sejarah diesel
Rudolph Diesel
Rudolph Diesel was born in Paris of Bavarian parents in
1858. As a budding mechanical engineer at the Technical University in Munich,
he became fascinated by the 2nd law of thermodynamics and the maximum
efficiency of a Carnot process and attempted to improve the existing thermal
engines of the day on the basis of purely theoretical considerations. His first
prototype engine was built in 1893, a year after he applied for his initial
patent, but it wasn't until the third prototype was built in 1897 that theory
was put into practice with the first 'Diesel' engine.
Diesel Cycle Operation
The Diesel cycle is the cycle used in the Diesel
(compression-ignition) engine. In this cycle the heat is transferred to the
working fluid at constant pressure. The process corresponds to the injection
and burning of the fuel in the actual engine. The cycle in an internal
combustion engine consists of induction, compression, power and exhaust
strokes.
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 (see valve timing chart).
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.
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.
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.).
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.
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.
