Four stroke engine: Definition, Working,
Cycle, Applications
Most of
the vehicles that rush on the road are powered by four-stroke IC engines. Based
on the working cycle these are further classified as petrol engines and diesel engines.
We notice four-stroke petrol engines in light or medium-duty
vehicles like Motorcycles, Cars, ATVs, etc. In this detailed article, we will
discuss four-stroke petrol engine
In this article, we’re
going to discuss:
What is Four stroke petrol engine?
Four
stroke petrol engine working cycle:
Parts:Construction
of four stroke petrol engine:
Terms
used in 4 stroke petrol engine:
What
are Four Strokes in Four Stroke Petrol Engine:
Working
of four stroke petrol engine:
Valve timing diagram:
Advantages
of four stroke petrol engine:
Disadvantages
of four stroke petrol engine:
Applications
of four stroke petrol engine
What
is Four stroke petrol engine?
The four stroke petrol engine is and I.C Engines that works
on a petrol cycle and completes a single power cycle within four strokes of the
piston or two revolutions of the crankshaft.
The term “four-stroke” indicates the engine in which a single
working cycle is completed within four strokes of the piston (suction,
compression, expansion, exhaust) while “Petrol” indicates the engine that works
on a petrol cycle (Otto cycle).
As
these engines use spark plugs for the combustion of the air-fuel mixture, these
are also referred to as 4-stroke spark ignition (SI) engines. It runs on the
fuels like gasoline, CNG, methanol, LNG, etc.
These engines are generally lighter and smaller in size than the
four-stroke diesel engines as it uses a lower compression ratio (8 to 12) in
comparison with 4 stroke diesel engines (14 to 24). Thus these engines are used
to run low to medium-duty automobiles.
Four stroke petrol engine working cycle:
The
four stroke petrol engine works on the principle of the Otto cycle. The cycle
was invented by the German engineer Nicolaus Otto and mainly consists of two
adiabatic and two isochoric processes.
It
comprises the following processes:-
a) Process (0-1):- During
this process, the piston sucks (pulls) the air-fuel mixture inside the cylinder
at constant pressure P1.
b) Process (1-2):- It
is an adiabatic compression process in which the piston compresses the air-fuel
mixture inside the cylinder from pressure P1 to P2.
c) Process (2-3):- It
is a constant volume heat addition process. In this process, a spark plug produces a spark, to ignite the fresh charge (Air
+ Fuel) which causes the pressure inside the combustion chamber to rise from P2
toP3.
d) Process (3-4):- It
is an adiabatic expansion in which the high-pressure gases expand inside the
cylinder from P3 to P4.
e) Process (4-1):-
It is a constant volume heat rejection, during which the heat is rejected
outside of the cylinder.
f) Process (1-0):- It
is a constant pressure process in which burn gases are released from the
cylinder into the exhaust Pipe.
Parts:
The
Four Stroke Petrol Engine Consists of Cylinder, Piston, Connecting Rod, Crank,
Inlet Port, Connecting Rod, Exhaust Port & Spark Plug.
These
engines basically consist of the following parts:-
1) Intake Port and intake
valve:- Inlet Port connects the cylinder to the intake manifold and
its opening is controlled with the help of the intake valve. During the open
period of the intake valve, a fresh charge from the intake manifold enters the
engine cylinder.
2) Exhaust Port and exhaust valve:- Exhaust
Port connects the cylinder to the exhaust manifold and its opening is
controlled with the help of the exhaust valve. Burn gases from Cylinder are
released into the exhaust pipe through the exhaust port.
3) Spark Plug:- Spark
Plug is connected to the cylinder head. The spark plug produces a spark in the
cylinder which helps to initiate the burning of fuel.
4) Cylinder:- The cylinder guides the
movement of the piston and also helps to form the enclosed cavity for the
combustion of the air-fuel mixture. It is connected between the cylinder head
and the crankcase
The
size of the cylinders used in automobiles varies as per power requirements.
The capacity of the engine is specified by the volume inside the
cylinder when the piston is at the bottom dead center.
5) Cylinder head:- The
cylinder head helps to form the upper enclosure of the combustion chamber. The
inlet and exhaust ports are constructed on the cylinder head and it also has a
provision for mounting the valves, spark plug, and valve actuating mechanism.
6) Piston: The piston reciprocates
inside the cylinder to continuously change the volume enclosed in the cylinder
which helps to perform the operations like suction, compression, expansion, and
exhaust.
7) Connecting Rod:
The connecting Rod connects the piston to the crank. One end of the connecting
rod is connected to the piston while another end of the connecting rod is
connected to the crank.
8) Crank and crankshaft:- The
big end of the connecting rod is connected to the crank. The crank and
crankshaft along with the connecting rod convert the reciprocating motion
of the piston into the rotary motion.
Construction
of four stroke petrol engine:
The 4 stroke petrol engine consists of stationary as well as
moving components.
The stationary components of the petrol engine are the cylinder, cylinder head,
crankcase, intake and exhaust manifold, spark plug, etc.Advertisements
The
moving components of the petrol engine are piston, connecting rod, crank and
crankshaft, intake and exhaust valves, etc.
The
cylinder head is mounted over the cylinder block and the piston is placed
inside the cylinder to form a cylinder cavity or combustion chamber.
The
piston easily reciprocates inside a cylinder and it is connected to the
crankshaft with the help of connecting rod. This helps to convert the
reciprocating motion of the piston into the rotary motion of the crankshaft and
vice versa.
The
inlet and exhaust valves are mounted into the cylinder head. The inlet valve
actuates to control the entry of fresh charge from the intake manifold into the
engine cylinder and the exhaust valve actuates to control the removal of
exhaust gases from a cylinder to the exhaust manifold.
Terms
used in 4 stroke petrol engine:
Following are some of the vital terms used in the case of the
4-stroke petrol engine, which will provide ease to understand the topic:-
TDC (Top Dead Centre):- It
is the nearest position of the piston from the cylinder head. When the piston
is at TDC, the volume inside the
cylinder enclosed by the piston is minimum.
BDC (Bottom Dead Centre):- It
is the farthest position of the piston from the cylinder head. When the piston
is at BDC, the volume inside the cylinder enclosed by the piston is maximum.
Stroke:- The travel of the piston
from the TDC to BDC or vice versa is known as stroke.
Stroke volume:- The
volume of the cylinder from the position TDC to the BDC is known as a stroke/swept volume. It is also known as the volume swept by the piston
from TDC to BDC.
Clearance volume:- When
the piston is at TDC, the Volume enclosed by the cylinder and piston is known
as clearance volume. It is the minimum volume inside the cylinder in the whole
cycle.
A/F ratio (Air-fuel ratio):- It
is the ratio of the mass of air to the mass of fuel present in the air-fuel
mixture.
What
are Four Strokes in Four Stroke Petrol Engine:
The
working of the 4-stroke petrol engine comprises of the following 4 important
strokes:-
1) Suction Stroke:- While
suction stroke, the piston moves from TDC to BDC with the intake valve in open
condition and the exhaust valve in closed condition.
During this stroke, as the piston moves from TDC to BDC, a partial
vacuum is created inside a cylinder that helps to suck (pull) the air-fuel
mixture from the intake manifold into
the cylinder.
At the end of the suction stroke when the piston reaches the BDC, the inlet
valve becomes closed.
2) Compression Stroke:–
During the compression stroke, the piston compresses the air-fuel mixture to
high pressure {inside the cylinder←Remove}. For this purpose, the piston moves from the BDC
toward TDC. During a compression stroke, both valves (Inlet and
Exhaust) remain in a closed condition. At the end of the compression
stroke, the spark plug produces the spark to ignite the air-fuel mixture.
Power Stroke / Expansion Stroke:- In
this stroke, the high-pressure combustion products push the piston downward
(toward BDC) to get expand. Hence, the piston gets power due to the expansion
of combustion products. During the power, stroke piston moves from the TDC
toward BDC. At the end of the power stroke, the exhaust port becomes open.
4) Exhaust Stroke:-
In Exhaust stroke, the piston moves from BDC to TDC. During this stroke, burn
gases are released to Exhaust Pipe through an exhaust port. The exhaust valve
becomes closed when the exhaust stroke is completed.
Working of four stroke petrol engine:
At
the start of the combustion cycle, when the piston is at TDC, the intake valve
becomes open and the exhaust valve remains in closed condition.
The
first stroke is the suction stroke during which the piston moves from TDC to
BDC. As the intake valve is in an open situation, the piston pulls the fresh
charge from the intake manifold, thus the cylinder gets filled with the fresh
charge. At the end of the suction stroke, when the piston reaches BDC, the
intake valve becomes closed.
Now
the piston moves from the BDC to TDC to compress the fresh charge trapped
inside the cylinder. During this stroke, both the intake and exhaust valve
remains in closed situation.
At
the end of the compression stroke, the spark plug produces the spark to ignite
the compressed air-fuel mixture. The combustion of the compressed charge
results in the generation of high-pressure combustion products inside the
combustion chamber.
These high-pressure combustion products push the piston from TDC
to BDC. The movement of the piston due to the expansion of the combustion
products is known as expansion stroke or power stroke. At the end of this power
stroke (at BDC), the exhaust valve becomes open.
Now
in the next stroke, the exhaust gases are expelled outside of the cylinder
through the exhaust port due to the upward motion of the piston from BDC to
TDC. At the end of this stroke, after reaching the piston to TDC, the exhaust valve
get closed.
After
Exhaust valve closes, the intake valve again becomes open and the fresh
Air-fuel mixture again enters the cylinder Therefore cycle continues.
As
the piston is connected to the crankshaft with the help of connecting rod, the
reciprocating motion of the piston gets converted into the rotary motion of the
crankshaft.
Valve
timing diagram:
The
valve timing diagram for four stroke petrol engine indicates the position of
the piston or the position of the crankshaft during the opening and closing of
the intake and exhaust valves, and generation of spark.
The
valve timing diagram is different for the theoretical cycle and for the actual
cycle. Lets discuss about each of them.
Theoretical valve timing diagram:-
The
above figure shows the valve timing diagram for a theoretical 4 stroke petrol
engine. As shown in the above figure, at the start of the suction stroke, the
intake valve opens exactly when the piston is at TDC and closes exactly when
the piston reaches to BDC.
The spark generation in this theoretical cycle occurs when the
piston is at TDC (starting of the expansion stroke). During the exhaust stroke,
the exhaust valve opens when the piston is at TDC and it closes when the piston
reaches to TDC
Actual valve timing diagram:-
In
the actual operation of the four stroke petrol engine, the inlet valve opens a
few degrees before the piston reaches TDC, thus due to the valve overlap, the
intake charge from the intake manifold helps to push the exhaust gases outside
of the cylinder.
The
inlet valve closes a few degrees after the BDC. During the suction stroke, when
the piston reaches the BDC, the intake charge not completely enters into the
cylinder and still the negative pressure is present in the cylinder.
Thus for completely feeding the intake charge into the cylinder, the closing of
the intake valve is delayed by a few degrees after the TDC.
AdvertisementsThe spark generation occurs just a
few degrees before the piston reaches to TDC. The combustion of intake charge
and building of pressure is not an instantaneous process and takes a certain
time. To avoid the delay in the building of pressure, the spark is generated a
few degrees before the piston reaches TDC so that the building of pressure
starts just after the piston reaches TDC.
The
exhaust valve opens a few degrees before the piston reaches to BDC. It occurs
to release the excessive pressure from the cylinder (at the end of the
expansion stroke) and thus it helps to avoid pumping losses during the upward
movement (exhaust stroke) of the piston.
The
exhaust valve closes a few degrees after the TDC. Thus the intake and exhaust
valve remains open for better scavenging
Advantages
of four stroke petrol engine:
The
four stroke petrol engine has the following advantages:-
1. The
4-stroke petrol engines work at high speed and low torque.
2. The
4-stroke petrol engines work at a comparatively lower compression ratio.
3. It
does not require a high-pressure fuel injection system and fuel injectors.
4. The
engines are light in weight.
5. The
working of 4 stroke petrol engine is less noisy.
6. Because
of the spark plug, cold starting is easier in 4-stroke petrol engines.
7. The
initial cost of 4 stroke petrol engine is less.
8. It
has a lower maintenance cost.
9. The
engine creates fewer vibrations.
Disadvantages of four stroke petrol engine:
The four stroke petrol engine has the following disadvantages:-Advertisements
1. It
requires an external device like a carburetor for the mixing of air and fuel.
2. The
fuel used by the four stroke petrol engine is highly volatile.
3. The
spark ignition system is required to ignite the compressed mixture.
4. The
cost of fuel used in four stroke petrol engine is comparatively high.
5. It
creates less torque than diesel engines.
6. The
4 stroke petrol engine has lower thermal efficiency.
7. It
is not suitable for heavy load applications due to the less torque generation.
8. This
engine consumes more fuel for a unit amount of power generation, Thus it has
lower specific fuel consumption.
9. Applications
of four stroke petrol engine:
The
four stroke petrol engine has applications in the following vehicles:-
Auto-rickshaw
- Scooter
- Motorcycle
- ATVs
- Petrol Cars
- Generators
- Light-duty transport vehicle
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