This is the inverted vertical direct-acting engine, with two or more cylinders placed side by side directly over the shaft. It has the great advantage that the shaft can readily be put at the low level necessary for screw propulsion. Two, three and four cranks are employed, the arrangement with four cranks being specially suitable, as has already been pointed out, when a balance of the engine at high speeds has to be secured. In vessels of high speed and power the engines are often arranged in twin sets, on two shafts with twin screw propellers.
The marine engine is always furnished with a surface con denser, consisting of a multitude of brass tubes about inch in diameter cooled by sea-water which is caused to circulate through the condenser by means of a circulating pump. This pump and the air pump are often driven independently of the main engine.
It is in marine practice that the most powerful reciprocating engines are still to be found, although in the largest and fastest vessels the turbine has taken their place. Another and more recent rival is the internal combustion engine, using the Diesel cycle of action, which by 1928 had found favour in many new liners. The reciprocating engine continues, however, to be much used for sea-going vessels, especially those of the "tramp" class, most commonly in the triple expansion form. Often the third stage of the expansion is performed in two cylinders, making four in all. This avoids the use of a very large cylinder, and secures an advan tage in the better balance which can be obtained with four cranks.
engine is non-condensing, except in a very few special cases, and the exhaust steam, delivered at the base of the funnel through a blast-pipe, serves to produce a draught of air through the furnace. In some instances a portion of the exhaust steam, amounting to about one-fif th of the whole, is diverted to heat the feed-water.
On the shaft are a pair of driving-wheels, whose frictional adhe sion to the rails furnishes the necessary tractive force. Nearly always a greater tractive force is secured by having two or more driving-wheels on each side, connected by a coupling-rod between pins on the outside of the wheels.
It is general to have under the front of the engine a group of smaller wheels which do not form part of the driving system. These are carried in a bogie, that is, a small truck upon which the front end of the boiler rests by a swivel-pin or plate which allows the bogie to turn, so as to adapt itself to curves in the line, and thus obviate the grinding of tyres and danger of derailment which would be caused by using a long rigid wheel base.
In inside-cylinder engines the cylinders are placed side by side within the frame of the engine, and their connecting-rods work on cranks in the driving shaft. In outside-cylinder engines the cylin ders are spread apart far enough to lie outside the frame of the engine, and work on crank-pins on the outsides of the driving wheels. Many modern locomotives combine the outside and inside arrangement, in order to get greater power, placing one or in some cases two cylinders within the frame, as well as two outside.
The principle of compounding has often been applied to loco motive engines, but without much advantage. (See LocomonvEs.)