Two-Stroke Engine Basics

Unlike four-stroke engines, a two-stroke engine completes its power cycle in just two piston movements. With each revolution, it accomplishes intake, compression, combustion, and exhaust processes, enabling a power stroke per revolution.

As the piston moves up, it compresses the air-fuel mixture in the crankcase. A vacuum is created below, drawing fresh mixture through the intake port. The compressed mixture above the piston is ready for ignition.

At the top of the compression stroke, the spark plug ignites the compressed air-fuel mixture. This causes a rapid expansion of gases, which forces the piston downward in a power stroke.

The piston's downward motion opens the exhaust port; burnt gases exit. Simultaneously, the fresh mixture from the crankcase enters the cylinder through the transfer port, preparing for the next cycle.

Two-stroke engines mix oil with fuel for lubrication. This mix lubricates the crankcase and cylinder but results in a smokier exhaust and a shorter engine life compared to four-stroke engines.

Two-stroke engines are often less fuel-efficient and emit more pollutants. Their simple design, however, offers advantages for lightweight applications such as chainsaws, jet skis, and mopeds.

Recent innovations, such as direct fuel injection and better scavenging techniques, have improved two-stroke engines' efficiency and reduced their environmental impact, revitalizing their application in modern machinery.