is the most convenient and is amply sufficient, except in case the diagram is excessively long. Ques. 699.—Having succeeded in drawing the ordinates across the face of the diagram, what is the next step? Ans.—The pressure represented by each line is measured from the exhaust line to the steam line, and so on, along the expansion curve throughout the length of the diagram, using for this purpose the scale adapted to the spring used, and having thus obtained measurements on each line, add all together and divide the sum total by the number of lines, which will give the mean forward pressure. To obtain the mean effective pressure, deduct the back pressure, which is represented by the distance of the exhaust line above the atmospheric line in a noncondensing engine, and in a condensing engine the back pressure is measured from the line of perfect vacuum. Ques. 700.-What is a planimeter? accurately measure the area of any plane surface, no matter how irregular the outline or boundary line is. Ques. 701.—What is the main requirement in ascertaining the M. E. P. of a diagram? Ans.-The prime requisite in making power calculations from indicator diagrams is to obtain the average height or width of the diagram, supposing it were reduced to a plain parallelogram instead of the irregular figure which it is. Ques. 702.-What advantage is gained by using the planimeter in measuring diagrums? Ans.-It shows at once the area of the diagram in square inches and decimal fractions of a square inch, and when the area is thus known it is an easy matter to obtain the average height by simply dividing the area in inches by the length of the diagram in inches. Having ascertained the average height of the diagram in inches or fractions of an inch the mean or average pressure is found by multiplying the height by the scale. Or the process may be made still more simple by first multiplying the area, as shown by the planimeter in square inches and decimals of an inch, by the scale and dividing the product by the length of the diagram in inches. The result will be tlie same as before, and troublesome fractions will be avoided. Ques. 703.-Having obtained the V. E. P., as shown by the diagram, how may the horse-power developed by the engine be ascertained? Ans.—The area of the piston (minus one-half the area of rod) multiplied by the M. E. P., as shown by the dia gram, and this product multiplied by the number of feet. traveled by the piston per minute (piston speed) will give the number of foot pounds of work done by the engine each minute, and if this product be divided by 33,000, the quotient will be the indicated horse-power (I. H. P.) developed by the engine. Ques. 704.—Mention two important factors in calculations of steam consumption. Ans.-In calculating the steam consumption of an engine, two very important factors must not be lost sight of, viz., clearance and compression. Especially is this the case in regard to clearance when there is little or no compression, for the reason that the steam required to fill the clearance space at each stroke of the engine is practically wasted, and all of it passes into the atmosphere or the condenser, as the case may be, without having done any useful work except to merely fill the space devoted to clearance. On the other hand, if the exhaust valve is closed before the piston completes the return stroke, the steam then remaining in the cylinder will be compressed into the clearance space and can be deducted from the total volume which, without compression, would have been exhausted at the terminal pressure. Ques. 705.-When, owing to light load and early cut-off, the expansion curve drops below the line of back pressure, how must the area of the diagram be calculated? Ans.—The area of the loop below the back pressure line must be subtracted from the remainder of the diagram. If the planimeter is used, the instrument will make the subtraction automatically, but if the diagram is divided into parts by ordinates, the pressure shown by the ordinates in the lower loop must be subtracted from that shown by the loop above the back pressure line in order to ascertain the M. E. P. or average pressure. Ques. 706.-What is meant by the adiabatic curve? The dotted line R C shows what the true adiabatic curve would be on the diagram, provided it could be realized. Ans.-If it were possible to so protect or insulate the cylinder of a steam engine that there would be absolutely no transmission of heat either to or from the steam during expansion, a true adiabatic curve or "curve of no transmission" might be ohtained. The closer the actual expansion curve of a diagram conforms to such a curve, the higher will be the efficiency of the engine as a machine for converting heat into work. |