Diagram of the indicator (vertical part), which gives a reading of pressure, invented by James Watt and Matthew Boulton in 1790, and the sliding board (horizontal part) and pencil tracer (attached to the indicator), invented by John Southern in 1796, which makes an 'indicator diagram', the pressure-volume graph (drawn on the sheet). |
about which he commented the following:
“The accumulated pressure during the motion of the piston from EF to DC will be expressed by the area EFcDE and the pressure during the whole motion by the area ABFcDA. Now it is well known that the area EFcDE is equal to ABFE multiplied by the hyperbolic logarithm:AD/AE = L ∙ AD/AE
and the whole area:ABFcDA = ABFE (1 + L ∙ AD/AE)
It having been granted that FLc is a rectangular hyperbola.”
Diagram of a Watt-type steam engine, illustrating that the “crosshead” (Ѻ), which connects to cord F, and hence is what moves the indicator “board” R back and forth, is the part of the steam where the piston rod (which is attached to the piston head), connects to the connecting rod (which connects to the crank and crank shaft), which turns the flywheel. [11] |
“Watt's indicator is perhaps the simplest form (above figures). Steam enters the indicator cylinder C, from the engine cylinder E. The pressure of the steam forces the [indicator] piston P, upward, compressing the spring, S, and raising the pencil, A. The sheet of paper R being moved at the same time by cord F which is attached to the crosshead (Ѻ) of the engine, will have described upon it a ‘diagram’ DD, which ‘indicates’, at every instant during a revolution of the engine, the pressure within the engine cylinder. At any instant, the height to which the pencil has been raised will be a measure of the pressure at that instant within the engine cylinder, whereas the horizontal distance through which the paper has been moved from either end (e.g., M) will denote the position of the [engine] piston in the engine cylinder at that instant. From this it follows that the length, L, of the diagram represents the length of the engine piston's stroke.”— Terrell Croft (1922), Steam-Engine Principles and Practice (pg. 41) [10]
“A string is simply attached to some point on the crosshead of the engine and to the board to which the paper is attached so that the paper and the piston move together. Since it is impractical to move the paper the same distance as the stroke of the engine, a device known as a ‘reducing motion’ is used so that the paper will move only four to six inches, even if the stroke of the engine is several feet.”— Bruce Babcock (2001), “The Story of the Steam Engine Indicator” [12]
“It was simple enough. A sheet of paper is pinned to a board, which moves to and fro with the reciprocating motion of the piston; at the same time a spring-loaded pressure gauge causes a pencil, held in a lever arm, to move at right angles to the motion of the board. As a result, the pencil automatically traces out the curve, relating the ‘pressure’ of the steam in the cylinder to the change in ‘volume’; that is to say it traces out the ‘indicator diagram’.”— Donald Cardwell (1971), From Watt to Clausius (pg. 80) [6]
The following is an indicator diagram for a Ranking cycle, by Terrell Croft (1922), which seems to show that the pencil of the indicator is first put at point Q on the diagram, then valve V1 is opened, letting steam in; but then at P, V1 is closed, at which point expansion occurs, the working body pushing the piston along curve PS, at the end of which valve V2 is opened, the steam exits to the condenser, whereby the vacuum then forms, via contact with the cold water (cold body) of the condenser, at which point the piston does returns to the top (upstroke), along curve SR, and at point R, valve V2 is closed, and the piston volume "compresses" along RQ, or something along these lines: [11]
“I am like a man parched with thirst in the expectation of relief, or a woman dying to hear (or tell) a secret, to know Southern’s mode of determining the power.”— George Lee (1796), “Letter to John Lawson” (Lawson was an employee of Watt and Bolton), Apr 12 [6]
“In the hands of a skillful engineer, the indicator is as the stethoscope of the physician, revealing the secret workings of the inner system, and detecting minute derangements in parts obscurely situated, and it also registers the power of the engine.”— William LeVan (1901), The Practical Management of Engines and Boilers (pg. 173)