piston displacement

TM-5-4240-501-14P Special Diver Air Support System (SDASS) NSN 4220-01-292-3708 Manual
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TM 5-4240-501-14&P Figure 3. What is "piston displacement"? It is the space displaced by the piston in its up and down movement or the volume shown above the piston in Figure 3. The bigger the bore and the longer the stroke, the greater the piston displacement. Displacement is computed by the following formula: Displacement = ^(Bore)2 X p X Stroke 4 Let us compute the displacement of a Model 6 engine which has a 2" bore and a 2" stroke. Using the above formula: 2X2 Displacement = X 3.1416 X 2 4 Displacement = 6.2832 cubic inches Our specification sheets show 6.28 cubic inches as the displacement for the Model 6 engine. The model numbers of the current engines indicate the approximate piston displacement. Model 60000 has 6.65 cubic inches; Model 14 has 14.21 cubic inches, etc. Piston displacement indicates the relative size of the engine, and usually horsepower is in direct proportion to size. THEORIES OF OPERATION Compression Figure 4. What do we mean when we say an engine has a 6 to 1 compression ratio? We mean that the space in the cylinder when the piston is at the top of the stroke is only one-sixth as great as when the piston is at the bottom of the stroke. Compression ratios do not tell us the horse-power of an engine. They do have a meaning as regards the efficiency of an engine. Generally, the higher the compression ratio, the greater the efficiency. However, as compression ratios are increased, the loads and stresses upon engine parts become more severe. Premium fuels may be required with high compression ratios. Experience has proven that compression ratios in the range of 5 -1 to 6 -1, currently used in Briggs & Stratton engines, are the best for the work and the conditions under which these engines must operate. Therefore premium fuel is not needed and "regular" is recommended. It is generally conceded that the valves are the most important factor in good compression. They operate under more severe conditions than any other parts of the engine. This is particularly true of the exhaust valve. Figure 5. The valves open and close in a little less than one revolution. When the engine is operating at 3000 RPM, each valve opens and closes in about 1/50 of a second. 3 14

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