SUBMITTED TO – Mr. ROHIT CHANDEL
BY- PERCY (B120020188)
SOURABH RANA (B120020282)
NAVNEET ARORA (B120020323)
Petrol engines come under the category of
Internal Combustion engines.
Petrol engines can be four stroke or two stroke.
4-stroke petrol engines work on Otto Cycle.
As the ignition in engine is due to spark,
therefore petrol engine is known as spark
ignition engine.
Four strokes of petrol engines is as follows:
1. Suction stroke
2. Compression stroke
3. Expansion or Working stroke
4. Exhaust stroke
During this stroke (also known as induction stroke) the piston moves from top dead centre (T.D.C) to bottom dead centre.
The inlet valve opens and proportionate fuel –air mixture is sucked in the engine cylinder.
This operation is represented by the line 5-1 (fig 5.32).
The exhaust valve remain closed throughout the stroke.
In this stroke, the piston moves (1-2) towards (T.D.C) and compresses the enclosed fuel air mixture.
The pressure of the mixture rises in the cylinder to a value of about 8 bar.
Just before the end of this stroke the operating –plug initiates a spark which ignites the mixture and combustion takes place at constant volume (line 2-3) (fig 5.32).
Both the inlet and outlet valve remain closed
during this stroke.
When the mixture is ignited by the spark plug the hot gases are produced which drive or throw the piston from T.D.C to B.D.C and thus work is obtained.
The flywheel mounted on the engine shaft stores energy during this stroke and supplies it during other three strokes.
The expansion of gases is shown by 3-4
(fig 5.32).
Both the valves remain closed during the start
of this stroke but when the piston reaches the
B.D.C the exhaust valve opens.
Removal of gases are accomplished during this stroke.
The piston moves from B.D.C to T.D.C.
The exhaust gases are driven out of the engine cylinder ; this is also known as scavenging.
This operation is represented by the line (1-5) (fig 5.32).
NOTE-the line 5-1 is below the atmospheric line.(this is due to the fact that owing to the restricted area of the inlet passages the entering fuel air mixture cannot cope with the speed of the piston.)
NOTE-the exhaust line 4-5 is slightly above
the atmospheric pressure line.(this is due to
restricted exhaust passages which do not
allow the exhaust gases to leave engine –
cylinder quickly).
The loop which has area 4-5-1 is called
negative loop; it gives the pumping loss due
to the admission of fuel air mixture and
removal of exhaust gases.
The area 1-2-3-4 is the total or gross work
obtained from piston and net work can be
obtained by subtracting the area 4-5-1 from
area 1-2-3-4.
o Fig shows a two stroke engine.
o The cylinder L is connected to a closed crank chamber C.C.
o During the upward stroke of the piston M , the gases in L are compressed and the same time fresh air and fuel(petrol) mixture enter the crank chamber through the valve V.
o When the piston moves downwards , V closes and mixture in C.C is compressed.
o The piston is moving upwards and is compressing an explosive change which has been supplied to L.
o Ignition takes place at the end of this stroke.
o The piston then travels downwards due to the expansion of the gases and near the end of this stroke the piston uncovers the exhaust port (E.P) and burnt gases escape through this port.(refer fig 3)
o The transfer port (T.P) then is
uncovered immediately.
o The compressed charge from
the crank chamber flow into
the cylinder and is deflected
upwards by the hump
provided on the head of the
piston.
o The piston then again start
moving from B.D.C to T.D.C
and the charge gets
compressed when E.P and T.P
is covered by the piston ;
hence cycle is repeated.
S.N
o
ASPECTS FOUR STROKE CYCLE
ENGINE
TWO STROKE CYCLE
ENGINE
1. Completion
of cycle
The cycle is completed in
4 strokes of the piston or
in the two revolution of
crankshaft.
The cycle is completed in 2
stroke of the piston or in
the one revolution of the
crankshaft.
2. Flywheel
required
Because turning
movement is not so
uniform ,therefore
heavier flywheel is
required.
More uniform movement
hence lighter flywheel is
required.
3. Valve and
valve
mechanism
4 stroke engine contains
valve and valve
mechanism.
Instead of valves it has
ports.
4. Volumetric
efficiency
Volumetric efficiency is
more due to more time of
induction.
Volumetric efficiency is less
due to lesser time for
induction.
5. Applications Cars , buses, tractors
etc.
Scooter ,motorcycles etc.
6. Power
produced for
same size of
engine
Because of one power
stroke for two
revolutions ,power
produced for same size
of engine is small.
Because of one power
stroke for one revolution,
power produced for same
size of engine is small.
7. Lubrication
required
Lesser rate of wear and
tear hence lesser
requirement of
lubricants .
Higher rate of wear and
tear and consequently
lesser requirement of
lubricants.
8. Initial cost Because of the heavy
weight and complications
of valve mechanism ,
higher is the initial cost.
Because of light weight and
simplicity due to valve
mechanism absence ,
lower is the initial cost.