charge. The air pushes
the exhaust gases out and acts as a buffer screen between the two mediums,
the charge and burnt gas. Air becomes the main component that is lost to the
atmosphere through the exhaust port.The charge that follows it is retained in
the cylinder to a greater extend
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Two Stroke Engine Using Reed Valves
FIGURE 2
SCHEMATIC DIAGRAM OF THE POSITION OF EXTRA REED
VALVES AT TRANSFER DUCTS
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Two Stroke Engine Using Reed Valves
CHAPTER 5
EXPERIMENTAL SET UP AND TEST PROGRAM
Experiments were carried out on a small capacity (55cc displacement
volume, developing 1.7kw at 5000 rpm) crankcase compression, loop
scavenged, two stroke S.I engine. This type of engine is used in light two
wheelers in India. The engine was coupled to eddy current dynamometer for
torque and speed measurement. An infrared gas analyzer was employed to
measure HC and CO emissions level in the exhaust cylinder. Two reed
valves are fitted at the upper ends of the transfer ducts connecting the
crankcase and cylinder.Polythene tube and a T-joint were used to inter
connect the air flowing through the two reed valves and make a common
connection. A standard needle lift valve (4.8mm diameter, total area of about
18.1 mm square) was employed to control air flow through the common
connection. Variable load tests were conducted at constant engine speeds of
3000 rpm and 4500 rpm.Performance and emissions were studied with
various amounts of air flow through the reed valves by changing the opening
area of control valve. Positions IA, IB and IC represent the area of opening
in the control valve of about 0.8 mm square, 1.16 mm square and 2.41 mm
square respectively. In a two wheeled moped or scooter continuous
adjustment of control valve during running condition is difficult and hence
an optimum area of opening of the control valve was found and experiments
were carried out with this opening area for entire range of engine operation.
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Two Stroke Engine Using Reed Valves
CHAPTER 6
RESULTS AND DISCUSSION
Performance and exhaust emissions with various amount of air flow
through reed valves corresponding to different area of opening of the control
valve positions IA, IB and IC are discussed below.
The variation of brake thermal efficiency with brake power for
various position of control valve opening for air flow through the reed
valves, from atmospheric air shown in figure. 3 at a constant engine speed of
3000 rpm. It is observed that improvements in brake thermal efficiency
gradually increase as the area of opening of the control valve increases from
control valve positions IA to IB, but starts decreasing from the position IC.
Air inducted through the reed valves during the upward movement of the
piston fills the transfer ducts and enters the cylinder first when the transfer
ports open, thereby reducing short-circuiting of the fresh fuel air mixture
that follows it. Hence higher brake thermal efficiencies are obtained for
control valve positions IA and IB.Test results indicate that at part load the
percentage improvement in the brake thermal efficiency is about 19.4% and
at full load it is about 17.2% for control valve positions IB when compared
to control valve fully closed condition.When the control valve opens more
than the optimum value, higher amounts of air enters through the secondary
circuit. These higher quantities of secondary air mixes with the fuel air
charge in the crankcase and makes the mixture very lean. As a result, the
overall mixture quality of the trapped charge will be affected leading to a
drop in the performance. This is the case with the control valve position IC.
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Two Stroke Engine Using Reed Valves
The variation of HC emissions with brake power at constant engine
speed of 3000 rpm is presented in the fig.4 for different positions of control
valve opening areas which vary the flow through the reed valves. Two stroke
S.I engine emits a large amount of HC in its exhaust as compared to a four
stroke engine. These unburned HC the exhaust arise due to mixture
shortcircuiting, composition of fuel blend etc.However a major contribution
of exhaust HC is due to short-circuiting during scavenging period of the
engine cycle. Hence the success of any scavenging system is reflected by the
amount of HC concentration in the exhaust. This problem was minimized to
a great extent with the new scavenging system. It is observed that HC
decreases considerably when the extra reed valves permits atmospheric air to
enter into the transfer ducts. The reductions in HC emissions are maximum
for control valve position IB and lower for other positions. Test results
indicate that at part load HC decreases from 6000 ppm to 3800 ppm and at
near full load from 5600 ppm to 3000 ppm for the optimum air flow as
compared to control valve fully closed condition.
CO emissions from S.I. engine are controlled primarily by the fuel air
equivalence ratio. For fuel rich mixtures CO concentration in the exhaust
increase steadily with increasing equivalence ratio, as the amount of excess
fuel increase. For fuel lean mixture CO concentration in the exhaust vary
little with equivalence ratio. Since two stroke S.I engines often fuel rich
mixtures, to compensate for the fresh charge, CO emissions are significant
and must be controlled. It is clear that any attempt to reduce short circuiting
of the fresh charge allows the use of lean fuel air mixtures; there by CO
emissions can be lowered. Incidentally the new system described in this
work thus both the function simultaneously and the reduction in CO
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Two Stroke Engine Using Reed Valves
emission obtained at a constant engine speed of 3000rpm as shown in fig.5.
When the secondary air enters the transfer duct and crank case through the
reed valves, it not only fills the transfer duct passage but also mixes with
delivered charge present in the crank case to a certain extent. Hence the
quality of the mixture trapped inside the cylinder will become fuel lean after
the exhaust port closes. As a result CO levels are significantly lower with all
positions of the control valve and it is lowest for maximum area of opening
of the control valve. Test result indicate that CO decreases from 5.6% to
1.3% by volume at part load and from 2.9% to 0.4% by volume at near full
load when the reed valve permit maximum air flow through the reed valves.
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Two Stroke Engine Using Reed Valves
FIGURE 3
VARIATION OF BREAK THERMAL EFFICENCY WITH BRAKE
POWER FOR DIFFERENT POSITIONS OF CONTROL VALVE AT
3000 RPM
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Two Stroke Engine Using Reed Valves
FIGURE 4
VARIATION OF HYDROCARBON EMMISIONS WITH BRAKE
POWER FOR DIFFERENT POSITIONS OF CONTROL VALVE AT
3000 RPM
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Two Stroke Engine Using Reed Valves
FIGURE 5
VARIATION OF CO EMMISIONS WITH BREAK POWER FOR
DIFFERENT POSITIONS OF CONTROL VALVE AT 3000 RPM
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Two Stroke Engine Using Reed Valves
CHAPTER 7
CONCLUSIONS
Based on experimental investigations with a new scavenging system
and its optimization in a small capacity, two stroke SI engine, following
conclusions are drawn
1. The fixing of the extra reed valves et the transfer duct in a two stroke
S.I engines leads to reduction in HC and CO emission levels and
considerable improvement in break thermal efficiency due to reduction in
short circuiting of the fresh mixture through the exhaust port during
scavenging process.
2. The amount of secondary air which enters into the transfer duct to
obtain better performance depends up on the throttle position and engine
capacity, since it will affect the mixture quality of the trapped charge. For
a small capacity engine, precious control of atmosphere air flow through
the reed valves is necessary in order to obtain maximum advantage in the
performance.
3. As it is physically not possible to provide constant adjustment of the
area of opening of the control valve for air to flow through the reed valve
in a two wheeled vehicle, an optimum position of area of opening is fixed
which lends overall improvement in the performance of the engine over
the entire range of engine operation.
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Two Stroke Engine Using Reed Valves
REFERENCES
1. H. Smith ‛The High Speed Two Stroke Petrol Engine.’
2. K.V.Gopalakrishnan, B.Nagalingam and P. Ramesh- Project Reports,
IIT-Madras.
3. Auto India.
4. S.J Magee, R. Douglas and G.P Blair ‛Reduction of Fuel
Consumption and Emissions for a Small Capacity Two Stroke Cycle
Engine.’
.
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Two Stroke Engine Using Reed Valves
ACKNOWLEDGEMENT
I express my deep gratitude to almighty, the supreme guide, for
bestowing his blessings upon me in the entire endeavour.
I owe great deal to the faculty of department of Mechanical
Engineering, MES College of engineering, Kuttippuram, for the successful
completion of the seminar.
I would like to place my deep indebtedness to Prof. George K. K.
Principal of MES College of Engineering, for his valuable tips.
I would to like to express my sincere thanks to Dr. T. N.
Sathyanesan, Head of Department of Mechanical engineering for all his
assistance.
I wish to express my deep sense of gratitude to Lecturer,
Mr. Krishnakumar.T.S, Department of Mechanical Engineering who
guided me throughout the seminar. His overall direction and guidance has
been responsible for the successful completion of the seminar.
Finally, I would like to thank all the faculty members of the
department of mechanical engineering and my friends for their constant
support and encouragement.
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Two Stroke Engine Using Reed Valves
ABSTRACT
Fresh charge loss during the scavenging process of a two stroke
S.I engine is known to be the principal reason for its high specific fuel
consumption and high hydrocarbon emission. In order to minimize the
fresh charge loss to the exhaust a new scavenging system had been
developed. To achieve better performance from this scavenging system,
control of air flow through reed valve is necessary, since it will affect the
mixture quality of the trapped charge. In this paper, improved
performance and reduction in exhaust emissions are reported by
optimizing the amount of atmospheric air entering through the reed
valves in a small capacity, two stroke S.I engine.
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Two Stroke Engine Using Reed Valves
CONTENTS
INTRODUCTION 1
PROPOSAL FOR MODIFICATION 3
REED VALVES 4
PRINCIPLE OF OPERATION 6
EXPERIMENTAL SETUP AND TEST PROGRAME 8
RESULTS 9
CONCLUSION 15
REFERENCES 16
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