TEE PRINCIPLES OF
Pig. 230.
Scale f inch = 1 foot.
UUROi/S DIAGRAM OP THE PATHS OP THE PISTON AND ORANK PIN.
relative proportion is, therefore, readily understood.No. 1 is the chord of supply, No. 2 that for expansion,No. 3 relates to exhaustion, and No. 4 represents
TEE MARINE ENGINE. 347
neutrality, or that portion of the stroke of the pistonwhen the port on the exhaust side is covered, often termedcompression; consequently, the piston for a period is
Fig. 231.
buegh’s diagram op the path op the crank pin in relation
TO THE SLIDE VALVE’S ACTION.
dovoid of pressure or vacuum. The length of the chord 1is due to two causes, which are the grade of expansionand the length of the connecting-rod. It will be noticedthat the chord at the plane line intersects with the circleslightly below the same. This last intersection is theangle that the crank assumes when the slide valve com-mences to open the port, and the vertical distance fromthe intersection to the plane line is due to the lead re-quired. The upper point of intersection, as before ex-plained, is subject to the curve assumed by the connecting-rod from a point, or distance, on the plane line to thecircle of the crank path. The length of chord No. 2 isregulated by the inside and outside laps of the slidevalve. The expansion of the steam is now in full opera-tion, and is released by the opening of the port on theexhaust side, hence the intersection of the chord atNo. 3. The length of the last-mentioned chord is morethan any other, due to the traverse of the valve, or thetime occupied in opening and closing the port for ex-haustion. Now in the case of an increase of supplysteam, the time for expansion and exhaustion would belessened in proportion, it being remembered that thecircle described by the crank pin cannot be increased ordecreased for a given length of stroke of piston. Thecircle, as before stated, is divided into four divisions,and the alteration in the grade of expansion or length ofconnecting-rod affects the whole proportionately. Theconcluding chord, No. 4, represents neutrality, or, asbefore stated, that portion of the stroke of the pistonwhere the vacuum and steam is cut off for a givenperiod, commonly known as compression. It will beremembered that the chord of expansion is due to theoutside and inside laps, i.e., when the valve is at theedge of the supply side of the port, the valve has to
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