In the present time four stroke engines is very popular in
automobile industries. Today we will learn about how four stroke petrol and diesel engine works. In most of car, buses, bikes, and scooters, we are using
four stroke engines because of its higher millage and sufficient power
(torque). A four stroke engine means that the piston passes two times from top
dead center to bottom dead center and crankshaft revolves two complete revolutions
in one power stroke (one time of fuel burns).
In present time two types of four stroke engines used in
automobile. These are
1. Spark ignition engine
(petrol engine)
2. Compression ignition
engine (diesel engine)
How Does Four Stroke Petrol Engine Work?
Four stroke spark ignition engine widely used in bikes,
sport cars because of its higher speed. In this type of engine combustion of
fuel ignite by the spark generate by an external spark plug. So it is known as
spark ignition engine. This engine used petrol as the fuel because of its
combustion temperature and other characteristics are suitable for this engine.
So it is also known as the petrol engine.
The four stroke engine works on Otto cycle. The power
generation process in the four stroke spark ignition engine is divided into
four thermal processes. Each process is run with one piston stroke. These
processes are known as intake stroke, compression stroke, expansion stroke,
exhaust stroke.
Intake stroke (suction stroke):
Air and fuel mixture enter into the cylinder during suction
stroke. In the suction stroke piston moves from top dead center to bottom dead center and simultaneously inlet valve opens. At this time the pressure inside
the cylinder is less than the atmospheric pressure so the air fuel mixture is
sucked in the engine cylinder through the inlet port. The inlet valve remains
open and exhaust valve remains close during this stroke. In this stroke the
volume of cylinder increases and the pressure remain unchanged. This operation
is represented by the line 1-2 in Otto cycle.
Compression stroke:
In this stroke the piston moves from bottom dead center to
top dead center and compressed the enclosed air fuel mixture which is drawn
into the cylinder during intake stroke. It compressed the air fuel mixture into
1/8 time to its original volume. Both inlet and exhaust valve remain closed
during this stroke. At the end of the
compression stroke when the piston reaches at top dead center the spark plug
generate a spark which ignites the air fuel mixture. In this stroke the
cylinder pressure increases and the volume of cylinder decreases. This stroke
is represented by the line 2-3 in the Otto cycle.
Expansion stroke (Power stroke):
At the end of the compression stroke, a spark generated by
the spark plug which ignite the air fuel mixture. When the mixture is ignited the
chemical process take place and produce combustion gases like carbon-di-oxide.
These gases increase the cylinder pressure and temperature. The pressure force
generated by the combustion of the fuel, exerts a force or thrust on piston and
throws it from top dead center to bottom dead center. Thus the work is obtained
in this stroke so it is also known as power stroke. During combustion of gases
the cylinder pressure is suddenly increases and the volume of cylinder remain
constant. This process is represented by the line 3-4 in Otto cycle. After
combustion of the fuel, high pressure is generated which expand the piston form
top dead center to bottom dead center. In the expansion the cylinder pressure
is decreases and the volume of cylinder increases. This operation is presented
by the curve line 4-5 in Otto cycle.
Exhaust stroke:
This is the last stroke of the cycle. The gases generated by
the combustion of fuel are useless after the complete expansion of the piston.
So it should be thrown out from the cylinder. At the end of the expansion
stroke the exhaust valve opens. This time the cylinder pressure is higher than
the atmospheric pressure. So these gases
are escape from the cylinder. During this oppression the cylinder volume
decreases and there is negligible change in the pressure of the cylinder. This
operation is represented by the line 5-1 in the Otto cycle.
At the end of the exhaust stoke again air fuel mixture is
drawn into the cylinder and this process running until the engine is start.
How Does Four Stroke Diesel Engine Work?
Four stroke compression ignition engines are widely used in
cars and heavy motor vehicle such as truck and buses because of its higher
torque at the lower speed. In this type of engine combustion of the fuel ignite
by the temperature increase during compression. So it is known as compression
ignition engine. This engine used diesel as the fuel so it is sometime called
diesel engine.
The working of the compression ignition engine is slightly
different of the spark ignition engine because of it works on Diesel cycle
instead of Otto cycle.
The power generation process in the four stroke compression
ignition engine is also divided into four thermal processes. Each process is
run with one piston stroke. These processes are known as intake stroke,
compression stroke, expansion stroke, exhaust stroke.
Intake stroke:
In the intake stroke of compression ignition engine only air
is drawn inside the cylinder. In this stroke
piston moves from top dead center to bottom dead center and simultaneously
inlet valve opens. At this time the pressure inside the cylinder is less than
the atmospheric pressure so the air is sucked in the engine cylinder through
the inlet port. Sometimes the air is drawn by use of a pump to increase engine
efficiency. The inlet valve remains open and exhaust valve remains close during
this stroke. In this stroke the volume of cylinder increases and the pressure
remain unchanged. This operation is represented by the line 1-2 in Diesel cycle.
Compression stroke:
In this stroke the piston moves from bottom dead center to
top dead center and compressed the enclosed air which is drawn into the
cylinder during intake stroke. It compressed the air into 1/22 time to its
original volume. Both inlet and exhaust valve remain closed during this
stroke. At the end of the compression
stroke when the piston reaches at top dead center, fuel is injected into the
cylinder by the injector. In this stroke the cylinder temperature and pressure of
cylinder reaches very high. This stroke is represented by the line 2-3 in the
Diesel cycle.
Expansion stroke
(power stroke):
At the end of the compression stroke, fuel is injected into
the cylinder by the injector in the form of fine spray. The compressed air
temperature is sufficient high to ignite the fuel. When the mixture is ignited
the chemical process take place and produce combustion gases like
carbon-di-oxide. These gases increase the cylinder pressure and temperature.
This pressure force generated by the combustion of the fuel, exerts a force or
thrust on piston and throws it from top dead center to bottom dead center. Thus
the work is obtained in this stroke so it is also known as power stroke. In
diesel a metered quantity of fuel is injected into the hot compressed air in
fine sprays by the injector and it starts burning at constant pressure. This
process is represented by the line 3-4 in Diesel cycle. After combustion of the
fuel, high pressure is generated which expand the piston form top dead center
to bottom dead center. In the expansion the cylinder pressure is decreases and
the volume of cylinder increases. This operation is presented by the curve line
4-5 in Diesel cycle.
Exhaust stroke:
This is the last stroke of the cycle. The gases generated by
the combustion of fuel are useless after the complete expansion of the piston.
So it should be thrown out from the cylinder. At the end of the expansion
stroke the exhaust valve open. This time the cylinder pressure is higher than
the atmospheric pressure. So these gases
are escape from the cylinder. During this operation the cylinder volume
decreases and there is negligible change in the pressure of the cylinder. This
operation is represented by the line 5-1 in the Diesel cycle.
At the end of the exhaust stoke again air is drawn into the
cylinder and this process running until the engine is start.
Today we have learnt about four stroke engine working. If you have any query regarding this article, ask by commenting.