The employment of air compressors for the injection of the oil is also a matter on which considerable difference of opinion exists, some authorities contending that greater efficiency is secured thereby as contrasted with mechanical injection. It has been asserted that the compressors employed with marine Diesel engines are larger than is necessary for the work they are called upon to do, with the engine even at full speed. It is essential, of course, that the compressor shall be of such power as to be equal to this stage of working, and there must be a certain amount of waste energy when the engine is running at less speed. The compressor must naturally be capable of meeting all demands. The air injection system is claimed to provide a better fuel constunption than any other method. It undoubtedly possesses an elasticity which no other system has so far provided and enables practically any grade of oil to be used apart from its other improvements. The Camellaird Fullagar engine is able to start from cold oil, with a specific gravity of .96, and containing even 4 per cent of ash. The mechanical injection system has certain advantages to recommend it, though it is not generally adopted, and the energy available for pulverizing the oil is not as great as in the case of the air compressor. It is clear from the statements made that the design of the marine Diesel engine has not yet reached perfection and that there exists a difference of opinion among engineers on many important points ; the progress made in its improvement has, however, been greater than has been the case of the steam engine This is shown in a table compiled by Sir Dugald Clerk some short time ago, which is as given below. The figures bring this aspect of the subject down to 1914, and practically represent the position at the present time.
Expressing the improvement indicated in percentage figures the table shows that with the steam engine the progress made represented by increased indicated horse-power was 81.10 per cent, and in brake horse-power 71.05 per cent for the internal combustion engine, the percentage progress during the period was in indicated horse-power obtainable 131.25 per cent and in brake horse-power 121.25 per cent.
It is evident from a study of the whole question that an important aspect of the Diesel engine construction in order that it may attain a greater amount of relia bility is that concerned with metallurgy and methods of manufacturing the different parts. Great importance, for instance, attaches to the manufacture of the castings, and it is a point which is being realized in this country. In some instances firms are setting up their own plant for the production of these, on which the reliability and efficiency of the engine so much depends. This will enable them to test every casting thoroughly before use. Among Continental manufacturers the greatest care is taken in the production of castings and in the selection of the metal, and for such vital parts as liners, cylinder covers, valves, and pressure parts, the cast iron is melted in the electric furnace or reverlxttory furnace, a slightly more costly method, but ensuring a high grade product. With an engine on which such great strains
are placed it is essential that the raw material and the ultimate product should pass the most rigid tests.
Another variation of recent invention is that known as the Sproule engine, the object of which is to provide an internal combustion engine having a steam engine " characteristic," that is to give high efficiency at light as well as full load, and to give an overload capacity with slightly falling efficiency and to be easily reversible.
The latest types of engines show distinct progress in design, and, where constructional and metallurgical difficulties have arisen, they are being removed by con stant study and experiment on the part of those engin eering firms ttlio have placed and kept British crafts manship in the forefront. The main lines along which development should be made are those leading to simpli fication of design and economy of production. Up to certain powers reliability and efficiency have been attained, and such excellent progress has been made in the designing and construction of larger units, that powers of 6,000 and over are anticipated by recognized authorities in the near future. The economic aspect of the application of the Diesel engine to marine work is an important part of the subject, and I have thought it preferable to retain this for treatment in a separate chapter.
Reference must also be made to what has been termed the Semi-Diesel engine, but more correctly known as the Hot-bulb type. The differences between these two classes of engine may be briefly stated. In the first place, in the true Diesel engine the heat required for combustion is induced within the engine by compression and as a part of the operation, whereas in the hot-bulb type the heat is produced artificially. The compression in the former may be stated at 450 lbs. compared with between 150 to 200 lbs. for the latter. Fuel injection and pulverization in the Diesel engine is produced by an air blast of from 800 to 1,000 lbs. the square inch, but in the other type this fuel injection is effected by a small plunge pump to each cylinder driving the oil into a heated chamber or hot-bulb ; the oil is ignited by the compression stroke. For starting up the engine a blow-lamp is user to heat the bulb, which, once the engine is running, remains heated by the explosion of the gas. In a few instances compressed air is employed for the injection of the fuel, but the method is not at all generally adopted.
The hot-bulb engine, being of the heavy, slow-running type is chiefly used by small commercial ships, such as coasting vessels, fishing boats, barges, and similar craft. The fuel best adapted to this engine are gas oils, solar oil, which is of higher specific gravity than fuel oil, and heavy paraffins. The sizes of this type of engine vary between 3 b.h.p. and 500 b.h.p. There are a considerable number of firms constructing these engines, for which there is an increasing demand, and, although the fundamental principles of design are the same in each engine, over forty varieties exist embodying some improvement in detail.