Dugald Clerk - Clerk Cycle

Clerk Cycle

In 1877 the Otto cycle was patented, immediately recognized to have a significant practical value. Clerk quickly followed with his concept of a two stroke engine of 1880, that would not infringe the Otto's patent (being a four stroke engine).

Clerk describes a Cambell engine as using his cycle, as follows : "It has two cylinders, respectively pump and motor, driven from cranks placed at almost right angles to each other, the pump crank leading. The pump takes in a charge of gas and air, and the motor piston overruns 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 wthout), 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 4-stroke cycle were part of this. Many years later the 2-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.