1923 Dunelt 499cc Supercharged Two Stroke Motorcycle

1923 Dunelt 499cc Two Stroke Motorcycle

Everything here comes from Historic Vehicles' page about Dunelt Motorcyles and their unusual supercharged, two-stroke engine:

Dunelt two-stroke cylinder with stepped piston – Montagu Cars and Motor Cycles

The patented Dunelt 499cc engine had a cylinder barrel with different upper and lower bore diameters, in which slid a ‘stepped’ piston with two matching diameters.

The pistons were double diameter and also bi-metallic, with an iron/steel section cast in for the ring grooves – not cheap to make.

Where a standard-bore engine had 499cc-limited mixture induction and transfer ability, the Dunelt engine could suck into its crankcase and transfer into the cylinder about 770cc of charge petrol-air mixture.

A deflector piston ensured that most of that mixture remained in the combustion chamber.

This basic form of supercharging had the desired effect and, in conjunction with a heavy flywheel, earned the Dunelt a reputation for being a torquey, ‘slogging’ engine that lent itself to sidecar applications, including commercial service and delivery tasks.

That wasn't the only unusual feature of the engine. It also had a one piece, forged crankshaft and a one piece connecting rod. Go read the whole thing. 

Also, a Dunelt motorcycle was the first to cross the desert from Cairo to Siwa Oasis and back in 1924.

Siwa Oasis

Siwa Oasis is 350 miles from Cairo as the crow flies.

The oasis is in a deep depression that reaches 62 ft below sea level. . . . The depression is fertile due to both natural flowing artesian wells and irrigation. . . . The oasis supports the cultivation of thousands of date palms and olives. - Wikipedia