| Two
Stroke Engine Using Reed Valves |
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 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).
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.
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
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