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Induction Stroke:-
The
induction stroke in a Diesel engine is used to draw in a new volume of
charge air into the cylinder. As the power generated in an engine is
dependent on the quantity of fuel burnt during combustion and that in
turn is determined by the volume of air (oxygen) present, most diesel
engines use turbochargers to force air into the cylinder during the
induction stroke.
From a theoretical
perspective, each of the strokes in the cycle complete at Top Dead
Centre (TDC) or Bottom Dead Centre (BDC), but in practicality, in order
to overcome mechanical valve delays and the inertia of the new charge
air, and to take advantage of the momentum of the exhaust gases, each of
the strokes invariably begin and end outside the 0, 180, 360, 540 and
720 (0) degree crank positions
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Compression Stroke:-
The
compression stroke begins as the inlet valve closes and the piston is
driven upwards in the cylinder bore by the momentum of the crankshaft
and flywheel.
The purpose of the compression
stroke in a Diesel engine is to raise the temperature of the charge air
to the point where fuel injected into the cylinder spontaneously
ignites. In this cycle, the separation of fuel from the charge air
eliminates problems with auto-ignition and therefore allows Diesel
engines to operate at much higher compression ratios than those
currently in production with the Otto Cycle.
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Compression Ignition:-
Compression ignition takes place when the fuel from the high pressure fuel injector spontaneously ignites in the cylinder.
In
the theoretical cycle, fuel is injected at TDC, but as there is a
finite time for the fuel to ignite (ignition lag) in practical engines,
fuel is injected into the cylinder before the piston reaches TDC to
ensure that maximum power can be achieved. This is synonymous with
automatic spark ignition advance used in Otto cycle engines.
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Power Stroke:-
The
power stroke begins as the injected fuel spontaneously ignites with the
air in the cylinder. As the rapidly burning mixture attempts to expand
within the cylinder walls, it generates a high pressure which forces the
piston down the cylinder bore. The linear motion of the piston is
converted into rotary motion through the crankshaft. The rotational
energy is imparted as momentum to the flywheel which not only provides
power for the end use, but also overcomes the work of compression and
mechanical losses incurred in the cycle (valve opening and closing,
alternator, fuel injector pump, water pump, etc.).
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Exhaust Stroke:-
The
exhaust stroke is as critical to the smooth and efficient operation of
the engine as that of induction. As the name suggests, it's the stroke
during which the gases formed during combustion are ejected from the
cylinder. This needs to be as complete a process as possible, as any
remaining gases displace an equivalent volume of the new charge air and
leads to a reduction in the maximum possible power.
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Exhaust and Inlet Valve Overlap:-
Exhaust
and inlet valve overlap is the transition between the exhaust and inlet
strokes and is a practical necessity for the efficient running of any
internal combustion engine. Given the constraints imposed by the
operation of mechanical valves and the inertia of the air in the inlet
manifold, it is necessary to begin opening the inlet valve before the
piston reaches Top Dead Centre (TDC) on the exhaust stroke. Likewise, in
order to effectively remove all of the combustion gases, the exhaust
valve remains open until after TDC. Thus, there is a point in each full
cycle when both exhaust and inlet valves are open. The number of degrees
over which this occurs and the proportional split across TDC is very
much dependent on the engine design and the speed at which it operates.
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