SI ENGINE
In two stroke engine, one
working cycle is completed in two strokes of the piston (i.e., one up and down
movement) or one revolution of the crankshaft. Hence, it is called as two stroke engine.
One working cycle consists of the following four processes.
1. Suction
2. Compression
3. Power
or expansion
4. Exhaust
The schematic diagram
of two-stroke engine.it consists of a cylinder of a cylinder in which all the
operations are performed. The piston reciprocates inside the cylinder.
The
piston is connected to the crankshaft through connecting rod and the crank.
Inside of the cylinder, valve ports are placed. Opening and closing of ports
are performed by the position of the piston.
In
the case of compression ignition (CI)engine, the spark plug is replaced by the
fuel injector. A two-stroke engine consists of ports instead of valves. In two
stroke engine, both the suction and compression process are carried out in the
first stroke, expansion and exhaust processes in the second stroke. Therefore, it develops power in each
revolution of the crank shaft.
WORKING
The
working principles of two stroke S.I engine is described as follows.
First stroke (suction and compression):
The
first stroke consists of the suction and compression processes. During the
first stroke, the piston moves upward from BDC to TDC. When the piston is at
BDC, the partially compressed air fuel mixture from crank case enters into the
cylinder through transfer port. Then the piston moves upward and compressed stroke,
the sparkplug produces spark, it will ignite the compressed high pressure fuel
air mixture.
When
the piston is at TDC, the inlet port opens and the air fuel mixture from the carburetor
enters into the crank. Thus one stroke of the piston is completed.
Second stroke (expansion or power and exhaust stroke);
When
air fuel mixture is ignited, both the pressure and temperature of the product
of combustion will suddenly increase. Therefore, the piston receiver power
impulse from the expanded gas and it pushes the piston downward and also
produces the power stroke. This process is described. During the expansion
stroke, some of the heat energy produced is converted into mechanical work.
During
downward stroke of piston, already entered air fuel mixture in the crank case
is partially compressed by the underside of the piston. This pre compression
process is called crank case compression.at the end of power stroke, the
exhaust port opens and burnt gases are sent out of the engine through this
port.
At
the same time, all the burnt gases are not exhausted .some portion of it will
remain in the cylinder. When the piston moves to BDC, the fresh air fuel
mixture from crank case enters into the cylinder to sweep out the burnt gases.
The process of sweeping out the exhaust gases with help of fresh air fuel
mixture is known as scavenging. The scavenging helps to remove the burnt gases
from the cylinder.