SI Engine – Parts, Working, Advantages, Disadvantages, and Applications

A spark ignition (SI)  engine is a type of internal combustion engine in which the air-fuel mixture is ignited using a spark from the spark plug.
The compression ratio of this engine is usually 6 to 10.
The SI Engine is commonly used in light-duty vehicles like motorcycles, cars, etc.
These engines are commonly known as gasoline engines in North America and petrol engines in most of the other countries like Britain.
But the SI engine should not be named as petrol Or gasoline engine as other fuels can also run on other fuels like autogas (LPG), methanol, ethanol,  compressed natural gas (CNG), hydrogen, etc.


Components of SI engine:

1) Inlet Valve:
Inlet valve in SI engine is the passage for the air-fuel mixture to enter into the cylinder of the engine.

2) Exhaust Valve:
The exhaust valve is used to remove the burnt or exhaust gases produced during the power stroke. All the exhaust gases escape through this valve.

SI Engine
SI Engine

3) Cylinder:
The cylinder is the container inside which all the processes during the four or two-stroke of the engine happens. The piston moves upward and downward inside this cylinder.

4) Piston:
It is the part of the engine that reciprocates during the power stroke and transmits the power to the crankshaft through the connecting rod.

5) Spark Plug:
The spark plug is used to ignite the compressed air-fuel mixture. It produces a spark at the end of the compression stroke.

6) Crankshaft:
It is used to convert the reciprocating motion of the crankshaft to rotary motion. It is connected with the post using the connecting rod.

7) Connecting Rod:
The connecting rod is used to connect the piston of the engine to the crankshaft so that reciprocating motion can be converted to rotary motion.

Working Of SI Engine:

The working of SI Engine is divided into four strokes:

1) Suction Stroke Or Charging Stroke:
In this stroke of a four-stroke engine, the air-fuel mixture enter enters into the cylinder.
At first, the piston moves downward and a vacuum is created in the cylinder. The system of the inlet valve is designed in such a way that the inlet valve opens at the same time when the vacuum is created.
As the vacuum is created in the cylinder and the inlet valve opens, the air-fuel mixture enters the cylinder from the carburetor through the inlet valve.
The piston moves from TDC ( Top Dead Center ) to BDC ( Bottom Dead Center ) in this stroke.
It is called suction stroke because in this stroke, the air-fuel mixture gets suck inside the cylinder and it is called charging stroke because charge enters in this stroke.
The inlet valve opens and the exhaust valve remains closed during this stroke.

2) Compression Stroke:
When the air-fuel mixture reaches the cylinder, the air-fuel mixture gets compressed in the compression stroke.
After the first stroke, the piston moves upward and the air-fuel mixture is compressed. Due to compression, the temperature and pressure of the air-fuel mixture get increased.
The compression stroke is completed when the piston reaches the TDC ( Top Dead Center). The piston moves from BDC to TDC in this stroke.
During this stroke, both the inlet and the exhaust valves remain closed.

3) Power Stroke or Expansion Stroke or Working Stroke:


In this stroke, the air-fuel mixture is ignited using spark, and combustion of air-fuel mixture happens due to which power is generated.
As the compression stroke ends, a spark is produced by the spark plug. This spark ignites the air-fuel mixture. The air-fuel mixture starts burning inside the cylinder due to which the pressure and temperature further increase and the air-fuel starts expanding suddenly.
and the air-fuel mixture gets combusted inside the cylinder.
At this time the mixture is in the topmost position and when the mixture expands, a very high thrust is produced which pushes the piston downward and the piston moves downward at high speed. As the piston moves downward, the crankshaft undergoes rotary motion.

The position moves from Top Dead Center (TDC) to Bottom Dead Center (BDC) in this stroke.
Both the inlet valve and the exhaust valve remain closed in this stroke.

4) Exhaust Stroke:
The exhaust gases or the burnt gases escape out of the cylinder in this stroke.
The piston reaches the Bottom Dead Center during the working stroke. After that, the piston moves upward from the BDC and will move towards TDC. When the piston moves upward, at the same time exhaust valve opens, and the burnt or the exhaust gases escape from the cylinder through the exhaust valve.
In this stroke, the exhaust valve opens and the inlet valve remains closed.

After the completion of the exhaust stroke, all the four-stroke repeats again and the engine continuously runs.

It takes two strokes for the crankshaft to complete one revolution. In four-strokes the piston completes two revolutions.

Advantages Of SI Engine:

1) SI Engines are low in cost
2) SI Engine produces less pollution as compared to CI Engine
3) These engines are smaller as compared to CI engine and hence requires less space.
4) These engines are lightweight.
5) The cost of an SI engine is lower as compared to CI Engine.
6) These engines require less maintenance cost.

Disadvantages Of SI Engine:

1) At high load, these engines are less economical.
2) These engines have high fuel consumption.
3) It has less efficiency as compared to the CI engine.
4) This engine has a knocking problem.

Applications Of SI Engine:

1) These engines are most commonly used in two-wheelers like motorcycles and scooters. Two-stroke SI engines are used in two-wheelers.
2) The radial SI Engine is used in small aircraft.
3) Si engines are also used in buses with CNG as fuel.
4) These engines are also used in land movers.
5) The four stroke SI engine is used in automobiles specially the four wheelers.

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