In the carburettor engine, fuel and air enter the cylinder in the form of an air-fuel mixture prepared by the carburettor.
The cycle of operations of the four-stroke carburettor engine occurs as follows.
Intake- the piston moves downwards. The intake valve is open. As a result of a vacuum produced by the downward movement of the the piston, air-fuel mixture prepared by the carburettor enters the cylinder through the intake port and, intermingling with the unscavenged exhaust gases, forms the combustible charge.
Compression-the piston moves upwards. Both the intake and the exhaust valves are closed. The volume above the piston diminishes and the combustible charge is compressed, which facilitates the evaporation of gasoline and mixing the gasoline vapor with air. By the end of the compression stroke, the pressure in the cylinder reaches 1 to 1.2 MPa and the temperature, 350 to 400°C.
power - burning of fuel and expansion of combustion gases. Both valves are closed. At the end of the compression stroke, the combustible charge is ignited by a spark. The piston moves down from TDC to BDC under the pressure of expanding combustion gases. The pressure of the gases reaches 2.5 to 4 MPa and their temperature, 2000°C.
exhaust-the piston moves upwards. The exhaust valve is open. The spent exhaust gases leave the cylinder through the exhaust port.