Marine Internal Combustion Engines

MARINE INTERNAL COMBUSTION ENGINES One of the principal reasons which has led to the adoption of the internal-combustion engine for marine propulsion is the thermal efficiency of this prime mover. The thermal efficiency of a diesel engine is 40% to 45%, according to size; whereas that of a reciprocating steam engine with coal-fired boilers is 2o% to 25%. As regards running, the fuel bill is about half that for an oil-fired boiler ship fitted with geared turbines; there is a very great saving of space occupied by the machinery, and a reduction in personnel. Against these advantages we have the greater initial cost, complication of parts, a greater number of spare parts have to be carried, and it is probable that the repair and upkeep bill will exceed that for steam units. For a given power the diesel compares unfavourably as regards weight with the steam tur bine and oil-fired boiler combination, and the head-room is greatly limited (this, especially from a naval point of view, is a marked disadvantage) ; but the possibilities of this prime mover are endless. The diesel has been and is being installed in large liners, and doubtless when a speed of, say, 25 knots can be reached, combined with a simple and reliable design of engine, this type of engine will be installed in the mammoth Atlantic record-breakers. The present types of marine internal-combus tion engines work either on the two-stroke or on the four-stroke principle ; but many competent authorities affirm that the marine oil engine of the future will be a double-acting two-stroke with port-scavenging, this design giving an engine of low initial cost and simple construction. The solid (or mechanical) injection of the fuel is likely to be adopted generally in preference to the air injection system, owing to the reduction in first cost that is ob tained by its use.

Operation of the Four-stroke Marine Diesel Engine.— The working principle of this engine is as follows : during the first downward stroke the piston draws air in through the suction valve ; during the return stroke the suction valve and all other communications with the atmosphere are closed and the air in the cylinder is compressed. Towards the end of the stroke, the fuel pump injects into the cylinder the necessary quantity of oil for the combustion stroke, so that when the piston arrives at dead centre the fuel burns rapidly, raising the pressure and temperature in the cylinder. During the next downward stroke of the piston the burnt gas is expanded, doing work; during the fourth stroke, the piston sweeps out the burnt gasses into the atmosphere through the open exhaust-valve, completing the f our cycles. Fig. 5 shows a section through one cylinder of a f our stroke marine engine by Burmeister and Wain. For single-screw ships, long-stroke engines running at low revolutions are em ployed in order to obtain a good propeller efficiency. For twin

screw ships, short-stroke engines are used, as higher speeds are permissible without reducing the propeller efficiency.

The Two-stroke Marine Diesel Engine.

A good example of this marine internal-combustion engine is the Sulzer two-siroke, a section through this engine being shown (fig. 6). The distinc tive feature of this design is the method of scavenging and re charging the cylinder ; i.e., removing the burnt products of the firing stroke and replacing these with a charge of fresh air. This is accomplished as follows : The scavenge air supplied by a pump is led to the scavenge trunk, which communicates with the cylin der through two ports at the bottom of the liner, the upper port being controlled by a valve. When the piston is at the top of its stroke, and combustion takes place in the usual manner, both ports are closed by the piston skirt. The piston then moves down and first uncovers the upper scavenging port, but the valve being closed no action occurs until the piston moves farther down and uncovers the exhaust ports situated opposite the scavenge ports. Exhaust commences immediately and relieves the pressure in the cylinder, so that when the lower range of scavenging ports is uncovered the air enters the cylinder and commences to sweep out the remainder of the exhaust products. The scavenging valve opens before the completion of the down stroke, so that the scavenging takes place through both upper and lower ports, and continues through the upper port until after the piston on its upward stroke has covered the exhaust port. This arrangement ensures a most effective means of clearing out the exhaust prod ucts, and also provides for an excess quantity of air by enabling the compression stroke to commence slightly above atmospheric pressure. In other words, supercharging is possible. In marine engines of over i,000 h.p. electrically driven scavenging pumps are provided.

The Diesel-electric Drive.

The application of this drive has been made to smaller ships. The employment of electric transmission allows comparatively small diesel engines, each coupled to a generator, to be used, the power being transmitted to motors on the propeller shafts. The arrangement and speed of the engines are independent of the propeller shaft, and the engines are short and not of excessive height. As there is no mechanical connection between the engines and the propeller shaft, the sub division of the ship with watertight bulkheads, to comply with the regulations, is simplified. In vessels fitted with this type of machinery it will be possible to use the same prime mover for both propelling and auxiliary purposes.