Sir Dugald Clerk KBE, FRS[1] (1854, Glasgow – 1932, Ewhurst, Surrey) was a Scottish engineer who designed the world's first successful two-stroke engine[2][3] in 1878[4] and patented it in England in 1881. He was a graduate of Anderson's University in Glasgow (now the University of Strathclyde), and Yorkshire College, Leeds (now the University of Leeds). He formed the intellectual property firm with George Croydon Marks, called Marks & Clerk.
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Clerk's name is also rendered as Clark in the archive of his works in the University of Glasgow. Most other sources, including The Encyclopædia Britannica[5] and official documents, list him under Clerk.
Dugald Clerk was the author of a comprehensive book covering the development of the oil and gas engine from its early inception, and including details of his own work in this area. The first edition was produced in 1886, and the notes here are taken from the 7th edition,[6] revised and updated up to 1896.
Clerk began work on his own engine designs in October 1876. Up to this time the commercial engines available had been the Lenoir engine from 1860, which worked on a double-acting 2-stroke cycle, but spent half of each stroke drawing gas into the cylinder. The Hugon engine was a slightly improved version, but both were quite inefficient (95 and 85 cubic feet of gas per HP hour respectively), neither used compression. The next commercial engine available (from 1867) was the Otto & Langen free piston engine, which used atmospheric pressure for the power stroke, and had half the gas consumption of the Lenoir and Hugon engines. It was in May 1876 that Otto developed his engine using the single-acting 4-stroke cycle with compression in the cylinder. Clerk decided to develop an engine using compression, but with the 2-stroke cycle, as he could see benefit to weight and smoothness of operation through having twice as many power strokes.
Clerk initially experimented with engines that "were identical to the Lenoir in idea, but with separate compression and a novel system of ignition", one of these was exhibited in July 1879. However it was not until the end of 1880 that he succeeded in producing the Clerk engine operating on the 2-stroke cycle, which became the commercial product. Clerk states "The Clerk engine at present in the market was the first to succeed in introducing compression of this type, combined with ignition at every revolution ; many attempts had previously been made by other inventors, including Mr. Otto and the Messrs. Crossley, but all had failed in producing a marketable engine. It is only recently that the Messrs. Crossley have made the Otto engine in its twin form and so succeeded in getting impulse at every turn."
In "Gas and Oil Engines",[6] Clerk refers to the significant earlier gas engine patents of Barnett in 1838 and Wright in 1833.
In 1877 the Otto cycle was patented, immediately recognized to have a significant practical value.[4] 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).[4]
Clerk describes a Cambell engine as using his cycle, as follows[6] : "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.
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[7] 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 out that it did not actually compress the fuel-air mixture, it simply moved the fresh mixture to the working cylinder to force out the gasses burnt previously.[3][4]
Clerk's original design did not allow the construction of smaller engines, as it required the aforementioned additional pumping cylinder for each working cylinder.[2][3] The crucial simplification of the concept, that made possible small yet powerful two stroke engines for mass markets, was patented by Joseph Day[3] in 1894.[2]