Internal combustion engines convert air and fuel into energy through combustion, powering the attached machinery. These engines can be categorized into two-stroke and four-stroke engines, depending on their mechanical complexity.

All internal combustion engines rely on a piston and a crankshaft. When combustion occurs, the piston is pushed down, rotating the crankshaft. As the piston rises, it further rotates the crankshaft. This repetitive process drives vehicles and other machinery with engines. The distinction between two-stroke and four-stroke engines lies in how often this piston-crankshaft rotation occurs.

Each up or down movement of the piston is called a “stroke.” In a four-stroke engine, the spark plug ignites the fuel/air mixture every four strokes, or every other full crankshaft rotation. In contrast, a two-stroke engine fires the spark plug every two piston strokes, or every full crankshaft rotation. Two-stroke engines are smaller, lighter, and more powerful relative to their size, mainly because they lack valves found in other engines.

In four-stroke engines, the fuel and air mixture only occupies the area above the piston. Conversely, in two-stroke engines, the mixture is present both above and below the piston. These design factors make two-stroke engines easier and cheaper to manufacture, which initially seemed advantageous. However, these characteristics eventually led to the phasing out of two-stroke engines by many modern countries and industries.