a port in the side of the cylinder at the out-end of its stroke to discharge the exhaust gases. When the pressure in the motor cylinder has fallen to atmosphere, the pump forces its charge into the back cover of the motor cylinder through a check valve, displacing before it the products of combustion through an exhaust port ; the motor piston then returns, compressing the contents of the cylinder into the compression space. The charge is then fired and the piston performs its working stroke. This is the Clerk cycle." The Clerk engine uses automatic 'poppet' type valves for inlet air and gas (one with spring assistance, one without), and a port in the cylinder uncovered by the piston for the exhaust valve. References to a Clerk engine with slide valve may refer to the earlier experiments with a Lenoir type engine. The ignition is by carrying an external flame, using a modification of a method he developed in 1878. Most engine designs that pre-dated the Otto engine (and Clerk engine), such as those of de Rivaz, the Niépce brothers, Jean Joseph Etienne Lenoir, Samuel Morey, and others, did use two-stroke engines, which were "natural" in the times of steam engine. Clerk's significant contribution was introducing Otto-styled compression to the two-stroke engine, bringing its efficiency up to date (for the 1880s). Several manufacturers adopted the Clerk cycle in the short term, though commercial aspects such as patents on the four-stroke cycle were part of this. Many years later the two-stroke engine for large capacity diesels using a turbocharger or supercharger has become common, for example in ships and railway locomotives. With open crankshafts, and the advantages of higher power to weight ratio, these engines are closely aligned with Dugald Clerk's concepts, and the Clerk Cycle. Pumping cylinder vs supercharger Clerk's engine was made of two cylinders – one working cylinder and an additional cylinder to charge the cylinder, expelling the exhaust through a port uncovered by the piston. Some sources consider this additional cylinder the world's first supercharger. Clerk himself states that "It is not a compressing pump, and is not intended to compress before introduction into the motor, but merely to exercise force enough to pass the gases through the lift valve into the motor cylinder, and there displace the burnt gases, discharging them into the exhaust pipe." Hence sources recognise it instead as a "pumping cylinder", pointing