Will KTM’s Clean Two-Stroke Engine Make It To The Street? | Cycle World

Will KTM’s Clean Two-Stroke Make It To The Street?

Transfer Port Fuel Injection cleans up the dirty two-stroke. Let’s hope there is a plated dual-sport bike coming soon to a dealer near you.

ktm clean two stroke engine illustration

In this side view we see that as in any simple crankcase-charged two-stroke engine, the KTM's charge air first enters the crankcase through a one-way reed valve, and is slightly compressed there as the piston descends. When the moving piston's top edge uncovers the transfer ports that join crankcase and cylinder, the slightly compressed air in the crankcase jets into the cylinder on a looping path which offers minimum mixing of fresh charge and exhaust gas. Fuel injectors in the tops of the transfer ducts add fuel to the entering air streams.

Illustration by Jim Hatch

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.

ktm clean two stroke engine illustration

Here we see the engine from behind and can see two of its transfer ducts in section. They conduct compressed air up from the crankcase and into the cylinder. A fuel injector, mounted in the top of each of these transfer ducts, sprays fuel against the moving air, thereby achieving a high speed difference between fuel droplets and airflow. This breaks up the fuel droplets, increasing their total surface area so that they rapidly evaporate to result in an easily ignited mixture of air and fuel vapor in the cylinder. The injection of fuel is timed such that none of it can reach the cylinder's exhaust port before it closes.

Illustration by Jim Hatch

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.