Before starting the topic, let’s think about the following questions:
Q.1 Why your bike does not start when spark plug is not working?
Q.2 How does the burning of gasoline drive your vehicle?
Q.3 Why two stroke vehicle’s average is less than that of four stroke?
Q.4 How does fuel enter inside the cylinder without a pump?
Four stroke spark ignition engine
is also known as the petrol engine and is widely used in bikes and cars as the power unit. It converts the chemical energy of fuel into mechanical energy by the piston. By knowing the working of this engine we can able to find out why our vehicle is not working properly.
The power generation process in a SI engine
is divided into four parts, each part known as the piston stroke. In an engine, stroke is refer to the maximum distance travel by the piston in a single direction. The piston is free to move only in upward and downward direction. In four stroke engine the piston move two time up and down and the crankshaft moves two complete revolution to complete four piston stroke. These are intake stroke, compression stroke, expansion stroke and exhaust stroke.
During the intake stroke of spark ignition engine, the piston
moves down form top end of the cylinder to the bottom end of cylinder and simultaneously intake valve opens. Due to the movement of piston low pressure or vacuum generates inside the cylinder. Due to this vacuum and the gravity action air fuel mixture enter into the cylinder through the intake valve. The intake valve remains open until the piston reach the lower end of the cylinder. After it the intake valve closes and seals the upper end of the cylinder.
After the piston passes bottom end of the cylinder, it starts moving up. Both valves are closed and the cylinder is sealed. The piston moves upward. This movement of piston compresses the air fuel mixture into a small space between the top of the piston and cylinder head. The mixture is compressed into 1/8 or less of its original volume. This compression ratio decides the power of engine. At the end of compression stroke the piston is at top end of the cylinder.
At the end of the compression stroke when the piston is at the top end of the cylinder an electric spark generated by the spark plug. The heat of the spark ignites the compressed air fuel mixture. The air fuel mixture then burn rapidly and produces high temperature up to 33000C. This high temperature causes very high pressure, which pushes down on the top of the piston. The connecting rod carries this force to the crankshaft, which turns to move the vehicle. At the end of power stroke the piston reach the bottom end of cylinder.
When the piston reaches the bottom end of cylinder on the power stroke, the exhaust valve opens. At this time due to the burn gases inside the cylinder, the pressure inside the cylinder is slightly high from atmospheric pressure. This pressure difference allows burn gases to escape through the exhaust port and the piston move through the top end of the cylinder. At the end of exhaust all burn gases escape and exhaust valve closed. Now again intake valve opens and this process keep on running until your vehicle remain in starting position.