In a four-stroke gasoline engine, which stroke begins the cycle?

• A. Power Stroke
• B. Intake Stroke
• C. Compression Stroke
• D. Exhaust Stroke

Answer: (B)

The first stroke is the intake stroke, where the cylinder is filled with air–fuel. As the piston moves downward and the intake valve is open, a vacuum draws in the fresh charge so the cylinder has something to compress, ignite, and expand in the subsequent strokes. After the intake comes compression (piston moves up to squeeze the charge), then the power stroke (combustion pushes the piston down to produce work), and finally the exhaust stroke (burnt gases are expelled). Since nothing can be burned or expanded without a fresh charge, the intake stroke naturally marks the start of each four-stroke cycle.