Principle of Four-Stroke Internal Combustion Engine?
2 Answers
The principle of a four-stroke internal combustion engine is as follows: 1. Intake stroke: The intake valve opens, and the exhaust valve closes. The piston moves from the top dead center to the bottom dead center. As the cylinder volume above the piston increases, a vacuum is created, and the pressure inside the cylinder becomes lower than atmospheric pressure. Under this pressure, fresh air or a fuel-air mixture is drawn into the cylinder. This process continues until the piston reaches the bottom dead center, and the intake valve closes. 2. Compression stroke: Both the intake and exhaust valves remain closed. The piston moves from the bottom dead center to the top dead center, compressing the fuel-air mixture inside the cylinder during its movement. 3. Power stroke: Both the intake and exhaust valves remain closed. Near the top dead center of the compression stroke, the spark plug installed above the cylinder head generates an electric spark, igniting the compressed combustible mixture. After combustion, the mixture releases a large amount of heat, rapidly increasing the pressure and temperature inside the cylinder. The maximum combustion pressure can reach 3–6 MPa, and the maximum combustion temperature can reach 2200°C–2500°C. The high-temperature and high-pressure gases push the piston rapidly toward the bottom dead center, performing work through the crankshaft connecting rod mechanism. 4. Exhaust stroke: Near the end of the power stroke, the exhaust valve opens. Since the pressure inside the cylinder is higher than atmospheric pressure, the high-temperature exhaust gases are rapidly expelled from the cylinder. This phase is known as the free exhaust phase, where the high-temperature exhaust gases quickly exit through the exhaust valve. As the exhaust process continues, it enters the forced exhaust phase, where the piston moves past the bottom dead center toward the top dead center, forcibly expelling the remaining exhaust gases from the cylinder. When the piston nears the top dead center, the exhaust process concludes. Due to the combustion chamber occupying a certain volume, it is impossible to completely remove all exhaust gases at the end of the exhaust stroke. The remaining portion of exhaust gases is referred to as residual exhaust.
I've been working with mechanics for thirty years, so I know this stuff inside out! A four-stroke engine is just like the human respiratory cycle, divided into four steps: intake, compression, power, and exhaust. The moment you start the engine, the piston moves down to draw the air-fuel mixture into the cylinder—this is the intake stroke. Then, the piston moves up to compress the gas; diesel engines can ignite at this stage, while gasoline engines have to wait for the spark plug to discharge. When the spark occurs, the air-fuel mixture explodes, pushing the piston down—this is when the real power is generated. Finally, the piston moves up again to push the exhaust gases out through the tailpipe. These four steps complete two rotations of the crankshaft and one rotation of the camshaft, with the valves opening and closing in a dance-like coordination. Modern cars even have variable valve timing technology to save fuel.
