An Engine is a device which transforms one form of energy into another form of Energy. An internal combustion engine is a heat engine where it undergoes different cycles of operations in a sequent manner to convert the thermal energy into useful work. In this article, we are going to discuss the 4 Stroke Engine working principle with the Spark ignition. So it is also known as the 4 Stroke Spark-Ignition Engines or 4 Stroke Petrol engine.

Working Principle of Engines

IC engines work on either Spark ignition or the Compression Ignition working principle.

Spark Ignition: Usually a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug.

compression ignition: Generally the Diesel Engines, where the combustion of the fuel is caused by the elevated temperature of the air in the cylinder due to the mechanical compression.

4 Stroke Petrol Engines | 4 Stroke Spark Ignition Engine

• In 4 Stroke Engine, the Thermodynamic cycle will be completed in the four strokes of the position or the two revolutions of the crankshaft.
• All the four strokes will be completed in the 720° of the crank rotation.
• During these four-strokes, there are five actions/events to be completed. They are suction and the compression combustion, expansion, exhaust.
• The combustion of the fuel will be taken care by the spark plug, so this is called the 4 Stroke Spark ignition engine also known as the 4 Stroke Petrol Engine.
• This 4 Stroke Petrol Engine was invented by Nicolaus A. Otto in 1876, so this Engine is also called as the Otto Engine.

The cycle of operation of a four-stroke petrol engine consists of the following strokes:

1. Suction or intake stroke,
2. Compression stroke,
3. Expansion or power stroke,
4. Exhaust stroke.

Suction or intake stroke

Suction Stroke starts when the piston is at the top dead centre(TDC) and about to move downwards. At this time the Inlet valve is opened and the exhaust valve is closed.

The suction created in the cylinder due to the movement of the piston downwards (Bottom Dead centre) the fuel mixture will be drawn into the cylinder.

When the piston reaches the bottom dead centre (BDC), the suction stroke will end.

Compression stroke

Compression Stroke starts right after the suction stroke completes. i.e the piston reaches the BDC.

During the Compression stroke, the piston moves from the Bottom Dead Centre(BDC) to the Top Dead Centre (TDC). Due to this the charge which is drawn into the cylinder will be compressed. During this stroke, both the inlet and exhaust valves are closed.

The charge which is occupied by the cylinder is compressed into the clearance volume which is available when the piston is at the TDC. As a result, the pressure and temperature of the air-petrol mixture or charge are increased.

Just before the piston reaches the TDC, the air-petrol mixture (charge) is ignited by a spark plug; suddenly burning of the air-petrol mixture takes place almost instantaneously.

These two strokes (i.e., suction and compression stroke) complete one revolution of the crankshaft.

Expansion or power stroke

During expansion stroke, both the inlet and exhaust valves remain closed.  The high pressure of the products of combustion pushes piston from Top Dead Centre (TDC) to Bottom Dead Centre(BDC).

It is also called as working stroke as the linear movement of the piston is converted into the rotational movement by the cranks shaft.

There is a flywheel is mounted on this crankshaft to collect the excess amount of the energy during the power stroke and the helps the remaining three ideal strokes.

Exhaust stroke

At the end of the expansion stroke, the exhaust valves are open to exhaust the burnt gases out of the cylinder.

During the exhaust stroke, the inlet valves will remain closed.

In this stroke, the piston starts moving from the Bottom Dead Centre(TDC) to the Top Dead Centre(TDC) and sweeps all the burnt gases from the cylinder into the atmosphere. now the cycle repeats from the first step again as the intake.

These two strokes (i.e., expansion and exhaust strokes) complete one revolution of the crankshaft.

Conclusion

The working principles of the internal combustion engines are the Spark ignition and the compression ignition. The 4 stroke engine is available in both the Spark ignition and the compression ignition working principles. We have discussed the 4 stroke engine with the spark ignition engine. also, read the 4 Stroke Compression ignition engine. If you have any thoughts, leave them in the comments section below.