The Otto Cycle consists of four strokes, only one of which is a power stroke. In some respects this may be regarded as a significant disadvantage, causing a loss of efficiency. In response to this problem, Dugald Clerk developed a two-stroke cycle, which is now sometimes called the Clerk Cycle. Although the Otto Cycle has been favoured in automobile design, the simpler and less costly two-stroke engine has predominated in applications including motorcycles, lawnmowers and chainsaws. The two-stroke principle has also been applied to large marine engines, which have vindicated Clerk's belief in its potential for greater efficiency.

Clerk first exhibited his two-stroke engine, manufactured by Messrs Thomson, Sterne & Co of Glasgow, in 1880. This engine essentially comprised a double cylinder configuration, in which one cylinder was known as the displacer and the other as the power cylinder. Gas and air were drawn into the displacer and slightly compressed before being transferred to the power cylinder. Both pistons drove the same crankshaft, with the displacer crank about 90 degrees in advance of the power cylinder crank.

The Stockport engine was a development from the Clerk engine where the displacer cylinder is opposite, but in line with, the power cylinder, with the crankshaft between them. Although the two-stroke Stockport was a commercial success, its maker, J E H Andrew & Co, began producing a four-stroke Stockport engine once Otto's patent had expired. Very few of the two-stroke Stockport engines remain today, making this engine a valuable exemplar of Clerk's approach to design challenges that are still being addressed by numerous engineers who share his aim of improving engine efficiency.

Debbie Rudder, Curator, and Noel Svensson, Powerhouse Volunteer, 2007

In 1882 Dugald Clerk was awarded Patent No. 4948 for 'Motive Power Engines Worked by Combustible Gas or Vapour' which document describes the engine.

J E H Andrew & Co Ltd of Stockport, near Manchester in England, began the manufacture of the horizontal two-stroke engine in 1882 to meet a growing demand for a more powerful engine than the Bisschop engine the company had been manufacturing. After about nine years, the company changed over to manufacture four-stroke engines following the expiration of the Otto-Langen and Crossley patents. Thus the date of manufacture of this engine is between 1882 and 1892. A trade journal of July 1891 carried an advertisement from J E H Andrew for a two-stroke engine similar in appearance to this engine except for a different nameplate.

In 1907 Henry Barraclough, then lecturer (and later Professor) in mechanical engineering at the University of Sydney, received a letter from Dugald Clerk, introducing him to the National Gas Engine Co Ltd and expressing Clerk's interest in meeting with Barraclough 'to chat about gas engines'. (Reference: University of Sydney Archives, Barraclough papers, series 32). Barraclough would have acquired the Stockport engine on this or another trip that he made for the purpose of equipping the Peter Nicol Russell Laboratory, where it was used for student experiments.

Tests conducted on Clerk engines in 1885 measured the gas consumption as 30 cuft/hp hr (1139 L/kW hr), for an 8 hp engine, and 24 cuft/hp hr (911 L/kW hr), for a 25 hp engine.

Gas engine, horizontal, Dugald Clerk two-stroke, 'Stockport', cast iron/steel, made by J E H Andrew & Co Ltd, Stockport, England, 1882-1892

The Stockport engine has two in-line horizontally opposed cylinders with the two pistons joined by a single piston rod. The power piston is also coupled to the crankshaft through a separate connecting rod. The cylinders, and flywheel, are mounted outboard of the crankshaft bearings, the cylinders being on the opposite side to the flywheel. The cylinders and bearing housings are mounted on a heavy base pedestal. When viewed from the side opposite the flywheel, the power cylinder is on the right. Between the cylinders and the flywheel can be seen the centrifugal governor, driven by a belt from the crankshaft, and the eccentric-operated valve that controls gas flow and ignition. The power cylinder is cooled by a water jacket. Cylinder bore is 165mm and piston stroke is 340mm.

The operational cycle of the engine was described in a leading trade journal in 1885 as follows: 'The engine is comprised of two cylinders horizontally opposed, one being the working cylinder and the other the charging cylinder. The admission of gas to the engine is controlled by a valve composed of two cones on a spindle and regulated by the governor. The method of operation was: during the out stroke of the charging piston, a charge of gas and air is drawn, from the point where they are mixed in the passage, into the charging cylinder. On the return or in-stroke of the piston, the slide valve will have moved sufficiently to open the port through which the charge is driven or forced into a reservoir. The charge is then admitted to the combustion chamber of the working cylinder, through the slide valve. The charge, under pressure, rushes into the combustion chamber driving before it, any non-explosive vapour.' After the air-gas mixture is admitted into the power cylinder it is compressed and then ignition occurs. During the next stoke of the piston the products of combustion expand until the exhaust ports are uncovered by the piston and the residual products are ejected by the incoming charge from the displacer chamber.

Made: J E H Andrew & Co Ltd; Manchester, England; 1882 - 1892

Cast in relief on the engine's base: 'J.E.H. ANDREW & COMPANY LIMITED STOCKPORT / SOLE MAKERS / ANDREW'S PATENTS' and below it: 'THE "STOCKPORT" / GAS ENGINE'. The number '376' is also engraved in a gold section on the wheel.

H10108

1882 - 1892

1530 mm

1530 mm

1255 mm