fighting and surviving

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More efficient systems
The cleanup began in the 1920s when cleaner and more efficient scavenging systems were developed. At the same time, the practical limits of engines increased from a small 100cc engine to 250cc and 350cc twin motorcycle engines in the 1930s.

Manufacturers spent a lot of time trying to solve the problem of the late closing exhaust port. The result was the "twingle" engine, where two cylinders in parallel share a combustion chamber.

In that engine, exhaust exits from one cylinder, and the fresh gases enter the other cylinder. The fresh gases then push the exhaust gas in front of them through the skinny u-shaped cylinders. Due to the geometry of the two rods on the same crank pin, the exhaust port actually closed before the transfers. This prevented a lot of fresh gas from escaping.

This design proved cleaner and more fuel efficient, and at the time it gave 2-strokes a new lease on life. The new concept also lent itself to supercharging with external pumps because the exhaust port closed earlier. In the 1930s the supercharged engine gave the 2-stroke a competitive edge in motorcycle racing. A supercharged 16-cylinder racecar engine of this design was developed in the 1930s and raced in the Indianapolis 500.

Challenges lead to power

After WWII, racing's rulemakers outlawed superchargers, and many again predicted the 2-stroke's death.

No way! Due to diligent experimentation, the simple loop-scavenged 2-stroke steadily improved its power output with features like rotary inlet valves; multiple scavenge and exhaust ports; and better exhaust systems.

Development of the expansion chamber exhaust led to the largest new increase in power. A modern expansion chamber exhaust uses sonic pressure waves to "over scavenge" the cylinder and draw fresh gases into the header pipe. It then uses the powerful return wave to push the fresh gases back into the cylinder under pressure. What had earlier been considered a drawback and a loss of fuel had now become a power advantage.

The result? Power output doubled and the engine ran cleaner. What had previously escaped as unburned gases was now pushed back and burned.

Shortly, the 2-stroke pushed the 4-stroke aside in the power game and dominated motorcycle racing. Only emission requirements prevented the 4-stroke from disappearing in motorcycles. But instead of investing technology into cleaning up emissions on 2-stroke engines, most motorcycle makers decided that it was easier to build 4-strokes since the emission technology already existed in the auto industry.

While motorcycle companies basically abandoned 2-strokes except for racing, outboard engine makers set out to clean it up because they still needed the lighter weight of a 2-stroke.

The 2-stroke's remaining drawback was unburned fuel escaping into the exhaust, especially at low rpm. To prevent this, injection systems like Orbital or Ficht inject fuel charges directly into the cylinders after clean air has scavenged the exhaust gases out the exhaust port. This reduces emissions because only fresh gases, but no fuel, enter the exhaust during scavenging.

Bombardier owns the "Ficht" technology and has a license from "Orbital." In addition, its own semi-direct injection (SDI), where a timed amount of fuel is injected into the back transfer passage late enough to prevent it from escaping out the exhaust port, is proving efficient.

Current challenges

Who knows, with continued development in injection technology, the 2-stroke engine may end up cleaner than a 4-stroke. In fact, Bombardier's new Evinrude and Johnson engines are cleaner and more fuel-efficient than the equivalent 4-strokes, plus a lot lighter and more compact.

Each of the snowmobile makers has an individual strategy now.

Yamaha is fully committed to 4-strokes and Bombardier is fully committed to 2-strokes in its Ski-Doo brand. Meanwhile, Polaris and Arctic Cat are working with both concepts.

A clean 2-stroke that meets emission standards has a clear advantage to 4-strokes in both power and weight. If Ski-Doo succeeds in its development, 2-stroke technology may even trickle back into motorcycles. In the meantime Yamaha continues to make its 4-stroke lighter to better compete with the cleaned up 2-strokes. How it will work out, only time will tell!