Working of the Two Stroke Engine

Report
MT 313 IC ENGINES
LECTURE NO: 02
(17 Feb, 2014)
Khurram
[email protected]
Yahoo Group Address: ICE14
HEAT ENGINES
I.C. Engines
Rotary
Reciprocating
Open Cycle
Gas Turbine
Wankel
Gasoline Diesel
E.C. Engines
Reciprocating Rotary
Steam
Stirling
Steam
Close Cycle
Gas Turbine
Internal Combustion Engine.
(I. C. Engine)
3
Classification of I. C. Engines
• Nature of Thermodynamic Cycle
• 1. Otto cycle engine.
• 2. Diesel cycle engine.
• 3. Dual combustion cycle engine.
• Type of the Fuel- used
•
•
•
•
•
1. Petrol engine.
2. Diesel engine.
3. Gas engine.
4. Bi-fuel Engine.
5. Dual Fuel Engine
Classification of I. C. Engines
• Number of Strokes
• 1. 2 - stroke engine
• 2. 4 - stroke engine
• Method of Ignition
• 1. Spark ignition engine, [S.I. Engine].
• 2. Compression ignition engine, [C.I.
engine].
• Number of Cylinders
• 1. Single cylinder engine.
• 2. Multi-cylinder engine.
Classification of I. C. Engines
• Position of the Cylinder
–
–
–
–
1. Horizontal engine
2. Vertical engine
3. V- engine.
4. Radial engine.
• Method of Cooling
–
–
1. Air cooled engine.
2. Water cooled engine
• Speed of the Engine
–
1. Low speed engine.
–
2. Medium speed engine.
–
3. High speed engine.
PARTS OF I.C. ENGINES
I C Engine Parts
I. C. Engine Terms and Definition
Spark plug
•
TDC (top dead center):
It is the top most position of
the piston towards head
side of the cylinder
Valve
Clearance
volume
TDC
Bore
Stroke
•
BDC (bottom dead center):
The lowermost position of
the piston towards the
crank end side of the
cylinder.
BDC
Piston
9
• Stroke:
It is the linear distance
traveled by the piston when
it moves from one end of
the cylinder to the other
end
• Bore:
It is the inside diameter
of the cylinder.
Stock to Bore Ratio:
L/d
d ˂ L Under Square
d =L Square
d ˃L Over Square
•
Swept volume or
(Displacement volume)
It is the volume swept
through by the piston in
moving between TDC and
BDC
Vs
•
= A x L = (π/4)d2 L
Clearance volume:
It is the volume contained in
the cylinder above the top of
the piston, when the piston is
at TDC.
•
•
•
Total volume = swept volume + clearance volume.
Compression ratio: “r”
It is the ratio of total cylinder volume to clearance
volume.
r = Total volume
=
VT
clearance volume
VC
Value of “r” for,
petrol engine lies between 7 to 9
Diesel engine lies between 15 to 22
• Cubic Capacity
VS x K
( K= No of Cylinders)
Working of 4-S Petrol Engine
• The petrol engines work on the principle of
“OTTO CYCLE”, also known as constant
Volume cycle.
• The engines operating on this cycle use either
petrol or other spirit fuels or the gases such as
LPG / CNG as their fuels.
•
•
In a 4-Stroke petrol engine, the charge is
admitted to the engine cylinder is a
homogeneous mixture of petrol and air.
Depending on the load on the engine, the fuel
and air is mixed in proper proportions and
sent in to the cylinder by a popular device
known as “carburetor”.
In a 4-stroke petrol engine there are four
main events taking place, they are
1. Suction
2. Compression
3. Working or power or expansion, and
4. Exhaust
So in a cycle there are four events to take
place, and each of this is performed
during a single stroke of the piston
Since ignition in these engines is due to a spark, they
are also called spark ignition engines .
INTAKE [Suction]: During the intake stroke, the piston
moves down ward, drawing a fresh charge of vaporized
fuel-air mixture, This operation is represented by the line
AB on the P-V diagram.
Pressure [P]
TDC
BDC
A
B
Volume [V]
2. Compression Stroke: During compression stroke, the piston moves from
BDC to TDC, thus compressing air petrol mixture. Due to compression, the pressure
and temperature are increased and is shown by the line BC on the P- V diagram. Just
before the end of this stroke the spark - plug initiates a spark which ignites the
mixture and combustion takes place at constant volume as shown by the line CD
Pressure [P]
D
TDC
C
BDC
A
B
Volume [V]
3. Working Stroke: The expansion of gases due to the heat
of combustion exerts a pressure on the piston. Under this
impulse, the piston moves from TDC to BDC and thus
the work is obtained in this stroke as shown by the line
DE
Pressure [P]
D
TDC
C
E
BDC
A
B
Volume [V]
4. Exhaust Stroke: At the end of the power stroke, the exhaust valve is opened
& greater part of the burnt gases escapes because of their own expansion. The
drop in pressure at constant volume is represented by the line EB. During
this stroke the piston moves from BDC to TDC and pushes the remaining
gases to the atmosphere. This stroke is represented the line BA on the P-V
diagram.
Pressure [P]
D
TDC
C
E
BDC
A
B
Volume [V]
P V diagram for
SI Engine / Otto cycle engine
D
TDC
C
Pressure
E
A
BDC
B
Volume
Theoretical Otto cycle
Diesel Engines
• This engine was invented in 1892 by a German
mechanical engineer named Rudolph Diesel.
• At first this engine was known as the
compression engine but later was named
Diesel after its inventor.
Diesel Engines
• Diesels come in two stroke and four stroke
versions and operate much like the gasoline
driven engines.
• Diesels have a greater compression ratio than
gasoline engines.
– Diesel
– Gasoline
16:1 – 23:1
6:1 – 12:1
Diesel Engine Principles of
Operation
• Intake Stroke
– The intake valve opens.
– The piston moves downward.
– Only air is pulled into the cylinder or pumped in
using a turbo charger (fan).
Diesel Engine Principles of
Operation
• Compression Stroke
– The upward movement of the piston compresses
the air increasing the temperature to approximately
538 degrees Celsius.
Diesel Engine Principles of
Operation
• Power Stroke
– As the piston reaches the
top, fuel is injected at just
the right moment and
ignited by the heat, forcing
the piston back down.
Diesel Engine Principles of
Operation
• Exhaust stroke
– The piston moves back to the top and pushes the
burned gases out of the exhaust valve or port.
Working of 4-S Diesel Engine
The basic construction of a four stroke diesel
engine is same as that of four stroke petrol engine.
Except that instead of a spark plug, a fuel
INJECTOR is mounted in its space.
Fuel injector injects the fuel in to the cylinder as a
fine spray at very high pressure
In case of diesel engine, the air enters the inside the
cylinder during suction, and it will get compressed during
the compression stroke. (i.e.. charge is only air)
At the end of the compression stroke the diesel is injected
in to the cylinder in the form of fine spray
When this fine spray diesel comes in contact with hot air
in the cylinder, it auto ignites and results in a combustion
of injected diesel fuel.
Since ignition in these engines is due to the temperature
of the compressed air, they are also called compression
ignition engines.
1. INTAKE [Suction]:
During the intake stroke, the piston moves down
ward, drawing a fresh charge [AIR]. This
operation is represented by the line AB on the PV diagram.
Pressure [P]
TDC
BDC
A
B
Volume [V]
31
2. Compression Stroke: During compression stroke, the
piston moves from BDC to TDC, thus compressing air.
Due to compression, the pressure and temperature are
increased and is shown by the line BC on the P- V
diagram. Just before the end of this stroke, a metered
quantity of Diesel is injected intoDthe hot compressed
C
air in the form of fine sprays
by means of fuel injector.
[P] at constant pressure shown by
The fuel startsPressure
burning
TDC
the line CD.
BDC
A
B
Volume [V]
3. Working Stroke: The expansion of gases due
to the heat of combustion exerts a pressure on
the piston. Under this impulse, the piston moves
from TDC to BDC and thus the work is
D
obtained in this stroke Cas shown
by the line DE
Pressure [P]
TDC
E
BDC
A
B
Volume [V]
4. Exhaust Stroke: At the end of the power stroke, the
exhaust valve is opened & greater part of the burnt
gases escapes because of their own expansion. The
drop in pressure at constant volume is represented by
the line EB. During this stroke the piston moves from
D
C
BDC to TDC and pushes the remaining gases to the
atmosphere. This
stroke
Pressure
[P] is represented the line BA on
TDC
the P-V diagram.
E
BDC
A
B
Volume [V]
P V diagram for
C.I. Engine / Diesel Cycle Engine
D
C
D
TDC
Pressure
E
A
BDC
B
Volume
Theoretical Diesel cycle
Comparison between Petrol & Diesel
Engine
1
Petrol engine
Diesel engine
It works on Otto cycle. It works on diesel cycle.
2
Air and petrol are
mixed in the
carburetor before they
enter into the cylinder.
Diesel is fed into the
cylinder by fuel injection
and is mixed with air
inside the cylinder.
3
It compresses a
mixture of air and
petrol and is ignited by
an electric spark.
(Spark Ignition)
It compresses only air
and ignition is
accomplished by the
heat of compression.
(Compression Ignition)
4
Cylinder is fitted with a
spark plug.
Cylinder is fitted with a
fuel injector.
5
Less thermal efficiency More thermal efficiency
and more fuel
and less fuel
consumption.
consumption.
6
Compression ratio
ranges from 4:1to10:1
Compression ratio
ranges from 12:1 to 22:1
7
Less initial cost and
more running cost.
More initial cost and less
running cost.
8
Light weight and
occupies less space.
Heavy and occupies
more space.
9
10
11
12
Difficult to start in cold
Easy to start even in
weather and requires
cold weather.
heater plugs.
They run for longer
Requires frequent
periods between
overhauling.
overhauls.
Fuel (petrol) is
Fuel (diesel) is cheaper
costlier and more
and less volatile.
volatile.
Used in heavy duty
Used in light
vehicles like cars,
vehicles like tractors,
motor cycle,
trucks, buses,
locomotives, etc.
scooters, etc.
Working of the Two Stroke Engine
• In a two stroke engine, a cycle is completed by
the two strokes of the piston.
• Out of the four strokes, the two strokes that are
eliminated are, suction and exhaust strokes.
• However, the exhaust process is achieved by
the admission of charge which is extremely
compressed, which drives out the burnt gases
out and this process is popularly called as
SCAVENGING.
In case of the two stroke engines instead of valves,
ports are used.
Ports in the cylinder liner, opened and closed by the
piston motion itself
Working of Two Stroke Petrol Engine
First stroke
Second stroke
Working of Two Stroke Petrol Engine
• First stroke (Downward) As
soon as the charge is ignited, the
hot gases force the piston to
move downwards, rotating the
crankshaft, thus doing the useful
work. During this stroke the inlet
port is covered by the piston and
the new charge is compressed in
the crank case as shown in the
fig.
• Further downward movement of the piston uncovers first
the exhaust port and then the transfer port.
• The burnt gases escape through the exhaust port.
• As soon as the transfer port opens, the
compressed charge from the crankcase
flows into the cylinder.
• As the compressed charge enters into
the cylinder, it pushes out the exhaust
gases from the cylinder.
• The process of removal of exhaust
gases by the fresh incoming charge is
known as scavenging.
• Second stroke: (upward)
Here the piston moves from BDC
to TDC, during the process the
exhaust port and transfer port are
covered and the charge in the
cylinder is compressed.
Simultaneously, vacuum is created
in the crankcase, and a new charge
is drawn into the crankcase
through the uncovered inlet port.
• The compressed charge is
ignited in the combustion
chamber by a spark
provided by the spark
plug and the cycle of
events is then repeated.
Working of Two Stroke Diesel Engine
First stroke
Second stroke
• First
stroke
(Downward)
Combustion starts once the diesel
is injected in to the hot
compressed air, the hot gases force
the piston to move downwards,
rotating the crankshaft, thus doing
the useful work. During this stroke
the inlet port is covered by the
piston and the new charge [air] is
compressed in the crank case as
shown in the fig.
• Further downward movement of the piston uncovers
first the exhaust port and then the transfer port.
• The burnt gases escape through the exhaust port.
• As soon as the transfer port opens, the
compressed charge from the crankcase
flows into the cylinder.
• As the compressed charge enters into the
cylinder, it pushes out the exhaust gases
from the cylinder.
• The process of removal of exhaust gases
by the fresh incoming air is known as
scavenging.
• Second stroke: (upward)
Here the piston moves from
BDC to TDC, during the
process the exhaust port and
transfer port are covered and
the fresh air in the cylinder is
compressed.
Simultaneously,
vacuum is created in the
crankcase, and a new charge
[air] is drawn into the crankcase
through the uncovered inlet
port.
49
• At the end of the compression diesel is injected to the
compressed air which is at a temperature higher than
the self ignition temperature of diesel. Hence, the
injected diesel auto ignites when it comes in contact
with hot air. And the cycle of events is then repeated.
Comparison between 4 - stroke &
stroke Engine
1
2
3
4 – stroke engine
One Working stroke for
every two revolution of
the Crank shaft
Turning moment on the
crank shaft is not even,
hence heavier flywheel
is required
2-
2 – stroke engine
One working stroke for
each revolution of the
crank shaft
Turning moment on the
crank shaft is more
even, hence lighter
flywheel is required
Less fuel consumption More fuel consumption.
6
Engine design is
complicated
Less output due to
mixing of fresh charge
with the burnt gases.
Lower thermal
efficiency
Engine design is
simple.
7
Lesser rate of wear and
tear.
Greater rate of wear
and tear.
8
It has inlet and exhaust
valves
It has inlet and exhaust
ports
4
5
More output due to full
fresh charge intake and
full burnt gases exhaust.
Higher thermal efficiency
9
10
11
12
13
Engine is heavy &
bulky.
It requires lesser
cooling and
lubrication
More initial cost
For the same power, the
engine is light and
compact.
It requires greater cooling
and lubrication. (consumes
more lubricating oil)
Less initial cost.
More running noise due to
Less running noise the sudden release of the
burnt gases.
Used in cars,
Used in mopeds, motor
trucks, buses,
cycles, scooters, etc.
tractors, etc.
Rotary (Wankel) Engine
• Designed in 1958 by a German scientist named
Felix Wankel.
• Wankel engines do not use pistons.
Wankel Engine
• The wankel engine uses a triangular shaped
rotor housed in an oval shaped cylinder.
• As the rotor is rotated it moves around the
cylinder producing the four basic functions to
produce a power stroke.
Wankel Engine Principles of
Operation
• Intake Stroke
– The production of
power begins with the
rotor at point A.
– The intake port is
uncovered allowing a
new air/fuel mixture
to
enter
the
combustion chamber.
Wankel Engine Principles of
Operation
• Compression Stroke
– As the rotor rotates the
combustion chamber
is reduced in size
compressing
the
mixture.
Wankel Engine Principles of
Operation
• Power Stroke
– At the highest point of
compression
the
air/fuel is ignited.
– The hot expanding
gases push on the
rotor causing it to
rotate.
Wankel Engine Principles of
Operation
• Exhaust Stroke
– The continued rotation
of the rotor uncovers
the
exhaust
port
allowing the exhaust
gas to escape.
– The cycle then repeats
when a new air/fuel
mixture is permitted to
enter the combustion
chamber.
Diesel VS Gasoline Engines
• Different type fuel (Diesel fuel).
• Diesel engines operate at a much higher
compression ratio.
• Diesel engines do not use spark plugs.
• Glow plugs are used to help get the engines
going on very cold days.
Diesel Engine Advantages
• Advantages
– Greater fuel economy (25% more efficient than
gasoline engines).
– Produces more power.
– Requires less maintenance.
Diesel Engine Disadvantages
• Disadvantages
– Must be constructed heavier to withstand the
higher pressures.
– Shortage of qualified mechanics in some areas.
Two Stroke Advantages
• Advantages
– Requires fewer moving parts to accomplish the
same amount of output as four stroke engines.
– Cheaper to maintain than four stroke engines.
– Smaller and simple in construction than four stroke
engines.
– Can work in any orientation.
Two Stroke Disadvantages
• Disadvantages
– Less fuel efficient than four stroke.
– Quicker wear of the engine’s moving parts.
– More polluting than four stroke engines since oil is
burnt with the fuel and air mixture.
Wankel Engine Advantages
• Advantages
– Less vibration than piston driven engines.
– A two rotary engine is as powerful as a six
cylinder piston engine.
– Power output can be increased by adding
additional rotors to the engine.
Wankel Engine Disadvantages
• Disadvantages
– Sealing the rotor in the odd shaped cylinder is
very difficult requiring costly maintenance.
– Construction cost are high for this engine.
– Shortages of trained mechanics to service this
engine type.
DIESEL
PETROL
Uses:
In diesel
engines, heating
systems
In petrol engines
Made
from:
Petroleum/ Crude oil Petroleum/ Crude Oil
Energy
content:
38.6 MJ/litre
Made by:
Fractional distillation Fractional distillation
34.6 MJ/litre
DIESEL
PETROL
Torque (for 1000 Nm @ 2000
10L
rpm
engine):
300Nm @ 4000 rpm
Power (for 490Hp @ 3500 rpm
10L
engine):
600Hp @ 5500 rpm
Power =
More torque at low
torque*RP speeds
M:
Runs at higher RPM
Autoignition
246°C
210°C
DIESEL
PETROL
CO2 emiss More than
Lower than diesel.
ion:
gasoline(petrol). Die
sel fuel produces
approximately 13%
more CO2 gas per
gallon of fuel burned,
compared to gas
(petrol) engines.
Viscosity:
increase at lower
temperatures
US
50 Billion gallons
Consumpti
No change
148 Billion gallons
Qusetions
• Define , Engine and Heat Engine?
• How I.C. engines are classified?
• What are the important basic components of an I.C.
engine?
• Define , Bore, Stroke, Clearance volume, compression
ratio, displacement volume and cubic capacity?
• Explain the working principle of the four stock S.I.
engine?
• Differentiate between C.I. and S.I.?
• Explain Otto cycle?
• Explain Diesel cycle ?
• Compare between Petrol and Diesel engines?

similar documents