Internal combustion engine including improved balance shaft structure, and personal watercraft incorporating same

An internal combustion engine for a personal watercraft, the personal watercraft comprising hull, a deck for supporting a rider thereon, and an engine mounted within the hull so as to be encircled from the bottom by the hull and from above by the deck,

• the engine comprising:
• a crankcase;
• a cylinder block, the cylinder block mounted to the upper side of the crankcase along a dividing plane;
• a crankshaft directed in a longitudinal direction of the hull;
• plural crankshaft bearings which rotatably support the crankshaft;
• at least one balance shaft for preventing secondary vibration;
• and,
• plural balance shaft bearings which rotatably support the balance shaft,
• wherein:
• the at least one balance shaft is arranged in parallel with the crankshaft, both the at least one balance shaft and the crankshaft disposed on the dividing plane;
• wherein the at least one balance shaft is positioned so as to extend between an end of the engine and a medial portion of the engine.

A personal watercraft, comprising a vessel body including a hull and a deck, the deck being capable of supporting at least one rider thereon, and an internal combustion engine mounted inside of the vessel body between the hull and the deck, the engine comprising:

• a crankcase;
• a cylinder block, the cylinder block mounted to the upper side of the crankcase along a dividing plane;
• a crankshaft directed in a longitudinal direction of the hull;
• a plurality of crankshaft bearings which rotatably support the crankshaft between the cylinder block and the crankcase;
• a first balance shaft for preventing secondary vibration;
• and,
• a balance shaft bearing which rotatably supports the first balance shaft,
• wherein the crankcase and the cylinder block each have a plurality of ribs formed on the interior surfaces thereof, the ribs of the crankcase and the cylinder block forming mutually opposed rib pairs, the mutually opposed rib pairs being spaced apart in a longitudinal direction of the engine,
• wherein the first balance shaft is arranged parallel to the crankshaft, each of the balance shaft and the crankshaft being disposed proximate the dividing plane;
• wherein one of said crankshaft bearings is situated on each of the rib pairs;
• wherein the balance shaft bearing fits on a selected mutually opposed rib pair;
• and the balance shaft bearing is closer to an adjacent mutually opposed rib pair as compared with the crankshaft bearing of the selected mutually opposed rib pair.