Kaksitahtimoottoreita

Kaksitahtimoottoreita, or two-stroke engines, are a type of internal combustion engine that complete a power cycle with two strokes of the piston during one crankshaft revolution. Unlike four-stroke engines, two-stroke engines have fewer moving parts, making them simpler, lighter, and often cheaper to produce. This design also allows for a power stroke to occur every revolution, potentially leading to higher power output for a given displacement compared to a four-stroke engine.

The two strokes in a two-stroke cycle are typically referred to as the compression stroke and the power stroke. During the compression stroke, the piston moves upwards, compressing the fuel-air mixture in the cylinder. Simultaneously, a fresh fuel-air mixture is drawn into the crankcase below the piston. As the piston reaches the top of its stroke, the spark plug ignites the compressed mixture, initiating the power stroke. The expanding gases push the piston downwards.

During the downward power stroke, the piston uncovers exhaust ports, allowing burnt gases to escape. As the piston continues its descent, it also uncovers transfer ports, which connect the crankcase to the cylinder. The compressed fuel-air mixture in the crankcase is then forced into the cylinder, scavenging the remaining exhaust gases and refilling the cylinder for the next cycle. This process of simultaneous intake, compression, combustion, and exhaust within two strokes is the defining characteristic of a two-stroke engine.

Due to their simplicity and power-to-weight ratio, two-stroke engines have been widely used in applications such as motorcycles, mopeds, outboard motors, chainsaws, and snowmobiles. However, they are generally less fuel-efficient and produce higher emissions than comparable four-stroke engines, particularly unburned hydrocarbons, due to the fuel-air mixture passing through the cylinder during the scavenging process. This has led to their decline in many applications, with stricter emission regulations favoring four-stroke designs or alternative technologies.