TM 5-4240-501-14&PFigure 3.What is "piston displacement"? It is the space displacedby the piston in its up and down movement or the volumeshown above the piston in Figure 3. The bigger the boreand the longer the stroke, the greater the pistondisplacement. Displacement is computed by thefollowing formula:Displacement = (Bore)2 X p X Stroke4Let us compute the displacement of a Model 6 enginewhich has a 2" bore and a 2" stroke. Using the aboveformula: 2X2Displacement =X 3.1416 X 24Displacement = 6.2832 cubic inchesOur specification sheets show 6.28 cubic inches as thedisplacement for the Model 6 engine.The model numbers of the current engines indicate theapproximate piston displacement. Model 60000 has 6.65cubic inches; Model 14 has 14.21 cubic inches, etc.Piston displacement indicates the relative size of theengine, and usually horsepower is in direct proportion tosize.THEORIES OF OPERATIONCompressionFigure 4.What do we mean when we say an engine has a 6 to 1compression ratio? We mean that the space in thecylinder when the piston is at the top of the stroke is onlyone-sixth as great as when the piston is at the bottom ofthe stroke.Compression ratios do not tell us the horse-power of anengine. They do have a meaning as regards theefficiency of an engine.Generally, the higher the compression ratio, the greaterthe efficiency. However, as compression ratios areincreased, the loads and stresses upon engine partsbecome more severe. Premium fuels may be requiredwith high compression ratios. Experience has proventhat compression ratios in the range of 5 -1 to 6 -1,currently used in Briggs & Stratton engines, are the bestfor the work and the conditions under which theseengines must operate. Therefore premium fuel is notneeded and "regular" is recommended.It is generally conceded that the valves are the mostimportant factor in good compression. They operateunder more severe conditions than any other parts of theengine. This is particularly true of the exhaust valve.Figure 5.The valves open and close in a little less than onerevolution. When the engine is operating at 3000 RPM,each valve opens and closes in about 1/50 of a second.314
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