Two-stroke motorcycles might be coming back

Tuesday, 14 July 2020

The question “stroke or smoke” might seem like choosing out of ways to die, but for motorcycle riders it’s asking the preference out of the two methods of combustion. “Stroke” refers to four-stroke engines, now the dominant technology while “smoke” is two-stroke, nicknamed as such because of the plumes of blue smoke the engines produce as a result of the burning of oil along with petrol.

Beginning in the 1980s and 1990s, two-stroke engines were phased out in favour of cleaner four-strokes, the purge culminating in the establishment of the MotoGP Grand Prix class in 2002, which saw 990cc four-strokes competing (and dominating) the older 500cc two-strokes. By 2003, no two-stroke GP machines were on the grid and public two-stroke motorcycles were limited to farm and off-road equipment.

That has largely been the case since but, despite the emissions issues, two-stroke tech still has merits. For example, power is produced with every rotation of the crank, whereas four-stroke engines produce power only every second rotation. Kawasaki is rumoured to be working on a supercharged two-stroke engine that could become a type of generator for a hybrid system as, if operated within a set RPM window, two-strokes can run quite cleanly.

Stroke or smoke? For the past few decades, four-stroke engines have dominated motorcycles but developments in two-stroke tech might mean the smokers are coming back.

Kawasaki isn’t the only one working on two-stroke tech, either. A man from Australia named Basil van Rooyen has designed a new type of engine called the “Crankcase Independent Two-Stroke” (CITS) which apparently meets the latest emissions regulations. The CITS avoids total loss lubrication, which means oil isn’t burnt along with petrol during combustion.

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The engine works by pressurising the sump, separating the fuel and oil. The pistons are connected to the conrods through a post that slides through a sealed “thimble”, which sits above the sealed sump and below the combustion chamber.

Van Rooyen’s design has apparently been well received by investors, with contracts to be signed in the next month or so. He posted a video of the system in action on YouTube in 2013, with the CITS adapted to an 800cc V-twin Suzuki Boulevard crankcase with modified Rotax 800 cylinder jackets and heads.

It could be used to power motorcycles at a range of power outputs, static electricity generators or even be responsible for the combustion part of a hybrid passenger vehicle, thanks to compact sizing, low vibrations, low emissions and relatively low manufacturing costs.

Personally, I prefer the sound of a four-stroke and the more usable powerband, but there could still be a place in the market for modern, cleaner two-strokes.