The two-stroke may be getting a second life.
Motorcycle maker KTM has developed a two-stroke engine with fuel injection that it’s launching in new enduro models this year, marking the first major use of the technology in a full-size production bike.
Two-strokes have long been prized for their simplicity, low manufacturing costs and compact size compared to similarly-powered four-strokes. But while all these factors are appealing to motorcycle designers, their reliance on carburation and the need to mix oil into the gasoline for lubrication leads to poor emissions and fuel economy. New European environmental regulations are the driving force behind KTM's move.
The issue is that adding features like fuel-injection to a two-stroke reduces its inherent advantages by increasing the complexity of the design. Small scooters have used them, however, and many snowmobile and outboard boat motors feature the technology, but the latter two don’t need to meet the same kind of emissions standards as street legal motorcycles and weight isn’t as much of a factor in those applications.
KTM hasn’t revealed all of the details about how its system works, but says it uses Transfer Port Injection, so the fuel will likely be injected into the intake stream, rather than directly into the compression chamber like many car engines do today. Honda has filed a patent for a general purpose two-stroke with direct fuel injection, but has not yet put it into production.
Being able to meter the introduction of gas into the intake more precisely should reduce or eliminate unburned fuel from making it into the exhaust. KTM says fuel consumption is improved and owners will no longer need to adjust carburetor jets, a time-honored and time-consuming tradition of two-stroke ownership. Pre-mixing the oil and gas will also not be required, which suggests the incorporation of an oil-injection system. A sketch of the high-tech engine shows that it will use an old school kick starter.
The engine will be available in 250cc and 300cc versions, and full details will be released in May when KTM takes the wraps off of the first bikes it will be offered in, including the KTM 250 XC-W that’s going on sale in the USA this fall.