Six Strokes of Genius
SIX STROKES OF GENIUS
WHILE THE WORLD'S major motorcycle manufacturers were busy weighing the advantages of two-stroke versus four-stroke engine technology, 47-year-old Australian wheat farmer Malcolm Beare designed and built a unique powerplant that combines a little bit of both.
His patented “six-stroke” design essentially places an inverted two-stroke engine atop a conventional four-stroke cylinder and bottom end. Below the fourstroke’s cylinder-head gasket, everything is conventional, so the concept can be transplanted onto an existing engine. But above that, he’s thrown away the cylinder head and fitted an ultra-shortstroke crankshaft that is driven at half engine speed like a conventional camshaft. This is attached via a short connecting rod to a piston that slides up and down past intake and
exhaust ports set into the cylinder wall, like those on a two-stroke.
Twin carburetors feed fuel/air mixture into the engine via a reedvalve block (thus preventing exhaust gases from exiting through the intake port), while a rotarydisc valve regulates exhaust timing by cutting off flow at the appropriate time to stop the gases
from returning into the cylinder.
During the compression and combustion strokes, the “upper” piston seals off both ports, trapping combustion pressure between the two pistons at TDC. Both pistons then descend as normal, which means that rather than robbing horsepower, the sixstroke’s “valvetrain” actually contributes to total engine output. Uncanny. What’s the point, you ask? The obvious engineering benefit is that by eliminating the four-stroke’s traditional poppet valves, you decrease friction, as well as remove any chance of floating or bending the valves. This in turn implies a theoretically higher rev limit, but according to Beare, the actual rev limit is determined by what the conventional lower crankshaft is able to bear. For that reason, he’s limited his Ducati Pantahbased V-Twin prototype to 9000 rpm, at which point he estimates
86 horsepower is delivered to the rear wheel-an impressive figure considering that a stock pre-1999 Ducati 900SS cranks out just 70 bhp. And there may be more to come through two-stroke expansion-chamber technology. But comparisons to a conventional four-stroke engine are sketchy, because if you add the prototype’s 602cc “upper” cylinders to its 744cc “lower” ones, the engine displaces 1346cc!
Riding the prototype six-stroke at Australia’s Calder Raceway revealed three assets: exceptional torque, crisp throttle response and reduced vibration. On top of that, Beare claims greatly improved fuel economy. One thing that is clear, however, is that in its present form, this isn’t a performance concept. Instead, what we have here is an inexpensive, yet elegant, hybrid engine design that may prove to be a viable alternative to the “dirty,” workaday two-stroke currently being phased out by all environmentally conscious countries. A sixstroke scooter, anyone?
Alan Cathcart
