# Stroke

 A diagram of a piston and cylinder that has a 30" stroke "length", from top to bottom. [1]
In engineering, stroke refers to one complete movement of the piston, in a piston and cylinder engine, in its reciprocating motion, either from bottom to top (up stroke) or from top to bottom (down stroke).

Overview
The following, made by Terrell Croft (1922), is an-illustrated diagram of the Watt-Southern indicator, or "Watt indicator", wherein, on the indicator diagram, the length L represents the graphical measure or distance of the stoke or "one direction" motion of the piston: [1]

 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. [1]
The operation of which is described as follows:

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]

The location of “crosshead”, which connects to cord F, which moves the indicator board R back and forth, is illustrated adjacent.
The following shows the piston, shown with valves V1 and V2 open, so make a down stroke: [1]

The following shows the piston, with valves V1 and V2 open, so take make an up stroke: [1]

The following shows the steam engine with automatic valves:

(add)

References
1. Croft, Terrell. (1922). Steam Engine Principles and Practices (stroke, pg. 3; up stroke / down stroke, pg. 4; crosshead, pg. 34; §: Steam-Engine Indicators and Indicator Practice, pgs. 40-83). Publisher.

External links
Stroke (engine) – Wikipedia.