On account of the cylinders of the four-cycle engine being filled by pure air, the combustion is very fine, and the consumption of fuel oil in b.h.p. is 15 to 20 per cent smaller than for a corresponding two-cycle engine, and as that quantity of oil which is burned without being transposed into useful work will be lost as heat in the exhaust gas, and will heat up the different parts of the engine, the four-cycle engine will work at a lower temperature, which will increase the durability of the engine, reduce the costs of upkeep, and allow uninterrupted non-stop runs for very long periods ; and many voyages have been made with Burmeister & Wain motor ships of a duration of fifty to sixty days without any stoppage.
The engine is built as an enclosed, forced lubricated engine. Tins design has been adopted from the very first of the motor ships, the M.S. Selandia, and the result of this was an absolute working reliability of the engines, with no wear and tear of the rotating parts, and therefore attendance to the machinery is independent of the number of cylinders and appurtenance:.
The Burmeister & Wain Diesel engine installations are constructed in such a way that not only the main propulsion power is delivered by Diesel engines, but also all the auxiliary machinery which is necessary in a modern. first-class ship is driven by Diesel engines, the power being transferred through electricity. All the necessary auxiliary pumps, such as cooling water pump. forced lubricating oil pump, bilge and sanitary pumps. auxiliary compressor, etc., are coupled to electro motors, driven from auxiliary Diesel dynamos, which Also give the necessary power for working the deck machinery, as electric steering gear, electric windlass, cargo winches, lighting and wireless telegraphy. This arrangement has effected considerable saying compared with the old manner of steam working auxiliaries, both during daily working at sea and during loading and unloading at port.
The best example I can select of the two-cycle type is that constructed by Messrs. Sulzer Bros.
The Sulzer engine is of the two-cycle type, with four or six cylinders working on one propeller shaft.
The cylinder head is of simple symmetrical construc tion. containing only a fuel valve and a starting valve in a common casing. Ample cooling spaces, ensuring efficient water circulation, are provided. The exhaust pipes are also efficiently cooled, as well as the partitions in the cylinder liner between the exhaust ports.
The pistons are cooled with sea water or with oil.
The cooling medium is conveyed to and from the piston by means of a patent device, which, while avoiding the use of all stuffing boxes or swivel points, nevertheless keeps perfectly watertight. The whole cooling system works unaer atmospheric pressure.
All the principal parts are fitted with forced lubrication, which experience has shown affords the best guarantee for reliability, and reduces wear to a minimum. The lubricating oil is cooled and filtered in its circulation in an easily accessible double filter.
Starting is effected by means of compressed air supplied from an amply dimensioned air injection compressor, and stored in a number of starting vessels for repeated manoeuvres. The air injection and starting vessels can be controlled from the engine platform. A fuel pump is fitted for each working cylinder, the individual parts being contained in a common casing. The air injection compressor is of the three stage type, with efficient cooling, separators, and safety valves between the stages. The valves of this air compressor are of the plate type of special construction, and can be easily and quickly adjusted, cleaned, or changed.
In the Sulzer Patent system of scavenging, in which two rows of ports are providea in the cylinder liner, the direction of the air currents causes a more thorough scavenging of all parts of the cylinder, and the exhaust gases are effectively expelled. With the system of scavenging through valves in the cylinder head, the scavenge air tends to take the shortest way to the exhaust ports, without properly displacing the products of combustion in the upper part of the cylinder.
This system of scavenging obviates the possibility of pre-ignition in the cylinder and explosions in the receiver on account of the valve which controls the admission of air to the ports being protected from the hot gases by the piston, and not coming in contact with them whilst at their highest temperature. The scavenge valve works always in the coolest part of the cylinder, and any entrance of unbunit fuel vapour into the scavenge air receiver is practically impossible.
Three other types which possess points of special interest ale the Camellaird-Fullagar, the Doxford, and the Vickers. It is essential that the student of this branch of engineering should be acquainted with these and I therefore deem it advisable to introduce brief descriptions of them. The two former are of the opposed piston type, and the latter is known as the solid-injcc.ion type.