TM 5-4240-501-14&PTHEORIES OF OPERATIONCompressionSection 14THEORIES OF OPERATIONCOMPRESSIONThe general subject of compression is a familiar one tomost mechanics. It has been discussed in detail byvalve manufacturers, ring manufacturers, pistonmanufacturers, and by makers of valve grindingequipment. The home mechanic, or handy-man, thinksnothing of getting out his grinding compound, lapping inthe valves and putting a new set of rings on the piston -all without knowledge of proper fit or tolerance. Whetherhe does the job right or not, he thinks it is easy. And, it iseasy. There is nothing difficult or mysterious aboutcompression, and the nice part is that a good job that willcreate lasting customer satisfaction is about as easy todo as a poor job.We must keep in mind, however, that the Briggs &Stratton engine is an air-cooled, single cylinder engine.The rules that hold true on liquid cooled, multi-cylinderengines do not always apply to Briggs & Strattonengines. For example:The operating temperature of a liquid-cooled engine isquite constant. The operating temperature of an air-cooled engine, however, may vary greatly with changesin air temperature, the load, and the speed. Thisnecessitates differences in tolerances and clearances ofparts like pistons, which must be fitted to Briggs &Stratton’s established clearances. These can differ fromthose used in most automotive engines.The advantages of a n air-cooled engine are many.There is no need for a complicated cooling system. Theengine is lighter in weight and occupies less space thanits liquid-cooled counterpart, and is comparatively easyto repair.Before we get into the mechanics of the subject, let usclarify some of the terms in common use.On single cylinder engines we think of goodcompression, not in terms of pounds of pressure persquare inch, but in terms of horsepower output. If theengine produces the power for which it was designed, webelieve the compression must be good. It is extremelydifficult to make an accurate compression test on asmall, one cylinder engine without expensive machinery.The reasons for this are the lack of a starter to crank theengine at a constant speed and the small displacementof the cylinder. Therefore, we do not publish anycompression pressure figures. As a simple compressiontest, give the flywheel a quick spin. If the flywheelrebounds on the compression stroke, the compression isat least good enough to start the engine.We talk about " compression" stroke and "power stroke".What are they? The Briggs & Stratton engine is a fourstroke cycle engine, or as it is commonly called, a fourcycle engine. It operates on the same principle as anautomobile engine. The crankshaft makes two completerevolutions to each power stroke of the piston.114
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