Snowmobile engines are very similar to the engines found in personal watercraft. Heavier touring models tend to use the four-stroke engine, while the lighter, sportier models use the two-stroke. An automobile engine sends power through a driveshaft, which directly rotates the axle and the wheels. However, a snowmobile engine links to a track drive, which rotates the tracks. The wheels on a snowmobile are essentially large gears with teeth spaced evenly with holes in the tracks. Every rotation of the gears powers the tracks and drives the snowmobile forward. The faster the engine, the faster the gears rotate, and the faster the tracks move.
Snowmobiles also have a clutch system that is basically a type of pulley-based CVT (Continuously Variable Transmission). This system consists of two pulleys, or clutches, connected by a drive belt. The primary clutch sits on the engine crankshaft. A pressure spring holds the primary's two halves apart when the engine rpm is low. As the engine begins to accelerate, the clutch weights generate enough centrifugal force to close the clutch, allowing the belt to move freely and transmit power. When riding uphill in deep powder snow, the clutch system keeps the engine at maximum RPM without shifting into a higher "gear." The secondary clutch connects to the track drive, which turns the wheels and powers the tracks. A spring in the secondary clutch operates the cams (wedges), which are sensitive to torque. As the engine's RPM increases and the primary transmits power, these cams squeeze together and tighten the belt. The process continues as the snowmobile accelerates. Once it reaches its top speed, the primary clutch closes, moving the belt into a higher "gear." Because the snowmobile needs less power to accelerate than it did to get going, the secondary clutch opens.
Unlike a manual or automatic transmission, this system is stepless and can smoothly go back and forth between an infinite number of "gears" depending on the speed and the amount of power needed . . . a GREAT and fun project to work on!