On the other hand, all gases act alike so far as their heat absorption is concerned, so that by using a material with a low evaporative unit, less fuel will be required to get the same expansion, which means the same power.
To illustrate this, let us assume that we have equal quantities of water, and of dioxide-of-carbon, and that is to be converted into a gas. It will take just double the amount of fuel to convert the water into a gaseous state. As both are now in the same condition, the law of heat absorption is the same from this time on.
The dioxide-of-carbon engine is one, therefore, which uses the vapor of this material, which, after passing through the engine, is condensed and pumped back to the boiler to be used over and over.
In like manner, also, ether, which has a low point of vaporization, is used in some engines, the principle being the same as the foregoing type.
Rotary Engines.—Many attempts have been made to produce a rotary type of steam engine, and also to adapt it for use as an internal combustion motor.
The problem is a complicated one for the following reasons: First, it is difficult to provide for cut-off and expansion. A rotating type, to be efficient, must turn at a high rate of speed, and this makes the task a more trying one. Second, the apparent impossibility of properly packing the pistons. The result is a waste of steam, or the gas used to furnish the power. Third, the difficulty in providing a suitable abutment so as to confine the steam or gas, and make it operative against the piston.
In [Fig. 115] is shown a type of rotary which is a fair sample of the characteristics of all motors of this form. It comprises an outer cylindrical shell, or casing, A, having a bore through the ends, which is above the true center of the shell, to receive a shaft B.
This shaft carries a revolving drum C of such dimensions that it is in contact with the shell at its upper side only, as shown at D, leaving a channel E around the other portions of the drum.