Nelitahtisissa

Nelitahtisissa is Finnish grammatical form used to refer to four-stroke engines or four-stroke configurations in general. In technical Finnish, nelitahtisissa appears as the inessive plural form of nelitahtinen, meaning “in four-stroke [engines].”

A nelitahtinen moottori is an internal combustion engine that completes its operating cycle in four strokes of the piston: intake, compression, power, and exhaust. During the intake stroke, the piston moves downward and draws the air-fuel mixture into the cylinder. In the compression stroke, the piston moves upward, compressing the mixture. The power stroke follows when the compressed mixture ignites (typically by a spark plug in gasoline engines or by compression in diesel engines), forcing the piston downward. The exhaust stroke then expels combustion gases through the exhaust valve. Valve timing, fuel delivery, and lubrication are managed by a camshaft and related systems, and many four-stroke engines use a separate lubrication circuit.

Compared with two-stroke engines, four-stroke designs generally offer higher fuel efficiency, lower emissions, and longer engine life, at the cost of greater mechanical complexity and heavier weight. They are widely used in automobiles, motorcycles, lawn and garden equipment, outboard motors, and stationary generators. Historically, the four-stroke cycle, also known as the Otto cycle, was developed in the late 19th century and became the dominant technology for automotive propulsion in the 20th century.

In Finnish usage, nelitahtisissa is commonly found in technical and educational contexts to describe engines or engine types in the plural locative form, as in nelitahtisissa moottoreissa.