CHAPTER LXVIII EXPANSION* Q. What is the principal advantage of steam over hot air as a motive fluid? A. The fact that it continually tends to expand until it attains the tension or pressure of the least resistance, without any extraneous force being expended. Q. In a locomotive what is the least resistance? A. That of the atmosphere, plus the friction in exhaust passages and stack. Q. What would be the minimum theoretical attainable resistance to expansion? A. That of a perfect vacuum, about 14.7 lbs. per square inch below that of the atmosphere, at sea level. Q. Why do you add "at sea level"? A. Because at high altitudes the atmospheric pressure is less, as proved by the barometer column or indication falling as the height increases. Q. Does the locomotive runner have anything to do with pressures in vacuo? A. Certainly; because all the functions of expansion are referred to absolute pressures; that is, those above vacuum. Q. When steam at 50 lbs. gage pressure is permitted to expand into double its volume, what would its gage pressure be? A. About (50 14.7) 2, above vacuum, or 32.35 lbs. absolute or 47.05 by the gage. Q. Is it advisable to use a high grade of expansion? A. Certainly, when it does not cost more to get it than it is worth. * See also under "Horse-Power" and "Indicator." Q. Explain that remark. A. The higher the grade of expansion with a given initial pressure, the greater the cylinder volume required to get a given average pressure; that adds to the weight, size and cost of the engine and the trouble with cylinder condensation and shocks of reversal of stroke. Q. Why not increase the initial pressure? A. That has already been done; but that calls for stronger, hence heavier boilers, and increased fuel consumption, to haul the extra boiler weight and get the higher steam pressure and temperature. Also interposes difficulties with valve and piston lubrication. Q. About where is the best paying limit found, these days? A. With average cut-off at about half stroke, and initial pressure of 250 lbs. by the gage. Q. What is the usual average maximum cylinder pressure as compared with boiler pressure? A. About 90 per cent of the latter. Q. When is this attained? A. In full gear only. Q. In order to get the advantage of great power with economical cut-off, what means are possible? A. To use either three cylinders instead of two, with maximum cut-off at half stroke, or greater cylinder diameter. Q. What is the objection to three cylinders? A. Mechanical complication. Q. To increased cylinder diameter? A. Excessive width, where there are bridges and tunnels; also, decreased traction, owing to lessening ratio of m. e. p. to boiler pressure, due to earlier cut-off; and the necessity of having extra starting ports. (See under Slide Valve.) Q. At what points of cut-off are the best coal rates obtained? A. At early cut-off. Thus in recent tests the best were 1.68, 1.52, 1.54, and 1.70 lbs. per i. h. p. hour, at 15 per cent cut-off (at speeds of 240, 280, 320, 360 r. p. m. respectively), while at 25 per cent cut-off they were 240 and 320 r. p. m., 1.80 lbs. Q. To what does this point? A. To desirability of longer cylinders to develop as much power at 15 per cent as with former 25 per cent. Q. What is the nominal expansion ratio? A. The quotient of the stroke length by the time of full steam. Q. What is the actual expansion ratio? A. The quotient of the sum of the cylinder volume and the clearance volume, by the sum of the volume filled up to the point of cut-off, and the clearance volume (all volumes in cubic inches). Thus if the clearance is 7 per cent of the cylinder volume, the actual expansion rate for cut-off at half stroke is 1.07 0.57 1.877. = Q. How do you find the net work done by the steam for one piston stroke, with a given cut-off? A. Find the actual expansion rate, allowing for clearance. Get its hyperbolic logarithm from the table. Add one thereto. Multiply the sum by the time of full steam plus the clearance in feet. From the product take the clearance. Multiply the remainder by the total initial pressure in pounds per square inch. That gives the total work done in foot pounds per square inch, on the piston. To get the negative work per square inch of back pressure in foot pounds per square inch, multiply the average back pressure in pounds per square inch by the stroke in feet. Take this produce from the total work per square inch, to get net work per square inch. Multiply the remainder by the net average piston area (deducting half the rod cross section); this will give the net foot pounds per single stroke. Q. How do you find the admission period needed for any given actual expansion ratio? A. Multiply the clearance proportion by one less than the actual expansion ratio; take this from one; divide by the actual expansion ratio. CHAPTER LXIX THE INDICATOR Q. What is a steam-engine indicator? A. An appliance enabling one to ascertain clearly, among other things, the pressure at each point of the stroke for each end of the cylinder. Fig. 266. Crosby Indicator. Q. How is it constructed? A. There is a small cylinder (see Fig. 266) which can be put in steam communication with the counterbore at either end of the cylinder (it is better to use one for each end). In this there plays accurately, but |