"Verbal" notes and sketches for marine engineers : a manual of marine engineering practice, intended for the use of naval and mercantile engineer officers of all grades, and students, and is specially compiled for the use of engineer officers preparing for examinations of competency at home or abroad . me power orthe same degree of uniformity of turning moment. The four-stroketherefore means a longer engine, and necessarily a heavier one alsoThe valve-gear of the two-stroke engine, being actuated b) a shaft withthe same rotational speed as the crank-shaft, is simpler than that in afour-stroke
"Verbal" notes and sketches for marine engineers : a manual of marine engineering practice, intended for the use of naval and mercantile engineer officers of all grades, and students, and is specially compiled for the use of engineer officers preparing for examinations of competency at home or abroad . me power orthe same degree of uniformity of turning moment. The four-stroketherefore means a longer engine, and necessarily a heavier one alsoThe valve-gear of the two-stroke engine, being actuated b) a shaft withthe same rotational speed as the crank-shaft, is simpler than that in afour-stroke engine, and the reversing arrangements are much lesscomplicated. The two-stroke, however, requires the addition of thescavenge arrangements which are absent from the four-stroke, and 620 Verbal Notes and Sketches the necessity for supplying the energy for working these makes themechanical efficiency less. On the question of efficiency, however, itmay be urged that the four-stroke engine has to overcome the frictionof the piston, &c., for what maybe called two idle strokes out of everyfour, and this must, to some extent, counti^rbalance the energy neces-sary to work the scavenging pumps. In the four-stroke engine all thehot used gases have to escape jiast the exhaust-valves, which thus may I. ?*; No. 24.—Exhaust Valve, , and Cam. become abnormally heated. On the other hand, in the two-strokeengine they have to pass the bars between the exhaust-ports, and it isthought by some that although these parts of the cylinder are water-jacketed, they must become over-heated and lose their accuracy ofsurface, and it must be remembered that all the piston-rings have topass these bars every stroke. Extended experience will be requiredto settle all these points. It may be mentioned that an engine is Internal Combustion Engines 621 beinf^ made on the four-stroke system in which the major portion ofthe_ exhaust passes out of the cyHnder throut^h ports preciselyas in the case of the two-
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