« НазадПродовжити »
port. When the crank pin has reached the back dead centre, the angle of the eccentric rod will be in reverse direction from its appearance in the cut, and the centre of the valve will be the same distance from the middle of its seat as is shown, but to the left, thus providing the lead opening to the back, or right-hand, admission port. If lever R should be moved further orward, the increased angle of the link would give a longer forward-and-back motion to the link-block and a lengthened travel of the valve; hooking up will have the opposite effect, until with lever R in mid-gear the only motion given to the valve would be its short motion equal to twice the lap plus lead. With the engine standing as shown in the cut, if lever R should be thrown into back gear, it is plain that, as the eccentric's movement is upward as the wheel turns backward, the link-block will be moved back and up, and the valve will again be moved in the right direction.
With a valve of inside admission, the eccentric must be located 180 degrees from the crank pin-just opposite its present position and this fixes the valve's advance and prospective travel in the opposite direction to the design here shown.
In this style of valve gear (as in Walschaerts') the lead is constant; proven when the crank pins are on either dead centre, by shifting the reverse lever from
one full gear position to the opposite extreme by which the valve will not be moved.
THE HOBART-ALLFREE CYLINDERS,
VALVES AND VALVE GEAR Cylinders and Valves.-The Hobart-Allfree cylinder and piston, valves and valve chambers, are shown in Figs. 42, 43, and 44. The advantages claimed in their use are increased pulling power and greater speed, economy in steam and fuel, cheaper cost of maintenance and more mileage secured from the engine, due to-reduced heat losses (short ports, and the insulation of the live steam); less steam used for a given cut-off (waste spaces in cylinders reduced to a minimum); higher ratio of expansion (as the result of dividing the steam distribution between two valves); perfect exhaust (for the same reason, and giving a better draft on the fire, with reduced back pressure); reduced “negative” work (very late exhaust closure only enough compression to provide for the cushioning of the piston); together with an increased turning effort on the drivers.
Of the four valve events embraced in the proper steam distribution in the cylinders of an engine, the single valve of any style can perform three perfectly: admission, cut-off, and release, but the fourth—known as the closure or compression event, is wrong. If the