Intake stroke
The piston moves from top dead center (TDC) to bottom dead center (BDC). The intake valve opens, and the filtered air enters the cylinder.
Explanation: The intake stroke is the first stroke of a four-stroke engine. During this stroke, the piston moves from TDC to BDC, creating a vacuum in the cylinder. The intake valve opens, allowing the filtered air to enter the cylinder. The air-fuel mixture is then compressed during the compression stroke.
Compression stroke
The intake valve closes, and the piston moves from BDC to TDC. The air introduced during the intake stroke is compressed in the combustion chamber. During this phase, the temperature of the air increases significantly, reaching approximately 700 to 800 degrees Celsius. This increase in temperature is mainly due to the high pressure to which the air is subjected.
Explanation: The compression stroke is the second stroke of a four-stroke engine. During this stroke, the piston moves from BDC to TDC, compressing the air-fuel mixture in the combustion chamber. The intake valve closes at the beginning of this stroke, sealing the air-fuel mixture in the cylinder. The compression ratio of an engine is the ratio of the volume of the cylinder with the intake valve open to the volume of the cylinder with the intake and exhaust valves closed. The higher the compression ratio, the greater the pressure and temperature of the air-fuel mixture at the end of the compression stroke.
Combustion stroke
The piston moves from TDC to BDC. The fuel is injected into the combustion chamber at a high pressure and temperature. The fuel ignites spontaneously when it comes into contact with the compressed air. The combustion is rapid and complete, and the pressure and temperature of the combustion gases increase significantly. The expanding combustion gases push the piston down, generating work.
Explanation: The combustion stroke is the third stroke of a four-stroke engine. During this stroke, the piston moves from TDC to BDC, while the fuel is injected into the combustion chamber and ignited. The combustion of the fuel produces hot, expanding gases that push the piston down, generating work.
Exhaust stroke
The piston moves from BDC to TDC. The exhaust valve opens, and the burned gases, which have a higher pressure than the outside air, exit the cylinder quickly until they reach a pressure similar to atmospheric pressure. The piston continues to move, expelling the rest of the burned gases from the cylinder through the exhaust valve.
At the end of this stroke, when the piston reaches the top dead center, the intake valve opens again, the exhaust valve closes, and the operating cycle begins again. For every two turns of the crankshaft, a complete cycle is produced. Useful work is generated during one of the four strokes of the cycle, precisely during the stroke that corresponds to the working time.
Explanation: The exhaust stroke is the fourth and final stroke of a four-stroke engine. During this stroke, the piston moves from BDC to TDC, while the exhaust valve is open and the burned gases are expelled from the cylinder. The opening of the exhaust valve creates a pressure differential between the cylinder and the atmosphere. This pressure differential forces the burned gases out of the cylinder. The piston continues to move until it reaches TDC, at which point the exhaust valve closes and the intake valve opens, beginning the intake stroke.