At last fall’s EICMA motorcycle show in Milan, Italy, reborn OSSA revealed two fuel-injected two-stroke-powered machines—one for trials, the other an enduro.
Two-stroke? Fuel-injected? Two-stroke streetbikes mostly disappeared after the mid-1980s because of their high unburned hydrocarbon emissions (UHC). The cause? With a carburetor fuel system and piston-controlled exhaust and scavenge ports, some fresh charge always leaks out through the last port to close—the exhaust. That is the big, bad, two-stroke UHC emissions problem.
If you substitute direct fuel injection, timed to enter the cylinder after exhaust port closure, no such leakage is possible and most of the two-stroke emissions problem is solved. As Bombardier discovered, the same desirable result can be achieved by injecting the fuel into a cylinder transfer port, timed so that none reaches the exhaust port before it has closed.
OSSA engineers knew the advantages of a two-stroke engine’s light weight and high torque in trials and enduro applications. They also knew that the moderate rpm of such engines made possible effective fuel injection that could meet applicable emissions standards. As Bombardier and others discovered, injecting fuel only “above the base gasket” allowed a two-stroke’s crankcase to become hot enough to cut power through charge heat expansion. OSSA’s 250/300i enduro therefore injects some fuel into the crankcase and some into the cylinder.
The enduro model has a power-broadening exhaust gate that is controlled by the same ECU that controls the innovative Kokusan fuel-injection system.
No single engine type is best for every application. It is the extreme fuel economy and high torque of marine two-stroke diesels that guarantee low shipping costs for exports and imports. Lightweight two-stroke engines able to meet relevant emissions standards also have a place in motorcycling, and it is refreshing to see a European manufacturer take the lead in proving it.