POWERHOUSE COLLECTION

Dobbie McInnes and Clyde oil engine indicator

Object No. 2006/156/1

This instrument is typical of the mechanical indicators used by engineers to test and improve the performance of internal combustion engines. It was made in Scotland in 1921 by Dobbie McInnes and Clyde and used in the maintenance of oil engines at a Kempsey power station. Analysis of indicator diagrams also contributed to the scientific understanding of energy, work and heat. Indicators have been used to analyse engine performance since the late eighteenth century. An indicator diagram shows how the pressure in an engine's cylinder varies during the engine's cycle; the diagram conforms to a standard shape if the engine is operating correctly, and the work done by the engine, and hence its power, can be calculated from the diagram. Thus an indicator is a tool for maintaining engines at optimum efficiency. Indicator diagrams also contributed to the basic understanding of engines, the development of high-speed steam engines from the early low-speed engines, and the elucidation of ideas in thermodynamics. James Watt invented the indicator around 1790 to visualise the changes occurring in the cylinder. His assistant John Southern added a pencil and chart to create a record of those changes. Later inventors improved the instrument and adapted it for use with different types of engine. In 1834, Clapeyron explained Carnot's thermodynamic concepts by relating the Carnot cycle to the shape of a typical indicator diagram. The indicator is also significant as the first type of instrument to record data as a graph. Graphical output of information by chart recorders attached to instruments was a major tool in many areas of science and technology until the late twentieth century, when chart recorders were largely supplanted by computers for acquisition and display of data. This particular instrument is representative of the mechanical indicators commonly used in the twentieth century by engineers in charge of maintaining internal combustion engines. It was used at a power station in Kempsey that only became redundant when this NSW town was connected to the state-wide electricity grid; hence it also represents the many small power stations that supplied local needs while the grid was being established. Debbie Rudder, Curator, 2006 Reference Hankins TL, 'Blood, dirt and nomograms' Isis, vol 90, 1999, pp 50-80.