The diesel engine is a type of internal combustion engine which ignites fuel by injecting it into hot, high-pressure air in a combustion chamber. In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.
Four-Stroke Cycle
The four-stroke cycle is completed in four strokes of the piston, or two
revolutions of the crankshaft. To operate this cycle the engine
requires a mechanism to open and close the inlet and exhaust valves.
Consider the piston at the top of its stroke, a position known as a top
dead centre (TDC).
The inlet valve opens and fresh air is drawn in as the
piston moves down. At the bottom of the stroke, i.e.
bottom dead centre (BDC), the inlet valve closes and the air in the
cylinder is compressed (and consequently raised in temperature) as the
piston rises.
Fuel is injected as the piston reaches the top
dead centre and combustion takes place, producing very high pressure
in the gases. The piston is now forced down by these gases
and the exhaust valve opens at the bottom dead centre. The final stroke is
the exhausting of the burnt gases as the piston rises to the top dead centre to
complete the cycle.
The four distinct strokes are known
as 'inlet' (or suction), 'compression', 'power' (or working stroke) and
'exhaust'.
These events are shown diagrammatically on a timing diagram. The angle of the crank at which each operation takes place
is shown as well as the period of the operation in degrees.
This diagram
is more correctly representative of the actual cycle than the simplified
explanation describing the four-stroke cycle. For different
engine designs the different angles will vary, but the diagram is typical.
Two-Stroke Cycle
The two-stroke cycle is completed in two strokes of the piston or one
revolution of the crankshaft. To operate this cycle where each
event is accomplished in a very short time, the engine requires several special arrangements. First, the fresh air must be forced in under
pressure.
The incoming air is used to clean out or scavenge the exhaust gases and then to fill or charge the space with fresh air. Instead of val*"js
holes, known as 'ports', are used which are opened and closed by the
sides of the piston as it moves.
Consider the piston at the top of its stroke where fuel injection and
combustion have just taken place.
Also read: Two-Stroke Engine Definition
The piston is forced
down on its working stroke until it uncovers the exhaust port. The burnt gases then begin to exhaust and the piston continues
down until it opens the inlet or scavenge port. Pressurised
air then enters and drives out the remaining exhaust gas. The piston, on
its return stroke, closes the inlet and exhaust ports.
The air is then
compressed as the piston moves to the top of its stroke to complete the
cycle. A timing diagram for a two-stroke engine is shown
in Figure 2.4.
The opposed piston cycle of operations is a special case of the
two-stroke cycle. Beginning at the moment of fuel injection, both pistons are forced apart one up, one down by the expanding gases.
The upper piston opens the exhaust ports as it reaches the end
of its travel. The lower piston, a moment or two later,
opens the scavenge ports to charge the cylinder with fresh air and
remove the final traces of exhaust gas. Once the pistons
reach their extreme points they both begin to move inward.
This closes
off the scavenge and exhaust ports for the compression stroke to take
place prior to fuel injection and combustion. This cycle is
used in the Doxford engine, which is no longer manufactured although
many are still in operation.
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