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Petrol Engine

Yogesh Ambekar
The petrol engine was introduced by engineers Gottlieb Daimler and Karl Benz (both from Germany), in 1885. It is considered as one of the biggest achievements in the automotive field. It uses petrol, known as gasoline in USA, as a fuel. It is made up of about 150 moving parts.
Petrol engines are compact and light in weight for the amount of power they produce. The rate at which it produces work is usually measured in horsepower or watts.

Kinds of Petrol Engine

Reciprocating petrol engines are distinguished in a number of ways. Some of them are as follows:
  • Type of compression
  • Valve arrangement
  • The way they are cooled
  • The way they are supplied with air and fuel
  • Number of piston strokes per cycle
  • Cylinder arrangement
Classification based on number of stokes per cycle:
  • Two Stroke
  • Four Stroke

Four stroke Petrol Engine

The four stroke engine is called so because the working of an internal combustion engine is divided into four stages, called four strokes of the engine.

Two Stroke Petrol Engine

From the name itself we get the idea about the functioning of the engine. The powerplant ignites fuel at every upward stroke, so there are two strokes for every ignition of fuel. They are called upward and downward strokes.
As the piston moves in upward direction from bottom to top in the first stroke, the air and fuel mixture gets compressed and ignited by a spark plug as the upward stroke comes to end. This results in an explosion of mixture, which forces the piston to move downwards thereby producing power.
Since they have less moving parts, they are light in weight. Also, the design is simple compared to four stroke powerplants. Big sized bulky two-stroke cycle engines have lubrication systems like that of four-stroke cycle engines.

Parts of a Petrol Engine

  • Cylinders
  • Cylinder block
  • Piston and Connecting rods
  • Cylinder head Crankcase
  • Valves
  • Crank shaft Flywheel
  • Exhaust system
  • Camshaft Fuel system
  • Lubrication system
  • Ignition system

Working of a Petrol Engine

Generally, the vehicles using petrol/gasoline engines have four strokes, as they are more efficient than two stroke engines and ensure complete combustion of fuel for its optimum use. The four-stroke cycle engine has four strokes, namely, intake, compression, power, and exhaust strokes.

Suction or intake stroke

Initially, when the engine is started, the piston moves down towards the bottom of the cylinder, which creates a low pressure at the top. Due to this, the intake valve opens and the fuel mixture containing petrol vapors and air are sucked in by the cylinder. The carburetor now decides in what ratio gasoline/petrol and air should be mixed.

The compression stroke

After this, the inlet valve gets closed. The piston now moves towards the top of cylinder and compresses the fuel mixture to one tenth of its initial volume. The temperature and pressure inside the cylinder increases due to the compression caused.

The power stroke

During this stroke, the inlet and exhaust valves remain closed. As the piston reaches near the top position, the spark plug produces an electric spark. Combustion is started by an ignition system that fires a high voltage spark through a field replaceable air gap called a spark plug.
The spark produced causes an explosion of the fuel. The hot gases expand and force the piston to move downwards. The piston is linked to the piston rod, and the piston rod to the crank shaft. They all move each other due to the link between them. The crank shaft is connected to the wheels. As the crank shaft moves, the wheels rotate and move the car.

The exhaust stroke

In this stroke, the exhaust valve remains open at the start. The piston is forced to move upwards because of the momentum gained. This forces the gases to move through the exhaust valve into the atmosphere. Now, the exhaust valve closes and the intake valve opens. After this, the four strokes of the engine are repeated again and again.


It is the heart of a gasoline engine. It measures the fuel and mixes it with the air in precise proportions. Old carburetors do spark advance by measuring the difference in pressure between the outside and inside of the carburetor.
The amount of throttle advance is also measured. The engine's remains, which may be carbon monoxide or unburned hydrocarbons, show how well the carburetor is working.
In new engines, a small computer is used to calculate these parameters and control one or more electric injectors. Most of the new cars use electronic fuel injection, as it allows the engine computer to precisely control the fuel air mixture, which increases energy efficiency and reduces pollution.


These engines are widely used in vehicles, portable power plants to supply the power to run pumps, and other machinery on farms. Many small boats, airplanes, trucks, and buses also use it.

Future scope

Continuous research is being carried to increase the fuel efficiency, reduce the pollutants, and to make it more light and compact. Recently, engineers at the University of Birmingham have made the smallest petrol engine that can replace conventional batteries. It is so tiny that it can be handled on a fingertip.