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The first step in proportioning the cylinder is to compute the approximate mean effective pressure from the assumed initial pressure, clearance, and cut-off, by the method already explained. Next assume the piston speed for the type of engine to be designed, and determine the piston area by the following formula:

This formula usually gives the diameter of the piston in inches and fractions of an inch, while it is desirable to make this dimension an even number of inches. This may be done by taking as the diameter the nearest whole number, and changing the piston speed to correspond. This is done by the use of the following equation.

In calculating the effective piston area, the area of the piston rod upon one side must be allowed for. The effective or average piston area will then be (2A - a)⁄2, in which A = area of piston, a = area of piston rod. This latter area must be assumed. After assuming a new piston diameter of even inches, its effective or average area must be used in determining the new piston speed. The length of stroke is commonly proportioned to the diameter of cylinder, and the piston speed divided by this will give the number of strokes per minute.

Example:—Find the diameter of cylinder, length of stroke, and revolutions per minute for a simple high-speed non-condensing engine of 200 I. H. P., with the following assumptions: Initial pressure, 90 pounds gage; clearance, 7 per cent; cut-off, 1⁄4; piston speed, 700 feet per minute; length of stroke, 1.5 times cylinder diameter.

By using the rules and formulas in the foregoing, we have:

M. E. P. = (90 + 15) × 0.63 - 17 = 49 pounds.

The nearest piston diameter of even inches is 16, which corresponds to an area of 201 square inches. Assume a piston rod diameter of 21⁄2 inches, corresponding to an area of 4.9 square inches, from which the average or effective piston area is found to be (2 × 201) - 4.9⁄2 = 198.5 square inches.

Determining now the new piston speed, we have:

Assuming the length of stroke to be 1.5 times the diameter of the cylinder, it will be 24 inches, or 2 feet.

This will call for 678.5 ÷ 2 = 340 strokes per minute, approximately, or 340 ÷ 2 = 170 revolutions per minute.

CHAPTER III

STEAM ENGINE DETAILS

Some of the most important details of a steam engine are those of its valve gear. The simplest form of valve is that known as the plain slide valve, and as nearly all others are a modification of this, it is essential that the designer should first familiarize himself with this particular type of valve in all its details of operation. After this has been done, a study of other forms of valves will be found a comparatively easy matter. The so called Corliss valve differs radically from the slide valve, but the results to be obtained and the terms used in its design are practically the same. The valve gear of a steam engine is made up of the valve or valves which admit steam to and exhaust it from the cylinder, and of the mechanism which governs the valve movements, the latter usually consisting of one or more eccentrics attached to the main shaft.

The Slide Valve