b. A general plan which embodies the recognized practice of good engineering.
c. Adaptation to the specific work which it is intended to perform, in size and in efficiency. It sometimes happens that good practice dictates the use of a comparatively uneconomical design.
2. Good construction, by which is meant—
a. The use of good material.
b. Accurate workmanship.
c. Skillful fitting and a proper “assemblage” of parts.
3. Proper connection with its work, that it may do that work under the conditions assumed in its design.
4. Skillful management by those in whose hands it is placed.
In general, it may be stated that, to secure maximum economical efficiency, steam should be worked at as high a pressure as possible, and the expansion should be fixed as nearly as possible at the point of maximum economy for that pressure. In general, the number of times which the volume of steam may be expanded in the standard single-cylinder, high-pressure engine with maximum economy, is not far from 1∕2√P, where P is the pressure in pounds per square inch; it rarely exceeds 0.75√P. This may be exceeded in double-cylinder engines. It is even more disadvantageous to cut off too short than to “‘follow’ too far.” With considerable expansion, steam-jacketing and moderate superheating should be adopted, to prevent excessive losses by internal condensation and reëvaporation; and expansion should take place in double cylinders, to avoid excessive weight of parts, irregularity of motion, and great loss by friction.
To secure this vitally important economy, it is advisable to seek some practicable method of lining the cylinder with a non-conducting material. This plan, as has been seen, was adopted by Smeaton, in constructing Newcomen engines a century ago. Smeaton used wood on his pistons, and Watt tried wood as a material for steam-cylinder linings. That material is too perishable at temperatures now common, and no metal has yet been substituted, or even discovered, which answers the same purpose. The loss will also be reduced by increasing the speed of rotation and velocity of piston. Where no effectual means can be found of preventing contact of the steam with a good absorbent and conductor of heat, it will be found best to sacrifice some of the efficiency due to the change of state of the vapor, by superheating it and sending it into the cylinder at a temperature considerably exceeding that of saturation. With low steam and slowly-moving pistons, it is better to pursue the latter course than to attempt to increase the efficiency of the engine by greater expansion.