Elements of Heat-Power Engineering Volume 1

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1912 Excerpt: ...D. Clerk, page 243. now toward the use of compression pressures in the neighborhood of 160 to 180 pounds because of the mechanical difficulties encountered with the higher pressures; and in this case a little under 30 per cent is extremely good thermal efficiency on the brake for modern engines, while the average operating value for good standard American types of stationary engines is about 25 to 27 per cent at rated load, and of course decreases with reduction in the load. (c) Besides the compression ratio, the thermal efficiency in general can also be increased by (1) Mixing the incoming charge more perfectly; (2) Producing fairly rapid and complete combustion at the compression end of the stroke (note, however, that too rapid combustion is not desirable); (3) Preventing loss of heat from the charge to surrounding metal during combustion and expansion. Many modern engines have elaborate mixing valves which cause thorough intermixing of gas and air before, or just at the time of, entering the cylinder. In high-efficiency engines the combustion space is made as nearly as possible spherical, hemispherical, or in the form of a short cylinder; and all pockets leading out of this space are avoided as far as possible. This results in less surface for the volume inclosed, and thus reduces heat loss to the metal and makes the combustion more rapid and complete for a similar reason. In pockets connecting with the combustion space the gases often burn long after combustion of the main part of the charge is complete. This can be prevented by placing the igniter in the pocket, and igniting the gas there first, in which case the rapid increase of temperature will cause a sudden pressure rise, blowing some of the burning gas into the main charge, thus causing very comp...