Steam Engine Indicators

Checking power Output with old Technology


| January/February 2006



I thought that the readers of Steam Traction might be interested in knowing more about what was happening in the photo on the back cover of the September/October 2005 issue of Steam Traction where I was photographed on a Baker 21-75 HP.

The gentleman on the ground behind me is Bill Newell, the engineer is Jason Newell, and the engine is the Newell family’s fine 21-75 A.D. Baker steam traction engine.

The reason for the smiles is that Bill, Jason and I have just succeeded in “taking a card” for the first time from the rod end of the cylinder on the Baker. The instrument attached to the cylinder hides Jason’s smile, but the twinkle in his eye is obvious. When I say we were successful in taking a card, what I mean is that we succeeded in using a 100-year-old engine analyzer, known as a steam engine indicator, to obtain a pencil and paper printout showing what was going on inside the Baker’s cylinder when the engine was running at 200 revolutions per minute and delivering 37 HP to the Prony brake. The engine is rated for 285 revolutions per minute but was governed down to 200 for the purpose of using the indicator. We did not attempt to determine the maximum horsepower.
The indicator we used was made by James L. Robertson & Sons, and attached to it was a Robertson-Thompson Improved Victor Reducing Wheel. By the time these instruments were manufactured, about 1900, the steam engine indicator had been in use for over 100 years. John Southern, an engineer who worked for James Watt, invented the indicator in 1796.

Steam Indicator Basics 

The steam engine indicator performs its function using two inputs taken from the engine: the pressure in the cylinder and the position of the piston. As the pressure in the cylinder rises and falls with each stroke, the change in pressure causes a small piston in the indicator to also rise and fall. This piston is connected to a delicate arrangement of levers and pivots that amplifies the movement so that a pencil lead on the end of one of the levers draws a line on a paper that is wrapped around a metal drum. In the photo, the white paper can be seen at the top of the indicator. The pencil lead is in the right-hand end of the small horizontal lever in front of the paper. If there were no other input, the pencil would simply draw a vertical line that would indicate nothing more than the maximum and minimum pressure in the cylinder.

Because a simple vertical line would be useless, one end of a string is wrapped around the metal drum that holds the paper, and the other end is attached to the crosshead of the engine. In the photo, you will note that a pipe nipple has been installed in the wrist pin where the grease cup is normally located. The purpose of this piece of pipe is to provide a place to attach the end of the string. As the crosshead moves back and forth, the string causes the paper to move in exact unison with it.

Once the indicator is attached to the cylinder, the pencil is rising and falling with the pressure of the steam, and the paper is rotating back and forth as the piston moves: The simultaneous movement of the pencil and the paper produces a plot showing the pressure at every point in the stroke of the engine.