The document discusses the four-stroke and two-stroke internal combustion engine cycles. The four-stroke cycle involves intake, compression, power, and exhaust strokes in two revolutions, while the two-stroke cycle completes these strokes in one revolution. Two-stroke engines are lighter, more powerful, and less expensive but are less efficient and more polluting than four-stroke engines. Four-stroke engines are more complicated with more parts but are more efficient, last longer, and pollute less. Internal combustion engines are widely used in vehicles, locomotives, aircraft, marine applications, lawn equipment, and industrial stationary equipment.