The Hildebrand and Wolfmüller (H&W for short) design, instead of using the new improvements of the 4-stroke engine, was a modification of one their earlier prototypes using steam power. As such, it had some strange design quarks compared to what we would recognize as a motorcycle.
Not using the usual bicycle type frame, the engine was placed horizontally under the riders feet. It was fed from a fuel tank and crude "vaporizer" mounted to the front down tube of the frame. Not having a proper carburetor, the fuel was regulated by turning a thumbscrew.
Also on the front down tube was the heating element for ignition source (attached to the fuel tank because everyone knows that crazy bikers like life on the edge). The ignition wasn't the low voltage spark type already in use elsewhere, but the more primitive "hot tube" ignitor. It was a small tube attached to the cylinder head and closed at one end. The heating element would heat a portion of that tube cherry red. When the piston came up on the compression stroke, the combustible mixture would be pushed into the tube. When it reached the heated portion it would ignite, beginning the power stroke.
The H&W design was unique in that the engine didn't have a traditional crankshaft. The connecting rods were attached directly to the rear wheel, which acted as a flywheel. This made it necessary for the rear wheel to be constructed as a heavy, solid disc. (Harley Fat Boy style) This just wasn't quite enough to keep the engine cycling smoothly so they used large rubber bands to help return the pistons to the top of stroke.
The rear wheel also acted as the camshaft, having an eccentric brass ring to work a long rod that would open and close the exhaust valve. The intake valve was of the "atmospheric" type used at the time, being pulled open by suction of the piston moving down in the cylinder and closed by a weak spring on the piston's upstroke.
Despite a few oddities of design, it made some impressive steps forward in other areas. As a converted steam machine, it kept the water reservoir in the rear mud guard. The water was now plumbed to cooling jackets around the engine cylinders to keep engine heat down... the world's first liquid cooled motorcycle. The lubricating oil for the engine was stored in one of the frame tubes, a method that other manufacturers would be using at least 70 years later. The H&W was also the world's first motorcycle to use air filled rubber tires. The tires were manufactured by a German company under license from Dunlap, who still produces motorcycle tires today. H&W, had the brand survived, could have also boasted of having the largest engine fitted to a motorcycle until recent times. The twin cylinder engine was 1498cc, that's 91 cubic inches, and at 240 rpms propelled the machine to a speed of 28 mph (45 km/h). A feat that must have been exciting considering that the only brake was a spoon that applied pressure to the front tire, very similar to today's bicycle brakes. It was actuated by a lever on the right side handlebar grip, just like today's motorcycles.
With poor records, estimates of total production range from 800 to 2000. The H&W proved expensive to build and the consumers complained about the hard starting and poor running of the design. Not being equipped with pedals or a clutch, starting the engine employed the "run and jump" method, while at the same time trying to find the right fuel adjustment with the aforementioned thumbscrew... a young man's sport for sure. After just three short years, Hildebrand and Wolfmüller closed their doors in 1897.
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