Working of 4 stroke petrol engine

The topic I will discuss with you today is the 4 stroke petrol engine. In case of four stroke cycle, one cycle of operation is completed in four strokes of piston and two revolutions of crankshaft.

Petrol engine is also called as spark ignition (SI) engine, because for ignition of mixture of air and petrol inside the cylinder, a spark plug is required.

All petrol engines work on otto cycle. The different operation in the cycle and each stroke is identified as per the function. Here valves are present instead of ports.

The operating of four stroke petrol engine is completed in 2 revolution of crankshaft and use of piston four stroke.

See this Explain working of 4 stroke diesel engine

Parts of four stroke petrol engine

Piston :

A disc on short cylinder fitting closely with in a tube which it moves up and down against a liquid or gas used in an internal combustion engine to derive motion or in a pump to impact motion.

Piston ring :

A ring on a piston sealing the gap between the piston and the cylinder wall.

Cylinder :

A cylinder is a central working part of a reciprocating engine or pump the space in which a piston travels.

Carburetor :

A device in an internal combustion engine for mixing air with a fire spray of liquid fuel.

Throttle :

A device controlling the flow of fuel on power to an engine.

Crankshaft :

A shaft driven by a crank.

Camshaft :

A shaft with one or more cams attached to it especially one operating the valves in an internal combustion engine.

Connecting rod :

A rod connecting two making parts in a mechanism especially that between the piston and crank pin an engine.

Push rod :

A rod operated by cans that opens and closes the valves in an internal combustion engine.

Rocker arm :

A rocking level in an engine, especially in an I-C engine, acts as a valve and is operate by a pushed from the camshaft.

Spark plug :

A device for firing the explosive mixture in an IC engine.

Inlet valve :

Actually inlet valve allows the air and fuel mixture to the engine during suction stroke.

Gudgeon pin :

A pin holding a piston rod and connecting rod together.

Valve spring :

A helical spring used to hold closed a value in the cylinder head of an IC engine.

Water jacket :

A casing containing water placed around something to protect it from extremes of temperature.

Exhaust valve :

After a complete cycle the burn gases exhaust by this valve.

4 stroke petrol engine working

Suction Stroke

In this stroke the piston is at very near to Top dead centre (TDC) and the inlet valve is open and outlet valve is closed. When the piston moves from TDC to BDC (Bottom dead centre) so that the air petrol mixture prepared by the carburetor comes through the inlet valve and fills the cylinder.

Compression stroke

In this stroke both the valve are closed. During this stroke the the mixture of air and fuel is compressed and automatically the pressure and temperature are increased due to compression. The piston moves from BDC to TDC. At the end of this stroke the air fuel mixture is ignite with the help of spark plug.

Working stroke 

It is also know as expansion stroke or power stroke. In this stroke both the inlet valve and exhaust valve are remain closed. At the beginning of the expansion stroke the gases are burned with the help of an electric spark. And there is a sudden increase in pressure and temperature. 

The volume inside the cylinder practically remains constant. Due to the increase in pressure, the piston is push down with a great force. The hot burnt gases expand due to the high speed of the piston.

During this expansion, some of the heat energy produced is convert into mechanical work. The piston moves from TDC to BDC. 

Exhaust stroke

This is the last stroke of the cycle. In this stroke the exhaust valve is open and inlet valve remains closed. The piston pushes the burnt gases out through exhaust valve into the atmosphere. The piston moves BDC to TDC.

Again the inlet valve opens and the same operations is repeat. And the engine cylinder is ready to suck the air fuel mixture again. 

Application of four stroke petrol engine

The four stroke petrol engines are the most commonly use engines in todays automobile sector and vehicles, both in case of single cylinder and multi cylinder applications.

Four stroke petrol engine commonly used in petrol cars, motorbikes, scooters, small propeller air crafts, formula one, small motor powered boats, auto rickshaw, water spray system etc.

Petrol engine and diesel engine difference

Petrol Engine :

1.  It works on the otto cycle in which the combustion of fuel takes place at a constant volume.
2. Usually a petrol engine exerts a small drag on the petrol and air during the suction stroke.
3. Here the carburetor is use to mix the air and petrol in the required ratio and supply it to the engine during the suction stroke.
4. The engine is design to produce a low compression ratio. It varies from 6 to 10.
5. The thermal efficiency is less (about 25%) due to the low compression ratio.
6. These are high speed engines R.P.M. varies from 300 to 5000.
7. Its starting cost is low but running cost is high because petrol is costlier than diesel.
8. The engine is easy to start because the compression ratio is less.
9. The maintenance cost is less.
10. The petrol engines are commonly use in light vehicles like scooters, motorcycles, cars. They are also use in airplanes. 

Diesel Engine :

1. It works on diesel cycle in which combustion at fuel takes place at constant pressure.
2. But the diesel engine only draws air during the suction stroke.
3. But at the end of the compression stroke an injector or atomizer is employ to inject the fuel.
4. But diesel engine is designed to produce high compression ratio. It varies from 15 to 25.
5. Thermal efficiency is more (about 40%) due to high compression ratio.
6. These are relatively low speed engine varies from 400 to 1200.
7. Its starting cost is high but running cost is low, because diesel is cheaper than petrol.
8. It is difficult to start the engine because compression ratio is high.
9. The maintenance cost is high.
10.  But diesel engines are generally employed in heavy duty vehicles like buses, trucks and earth moving machines etc.

So friends, this was some important information about 4 stroke petrol engine. From this article we learned how 4 stroke petrol engine works, and know some of its parts and also know its application. Hope you will get to learn a lot from this article, Thank you.