What is the difference between 4-stroke and 2-stroke engines?

[EzraAnn]

The differences between two-stroke and four-stroke engines are as follows: Structural differences: Structurally, the two-stroke engine is relatively simpler, mainly composed of cylinder head, cylinder, piston, piston rings and other components, with intake, exhaust and transfer ports on the cylinder block; the opening and closing of these ports are determined by the piston's position. Compared to four-stroke engines, it lacks complex valve mechanisms and lubrication systems, and generally uses air cooling, making its structure much simpler. Performance differences: Performance-wise, when the crankshaft speed is the same, a two-stroke engine performs twice as many power strokes per unit time as a four-stroke engine. Theoretically, the power of a two-stroke engine should be twice that of a four-stroke engine (but in reality, it's only 1.5~1.7 times), with higher specific power, better dynamic performance, and less engine vibration. In addition, in terms of energy saving and environmental protection, four-stroke engines are more energy-efficient and environmentally friendly than two-stroke engines, with higher fuel utilization. Moreover, two-stroke engines are lighter in weight, cheaper to manufacture, have lower failure rates, are easier to maintain, and more convenient and flexible to use. Two-stroke working principle: First stroke: The piston moves upward from the bottom dead center, and after the three ports are closed simultaneously, the mixed gas entering the cylinder is compressed; when the intake port is exposed, the combustible mixture flows into the crankcase. Second stroke: When the piston is compressed near the top dead center, the spark plug ignites the combustible mixture, and the expanding gas pushes the piston downward to perform work. At this time, the intake port is closed, and the combustible mixture sealed in the crankcase is compressed; when the piston approaches the bottom dead center, the exhaust port opens, and the exhaust gas rushes out; then the transfer port opens, and the pre-compressed combustible mixture rushes into the cylinder to expel the exhaust gas, completing the scavenging process. Four-stroke working principle: Intake stroke: The intake valve (L) opens, the piston moves downward, and the mixture of fuel and air enters the cylinder. When the piston reaches the lowest point, the intake valve closes. Compression stroke: Both the intake and exhaust valves are closed, the piston moves upward, compressing the fuel-air mixture. When the piston reaches the top, the compression stroke ends, converting mechanical energy into internal energy. Power stroke: The spark ignites the mixture, the burning gas expands rapidly, pushing the piston downward, converting internal energy into mechanical energy. Exhaust stroke: The exhaust valve (R) opens, the piston moves upward, expelling the burned exhaust gas. When the piston reaches the top, the exhaust valve closes.