Two-stroke lovers have dreamed of this day: when a major manufacturer applies modern technologies to produce powerful two-stroke bikes once more—with legal low emissions.
Two-stroke road bikes pretty much disappeared in the US after 1984 because carbureted two-strokes must scavenge (refill, while pushing exhaust gas out) their cylinders with fuel-air mixture. Because exhaust and transfer (fresh charge) ports must be open simultaneously for roughly 120 degrees, it was inevitable that such engines would “short-circuit” some fuel to the exhaust, producing high emissions of unburned hydrocarbons and using roughly 30 percent more fuel than a four-stroke engine of the same power.
Then in the later 1980s/early ’90s came direct fuel injection, promising to cut two-stroke emissions and fuel consumption to four-stroke levels. By eliminating the carburetor and injecting the fuel directly into the combustion chamber after the exhaust port had closed, it was made impossible for raw fuel to enter the exhaust. Because a two-stroke’s exhaust port typically closes at 83 to 90 degrees BTDC and ignition occurs at or slightly after 20 degrees BTDC, that left roughly one-fifth of a revolution for getting the fuel into the cylinder and evaporating it into easily ignited vapor. An injector capable of those tasks was highly specialized and therefore expensive. But DFI showed that cleaning up two-stroke emissions and reducing fuel consumption could be accomplished.
Bombardier then found another path to the same goal—transfer port injection, or TPI. Transfer ducts move the fuel-air mixture from its pre-compression in the crankcase, up into the cylinder. If injection into these ports is timed correctly, three things happen: 1) no fuel reaches the exhaust port before it closes, 2) more time is made available for getting the fuel into the engine, making expensive DFI injectors unnecessary, and 3) air velocity up through the transfer ducts is often close to sonic, ideal conditions for break-up and evaporation of fuel droplets are created.
KTM’s TPI aims two injectors against the flow in the transfer port pair farthest from the exhaust port. Its 66.4 x 72.0mm 250 single makes a claimed 49 hp at 8,500 rpm, and its 293cc 72.0 x 72.0mm single makes 53.2 hp at the same revs. Because two-strokes are simple and light, these 228-pound-claimed off-road competition bikes will have excellent performance.
What about smoke from lube oil? It is cut by up to 50 percent by load-adjusted delivery from a pump delivering fuel-to-oil ratios from 80:1 to 110:1.
KTM knows that the performance of these bikes will exert serious pressure on heavier existing four-strokes. What market forces might that unleash? It’s just a competition engine in the US for now, but a 53-hp, 230-pound street-legal dirt bike would be hard to resist.