Ask Kevin: Why No Modern Two-Stroke Streetbikes?

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Question: I theorize that one of the main reasons we don't have modern two-stroke streetbikes is that consumers prefer the sound and feel of a four-stroke. And when I look at the 40-year-old two-stroke technology of my three Kawasaki triples, I wonder what a truly modern two-stroke street bike would be like. If manufacturers used the latest technology, what would be the configuration and output of modern large capacity two-stroke for, say, a sportbike or a naked standard?

Joseph Byrd

Cedar Rapids, Iowa

Answer: Two-strokes left the market because they could not meet steadily-tightening EPA standards for vehicle exhaust emissions. The very feature that makes two-strokes attractive—the simplicity of having only three moving parts (crankshaft, con-rod, and piston—was also their undoing.

A four-stroke engine has a separate piston stroke for each of the four functions necessary to a spark-ignition engine: intake, compression, power, and exhaust. This separation is made possible by the provision of mechanically operated valves for intake and exhaust functions.

In simple two-strokes, valving is provided by the motion of the piston, sliding past port openings in the cylinder wall. The above four functions must be crowded into just two piston strokes. As the piston descends on the power stroke, at roughly half-stroke it begins to uncover a large exhaust port or ports, and exhaust gas begins to leave the cylinder. A quarter-stroke later, the piston uncovers a set of fresh-charge transfer ports. Fresh air and fuel have meanwhile been drawn into the crankcase, and the descent of the piston compresses this fuel-air mixture. As the transfer ports open, this mixture begins to jet into the cylinder through them.

Here is the problem: Even with the best-possible aiming of the transfer ports (to keep the entering fresh charge away from the exhaust port, and send it on a long looping path that fills the cylinder), some fresh charge does “short-circuit”, and flow directly out the exhaust. This loss of fresh charge to the exhaust is a direct consequence of having exhaust and transfer ports open at the same time. In a four-stroke, this is prevented by the use of mechanically operated valves.

How much charge is short-circuited? A good four-stroke needs about 0.5 pounds of fuel to make one horsepower for one hour, but a well designed two-stroke (with correctly aimed transfer ports and expansion chamber exhaust) needs more like 0.62 pounds. The difference, about 25 percent, is the fuel lost directly out the exhaust port. This was acceptable before 1980 because fuel was cheap and the big emissions reductions were then being achieved with cars. Yamaha eventually put catalytic mufflers on its RZ350 model, but the heat generated from the post-combustion of the short-circuited fuel was a concern. Two-stroke road models were not offered after 1984.

There are effective technologies for eliminating two-stroke fuel short-circuiting, and they are Direct Fuel Injection (DFI) and Indirect Fuel Injection (IFI). In a DFI two-stroke—such as certain models of snowmobile and personal watercraft engines—the engine's intake and transfer processes handle only pure air, and the fuel is rapidly injected only after the cylinder's exhaust port has closed. This, by making it impossible for any fuel to be short-circuited out the exhaust, transforms the two-stroke engine into very attractive low-emissions powerplant. Major automakers worldwide (including, GM, Ford, Toyota, Chrysler, Peugeot, etc.) ran at least 22 different DFI two-stroke development programs in the period 1986 to 1994.

Indirect Fuel Injection achieves the same result as DFI, but by injecting the fuel upstream and before exhaust port closure, timed in such a way that none of it can reach the exhaust port before it closes. The advantage of IFI is that its earlier injection provides more time for fuel evaporation, allowing use of a less specialized type of fuel injector than is required for DFI, saving production cost.

I rode an Aprilia 50cc scooter, equipped with the Orbital DFI Combustion Process, several years ago, and can testify to how different it was. First, upon starting, the engine idles as smoothly and steadily as a four-stroke. As you ride away, the engine continues to fire regularly. Why? When a simple two-stroke idles, a very small volume of fresh charge mixes with a cylinder fulll of exhaust gas. The usual result is a misfire. But after one or two more cycles, enough fresh charge accumulates in the cylinder that the spark plug is finally able to ignite it. So the result is an irregular series of firings with one or more misfires between—the classic "ring-ding idle." As the rider begins to open the throttle, more fresh charge is admitted, so the engine misfires less often, at first eight-stroking, then four-stroking, and finally, at about 30 percent throttle, firing smoothly all the time.

In the Orbital Aprilia 50, the idle mixture is directed toward the spark plug, so there is no misfiring at idle. As the throttle is opened, firing remains smooth.

Orbital’s is not the only two-stroke DFI system. Yamaha ran a Hitachi DFI on some of its outboard motors and Outboard Marine (now owned by the Canadian Bombardier firm) equipped its two-stroke outboards first with the Ficht DFI, then developed the more sophisticated E-Tec DFI. DFI injectors are more expensive than automotive port injectors because they must inject and evaporate the fuel in the very short time between exhaust port closure and ignition (about 1/5 of a crank revolution).

Why did the automakers not produce the two-stroke engines they were evaluating 25 years ago? It was decided that since no one could predict what the Environmental Protection Agency would require next, it was safest and most economical to continue with what they knew best—four-strokes. But their research did make it clear that emissions-compliant two-strokes were technically feasible.

When I think of a notional future two-stroke sportbike, I first think of an engine within the speed capability of the E-Tec DFI system—let’s say an 8000-rpm 500 twin or 750 triple. With the sophisticated porting of present-day snowmobile two-strokes, such engines would easily make 100 hp (500) and 150 hp (750), yet engine weight would be 90 to 120 pounds. The result could be a new kind of sportbike whose performance would be based more on light weight and handiness than just upon brute power.

Why did former two-stroke builders Yamaha, Suzuki, and Kawasaki switch to four-strokes? The large automakers had the R&D clout to develop four-stroke emissions technology, after which the motorcycle makers could essentially buy that technology "off the shelf," avoiding the R&D cost. It was thus cheaper to build four-strokes with cheap, proven emissions technology than to pioneer their own two-stroke emissions solutions year by year, as EPA tightened the requirements.

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