4-Stroke Diesel Engine:-
A 4-stroke engine that uses diesel as
a fuel is called 4-strokeDiesel engine. In diesel engine we do not have
required any spark plug for ignition of fuel. Because in diesel engine after
compression of fresh air diesel is injected with high pressure. Due to friction
between diesel and air ignition takes place and power stroke takes place. Because
ignition takes place due to compression of fresh air and diesel so that diesel
engine also called compression ignition (CI) engine.
4-stroke
diesel engine works on diesel cycle given below.
Diesel
cycle:-
The Diesel
cycle is a combustion process of a reciprocating internal combustion engine.
In it, fuel is
ignited by heat generated during the compression of air in the combustion
chamber, into which fuel is then injected. This is in contrast to igniting the
fuel-air mixture with a spark plug as
in the Otto
cycle (four-stroke/petrol) engine. Diesel engines are used
in aircraft, automobiles, power generation, diesel-electric locomotives,
and both surface ships and submarines.
The Diesel cycle is assumed to have constant pressure during
the initial part of the combustion phase ( to in the diagram, below). This is an
idealized mathematical model: real physical diesels do have an increase in
pressure during this period, but it is less pronounced than in the Otto cycle.
In contrast, the idealized Otto cycle of a gasoline engine approximates a constant volume process during that
phase.
The
Idealized Diesel Cycle:-
p-V Diagram for the
ideal Diesel cycle. The cycle follows the numbers 1-4 in clockwise direction.
The image on the left shows a p-V diagram for the ideal Diesel cycle; where is pressure and V the volume or the specific volume if the process is placed on a unit mass basis. The
ideal Diesel cycle follows the following four distinct processes:
·
Process
1 to 2 is isentropic compression of the
fluid (blue)
·
Process
2 to 3 is reversible constant pressure
heating (red)
·
Process
3 to 4 is isentropic expansion (yellow)
·
Process
4 to 1 is reversible constant volume cooling (green)
The Diesel engine is a heat engine: it converts heat into work. During the bottom
isentropic processes (blue), energy is transferred into the system in the form
of work but
by definition (isentropic) no energy is transferred into or out of the system
in the form of heat. During the constant pressure (red, isobaric) process, energy enters the system as heat .During the top
isentropic processes (yellow), energy is transferred out of the system in the
form of ,but
by definition (isentropic) no energy is transferred into or out of the system
in the form of heat. During the constant volume (green, isochoric) process, some of energy flows out of the system as heat
through the right depressurizing process .The work that leaves the system is
equal to the work that enters the system plus the difference between the heat
added to the system and the heat that leaves the system; in other words, net
gain of work is equal to the difference between the heat added to the system
and the heat that leaves the system.
·
Work
in (Win) is done by the piston
compressing the air (system)
·
Work
out (wout) is done by the working
fluid expanding and pushing a piston (this produces usable work)
·
Heat
out (Qout) is done by venting the
air
The net work produced is also represented by the area
enclosed by the cycle on the P-V diagram. The net work is produced per cycle
and is also called the useful work, as it can be turned to other useful types
of energy and propel a vehicle (kinetic energy) or produce electrical energy. The summation of many such
cycles per unit of time is called the developed power. The is also called
the gross work, some of which is used in the next cycle of the engine to
compress the next charge of air.
Parts of 4-stroke
diesel engine:-
Cylinder head:
The top end of cylinder is
closed by means of removable cylinder head.The inlet valve, exhaust valve,
spark plug, injector etc. are bolted on the cylinder head. The main function of
cylinder head is to seal the cylinder block and not to permit entry and exit of
gases on cover head valve engine.
Cylinder block:
The main
function of cylinder is to guide the piston. It is in direct contact with the
products of combustion so it must be cooled. For cooling of cylinder a water
jacket (for liquid cooling used in most of cars) or fin (for air cooling used
in most of bikes) are situated at the outer side of cylinder. At the upper end
of cylinder, cylinder head and at the bottom end crank case is bolted.
Connecting
Rod:
It connect piston to the crank shaft and transmit the motion and thrust of piston to crank shaft. The lower end of connecting rod is connected to the piston and the bigger is connected to the crank shaft
It connect piston to the crank shaft and transmit the motion and thrust of piston to crank shaft. The lower end of connecting rod is connected to the piston and the bigger is connected to the crank shaft
Crank Shaft:
It is located in the bottom
end of cylinder block. It transmits the reciprocating motion of piston into
rotary motion. This rotary motion used to rotate wheels of the vehicle.
Oil
Sump:
It is bolted at the lower end of the cylinder block. All the oil for lubricating the movable parts is placed in it.
It is bolted at the lower end of the cylinder block. All the oil for lubricating the movable parts is placed in it.
Camshaft:
It is fitted either in the cylinder head or at the bottom of the cylinder block. It is use to open or close valves at proper timing in multi-cylinder engine
It is fitted either in the cylinder head or at the bottom of the cylinder block. It is use to open or close valves at proper timing in multi-cylinder engine
Valves:
It is fitted on the cylinder head. It regulates the flow of air fuel mixture inside the cylinder and exhaust gas outside the cylinder block. When both inlet and exhaust valves are closed no pressure can go inside or outside of cylinder block.
It is fitted on the cylinder head. It regulates the flow of air fuel mixture inside the cylinder and exhaust gas outside the cylinder block. When both inlet and exhaust valves are closed no pressure can go inside or outside of cylinder block.
Injector:
It is used in Diesel engine (Compression Ignition). It is fitted on the cylinder head. It is used to inject fuel in spray form inside the cylinder at the end of compression stroke.
It is used in Diesel engine (Compression Ignition). It is fitted on the cylinder head. It is used to inject fuel in spray form inside the cylinder at the end of compression stroke.
Push
Rod:
It is used when the camshaft is situated in the bottom of the cylinder head. It regulates the timing of valves open and close through rocker arm and camshaft.
It is used when the camshaft is situated in the bottom of the cylinder head. It regulates the timing of valves open and close through rocker arm and camshaft.
Fly wheel:
A flywheel is a rotating mechanical device that
is used to store rotational energy. The amount of energy stored in a flywheel is proportional to the square of its
rotational speed.
Stroke:
Distance covered by piston from top dead center to bottom dead
centre or bottom dead centre to top dead center is called stroke.
Stroke Leangth:
The leangth
of stroke is called stroke leangth.
Pre ignition chamber:
In a diesel engine, fuel is
introduced as the piston approaches the top dead center of its stroke. The fuel
is introduced under high pressure either into a pre combustion chamber or directly into the piston-cylinder combustion
chamber. Combustion starts in this chamber therefore it is called pre
ignition chamber.
Diesel engine:
There is no electrical sparking plug in an
auto-ignition diesel engine; the heat of compression raises the temperature
of the air in the cylinder sufficiently to ignite the diesel when this is
injected into the cylinder; after the compression stroke. The CR will
customarily exceed 14:1 and ratios over 22:1 are common. The appropriate
compression ratio depends on the design of the cylinder head. The figure is
usually between 14:1 and 23:1 for direct injection engines, and between 18:1
and 23:1 for indirect injection
Injection pump:
An Injection
Pump is the device that pumps
diesel (as the fuel) into the cylinders of a diesel engine. Traditionally,
the injection pump is driven indirectly from the crankshaft by
gears, chains or a toothed belt (often the timing belt) that also drives the camshaft. It rotates at half
crankshaft speed in a conventional four-stroke diesel engine. Its timing is such that the fuel is injected only
very slightly before top dead centre of
that cylinder's compression stroke. It is also common for the pump belt on
gasoline engines to be driven directly from the camshaft. In some systems
injection pressures can be as high as 200 MPa (30,000 PSI).
Rocker arm:
A rocker arm (in the context of an internal combustion engine of automotive, marine, motorcycle and reciprocating
aviation types) is an oscillating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to
open it. One end is raised and lowered by a rotating lobe of the camshaft
(either directly or via a tappet (lifter)
and pushrod) while the other end
acts on the valve stem. When the camshaft
lobe raises the outside of the arm, the inside presses down on the valve stem,
opening the valve. When the outside of the arm is permitted to return due to
the camshafts rotation, the inside rises, allowing the valve spring to close
the valve.
The
power generation process in four stroke diesel engine is also divided into four
parts. Each part is known as piston stroke. In IC engine, stroke is referred to
the maximum distance travel by the piston in a single direction. The piston is
free to move only in upward and downward direction. In four stroke engine the
piston move two time up and down and the crankshaft move two complete
revolution to complete four piston cycle. These are suction stroke, compression
stroke, expansion stroke and exhaust stroke.
Suction stroke:
In
the suction stroke or intake stroke of diesel engine the piston start moves
from top end of the cylinder to bottom end of the cylinder and simultaneously
inlet valve opens. At this time air at atmospheric pressure drawn inside the
cylinder through the inlet valve by a pump. The inlet valve remains open until
the piston reaches the lower end of cylinder. After it inlet valve close and
seal the upper end of the cylinder.
Compression stroke:
After
the piston passes bottom end of the cylinder, it starts moving up. Both valves
are closed and the cylinder is sealed at that time. The piston moves upward.
This movement of piston compresses the air into a small space between the top
of the piston and cylinder head. The air is compressed into 1/22 or less of its
original volume. Due to this compression a high pressure and temperature
generate inside the cylinder. Both the inlet and exhaust valves do not open
during any part of this stroke. At the end of compression stroke the piston is
at top end of the cylinder.
Power
stroke:
At
the end of the compression stroke when the piston is at top end of the cylinder
a metered quantity of diesel is injected into the cylinder by the injector. The
heat of compressed air ignites the diesel fuel and generates high pressure
which pushes down the piston. The connection rod carries this force to the
crankshaft which turns to move the vehicle. At the end of power stroke the
piston reach the bottom end of cylinder.
Exhaust stroke:
When
the piston reaches the bottom end of cylinder after the power stroke, the
exhaust valve opens. At this time the burn gases inside the cylinder so the
cylinder pressure is slightly high from atmospheric pressure. This pressure
difference allows burn gases to escape through the exhaust port and the piston
move through the top end of the cylinder. At the end of exhaust all burn gases
escape and exhaust valve closed. Now again intake valve open and this process
running until your vehicle starts.
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