The superheating of the steam for our own engine was not affected by the connection of our steam-pipe with those of the other engines. The explanation of this phenomenon seemed to be that as our boiler generated far more steam than our own engines required, the current was always from our pipe into the connected pipes.
Scale 20
Diagram from Allen Engine, back end of cylinder, at Fair of American Institute, 1870.
I was here first made alive to the enormous waste of steam in the feed-pumps, a separate one for every boiler, including our own. In these the steam has to follow full stroke, at a pressure sufficient, on the larger area of the steam piston, to overcome the pressure in the boiler. Moreover, the extreme heat interval between the temperatures of the entering and the exhaust steam and the slow motion, permitting the walls of cylinder, heads and piston to be cooled very deeply by the exhaust, produces the condensation of probably from five to ten times as much steam as is usefully employed, differing according to the rate of piston motion. I began to rather admire the practice of the English, who knew nothing about boiler feed-pumps, except those on the engine, and I certainly wonder that the genius did not arise long before he did, who first thought of exhausting the feed-pump into the feed-water under atmospheric pressure only, so returning to the boiler all the heat received in the pump that is not converted into the work of overcoming the boiler pressure and the atmospheric resistance or lost in external radiation.
The above [diagram] represents the performance of this engine in its regular work. It shows distinctly the compression curve, the points of cut-off and release, and the back pressure required to expel the exhaust. It will be seen that the expansion fell to 5 pounds below the atmosphere. I have added to it a line representing the waste room in ports and clearance, and the theoretical expansion curve plotted according to the law of Mariotte, showing the expansion terminating 2.5 pounds above this curve, from the re-evaporation already noted and the heat abandoned by the steam as the pressure fell.
After the close of the fair this engine was run on several days, under a variety of loads applied by a Prony brake, in the presence of a number of engineers and others who had been invited to witness the trials. Of the diagrams taken on these trials, I find that I have preserved only the two here shown, namely, a single friction diagram from the back end of the cylinder, on a scale of 20 pounds to the inch, and a diagram showing large power, taken from the front or crank end, on a scale of 24 pounds to the inch. The former shows the trifling loss from friction in this engine. I have measured this card, and find the mean effective pressure, or difference between the areas showing the excess of the forward over the back pressure, to be 1.1 pounds on the square inch, which, assuming the opposite card to be equal with it, was the friction of the engine. The exhaust line shows the power required to reverse the direction of motion of the exhaust, which at the end of the stroke was rushing back into the cylinder.
The latter is especially interesting as showing the identity of the expansion curve with the theoretical, three points on which are marked by the crosses. The sharp reaction of the indicator while the crank was passing the dead center will also be observed.
After this trial I made a careful comparison of the diagrams taken under the different loads with the friction diagrams, and found the uniform results to be that the friction diagrams subtracted from the diagrams taken under the load left in each case, of six different loads, exactly the same effective work done that was shown by the brake.
