Four Stroke Diesel Engine
Already early Rudolf Diesel (1858 –
1913) was interested in engines. In his youth he was fascinated by the
engines of Lenoir and the steam engines that were usual at his time.
During his study he learned of his teacher, professor Linde, a famous
inventor, that the thermal engine could reach by far a better
performance. He referred to the young Frenchman Sadi Carnot (1796 –
1832), who discovered the Carnot’ cyclic process, a physical principle
that describes the ideal process of the burn in an engine . Diesel was
pursued from now on by the thought to build such an engine. 1890, Diesel
had the crucial idea, how the cumbustion process could be improved: The
engine takes in just air, which is to be compressed now to a pressure
of about 200 bar. At this point, heavy fuel (such as crude oil or
petroleum) gets injected by an injector in the air that is heated up
because of the huge pressure. The high themperature leads immediately to
the inflammation of the fuel by autoignition, which makes a spark plug
unnecessary.
There are different types cycles in
thermodynamics.Such as Otto cycle,Carnot vapour cycle,Diesel cycle
etc.Out of that cycle diesel engine works on diesel cycle.This cycle is
also known as constant pressure cycle.Diesel engine is mostly employed
in Stationary Power plants,Ships,Heavy Motor Vehicles.
In Petrol Engine,the air-fuel mixture
after being compressed in the engine cylinder to a high pressure,is
ignited by an electric spark from a spark plug.In diesel engine,diesel
oil and light and heavy oil used as fuel.This fuel is ignited by being
injected into the engine cylinder containing air compressed to a very
high pressure,the temperature of this air is sufficiently high to ignite
the fuel.That is why there is no spark plug used in diesel engine.This
high temperature compressed air used in the form of very fine spray is
injected at a controlled rate so that the combustion of fuel proceeds at
constant pressure.
Diesel Engine is mainly worked on below strokes.
1)Suction Stroke:-
In this stroke,the piston moves down
from the top dead centre.As a result,inlet valve opens and air is drawn
into the cylinder.After sufficient quantity of air with pressure is
drawn,suction valve closes at the end of the stroke.The exhaust valve
remains closed during this stroke.
2)Compression Stroke:-
In this stroke,piston moves up from the
bottom dead centre.During this stroke both inlet and exhaust valve are
closed.The air drawn into the cylinder during suction stroke,is
entrapped inside the cylinder and compressed due to upward movement of
the piston.In diesel engine,the compression ratio used is very high as a
result,the air is finally compressed to a very high pressure up to 40
kilogram per centimeter square,at this pressure,the temperature of the
air is reached to 1000 degree centigrade which is enough to ignite the
fuel.
3)Constant Pressure Stroke:-
In this stroke,the fuel is injected into
the hot compressed air where it starts burning,maintaining the pressure
constant.When the piston moves to its top dead centre,the supply of
fuel is cut-off.It is to be said that the fuel is injected at the end of
compression stroke and injection continues till the point of
cut-off,but in actual practise,the ignition starts before the end of
compression stroke to take care of ignition tag.
4)Working Or Power Stroke:-
In this stroke,both inlet and exhaust
valve remain closed.The hot gases (which are produced due to ignition of
fuel during compression stroke) and compressed air,now expand
adiabatically,in the cylinder pushing the piston down and hence work is
done.At the end of stroke,the piston finally reaches the bottom dead
centre.
5)Exhaust Stroke:-
In this stroke,the piston again moves
upward.The exhaust valve opens,while inlet and fuel valve are closed.A
greater part of the burnt fuel gases escape due to their own
expansion.The upward movement of the piston pushes the remaining gases
out through the open exhaust valve.Only a small quantity of exhaust
gases stay in the combustion chamber.At the end of exhaust stroke,the
exhaust valve closes and the cycle is thus completed.
As there is some resistance while
operating in inlet and exhaust valve and the some portion of burnt gases
remains inside the cylinder during the cycle, resulting the pumping
losses.This pumping losses are treated as negative work and therefore
subtracted from actual work done during the cycle.This will give us net
work done from the cycle.
Application of the diesel engine
The diesel engine still uses above all
heavy fuels such as fuel oil, which are a bit more expensive than
gasoline. Because of this reason, cars with otto engines that are more
harmful for the environment are often preferred to the ones using diesel
engines. Only larger diesel engines are competitionsless. For example,
in large ships or in older locomotives huge diesel engines with many
cylinders are used for driving. The diesel two stroke engine, which can
also be constructed, was less successful.
Major advantages
Diesel engines have several advantages over other internal combustion engines:.
-
They burn less fuel than a petrol engine performing the same work,
due to the engine’s higher temperature of combustion and greater
expansion ratio. Gasoline engines are typically 30 percent efficient
while diesel engines can convert over 45 percent of the fuel energy into
mechanical energy .
-
They have no high voltage electrical ignition system, resulting in
high reliability and easy adaptation to damp environments. The absence
of coils, spark plug wires, etc., also eliminates a source of radio
frequency emissions which can interfere with navigation and
communication equipment, which is especially important in marine and
aircraft applications.
-
The life of a diesel engine is generally about twice as long as that
of a petrol engine due to the increased strength of parts used. Diesel
fuel has better lubrication properties than petrol as well.
-
Diesel fuel is distilled directly from petroleum. Distillation
yields some gasoline, but the yield would be inadequate without
catalytic reforming, which is a more costly process.
-
Diesel fuel is considered safer than petrol in many applications.
Although diesel fuel will burn in open air using a wick, it will not
explode and does not release a large amount of flammable vapor. The low
vapor pressure of diesel is especially advantageous in marine
applications, where the accumulation of explosive fuel-air mixtures is a
particular hazard. For the same reason, diesel engines are immune to
vapor lock.
-
For any given partial load the fuel efficiency (mass burned per
energy produced) of a diesel engine remains nearly constant, as opposed
to petrol and turbine engines which use proportionally more fuel with
partial power outputs
-
They generate less waste heat in cooling and exhaust.
-
Diesel engines can accept super- or turbo-charging pressure without
any natural limit, constrained only by the strength of engine
components. This is unlike petrol engines, which inevitably suffer
detonation at higher pressure.
-
The carbon monoxide content of the exhaust is minimal, therefore diesel engines are used in underground mines.
-
Biodiesel is an easily synthesized, non-petroleum-based fuel
(through transesterification) which can run directly in many diesel
engines, while gasoline engines either need adaptation to run synthetic
fuels or else use them as an additive to gasoline (e.g., ethanol added
to gasohol).
p-V Diagram for the Ideal Diesel cycle
p-V Diagram for the Ideal Diesel cycle.
The cycle follows the numbers 1-4 in clockwise direction. In the diesel
cycle the combustion occurs at almost constant pressure and the exhaust
occurs at constant volume. On this diagram the work that is generated
for each cycle corresponds to the area within the loop.
Diesel engine applications
The characteristics of diesel have different advantages for different applications.
Passenger cars
Diesel engines have long been popular in bigger cars and have been used in smaller cars.
Railroad rolling stock
Diesel engines have eclipsed steam
engines as the prime mover on all non-electrified railroads in the
industrialized world. While electric locomotives have now replaced the
diesel locomotive almost completely on passenger traffic in Europe and
Asia, diesel is still today very popular for cargo-hauling freight
trains and on tracks where electrification is not feasible.Most modern
diesel locomotives are actually diesel-electric locomotives: the diesel
engine is used to power an electric generator that in turn powers
electric traction engines with no mechanical connection between diesel
engine and traction.
Other transport uses
Larger transport applications (trucks, buses, etc.) also benefit from the diesel’s reliability and high torque output.
In merchant ships and boats, the same
advantages apply with the relative safety of diesel fuel an additional
benefit. The German pocket battleships were the largest diesel warships,
but the German torpedo-boats known as E-boats (Schnellboot) of the
Second World War were also diesel craft. Conventional submarines have
used them since before the First World War, relying on the almost total
absence of carbon monoxide in the exhaust. American World War II
diesel-electric submarines operated on two-stroke cycle as opposed to
the four-stroke cycle that other navies used.
Non-transport uses
Diesel engines are also used to power
permanent, portable, and backup generators, irrigation pumps, corn
grinders, and coffee de-pulpers.
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