Just like an S.I engine, C.I engine also comprises of four
strokes for the entire combustion process to occur. Unlike a typical S.I engine
where air and fuel are mixed and supplied to the combustion chamber, only air
is sent during the suction stroke in C.I engine since air and diesel mixture is
a heterogeneous mixture. Diesel is injected at the end of compression stroke.
C.I engines don't require spark plugs to ignite the fuel.
SUCTION STROKE:
The inlet valve is open during the suction stroke allowing the
air from the atmosphere to enter the combustion chamber. The piston moves from
t.d.c to b.d.c, thereby creating suction and sucking more air. The inlet valve
closes after the piston reaches b.d.c and the air gets trapped inside the
combustion chamber. Due to this piston movement, a very low pressure of about
0.1 bar or less is created inside the cylinder.
COMPRESSION STROKE:
Now both the inlet and exhaust valves remain closed and the
piston moves upward from b.d.c to t.d.c. This compresses the air inside the
cylinder and increases the pressure and temperature. The pressure achieved is
around 30 to 50 bar and the temperature reached is about 600 °C. This
pressure and temperature is enough to ignite the fuel that will be injected at
the end of compression. The compression ratio of a C.I engine varies from 16:1
to 20:1.
POWER STROKE:
Both the inlet and the exhaust valves remain closed. Diesel is
injected into the combustion chamber at the end of compression stroke. The rate
of combustion will depend on how fine the diesel is atomized by the fuel
injection system. The higher the injection pressure, the finer will be the
atomization of diesel. The highly pressurized diesel is injected into the
heated air and the heat inside the combustion chamber due to compression is
enough to ignite the fuel. The intense heat will vaporize the fuel droplets and
burn it releasing heat energy. This energy pushes the piston from t.d.c to
b.d.c and the energy is used to drive the crankshaft.
EXHAUST STROKE:
The exhaust valve opens and now the piston moves from b.d.c to
t.d.c pushing the exhaust gases out. This completes the four strokes of C.I
engine. The exhaust gases are sent to the atmosphere through the exhaust pipe.
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