It is a remarkable fact that a number of builders of blowing engines should balk at the idea of running a steam-driven blowing engine more than 40 revolutions, yet regularly design and build gas engines of the same stroke to run from 70 to 85 revolutions, and the situation is made the more remarkable by the fact that the reciprocating parts of the gas engine are about 50 per cent. heavier than those of the steam blowing engine of similar capacity. This, with their high rotative speeds, double those of many steam engines, makes the inertia about six times as great as it is in the steam engine. The set of indicator cards from a gas-driven blowing engine, obtained through the kindness of the William Tod Company of Youngstown, and shown by Figs. 120, 121, and 122 (page 184) are from a 42 and 80 X 60 gas engine with a rated speed of 65 revolutions, with reciprocating parts weighing 94,500 lbs. for each side.

From these I have prepared the net pressure diagrams for a pair of gas cylinders and the corresponding air cylinder, shown in Fig. 123 (page 185). These have been reduced into a combined net pressure diagram of both sets of cylinders, shown in Fig. 124 (page 186), on which is drawn the line of inertia effect at 65 revolutions and in dotted lines the inertia effect at 60 revolutions.

In Fig. 125 (page 187) is shown the resultant of these combined air gas, and inertia diagrams at 65 revolutions. It will be seen that the equalization of power developed and power absorbed by this means is very good, but that the speed is, in this case, if anything a little too high, and that a better equalization would be obtained at 60 revolutions.

The stroke of this engine is 5 ft., that of the horizontal cross-compound, of which inertia and combined diagrams were presented earlier, was 6 ft., only 20 per cent. more. It seems remarkable that this engine should be run at a speed of 65 revolutions, while the rated speed of the steam engine was only 40 revolutions, although it would obviously do better work if its detailed proportions were right, at 55 revolutions, than it would at 40, with an increase in output of practically 40 per cent. and without any countervailing disadvantage.

I think the explanation is that the gas engine simply will not run properly at low speeds on account of the tremendous variations in pressure, and the reversals of stress consequent on them, so that engine builders have been compelled to run them at a proper speed whether they would or not; while in the case of the steam engine the reversals of stress were so much less severe that the engine could endure them without the equalizing influence of the inertia effect, and the obvious advantages of higher speed have been ignored because the engine could get along without them.

Fig. 120.

Fig. 121.

Fig. 122.

Figs. 120-122. Tod gas engine cards.

It may be of interest also to point out here the "spike" of pressure due to the reexpansion of the air in the clearance space which increases the total pressure on the running parts of the engine 15 or 20 per cent. above what it should be at the beginning of the stroke.

It would seem that by delaying the ignition very slightly the explosion line of the diagram could be made to slope forward by a very small amount so as to offset the pressure from the reexpansion of the clearance air and that this would not lessen the area of the diagram by an appreciable amount, while it would reduce the maximum stress on the whole engine to an important extent.