The Story of the Steam Engine Indicator

Engine indicators, used to analyze the performance of an engine, have been around for more than 200 years


| July/August 2001



An early steam engine indicator

An early steam engine indicator

"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 Barnet LeVan 

The Invention Of The Indicator

When a mechanic attaches an engine analyzer to the engine of your vehicle he may be using state-of-the-art equipment, but what he is doing has been done by mechanics for more than 200 years. The first instrument for analyzing the performance of an engine, and even recording the results on paper, was invented some time shortly before 1800. Most writers attribute the invention to James Watt, but others (Kalman DeJuhasz and The Victoria and Albert Museum) attribute it to John Southern, an engineer who worked for Watt. This instrument was named the steam engine indicator by its inventor, a name that continues to be used today. Bolton and Watt, the steam engine manufacturing company that employed Southern, realized the tremendous competitive value of owning such an instrument and consequently kept the existence of the invention so secret that they did not even attempt to get a patent on it. It appears that the secrecy surrounding Watt's indicator outlived Watt by nearly a hundred years. As late as 1900, Cecil Peabody, in his book Manual of the Steam Engine Indicator, stated that the exact form of the original indicator was not known and he thus proceeded to "consider it from the form ascribed to it by tradition." Most texts described the principle of the Watt indicator with sketches similar to Figure 1. The true shape of the indicator did not come to light until 1906 when the Science Division of the Victoria and Albert Museum in London (Now the Science Museum) published a photo of it in a catalog of their mechanical engineering collection. It appears from the catalog that the museum had not acquired the indicator until 1890. The only other photo that I found of a Watt indicator was in a 1934 text, The Engine Indicator, Its Design, Theory and Special Applications, written by DeJuhasz. This photo was of an indicator in the Science Museum in London but not the same indicator shown in the 1906 catalog. However, it appears to be the same indicator that I saw on display there in 1999. I checked with the Smithsonian Institution in Washington, D.C., to see if they had a Watt indicator, or if they knew of one in the U.S. They replied that they did not know of any Watt indicators in the U.S. but that they have a replica on display that was built in the museum in the 1920s. Figure 2 shows a Watt indicator.

So, What Is an Indicator and How Does It Work?

The indicator simply records, on a piece of paper, the pressure in one end of a steam cylinder as the piston of the engine moves from one end of its stroke to the other and back. To illustrate this I will use one of the sketches of a Watt indicator shown in Figure 1. When the slide valve opens and admits steam to the cylinder, the pressure in the cylinder rises and causes the pencil on the top of the plunger to rise. If the paper is not moving, the pencil will simply draw a vertical line as the pressure rises and falls and as the steam is admitted, expands, and is exhausted from the cylinder. However, if the paper can be caused to move back and forth, as the piston moves back and forth and as the pressure rises and falls, the pencil will draw a diagram indicating the pressure in the cylinder at every point of the stroke. To accomplish this, 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. (More about reducing motions later.) The vertical movement of the pencil is limited to about two to three inches by selecting the appropriate spring. Figure 3 shows the location of the ports where such an indicator could be attached to my 45 HP J. I. Case engine.

Figure 4 shows a rather idealized diagram similar to one created by the pencil on an indicator. This diagram shows the pressure in the cylinder at various points of the stroke. If the engine is running under a full load, the vertical line at the left shows how the pressure rises in the cylinder when the inlet valve opens and the piston is at that end of its stroke. As the piston moves to the right and the valve stays open, the pressure remains at its highest along the steam line. If both the throttle valve and the governor valve are wide open, this pressure is only slightly lower than the boiler pressure because of pressure-drop in the pipes, fittings, governor and throttle valve. At the point marked "cut-off" the inlet valve closes, and no more steam is admitted to the cylinder. (If you have ever wondered why a steam engine will not start unless the crank is in certain quadrants, it is because, in two quadrants, the piston is beyond this cut-off point.) Steam engines operate most efficiently when they effectively use the curved portion of the diagram marked "Expansion." In this area, no more steam is admitted, and the engine simply extracts power from the steam as it expands. (When you "hook-up" the reverse lever on a steam engine, you are moving the cut-off point to the left so that less steam is admitted and more advantage is taken from the expansion of the steam. Later cut-off gives more power when it is needed, but you will use much more fuel to get that power.) At the point marked "release" the exhaust valve opens and releases any remaining steam. This steam is swept from the cylinder as the piston moves back toward the end of the cylinder where we began. However, before it reaches the end of the cylinder, the exhaust valve closes prior to the inlet valve opening so that some of the steam in the cylinder is compressed, cushioning the piston as it comes to a stop at about the time the inlet valve opens. This compression serves a dual purpose in that it also raises the temperature of the trapped steam and of the cylinder, thus reducing the condensation of the freshly admitted steam. The cycle now begins all over again.

The Development of the Indicator

It is not too hard to imagine how extremely valuable this information was to the Bolton and Watt engineers, when, for the first time, they were able to see what was happening inside of the cylinders of their engines. And it is not hard to imagine the advantage that they enjoyed while their competitors were still guessing as to how to design and set their valves.

The McNaught Indicator

However, in the 1820s or 1830s an Englishman by the name of John Farey, while visiting Russia, discovered an indicator attached to one of Bolton and Watt's engines and immediately recognized its value to engineers. When he returned to England, he arranged with a man named William McNaught from Glasgow, Scotland, to build indicators for him. According to DeJuhasz, McNaught not only built the indicators, he also recognized some of the shortcomings of the Watt indicator and made major improvements in the design before beginning production sometime between 1825 and 1830. These improvements were necessary because in the years since the indicator was invented, the design of steam engines had advanced to where they were using higher and higher steam pressures and were running at ever-faster speeds. The same conditions that made the Watt indicator obsolete were the factors that eventually doomed every mechanical indicator that was designed to overcome them.

jaynan mckelvey
8/27/2012 4:15:00 AM

How much would one of these go for today? I have one that shows it is a left hand L. P. Cylinder, pat. date is 12-1878. Just wondering.