KTM's new EXC TPI engines for 2018 use transfer port fuel injection to overcome many of the emissions issues that two-strokes face 111 111 111 111 KTM's new EXC TPI engines for 2018 use transfer port fuel injection to overcome many of the emissions issues that two-strokes face 111 111 111 111 Courtesy of KTM

Trevitt's Blog: Can Two-Strokes Make a Comeback?

Written by  Andrew Trevitt on Friday, 07 April 2017 12:16

KTM and Husqvarna recently announced that they will be producing two fuel-injected two-stroke models for 2018, as part of their respective enduro lineups. (The Husqvarnas will be essentially rebadged KTMs, as are most of the company's models.) While most manufacturers, including KTM and Husqvarna, offer a range of two-stroke dirt bikes, there have been few two-stroke street bikes of any note since the demise of the Yamaha RZ350 in the mid-nineties. The KTM and Husqvarna announcements, even though covering enduro models, could be the beginning of a trend that sees two-strokes making a comeback in the on/off-road market, and perhaps eventually street bikes.

Two-strokes offer less weight, fewer moving parts, less friction and - of course - twice the number of power strokes than four-strokes, for substantially more performance in a given displacement. On the downside, however, emissions and fuel economy are significantly worse. There are two issues here: One is that in the two-stroke cycle, the transfer ports (that "transfer" the fuel/air mixture from the crankcase into the cylinder) are open at the same time as the exhaust port, and for a significant portion of the cycle. During this time, unburnt fuel can go directly out the exhaust, affecting emissions considerably. The second issue is that, because the fuel/air mixture in a conventional two-stroke passes through the crankcase, the lubricating oil for the big-end and main bearings ends up being burnt along with the fuel, also affecting emissions. The writing was on the wall for two-stroke street bikes in the early eighties, with increasingly strict emissions laws being more and more difficult for the manufacturers to comply with.

In the mid-nineties, Bimota manufactured the V-Due, a 500 cc two-stroke V-twin street bike. The V-Due worked around the emissions issues by using fuel injection and forced lubrication for the bottom end. Ideally, a two-stroke would use direct fuel injection, where fuel is injected into the combustion chamber (rather than the throttle body) after the exhaust port is closed, to minimize emissions. This technology has issues of its own, however, and the V-Due used transfer port injection. While not an optimum solution, in this setup only air goes through the throttle body, into the crankcase and up the transfer ports; the fuel is finally injected in the transfer ports, where it can't pick up the lubricating oil. As well, the exhaust port can be almost closed when the fuel is introduced, minimizing how much goes directly out the exhaust unburnt.

With a separate lubrication system for the V-Due's bottom end, and only air going through the crankcase, the amount of oil that made it to the combustion chamber was also minimized. While the bike did pass US emissions standards at the time, it had significant issues with rideability attributed to the fuel injection, and eventually the system was scrapped altogether in favour of carburetors. Even then the model had continuing issues, and was largely blamed for the company's bankruptcy.

While little was revealed in the KTM and Husqvarna press releases, the KTM version did indicate that the new bikes use transfer port injection, like the V-Due. Certainly the technology has progressed significantly since the V-Due's time, and KTM promises "a completely new experience in terms of power delivery and rideability." KTM, and other manufacturers, have surely been working on two-stroke fuel injection for some time, and the technology is very common in the marine and snowmobile market. If the new KTMs deliver on those promises of rideability and power delivery, it may open the floodgates for the other manufacturers to follow suit.

What will the holdup be for street bikes? The RZ350 and V-Due had a difficult time meeting the relatively relaxed emissions standards of their time, and the current Euro 4 standard is extremely difficult even for clean-burning four-strokes to meet. Load up a two-stroke with direct injection, an elaborate lubrication system, exhaust valves and other extras to meet today's standards, and cost, weight and complexity quickly approach the four-stroke realm. (Note on the image above all the extra equipment tacked onto the cylinder of the KTM engine.) Additionally, in the last 20 years since the V-Due, four-stroke technology has improved considerably and closed the gap to two-stroke performance.

The media launches of the new KTM and Husqvarna models are mid-May, at which time we'll know more about the technology used and if it could potentially be applied to street bikes. It may be wishful thinking on my part, but I don't think I'm alone in wondering how something like an up-to-date RZ500 would compare to a current four-stroke litrebike.

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Last modified on Wednesday, 12 April 2017 00:23
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