Marine Engineering

MARINE ENGINEERING. There are three distinct types of marine engines, and these may be grouped as follows: (I) steam engines with coal or oil-fired boilers; (2) internal combustion engines, such as the diesel or semi-diesel engines; and (3) the above two types with electrical transmission of power between the prime mover and the propeller shaft; viz., engines with turbo-electric or diesel-electric drives. Of course, group (I ) includes reciprocating steam engines, turbines (geared or otherwise) or combined turbine and reciprocating engines.

These types are described on subsequent pages.

Reciprocating Marine Steam Engines.

All marine recip rocating engines now installed are stage-expansion engines, that is, steam expands in stages from the highest pressure cylinder, through the intermediate stages, and finally exhausts into the condenser, where it is condensed and returned to the boiler in the form of feed water.

Stage-expansion engines may be grouped as follows . Com pound:—In this type there are two cylinders, the high pressure and the low pressure. Steam actuates the piston in the high-pres sure cylinder, and from there it passes into the low-pressure cylinder, completes its work in this cylinder, and finally exhausts to the condenser. Triple expansion.—In this engine, as its name implies, steam exerts its power in three stages before exhaust ing to the condenser. Quadruple expansion.—In this type the steam is used four times before exhausting into the condenser, the usual design being to fit two intermediate cylinders.

General Construction.

The engine cylinders are supported by columns, which also act as a guide for the crossheads. This crosshead forms a hinged joint between the piston rod and the connecting rod, which is provided with turned pins on either side, connected to the forked end of the connecting rod, and provided with suitable bearings. The connecting rod is in turn connected to the crank with suitable bearings. The eccentrics governing the valve motion are keyed on to the shaft, the eccentric straps usu ally being made of a brass composition, lined with white metal. Link motion affords a means of quick reversal, and also allows an earlier or a later cut-off of the steam by regulating the valve admitting the steam to the cylinders. The link operates in con junction with two eccentrics, and is actuated by means of a small steam reversing engine, or by a hand wheel connected with a re versing shaft. The reversing mechanism is placed along the front of the engine. Fig. i shows details of the high-pressure and low-pressure pistons of a high-powered marine reciprocating steam engine.

The advent of the marine steam turbine, the Lentz Double Com pound, and the marine internal-combustion engines, and the elec tric transmission of power for propelling machinery certainly has led many shipbuilders to install one or the other of these types, but it is interesting to note that in some recent large liners the carefully balanced triple-expansion engine has been installed.

Experiments are being carried out to utilize steam at a much higher pressure than formerly, and excellent results are expected.

Combined Reciprocating Engines and Steam Turbines.— This method ensures that the whole of the expansive energy of the steam is utilized to its fullest extent. An excellent example of this arrangement is the White Star "Olympic." The arrangement has proved an economical one, and is as follows: each wing pro peller shaft is driven by a reciprocating engine, the low pressure asset in the case of warships and passenger steamers; (d) engines are placed well down in vessel, an all-important point in war ships; (e) there is less expenditure of lubricating oil; (1) there is no cylinder lubrication, thus clean feed water is returned to the boilers; (g) there is absence of vibration, a big factor in warships and passenger steamers; smaller attendance is re quired than in the case of reciprocating steam engines; (i) there is superior governing (balance), good parallel running and even turning moment ; (j) priming (the passage of water mixed with the steam from the boilers), which would fracture the cylinder covers of a steam reciprocating engine, has no material effect on a marine steam turbine ; (k) overloading (forcing the turbines) can be indulged in within reasonable limits; (1) it is more eco nomical than the reciprocating marine steam engine except at the lowest speeds.

The large-sized turbines are more economical in steam per horse-power developed than the best triple or quadruple expan sion engine, as the turbine is able to take full advantage of the whole of the expansive energy of the steam. Two well-known types of marine turbines in general use are the combined impulse and reaction turbine and the impulse turbine. There are naturally many modifications of these types.

Combined Impulse and Reaction Turbines.

This arrange ment consists of the usual reaction turbine with a single impulse stage, in the nozzles of which there is a considerable pressure and consequently loss of heat, giving a large proportion of effective work in this stage. The effect of this is the shortening of the turbine with economy, and permitting for moderate powers of the complete expansion of the steam on a single shaft. There is a saving in weight and space, the machinery arrangement is simplified, and the propulsive efficiency is improved with slower propeller speed. The nozzles are grouped in a box on the forward side of the cylinder, the impulse rotor blading is fitted on a rim, cylinder of which exhausts into a Parsons low-pressure turbine driving the central shaft.