Understanding the basics of four stroke engine

1. SCIENCE OF AERONAUTICS
AND
ENGINEERING EDUCATIONAL TECHNICS
CH. PURUSHOTHAM
AERONAUTICAL ENGG.
Four stroke Engine
DEPARTMENT OF AERONAUTICAL ENGINEERING

2. Four-stroke engine
A four-stroke engine (also known as four-cycle) is an internal
combustion engine in which the piston completes four separate strokes which
constitute a single thermodynamic cycle. A stroke refers to the full travel of
the piston along the cylinder, in either direction. The four separate strokes are
termed:
1. Intake:
This stroke of the piston begins at top dead center. The
piston descends from the top of the cylinder to the bottom of
the cylinder, increasing the volume of the cylinder. A
mixture of fuel and air is forced by atmospheric (or greater
by some form of air pump) pressure into the cylinder through
the intake port.
2. Compression:
With both intake and exhaust valves closed, the piston
returns to the top of the cylinder compressing the air or fuel-
air mixture into the cylinder head.
3. Power:
This is the start of the second revolution of the cycle.
While the piston is close to Top Dead Centre, the compressed
air–fuel mixture in a gasoline engine is ignited, by a spark
plug in gasoline engines, or which ignites due to the heat
generated by compression in a diesel engine. The resulting
pressure from the combustion of the compressed fuel-air
mixture forces the piston back down toward bottom dead
centre.
4. Exhaust:
During the exhaust stroke, the piston once again returns to
top dead centre while the exhaust valve is open. This action
expels the spent fuel-air mixture through the exhaust valve(s).

3. There are different types cycles in thermodynamics. Such as Otto cycle, Carnot vapour cycle,
Diesel cycle etc. Out of that cycle diesel engine works on diesel cycle. This cycle is also
known as constant pressure cycle. Diesel engine is mostly employed in Stationary Power
plants, Ships, Heavy Motor Vehicles.
In Petrol Engine, the air-fuel mixture after being compressed in the engine cylinder to a high
pressure, is ignited by an electric spark from a spark plug. In diesel engine, diesel oil and
light and heavy oil used as fuel. This fuel is ignited by being injected into the engine cylinder
containing air compressed to a very high pressure, the temperature of this air is sufficiently
high to ignite the fuel. That is why there is no spark plug used in diesel engine. This high
temperature compressed air used in the form of very fine spray is injected at a controlled rate
so that the combustion of fuel proceeds at constant pressure.
Advantages of 4 Stroke Engines:
1. You don’t have to pre mix the oil in
to the fuel.
2. The engine lasts longer than 2-stroke
and won’t cease as quickly.
3. Quieter than 2-stroke
4. Cleaner running
5. More torque
6. Last longer
7. Run much cleaner than 2 strokes
8. More efficient use of gas
Disadvantages of 4 Stroke Engines:
1. Heavier than a 2-stroke
2. Cant turn upside down
3. Slower and less sharper acceleration than
2-Stroke
4. Slightly larger in size.
5. Complicated
6. Half as powerful as two stroke engines

4. Engine Parts and Their Functions
Engines can come in several different varieties, with various parts depending on the
type of vehicle it is. For example, a truck engine would be different as compared to the
engine of a regular car due to the amount of power required. Today, many vehicles make use
of the internal combustion engine, however with slight variations according to vehicle types,
having some added features or components. Developed in the 19th century, this type of
engine still remains a popular choice and it continues to benefit from the technological
advances in engineering. Today’s advanced engines may even feature computerized controls
and advanced systems to make them more efficient, durable and powerful. With advances
come several components which all work together to allow the engine to perform the required
tasks. In order to be able to understand how an engine works, it is important to understand
what the different engine parts are.
Many of the automotive engines used in the industry today are four-stroke internal
combustion engine that use either gasoline or diesel as a fuel. These engines get their name
‘four-stroke’ from the four distinct phases that occur in the engine during operation. Being
the first phase, fuel and air are taken into the combustion chamber, earning this phase the
name; intake phase. A piston is then used to compress the fuel in the next phase. Thereafter a
spark is used to ignite the fuel to cause a controlled explosion. This explosion provides the
engine with the energy required to drive the car forward. The ignition of the fuel varies in
diesel powered engines and gasoline powered engines. Gasoline powered engines make use
of a spark to ignite the fuel. The spark is generated through electrical components. On the
other hand, the fuel in a diesel engine is ignited through compression and does not require an
extra electrical component. After the ignition phase, the final part in the four-stroke phase is
the exhaust phase. During which, the unused fuel and carbon emissions are let out of the
combustion chamber to allow new fuel and gas to enter the space, allowing the process to
start over again.
The core component of an engine is the cylinder that houses the pistons. In a regular car
engine, it can have anywhere from four to eight cylinders. The arrangement of the cylinder
can pose different advantages and disadvantages. Depending on the size and type of vehicle,
manufacturers opt for differing number of cylinders to match the requirements of the vehicle.
The movements of the pistons in the cylinders provide the engine with power for the vehicle
to function.

5. Components of an Engine
Cylinder block
Cylinder is the main body of IC engine. Cylinder is a part in which the intake of fuel,
compression of fuel and burning of fuel take place. 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. The upper side of cylinder
is consists of a combustion chamber where fuel burns. To handle all this pressure and
temperature generated by combustion of fuel, cylinder material should have high compressive
strength. So it is made by high grade cast iron. It is made by casting and usually cast in one
piece.
Pistons & Piston Rings :
A piston is a component of reciprocating engines. It is located
in a cylinder and is made gas-tight by piston rings. Its purpose
is to transfer force from expanding gas in the cylinder to
the crankshaft via a piston rod and/or connecting rod. This is a
cylindrical piece of metal that is located inside the cylinder of
the engine. Piston rings are located between the piston and the
cylinder in which the piston is located in. They provide a
sealing edge between the exterior of the piston and the interior
of the cylinder. The purpose of these engine parts is to seal the
space and prevent the fuel and air mixture on one side of the
piston from leaking into the sump during the combustion or
compression process and also prevent the oil in the sump from
leaking into the combustion area as it would get burnt and lost, deterring the movement of the
piston.
Spark Plug
In a spark ignition engine, a mixture is ignited by an electric
spark from a spark plug — the timing of which is very precisely
controlled. Almost all gasoline engines are of this type. Diesel
engines timing is precisely controlled by the pressure pump and
injector. The normal plug distance between the spark plug is the 1mm
apart. and the voltage is 3000v at normal atmospheric conditions, As
mentioned earlier, gasoline engines make use of a spark to ignite the
fuel and cause a controlled explosion in the engine. The spark plug in
these engines supplies the spark that is required to ignite the air and
fuel mixture.

6. Valves :
These engine parts allow for fuel and air to enter the combustion
chamber and later let the exhaust out. They remain sealed during the
combustion process and only open when required.
Connecting rod and Crankshaft :
Most reciprocating internal combustion engines end up turning a
shaft. This means that the linear motion of a piston must be
converted into rotation. This is typically achieved by a crankshaft.
The connecting rod connects the piston to the crankshaft. As the
piston moves up and down due to the controlled explosions, it
causes the connecting rod to move. This then cause the crankshaft
to move as well as it is connected to the connecting rod, in a circular
motion due to the configuration of the piston, connecting rod and crankshaft.
Injector :
Injector is usually used in compression ignition engine. It sprays the
fuel into combustion chamber at the end of compression stroke. It is
fitted on cylinder head.
Camshaft
Camshaft is used in IC engine to control the opening and
closing of valves at proper timing. For proper engine output
inlet valve should open at the end of exhaust stroke and closed
at the end of intake stroke. So to regulate its timing, a cam is
use which is oval in shape and it exerts a pressure on the valve
to open and release to close. It is drive by the timing belt which
drives by crankshaft. It is placed at the top or at the bottom of
cylinder.
Sump :
Surrounding the crankshaft, the sump contains some amount of oil.