Four stroke diesel engine

SUCTION STROKE: With the movement of the piston
from T.D.C. to B.D.C. during this stroke, the inlet valve
opens and the air at atmospheric pressure is drawn
inside the engine cylinder; the exhaust valve however
remains closed. This operation is represented by the
line 5-1

COMPRESSION STROKE : The air drawn at atmospheric
pressure during the suction stroke is compressed to
high pressure and temperature as the piston moves
from B.D.C. to T.D.C. Both the inlet and exhaust valves
do not open during any part of this stroke. This
operation is represented by 1-2

POWER STROKE OR EXPANSION STROKE : As the piston
starts moving from T.D.C to B.D.C, the quantity of fuel
is injected into the hot compressed air in fine sprays by
the fuel injector and it (fuel) starts burning at constant
pressure shown by the line 2-3.
At the point 3 fuel supply is cut off. The fuel is injected
at the end of compression stroke but in actual practice
the ignition of the fuel starts before the end of the
compression stroke. The hot gases of the cylinder
expand adiabatically to point 4. Thus doing work on the
piston.
EXHAUST STROKE : The piston moves from the B.D.C.
to T.D.C. and the exhaust gases escape to the
atmosphere through the exhaust valve. When the piston
reaches the T.D.C. the exhaust valve closes and the
cycle is completed.