MARINE ENGINES. Marine engines have dur ing recent years tended to one general type. For special services on inland waters a great num ber of various specialties are found, but in sea going steamers the type in almost universal use is the vertical (i.e., the piston moves vertically), inverted (i.e. the cylinder is above the crank), direct-acting (i.e. the connecting rod joins the crosshead directly to the crank), triple (or quadruple) expansion engine. Engines differ as regards fittings and attachments, length of stroke and revolutions, weight, speed, etc., but a de scription of the general type will give the es sential features of all.
Naval engines are built lighter, have a shorter stroke, and run at higher speeds than those in the merchant service. In a triple-expansion engine the steam works expansively in three successive stages, in a quadruple in four. The reason for the introduction of the multiple-expansion en gine is the greater economy obtained when steam is used expansively over a greater range. This cannot be efficiently accomplished in a single cylinder owing to various causes chiefly due to liquefaction, hence compound (two stage) en gines were then the triple and quad ruple. The economy gained by the compound over the simple is about 50 per cent., by triple over compound about 25 per cent., and by quadruple over triple about 10 per cent. In the quadruple the gain in economy is obtained by considerable increase in weight, so that for most services the real gain of the quadruple is questionable. The type of engines will depend largely on the steam pressure employed. For a pressure of 40 to 90 pounds a compound; up to 190 pounds the triple; above 190. the quadruple may be used if space and weight are not very important. At present the pressure used is ordinarily between 140 and 250 pounds per square inch above the atmosphere. A great advantage of multiple-expansion engines, and a cause for their adoption, is the more even turning effect and better balancing obtained. The triple-expansion engine has either three or four cylinders, more often three, arranged in suc cessive order, H.P. (high pressure), I.P. (inter mediate pressure). L.P. (low pressure). each cyl
inder being attached by means of its piston and connecting. rod to its own crank on the erank shaft, which is usually made in interchangeable sections, one for each crank. Cranks are usually set at 120° from each other to obtain even turn ing effect. Four cylinders are used when the L. P. cylinder would be too large to be con veniently fitted or built, or to obtain a better balancing of the engine and reduce vibrations. The sequence of the cylinders is then high pres sure. intermediate pressure, low pressure, low pressure; or, on what is known as the Yarrow Schlick-Tweedy system, low pressure, high pres sure, intermediate pressure, low pressure, with cranks set at right angles. Here the crank shaft is generally in two sections.
The course of steam in a triple-expansion en gine would be as follows: Leaves main steam pipe. passes through separator, then throttle, and into high-pressure valve chest. The movement of the valve opens and closes the steam ports at pressure cylinder for 0.0 to 0.75 of the stroke and then cuts off. The steam in the cylinder then expands, continuing to move the piston. Just before the end of the stroke the valve opens to ex haust and at about the same time begins to allow steam to enter on the other side of the piston; this results in cushioning at the end of the stroke.
The steam having been reduced about 50 to GO per cent. in pressure and correspondingly in tem perature, leaves the high pressure cylinder, and passes to the intermediate pressure receiver, which is really the intermediate pressure valve chest. From here it enters through the intermedi ate pressure valve and does its work in the inter mediate pressure cylinder, being again reduced in temperature and pressure. On leaving the intermediate pressure cylinder the steam is gen erally at about atmospheric pressure. Then it is conducted to the low pressure receiver and goes through its third stage of working and ex pansion. On leaving the low pressure it goes through the exhaust pipe to the main condenser where it is condensed, and then as water and vapor it is pumped by the air pump to the hot well or feed tank, and thence to the boiler, where it is reevaporated.