The Hildebrand & Wolfmüller patent of 20 January 1894, No. 78553 describes a
two-cylinder,
four-stroke engine, with a bore and stroke of . It produced @ 240 rpm propelling a weight of up to a maximum speed of .The fuel-air mixture from the
surface carburettor was regulated by a valve operated by controls on the handlebar. The engine, which was positioned flat in the frame, used
hot tube ignition with the tube heater in front of the cylinder heads. The tube heater was ventilated by the lower
downtubes while the upper downtubes held lubricating oil. Some design details were carried over from a steam-powered prototype made by the Hildebrand brothers in 1889, including the water tank shaped to form the rear mudguard and the connecting rods of the engine driving the rear wheel directly. The water tank was repurposed to supply water to the
cooling jackets surrounding the cylinders. The steam-powered prototype had a double-acting cylinder, applying power to the piston in both directions. With no double action available in the petrol engine, and no
flywheel effect apart from the movement of the rear wheel, the return impulse for the piston was provided by heavy rubber bands. The intake valves were operated by the suction caused by the intake stroke, while the exhaust valves were operated by an eccentric brass ring on the rear wheel and a device at the cylinder head that opened each cylinder's valve alternately. ==Production run==